1 Introduction

Recently, intense Terahertz (THz) electrical fields have attracted much interest in the field of material science, especially in semiconductor physics. High-power THz radiation reveals various nonlinear phenomena in semiconductors, such as multi-photon absorption, light-induced ionization, and saturated absorption [1, 2]. The use of high electrical fields of THz waves is expected to reveal many as-yet-undisclosed phenomena.

To produce intense THz waves, generation using a free electron laser [3] and THz pulse generation by tilted-pulse-front femto-second pulses [4] have been developed. Novel results have been reported from their use. More compact systems without time-sharing and with longer duration of high electrical field are anticipated for the exploration of new fields.

As a practical matter, sensitive THz-wave detection is crucial for all applications. Generally, thermal detectors such as bolometers, pyroelectric detectors, and Golay-cell detectors are commercially available. Most of them cover a wider region of THz frequencies. Maturely developed devices provide stable performance. However, cryogenic cooling is necessary for high sensitivity in bolometer. In other cases, low sensitivity at room temperature restricts detailed measurements.

A technique requested for generation and detection has been nonlinear optical wavelength-conversion. For compact, tunable-wavelength THz-wave generation, a THz-wave parametric oscillator [5], a THz-wave parametric generator [6], and difference frequency generation using inorganic or organic nonlinear optical crystals [7, 8] have been demonstrated. Their emission power has been improved for about 15 years, as presented in Fig. 1. Moreover, THz-wave detection using up-conversion from THz waves to light has been developed [9–11].

Fig. 1
figure 1

Development of THz-wave parametric sources and difference frequency generation (DFGs). Red squares represent THz-wave parametric sources. Black circles show DFGs. Various nonlinear optical crystals such as LiNbO3 for the THz-wave parametric source and GaP, GaSe, ZGP, DAST, and BNA for DFG are included

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Particularly for wavelength conversion, LiNbO3 crystal is a promising material because of its high nonlinearity and high resistance to optical damage. Additionally, the material has a good figure of merit (FOM) on conversion efficiency considering the effect of THz-wave absorption [4]. Stoichiometric LiNbO3 of 18.2 pm2 cm2/V2 [4] is comparable to that of organic nonlinear crystal with higher nonlinearity [12].

To obtain an intense THz-wave emission, an efficient method must be chosen from among the various existing nonlinear processes in LiNbO3. Some processes are characterized as wavelength-conversion processes, such as χ(2) process of harmonic generation. Others are characterized as parametric mixing or optical rectification, or the χ(3) process of Raman [13]. Regarding other phenomena, multi-photon absorption or subsequent cascade processes after first wavelength-conversion might be discussed. Otherwise, the large absorption coefficient of LiNbO3 in the THz region [14] suggests that it might reduce the output power.

Stimulated Brillouin scattering (SBS) is a crucial consideration. An intense pump beam excites acoustic phonons in LiNbO3. Then SBS occurs within long temporal pulse-width of the pumping. Generally, steady-state Stokes amplification in the SBS process starts around the ten-fold period of the lifetime of the acoustic phonons [15]. The build-up time of LiNbO3 is known to be 1.5 ns [16]. Additionally, SBS gain of about 104 /cm in LiNbO3 is calculated when the pump energy of 30 mJ/pulse is irradiated with the pulse width of 10 ns at FWHM and the beam size of 1 mm2. The SBS gain is much higher than the parametric gain of 10 /cm [17] in LiNbO3. Therefore, the suppression of SBS is important for efficient parametric wavelength-conversion in LiNbO3.

Herein, we propose the use of sub-nanosecond pump pulses from a micro-chip Nd:YAG laser with a post amplifier to obtain efficient excitation of LiNbO3, thereby generating a monochromatic beam. Additionally, for intense THz-wave generation, an experimental condition was optimized to suppress the gain leak of the pumping that arose by cascade nonlinear optical processes. Consequently, tens-of-kilowatt peak intensity of the THz wave with wide tunability in injection-seeded THz-wave parametric generator (is-TPG) is reported herein. Moreover, the high conversion technique is applicable to an inverse process from THz wave to the near-infrared beam. Therefore, sensitive THz-wave detection is demonstrated, which is more than that of 4 K bolometer.

2 Kilowatt Peak-power, Tunable THz-wave Generation using LiNbO3

Actually, THz-wave parametric generators and oscillators using LiNbO3 crystal have been developed from the late 1990s [18] after pioneering studies conducted in the late 1960s to early 1970s [19, 20]. The LiNbO3 crystal, which covers generation from sub-THz to 3 THz or more, has been expected to open new fields of THz applications because high-power THz-wave sources were lacking during early THz research [21, 22]. Much effort has been spent on the development of high-power THz-wave generation, gradually improving the THz-wave emission power.

Recently, a revolutionary idea was reported for gain concentration to generate strong THz waves. Suppressing SBS is important for LiNbO3 crystal because of the comparable lifetime to that of intense pump excitation with nanosecond pulse duration. Additionally, suppressing the generation of a higher-order Stokes beam is crucial for the reason described below.

Generation of the coherent THz wave is based on stimulated scattering via phonon polaritons in the LiNbO3 crystal, which has a transverse optical phonon mode (A1-symmetry soft mode) [5] that is Raman-active and far-infrared-active. In the Raman scattering process via this mode, only the wave polarized along the c-axis of the crystal is involved [5]. This mode, which is strongly coupled with far-infrared light, forms an elementary excitation wave called a polariton. Then, the parametric process is predominant, generating a signal beam and an idler beam. Here, the Stokes wave is defined as the idler. The THz wave is regarded as the signal.

Applying an intense pump beam to the crystal, a first Stokes is generated through the process described above. When a high-power THz wave is generated, the Stokes beam is also of high power. Therefore, the Stokes beam becomes a new pumping beam, as shown in the inset of Fig. 2. As a cascade, higher-order Stokes beams can be generated by their own non-collinear phase-matching condition until the pump beam intensity cannot provide sufficient gain to the cascade process. The generated THz waves in the parametric process have the same frequency. However, the phase-matching angles mutually differ. Furthermore, THz waves in high-order cascade process are generated apart from the coupling-out surface. They are considerably attenuated because of large absorption property of LiNbO3 crystal in the THz frequency region.

Fig. 2
figure 2

Experimental setup of an injection-seeded terahertz-wave parametric generation pumped by a sub-nanosecond near-infrared pulse beam from a microchip Nd:YAG laser with a post amplifier. The inset figure shows the non-collinear interaction of the cascade parametric process

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Here, a high-order Stokes beam has a small angle to the respective pump beam because of non-collinear phase-matching. The Stokes beams move in the crystal for higher order, where the generated THz wave is diminished remarkably by large absorption of LiNbO3. Consequently, high gain of the original pump beam is degraded for intense THz-wave generation.

Considering the effects of SBS and cascade process, a sub-nanosecond pulse width of a pump laser was used. An optimum optical arrangement including beam size, beam collimation, and crystal length was used for the experiment. Figure 2 portrays the experimental setup, which is based on an injection-seeded THz-wave parametric generator [23]. The cavity-less design is effective for a small roundtrip with use of a short pulse. The pumping source was a diode-end-pumped, single-mode, stable linearly polarized microchip Nd3+:YAG laser that was passively Q-switched by a [110]-cut Cr4+:YAG saturable absorber. The microchip configuration enables low-order axial and transverse mode laser oscillation, of which the linewidth is less than 0.009 nm. The laser delivers more than 1 MW peak power giant-pulses (> 500 μJ/pulse) with 420 ps pulse width at 100 Hz repetition rate with a M2 factor of 1.09. The laser is free from electrical noise, in contrast to active Q-switched lasers. Additionally, such fixed, passive Q-switching provides stable peak power, with less than +/− 2 % power jitter [24].

The pumping beam from the microchip laser was amplified in two amplifiers in a double-pass configuration and one more post-amplifier. Each amplifier used 0.7-at.% Nd3+ doped YAG with 3 mm diameter and 70 mm length. The gain medium was excited from the direction perpendicular to an optical axis by 200 W laser diodes with wavelength tuned at 808 nm. In the double-pass configuration of a first stage amplifier, a circular polarization of the laser beam generated using a quarter-wave plate provides efficient enhancement of the power because of its homogeneous gain-consumption. The amplified laser beam was extracted by reflection at a polarization beam splitter. Moreover, the post-amplifier enhanced the beam power up to 48 MW/pulse (20 mJ/pulse energy). The spatial pattern of the amplified beam was a Gaussian-like pattern.

For the injection seeding, the CW emission of an external-cavity diode laser (ECDL, Velocity 6300; New Focus Inc.) was used. The seeding beam was amplified by an optical Yb-doped fiber amplifier (YAD-1-PM; IPG Photonics Corp.). The maximum power was 800 mW. The beam was overlapped with the pump beam in the LiNbO3 crystal. At the same time, the seed beam was passed through optics satisfied with achromatic injection-seeding [25].

The pump laser beam was coupled into a congruent 5-mol% MgO-doped LiNbO3 (MgO:LiNbO3) crystal with 50 mm length and 4 × 5 mm2 cross section for the beam input. A non-doped Si-prism coupler placed on the y-surface of the nonlinear crystal acted as an efficient output coupler for the THz waves, which avoided the total internal reflection of the THz waves at the boundary between the crystal and the air.

The diameter of the pumping beam was controlled to about 1 mm (full width at half maximum, FWHM) at the crystal. To reduce the THz absorption by the MgO:LiNbO3 crystal, the pump beam was propagated close to the y-surface. The distance between the y-surface and the beam center was adjusted precisely to obtain a maximum THz-wave output. It was approximately equal to the pumping beam radius. The generated THz-wave output was measured using a pre-calibrated pyroelectric detector (SPI-A-65 THz; Spectrum Detector Inc.) with responsivity of 76 kW/V at 1.8 THz or using a general pyroelectric detector (Jasco Corp.) without power calibration.

Figure 3 presents the peak output power of terahertz-wave at 1.8 THz as a function of the input pumping energy. When increasing the pumping energy, the THz-wave radiation is initially detected with pumping energy of approximately 6 mJ/pulse. The peak power of the pump beam was 12 MW. The power concentration was 1.5 GW/cm2. The THz-wave output power increased monotonically. The peak power reached about 120 W at 1.8 THz for 14 mJ/pulse pumping energy (28 MW peak power; 3.5 GW/cm2 power density). The seeding power was 80 mW (CW).

Fig. 3
figure 3

Peak power of terahertz-wave output at 1.8 THz as a function of the pump beam input energy

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The THz-wave emission was enhanced much more than in the earlier experiment. Moreover, results show that additional seeding power provides higher emissions under this high pump excitation. In the following experiment, the seeding power was increased to 500 mW (CW). Then the THz-wave emission was detected using the calibrated pyroelectric detector. Figure 4 presents an output signal for 1.8 THz emission from the detector. The dots represent the output signal. The solid line shows the average during 10 ms. The pulsed THz waves were generated with the repetition rate of 100 Hz (10 ms). However, the detector has slower temporal response below 5 Hz. The chopper was inserted between the is-TPG and the detector with chopping frequency of 3 Hz. We estimated the energy and peak power from the calibrated average power. The measured peak power of the THz-wave was about 1 kW at 1.8 THz in this case.

Fig. 4
figure 4

Experimental results for the detection of an intense THz-wave emission with a pyro-electric detector at 1.8 THz

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Experiment results show that the idler wave intensity was sufficiently high for generating other Stokes beam as the cascade process. The pumping beam power concentration was controlled by changing the beam diameter. The crystal length was controlled precisely to obtain a maximum THz-wave output. Then, the better condition in the past experiment was accomplished and intense THz-wave output with peak power of about 50 kW at 1.9 THz was obtained. The maximum output around 1.9 THz resulted from the gain peak, as reported from an earlier study [26] .

Figure 5 depicts THz-wave output as a function of frequency. Wide tunability of 0.7–2.9 THz was obtained by changing the seeding beam wavelength, which was actually possible only to tune the ECDL wavelength. Consequently, the frequency range with peak power of more than 10 kW was 1.3–2.7 THz. The range with more than 1 kW was 0.8–2.9 THz. Peak power of more than 100 W was obtained in the whole measured range.

Fig. 5
figure 5

Peak power of terahertz-wave output as a function of frequency

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For decreasing power in the lower frequency-range below 1 THz, the parametric gain is reduced because it leaves the actual gain center on 2 THz. However, greater absorption at frequencies higher than 3 THz limits intense THz-wave emissions.

The THz-wave emissions from is-TPG are stable with fluctuation of root-mean-square of less than 5 %. Stable emissions are useful in many potential THz-wave applications that require long-term measurements. Figure 6 depicts a demonstration of THz-wave imaging. The sample was a test pattern based on USAF 1951, which was fabricated by A. Dobroiu and C. Otani at RIKEN. The pattern was printed with metallic ink on paper. Furthermore, the imaging measurement was done by scanning the sample on which a THz wave was focused with a Tsurupica lens. The scanning area was 40 × 40 mm2 with a step of 100 μm. Every pixel was depicted with two THz-wave pulse averaging. The sample was scanned step-by-step using a motorized x–z stage. The THz-wave image measurement was acquired in about five and half hours. Consequently, a clear image was obtained because of the stable THz-wave emission from the is-TPG. The THz frequency was 2.2 THz. The spatial resolution was less than 500 μm.

Fig. 6
figure 6

THz-wave image of a test pattern modeled on USAF 1951

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3 Highly Sensitive THz-wave Detection using Nonlinear Up-conversion in LiNbO3

Sensitive THz-wave detection performed at room temperature is necessary for actual THz-wave applications. A THz-wave detection by nonlinear wavelength-conversion is a promising detection method because of its high sensitivity and its rapid temporal response at room temperature. Some researchers have obtained effective THz-wave detection using inorganic or organic nonlinear optical crystals such as LiNbO3 [10], GaAs [27], GaP [28], and DAST [11]. The advantage of THz-wave detection via nonlinear optical up-conversion is leveraged using advanced photonic devices such as a p-i-n photodetector, an avalanche photodetector, and a photomultiplier.

The LiNbO3 crystal is a potential material for converting wavelengths between THz wave and near-infrared (near-IR) light, as occurs with intense THz-wave generation. We inferred that the crystal can be used not only as nonlinear up-conversion but also for parametric amplification to detect THz waves. Mixing THz wave with an intense near-IR pump beam provides generation of a signal light at a different frequency because of the stimulated polariton scattering. The up-converted signal is amplified parametrically by a LiNbO3 optical parametric gain. The signal light intensity is proportional to that of the incident THz-wave radiation. It also preserves the phase information of the input THz wave.

Figure 7 presents the experimental setup of THz-wave detection used for the nonlinear up-conversion effect of MgO:LiNbO3. The THz-wave source was the developed is-TPG. As a pump source, the micro-chip Nd:YAG laser passing through the is-TPG was reused for THz-wave detection. The THz wave from the is-TPG was collimated with a cylindrical lens and was focused to another MgO:LiNbO3 crystal for detection through a non-doped Si-prism input-coupler. The MgO:LiNbO3 length was 65 mm. Timing of the pump beam and THz wave coupled into the crystal was matched by adjusting both path lengths from the is-TPG to make them equal.

Fig. 7
figure 7

Experimental setup of terahertz-wave generation and detection

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The incident angle between the THz wave and the pumping beam are expected to satisfy the non-collinear phase-matching condition, which is the same condition at THz-wave generation. Actually, the phase-matching angle was arranged precisely to yield the maximum output of the signal. The pump beam was irradiated as close as to the edge of input cross section where the Si-prism coupler was set for THz-wave input. Consequently, by making a better condition for interaction, the detection signal was obtained efficiently.

Images of the generated signal lights are presented in Fig. 8. The lights were visualized using a near-IR laser beam visualizer. In the figure, the images aligned up and down indicate THz-wave detections of which the frequencies were tuned respectively to 1.0 THz, 1.3 THz, 1.6 THz, 1.9 THz, and 2.2 THz. The signals were generated with a non-collinear phase-matching condition. Therefore, the spectroscopic detection was obtained by calibrated horizontal position as a frequency. The THz-wave frequency is known at the THz-wave source, but the THz-wave frequency is also derived from monitoring both wavelengths of the detected signal light and the pump beam with an optical spectrum analyzer. Therefore, once the relation between the signal wavelength and spatial position is calibrated, a linearly arrayed photodetector can be useful for THz-wave spectroscopic detection.

Fig. 8
figure 8

Images of up-converted signals at 1.0 THz, 1.3 THz, 1.6 TH, 1.9 THz, and 2.2 THz

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The pump beam brightness shown in Fig. 8 depicts pump depletion, a changing beam pattern, and changing spatial distribution of pump power after nonlinear optical interactions at both generation and detection. However, the signal light brightness depends on the up-conversion condition, the THz-wave input power, and absorption loss during THz-wave propagation in air.

The signal light temporal profile at 1.8 THz was measured using an InGaAs photodetector with 12.5 GHz cut-off frequency (818-BB-35; Newport Corp.) as shown in Fig. 9. The solid line is the measured temporal waveform of the signal. The red dashed line is a Gaussian fitting curve. The gray dashed line shows a signal with no input THz wave. A Gaussian-like temporal profile was found, which was averaged for 10 pulses at the repetition rate of 100 Hz. The pulse width was about 250 ps FWHM, as measured using an oscilloscope (DPO7354; Tektronix Inc.) with 3.5 GHz bandwidth. The smooth profile shows a single longitudinal mode. Efficient nonlinear up-conversion was obtained for monochromatic THz-wave detection. When the input THz-wave was blocked, the up-converted signal disappeared. The photodetector received stray light and parametric fluorescence at a quite low level of detection. Actually, the noise level is almost equal to the electrical noise level of the photodetector in this case.

Fig. 9
figure 9

Temporal waveform of up-converted signal light measured using an InGaAs photodetector with 12.5 GHz cut-off frequency. The solid line is the measured temporal waveform of the signal. The red dashed line is a Gaussian fitting curve. The gray dashed line shows a signal with no input THz wave

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Next, the sensitivity and dynamic range of nonlinear optical up-conversion detection using MgO:LiNbO3 with the sub-nanosecond pumping is described. Figure 10 depicts the relation between the input THz-wave energy and up-converted signal energy. Pump energy of 10 mJ/pulse was constantly coupled into the nonlinear crystal for any input energy of the THz wave. The input THz-wave energy was calibrated at maximum with a pre-calibrated THz-wave detector, which was about 1 μJ/pulse at 1.8 THz.

Fig. 10
figure 10

Measured up-converted signal energy as a function of the input THz-wave energy

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Coupling energy of THz wave into MgO:LiNbO3 was controlled by two sets of THz-wave attenuators. One THz-wave attenuator set consists of four thin-film attenuators with different THz-wave transmissions: 30 %, 10 %, 3 %, and 1 % (TFA, Tokyo Instruments, Inc.). The sets are useful as single attenuators or in combination as tuned flexibly. The total transmission through the attenuators was reduced discretely from 100 % to 8 × 10-9 %.

The detection of the up-converted signals was measured using a broadband power detector (13PEM001; Melles Griot) and an InGaAs photodetector with 2 GHz cut-off frequency (818-BB-30; Newport Corp.). In detecting intense THz waves, the conversion efficiency was high. A strong up-converted signal light was emitted. The intense radiation was monitored easily with the near-IR visualizer portrayed in Fig. 7. Then, the power detector was used during the generation of intense signal lights. In contrast, the InGaAs photodetector was used in the lower power range below 100 μJ/pulse to achieve highly sensitive detection. In this experiment, the detector was exchanged at input THz-wave energy of 9 pJ/pulse, with detection signal energy of 100 μJ/pulse, where the InGaAs photodetector was calibrated using the power detector.

Minimum THz-wave detection was obtained down to energy of about 80 aJ/pulse. The remarkably sensitive detection exceeds that of a cryogenically cooled (4.2 K) Si bolometer by more than one order in minimum detection. The Si bolometer sensitivity is about 1 fJ/pulse. Additionally, the dynamic range of the detection covers the notably wide range of 80 aJ/pulse – 1 μJ/pulse, which is about 100 dB. This result is attributable to the high parametric gain of MgO:LiNbO3, which is especially effective in small THz-wave input. Actually, about 108 times the photon number of near-IR signal light was extracted compared with that of the input THz wave at minimum detection. The photon number of the input THz wave was estimated as about 67,000. Coherently parametric amplification in the MgO:LiNbO3 crystal excited by the pump beam increased the photon number of the near-IR signal.

Based on this result, a noise equivalent power (NEP) of the up-conversion detection was calculated as about 18 pW/Hz1/2 from the detected minimum peak power of the THz wave and the bandwidth of the InGaAs photodetector. Optical detectors in the near-IR/visible region offer excellent performance with very high efficiency, low NEP, and large bandwidths. Consequently, if it were possible to convert THz radiation to visible/near-IR, even at a low efficiency, then sensitive THz-wave detection would be achievable.

Moreover, up-conversion detection is based on coherent nonlinear optical interaction. Therefore, the THz-wave phase information is detectable. Phase detection is not described herein, but THz-wave detection based on nonlinear optical up-conversion potentially provides both amplitude and phase detection.

4 Conclusions

Wavelength conversion between near-IR light and THz wave using MgO:LiNbO3 nonlinear optical crystal is a promising technique for intense THz-wave generation and sensitive THz-wave detection. As described herein, we proposed precise control of nonlinear optical processes in MgO:LiNbO3 such as stimulated Brillouin scattering and cascade nonlinear optical process. Consequently, about 50 kW peak-power THz-wave generation with wide tunability was achieved using an injection-seeded THz-wave parametric generator pumped by optically amplified near-IR pulses with sub-nanosecond duration. Intense power indicating high electrical fields of about 500 kV/cm is expected at a spot of the focused beam. Additionally, for sensitive THz-wave detection, minimum detection of about 80 aJ/pulse at 1.8 THz was obtained by nonlinear up-conversion from THz wave to near-IR light. Simultaneously, the remarkable dynamic range of 100 dB was demonstrated.

The THz-wave generation and detection function at room temperature. The system’s compact size provides potential benefits for real and novel THz-wave applications.