Ultrashort electromagnetic radiation in the frequency region between 0.1 and 10 THz is of great interest for a variety of applications, such as optical spectroscopy, biomedical imaging, communications, and security systems [1-3]. In particular, studies on terahertz (THz) nonlinear phenomena in various materials, often require intense ultrashort THz pulses with microjoule-level energy and electric field strength of >100 kV/cm, are currently regarded as an unexplored new research area [4-7]. Since the availability of compact high-power ultrashort THz sources is quite limited yet, development of such sources is an essential step for advanced nonlinear spectroscopy and applications in the THz frequency range. There are different methods to generate ultrashort THz pulses depending on the desired frequency range and the purpose of applications. Thanks to both technical advances in femtosecond laser systems and photoconductive (PC) antennas based on semiconductor materials, several generation techniques are wellestablished for single-cycle THz pulse generation in the frequency range around 1 THz. The PC antenna-based THz emitters are widely used for THz time-domain spectroscopy (THz-TDS) to find molecular fingerprints of matters and to extract their optical constants [8]. Although such THz-TDS systems exhibit an excellent signal-to-noise ratio (SNR, ratio of the peak-to-peak signal amplitude and the root-meansquare noise amplitude) of >100000:1 in the temporal THz waveform [9], achievable THz pulse energies are limited and cannot exceed 1 μJ because of the limited illumination of excitation pulses on the PC antenna [10]. In last decade, optical rectification in second-order nonlinear optical crystals, such as ZnTe, GaSe, and DAST, has been emerging as an efficient alternative method for generating more intense single-cycle THz pulses. However, the magnitude of the THz pulse energy is often limited by several factors, such as absorption, phonon resonances, and photorefractive damages in nonlinear crystals [11, 12]. In addition, the group velocity of the ultrashort optical excitation pulses has to be well-matched to the phase velocity of the newly generated THz pulses in the nonlinear medium for efficient frequency conversion [13].

Lithium niobate (LiNbO₃) is a well-known nonlinear optical crystal and a potential candidate for efficient THz wave generation due to its large second-order nonlinear coefficient (d_eff = 168 pm/V). The relatively large band-gap (E_g = 3.8 eV) of LiNbO₃ allows us to use near-infrared excitation sources without two-photon absorption [14]. However, the large index dispersion and mismatch in LiNbO₃ inhibit efficient phase matching in a collinear geometry. Since Hebling et al. suggested a non-collinear phase matching in a prism-cut LiNbO₃ using a tilted pulse-front pumping (TPFP) scheme and reported its proof-of-principle experiment for efficient THz pulse generation in 2003, many efforts have followed to improve both THz pulse energy and spectral bandwidth by utilizing different configurations and excitation sources [15-17]. As a result, it turned out that there was a tradeoff between the pump-to-THz conversion efficiency and the spectral bandwidth for THz pulse generation. It is not a trivial task to achieve simultaneously high-energy and broadband THz pulses via optical rectification, because the frequency conversion for high-frequency ranges of >3 THz is strongly limited by the low intensity distribution at both spectrum ends of the excitation pulses and the phase matching condition [18]. Up until now, most of activities dealing with this technique were focused on generation of high pump-to-THz conversion efficiency with moderate spectral bandwidths. Fülöp et al. reported sub-millijoule THz pulse generation based on a Yb:YAG amplifier system delivering 1.3-ps excitation pulses at 1030-nm wavelength and 10-Hz repetition rate. A high pump-to-THz energy conversion efficiency of 2.5 × 10^-3 was achieved, but the spectral peak of the generated THz pulses was shifted to a quite low spectral range of 0.25 THz [19]. In addition, a low-repetition-rate THz system is not well-suited for applications in precise THz-TDS and nonlinear spectroscopy because of low SNR. Very recently, Huang et al. reported the highest pump-to-THz energy conversion efficiency of 3.8 × 10^-2 by employing 1-kHz, 680-fs excitation pulses at 1030 nm. In this experiment, the prism-cut LiNbO₃ was cryogenically cooled down to 150 K in a dewar to reduce thermal loads and absorption behavior in LiNbO₃ [20]. In the present work, high-power single-cycle THz pulses with a diffraction-limited Gaussian profile were generated at room temperature by utilizing the TPFP technique at 800-nm wavelength. Although the previous study (Hirori et al. in [16]) demonstrated similar high-power THz pulse generation via TPFP, the 4f-lens arrangement led to an asymmetric propagation of the generated THz beam in vertical and horizontal directions. We adopt an optimized single-lens focusing geometry to generate radially-symmetric THz radiation from the prism-cut LiNbO₃ crystal. The system delivered a maximum average power of 3.3 mW, which corresponds to 3.3-μJ single pulse energy, and the highest SNR (~1:15000) ever achieved at 1-kHz repetition rate. A pump-to-THz energy conversion efficiency of 1.36 × 10^-3 was obtained with 2.4-W pumping.

For THz pulse generation, we used a 1-kHz Ti:sapphire regenerative amplifier (Spitfire Ace, Spectra Physics), delivering 4-W average output power with pulse duration of 100 fs at 800 nm, as the excitation source. Figure 1 shows the schematic configuration of a high-power THz pulse generation and detection system. This system can be further used for THz-TDS. The optical beam was divided by a 92:8 beam splitter for THz generation and detection.

The tilting angle (γ) of the pump pulse-front was controlled by an Au-coated diffraction grating (1800 grooves/mm) and a lens of 150-mm focal length using the following relation: where m is the diffraction order, λ_p is the pump wavelength, d is the grating groove number, is the group index of the pump pulse, β₁ and β₂ are the magnification factors of the lens for the pump wave front and the grating image, respectively, and θ_d is the diffraction angle [16].To optimize the efficiency of THz pulse generation, we chose two magnification factors (β1=β2 ≈ 0.591) and the diffraction angle (θ_d=56.1°) by using the Eqs. (1) and (2), and the grating equation. After passing through a half-wave plate, the vertically-polarized excitation beam was focused on the 1.3-mol% MgO-doped prism-cut stoichiometric LiNbO₃ crystal with a dimension of 9×9×9 mm³ by a single spherical lens with 150-mm focal length. Such single-lens focusing geometry makes wave front images of the tilted pulse and the diffraction grating flat inside LiNbO₃ without distortion [14]. The incident facet of LiNbO₃ crystal was cut under an angle of 63° that can be calculated from the relation

The horizontal (d^I_H) and the vertical diameters (d^I_V ) of the pump beam at the entrance-facet of crystal were 4 and 9 mm, respectively. The THz pulses generated were subsequently guided with four off-axis parabolic mirrors of different focal lengths to the detection stage. The average power of the THz pulses was measured by using a calibrated pyroelectric detector (THZ5B-MT-DZ, Gentec-EO) and the time trace of the THz electric field was recorded by an electro-optic sampling method in a 0.3-mm-thick GaP crystal. Six Si wafers were used to suppress the nonlinear response in the electro-optic sampling. The THz beam profile at focus was measured by using a pyroelectric camera (Pyro CAM-III, OPHIR). All of the measurements were carried out under dry air purging at room temperature of 20℃.

Figure 2 shows the generated THz average power versus the incident pump power, measured at the sample position of Fig. 1. The THz average power increased quadratically with increasing pump power. Above pump powers of 800 mW, the experimental data began to deviate slightly from the square-law curve (solid line). This might be mainly caused by the occurrence of free carrier absorption of THz pulses in LiNbO₃ [19]. In the pulse-front-tilted excitation scheme with combination of grating-lens-LiNbO₃, we were able to generate intense THz pulses with average powers up to 3.3 mW with 2.4-W pumping. Once the pump power exceeded 2.45 W, the photorefractive effect was observed in the crystal and this led to a slight decrease of the conversion efficiency. At a pump power of 2.4 W, the pump-to-THz energy conversion efficiency of 1.36 × 10^-3 was obtained and the corresponding photon conversion efficiency was evaluated to be 74.8% by taking into account the peak frequency of 0.62 THz.

Figure 3 shows the time trace of the generated THz pulses measured at the maximum average power by electrooptic sampling and the corresponding spectrum was obtained by fast Fourier transform (FFT). The THz electric field strength is given by where I_a and I_b denote the signal amplitudes measured by two separate detectors. n_opt and r₄₁ are are the optical refractive index at 800 nm and the electro-optic coefficient of GaP, respectively. Furthermore, t_si and t_GaP are the Fresnel transmission coefficients of Si wafer and GaP, and l is the thickness of GaP [21]. As can be seen in the temporal profile of the THz pulse in Fig. 3(a), the peak-to-peak electric field strength amounted to 350 kV/cm, which was calculated with n_opt = 3.2, r₄₁ = 0.88 pm/V, t_si = 0.523, t_GaP = 0.46, and l = 0.3 mm taking into account the reflectivity of parabolic mirrors. The inset of Fig. 3(a) shows a 2000-fold-magnified feature of the leading edge in the temporal THz waveform. We obtained a SNR as high as ~1:15000 with a time constant of 300 ms when six Si attenuators were used. It is noted that this value is, to our knowledge, the highest one ever achieved with a THz system operating at 1-kHz repetition rate. Figure 3(b) shows the corresponding FFT spectrum. The center frequency and the spectral bandwidth were measured to be 0.62 THz and 1.1 THz, respectively. The water absorption, which usually occurs in this THz spectral region, could be almost suppressed by dry air purging.

An optimal focal length of the first parabolic mirror M1 was chosen through the measurement of the THz beam divergence. As shown in Fig. 4(a), the THz beam radius from the emitter to far-field was evaluated by the knifeedge method. We found that the radially-symmetric radiation of THz beams could be easily achieved by the optimized single-lens focusing scheme. As shown in inset of Fig. 4(a) and Eq. (5), the horizontal diameter ( mm) of the pump beam projected at the exit-facet nearly equals the vertical diameter () of the pump beam at the same facet.

The half divergence angles of the THz beam at the horizontal and vertical direction were measured to be 82.54 and 81.32 mrad, respectively

Figure 4(b) shows the spatial intensity distribution of the THz beam at the focal point between the parabolic mirrors M2 and M3. The radial symmetric cross-section of the THz beam spot with a diameter of about 2 mm is clearly captured by the pyroelectric camera. The M²-factor of 1.2 measured at this position implied that the generated THz beam exhibits a nearly diffraction-limited Gaussian profile.

Figure 5 shows the measured long-term stability of our THz pulse generation system. The time-trace of the THz electric field strength was recorded during 20 hours. Neither phase shift nor temporal distortion of the electric field was observed in the THz waveform. The peak value of the electro-optic sampling signal was also measured for the same period with N = 6000 sampling. The mean value of the electric field at the maximum with a standard deviation was estimated to be 201.1±0.6 kV/cm, which corresponds to an error of only < 0.3%. Finally, we could achieve an exceptionally high SNR of about 1:15000.

We demonstrated efficient THz pulse generation in a prism-cut LiNbO₃ via optical rectification based on TPFP at 800 nm. It was shown that our THz generation system can generate 3.3-mW THz pulses with electric field strength as high as 350 kV/cm and the pump-to-THz energy conversion efficiency of 1.36 × 10^-3 at room temperature. We expect that such a high-power THz pulse generation system delivering nearly diffraction-limited 1-kHz THz pulses with extremely high signal-to-noise ratio (~1:15000) is very well suited for sensitive and precise THz applications in nonlinear and time-domain spectroscopy, imaging, and metrology.