Preface

Moving to smart grids to cyber-physical–social systems, moving from 5G to 6G, and moving from navigation assistance to intelligent driving, our world is changing and ubiquitous connections are beyond imagination. Emerging terahertz (THz) technologies, which could potentially change people's lives in every aspect, motivate us to merge, expand, and utilize THz bands for a variety of new designs, materials, and technique architectures by bundling integrated circuits, signal processing, energy harvesting, sensing, imaging, tomography, and data transmission.

With the increase of mobile traffic, several limitations in the existing social–physical world have emerged. Fortunately, new THz band and intelligent transportation system (ITS) technologies are promising candidates to combat these limitations. THz is an underdeveloped ultra-wideband zone between microwave and infrared that bridges the gap between electronics and optics. ITS can benefit from deep learning-based traffic smart electronics, control circuits, and vehicle-to-anything connections through road networks. To build extended wide-band systems that meet the aforementioned needs, researchers and engineers have introduced new advances in hybrid electro-photonics to THz technology and ITS fundamental designs. Great attention has been drawn to in-depth elaboration on information awareness and intelligent info-energy fusion.

THz has many applications across different research disciplines due to the advantages of electronics and optics. It is shaping a future wireless world and promoting initiatives on exploring brand-new integrated circuits for ITS, reconfigurable intelligent surfaces (IRS), nondestructive evaluation, simultaneous wireless information and power transfer, security, imaging, and signal processing.

This book provides a thoughtful and comprehensive understanding of the state of the art in THz science as well as novel technologies that promote essential aspects of contemporary ITS. It includes new research across different disciplines, from theoretical basis to experimental design, and from featured architectures to practical applications. The book is divided into four sections: "THz Science", "System Dynamics", "Smart City Essentials", and "Intelligent Device".

Section 1 discusses the science of THz technology. Chapter 1, "Broadband Terahertz Emission from Photoconductive Devices", discusses recent advances and studies on photoconductive components for THz emission, in which several materials have been employed to be photoconductive. It covers materials, quantum dots, nanostructure, dielectric materials, and the grating structure on photoconductive surfaces. It also examines screening effects. Chapter 2, "Studies of Terahertz Sources and Their Applications", focuses on THz radiation and its generation mechanism through laser-plasma interactions. It is pointed out that higher-power THz sources and their corresponding detector sets may be required for widespread applications.

High-frequency harmonics could cause serious system performance decay in intelligent automatic systems. If they are not smoothed, best performance cannot be achieved in an actual intelligent system. Section 2 includes Chapter 3, "Research and Application of PID Controller with Feedforward

Filtering Function", which studies industrial automatic system dynamics and proposes a new design for a PID controller with a filtering function, which offers weak noise amplification and strong harmonics' reduction with a smoothing effect.

Section 3 discusses advances in intelligent urban road networks that promote the development of smart cities. Chapter 4, "Traffic State Prediction and Traffic Control Strategy for Intelligent Transportation Systems", provides a comprehensive review of traffic status prediction and control techniques. Two deep learning approaches, convolutional neural network (CNN) and long short-term memory (LSTM) models are combined and realized for hybrid traffic state prediction with improved accuracy. Moreover, decentralized multi-agent advantage Actor-Critic technique and Nash Q learning is introduced for traffic signal control applications, being able to converge to the local optimum and overcome the scalability issue induced by city traffic policy updating for district neighborhood(s). Chapter 5, "Vehicle-To-Anything: The Trend of Internet of Vehicles in Future Smart Cities," discusses a variety of emerging technologies for intelligent roads and vehicles. Vehicle-to-anything (V2X) involves many different formats (V2V, V2R, V2P, V2I, and V2N) and all the elements on road with appropriate interconnection, including vehicles, infrastructure elements, people, information networks, and so on. Cooperative sensing, information query, intersection assistance, collision avoidance, and visible light positioning are introduced, which can be readily integrated into existing traffic infrastructure. For example, streetlights, direction signs, and traffic control signals can all be parts of V2X, often without the need for redeploying for enhancement of their basic functionalities. In dense urban areas, an increasing number of vehicles, especially private cars, raises a high demand for enhanced road capacity. Chapter 6, "Prediction of Large Scale Spatio-temporal Traffic Flow Data with New Graph Convolution Model", examines traffic flow as an important road condition, of which prompt and accurate prediction will provide better and more quickly adjusted traffic control and flow guidance. Part of smart city essentials is upgrading existing traffic control systems from "passive adjust" to "active control" and providing dynamic and flexible traffic control strategies. Thus, the chapter introduces a GCN-based traffic flow prediction model. Public traffic datasets worldwide are used for traffic prediction experiments, which include traffic data sources, data contents and data acquisition addresses, and a complete data processing process.

Intelligent devices make everyday life easier. Apart from mobile phones, mobility is an important factor to measure the convenience of an intelligent device. Section 4 discusses intelligent devices. Chapter 7, "Low-Cost Simple Compact and Portable Ground-Penetrating Radar Prototype for Detecting Improvised Explosion Devices", discusses the development and fabrication of a portable, compact, and low-cost continuous-wave (CW) ground penetrating radar (GPR) prototype operating at a single frequency of 920 MHz that applies enhanced isolation antennas. This device is demonstrated to be capable of detecting both metal and non-metal targets buried in soil as well as in a sandpit with high sensitivity. For example, it can detect a small bunch of wires buried 20 cm in the soil. Its maximum depth of detection in semidry soil is 65 cm for a metallic circular plate with a radius of 12.5 cm.

In summary, dedicated advances and the latest technologies in THz science, system dynamics, smart city essentials, and intelligent devices are presented in four sections of this volume. Though a fundamental understanding is provided for engineering and science communities on several selected topics, there exist many unmined work directions and research challenges. The chapters in this book can

facilitate technology research and promote more excellent work in investigating the challenges in intelligent electronics and circuits fields for the development of a modern information society.

The editor would like to take this opportunity to thank all authors for their excellent contributions, the reviewers for their efforts in ensuring the high quality of the chapters, and the staff at In-Tech for their consistent effort and support.

> **Mingbo Niu** Chang'An University Shaanxi, China

Section 1 THz Science

#### **Chapter 1**

## Broadband Terahertz Emission from Photoconductive Devices

*Salman Alfihed and Abdullah Alharbi*

#### **Abstract**

This chapter explores the terahertz (THz) emission from biased semiconductor photoconductive devices. The photoconductive device is an optoelectronic device that is able to emit broadband THz radiation under the optical excitation, by an ultrafast laser, in the existence of a bias field. This chapter explains the basic principle of photoconductive devices with focusing on the main device components, being the photoconductive material and the photoconductive structure. Then, various materials and structures are discussed toward improving the performance of the photoconductive THz emitters. Furthermore, the main limitations and considerations are presented with insight into the different saturation and screening effects due to the bias field and pump fluence. Ultimately, the recent advances and studies of photoconductive THz emitters are presented in terms of material and structure, including the quantum dots, the nanostructure, the use of dielectric materials, and the grating structure on the photoconductive surfaces.

**Keywords:** photoconductive device, photoconductive THz emitter, semiconductor THz emitter, broadband THz emission, photoconductive antenna

#### **1. Introduction**

Since the ultrafast (femtosecond) laser was demonstrated in the 1980s, the field of terahertz (THz) technologies has emerged with an array of applications appearing in different areas, from spectroscopy and sensing to imaging and high-speed communications [1, 2]. Terahertz radiation is nonionizing radiation and has low photon energies, thus having less chance of tissues, cells, and DNA damage during the spectroscopic, sensing, and imaging applications. In addition, the terahertz radiation can be transmitted through some opaque objects in visible light, which opens an array of detection and security applications. The late development of the THz applications is due to the challenges in the generation and detection within the THz band. Its frequencies of 0.1 to 10 THz (30 μm to 3 mm), sandwiched between the electronic and optical frequencies, cannot be generated by conventional electronics or optical methods [3]. This is because the conventional electronics technologies are insufficient to produce broadband waves at these relatively high frequencies. On the other hand, conventional optical technologies cannot emit THz frequencies due to a fundamental issue; there is no material with a bandgap energy corresponding to the THz frequencies [4]. Fortunately, various ultrafast laser and semiconductors approaches have been examined and established. That leads to demonstration of the first emission of pulsed THz radiation using a dipole photoconductive antenna in 1988 by Smith *et al.* [5]. After that, the photoconductive devices were used widely

to emit and detect broadband THz radiation; such devices have been developed with regards to their materials and structures to enhance the emission and detection of THz radiation. Nevertheless, the THz emission can be done mainly by two main methods, being optical rectification based on electro-optic (EO) crystal and photoconductive THz emitters based on semiconductors [6, 7].

The photoconductive THz emitter is an optoelectronic device with three main components, being the photoconductive materials, the photoconductive electrodes, and the lens [8] (**Figure 1**). The photoconductive material is a semiconductor having bandgap energy compatible with the photon energy of the ultrashort laser pulses. In addition, the photoconductive material should have optimum characteristics, including carrier lifetime, carrier mobility, breakdown voltage, and dark resistivity [9]. The carrier lifetime is preferable to be short. However, in the case of the photoconductive detector, it must be in the subpicosecond range. A higher breakdown voltage, carrier mobility, and dark resistivity are fundamental characteristics to assure a better photoconductive THz emitter performance in the form of higher radiated power, higher SNR, and broader bandwidth. The second

#### **Figure 1.**

*Illustration of the photoconductive device, as in (a) it shows a schematic diagram of the photoconductive THz emitter, and in (b) it shows the semiconductor band structure under the applied electric field. © IOP publishing. Reproduced with permission. All rights reserved [8].*

#### *Broadband Terahertz Emission from Photoconductive Devices DOI: http://dx.doi.org/10.5772/intechopen.102930*

component is the photoconductive electrodes. The photoconductive electrodes are two metal electrodes patterned on top of the device, having a gap in between, namely, a photoconductive gap. The design and dimensions of such a gap will influence the device's performance. The last component is the lens. The lens is typically integrated with the emitter to accumulate the radiated field; the radiated field will then be focused on the targeted radiation path.

The photoconductive THz emitter can generate the THz radiation following photoexcitation of its photoconductive gap by an ultrashort laser pulse. When the laser pulse is focused into a photoconductive gap, the laser pulse generates free electrons and holes within the semiconductor, having a rate proportional to the laser pulse [10]. The free carriers will then accelerate under a bias field, controlled by the bias voltage, *V*, produce a transient photocurrent, and ultimately drive the emitter to emit far-field radiation with frequencies spanning into the THz spectrum [11].

This chapter presents the photoconductive devices for THz generation, with insights into their components, limitations, and considerations, and recent progress in this field. In Section 2, a number of photoconductive materials are discussed, the influence of the material and material's characteristics are addressed. In Section 3, the photoconductive electrodes (structure) are considered. This includes different structures characteristics based on their size, being a large aperture antenna, a small aperture antenna, and the plasmonic antenna, discussing the influences on the photoconductive THz emitters' performance. In Section 4, the limitations of the THz emission by photoconductive devices are discussed. The presented limitations are mainly related to the bias field and optical (pump) fluence, which appear in the form of radiated power saturation. In addition, the underlying physics of the space charge and near-field saturation is provided. Ultimately, in Section 5, the recent advances in photoconductive devices technology are given, including the integrated devices and the system-on-chip technologies.

#### **2. Photoconductive materials**

In general, the photoconductive THz emitter performance differs based on the photoconductive material and structure. Therefore, the photoconductive material will be the focus of this section. The optimum photoconductive materials would be crystal lattices with a direct bandgap between the valence and conduction bands. This bandgap determined the absorption wavelength of the exciting laser pulse. Other factors that play a significant role in choosing suitable materials are low carrier lifetime and high carrier mobilities. The most studied materials for photoconductive devices are gallium arsenide (GaAs), indium gallium arsenide (InGaAs), quantum well of InGaAs, indium aluminum arsenide (InAlAs), and a combination of group III-VI materials. This section will explore the photoconductive materials GaAs, ion-implantation in GaAs, InGaAs, and multi-quantum wells InGaAs/InAlAs.

Gallium Arsenide (GaAs) is a III–V semiconductor that has a bandgap of (Eg ∼ 1.42 eV at 300 K) corresponding to the emission wavelength of 880 nm [12]. GaAs is compatible with the titanium-doped sapphire (Ti: sapphire) femtosecond pulsed laser sources typically used to illuminate the photoconductive THz emitters. The GaAs has been the most common material and is typically employed in semi-insulating (SI)-GaAs, low temperature-grown (LT)-GaAs, or ion-implanted GaAs. The SI-GaAs grown by liquid-encapsulated Czochralski at 450–600°C [13] is typically a single crystal that has a high resistivity (>107 Ω cm) and a high electron mobility (μ > 7000 cm2 /Vs) [14]. The SI-GaAs is considered a cost-effective

substrate and has become widely used material for photoconductive devices. However, the research was ongoing to shorten the carrier lifetime. The LT-GaAs grown on SI-GaAs is proved to reduce carrier lifetime two orders of magnitude to below 1 ps compared to SI-GaAs (t > 100 ps) and efficiently generate broadband THz radiations of over 1 THz with high resistivity (107 Ω cm) and reasonable mobility μ (100–300 cm2 /Vs) [15]. The growth is typically done by molecular beam epitaxy (MBE) on the surface of SI-GaAs substrate and growth temperature to between 200°C and 300°C in an arsenic-rich environment [16]. In such a case, it yields a high level of crystallinity, which means higher carrier mobilities and point defects due to excess As precipitants. Higher mobility leads to fast response, and point defects significantly reduce the lifetime (below 400 fs). These point defects act as recombination centers [15]. Increasing the temperature above 250°C will increase the lifetime to be greater than 50 ps. Tani *et al*. conducted a direct between LT-GaAs and SI-GaAs and studied the effect of growth temperature and anneal time effects where the carrier lifetime of LT-GaAs grown at 250°C and followed by post-growth annealing at 600°C for 5 min was found to be a 0.3 ps [17]. However, the process conditions for LT-GaAs are not easy to reproduce due to unreliable temperature monitoring below 400°C.

An alternative approach is using the ion-implantation technique to create point defects and reduce the lifetime in SI-GaAs by implementing arsenic, oxygen, nitrogen, carbon, and hydrogen (proton). Implanting H+ ions are shown to decrease the carrier lifetime of GaAs to sub-picosecond. Then several groups studied the effect of As+3 ion implantation of SI-GaAs and introduced excess As+3 impurities within the crystal structure similar to LT-GaAs [11]. However, the ion-implantation technique of As+3 (GaAs∶ As+3) improved the controllability of the excess As+3 concentration and uniformity as compared to LT-GaAs, making it more reproducible than LT growth [11]. Salem *et al.* [18] characterized the GaAs:As+ and unprocessed GaAs implanted with various other ions, including hydrogen, oxygen, and nitrogen. Their study revealed the lowest THz pulse intensity was observed in the GaAs∶N3− and all devices saturated at a higher pump fluence than nonprocessed GaAs emitter except GaAs:N+ . A study by Liu *et al.* [19] revealed that using multi-energy implanted As+ ions leads to a shorter THz pulse and a higher bandwidth response than using single-energy ions.

The InGaAs are also employed as photoconductive material. It is a great advantage of the III-V compound to engineer the bandgap by changing the composition ratio. For example, the bandgap of the ternary compound indium gallium arsenide (InxGax-1As) can be potentially varied from 1.42 eV (x = 0) to 0.36 eV (x = 1). From a practical point of view, the protentional to achieve 0.8 eV (1550 nm optical excitation) was the motivation for investigating this material for THz applications. Doping InGaAs by iron has been demonstrated to provide required recombination sites for a subpicosecond carrier lifetime, higher optical pump saturation power, and higher breakdown voltage. Wood *et al.* [20] investigated the InGaAs∶Fe2+ emitter that is grown by using Metal organic chemical vapor deposition (MOCVD) across 830-nm to 1.55-μm optical excitation and found the highest THz power at the 1.2-μm excitation wavelength. This study shows precise control of the Fe-doping added during the epitaxial growth process and the strength of engineering the bandgap of III-V materials compound.

Heterostructure devices consisting of alternate InGaAs/InAlAs multilayer stacks (multiquantum wells) have been proposed [21] as potential materials for photoconductive devices and achieve high performance at 1550 nm comparable to LT-GaAs excited at 800 nm. Sartorius *et al.* demonstrated the first InGa(Al)As-based THz photoconductive devices operating at 1.5 μm [22]. In their device, an MQW comprised of 12 nm InGaAs∶Be2+/8 nm InAlAs as the photoconductive region was grown using standard low-temperature methods on an InP substrate. Moreover, the


*Broadband Terahertz Emission from Photoconductive Devices DOI: http://dx.doi.org/10.5772/intechopen.102930*

#### **Table 1.**

*Summary of some photoconductive materials with the advantages, disadvantages, active layer, and the operating wavelength.*

material dark resistivity increases by four orders of magnitude comparable to bulk InGaAs due to the presence of Be2+ during the growth and the insertion of InAlAs, which has a higher dark resistivity than the InGaAs∶Be2+.

In addition to the GaAs, and InGa(Al)As, many other materials of group III-V such as InAs [23], InSb [23], GaSb [24], GaAsSb [25], and doped InGaAs [26], GaInSb [25] are studied as photoconductive material. Choosing the materials highly depends on the application and operating wavelength. Although LT-GaAs is still the most used material for photoconductive devices and is the most efficient material for 800 nm. However, it exhibits poor absorption at 1.55 μm, where other materials such as InGaAs or InGaAs/InAlAs heterostructure become more attractive. **Table 1** summarized some of the photoconductive materials with the advantages, disadvantages, active layer, and the operating wavelength.

#### **3. Photoconductive structure**

The photoconductive devices for THz emission have been developed extensively to fulfill the demand for high-performance THz emitters—and thus be essential for spectroscopic and imaging applications. The development of the emitters' structure is related to its design and dimensions and how that is attributed to the high performance of the THz emission. The performance of the photoconductive THz emitters is determined in the form of radiated power (or the THz spectral amplitude), SNR,

and bandwidth. It is worth noting that the bandwidth here manifests itself as is the maximum frequency in the THz spectral amplitude, as a function of frequency, *f*, before the noise level of the measurements system. This chapter will provide an overview of such development regarding the design and structure of photoconductive THz emitters, with insights into the different emitter structures based on the dimensions, such as large-aperture and interdigitated electrodes THz emitters, small aperture THz emitters, and plasmonic THz emitters.

In the large-aperture and interdigitated electrodes photoconductive THz emitters, the gap between the two electrodes can be large as 4 mm to 130 μm [11]. Such a gap will allow a high level of optical excitation before reaching the saturation issues. Thus, the importance of such emitters stems from the need to scale up the radiated power, which is influenced by the incident optical power. A molded has been developed by Darrow *et al.* to study and predict the saturation in large-aperture photoconductive THz emitter, which is expected to have the saturation at higher pump fluence than the small-aperture THz emitter does [27].

In the small-aperture photoconductive THz emitter (dipole antenna), the gap between the two electrodes is smaller than in the large-aperture THz emitters, typically below 200 μm. In this case, it will be more difficult to align the laser spot within the PC gap. Although these emitters experience the saturation issues at lower pump fluence, in comparison with the large-aperture THz emitters, these emitters provide broader bandwidth over the large-aperture THz emitters. Our recent work on the design and structure of photoconductive THz emitters based on SI-GaAs examined the influence of bowtie structure characteristics on the THz spectral amplitude and bandwidth [28]. It is found that the bandwidth can be improved from 3.4 THz to 3.7 THz by changing the design of electrode structure from a sharp bowtie to an asymmetric bowtie structure at the same photoconductive gap. That could be attributed to the smaller capacitance of the sharp bowtie structure over the asymmetric bowtie structure, which results in a shorter resistance-capacitance (RC) time constant. The RC time constant, *τ*RC, can be related to the gap conductance (of the photoconductive antenna), *G*(*t*), the gap capacitance, *C*, and the antenna/transmission line impedance, *Z*0 as [28]:

$$
\pi\_{\rm RC} = \heartsuit \Sigma\_0 \mathsf{C} / (\heartsuit \Sigma\_0 \mathsf{C} \mathsf{G} \{ t \} \mathsf{+1}),
\tag{1}
$$

**Figure 2** illustrates the biased photoconductive gap with its equivalent circuit, here the redistribution of charge on the electrodes, can be seen as incident voltage waveform, *vi*(*t*), reflected voltage waveform, *vr*(*t*), and transmitted voltage waveforms, *vt*(*t*).

The plasmonic THz emitter is introduced by Berry *et al.* by means of increasing the THz radiated power. An improvement of up to 50 times of the THz radiated power is observed using a plasmonic structure compared to a conventional photoconductive THz emitter [29]. The fabricated plasmonic THz emitter was based on LT-GaAs to provide an ultrafast photoconductor response. The distance between the two electrodes was 60 μm; the antenna structure was bowtie without fine tips; the maximum width of the antenna was 100 μm; the minimum width was 30 μm. In the plasmonic structure, the grating structure had a gap width of 100 nm, and the deposited gold had the same width of 100 nm. The optical pump focused on the gap close to the anode to maximize the THz radiation [29, 30]. **Figure 3** shows a comparison between the conventional photoconductive antenna and the plasmonic photoconductive antennae as in (a) the conventional photoconductive THz emitter and (b) the plasmonic

#### **Figure 2.**

*A biased photoconductive gap at bias voltage, Vb. The inset shows an equivalent circuit of the photoinductive gap having the electrodes with incident voltage waveform, vi(t), reflected voltage waveform, vr(t), and transmitted voltage waveforms, vt(t). The gap conductance is shown as G(t) and the gap capacitance is shown as C. this figure is reprinted from [28].*

#### **Figure 3.**

*A schematic diagram of the photoconductive device shows in (a), the conventional photoconductive THz emitter, and in (b), the plasmonic photoconductive THz emitter. © IOP publishing. Reproduced with permission. All rights reserved [31].*

photoconductive THz emitter [32]. In addition to the plasmonic metal structure, the silver nano-islands on the photoconductive surface improve the performance of the photoconductive THz emitter [33]. Georgiou *et al.* have been recently demonstrated a 3-dimensional photonic-plasmonic photoconductive device. The performance of the THz emission is enhanced by employing a periodic array of nanopillars, which raise the optical absorption on the device surface and optimize the collection efficiency by converting each nanopillar into a single (nano) photoconductive switch. As a result, nearly the overall generated current and the bandwidth are increased by 50-fold and five times, respectively. However, such a device will request high-tech technology to fabricate it [34]. On the contrary, the metal-based nano-islands structures enhance the photoconductive THz emission with a less complicated fabrication process. It is worth noting that these metal nano-islands can be produced by fine-tuning the deposition and thermal annealing process. Thus, it will offer less complexity during the fabrication in compression of the above-mentioned plasmonic structure.

Overall, the photoconductive structure plays a significant role in the performance of the photoconductive THz emitters. The large-aperture and interdigitated electrodes THz emitters mitigate the influence of saturation for scaling up the THz emission with the optical influence. The small aperture THz emitter (dipole antenna) shows a broader bandwidth, which allows discovering a more comprehensive range of THz frequencies. In addition, the recent studies on plasmonic devices present their significance to the photoconductive THz emitter performance. It also steers the future research and development of high-performance photoconductive devices for spectroscopy, sensing, and imaging applications.

#### **4. Limitations of the THz emission by photoconductive devices**

The underlying physics of the THz emission by photoconductive devices is the core of this section, which helps understand these devices' behavior. The photoconductive THz emission scales linearly with the applied bias field and pump fluence. However, that can be precise only in the ideal case, at low levels of bias field and/or optical excitation. Higher levels of bias field influence the photoconductive THz emitters' performance. Such influence can be seen as thermal effects, space-charge-limited current effects, etc. In addition, the photoconductive device has a limitation at a higher bias field correlated to the breakdown voltage of the photoconductive material. The pump fluence also has an impact, but that can be observed as the saturation of the THz radiation. The saturation (screening) of the THz radiation is mainly associated with two different mechanisms, being space-charge and near-field screening. This section will explore the limitation of the photoconductive THz emission with insights into the material and structure implications on photoconductive THz emitter's performance.

The THz radiated power (or the THz field amplitude, *E*THz) can be scaled with the bias field, *E*b, in three different mechanisms, based on the bias filed value and the photoconductive material, being a superliner (red), a linear (blue), and a sublinear (yellow), as shown in **Figure 4**. The superliner behavior is seen clearly with the photoconductive THz emitters based on GaAs. This superlinearity is associated with the space-charge-limited current due to the deep EL2 traps states [29, 35]. The superliner behavior is associated with the limitations within the semiconductor. This limitation manifests itself as Joule heating within the semiconductor and is observed in the photoconductive THz emitters based on InP [32]. Collier et al. have studied the Joule heating limitation in the photoconductive THz emitters based on InP. They found a correlation between the surface quality and the carrier lifetime, which directly affects Joule heating. In

#### **Figure 4.**

*The scaling of THz radiated power as the THz field amplitude, ETHz, with the bias field, Eb, in three different methods, being a superliner (red), a linear (blue), and a sublinear (yellow).*

the textured InP photoconductive THz emitters, the carrier lifetime is decreased, which reduces the photocurrent and ultimately diminishes the Joule heating [32].

The pump fluence impacts the radiated THz power in the form of saturation (screening). At a higher level of optical excitation, the radiated THz power will be saturated. This saturation can be classified into two mechanisms, being spacecharge and near-field screening. However, each screening status differs based on the photoconductive characteristics (material and structure) and optical characteristics (pump fluence). It is worth noting that transient mobility (mobility as a function of pump fluence) plays an important role in the emitted THz power and thus in the screening of the THz field [30]. The mechanisms of these two screening effects are explained in the next paragraph.

In the space-charge THz screening, the limitation of the photocurrent within the photoconductive gap is due to the high carrier densities within the photoconductive

gap, affected by the high pump fluence. The charges drift in the opposite direction. Thus, the bias field screens and ultimately limits the radiated THz field [36]. In the near-field THz screening, the direction of the radiated THz field is in the opposite direction of the bias field, which limits the linear scale of the THz radiated field with the pump fluence, as increasing the pump fluence will raise the carrier densities within the semiconductor [37]. At the same pump fluence, the carrier densities in the emitter with a large photoconductive gap will be smaller than in the emitters with a small photoconductive gap. Thus, a large photoconductive gap emitter leads to scaling up the radiated THz power for higher levels, which increases the total emitter performance, before reaching the screening issues [30].

Overall, the main limitations of the THz emission by photoconductive devices can be related to the applied bias field and the exciting pump fluence. The two limitations are correlated with the photoconductive material and structure characteristics. These two limitations prevent the THz field amplitude from scaling linearly with the bias field and pump fluence. Thus, it is essential to design the photoconductive THz emitter carefully. Furthermore, the photoconductive material must be chosen judiciously to meet the demand of the high-radiated THz field for the aforementioned advanced applications.

#### **5. Recent advances in photoconductive devices**

A number of the recent advances and research in the field of photoconductive devices are discussed in this section, with insight on the development of the material and structure to enhance the photoconductive THz emission for spectroscopic, sensing, and imaging applications. The section will explore different approaches including:


The development of the photoconductive THz emission using such new approaches is notable. The quantum dots are mainly related to photoconductive materials. In contrast, the nanostructured electrodes, dielectric metasurfaces, and Grating photoconductive devices are associated with the photoconductive structure. Here, the main interest is to focus on improving the THz emission using these different approaches and the potential enhancement of these devices.

The quantum dots have been employed to boost the photoconductive THz emitters' performance. Gorodetsky *et al.* used InAs quantum dots in bulk GaAs. The short carrier lifetime has captured with the dots in such devices and maintains high carrier mobility [38, 39]. The photo-electronic priorities of the quantum dots can be managed by controlling the characteristics of these dots during epitaxial growth. It is worth noting that quantum dots have three-dimensional effects compared with a one-dimensional effect in the quantum wells. In Gorodetsky *et al.* study, the active regain consists of InAs quantum dots layer (1–2 nm), InGaAs wetting (5 nm), and GaAs spacer (35 nm). At the top of these layers, an LT-GaAs layer is grown with a 30-nm thickness, the observed boost of such structure is about 5-fold at 1.0 THz [37]. In addition, GaAs with ErAs quantum dots has been demonstrated for exaction laser having a 1550-nm wavelength [40]. The observed conversion rate

(from optical to THz power) is 0.18%. However, this result was obtained by using a resonant slot antenna.

Nanostructure electrodes of the photoconductive device show an improvement of the photoconductive THz generation, even without a plasmonic effect. Although the plasmonic photoconductive THz emitter is one of the breakings through in the THz generation and detection field, the nanostructure has its encasement on the performance of such devices [41]. Singh et al. examined an antenna nanostructure fabricated by utilizing an electron-beam lithography system (EBL), having a 5-nm titanium layer and a 25-nm gold layer. Hilbert-fractal design is used with different line widths up to 140 nm. An improvement of the emitted THz power by an approximate factor of two is observed using this nanostructure.

Dielectric metasurfaces in photoconductive terahertz devices can be used as an alternative method to enhance the photoconductive THz emitters' performance instead of the plasmonic structure [42]. Although the plasmonic structure delivers better THz field improvement over the dielectric structure, the dielectric structure has a substantial characteristic which is the lack of dissipation [43]. In addition, the optical absorption of the incident light (laser) onto the photoconductive device can be improved by reducing the Fresnel losses, which can be done by using thin films of dielectric materials on top of the photoconductive gap. These dielectric materials (thin films) include SiO2, Si3N4, Al2O3, and TiO2 [44, 45]. **Figure 5** shows a bowtie antenna having a layer of TiO2 being coated on the photoconductive gap, in (a) the schematic view of the photoconductive THz emitter, (b) the SEM image of the photoconductive THz emitter, and (c) the THz spectral amplitude obtained with using TiO2 layer (red) and without using TiO2 layer (black), "from [45]".

The grating structure manifests itself as a periodic array of grooves, lines, slits, etc. The grating structure of the photoconductive devices for THz generation has been studied according to the effective medium approximations (or effective medium theory). The theory can be applied to describe the interaction of light with the grating structure (subwavelength) [46]. Chia et al. have modeled and simulated the influence of grating structure on the THz emission performance by COMSOL Multiphysics software with an insight into the effects of grating geometrical parameters. The author funds an improvement of about 1.63 of the photocurrents obtained by an optimized grating structure of photoconductive THz emitter over the planer emitter structure. This is due to the higher photon absorption, which leads to and leads to more carrier generation within photoconductive material, thus higher photocurrent is observed [46]. **Figure 6** shows the simulated grating structure of LT-GaAs and its effects, as in a) the upper diagram shows the surface of

#### **Figure 5.**

*The bowtie photoconductive antenna with TiO2 layer, coated on the photoconductive gap, in (a) the schematic view of the photoconductive THz emitter, (b) the SEM image of the photoconductive THz emitter, and (c) the THz spectral amplitude obtained with using TiO2 layer (red) and without using TiO2 layer (black). This figure is reprinted from [45] with the permission of AIP publishing.*

**Figure 6.**

*The simulated grating structure of LT-GaAs, as in (a) the upper diagram shows the surface of planner photoconductive THz emitter, the lower diagram shows grating structure of the photoconductive THz emitter, and (b) the normalized electronic concertation obtained by the two different simulated photoconductive THz emitters. This figure is reprinted from [46].*

planner photoconductive THz emitter, the lower diagram shows grating structure of the photoconductive THz emitter used in the simulation, and b) the normalized electronic concertation obtained by the two different simulated photoconductive THz emitters, "from [46]".

Nowadays, the development of photoconductive devices regarding materials and structure is a hot research topic. Several publications have discussed many schemes to achieve higher performance of THz generation by photoconductive devices to facilitate the applications in cutting-edge technologies such as THz spectroscopy, THz sensing, and THz imaging. For photoconductive materials, the research focuses on the quantum dots as well as promotes material properties such as the carrier lifetime and carrier mobility. For the photoconductive structure, the implementation of plasmonic and nanostructures shows its advantage for the aforementioned applications. However, utilizing some novel ideas such as grating structure and a precise selection of the dielectric material is demonstrated to boost the performance of photoconductive devices further.

#### **6. Conclusion**

This chapter presented the photoconductive devices for THz emission. Several materials have been employed as photoconductive materials. However, GaAs is a typical material for these applications, particularly for the sapphire femtosecond pulsed laser sources, which emit at the same range of the bandgap energy of GaAs. Furthermore, several photoconductive structures have been employed. The plasmonic structure shows the highest impact of the photoconductive THz emitters' performance over the microstructure photoconductive THz emitters. On top of that, the screening effects of the THz field amplitude is an issue limiting the linear scaling of the THz field with the pump fluence. Such limitations can be diminished using a large-aperture photoconductive antenna. At the end of this chapter, the improvement of these devices' performance has been considered by viewing some recent work in this area. The work has also presented the influence of the quantum dots, the nanostructured electrodes (nonplasmonic) of the photoconductive device, the dielectric materials in photoconductive terahertz devices, and the grating

structure on the photoconductive surface. It is hoped that the presented work can lay a role in continuing advancements of photoconductive devices.

### **Conflict of interest**

The authors declare no conflict of interest.

### **Author details**

Salman Alfihed\* and Abdullah Alharbi King Abdulaziz City for Science and Technology, Riyadh, Saudi Arabia

\*Address all correspondence to: salfihed@kacst.edu.sa

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 2**

## Studies of Terahertz Sources and Their Applications

*Sukhmander Singh, Shravan Kumar Meena, Ashish Tyagi, Sanjeev Kumar, Man Raj Meena and Sujit Kumar Saini*

#### **Abstract**

The contributed chapter discuss the applications of terahertz radiations and its generation mechanism through laser plasma interactions. The methods of generation of terahertz radiations from plasma wake field acceleration, higher harmonic generation and the laser beat wave plasma frequency are reviewed. The nonlinear current density oscillate the plasma at beat wave frequency under the effect of ponderomotive force and excite the terahertz radiation at beat wave frequency. The current state of the arts of the methods of generation has been incorporated. The mathematical expression of ponderomotive force has been derived under the influence of gradient of laser fields. In additions, the future challenge and their overcomes are also been discussed.

**Keywords:** electromagnetic waves, THz radiation, beat wave, ponderomotive, nonlinear, plasma, harmonic, detectors

#### **1. Introduction**

THz radiation has applications in broadband THz communications, basic science, security, pharmaceutical industries, manufacturing and medicine science. The Terahertz (THz) frequency region, which was difficult accessible frequency region range (0.1–30 THz) lies between the microwave and infrared bands in electromagnetic spectrum. This THz region is also defined as borderline of high frequency region of the microwave band and long wavelength region of far infrared light. Radiation at 1 THz has a period of 1 ps, wavelength 300, wave number 33/cm and photon energy 4.1 meV. It has peak field at 100 MV/cm between 15 THz and 50 THz that provide major momentum to the investigating materials. Therefore THz waves allow direct access to molecular rotations, lattice vibrations and spin waves (low-energy excitations) in contrast to excitations of valence electrons stimulated by optical waves. The THz radiation is non-ionizing and nondangerous for living cells. These radiations can penetrate through plastics, metals, textiles, paper and woods which assists to identify the explosives and drugs. In most of the cases, the vibrational modes of oxygen, water and carbon monoxide (molecules of drugs and explosives) lies in the THz region, therefore during the investigation, those ingredients display distinctive absorption lines in the THz frequency range.

Electromagnetic waves have an applications in medical imaging, broadcasting, WiFi, and treating cancer. The Sequential arrangement of electromagnetic waves are shown in **Table 1**.

We also summarize some of the main applications for each range.


**Table 1.** *The electromagnetic spectrum.*

#### **2. Application of terahertz radiation**

THz radiation technology have a substantial presentation in the field of engineering, science, biomedical engineering, astrophysics, environmental engineering, information science, technology and plasma physics.

#### **2.1 In biomedicine**

Terahertz waves are useful for the diagnosis of disease since every organisms have a unique response to THz wave. THz tomography get absorption rate distribution and three-dimensional distribution of the refractive index of materials in computer assisted tomography [1–29].

#### **2.2 Quality control and safe monitoring**

THz radiation is used to observer the process of food processing, weapons, drugs and explosives. THz electromagnetic waves are completely harmless to humans owing to its strong capacity of penetration [2–29].

#### **2.3 Non damaging testing**

The penetration length of radiations are measured by THz time-domain spectroscopy. The safety and penetrable properties of THz waves are useful for

nondestructive testing. THz waves can penetrate a few inches thick foam. Foam used in space shuttle has very low refractive index and absorption variation, although this change can be observed to detect the defects [1–10].

#### **2.4 Astronomy and atmospheric research**

The atmospheric molecules (nitrogen, water, oxygen and carbon monoxide) have excitation energy in the terahertz range, therefore these molecules can be detected with THz radiation to monitor the atmospheric environmental and ozone layer as well as space research [3–12]. THz technology can be employed in astronomy and Earth observation to monitor the weather. The ultra-high frequency of THz radiation provide better digital signal processing and imaging.

#### **2.5 Wireless communication and networking**

THz band has higher frequency, wider bandwidth and greater channel than the microwave and 10 Gbps wi-fi transmission speeds may be obtained by means of THz communique, which is some hundred or maybe hundreds of instances faster than contemporary ultra-Wideband technology [3–12].

#### **2.6 Secure communication**

The small power of THz radiations is used to gain long-range space communications because of its low attenuation characteristic. THz radiation has wider beam width against space optical communication, which make it suitable to pointing in the long-distance space communication [1–6].

#### **2.7 Chemical and biological agent detection**

Terahertz radiation is very sensitive to molecules and surrounding environment. Therefore, terahertz technology is used in chemical detection and identifications of the chemical and biological agents [1–16].

#### **2.8 Medical applications**

Ionizing radiation is a kind of radiation that carries photon energy large enough to detach electrons from atoms or molecules, leading to their ionization. X-ray radiation may be harmful for humans. Typical X-ray imaging systems for medical purpose involves photon energies close to 100 keV. Hence, a person may get exposures of a high dose. Since the detection resolution is restricted by diffraction, therefore due to shorter wavelength of the terahertz radiation, it provide better spatial resolution in imaging. Moreover, the vibrational and rotational transition energies of the biomolecular constituents of tissue lies in the THz frequency range, which offer good spectroscopic information of biological tissues. The low photon energy of the radiation is nonionizing and there is negligible scattering from tissues. An exposure of a high dose of ionizing radiation may damage DNA of a human body and may increases the possibility of developing cancer. When the energy is larger than 10 eV, we say the radiation is ionizing. Therefore the water content presents in the tissue can provides good contrast between the healthy and diseased states of tissues using timedomain spectroscopy based on terahertz radiation. The time-domain spectroscopy provide quasi 3D information in the broad frequency range to investigate the desired information. Although terahertz technology is still young and there have been no major commercial applications in the medical science.

#### **2.9 Quality control and pharmaceutical applications**

X-ray photoelectron spectroscopy, Fourier transform infrared and laser induced breakdown spectroscopy are destructive for the medicine tablet to investigate the uniformity of the coating. The non-uniform of the coating or surface defects on the tablets leads to lacks of the desired dose delivery. THz waves have penetrating behavior because of its electromagnetic nature. Terahertz image can be optimized for performing 3D analysis on tablets to determine coating integrity and thickness.

#### **3. Food applications of terahertz spectroscopy**

#### **3.1 Terahertz sources**

An electronic and photonic materials based methods have been built to generate THz radiation in the recent years and these methods are tabulated in **Table 2**. These sources complement laser-based and other table-top THz sources, which are limited to lower average powers, lower peak fields and lower repetition rates.


**Table 2.** *Sources of THz.*

#### **4. Ponderomotive force**

The nonlinear process arise, when a very high intensity electromagnetic wave interacts with a plasma and the force due to radiation pressure is coupled to the plasma particles and it is called ponderomotive force. Self-focusing of laser light in a plasma is a direct effect of ponderomotive force. When a gas is ionized by propagating a laser thought it, a force exert on the medium at ionization front. A laser beam causes a radially directed ponderomotive force in a plasma which forces plasma out of the beam and dielectric constant becomes higher inside the beam than outside. The plasma acts as a convex lens focusing the beam to a smaller diameter. Here we derive the expression for the ponderomotive force [35].

*Studies of Terahertz Sources and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.101685*

Equation of motion of particle under the electromagnetic fields

$$m\_{\varepsilon} \frac{d\overrightarrow{v}}{dt} = -e\left[\overrightarrow{E}\left(\overrightarrow{r}\right) + \overrightarrow{v} \times \overrightarrow{B}\left(\overrightarrow{r}\right)\right] \tag{1}$$

Non-linearity in the system comes partly from the *v* ! � *<sup>B</sup>* ! term which is a second order term and assuming that *v*<sup>0</sup> ! <sup>¼</sup> *<sup>B</sup>*<sup>0</sup> �! <sup>¼</sup> <sup>0</sup>*.* The other part of nonlinearity comes from evaluating E! at the actual position of the partical. Let the wave electric field is

$$
\overrightarrow{E}\left(\overrightarrow{r}\right) = \overrightarrow{E\_S}\left(\overrightarrow{r}\right)\cos\left(\alpha t\right) \tag{2}
$$

We expand E(r) about point *r* ¼ *r*<sup>0</sup>

$$m\_{\epsilon} \frac{d\overrightarrow{\nu\_1}}{dt} = -e\overrightarrow{E}\left(\overrightarrow{r\_0}\right) = -e\overrightarrow{E\_S}\left(\overrightarrow{r\_0}\right)\cos\left(\alpha t\right) \tag{3}$$

After integrating over time, we get

$$
\overrightarrow{w\_1} = \left(\frac{e}{m\_\epsilon \alpha}\right) \overrightarrow{E\_S} \left(\overrightarrow{r\_0}\right) \sin\left(\alpha t\right) \tag{4}
$$

Again, integrating over time, we get

$$
\overrightarrow{r\_1} = -\left(\frac{e}{m\_e \alpha^2}\right) \overrightarrow{E\_S} \left(\overrightarrow{r\_0}\right) \cos\left(\alpha t\right) \tag{5}
$$

Now, according to Faraday's law

$$
\vec{\nabla} \times \vec{E} = -\frac{\partial \vec{B}}{\partial t} \tag{6}
$$

It implies

$$
\overrightarrow{B\_1} = -\left(\mathbf{1}/a\right)\overrightarrow{\nabla} \times \overrightarrow{E} \tag{7}
$$

The Taylor expansion of Eq.(2) about point *r* ! ¼ *r*<sup>0</sup> !

$$
\overrightarrow{E\_S} \left( \overrightarrow{r} \right) = \overrightarrow{E\_S} \left( \overrightarrow{r\_0} \right) + \left( \overrightarrow{r\_1} \cdot \overrightarrow{\nabla} \right) \overrightarrow{E\_s} \left( \overrightarrow{r} = \overrightarrow{r\_0} \right) + \dots \tag{8}
$$

Putting the value in equation in (1) from Eq. (7) and (8), we get

$$m\_{\varepsilon} \frac{d\overrightarrow{v\_2}}{dt} = -\varepsilon \left[ \left( \overrightarrow{r\_1} \cdot \overrightarrow{\nabla} \right) \overrightarrow{E\_s} \left( \overrightarrow{r\_0} \right) \cos \left( \alpha t \right) + \overrightarrow{v\_1} \times \overrightarrow{B\_1}^{\prime} \right] \tag{9}$$

On solving the equation and taking the average over time.

$$\left(m\_{\epsilon}\frac{d\overrightarrow{v\_{2}}}{dt}\right)\_{\text{avg}} = -\left(\epsilon^{2}/4m\_{\epsilon}\alpha^{2}\right)\overrightarrow{\nabla}\left(E\_{\epsilon}^{\ \ 2}\right)\_{\text{avg}}\tag{10}$$

Or

$$\left(m\_{\epsilon}\frac{d\overrightarrow{v\_{2}}}{dt}\right)\_{\text{avg}} = -\left(\epsilon^{2}/2m\_{\epsilon}\alpha^{2}\right)\overrightarrow{\nabla}\left(E^{2}\right)\_{\text{avg}}\tag{11}$$

So the left hand side in equation (11) is the effective force on a single electron, which can be denoted by fNLe*:*

Similarly force on ions can be written

$$\overrightarrow{f\_{NLi}} = \left(m\_i \frac{d\overrightarrow{v\_2}}{dt}\right)\_{\text{avg}} = -\left(\mathfrak{e}^2/2m\_i\omega^2\right)\overrightarrow{\nabla}\left(E^2\right)\_{\text{avg}}\tag{12}$$

So the resultant force on plasma due to ions and electrons is

$$\begin{split} \overrightarrow{F\_{rdt}} &= n\_0 \left( \overrightarrow{f\_{N\&i}} + \overrightarrow{f\_{N\&\epsilon}} \right) \\ &= \left( \frac{n\_{\epsilon 0} e^2}{2m\_{\epsilon} \alpha^2} \right) \left( 1 + \frac{m\_{\epsilon}}{m\_i} \right) \overrightarrow{\nabla} \left( E^2 \right)\_{\text{avg.}} \end{split} \tag{13}$$

Since the mass of the ions are much greater than the mass of the electrons, we get

$$
\overrightarrow{F\_{\text{rdtt}}} = -\left(\omega\_{\text{pr}}\angle\_{\text{o}}\right)\overrightarrow{\nabla}\left(\mathbf{e}\_{\text{0}}\mathbf{E}^{2}/\mathbf{2}\right)\_{\text{avg}}\tag{14}
$$

This is called Ponderomotive force.

#### **5. Non-linear current due to laser beating**

Let us consider two different color laser beams co-propagating in a corrugated plasma having electric field profiles as follows

$$\overrightarrow{E\_1}^{\rightarrow} = \hat{j}E\_0 \varepsilon^{-\left(\chi^2/a\_0^2\right)} \mathbf{e}^{i\left(\mathbf{k}\_{\mathbf{1}\mathbf{x}} - a\_{\mathbf{1}\mathbf{t}}\mathbf{t}\right)}\tag{15}$$

$$\overrightarrow{E\_2}^{\prime} = \hat{j}E\_0 \mathbf{e}^{-\left(\mathbf{y}^2/a\_0^2\right)} \mathbf{e}^{i(\mathbf{k}\_2 \mathbf{x} - \mathbf{u}\_2 \mathbf{t})} \tag{16}$$

The equation of motion of plasma electrons in a field

$$m\frac{\partial \overrightarrow{\boldsymbol{v}}\_{\epsilon}}{\partial t} = -e\overrightarrow{\boldsymbol{E}}\tag{17}$$

Lasers impart oscillatory velocity to electrons

$$
\overrightarrow{v\_1} = \mathbf{e} \,\overrightarrow{E\_1} / i\mathbf{o}\_1 m \tag{18}
$$

$$
\overrightarrow{v\_2} = \mathbf{e} \,\overrightarrow{E\_2} \, / i \mathbf{o}\_2 m \tag{19}
$$

The corresponding ponderomotive force

$$
\overrightarrow{f\_p}^{nl} = \frac{-e^2}{2m\alpha\_i^2} \overrightarrow{\nabla} \left( E\_{avg}^2 \right) \tag{20}
$$

In terms of two components of electric field

$$\overline{f}\_p^{\text{val}} = \frac{-e^2}{2mo\_1 o\_2} \vec{\nabla} \left( \overrightarrow{E\_1} \, \overline{E\_2}^\* \, ^\* \right) \tag{21}$$

We know that

$$
\overrightarrow{F} = -\varepsilon \nabla V \tag{22}
$$

*Studies of Terahertz Sources and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.101685*

By comparing the Equation (21) and (22)

$$V = \frac{\mathcal{e}}{2m\alpha\_1\alpha\_2} \vec{\nabla} \left( \overrightarrow{E\_1} \, \overrightarrow{E\_2}^\* \, \right) \tag{23}$$

the equation (19) deduce that

*Ei* <sup>¼</sup> *imω<sup>v</sup> <sup>e</sup> :* (24)

So, from the equation (18) and (19) , we get

$$V = \frac{-m}{2\varepsilon} \left( \overrightarrow{v\_1} . \overrightarrow{v\_2}^\* \right) \tag{25}$$

From the equation (22)

$$
\overrightarrow{f\_p}^{nl} = \frac{-\varepsilon^2}{2m\alpha\_1\alpha\_2} \overrightarrow{\nabla} \left( \overrightarrow{E\_1} \, \overrightarrow{E\_2}^\* \, \right) \tag{26}
$$

Putting the values of electric filed *E*<sup>1</sup> ! and *E*<sup>2</sup> ! -

$$\overrightarrow{f\_{\,p}}^{nl} = \frac{-e^2}{2m\alpha\_1\alpha\_2} \overrightarrow{\nabla} \left( E\_0^2 e^{-\left(2\overline{y}^2/a\_0^2\right)} e^{i[(k\_1-k\_2)\mathbf{x}-(\alpha\_1-\alpha\_2)t]} \right) \tag{27}$$

Or

$$
\overrightarrow{f\_p}^{nl} = \frac{-\varepsilon^2 E\_0^2}{2m\alpha\_1\alpha\_2} \overrightarrow{\nabla} \left( e^{-\left(2\overline{\gamma}^2/a\_0^2\right)} e^{i\left(k'\mathbf{x} - \alpha'\mathbf{t}\right)} \right) \tag{28}
$$

Here,

$$k'=(k\_1-k\_2)\text{ and }\alpha'=(\alpha\_1-\alpha\_2)\tag{29}$$

This oscillatory current is the source for the emission of THz radiation at the beating frequency.

#### **6. Generation of terahertz radiation**

A wide range of THz sources are now commercially accessible, although they are big and relatively expensive to run. As a result, much research is being done in order to develop appropriate THz sources. Various organizations across the world have devised various techniques for producing THz sources [5, 6]. Traditional THz sources are based on electro-optic crystals such as ZnSe, GaP, LiNbO3, or photo conductive antennas as well as super-luminous laser pulse interactions with large band gap semiconductors and dielectrics [7–13]. Laser plasma interaction, optical recitation, solid state electronic devices and many complex methods are being used to generate THz radiations [1–20]. The output of such sources can be harmonically multiplied to the THz range. Recent improvement in the field of quantum cascade lasers, laser emission is achieved through the use of inter-sub-band transitions in a periodic repetition of layers of two different compositions, or super-lattice structure. A super-lattice is a periodic structure of quantum wells and barriers. The photon emitted by the super-lattice is due to the intersub-band transition in the

super-lattice. Such transitions can be specified by the thickness of the coupled wells and barriers. Therefore, by toiloring the periodicity of the super-lattice to specific well-barrier thickness, THz radiations of specified energy range can be generated. Although the idea of inter-band emission was known since 1971, the crystal growth technology for creating quantum cascade lasers is relatively new and expensive. Terahertz radiation in the frequency range 0.1–10 THz, lies between the MW and IR region and has potential uses in a wide range of fields. That why researchers are interested in this portion of the spectrum [1–5].

#### **7. Schemes based on laser plasma interaction**

Terahertz can be generated from nonlinear plasma medium. The following below mentioned schemes are commonly used for the generation of THz radiations.


The high power laser beams changes the index of refraction of the plasma medium due to non-linear processes, called Self-focusing of laser beam. The increasing intensity of electric field enhances the index of refraction of plasma and the plasma shows similar behaves as a converging lens (**Figure 1**). Further, intensity of self-focusing region rises as the beam enters into medium, until the divergence effect occurs.

A work has been done to investigate terahertz generation in magnetized plasma using self-focusing of hollow Gaussian laser beam [36]. The hollow Gaussian filamented laser propagates parallel to magnetic field and interact with electron plasma wave to produce terahertz radiations. The study shows that intensity of emitted radiations is highly sensitive to the order of hollow Gaussian laser beam. Terahertz generation by amplitude-modulated self-focused Gaussian laser beam in ripple density plasma has also been studied in Ref. [37]. In this system, a current is generated by transverse component of ponderomotive force on electrons as a result the radiation is being driven at the modulation frequency (taken into terahertz domain). It is found that in comparison to without self-focusing to self-focusing an enhancement has been seen in terahertz generation which supported by numerical

**Figure 1.** *Self focusing of LASER beam in plasma.*

#### *Studies of Terahertz Sources and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.101685*

simulation. In rippled density plasma by using cosh-Gaussian lasers terahertz generation has been studied in Ref. [38]. The laser exerts a ponderomotive force along the transverse direction as a result electrons oscillate which get coupled with the density ripple to generate terahertz radiation. It is found that by changing the decentered parameter of laser there exist a notable change in magnitude, amplitude and conversion efficiency of terahertz radiations. Terahertz radiations can also be generated by using relativistic self-focusing hollow Gaussian laser in magnetoplasma [39]. Due to relativistic effect the change in electron mass occurs at high intensity which leads to produce nonlinear effects in plasma leading to the self-focusing of hollow Gaussian laser beam. Hassan et al. defined that when two Laguerre-Gaussian laser beam is gone through the cross-focusing then it generates THz [40]. The amplitude of THz can be enhanced with the help of large amplitude density ripple.

Kumar et al. [41] numerically investigated that THz yield increases sufficiently under the effect of self-focusing and defocusing of amplitude-modulated Gaussian laser beam in rippled density plasma. Hong et al. [42] studied the propagation of a Gaussian and hollow Gaussian laser beam in a tapered plasma to figure out the ponderomotive self-channeling and relativistic self-focusing effects. It has been concluded that, when transverse plasma density is homogeneous, its focusing ability is robust than that of the hollow Gaussian laser. Vhanmore et al. [43] used asymmetric elegant Hermite-cosh-Gaussian to study the self-focusing in magnetized plasma. Kumar et al. [44] analytically studied relativistic self-focusing and particle-in-cell simulations, which reveals that the self-focusing is less sensitive to laser amplitude variation in deeper plasma channels for millimeter range plasma channels present scheme is being valid.

#### **7.1 Wake field terahertz scheme**

Laser wake field scheme depends on the interaction of a laser beam with a plasma. A conical emission in the forward direction is produced by laser pulse under the influence of oscillating electrons which involve Cerenkov mechanism is called the laser wake field. Tajima and Dawson explained this scheme [45] as follows. The ponderomotive force originated by laser pulse envelope pushes away the background plasma electrons. The resulting force from the charge separation initiates a density oscillation, after the laser pulse left away the plasma. In this situation, group velocity of the laser equal to the phase velocity of the density oscillation. The same charge fluctuation is known as plasma wave or plasma wake. Self-trapped background plasma electrons produce electron bunch in the wake. Sheng et al. [46] detected powerful coherent emission of terahertz radiation in inhomogeneous plasma, when laser pulse is incident obliquely in laser wake field. It has been observed that the duration of terahertz, frequency and bandwidth depend on laser pulse duration and plasma density profile.

Gupta et al. [47] reported that plasma-density modulation and magnetic field can assist in electron energy enhancement by improving the electron trapping in laser wakefield acceleration (LWFA). Gupta et al. [48] also investigated the acceleration of electrons by the plasma waves in a density rippled inhomogeneous plasma. Gopal and Gupta [49] explored the use of asymmetric laser pulses (of sharp rising front) for optimization and control of electron beam in LWFA and reported that an asymmetric laser pulse reduces the beam emittance, enhance injection and can help in controlling the beam spreading to generate a high-quality monoenergetic beam. Yoshii et al. [50] employed the particle in cell simulation to generate the Cherenkov wake field by a short laser pulse to realize THz radiation in a magnetized plasma. Gopal et al. [51] have also suggested a method of enhancing the magnetic field strength in laser pulse interaction with plasma. Esarey et al. [52] have reviewed the physics of the plasma

beat wave accelerator, laser wakefield accelerator and self-modulated laser wakefield accelerator. These sources are capable to handle the strong electric field of order 100 GV/m from an intense laser. Hofmann simulated the performance of quadrupoles and solenoids in focusing and energy selection of laser accelerated protons [53]. Döpp et al. proposed the use of longitudinal density tailoring to reduce the beam chirp at the end of the accelerator [54].

#### **7.2 Beat wave schemes**

For the Generation of efficient THz radiation at different frequencies, various experiments have been conducted on lasers beating in a corrugated plasma. THz radiation generated by beating of two lasers yields more tunability and efficiency.

The basic mechanism to generate THz radiation is as follows:

Consider two laser beams having different wave numbers and frequencies propagating in a corrugated plasma. The laser beams exert a ponderomotive force on electrons. As a result these electrons, drives longitudinal oscillations (at beat frequency) adjacent to plasma frequency as shown in **Figure 2**. The generated beat wave decays (parametrically) into a terahertz wave and a plasma wave. The generated terahertz wave is (plasma channel) transverse magnetic mode with finite longitudinal component of the electric field.

Malik et al. [55] used super-posed femtosecond laser pulses to generate the THZ from a gas jet through oscillatory current density. The emission of THz radiation occur through oscillating dipoles. Hamster et al. [56] used 100 femtosecond (1 TW) laser focused onto gas through wakefield. The electrons execute oscillatory motion and produce terahertz radiation under the influences of ponderomotive force. Yampolsky and Frainman [57] reported the four-wave coupling scheme in a plasma filled capillary for the amplification of terahertz radiation. Kukushkin has produced the THz radiations in semiconductors using crossed alternating electric field and static magnetic field [58]. The external dc magnetic field used to increase the field of emitted radiations [59]. Jafari et al. have investigated that the generation of THz radiation by nonlinear coupling of two color laser beam which have Gaussian field in a plasma with multi-ion species through ponderomotive force on laser in plasma [60]. The radiated THz emission strongly depends on the density of ionic species. Result shows that the maximum value of the amplitude of THz found in a specific range of laser intensity. Li et al.

**Figure 2.** *THZ wave generation in beat wave mechanism.*

*Studies of Terahertz Sources and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.101685*

investigated the contribution of the optical rectification for the generation of THz radiation by two color laser pulse by including the pump power of laser, rotation angle of Beta Barium Borate crystal and the numerical aperture of lens. All of the above factor dramatically affects the intensity of the radiation of THz wave [61]. Bhasin and Tripathi [62] used optical rectification of a x-mode picosecond laser pulse in rippled density magnetized plasma to generate THz radiation. Malik et al. investigated the scheme of the generation of the THz waves with two color laser in clustered plasma. The cluster plasma produces third order nonlinearity and resulted nonlinear current density produce the terahertz radiation. This scheme do not require magnetic field and the density gradient to generate the beat frequencies. The THz conversion efficiency depends on the cluster parameter. It is concluded that the surface plasmon resonance enhances the THz generation efficiency and THz power falls down with THz frequency [63].

In Ref. [64], spatial-Gaussian lasers has been used in a periodic density plasma to excites the radiation which shows depends on the laser-beam width and amplitude. Malik et al. employed two spatial-triangular laser beams for the excitation of terahertz radiation and reported the THz field <sup>10</sup><sup>5</sup> kV/cm and the efficiency <sup>10</sup><sup>2</sup> correspond to the laser intensity <sup>10</sup><sup>14</sup> W/cm<sup>2</sup> [65]. Malik and Malik [66] also suggested the mechanism for the generation of tunable terahertz radiation under the application of two femtosecond laser pulses. Dai and Liu [67] studied Terahertz emission in a gaseous plasma (generated by two lasers) with intensity of 5.00 <sup>10</sup><sup>14</sup> W/cm<sup>2</sup> . Kumar et al. [68] studied Beat excitation of terahertz radiation from two different frequency infrared lasers of TM/TE mode propagating along z direction in a rippled density semiconductor waveguide slab in a magnetic field (applied transvers to it) and the terahertz yield is significantly higher in the TM mode laser beating than in the TE mode laser beating. Malik and Malik [69–72] investigated the role of an external DC magnetic field in tuning the frequency and power of terahertz radiation. Varshney et al. [73, 74] proposed a scheme for the generation of THz radiation from rippled density magnetized plasma by beating of extraordinary mode lasers.

Malik and Singh [74] used two super-Gaussian lasers to generate the highly focused terahertz radiation by frequency mixing. Chaudhary et al. [75] used Hermite cosh Gaussian lasers to generate the efficient intensity distribution of tunable terahertz radiation. Manendra et al. [76] used hollow sinh super-Gaussian laser beams to generate polarized terahertz wave by photo mixing of two-color laser. The efficiency and the field amplitude increases with electron temperature. Zhang et al. [77] did two-dimensional particle-in-cell simulations of ultra-intense relativistic laser plasma interaction of solid target to generate the terahertz pulses by coherent transition radiation and THz radiation energy increased by 10 times. Manendra et al. [78] investigated the effect of electron temperature on intensity and efficiency of terahertz generated by laser beating in inhomogeneous plasma [79]. Manendra et al. [78–80] used radially polarized lasers having a top-hat envelope profile [s (profile index) ≥ 1] in density modulated hot plasma and concluded that the conversion efficiency increased by 5 times, at the electron thermal velocity 0.2c, where c is speed of light. Liu et al. [81] did two-dimensional particle-in-cell simulations to study the terahertz wave propagating in the stagnation region of a reentry plasma sheath and these investigations are useful to study the attenuation of radio waves in atmosphere communication.

#### **7.3 Resonant third harmonic generation**

Higher Harmonic Generation are used to generate a highly coherent radiation sources in the soft x-ray region of the spectrum. When the electric field of the order 1013–10<sup>14</sup> W/cm<sup>2</sup> of laser interacts with a molecular gas, higher harmonics are produced through nonlinear process laser field. High Harmonic Generation sources has applications in plasma diagnostics, molecular dynamics and in solid state science [82, 83].

Some nonlinear optical crystal describes the formation of the field at the sum frequency of the source fields ω3 = ω1 + ω2, where, ω1, and ω2 are the frequencies of the sources fields. The crystal produces polarization at a combination of their frequencies and the resultant field oscillate at a frequency ω3. In second order harmonic generation the resulting field oscillate at 2ω frequency as shown in **Figure 3**.

Cook and Hochstrasser defined that, when we focus the fundamental and second harmonic laser simultaneously whose peak intensities is the order of 10<sup>14</sup> then it generates the THz radiation [84]. Panwar et al. [85] studied the effect of nonuniform rippled plasma channel on resonant third harmonic laser radiation generation, strongly enhancement of the self-focusing plasma channel non-uniformity and compression of main laser pulse at lower powers and the self-focusing reduces the effectiveness of the third harmonic power because of the compression of main laser in a deeper plasma channel.

Kumar et al. [86] generated the 20 times frequency of the incident wave (highfrequency O-mode radio wave) by nonlinear reflection through ionospheric grating. Kumar and Tripathi [87] studied the parametric coupling of a high amplitude lower hybrid wave with the ion cyclotron instability in tokamak, driven by neutral beam converted ion beam and coupling would be strong when the ion cyclotron wave and the wave numbers of the pump are perpendicular to each other, advanced stage operations of a tokamak as ITER is relevant to it. Kumar [88] investigated the generation of Terahertz radiation by second-order nonlinear mixing of laser and its frequency shifted second harmonic in a rippled density plasma, and emission of THz radiation is maximized when the polarization of the lasers being aligned and also results are accordant with the recent experimental results. Surface Plasmon resonance are used in photonic devices and surface enhanced Raman scattering. Kumar et al. [89] used metal–vacuum of circular surface grating to excite the surface plasma wave and its intensity depends on dimensions of the grating. Tyagi et al. [90] investigated the procedure of third harmonic generation by laser magnetized plasma interaction, and the phase matching condition for the up shifted frequency is satisfied and the laser frequency is not too far from the upper hybrid frequency. Kumar et al. [91] investigated the process of generation of Smith–Purcell terahertz radiation of 10 mW at 10 THz by mixing of two co-propagating lasers passing over a periodic metallic grating.

Kumar and Kumar [92] proposed a scheme of a planar array of nanotube for generation of THz radiation by passing an ultrashort electron bunch. The emitted THz radiation generates at natural frequency of ωp/√2, which is the frequency of electron cylinder.

**Figure 3.** *Mechanics of second order harmonic generation.*

#### *Studies of Terahertz Sources and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.101685*

It is revealed that the intensity of terahertz radiations is highly sensitive to the magnetic field and the index of super Gaussian beams [93, 94]. By using beating of two super Gaussian lasers in plasma with DC electric field in transverse direction terahertz radiation can be generated [95]. Terahertz radiation generated due to ponderomotive force which acts on electron plasma wave and make them oscillate at frequency difference of two lasers which generates a nonlinear current having frequency in terahertz domain. It is found that the amplitude of terahertz radiation can be enhanced by index of two lasers as well by DC electric field. Investigations are going on for the generation of intense picosecond THz pulses via nonlinear optical methods such as optical rectification. A work has been done to generate terahertz radiations using optical rectification of a super-Gaussian laser beam in rippled density plasma [96]. The change in intensity cause a ponderomotive force in transverse direction which makes electrons oscillates and as a consequence terahertz radiation is being produced. The phase matching is provided by ripple of plasma. In a collision less magnetoplasma terahertz radiation can be produced by using two cross focused Gaussian laser beams [97]. When the applied magnetic field is increased the focusing of lasers increases due to this a nonlinear ponderomotive force acts upon electron plasma waves causing electrons oscillations and a nonlinear current is produced at the terahertz frequency domain. It is found that the amplitude of generated terahertz radiation increases with magnetic field and the cross focusing of two laser beams. The optimization of laser-plasma parameters gives the normalized terahertz power of order 10k. The relativistic focusing of two co-axial Gaussian laser beams into ripple density plasma has been investigated by Kumar et al. [98]. When two lasers propagate into ripple density plasma, then the ponderomotive force reinforce the electrons to oscillates into the transverse direction and these oscillations gets coupled with ripple density of plasma and produce a nonlinear current at terahertz frequency. The study suggest that the amplitude of THz radiation can be enhanced by relativistic ponderomotive focusing of two lasers and also the conversion frequency of the order of 10–<sup>3</sup> can be achieved. The terahertz generation in collisional plasma using two cross focused laser beams has been studied by Sharma and Singh [99]. The optimized parameters of lasers provide the radiated power of the order 0.23 MW. The applied static electric field, ripple density of plasma and the collision frequency of electron allow the generation of the terahertz radiation. Singh et al. presented a scheme for the generation of strong THz radiation through optical rectification of shaped laser pulse in magnetized plasma [100]. The THz yield increases with the increasing strength of the background magnetic field and the sensitivity depends on the ripple wave number. The emitted power is directly proportional to the square of the amplitude of the density ripple. The enhancement in terahertz generation can be achieved by increasing strength of background magnetic field. It is found that the power of emission is directly proportional to the square of amplitude of ripple. Singh et al. further employed hyperbolic-secant and Gaussian shapes of laser beam to generate the terahertz radiation through optical rectification of a laser pulse in magnetized ripple density plasma. The amplitude of the terahertz radiation shows dependence on the laser beam, laser profile index and the density ripple. When cyclotron frequency approaches to the THz frequency, the THz field amplitude reaches its maximum value [101]. The normalized amplitude of the radiation of order 10–<sup>2</sup> has been realized. A work has been done to study the generation of terahertz radiations using optical rectification of an amplitude modulated super-Gaussian laser beam propagating into a periodic density plasma with a transverse magnetic field applied on it [102]. The transverse ponderomotive force arises due to the non-uniform spatial variation in laser intensity. The terahertz field amplitude increases with magnetic field strength, modulation index and ripple parameters. Efficiency of the order of 10–<sup>5</sup> of terahertz wave is

achieved. The relativistic ponderomotive force and nonlinear phenomena excite the modulation instability. Jha et al. [103] studied the modulation instability due to the propagation of a laser pulse through a magnetized plasma. It has been depicted that the transverse magnetization of the plasma reduces the modulation instability. In magnetized plasma, the peak spatial growth rate of instability decreased by almost 14 percentage in contrast to the unmagnetized plasma case. Kumar and Tripathi [104] examined Rayleigh scattering of a Gaussian laser beam from clustered gases. According to the model, the clusters expand under laser-induced heating and hydrodynamic pressure and approach towards plasma resonance. When the cluster electrons reach the plasma frequency of √3 times the laser frequency, it produces resonantly enhanced Rayleigh scattering. Magesh and Tripathi [105] investigated the laser excitation of electrostatic eigenmodes of a plasma (having parabolic density profile) in an azimuthal magnetic field. Singh et al. [106] proposed the THz radiation generation by the interaction of the pump upper hybrid wave and the laser (extraordinary wave). In this mechanism, the non-linear interaction between the two waves creates a non-linear current at their frequency difference, which can be brought in the THz range under the appropriate pump frequency and phase matching conditions. In the same research area of THz generation, Hassan et al. [107] studied the interaction of a high-intensity laser beam with density ripple in collisionless magnetized plasma under the paraxial ray approximation to produce THz radiation.

Kumar and Tripathi investigated the schemes of terahertz radiation generation using different methods such as non-linear mixing of laser pulses of finite spot size in clustered gas [108–110], laser bunched electron beam in a magnetic wiggler and optical mixing of laser pulses of finite spot size in a rippled density unmagnetized plasma. In an another study, Rajouria et al. [111] proposed that the relativistic mass and non-linearity increases the resonance absorption of the laser pulse in a density gradient plasma. K K and Tripathi [112] used carbon nanotubes array to investigate the linear and non-linear interaction of laser. The surface plasmon resonance increases, when the laser imparts oscillatory velocity and excursion to electrons in the nanotubes. Kumar et al. [113] have studied the non-linear mixing of laser pulses in a rippled density magnetized plasma. It was obtained that the electron drift induced by lasers' ponderomotive force couples with the density ripple and produces a non-linear current that resonantly drives the THz at the beat frequency. Liu et al. [114], developed an analytical formalism for broadened surface plasmon resonance and enhanced X-ray emission is achieved in a non-uniform clusters with high power lasers. Kumar et al. [115] explored the laser beat wave excitation of THz radiation in a hot plasma with a step density profile, where enhanced yield is achieved due to the coupling with the Langmuir wave at plasma frequency near THz frequency.

Bakhtiari et al. [116] proposed a scheme for improving terahertz radiation efficiency by the interaction of two Gaussian laser array beams in an electron-neutral collisional plasma. They optimized that high efficiency of up to 0.07% can be achieved using array beams, which is almost three times higher than the maximum efficiency achieved by a single Gaussian laser beam. An analytical study has been presented by Sharma et al. [117] for the generation of terahertz radiation due to transverse wakefields produced by the propagation of a circularly polarized laser pulse in a homogeneous, underdense, and axially magnetized plasma. Sobhani et al. [118] demonstrated the vital role of pump depletion and cross-focusing effects in the generation of twisted THz radiation in a non-linear plasma medium. Lehmann and Spatschek [119] discussed the generation of plasma gratings in underdense plasma by counterpropagating laser pulses, which can act as plasma photonic crystals for high-power lasers. In Ref. [10, 120] review of recent progresses in the generation, detection and application of intense terahertz radiation has been reported.

#### **8. Detection of terahertz pulses**

The electro-optic sampling is used to detect the THZ radiations which is opposite of optical rectification mechanism in nonlinear crystals. Coherent detection process is normally used so that the amplitude and phase of radiation can be detected. Other way of detection of Terahertz wave is electro optic effect. This detection method is based on the process in which electric field at terahertz frequency induce a birefringence in an optically transparent material. The relation between the magnitudes of effect is directly proportional to the state of the field. Fourier transform of the temporal pulse give the THz spectrum.

The direct and coherent detectors are mainly used to identify the terahertz radiations. The direct detector measure the average power and the coherent detectors measure the instantaneous value of electric field.

#### **8.1 Direct detectors**

The Bolometer, Golay cell and the pyroelectric are used to measure the average power of broadband THz pulses. The bolometer work on the principle of temperature-dependent electrical resistivity which contains of a sensing material. As soon as it absorb the incoming photons, its shows the change of its electrical resistivity when it is illuminated by incoming radiations.

#### **8.2 Coherent detectors**

The atto-second technologies are used to measure the amplitude and phase of the electric field oscillation of an EM wave at THz frequency. Although this technique is difficult at optical frequencies, therefore it is achieved in the radio wave frequency range with the oscilloscope.

The THz radiation photon have energies of the order of few meV than the photons of optical frequencies. Therefore the ambient background and thermal noise disturb the measurement of THz radiations. So, it has become necessary to extract the background noise from the interested signals. The researcher use the Phase Sensitive Detection linked with an instrument called Lockin Amplifier.

#### **9. Challenges in the terahertz radiation spectroscopy**

There are various challenges in the field of terahertz radiation spectroscopy and imaging. The first issue is that terahertz radiation are strongly absorbed by the polar liquid (water) which presents in all the tissues, so they cannot penetrate much deeper into the moist tissues [1, 3, 4].

The other challenges in the terahertz radiation spectroscopy and imaging are resolution and its slowness mechanism in comparison to previously established ways of imaging, which produces thousands of pixels per second while the terahertz have the speed 1 pixel in several seconds. For any conventional way of imaging the diffraction is limited by wavelength of the radiation. In the case of terahertz imaging, it lies in the range of one micrometer to 3 mm which do not give enough detail images for most of the medical applications. This shortcoming can be overcome by near field imaging.

#### **10. Conclusion**

This chapter contribute the applications and generations of THz technology in the field of security, medicines, science and biomedical engineering. The biomacromolecules and certain drugs are detected by using of THz spectroscopy. THz imaging has been employed to diagnostics of cancers, treatment skin burn and dental related diagnostics. The biological effect of THz is still required to further explore the research area. The nonthermal effects of THz radiation on human DNA is needed to pay more attention. For the widespread applications of THz, we need higher-power THz sources and their THz detectors.

#### **Author details**

Sukhmander Singh<sup>1</sup> \*, Shravan Kumar Meena<sup>2</sup> , Ashish Tyagi<sup>3</sup> , Sanjeev Kumar<sup>2</sup> , Man Raj Meena<sup>4</sup> and Sujit Kumar Saini<sup>2</sup>

1 Plasma Waves and Electric Propulsion Laboratory, Department of Physics, Central University of Rajasthan, Ajmer, Rajasthan, India

2 Department of Physics, Motilal Nehru College, University of Delhi, Delhi, India

3 Department of Physics, Swami Shraddhanand College, University of Delhi, Delhi, India

4 Department of Physics, Gargi College, University of Delhi, New Delhi, India

\*Address all correspondence to: sukhmandersingh@curaj.ac.in

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Studies of Terahertz Sources and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.101685*

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