Open Access
Add to BibSonomyAbstract
A cylindrical lens telescope tilted-pulse-front pumping scheme was proposed for high energy terahertz (THz) pulse generation. This scheme allows higher pump energy to be used with lower saturation effects under high pump fluence, and higher THz generation efficiency was achieved within large range of pump energy. The optimum pump pulse duration and crystal cooling temperature for THz generation in LiNbO3 (LN) crystal were also researched systematically. Excited by 800-nm laser, up to 0.19 mJ THz pulse energy and 0.27% conversion efficiency was demonstrated under 800-nm 400-fs laser excitation with ~100-mJ pulse energy and 150-K LN cooling temperature.
© 2016 Optical Society of America
1. Introduction
High-energy THz pulses are attractive for many applications such as investigation of nonlinear charge dynamics [1], nonlinear interaction with nanostructured materials [2], molecular alignment [3], high harmonic generation [4], and so on. These remarkable results of THz induced studies clearly show the application potential of intense THz pulses [5]. Optical rectification (OR) of laser pulses has emerged as the most powerful way to generate high-energy THz pulses [6–11] and resulted in the highest THz pulse energy to date [8]. Within this category, OR with tilted-pulse-front in LiNbO3 (LN) crystal is of particular interest due to its compatibility. This approach had produced 0.4 mJ THz pulses [12] and 3.8% optical-to-THz conversion efficiency [13] pumping by 1030 nm laser.
Although 1030 nm laser excitation yields much higher conversion efficiencies, 800 nm sources based on Ti:sapphire systems prevail in ultrafast laser technology as the most accessible and widely employed sources. Consequently, it is of great value in pursuit of accessible high-energy THz pulse sources pumping by 800 nm laser. Single-cycle THz pulses with 50 μJ energy had been generated with 5 × 10−4 laser to THz conversion efficiency by 800 nm laser pump [14]. However, recent experiment and calculations have predicted that for 800 nm pumping three-photon absorption (3PA) leads to considerable impact of free carrier absorption (FCA) on THz generation, and the THz conversion efficiency goes saturated when pump fluence exceeds certain threshold [14–17]. The spectral broadening (cascading effects) of the pump pulse may be another limited factor of the high-energy THz pulse generation [18–20]. In [21], it had been investigated specifically the limits of THz conversion efficiency with 800 nm laser system below 8 mJ pumping in LN crystal. Experiments on high efficient THz generation under higher energy 800 nm pumping are lack, and how to maintain high conversion efficiency and how to achieve high pulse power generation under high-energy laser pump are required.
In this paper, driven by 800-nm laser based on Ti:sapphire systems, up to 0.19 mJ energy and 0.27% conversion efficiency terahertz pulses generation was demonstrated at 150 K LN cooling temperature. A new cylindrical lens telescope (CLT) tilted-pulse-front pumping (TPFP) scheme was proposed in this work for efficient THz generation under up to 100 mJ laser pulse excitation. The main advantage of our proposed CLT-TPFP scheme is that it allows higher pump energy to be used with lower saturation effects under high pump fluence, hence, higher THz generation efficiency are maintained within a wider and larger range of pump energy. Another key aspect is to use the optimum pump pulse duration and crystal cooling temperature for THz generation in LN crystal with 800-nm femtosecond laser pumping. The optimum experimental conditions and results were researched systematically and given in this paper too.
This paper is organized as follows. Section 2, experimental setup, describes the proposed CLT-TPFP scheme along with commonly used lens telescope (LT) TPFP for comparison, and experimental methods. Section 3, results and discussion, presents the optimization for efficient THz generation under crystal temperature effect and pump pulse duration effect, and the 0.19-mJ THz pulse generation and detailed discussion. Conclusion and outlook are drawn in Sec. 4.
2. Experimental setup
The CLT-TPFP setup we proposed and used in our experiment is shown in Fig. 1(a), along with LT-TPFP setup for comparison. A Ti:sapphire system was used as the pump laser with 800-nm central wavelength, 33-fs pulse duration. The initial spectral FWHM bandwidth was about 23 nm, and a variable width slit was placed in the stretcher of the Ti:sapphire chirped pulse amplification system, which allowed for tuning the spectral bandwidth and consequently Fourier-limited (FL) pulse duration. An intensity autocorrelation and a single-shot spectrum analyzer were used to characterize the duration and bandwidth of pump pulses respectively. The pulse energy of laser was monitored by a laser energy detector in front of the cryostat. Up to ~100 mJ/pulse was used in the experiment. The original pump beam diameter from the Ti:sapphire laser system was ~28 mm at 1/ e2 of its intensity. And then the wave front of the pump pulses was tilted by a 1200-lines/mm grating. A λ/2 plate in front of the grating was used to maximize the diffracted efficiency of the grating, while the other λ/2 plate was used to rotate the polarization of the pump light diffracted from the grating to parallel to the optic axis of LN crystal.
Fig. 1 (a) The CLT-TPFP setup proposed and used in the experiments, along with commonly used LT-TPFP setup. L: lenses, CL: cylindrical lenses, VW: vacuum window, FS: fused silica window, PTEF: Teflon window. Images of output THz beam under the LT-TPFP scheme (b) and that under the proposed CLT TPFP scheme (c). (d) The THz pulse voltage signal of the pyroelectric detector recorded by a storage oscilloscope.
The CLT-TPFP scheme consists of one 250-mm focal length, 60 × 62 mm2 size and one 70-mm focal-length, 30 × 32 mm2 size cylindrical lenses in confocal arrangement, and the pumped spot diameter on the LN crystal was 28 × 6.5 mm2. In principle, the pumped spot size can be further increase, which is only limited by the optical element and LN crystal diameter. The commonly used LT-TPFP scheme consists of one 175-mm focal length, 50-mm diameter and one 50-mm focal length, 25-mm diameter lenses in confocal arrangement, and the pump beam cross-section diameter on LN crystal was 6.5 mm. A 5.0% MgO-doped congruent LN prism with excitation area of 30 × 30 mm2 was used for OR. And the LN crystal was placed inside a closed-cycle helium cryostat with a fused silica input window for laser and a Teflon window for THz pulse emission. The optical elements in front of the second lens were placed in a vacuum room to avoid the air ionization, and a fused silica window was used as vacuum window. Table 1 shows the experimental parameters used for THz generation.
Table 1. Overview of the CLT- and LT- TPFP experimental parameters.
A calibrated pyroelectric detector (Microtech Instruments) with 2 × 3 mm2 active area and a cone-shaped metallic input was used to measure the energy of the THz pulses, and a polymethylpentene (TPX) THz lens with 100-mm focal-length, 50-mm diameter was used to focus the THz beam emerging from the LN crystal before the detector. In order to avoid saturation of the pyroelectric detector, a silicon wafer of 1 mm thickness with 50% measured transmission and a cardboard plate with 44% transmission were placed in the front of the detector to attenuate the THz pulses. The transmissions of Teflon window (75%) on the cryostat and THz lens (74%) before the detector were also taken into account. All the transmission of the filter assembly was carefully measured. A storage oscilloscope was used to record the voltage signal of the pyroelectric detector, and the THz energy WTHz was calculated from the voltage modulation Vm of the recorded trace (as shown in Fig. 1(d)) by WTHz = C·Vm·τ/S [12], where the sensitivity S = 1200 V/W was obtain from factory calibration, while the correction factor C≈1 and the time constant τ = 31 ms were fitting of the recorded trace. A 320 × 240 pixel uncooled microbolometer THz camera with 23.5 μm pixel pitch was used to measure the THz intensity image outside the cryostat. The measured THz beam images of the LT-TPFP scheme and the CLT-TPFP scheme were shown in Fig. 1(b) and 1(c), respectively.
Previous experiments and theoretical model show that optimal pump beam diameter and pump fluence for maximum THz conversion exist in THz generation from LN crystal under TPFP scheme [16, 17]. The advantage of our proposed CLT-TPFP scheme is that it only minifies one dimension of the pump beam, so that it maintains the optimal pump fluence and optimal 1-D optical beam diameter for highest THz conversion efficiency under higher pump energy by increasing the optical beam size in the other dimension.
3. Results and discussion
3.1 Crystal temperature effect on THz generation efficiency
Cooling the LN crystal to cryogenic temperature and tuning the pump pulse FL duration were previously demonstrated as the two most efficient ways to enhance the THz generation efficiency [7,22]. In this section, we first investigated the relationship of the THz generation efficiency at different LN cooling temperature under LT-TPFP scheme. The incident and emergence angles of the grating were readjusted to optimize the pulse front tilt for the phase matching at each temperature.
As shown in Fig. 2(a), an efficiency-enhancement factor of more than 2 times was achieved when temperature changes from 295 K (room temperature) to 150 K, while in the range of 150 K to 100 K only a slight change can be observed. We further investigated the THz conversion efficiency as a function of LN temperature with different pump fluence, which are shown in Fig. 2(b). As can be seen, there is optimum LN cooling temperature for different pump fluence, and for high pump fluence (≥16.5 mJ/cm2) the optimum temperature is about 150 K.
Fig. 2 Measured THz generation efficiency (a) vs. pump energy and fluence at different LN temperature, and (b) vs. LN temperature at different pump fluence.
3.2 Pump pulse FL duration effect on THz generation efficiency
We first tuned the pump laser spectral bandwidth by using a variable width slit placed in the stretcher of the Ti:sapphire system, and investigated the dependence of the pump pulse FL duration (FWHM) on the spectral bandwidth (FWHM). The measured power spectral bandwidth and pulse duration are shown in Fig. 3(a) and 3(b).
Fig. 3 Spectral bandwidth used in the experiment (a), and the pulse duration vs. FWHM power spectral bandwidth (b), the inset is the measured result of 21 nm FWHM bandwidth with an intensity autocorrelation.
Then we investigated the relationship of the THz generation efficiency with the pump energy at different pump pulse duration with 150 K LN cooling temperature. In [23], a similar measurement was carried out with 1030 nm but not 800 nm center wavelength of pump pulse. According to previous calculations [16], there is optimum incident position on LN crystal for different pump pulse durations; hence, in our experiments the incident position of pump beam was readjusted to achieve the highest THz generation for each measurement with different pump pulse durations. The experimental results are shown in Fig. 4(a) and 4(b). The THz generation efficiency in Fig. 4(a) exhibit nearly constant or even decreasing dependence on pump energy. This may because the pump energy (fluence) is larger than 3 mJ (~3mJ/cm2) in the experiment, and at such high pump fluence, some strong nonlinear effects such as FCA, cascading effects, and self-phase modulation may result in the saturation of the THz conversion efficiency. An efficiency-enhancement factor of 3~4 times was achieved by tuning the pump pulse duration from 33 fs to 400 fs, while from 400 fs to 510 fs the efficiency slightly decreases. The optimum pulse duration of 400 fs is consistent with the predicted optimum duration [7], which is a little shorter than the optimum duration (500 fs) in room temperature. The optimum conditions of 400 fs pumping laser and 150 K cooling temperature were father used in the following experiments.
Fig. 4 THz generation efficiency (a) as functions of pump energy and pump fluence with different pump pulse durations, and (b) as a function of pump pulse duration under different pump fluence.
3.3 THz pulse generation with 0.19 mJ energy
In this section, we used the proposed CLT-TPFP scheme, along with LT-TPFP scheme for comparison, to achieve highest possible THz pulse energy. With the optimum conditions of 400 fs pulse duration pumping laser and 150 K cooling temperature, we used the LT- and CLT-TPFP scheme to investigate the THz generation with increasing pump energy, respectively. The pumped beam cross-section diameter of CLT-TPFP scheme on LN crystal was 28 × 6.5 mm2, which is much larger than the diameter of LT-TPFP scheme (6.5 mm in diameter), shown in Tab.1. For LT-TPFP scheme, the maximum pump energy used was 18.7 mJ, and the corresponding maximum fluence was 56.0 mJ/cm2. While for CLT-TPFP scheme, up to 99.8 mJ/pulse and corresponding fluence of 69.8 mJ/cm2 pump laser was used for THz generation.
The measured THz pulse energies as function of the pump energy and fluence are shown in Fig. 5(a) and 5(b), respectively. For LT-TPFP scheme, increase of the THz energy with increasing pump energy was observed when the pump energy is below 14.4 mJ (43.2 mJ/cm2), and the achieved maximum THz pulse energy was 30.7 μJ under 18.7-mJ pump energy. While for CLT-TPFP scheme, the THz pulse energy is almost in direct proportion to pump energy below 70 mJ (49 mJ/cm2). The maximum THz energy achieved with the CLT-TPFP scheme was 0.19 mJ under 99.8-mJ pump energy.
Fig. 5 Generation of THz pulse energy as a function of pump energy (a) and fluence (b). Blue square and red circle represent results with the CLT- and LT- TPFP scheme, respectively. Previous published experiment results with 800-nm pumping under LT-TPFP scheme are also shown for comparison.
The corresponding THz generation efficiency as function of the pump energy and fluence are shown in Fig. 6. As can be seen in Fig. 6(a), for CLT-TPFP scheme, highest conversion efficiency of 0.27% can be observed under 33.7-mJ pump energy (23.6 mJ/cm2 pump fluence). The efficiencies of CLT-TPFP scheme are higher than that of LT-TPFP scheme and than those from most of previous published experimental results under 800-nm pumping, which is consistent with our previous expectancy. For LT-TPFP scheme, the efficiency reaches its highest value of 0.2% with 4.6-mJ pump energy (13.6 mJ/cm2 pump fluence), and then decreases with the pump energy. THz generation efficiency as a function of pump energy decreases much more quickly than that with CLT-TPFP scheme. Interestingly in Fig. 6(b), although the CLT-TPFP scheme’s efficiency is a little larger than the LT-TPFP scheme with the same pump fluence, the trend of efficiency variation vs. pump fluence of CLT-TPFP scheme is similar to that of LT-TPFP scheme.
Fig. 6 THz generation efficiency as a function of pump energy (a) and fluence (b) with the CLT- and LT -TPFP scheme. Previous published experiment results with 800-nm pumping under LT-TPFP scheme are also shown for comparison.
To explore the reason for this behavior, we investigate the intensity distribution of the pump beam for the LT-TPFP and CLT-TPFP scheme with the same pump energy and the same average pump fluence, respectively. Figure 7 shows the horizontal and vertical intensity distributions of the pump beam cross-section which were measured by using a lens and a CCD camera. From Fig. 7(a), one can find that under the same laser pump energy, the horizontal and vertical intensity distributions of CLT-TPFP scheme pump beam are much lower than that of LT-TPFP scheme, and low pump intensity may result in weaker saturation effect in LN crystal. Hence, higher THz conversion efficiency was achieved with the same pump energy under CLT-TPFP scheme than that under LT-TPFP scheme. While with the same average fluence pumping, as shown in Fig. 7(b), the peak horizontal and vertical intensity distributions of CLT-TPFP scheme are the same as that of LT-TPFP scheme, but the former pump beam vertical diameter is much larger than the latter’s diameter, and larger pump area means more pump laser converts to THz waves. It is the reason why the CLT-TPFP scheme’s efficiency is higher than the LT-TPFP scheme with the same pump fluence. In addition, since the pump beam intensity is nearly Gaussian distribution, and the THz efficiency is not exactly linear with pump fluence as some strong nonlinear effects may result in the saturation of the THz conversion efficiency at such high energy pumping, hence, although the CLT vs. LT scheme nearly have a 4:1 pump beam area relationship, the efficiency of CLT is not 4 times higher (but still higher) than that of LT scheme.
Fig. 7 The horizontal (H.) and vertical (V.) intensity distributions of pump beam cross-section for the LT and CLT TPFP scheme under the same pump energy (a) and the same average pump fluence (b). The intensity distributions of LT (c) and CLT (d) pump beam.
Although we had not measure the generated THz pulse spectra in this paper, one should note that the THz pulse spectra bandwidth should go down with longer pulse duration pump, as discussed in [23]. Broader THz spectra require shorter FL pump pulse duration, which inherently results in lower conversion efficiencies, as shown in Fig. 4.
4. Conclusion
In conclusion, a CLT-TPFP scheme was proposed in this paper, which allows higher energy laser pumping with efficient optical-to-THz conversion. The saturation effect of THz generation under high pump energy was reduced in this scheme, and higher THz generation efficiency was maintained within large pump energy range of 4 mJ to 100 mJ. The optimum pump laser spectral bandwidth and crystal cooling temperature for OR in LN crystal with 800-nm femtosecond laser pumping were discovered, which are ~150 K and ~400 fs, respectively. Excited by 800-nm Ti:sapphire laser, up to 0.19 mJ energy and 0.27% conversion efficiency of terahertz pulse generation was achieved by using the proposed CLT-TPFP scheme. THz generation efficiency as a function of pump energy and fluence in CLT- and LT-TPFP scheme were discussed in detail. The proposed CLT-TPFP scheme also can be used in other excitation laser systems with different wavelengths (such as 1030 nm [12,13]) with TPFP scheme, which are expected to maintain high optical-to-THz conversion efficiency under higher pump energy and to consequently achieve higher energy THz pulse generation than conventionally used LT-TPFP scheme.
Acknowledgments
The authors would like to thank the sponsors of this work— National Key Basic Research Program of China (No. 2015CB755405), the National Natural Science Foundation of China (No. 61427814), and Foundation of President of China Academy of Engineering Physics (No. 201501033).
References and links
1. M. C. Hoffmann, J. Hebling, H. Y. Hwang, K.-L. Yeh, and K. A. Nelson, “THz-pump/THz-probe spectroscopy of semiconductors at high field strengths,” J. Opt. Soc. Am. B 26(9), A29–A34 (2009). [CrossRef]
2. R. Shimano, S. Watanabe, and R. Matsunaga, “Intense terahertz pulse-induced nonlinear responses in carbon nanotubes,” J. Infrared Millim. Terahertz Waves 33(8), 861–869 (2012). [CrossRef]
3. S. Fleischer, Y. Zhou, R. W. Field, and K. A. Nelson, “Molecular Orientation and Alignment by Intense Single-Cycle THz Pulses,” Phys. Rev. Lett. 107(16), 163603 (2011). [CrossRef] [PubMed]
4. E. Balogh, K. Kovacs, P. Dombi, J. A. Fülöp, G. Farkas, J. Hebling, V. Tosa, and K. Varju, “Single attosecond pulse from terahertz-assisted high-order harmonic generation,” Phys. Rev. A 84(2), 023806 (2011). [CrossRef]
5. T. Kampfrath, K. Tanaka, and K. A. Nelson, “Resonant and nonresonant control over matter and light by intense terahertz transients,” Nat. Photonics 7(9), 680–690 (2013). [CrossRef]
6. J. Hebling, G. Almasi, I. Kozma, and J. Kuhl, “Velocity matching by pulse front tilting for large area THz-pulse generation,” Opt. Express 10(21), 1161–1166 (2002). [CrossRef] [PubMed]
7. J. A. Fülöp, L. Pálfalvi, M. C. Hoffmann, and J. Hebling, “Towards generation of mJ-level ultrashort THz pulses by optical rectification,” Opt. Express 19(16), 15090–15097 (2011). [CrossRef] [PubMed]
8. C. Vicario, A. V. Ovchinnikov, S. I. Ashitkov, M. B. Agranat, V. E. Fortov, and C. P. Hauri, “Generation of 0.9-mJ THz pulses in DSTMS pumped by a Cr:Mg₂SiO₄ laser,” Opt. Lett. 39(23), 6632–6635 (2014). [CrossRef] [PubMed]
9. K.-L. Yeh, M. C. Hoffmann, J. Hebling, and K. A. Nelson, “Generation of 10μJ ultrashort terahertz pulses by optical rectification,” Appl. Phys. Lett. 90(17), 171121 (2007). [CrossRef]
10. H. Hirori, A. Doi, F. Blanchard, and K. Tanaka, “Single-cycle terahertz pulses with amplitudes exceeding 1 MV/cm generated by optical rectification in LiNbO3,” Appl. Phys. Lett. 98(9), 091106 (2011). [CrossRef]
11. J. A. Fülöp, L. Pálfalvi, S. Klingebiel, G. Almási, F. Krausz, S. Karsch, and J. Hebling, “Generation of sub-mJ terahertz pulses by optical rectification,” Opt. Lett. 37(4), 557–559 (2012). [CrossRef] [PubMed]
12. J. A. Fülöp, Z. Ollmann, C. Lombosi, C. Skrobol, S. Klingebiel, L. Pálfalvi, F. Krausz, S. Karsch, and J. Hebling, “Efficient generation of THz pulses with 0.4 mJ energy,” Opt. Express 22(17), 20155–20163 (2014). [CrossRef] [PubMed]
13. S. W. Huang, E. Granados, W. R. Huang, K. H. Hong, L. E. Zapata, and F. X. Kärtner, “High conversion efficiency, high energy terahertz pulses by optical rectification in cryogenically cooled lithium niobate,” Opt. Lett. 38(5), 796–798 (2013). [CrossRef] [PubMed]
14. A. G. Stepanov, S. Henin, Y. Petit, L. Bonacina, J. Kasparian, and J.-P. Wolf, “Mobile source of high-energy single-cycle terahertz pulses,” Appl. Phys. B 101(1-2), 11–14 (2010). [CrossRef]
15. A. G. Stepanov, L. Bonacina, S. V. Chekalin, and J.-P. Wolf, “Generation of 30 μJ single-cycle terahertz pulses at 100Hz repetition rate by optical rectification,” Opt. Lett. 33(21), 2497 (2008). [CrossRef] [PubMed]
16. S.-C. Zhong, Z.-H. Zhai, J. Li, L.-G. Zhu, J. Li, K. Meng, Q. Liu, L.-H. Du, J.-H. Zhao, and Z.-R. Li, “Optimization of terahertz generation from LiNbO3 under intense laser excitation with the effect of three-photon absorption,” Opt. Express 23(24), 31313–31323 (2015). [CrossRef] [PubMed]
17. S. B. Bodrov, A. A. Murzanev, Y. A. Sergeev, Y. A. Malkov, and A. N. Stepanov, “Terahertz generation by tilted-front laser pulses in weakly and strongly nonlinear regimes,” Appl. Phys. Lett. 103(25), 251103 (2013). [CrossRef]
18. K. Ravi, W. R. Huang, S. Carbajo, X. Wu, and F. Kärtner, “Limitations to THz generation by optical rectification using tilted pulse fronts,” Opt. Express 22(17), 20239–20251 (2014). [CrossRef] [PubMed]
19. K. Ravi, W. R. Huang, S. Carbajo, E. A. Nanni, D. N. Schimpf, E. P. Ippen, and F. X. Kärtner, “Theory of terahertz generation by optical rectification using tilted-pulse-fronts,” Opt. Express 23(4), 5253–5276 (2015). [CrossRef] [PubMed]
20. X. Wu, S. Carbajo, K. Ravi, F. Ahr, G. Cirmi, Y. Zhou, O. D. Mücke, and F. X. Kärtner, “Terahertz Generation in Lithium Niobate Driven by Ti:Sapphire Laser Pulses and its Limitations,” Opt. Lett. 39(18), 5403–5406 (2014). [CrossRef] [PubMed]
21. F. Blanchard, X. Ropagnol, H. Hafez, H. Razavipour, M. Bolduc, R. Morandotti, T. Ozaki, and D. G. Cooke, “Effect of extreme pump pulse reshaping on intense terahertz emission in lithium niobate at multimilliJoule pump energies,” Opt. Lett. 39(15), 4333–4336 (2014). [CrossRef] [PubMed]
22. J. A. Fülöp, L. Pálfalvi, G. Almási, and J. Hebling, “Design of high-energy terahertz sources based on optical rectification,” Opt. Express 18(12), 12311–12327 (2010). [CrossRef] [PubMed]
23. C. Vicario, B. Monoszlai, C. Lombosi, A. Mareczko, A. Courjaud, J. A. Fülöp, and C. P. Hauri, “Pump pulse width and temperature effects in lithium niobate for efficient THz generation,” Opt. Lett. 38(24), 5373–5376 (2013). [CrossRef] [PubMed]
