We developed a difference frequency generation (DFG) source with an organic nonlinear optical crystal of DAST or BNA selectively excited by a dual-wavelength β-BaB2O4 optical parametric oscillator (BBO-OPO). The dual-wavelength BBO-OPO can independently oscillate two lights with different wavelengths from 800 to 1800 nm in a cavity. THz-wave generation by using each organic crystal covers ultrawide range from 1 to 30 THz with inherent intensity dips by crystal absorption modes. The reduced outputs can be improved by switching over the crystals with adequately tuned pump wavelengths of the BBO-OPO.
©2012 Optical Society of America
Nonlinear optical effects in nonlinear optical crystals are extremely useful in generating monochromatic THz waves with high peak power, narrow bandwidths, and ultrawideband frequency tunability [1,2]. Recently, a remarkable breakthrough has been achieved by obtaining a radiation peak power considerably exceeding the kW level by using a microchip Nd:YAG laser pumping and injection seeding technique under the optical parametric generation (is-TPG) in inorganic Mg:LiNbO3 crystals . The spectral bandwidth of the is-TPG is no more than 5 GHz, which is quite narrow compared to other THz-wave sources. However, the tunable frequency range still remains around 1–3 THz.
Other THz-wave sources based on the difference frequency generation (DFG) scheme are also attractive from the viewpoint of enhanced ultrawideband THz frequency tunability . The low dispersion of the refractive index covering an extremely wide range yields collinear phase matching . In addition, organic nonlinear optical crystals provide high conversion efficiency because of large nonlinear susceptibilities; e.g., 1010 pm/V at 1318 nm in 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) and 234 pm/V at 1064 nm in N-benzyl-2-methyl-4-nitroaniline (BNA).
In actual, to attain ultrawideband THz frequency tunability, a dual-wavelength pump sources with wideband wavelength tunability are required simultaneously with wide phase matching in the crystals. So far, a dual-wavelength KTiOPO4 optical parametric oscillator (KTP-OPO) pumped with 532-nm lights has been developed as a dual-wavelength pump source for DFG by using DAST and BNA [6,7]. The dual-wavelength could be quickly controlled to satisfy the phase matching condition and for tuning the THz-wave frequency continuously or randomly. However, the necessary wavelengths of a dual-wavelength pump source depend on the inherent phase-matching condition of the nonlinear crystal for THz-wave generation. For DAST-DFG, tunability from 1.3 to 1.8 μm is required for ultrawideband THz-wave generation from 1 THz up to 40 THz. On the other hand, tunability from 0.8 to 1.3 μm is required for BNA-DFG. In addition, new nonlinear organic materials with large nonlinear susceptibilities have been recently developed and utilized for wideband THz-wave generation [8–13]. The Er-doped (70 fs, 47.8 MHz) fiber laser with a center wavelength of 1560 nm was used to pump organic DASC and BDAS-TP crystals for THz-wave generation via an optical rectification process . A dual-wavelength between 1.3 and 1.7 μm were found to be optimal for efficiently generating the wideband THz-waves in DSTMS . Moreover, dual-wavelength from 1.2 to 1.54 μm was used for the wideband THz-waves generation in OH1 crystals via the DFG process . These results indicate that the required tuning range of dual-wavelength for almost all organic nonlinear materials stays in the range of 0.8–1.8 μm.
From the viewpoint of wideband wavelength tunability, conversion efficiency, and high damage threshold, OPOs are potential sources. In particular, an OPO based on a β-BaB2O4 (BBO) crystal pumped with 355-nm light is a most suitable candidate as a dual-wavelength pump source for an efficient ultrawideband THz-wave generation by using the DFG process in organic materials.
In this paper, we report on the development of an ultrawide tunable THz-wave source based on organic nonlinear optical crystals of DAST and BNA pumped with a dual-wavelength BBO-OPO instead of the KTPO-OPO.
2. Experimental setup
Figure 1 shows the system of DAST- and BNA-DFG pumped with a dual-wavelength BBO-OPO. The original pump source was a basic Nd:YAG laser that produces a 1064-nm wavelength light with a pulse duration of 12 ns at a repetition rate of 10 Hz and the maximum pulse energy is 1500 mJ (SOLAR, LQ929). Typical operating pulse energy of 1064-nm light at the experiment was 120 mJ. By using a first Lithium triborate (LBO) crystal with a cutting angle of θ = 90° and ϕ = 11.6°, the second harmonic radiation with the wavelength of 532 nm was obtained. Then, the 355-nm light as a pump source for the dual-wavelength BBO-OPO can be obtained by the sum frequency generation process of the 1064-, 532-, and 355-nm lights in the second LBO crystal whose cutting angle is θ = 42.2° and ϕ = 90°. The output power of 355-nm light was 30 mJ with the 25% conversion efficiency from the fundamental 1064-nm light to the 355-nm light because of the each 50% conversion efficiency at the respective LBO crystals.
After the 355-nm pump light was collimated to suppress the beam divergence, it was guided toward the dual-wavelength BBO-OPO system, which consists of 5 mirrors and two BBO crystals. In order to efficiently generate THz waves with narrow spectral bandwidths, the spectral bandwidths of the dual-wavelength pump beams need to be narrow. In general, an etalon or grating is inserted into the cavity or injection seeding is performed to reduce the spectral bandwidths of the OPO [14–18]. However, the insertion loss and complexity of the cavity control are attendant problems. In the system proposed here, a simple BBO-OPO without wavelength selective elements based on type II phase matching, which can serve relatively narrower OPO linewidths compared with those in type I phase matching, was employed [19,20]. Figure 2 shows the calculated tuning and gain curves of the BBO-OPO with type II phase-matching condition when pumped with 355-nm light. The idler lights with a wavelength range 0.8–1.8 μm are available for DAST and BNA pumping. Two BBO crystals of size 10(W) × 5(H) × 15(L) mm were cut for the type II phase matching at θ = 38° and ϕ = 30° and used in the system. The edges of the BBO crystals were coated with high damage threshold protective coating, which also acted as a broadband antireflection coating.
Because it was expensive and difficult technically to construct cavity mirrors to fulfill the complicated broadband transmission/reflection properties, ordinary commercial mirrors were used to construct the dual-wavelength BBO-OPO. The reflectance of the mirrors measured using an FT spectrometer is shown in Fig. 3 . The M1 mirror had broadband reflection (approximately 80%) for the entire range of wavelengths relevant to this BBO-OPO. The M2 mirror was an output coupler with a broadband dielectric coating. It had high reflectivity for the signal lights of wavelengths of 400–650 nm and could transmit idler lights with wavelengths above 800 nm to ensure a singly resonant OPO operation. These idler lights are required and used for the THz-wave generation via the phase-matched DAST and BNA-DFG. The three Mp mirrors were set to reflect the 355-nm pump beam. These Mp mirrors were allocated inside the cavity to protect the M1 and M2 mirrors from optical damage because of the intense 355-nm pump beam . The reflected 355-nm pump beam was safely dumped by the isolator installed in the front of the cavity. Two BBO crystals were mounted on the galvano scanner with walk-off compensated configuration  and the angular phase matching condition could be controlled independently and randomly with a 1-ms time response. The cavity length was 15 cm, which allows 10 round trips of the resonating signal waves during the 355-nm pump pulse duration. Two extracted idler waves with a wavelength difference corresponding to the THz photon energy were injected into the organic DAST or BNA crystals for the collinearly phase matched DFG process. The generated THz waves were first collimated with the first parabolic mirror, and then focused with the second parabolic mirror. Finally, the THz waves were detected by the Si bolometer or pyroelectric detector after the two pump idler beams were thoroughly cut off by low-pass filters.
The circles and squares in Fig. 4 show the tunability of the dual-wavelength BBO-OPO pumped at 355 nm as a function of internal angles. The internal angles were controlled by changing the angles of BBO crystals mounted on the galvano scanner. These data were well fitted with the theoretical tuning curve for the idler waves. Although the upper limit of the wavelengths measured by a spectrum analyzer was 1750 nm, the tunability from roughly 0.8–1.8 μm, which is required for the phase-matched DAST- and BNA-DFG, was successfully obtained. Output pulse energy of the dual-wavelength BBO-OPO depends on wavelength as shown in Fig. 5 and pump pulse energy of 355-nm lights. Especially, higher 355-nm pump pulse energy is desirable from the viewpoint of the conversion efficiency. However, in order to avoid cavity mirrors damages, 355-nm pump pulse energy was fixed at 30 mJ. In this case, roughly 3 mJ pulse energy of the dual-wavelength BBO-OPO was obtained at the dual-wavelength of around 1 μm with a conversion efficiency of approximately 10%. The efficiency is low compared to other results reported ever due to no optimized cavity mirrors. In addition, the spectral bandwidths remained at less than 60 GHz throughout the entire tuning wavelengths range as shown in Fig. 5. These values are sufficient for the THz-wave generation by DFG. Example of spectrum power balance of dual-wavelength was shown in Fig. 6 . Here, one wavelength is fixed at 1.3 μm, which is useful for DAST-DFG, and the other was changed. The balances were inconsiderably degraded for DFG as the wavelength difference became large.
3. Ultra-widely tunable THz-wave generation using DAST and BNA pumped with a dual wavelength BBO-OPO
Figures 7(a) and 7(b) show typical output spectra of DAST- and BNA-DFG, respectively, pumped by the dual-wavelength BBO-OPO. Here, the dual-wavelength was independently controlled to satisfy respective phase matching conditions with the tuning ranges of from 1.2 to 1.6 μm for DAST-DFG and from 0.8 to 1.3 μm for BNA-DFG. Especially, THz-wave intensity and frequency tunability of BNA-DFG can be much improved by pumping with the BBO-OPO compared to previous report  where dual-wavelength tuning range was limited only from 0.75 to 0.95 μm. Each THz-wave output spectrum has several intensity dips mainly caused by the water vapor absorptions and intrinsic phonon absorptions of the crystals. The water vapor absorptions have a notable influence on the intensity dips at frequencies below 10 THz . On the other hand, strong phonon absorptions are responsible for the intensity dips observed at 8.7, 15.2, from 20 to 23, and at 29.5 THz in the case of DAST-DFG  and at 7.4, 13.6, from 15 to 17, and at 19.6 THz for BNA-DFG .
Water vapor absorption can be avoided by replacing the atmospheric air with dried nitrogen gas or vacuum. On the other hand, inherent phonon absorptions are inevitable. By using a variety of nonlinear crystals with different thickness, the intensity dips caused by the range of such phonon absorptions can be compensated. For example, Fig. 8 shows a prospective THz-wave output spectrum when DAST- and BNA-DFG are integrated and the partial compensations of the intensity dips resulting from the respective phonon absorptions are achieved. By using other types of organic nonlinear crystals, a much higher and wider THz-wave output spectrum without any intensity dips must be obtained. In a real application, a quick and random switching of nonlinear crystals depending on the desired THz frequency under the synchronization of the pulse laser operation is practically important. To achieve this, we introduced a rotatable circular mount that can hold a number of nonlinear crystals in the DFG system. The selective switching of the crystals corresponding to the intended THz frequency is achieved by rotating the mount in synchronization with the pulse operation of the optimum two pump wavelengths from the dual-wavelength BBO-OPO. Even after the mount is rotated, the position of a target crystal can be coaxially kept on the path of the two pump beams, as shown in Fig. 1. Because the developed dual-wavelength BBO-OPO has enough wide tunable range to pump various organic nonlinear materials such as DAST, BNA, DASC, BDAS-TP, DSTMS, and OH1, our system can be easily adopted for use with these materials. A higher and wider THz-wave generation without any intensity dips can be expected when several types of organic nonlinear crystals and the dual-wavelength BBO-OPO are used simultaneously.
In order to create a coherent THz-wave source with continuous ultrawideband frequency tunability based on the DFG scheme by using organic nonlinear DAST and BNA materials, a dual-wavelength BBO-OPO pumped with 355-nm light was developed. The tunability of two wavelengths from 800 to 1800 nm, which can be applied to DAST and BNA as well as to almost all other reported organic nonlinear crystals, was successfully obtained by the dual-wavelength BBO-OPO. With the dual-wavelength BBO-OPO as a two pump light source for the DFG process, an ultrawideband THz-wave generation can be demonstrated in DAST and BNA crystals. Our system including the dual-wavelength BBO-OPO and a rotatable mount to hold several nonlinear organic crystals is a promising scheme to generate a higher, flatter, and wider THz-wave spectrum by DFG when several types of organic crystals are used simultaneously.
The authors would like to thank Professor H. Ito of RIKEN, Professor K. Kumano of Tohoku University, Professor H. Hashimoto of Osaka City University, Dr. K. Miyamoto of Chiba University, and Dr. S. Ohno of Tohoku University for useful discussions and comments. We would also like to thank Ms. M. Saito for her cooperation in the DAST and BNA crystal growth and Mr. C. Takyu for the dielectric coating of several optical components. Finally, the authors would like to express their appreciation to Ms. C. Suzuki, Mr. A. Harako, and Mr. Y. Usuki of Furukawa Co., Ltd. for the synthesis and purification of the BNA ingredients. This work was partially supported by the Strategic International Cooperative Program (Japan–Singapore) and the Strategic International Cooperative Program (Japan–France), and Collaborative research based on Industrial Demand of the Japan Science and Technology Agency (JST), and JSPS KAKENHI (19206009), KAKENHI (23760058), KAKENHI (23360045), KAKENHI (23560053), KAKENHI (228044), KAKENHI (23760001), KAKENHI (24560535), Sakura-project (I2012651) and RIKEN Grant and RIKEN Incentive Research Grant.
References and links
1. K. Kawase, J. Shikata, and H. Ito, “Terahertz parametric source,” J. Phys. D Appl. Phys. 34, R1–R14 (2001).
2. V. Ya. Aleshkin, A. A. Antonov, S. V. Gaponov, A. A. Dubinov, Z. F. Krasil’nik, K. E. Kudryavtsev, A. G. Spivakov, and A. N. Yablonskii, “Tunable source of terahertz radiation based on the difference-frequency generation in a GaP crystal,” JETP Lett. 88(12), 787–789 (2008). [CrossRef]
3. S. Hayashi, K. Nawata, H. Sakai, T. Taira, H. Minamide, and K. Kawase, “High-power, single-longitudinal-mode terahertz-wave generation pumped by a microchip Nd:YAG laser,” Opt. Express 20(3), 2881–2886 (2012). [CrossRef] [PubMed]
4. H. Ito, K. Suizu, T. Yamashita, A. Nawahara, and T. Sato, “Random frequency accessible broad tunable terahertz-wave source using phase-matched 4-dimethylamino n-methyl 4-stibazolium tosylate crystal,” Jpn. J. Appl. Phys. 46(11), 7321–7324 (2007). [CrossRef]
5. I. Katayama, R. Akai, M. Bito, H. Shimosato, K. Miyamoto, H. Ito, and M. Ashida, “Ultrabroadband terahertz generation using 4-n,n-dimethylamino-4′-n′-methyl-stilbazolium tosylate single crystals,” Appl. Phys. Lett. 97(2), 021105 (2010). [CrossRef]
6. T. Taniuchi, J. Shikata, and H. Ito, “Tunable terahertz-wave generation in DAST crystal with dual-wavelength KTP optical parametric oscillator,” Electron. Lett. 36(16), 1414–1416 (2000). [CrossRef]
8. H. Nakanishi, H. Matsuda, S. Okada, and M. Kato, “Organic and polymeric complexes for nonlinear optics” in Proc. MRS Int. Mtg. Adv. Mater. 1, 97–104 (1989).
9. H. Hashimoto, Y. Okada, H. Fujimura, M. Morioka, O. Sugihara, N. Okamoto, and R. Matsushima, “Second-harmonic generation from single crystal of n-substituted 4-nitroanilines,” Jpn. J. Appl. Phys. 36(Part 1, No. 11), 6754–6760 (1997). [CrossRef]
10. L. Mutter, F. Bruner, Z. Yang, M. Jazbinsek, and P. Günter, “Linear and nonlinear optical properties of the organic crystal DSTMS,” J. Opt. Soc. Am. B 24(9), 2556–2560 (2007). [CrossRef]
11. F. D. Brunner, O. P. Kwon, S. J. Kwon, M. Jazbinsek, A. Schneider, and P. Günter, “A hydrogen-bonded organic nonlinear optical crystal for high-efficiency terahertz generation and detection,” Opt. Express 16(21), 16496–16508 (2008). [CrossRef] [PubMed]
12. T. Matsukawa, M. Yoshimura, Y. Takahashi, Y. Takemoto, K. Takeya, I. Kawayama, S. Okada, M. Tonouchi, Y. Kitaoka, Y. Mori, and T. Sasaki, “Bulk crystal growth of stilbazolium derivatives for terahertz waves generation,” Jpn. J. Appl. Phys. 49(7), 075502 (2010). [CrossRef]
13. T. Notake, K. Nawata, H. Kawamata, T. Matsukawa, and H. Minamide, “Solution growth of high-quality organic n-benzyl-2-methyl-4-nitroaniline crystal for ultra-wideband tunable DFG-THz source,” Opt. Mater. Express 2(2), 119–125 (2012). [CrossRef]
14. J. G. Haub, M. J. Johnson, A. J. Powell, and B. J. Orr, “Bandwidths characteristics of a pulsed optical parametric oscillator: application to degenerate four-wave mixing spectroscopy,” Opt. Lett. 20, 1637–1639 (1995). [CrossRef] [PubMed]
15. J. M. Boon-Engering, W. E. van der Veer, J. W. Gerritsen, and W. Hogervorst, “Bandwidth studies of an injection-seeded b -barium borate optical parametric oscillator,” Opt. Lett. 20(4), 380–382 (1995). [CrossRef] [PubMed]
16. J. G. Haub, R. M. Hentschel, M. J. Johnson, and B. J. Orr, “Controlling the performance of a pulsed optical parametric oscillator: a survey of techniques and spectroscopic applications,” J. Opt. Soc. Am. B 12(11), 2128–2141 (1995). [CrossRef]
18. S. Das, “Narrow linewidth pulsed optical parametric oscillator,” Pramana J. Phys. 75(5), 827–835 (2010). [CrossRef]
19. G. Anstett, G. Goritz, D. Kabs, R. Urschel, R. Wallenstein, and A. Borsutzky, “Reduction of the spectral width and beam divergence of a BBO-OPO by using collinear type-II phase matching and back reflection of the pump beam,” Appl. Phys. B 72(5), 583–589 (2001). [CrossRef]
20. W. R. Bosenberg and C. L. Tang, “Type II phase matching in a β-barium borate optical parametric oscillator,” Appl. Phys. Lett. 56(19), 1819–1821 (1990). [CrossRef]
21. The HITRAN database, http://www.cfa.harvard.edu/hitran/
22. S. Ohno, K. Miyamoto, H. Minamide, and H. Ito, “New method to determine the refractive index and the absorption coefficient of organic nonlinear crystals in the ultra-wideband THz region,” Opt. Express 18(16), 17306–17312 (2010). [CrossRef] [PubMed]