**1. Introduction**

Terahertz waves (wavelength, 30–3000 μm; frequency, 10–0.1 THz) are important not only in the fields of basic science, such as molecular spectroscopy, molecular optics, plasma measurement, charged particle acceleration, and radio astronomy, but also in numerous applications, such as broadband wireless communication, nondestructive inspection, high precision radar,

© 2016 The Author(s). Licensee InTech. 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. © 2018 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.

and global environmental measurement, since they have higher directivity like infrared than microwaves and higher transmittances in the atmosphere and in soft materials like microwave than infrared. Therefore, high-peak-power, narrow-linewidth (high-brightness), and continuously tunable terahertz-wave sources that could be widely used in such applications are required. The terahertz (THz) region is relatively unexplored, because of the lack of the commercially available high-brightness and continuously tunable sources, high-sensitive and fast detectors, and optics, which has resulted in what is called the frequency gap [1–3]. Over the past two decades, there has been striking growth in the region of science and engineering, which has become a vibrant, international, cross disciplinary research activity [4]. Wavelength (frequency) conversion in nonlinear optical materials is an effective method for generating high-brightness and continuously tunable terahertz waves owing to the high conversion efficiency, bandwidth, wide tunability, and room temperature operation. A terahertz-wave source using parametric wavelength conversion based on lithium niobate (LiNbO3 ) crystals was first proposed in 1960s [5, 6] and realized in the mid-1990s with the terahertz-wave parametric oscillator [7]. At that time, the tuning range and the observed maximum peak output power of terahertz wave were from 1.1 to 1.6 THz (270–184 μm) and several milliwatts, respectively. By using the current injection-seeded terahertz-wave parametric generator (is-TPG), the tuning range expanding from 0.39 to 5.0 THz (750–60 μm) and the peak output power exceeding 55 kW [8–11] were observed, representing an increase by 10 times and seven orders of magnitude, as shown in **Figure 1**. **Table 1** lists the characteristics of three typical intense terahertz-wave sources: our injection-seeded terahertz-wave parametric generator (is-TPG), well-known intense terahertz-wave sources, a far-infrared free-electron laser (FIR-FEL) [12], and terahertz-wave pulse generation through optical rectification using a tilted-pulsefront

excitation (OR) [13]. The is-TPG is one of the brightest sources in the terahertz region with a wide tuning range. We explain in this chapter how the high-brightness terahertz waves are

*)<sup>2</sup> × linewidth]*. OR generates broadband terahertz waves.

High-Brightness and Continuously Tunable Terahertz-Wave Generation

**Table 1.** Characteristics of typical intense terahertz-wave sources: our injection-seeded terahertz-wave parametric generator (is-TPG), terahertz-wave pulse generation through optical rectification using a tilted-pulse-front excitation (OR), and a narrowband free-electron laser that works in the far-infrared region (FIR-FEL). The brightness temperature

When the intense laser beams pass through a nonlinear optical crystal, the transverse photon and phonon wave fields become coupled and behave as new mixed photon-phonon states called polaritons. Broadband terahertz-wave generation results from efficient parametric scattering of laser light via polaritons [5, 6]. The polaritons exhibit phonon-like behavior in the resonant frequency region (near the transverse optical (TO)-phonon frequency *ωTO*); however, they behave like photons in the nonresonant low-frequency region, as shown in **Figure 2**. Generation of narrowband terahertz waves can be achieved by applying an optical resonator (in the case of the terahertz-wave parametric oscillator (TPO)) or injecting a "seed" (in the case of the injection-seeded terahertz-wave parametric generator (is-TPG)) for the idler wave [14]. The wide tunability is accomplished simply by changing the angle between the incident pumping beam and the resonator axis (in the case of TPOs) or the wavelength and axis of the

) crystal.

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generated via acoustic phonons of a nonlinear lithium niobate (LiNbO3

**Figure 2.** (Left) Dispersion relation of the polariton. (Right) Noncollinear phase-matching condition.

**2. Terahertz-wave parametric generation**

) is calculated as *kBT<sup>B</sup> = Peak power/[(M<sup>2</sup>*

(T<sup>b</sup>

**Figure 1.** Development of parametric sources using LiNbO3 in our group. Points represent the peak output power during 1–3 THz in each paper.

High-Brightness and Continuously Tunable Terahertz-Wave Generation http://dx.doi.org/10.5772/intechopen.75038 31


**Table 1.** Characteristics of typical intense terahertz-wave sources: our injection-seeded terahertz-wave parametric generator (is-TPG), terahertz-wave pulse generation through optical rectification using a tilted-pulse-front excitation (OR), and a narrowband free-electron laser that works in the far-infrared region (FIR-FEL). The brightness temperature (T<sup>b</sup> ) is calculated as *kBT<sup>B</sup> = Peak power/[(M<sup>2</sup> )<sup>2</sup> × linewidth]*. OR generates broadband terahertz waves.

excitation (OR) [13]. The is-TPG is one of the brightest sources in the terahertz region with a wide tuning range. We explain in this chapter how the high-brightness terahertz waves are generated via acoustic phonons of a nonlinear lithium niobate (LiNbO3 ) crystal.
