**1. Introduction**

#### **1.1 Brief introduction on terahertz radiation**

Terahertz (THz) is a multiple of the unit of frequency in the International System, equal to 1012 Hz. In the present context we use it for representing the frequency of electromagnetic waves in the range of 0.1 to 30 THz (wavelength 10 μm to 3000 μm), which confines with microwaves and infrared (IR) light. The THz radiation lies on a transition region from macroscopic classical theory to microscopic quantum theory and from classical electronics to photonics. Sometimes, especially in the past, people refer to this region with the expression "Terahertz gap". The reason for this is that both the generation and detection of THz electromagnetic waves is quite challenging compared to well-established techniques in the microwaves and infrared regions, so that this important part of the electromagnetic spectrum was inaccessible until very recent years. Since the first artificial THz wave

was produced and detected by means of ultrashort laser pulses in the early 90's, a strong research effort began with the aim to improve our ability to produce, control, manipulate and detect this kind of waves [1], so that now, 30 years later, the expression "THz gap" is not so often employed anymore.

The importance of THz spectroscopy in fundamental science is obvious because many important low-energy excitations are present within this range in practically all kind of materials: emerging materials [2], semiconductors [3], ferroelectrics [4], superconductors [5], polymers [6], photonic crystals [7], liquids [8–10], gases [11] and biological systems [12]. Moreover, THz technology has a high impact in many different areas too. We shall not cite them all here, but as an example the application of THz sensing and imaging to biomedical and biological studies and laboratory practices will revolutionize, in our opinion, these fields in a close future [13]. One of the great advantages in this case, is the very low photon energy, which is much safer than X-rays for living tissues imaging [14].

The unique performance of THz has far-reaching implications in areas such as communications [15], radar [16], astronomy [17], safety inspection (airports) [18], etc. Terahertz is a new source of radiation with many unique advantages: as an example, it is possible to combine spatial and temporal resolution (a typical THz pulse has a time duration of about 1 ps and a wavelength of about 100 μm) in order to perform not only imaging but also time-resolved measurements on an ultrafast ps time-scale. As a result, THz research has a great value to national economy and national security [19].

#### **1.2 THz Spectroscopy**

For many years, IR and Raman spectroscopies have been used to investigate the properties of molecular vibrations and rotations. THz Spectroscopy can complement those two techniques, for mainly two reasons: from the one hand, it can access higher wavelengths compared to IR and Raman, and from the other hand it can access the complex transmission or reflection coefficients rather than their real squared value (power spectrum), as will be shown in the following [20].

**Figure 1** shows the position of the THz range in the electromagnetic spectrum. At present, there is no generally accepted definition of the upper and lower limits of THz radiation frequency. The high frequency of THz spectrum overlaps with the far IR spectrum, and the low frequency overlaps with the microwave frequency band. Most of the authors set the limits of this range to 0.1–3 THz, because this is the emission region of the most commonly used sources (photoconductive antennas and optical rectification), but here we prefer to extend this range up to 30 THz, because this is the upper limit of our source based on air-plasma, which will be described in the following, and this emission scheme is essential to achieve the THz Hyper-Raman effect, as it will be shown later.

**43**

(ω

*TeraVision: A LabVIEW Software for THz Hyper-Raman Spectroscopy*

A major reason for the rapid development of terahertz spectroscopy is the progress of ultrafast lasers (femtosecond pulses) by means of the Chirped Pulse Amplification technique, which was introduced by Donna Strickland and Gérard Mourou at the University of Rochester in the mid-80's for which they received the Nobel Prize in Physics in 2018 [21]. These ultrashort laser sources gave birth to several new generations of spectrometers in the early 90's, including THz. The THz spectrometer is capable of generating and detecting pulses of coherent THz radiation, but most importantly it is able to detect both amplitude and phase of the THz wave, and thus it allows retrieving the full complex dielectric function of the target material without resorting to Kramers-Kronig relationships, as it happens in standard IR spectroscopy. Moreover, some recent advances in both the fs laser sources and the THz emission efficiency and detection sensitivity, allowed the researchers to make use of intense THz pulses to control the material relevant parameters [22],

THz Time Domain Spectroscopy (THz-TDS) is the most widespread technology among general THz spectroscopies. We give here a brief description of it, although

TeraVision software is able to control standard THz-TDS measurements besides the THYR Spectroscopy, and, second, THz-TDS is a good way for inexperienced readers to approach the topic of THz Spectroscopy in general. The name of this technique is quite straightforward as the spectroscopic information is not retrieved directly in the frequency domain, but in the time domain, and then converted by simple Fourier Transform. The THz pulse can be generated in several ways. The most commonly employed generation mechanisms are photoconductive antennas [23], optical rectification [24] and air-plasma four waves mixing [25]. The first method creates a strong and short burst of current in between two electrodes with applied bias voltage when a population of free charges is suddenly created by a fs-laser optical pulse in a semiconductor slab. The second method converts the fs-pulse bandwidth into THz by nonlinear optical difference frequency conversion (

often called "optical rectification", this expression more properly refers to static electric fields generated by light) by means of opportune nonlinear crystals (ZnTe, GaSe, GaP and many others have been proved to work best on different spectral ranges). The key point here is that due to the very large bandwidth of fs-pulses the

rather creates a low-energy band whose extension depends on the laser pulse bandwidth as well as the dielectric properties of the nonlinear crystal. The third method is based on laser ionization of air molecules, converted by strong laser intensity into plasma, which is a highly non-linear medium. The contextual presence of fundamental and double frequency photons (created by a suitable nonlinear crystal Beta Barium Borate β-BBO, placed few centimeters in front of the plasma) generates many four-waves mixing frequencies: some of them are falling into the visible domain, as possible to observe in **Figure 2** on the mirror surface, and should be filtered out by a high resistivity thick silicon wafer, but one particular linear

ωωω − − ) is capable of producing the THz radiation. Also, here as in previous point, the subtraction of two photons at fundamental laser frequency from one photon at double frequency, is not a static field

= 0 ) because of the large bandwidth of the fs-laser pulses. This is the generation

For detection, it is possible to use any of the above mentioned techniques, but reversed. In all cases, the detection works as follows: the THz pulse is sent to the sample and it gets reflected or transmitted. Then, it is sent to a detector which will

ω ω ω ω− ,

− ) does not always vanish, but it

it is not the main topic of this Chapter, for two different reasons. First, the

*DOI: http://dx.doi.org/10.5772/intechopen.96663*

rather than being limited to measure its spectrum.

difference between all possible frequencies (

combination of those frequencies ( 2

scheme which is necessary for THYR Spectroscopy [26].

**1.3 THz time domain spectroscopy**

**Figure 1.**

*The terahertz region in the electro-magnetic spectrum.*

*TeraVision: A LabVIEW Software for THz Hyper-Raman Spectroscopy DOI: http://dx.doi.org/10.5772/intechopen.96663*

A major reason for the rapid development of terahertz spectroscopy is the progress of ultrafast lasers (femtosecond pulses) by means of the Chirped Pulse Amplification technique, which was introduced by Donna Strickland and Gérard Mourou at the University of Rochester in the mid-80's for which they received the Nobel Prize in Physics in 2018 [21]. These ultrashort laser sources gave birth to several new generations of spectrometers in the early 90's, including THz. The THz spectrometer is capable of generating and detecting pulses of coherent THz radiation, but most importantly it is able to detect both amplitude and phase of the THz wave, and thus it allows retrieving the full complex dielectric function of the target material without resorting to Kramers-Kronig relationships, as it happens in standard IR spectroscopy. Moreover, some recent advances in both the fs laser sources and the THz emission efficiency and detection sensitivity, allowed the researchers to make use of intense THz pulses to control the material relevant parameters [22], rather than being limited to measure its spectrum.

#### **1.3 THz time domain spectroscopy**

*LabVIEW - A Flexible Environment for Modeling and Daily Laboratory Use*

sion "THz gap" is not so often employed anymore.

than X-rays for living tissues imaging [14].

Hyper-Raman effect, as it will be shown later.

*The terahertz region in the electro-magnetic spectrum.*

national security [19].

**1.2 THz Spectroscopy**

was produced and detected by means of ultrashort laser pulses in the early 90's, a strong research effort began with the aim to improve our ability to produce, control, manipulate and detect this kind of waves [1], so that now, 30 years later, the expres-

The importance of THz spectroscopy in fundamental science is obvious because many important low-energy excitations are present within this range in practically all kind of materials: emerging materials [2], semiconductors [3], ferroelectrics [4], superconductors [5], polymers [6], photonic crystals [7], liquids [8–10], gases [11] and biological systems [12]. Moreover, THz technology has a high impact in many different areas too. We shall not cite them all here, but as an example the application of THz sensing and imaging to biomedical and biological studies and laboratory practices will revolutionize, in our opinion, these fields in a close future [13]. One of the great advantages in this case, is the very low photon energy, which is much safer

The unique performance of THz has far-reaching implications in areas such as communications [15], radar [16], astronomy [17], safety inspection (airports) [18], etc. Terahertz is a new source of radiation with many unique advantages: as an example, it is possible to combine spatial and temporal resolution (a typical THz pulse has a time duration of about 1 ps and a wavelength of about 100 μm) in order to perform not only imaging but also time-resolved measurements on an ultrafast ps time-scale. As a result, THz research has a great value to national economy and

For many years, IR and Raman spectroscopies have been used to investigate the properties of molecular vibrations and rotations. THz Spectroscopy can complement those two techniques, for mainly two reasons: from the one hand, it can access higher wavelengths compared to IR and Raman, and from the other hand it can access the complex transmission or reflection coefficients rather than their real squared value (power spectrum), as will be shown in the following [20].

**Figure 1** shows the position of the THz range in the electromagnetic spectrum. At present, there is no generally accepted definition of the upper and lower limits of THz radiation frequency. The high frequency of THz spectrum overlaps with the far IR spectrum, and the low frequency overlaps with the microwave frequency band. Most of the authors set the limits of this range to 0.1–3 THz, because this is the emission region of the most commonly used sources (photoconductive antennas and optical rectification), but here we prefer to extend this range up to 30 THz, because this is the upper limit of our source based on air-plasma, which will be described in the following, and this emission scheme is essential to achieve the THz

**42**

**Figure 1.**

THz Time Domain Spectroscopy (THz-TDS) is the most widespread technology among general THz spectroscopies. We give here a brief description of it, although it is not the main topic of this Chapter, for two different reasons. First, the TeraVision software is able to control standard THz-TDS measurements besides the THYR Spectroscopy, and, second, THz-TDS is a good way for inexperienced readers to approach the topic of THz Spectroscopy in general. The name of this technique is quite straightforward as the spectroscopic information is not retrieved directly in the frequency domain, but in the time domain, and then converted by simple Fourier Transform. The THz pulse can be generated in several ways. The most commonly employed generation mechanisms are photoconductive antennas [23], optical rectification [24] and air-plasma four waves mixing [25]. The first method creates a strong and short burst of current in between two electrodes with applied bias voltage when a population of free charges is suddenly created by a fs-laser optical pulse in a semiconductor slab. The second method converts the fs-pulse bandwidth into THz by nonlinear optical difference frequency conversion (ω ω− , often called "optical rectification", this expression more properly refers to static electric fields generated by light) by means of opportune nonlinear crystals (ZnTe, GaSe, GaP and many others have been proved to work best on different spectral ranges). The key point here is that due to the very large bandwidth of fs-pulses the difference between all possible frequencies (ω ω− ) does not always vanish, but it rather creates a low-energy band whose extension depends on the laser pulse bandwidth as well as the dielectric properties of the nonlinear crystal. The third method is based on laser ionization of air molecules, converted by strong laser intensity into plasma, which is a highly non-linear medium. The contextual presence of fundamental and double frequency photons (created by a suitable nonlinear crystal Beta Barium Borate β-BBO, placed few centimeters in front of the plasma) generates many four-waves mixing frequencies: some of them are falling into the visible domain, as possible to observe in **Figure 2** on the mirror surface, and should be filtered out by a high resistivity thick silicon wafer, but one particular linear combination of those frequencies ( 2ωωω − − ) is capable of producing the THz radiation. Also, here as in previous point, the subtraction of two photons at fundamental laser frequency from one photon at double frequency, is not a static field (ω = 0 ) because of the large bandwidth of the fs-laser pulses. This is the generation scheme which is necessary for THYR Spectroscopy [26].

For detection, it is possible to use any of the above mentioned techniques, but reversed. In all cases, the detection works as follows: the THz pulse is sent to the sample and it gets reflected or transmitted. Then, it is sent to a detector which will

#### **Figure 2.** *Plasma filament for THz generation.*

change its electric or optical behavior according to the THz excitation, and namely to the value (and sign) of the THz electric field at a specific time. As an example, the THz pulse will change the microcurrent which flows in between two electrodes on a semiconductor substrate (photoconductive antennas) but this current will exist only when a second fs-laser optical pulse hits the semiconductor to create free carriers. Given the much shorter duration of the fs pulse as compared to the THz one, the microcurrent will be proportional not to the integrated THz pulse energy, but rather to the value of the THz electric field in time, where the time is given by the delay between the THz and the probing optical pulse. In a similar fashion, one can exploit the transient birefringence of certain nonlinear crystals in order to change the polarization state of the probing optical pulse. Again this transient change depends on the time delay between THz and optical pulses and therefore it gives a time-scan of the temporal profile of the THz pulse (so-called "electro-optic sampling"). Finally, the symmetry breaking induced by the THz electric field in a centro-symmetric medium (usually air) will produce a nonlinear effect when coupled with a strong optical pulse, such as for instance optical Second Harmonic Generation (SHG), and again its intensity will be proportional to the amplitude of the THz electric field responsible for the symmetry breaking (this is the so-called Air Biased Coherent Detection - ABCD).

THz-TDS has the following characteristics:

1.THz-TDS technology can be used for qualitative identification [27], because it can effectively detect the physical and chemical information of materials in the THz band. And it is also a non-destructive detection method.

**45**

Raman - THYR.

*TeraVision: A LabVIEW Software for THz Hyper-Raman Spectroscopy*

niently and quickly by using THz-TDS technology.

detection sensitivity and stable operation at room temperature.

technology will have a broad commercial application prospect [32].

2.The amplitude and phase information of various materials such as dielectrics [28], semiconductors [3], gas molecules [11], biological systems [12, 29] (proteins, DNA, etc.) and superconductors [5, 30] can be obtained conve-

3.In conductive materials, THz radiation can directly access the carrier dynamics. THz-TDS can effectively be used as a non-contact and ultrafast multimeter for

4.Due to the transient nature of THz radiation, THz-TDS techniques can be used to measure relaxation-times and the low-energy excitations dynamics in

Besides, THz-TDS technology also has the advantages of wide bandwidth, high

Despite a huge amount of developments in the last 20 years, THz-TDS still needs some improvements on the spectral resolution and spectral range. In the near future, THz-TDS technology will be a powerful tool for uncovering and analyzing ultrafast phenomena in basic sciences such as physics, chemistry, and biology. At the same time, with the reduction of laser cost, the emergence of more efficient THz emitters and detectors, and more compact and advanced optical design, THz-TDS

The standard THz-TDS technique has already proved to be very effective. Nonetheless, it is important to highlight that, despite the use of nonlinear optics to generate and reveal THz radiation, this is a linear spectroscopic technique, i.e. it probes the linear dielectric function of the target material. Sometimes, it could be not so easy to access the target observables, such as the low-frequency excitations (phonons, magnons, excitons, etc.). For example, a large absorption can obscure a significant part of the spectral range, the index of refraction and/or the dielectric function could be difficult to interpret if the target materials have a complicated structure, the background from the bulk can overcome the weaker superficial contribution, which is perhaps the real target of the experiment, and finally some low-energy excitations are simply out-of-reach because of symmetry, i.e. they just do not show up in the linear parameters due to selection rules. Accessing the nonlinear optical parameters can in principle remove most of these hurdles. With this aim in mind, a new spectroscopic technique has been developed by our group a couple of years ago. Through the application of this new technology, femtosecond laser pulses and intense sub-picosecond broadband THz pulses produce THzoptical four-wave mixing in the material. The spectrum of the resulting signal is decomposed both in wavelengths and time and it appears as two distinct frequency

ω

*<sup>L</sup>* , where

ω

*<sup>L</sup>* is the optical

*DOI: http://dx.doi.org/10.5772/intechopen.96663*

conductivity measurements.

general, with very good resolution [31].

**1.4 THz Hyper-Raman time domain spectroscopy**

side bands around the optical SHG central frequency 2

central frequency of the fundamental fs-pulse. This effect resembles the wellknown Hyper-Raman (HYR) effect, and therefore it has been named THz Hyper-

In standard all-visible HYR, the coupling between optical frequencies happens through a high-order nonlinear susceptibility tensor. The model at the base of the THYR effect, instead, is a regular four-wave mixing effect, in which one of the optical photons is replaced by a THz photon. The mathematical formalism is therefore that of a nonlinear Raman effect, but because of the very small THz photon energy, the sidebands are appearing very close to the SHG central frequency

change its electric or optical behavior according to the THz excitation, and namely to the value (and sign) of the THz electric field at a specific time. As an example, the THz pulse will change the microcurrent which flows in between two electrodes on a semiconductor substrate (photoconductive antennas) but this current will exist only when a second fs-laser optical pulse hits the semiconductor to create free carriers. Given the much shorter duration of the fs pulse as compared to the THz one, the microcurrent will be proportional not to the integrated THz pulse energy, but rather to the value of the THz electric field in time, where the time is given by the delay between the THz and the probing optical pulse. In a similar fashion, one can exploit the transient birefringence of certain nonlinear crystals in order to change the polarization state of the probing optical pulse. Again this transient change depends on the time delay between THz and optical pulses and therefore it gives a time-scan of the temporal profile of the THz pulse (so-called "electro-optic sampling"). Finally, the symmetry breaking induced by the THz electric field in a centro-symmetric medium (usually air) will produce a nonlinear effect when coupled with a strong optical pulse, such as for instance optical Second Harmonic Generation (SHG), and again its intensity will be proportional to the amplitude of the THz electric field responsible for the symmetry breaking (this is the so-called Air Biased Coherent

1.THz-TDS technology can be used for qualitative identification [27], because it can effectively detect the physical and chemical information of materials in the

THz band. And it is also a non-destructive detection method.

**44**

Detection - ABCD).

**Figure 2.**

*Plasma filament for THz generation.*

THz-TDS has the following characteristics:


Besides, THz-TDS technology also has the advantages of wide bandwidth, high detection sensitivity and stable operation at room temperature.

Despite a huge amount of developments in the last 20 years, THz-TDS still needs some improvements on the spectral resolution and spectral range. In the near future, THz-TDS technology will be a powerful tool for uncovering and analyzing ultrafast phenomena in basic sciences such as physics, chemistry, and biology. At the same time, with the reduction of laser cost, the emergence of more efficient THz emitters and detectors, and more compact and advanced optical design, THz-TDS technology will have a broad commercial application prospect [32].
