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Abstract
Matter manipulation in terahertz range calls for a strong-field broadband light source. Here, we present a scheme for intense terahertz generation from DSTMS crystal driven by a high power optical parametric chirped pulse amplifier. The generated terahertz energy is up to 175 µJ with a peak electric field of 17 MV/cm. The relationship between terahertz energy, conversion efficiency, and pump fluence is demonstrated. This study provides a powerful driving light source for strong-field terahertz pump-probe experimentation.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent decades, the development of electronic technology, semiconductor technology and laser technology, especially the progress of ultrafast laser technology, has promoted the intensive research and rapid development of strong field terahertz (THz) generation and application. High-field THz light source has aroused great interest among researchers in various fields, and has been applied to the cutting-edge research of superconducting excitation, material phase transition and molecular manipulation [1]. For intense THz generation, laser driven optical rectification (OR) in nonlinear crystals is one of the most widely used methods which has the advantages of high peak field, excellent stability and relatively low cost [2–5]. Benefitting from the large nonlinear coefficient, high damage threshold and perfect optical properties, lithium niobate (LN) crystal has been used for THz generation via OR [2,6–10]. By using laser pulse front tilting and cryogenic cooling technologies, Baolong Zhang et al. achieved 1.4 mJ THz pulses in LN under the excitation of 214 mJ femtosecond laser pulses. The energy conversion efficiency reached 0.7%, and the THz peak electric field reached 6.3 MV/cm [11]. However, the phase matching condition in LN is harsh and the generated THz spectrum is mainly below 1 THz. Other nonlinear inorganic crystals such as ZnTe [12] and GaP [13] have also been used to produce THz radiation through OR with much lower conversion efficiency. Recently, organic crystals such as OH1 [14,15], DAST [16,17] and DSTMS [14] have been used for THz generation because of their high second-order nonlinear coefficients [18,19] and the fact that phase matching condition can be easily satisfied at certain pump wavelength. DAST has high nonlinear optical susceptibility d111 = 210 ± 55 pm/V [20], which is dozens of times that of LN. And THz generation of up to 15 THz has been confirmed using DAST [21,22]. However, many absorption features appear in the generated THz spectrum. The strongest of these, at 1.1 THz, is attributed to transverse optical (TO) phonon resonance [23]. With similar chemical composition as DAST [24], DSTMS shows a comparable second-order susceptibility but much lower absorption at THz range. Moreover, DSTMS can be grown to large-area bulk single crystal, which has the ability to afford high pump energy [25]. The suitable pump wavelength for DSTMS crystal is 1.3∼1.5 µm [26]. The reason is that the group velocity of pump light doesn’t change much in this waveband and can achieve velocity matching with the THz, in addition, the absorption of crystal for pump light in this waveband is low. C. Vicario et al reported on high-field THz transients with 0.9 mJ pulse energy produced in a 400 mm2 partitioned organic crystal by OR of a 30 mJ Cr:Mg2SiO4 laser pulse centered at 1.25 µm wavelength [27]. Pumping by a three-stage optical parametric amplifier (OPA), M. Shalaby et al reported on a regime of extremely bright PW/m2 level THz radiation with peak fields up to 8.3 GV/m and 27.7 T. The experimental scheme relies on finding the optimum settings of pump wavefront curvature and post generation beam divergence. The high peak field was achieved by fine tuning the pump wavefront and optimizing the broadband THz beam transmission for tight focusing [14]. Since the output capability of OPA is greatly restricted by the pump energy, it is not suitable for generating energies of tens of millijoules or higher. While optical parametric chirped pulse amplifier (OPCPA) has the capability to reach hundreds, or even thousands, of millijoules output energy which offers a powerful pump laser for intense THz generation.
In this letter, we present the intense THz generation from DSTMS crystal pumped by a high power OPCPA. The electric field and spectrum of the THz radiation are characterized by electro-optical (EO) sampling measurement. The relationship between conversion efficiency and pump fluence is also demonstrated. With 10 mJ pump energy, the generated THz pulse energy reaches 175 µJ, and the electric field is about 17 MV/cm at the focus. This study promotes the intense THz generation based on OPCPA facilities and provides the possibility for multiband high field pump-probe experiments.
2. Experimental setup
The experimental setup is shown in Fig. 1. High power infrared OPCPA pulses with 13 mJ pulse energy, 137 fs pulse duration, 1.45 µm center wavelength and 20 Hz repetition rate serve as pump of the system [28]. The OPCPA is seeded by a BBO based optical parametric amplifier (OPA) driven by a commercial Ti:sapphire laser (Astrella, Coherent Inc.). After being chirped by a Öffner-type stretcher, the signal pulse of the OPA is injected into a two-stage KTA based OPA which is pumped by a high power Nd:YAG laser. Through a two-grating compressor, the amplified signal pulse is compressed to 137 fs with 13 mJ pulse energy. The infrared laser pulses are divided by a beam splitter (R:T = 9:1). The reflection and the transmission pulses are used to pump the DSTMS crystal and probe the THz wave respectively. The DSTMS crystal is a single crystal with 12 × 8 mm2 usable area and 420 µm thickness. In the organic crystal DSTMS, OR takes place and the generated THz pulses propagate in the same direction as the residual infrared beam which is then filtered out by a 2 mm-thick high-density polyethylene (HDPE) plate. The infrared pump beam is collimated, with a diameter of 7 mm. After beam expansion by two off axis parabolic mirrors (OAP), the generated THz pulses are focused to a 50 µm thick GaP for electric field characterization via EO sampling. The probe beam passes through a time delay line, and combines with the THz pulses through a hole in the center of OAP3.
Fig. 1. Schematic of the experimental setup. OPCPA, optical parametric chirped pulse amplifier; BS, beamsplitter (R:T = 9:1); HDPE, high density polyethylene; OAP 1, 2, 3, off axis parabolic mirrors; VND, variable neutral density filter; L, lens; QWP, quarter wave plate; WP, Wollaston polarizer; BPD, balanced photo diode.
3. Results and discussion
Figure 2 shows the spectral changes of pump light before and after DSTMS crystal. It can be seen that after passing through the DSTMS crystal, the spectrum of pump light shifts to the long wavelength and broadens to over 2 µm. The reason for this phenomenon can be explained as follows. The nonlinear effect of OR occurs when the pump pulse is injected into the DSTMS crystal, and the nonlinear difference frequency occurs between the high frequency and low frequency components of the spectrum. The frequency down conversion of high frequency photons generates low frequency photons and THz photons, and the resulting low frequency photons can continue to generate lower frequency photons and THz photons by frequency down conversion. This cascaded difference frequency process makes the spectral peak shift to the long wave direction, and the intensity of the short wave band decreases relatively. Because the nonlinear optical susceptibility χ(2) and the electro-optic coefficient r111 of DSTMS are large, meanwhile, the pump light polarization fulfills the optimal THz generation conditions. Therefore, the additional nonlinear refractive index of DSTMS is mainly originating from the quasi-χ(3) effect due to combination of the cascaded 2nd-order OR process and the linear EO effect [29]. The contribution from the intrinsic χ(3) nonlinearity of DSTMS should be negligible. The additional nonlinear refractive index of DSTMS is proportional to the pump intensity. Therefore, the additional refractive index caused by different parts of the pump pulse is different, and the refractive index of DSTMS is a function of time. This makes different parts of the pump pulse through DSTMS bring different phase changes, instantaneous frequency is different. Because the intensity of the pump light increases and then decreases with time, the instantaneous frequency of different parts can be higher or lower than the center frequency of the pump pulse. That is, the spectrum expands in both long and short wave directions, which is an approximate symmetric broadening of the spectrum. After spectrum broadening, each frequency component has a cascaded frequency conversion based on phase mismatch [30]. Each frequency component can be regarded as fundamental wave, and nonlinear second harmonic generation will occur in DSTMS to generate corresponding second harmonic waves. However, because the phase matching condition required by SHG is not satisfied, which means the phase mismatch is serious, so the fundamental wave converts to the second harmonic wave within the “characteristic distance of the interaction of fundamental and harmonic wave” and the conversion is weak. After this characteristic distance, the harmonic wave converts back to the fundamental wave. This reciprocal frequency conversion progress is repeated as the pump pulse propagates through the DSTMS crystal. The absorption coefficients of the harmonic waves corresponding to each frequency component are obviously different in DSTMS. Since the absorption coefficient of DSTMS crystal is much higher for wavelength below 0.7 µm, the harmonic waves of the higher frequency (shorter wavelength) components are strongly absorbed in the crystal and only the harmonic waves of the lower frequency (longer wavelength) components can convert back to the fundamental waves. Therefore, the intensity of the residual pump light spectrum output by DSTMS is relatively weak in the short wave band. With a pump pulse duration of 137 fs, as shown in the inset of Fig. 2, the output infrared pulses are self-compressed, because of the spectrum broadening and cascaded SHG progress [30]. The THz generation is affected by tuning the OPCPA compressor, and the highest conversion efficiency is achieved when the chirp is 0, that is, when the shortest pump pulse is applied.
When the THz electric field propagates in GaP, it will arouse the linear EO effect of GaP which causes a phase change Δф of the co-propagating probe pulse. Δф is related to the time delay between THz and probe pulses, through which the THz time domain electric field can be determined. In the experiment, the THz waveform is characterized by EO sampling with a 50 µm thick GaP crystal (the blue solid line in Fig. 3(a)). The main lobe of THz pulse contains about 1.5 THz cycles. Further, the time domain envelope and intensity curves can be obtained, as shown in the pink curves and the brown curve of Fig. 3(a), respectively.
Fig. 3. Time and frequency domain descriptions of the THz pulse. (a) THz waveform measured by EO sampling (blue solid line). The THz pulse envelopes (pink dotted lines). The THz pulse intensity (brown solid line). (b) THz spectrum.
Figure 3(b) shows the measured THz spectrum obtained by Fourier transform of the electric field. The main spectrum components of THz pulse are below 4.5 THz. The carrier frequency of the pulse can be calculated to be about 2 THz according to the spectrum.
When measuring the THz energy in the experiment, the THz wave generated by DSTMS is filtered by 2 mm HDPE and then enters the THz energy meter (THZ5I-BL-BNC, GENTEC-EO Inc.). The energy meter is attached with a high-resistance silicon wafer. Thus, the THz wave generated by DSTMS is filtered by the HDPE and high-resistance silicon before being converted into electrical signal by thermoelectric conversion, which is read by an oscilloscope. According to the THz spectrum and the transmission efficiency of the HDPE and silicon filters, the THz transmittance is 81.46% and 54.18% respectively.
For incident light of different wavelengths, the light energy required to make the THz energy meter generate 456 mV electrical signal is also different. At this time, the value of light energy is called the “correction factor”. Thus, the correction factor for the total energy of the THz pulse is related to the THz spectrum. Taking both the THz spectral distribution and the wavelength-dependent correction factor into consideration, the correction factor corresponding to the whole THz pulse can be calculated as 4.8365. And the truly reliable THz energy can be obtained by combining the readings of the energy meter.
Figure 4 shows the relation curve between the pump fluence and THz energy as well as the relation curve between the pump fluence and conversion efficiency. The pump light spot on the surface of the DSTMS can be regarded as a circle with a diameter of 7 mm. It can be seen that with the increase of pump fluence, the THz energy basically increases, while the conversion efficiency increases to saturation and then decreases slightly. Theoretically, the THz pulse energy JTHz which is generated by OR is proportional to the square of the pump pulse energy Jopt and inversely proportional to the area Aopt of the crystal illuminated by pump light [31]
Fig. 4. The relation curve between the pump fluence and THz energy (orange solid line). The relation curve between the pump fluence and conversion efficiency (cyan solid line). Inset: THz spot profile.
When the pump fluence is less than 15.6 mJ/cm2, the relation curve between the pump fluence and THz energy is in good agreement with Eq. (1). When the pump fluence continues to rise, the increasing rate of the THz energy decreases gradually and is no longer proportional to the square of pump fluence. When the pump fluence reaches 26 mJ/cm2 (corresponding to ∼10 mJ pump pulse energy), the maximum THz energy is 175 µJ. And the conversion efficiency correspondingly reaches 1.73%. At this time, the pump light intensity on the crystal surface is already strong enough to cause signs of damage on crystal, so the pump power is not increased any more. In the experiment, the pump fluence of 15.6 mJ/cm2 is the inflection point of the THz energy curve and also the starting point of saturation of conversion efficiency. From this point on, Keff should be regarded as a function of the pump fluence (Jopt/Aopt) of pump pulse. The main mechanism of this saturation effect lies in the nonlinear process of three-photon absorption of pump light by DSTMS [29]. The photon energy absorbed by DSTMS will be released in the form of fluorescence. Such absorption leads to the decrease of pump energy available for OR effect, resulting in the saturation of conversion efficiency. When the pump energy is 7.7 mJ, the maximum conversion efficiency is about 2%. And the pump fluence is 20 mJ/cm2. The conversion efficiency even decreases slightly when the pump fluence is greater than 20.8 mJ/cm2 (corresponding to the pump energy greater than 8 mJ). Both of the threshold and saturated pump fluence for THz generation are related to crystal quality, pump spot, pulse duration and other parameters. Better crystal uniformity, smoother pump spot, and wider pulse duration help raise the threshold and saturated pump fluence.
The THz peak electric field ETHz is about 17 MV/cm, which is calculated according to the formula [32]
4. Conclusions
In summary, we have pumped organic crystal DSTMS with a high energy OPCPA to generate THz wave with pulse energy of 175 µJ. The THz electric field and spectrum are characterized by EO sampling. The THz spectrum covers 0.1-4.5 THz and the THz electric field reaches about 17 MV/cm. The efficiency variation in the process of generation, time domain waveform and frequency spectrum are analyzed. This study provides a powerful driving light source for strong field THz pump-probe experiment.
Funding
National Key Research and Development Program of China (2022YFA1604401); National Natural Science Foundation of China (62105346); Scientific Instrument Developing Project of the Chinese Academy of Sciences (YJKYYQ20200031); CAS Project for Young Scientists in Basic Research (YSBR-059); Basic research project of Shanghai Science and Technology Innovation Action Plan (20JC1416000); 100 Talents Program of CAS; Shanghai Pilot Program for Basic Research – Chinese Academy of Science, Shanghai Branch.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are available from the authors upon reasonable request.
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