1. Introduction

In recent years, the technologies available in the terahertz frequency range, connecting the microwaves and the infrared, have made great progress with the development of new methods for generating and detecting the terahertz radiation. Improved sources and detectors allow terahertz radiation to be applied in solving real-world problems, such as in material science, solid state physics, molecular analysis, atmospheric research, biology, chemistry, drug and food inspection, and gas tracing. From the point of view of applied researches using terahertz waves, there is a growing demand for sources that are simple and compact and provide a high-power output.

In our laboratory, we have been focusing on the development of a terahertz-wave source for spectroscopic imaging and sensing in the terahertz region. There are several ways to achieve this. The widely used process is the optical rectification or photoconductive switching produced by using femtosecond laser pulses [1,2]. Applied research, such as terahertz time-domain spectroscopy (THz-TDS), makes use of the good time resolution and the ultrabroad bandwidth up to the terahertz region. Novel tunable sources already exist in the sub-terahertz (several hundred gigahertzes) frequency region, such as the backward-wave oscillator (BWO). However, the output power of a BWO rapidly decreases in the frequency region above 1 THz, and its tuning capability is relatively limited. Only a few sources bring together qualities such as high power, narrow linewidth, wide tunability, room temperature operation, compactness, and ease of use. Terahertz-wave parametric generation is based on an optical parametric process in a nonlinear crystal [3,4]. The principles of the terahertz-wave parametric generator (TPG) [5–8] and the terahertz-wave parametric oscillator (TPO) [9–11] allow building systems that are not only compact, but also operate at room temperature, making them suitable as practical sources. In principle, both a narrow linewidth and a wide tunability are possible in injection-seeded TPG or TPO (is-TPG or is-TPO) systems with single-longitudinal-mode near-infrared lasers as seeding beams [12–16].

In previous reports, these sources were pumped using a flashlamp or laser-diode-pumped active Q-switched Nd:YAG lasers with long pulse widths of 5–25 ns. The output energy of the terahertz wave increases with the pump energy, but eventually the damage threshold of the crystals is reached. In this paper, we introduce a high-peak-power, single-longitudinal-mode, widely-tunable and small-sized terahertz-wave parametric source pumped by an amplified microchip Nd:YAG laser, seeded with the idler wave provided by an external-cavity diode laser (ECDL). We show how the output peak power was further enhanced and how the is-TPG was reduced to A3 paper size by using a passively Q-switched small pump source having a short pulse width. These characteristics of the pump beam permit high-intensity pumping without thermal damage to the crystal surface. The higher-intensity pumping allows a higher terahertz-wave output than those previously reported.

2. Terahertz wave parametric generation

When a strong laser beam propagates through a nonlinear crystal, photon and phonon transverse wave fields are coupled and behave as new mixed photon-phonon states, called polaritons. The generation of the terahertz-wave results from the efficient parametric scattering of laser light via a polariton, that is, stimulated polariton scattering. The scattering process involves both second- and third-order nonlinear processes. Thus, strong interaction occurs among the pumping beam, the idler wave, and the polariton (terahertz) wave. The principle of tunable terahertz-wave generation is as follows. The polaritons exhibit phonon-like behavior in the resonant frequency region (near the transverse optical (TO)-phonon frequency ω_TO), however, they behave like photons in the non resonant low-frequency region, as shown in Fig. 1 . A signal photon at terahertz frequency (ω_T) and a near-infrared idler photon (ω_i) are created parametrically from a near-infrared pumping photon (ω_p), according to the energy conservation law ω_p = ω_T + ω_i (p: pumping beam, T: Terahertz-wave, i: idler wave). In the stimulated scattering process, the momentum conservation law k_p = k_i + k_T (noncollinear phase-matching condition, Fig. 1) also holds. This leads to the angle-dispersive characteristics of the idler and terahertz waves. Thus, broadband terahertz waves are generated depending on the phase-matching angle. Generation of a coherent terahertz-wave can be achieved by applying an optical resonator (in the case of the terahertz-wave parametric oscillator (TPO)) or injecting a “seed” for the idler wave (in the case of the injection-seeded terahertz-wave parametric generation (is-TPG)). Continuous and wide tunability is accomplished simply by changing the angle between the incident pumping beam and the resonator axis or the seeding beam.

 

Fig. 1 (left) Dispersion relation of the polariton, an elementary excitation generated by the combination of a photon and a transverse optical phonon (ω_TO). The polariton in the low-energy region behaves like a photon at terahertz frequency. Due to the phase-matching condition as well as the energy conservation law, both of which hold in the stimulated parametric process, the tunable terahertz-wave is obtained by the control of the wave vector k_T. (right) Noncollinear phase-matching condition using MgO:LiNbO₃.

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The bandwidth of the TPG is decided by the parametric gain and absorption coefficients in the terahertz region. Figure 2 shows the calculated gain and the absorption coefficient at several pump intensities [16]. The gain curve has a broad bandwidth of around 3 THz, with a dip appearing at around 2.6 THz. This is because the low frequency modes of doped MgO in the MgO:LiNbO₃ work as a crystal lattice defects for LiNbO₃ [14].

 

Fig. 2 Calculated gain and absorption coefficient of MgO:LiNbO₃ pumped at 1064 nm.

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3. Experimental setup

The experimental setup, shown in Fig. 3 , consists of a pumping source (Microchip Nd:YAG laser), amplifiers, seeding source (ECDL) and the nonlinear crystal (MgO:LiNbO3). The pumping source is a diode end-pumped single-mode microchip Nd³⁺:YAG laser passively Q-switched by Cr⁴⁺:YAG saturable absorber. This microchip configuration enables the low order axial and transverse mode laser oscillation, whose linewidth is below 0.009 nm. The laser delivers more than 1 MW peak power pulses (> 500 µJ/pulse) with 420 ps pulse width at 100 Hz repetition rate with a M² factor of 1.09. This laser is free from electric noise compared with active Q-switched lasers. Additionally, this kind of fixed passively Q-switching provides a stabilized peak power, with less than +/− 2% power jitter [17,18]. The pumping beam from the microchip laser is amplified in two amplifiers in double pass configurations. In each amplifier is employed 0.7 at.% doped Nd³⁺:YAG with a diameter of 3 mm and a length of 70 mm, transversely pumped by 200 W laser diodes (tuned at 808 nm) in a three-fold geometry. Beam extraction is through a polarization beam splitter (PBS). The pumping beam diameter on the crystal is about 1 mm (FWMH). We used a 50-mm-long nonlinear MgO:LiNbO₃ crystal with antireflection coating for a wavelength of 1064 nm. A Si-prism array placed on the y surface of the MgO:LiNbO₃ crystal acts as an efficient output coupler for the terahertz waves to avoid the total internal reflection of the terahertz waves on the crystal output side. For an efficient terahertz wave emission, the pumped region within the crystal must be as close as possible to the Si-prism array, because of the large absorption coefficient of the MgO:LiNbO₃ crystal in the 1 ~3 THz range (10 ~100 cm⁻¹). The distance between the y surface and the beam center was precisely adjusted to obtain a maximum terahertz-wave output, and it was approximately equal to the pumping beam radius. The terahertz-wave output extracted through the Si-prism array was measured using a pyroelectric detector. All the components, except for the detector in Fig. 3, can be mounted on an A3 paper (29.7 × 42 cm).

 

Fig. 3 Experimental setup for the is-TPG. A Si-prism array is placed on the y-surface of the MgO:LiNbO₃ crystal.

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4. Results

Figure 4 shows the peak output power of terahertz-wave as a function of input pumping energy. As the pumping energy increases, the terahertz wave starts being detected at a pumping energy of approximately 6 mJ/pulse (12 MW (peak), 1.5 GW/cm²), then increases monotonically. The observed maximum output peak power of the terahertz wave was about 120 W (@ 1.8 THz) for a pumping energy of 14 mJ/pulse (28 MW (peak), 3.5 GW/cm²) and a seeding power of 80 mW (CW); the output power was measured by a calibrated pyroelectric detector (SpectrumDetector Inc.: SPI-A-65 THZ). Then, by blocking the seeding beam, the output peak power decreased to less than 100 mW. This value was the highest peak output power achieved in our research. The output power of the terahertz wave appears to start saturating when the pumping energy exceeds 12 mJ/pulse (24 MW (peak), 3 GW/cm²). The intensity of the idler wave was sufficiently high for generating other TPG. This leads to second storks wave generation as a cascade process. In the experiment, the pumping intensity (the pumping energy and the beam diameter) and crystal length were precisely controlled to obtain a maximum terahertz-wave output to avoid this process.

 

Fig. 4 Input energy dependence of peak output power of the terahertz-wave at 1.8 THz.

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It was possible to tune the wavelength of terahertz wave using the ECDL as a tunable seeder. When the pumping energy was about 14 mJ/pulse (28 MW (peak)) and seeding power 80 mW (CW), a wide tunability from 1.2 – 2.8 THz was observed as shown in Fig. 5 , by changing both the wavelength and the incident angle of seeding beam. The tuning curve has a broad bandwidth, with a flat region around 1.6 – 2.2 THz. The tuning range is limited by the TPG gain and absorption, as shown in Fig. 2. In the low or high frequency regions, below 1.4 THz or above 2.6 THz, the terahertz-wave output decreased because of a low gain or a high absorption coefficient in each region.

 

Fig. 5 The wide tunability of an is-TPG.

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The Fourier transform limit of the spectral width was calculated from the pulse duration of the terahertz wave as measured by a Schottky barrier diode. The typical pulsewidth of the terahertz wave was less than 120 ps. Figures 6(a) and 6(b) show that the wavelength and linewidth of the terahertz-wave was near the Fourier transform limit, as found by a scanning Fabry–Perot etalon consisting of two Ni metal mesh plates with a 65 μm grid. The displacement of one of the metal mesh plates corresponds directly to half of the wavelength. We measured the linewidth of the terahertz wave with a wavelength of 220 μm. The free spectral range (FSR) of the etalon was about 12 GHz, and the linewidth was measured to be less than 5 GHz. This figure also demonstrates the stability of the spectrum and output during the 10 min scan. The merit of an injection-seeded TPG lies in its output stability due to the mode-hop-free characteristic, since it has no cavity.

 

Fig. 6 Wavelength and linewidth of the terahertz-wave measured with a scanning Fabry–Perot etalon consisting of two metal mesh plates. (a) The stability of the spectrum is demonstrated and the displacement between the two periods (220 μm) corresponds directly to the wavelength. (b) The FSR of the etalon is 12 GHz and the linewidth of the terahertz wave is measured to be less than 5 GHz.

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5. Conclusion

We demonstrated a high-peak-power, single-longitudinal-mode and widely-tunable terahertz-wave source pumped by an amplified microchip Nd:YAG laser. This source generates high-peak-power terahertz wave (> 120 W @ 1.8 THz) with narrow linewidth (< 5 GHz). It is tunable from 1.2 to 2.8 THz with a flat tuning curve. We expect this compact source (footprint: A3 paper, 29.7 × 42 cm) to offer good advantages for many applications especially for the measurement of samples with strong absorption.