Tunable Terahertz Source Based on Optics Pumped Nonlinear Crystals

De-Gang Xu, Peng-Xiang Liu, Yu-Ye Wang, Kai Zhong, Wei Shi, and Jian-Quan Yao

Tunable Terahertz Source Based on Optics Pumped Nonlinear Crystals

De-Gang Xu, Peng-Xiang Liu, Yu-Ye Wang, Kai Zhong, Wei Shi, and Jian-Quan Yao

——Recent progresses about optical pumped tunable terahertz (THz) sources are interviewed, including THz parametric oscillation (TPO) and difference frequency generation (DFG). We develop high efficiency and high power surface-emitted TPO, as well as DFG with nonlinear crystals. A novel scheme for the high efficiency DFG source based on the Cherenkov phase-matching technology is comprehensively investigated in both bulk crystals. The widely tunable optical THz radiation is also researched based on the organic nonlinear 4-N,N-dimethylamino-4′-N′-methylstilbazolium 2,4,6-trimethylbenzenesulfonate (DSTMS) crystal.

Index Terms——Cherenkov phase-matching, difference frequency generation, nonlinear optics, parametric oscillation.

1. Introduction

The terahertz (THz) wave is considered to have great values and plentiful applications, such as in material science, analysis of molecular spectra, information and communication technology, biology and medical science, nondestructive evaluation, and national security[1]. High-power widely tunable THz sources are required in the above practical applications of THz technologies. Apromising approach for monochromatic THz generation is based on the second-order nonlinear optical effect, e.g. THz parametric oscillation (TPO) and difference frequency generation (DFG), due to following advantages: availability of both the low cost laser pump source and nonlinear crystal, continuous and wide tunablity, operating at room temperature, and compactness of system. The main drawback of this method is the low output power and conversion efficiency. An amount of investigation has been made to improve the efficiency, including on the nonlinear materials (e.g. GaSe[2], GaP[3], GaAs[4], LiNbO3[5]and DAST[6]) as well as on the phase-matching configurations (e.g. birefringence[2], noncollinear[4],[5], and quasi phasematching[3]).

In this paper, we summarize our recent results on the monochromatic and tunable THz generation based on the nonlinear optical frequency conversion technology, including high power and high efficiency surface-emitted TPO, DFG with the organic DSTMS nonlinear crystal, and Cherenkov phase-matching in both bulk crystals.

2. High Power THz Parametric Oscillation with Surface-Emitted Geometry

High power monochromatic THz-wave generation was achieved by surface-emitted (SE) TPO with MgO doped LiNbO3[7],[8], due to the high coupling efficiency of the geometry and the high damage threshold of the nonlinear medium. A high-energy, low-threshold THz-wave output has been experimentally demonstrated with an intracavity terahertz-wave parametric oscillator based on a surface-emitted configuration, which was pumped by a diode-side-pumped Q-switched Nd:YAG laser. Different beam sizes and repetition rates of the pump light have been investigated for a high-energy and high-efficiency THzwave generation. The maximum THz-wave output energy of 283 nJ/pulse was obtained at 1.54 THz under an intracavity 1064 nm pump energy of 59 mJ. A continuously tunable range from 0.75 THz to 2.75 THz was realized.

The schematic diagram of the compact intracavity TPO with the pentagonal MgO:LiNbO3crystal is shown in Fig. 1. The pump module consisted of three fifty-bar stacked diode arrays emitting at 808 nm with a repetition rate of 10 Hz and pulses width of 200 μs. The Nd:YAG rod (5 mm in diameter, 115 mm in length) was plane-parallel polishedand antireflection coated on both ends at 1064 nm. The overall pump wave cavity was formed by two flat mirrors,M1andM4. The mirrorM1was high reflectivity at 1064 nm wavelength, and theM4mirror was 30% transmission at 1064 nm. The KD*P and polarizer were used as a Q-switch and were mounted between the laser head andM1mirror. An aperture with diameters of 3 mm and 3.5 mm was used in the cavity to limit the pump beam size. The TPO cavity for the Stokes wave was formed by a pair of plane-parallel mirrors,M2andM3. It was placed in the pump wave cavity to be consistent with the noncollinear phase-matching geometry. The TPO cavity with a length of 155 mm was eudipleural and compact. The pump wave passed through the cavity at the edges ofM2andM3. The nonlinear gain medium was a 5 mol% MgO doped LiNbO3crystal. The pentagonal crystal was cut from a rectangular crystal whose dimensions were 70×46×5 mm3in thex,y,zdirections respectively, and the cutting angles are shown in Fig. 1. All the end surfaces which transmitted the pump wave were polished and uncoated.

Fig. 1. Schematic diagram of the intracavity THz parametric oscillator with surface-emitted configuration.

Fig. 2. Optical conversion efficiency for THz-wave generation increased with the diode pump energy under different pump beam diameters: (a) THz-wave output energy and (b) conversion efficiency under different pump beam sizes for the intracavity surface-emitted TPO.

Fig. 2 (a) shows the THz-wave output energy at 1.54 THz under different pumping beam sizes. The maximum output energies were 201 nJ/pulse and 283 nJ/pulse under diode pump energy of 595 mJ when the pump beam sizes were 3 mm and 3.5 mm in diameter, respectively. Fig. 2 (b) shows that the optical conversion efficiency for THz-wave generation increases with the diode pump energy under different pump beam diameters. The maximum conversion efficiencies of 3.8×10-6and 4.8×10-6were achieved under diode pump energy of 595 mJ for 3 mm and 3.5 mm pump beam spots, respectively, which corresponded to intracavity 1064 nm pump energies of 52.8 mJ and 59 mJ. The corresponding photon conversion efficiencies were 0.07% and 0.088% for the two cases, respectively. Fig. 3 shows the tunable output characteristics of the THz-wave under the intracavity 1064 nm pump energy of 48 mJ. A widely-tunable range from 0.75 THz to 2.75 THz was realized.

Fig. 3. Measured THz-wave tunable output characteristics under the 1064 nm pump energy of 48 mJ.

3. Monochromatic Cherenkov THz Source

Cherenkov phase-matching in LiNbO3[9]is a promising configuration for obtaining a high-energy high-efficiency tunable THz source, as the PM condition is automatically satisfied. The high-efficiency Cherenkov-type difference frequency generation was achieved by developing the dual-wavelength pump source, which was based on a singly resonant near-degenerated optical parametric oscillator. Total utilization efficiency of pump energy was considerably increased by recycling the residual fundamental wave. The high energy of Cherenkov-type monochromatic THz generation was achieved to be 1.58 nJ/pulse. A tuning range of THz source was 0.1 THz to 3.2 THz.

The experimental setup is shown in Fig. 4. Second-harmonic (λ0/2) and residual fundamental (λ0) waves, output from a frequency-doubled Nd:YAG laser (15 ns, 10 Hz), were separated by a dichroic mirror (DM: 1064HR/532AR). The second-harmonic waves worked as the pump wave of a singly-resonant OPO (SR-OPO), which consisted of two flat mirrors (M1andM2:T＞95% at 532 nm andR≈ 99% at 1064 nm), a potassium titanium oxide phosphate (KTP) crystal (8×7×21 mm3, cut atθ=90°andϕ=24.5°) and a Glan prism (GP). The SR-OPO operated around the degenerated point ofλ0(1064.4 nm) with type-II critical phase matching. The P-polarized signal wave (λ1) transmitted through GP (T＞98%) and resonated inside the cavity (130-mm-long). The S-polarized idler wave (λ2) was highly reflected by GP (R＞99%) and output from the cavity, which was tuned in the range of 1064.8 nm to 1090.2 nm (ϕ-angle tuning). The residual fundamental wave was converted into s-polarization by a half wave-plate (HWP) and a Brewster polarizer (BP), and mixed withλ2by a beam combiner (BC) with a ratio of 1:9 (λ0:λ2). An aperture (A, with a diameter of 3 mm) was used for the alignment to make sure the two beams were well overlapped. The combined beam passes through A and was focused by a cylindrical lens (f=100 mm) into a Si-prism coupled MgO:LiNbO3crystal (10×5×25 mm3, c-cut).

With a Si-prism coupled MgO:LN crystal, we obtained monochromatic THz-wave via Cherenkov phase-matched DFG betweenλ0andλ2. Dependence of THz outputETon the high-frequency pump energyE0was measured (at 0.78 THz) for fixed idler energyE2(=2.88 mJ). According to Fig. 5, a linear relationship is exhibited by data points (squares). The highest output energy: 1.58 nJ/pulse was achieved at the pump energy ofE0=8.65 mJ (withEF=113.6 mJ andESH=18.2 mJ). This result was 3-fold larger than the previous value[10], which even required critical control of pump incidence angle with a precision of 1°. Total energy conversion efficiency from the input fundamental wave (1064 nm) to the output THz-wave wasη1=ET/EF=1.39×10-8, and the corresponding photon conversion efficiency (including the processes of SHG, OPO, and DFG, as well as the loss at the beam combiner) was 5×10-6, which should be multiplied by 2 if we considered both the two parts of Cherenkov wedge.

Fig. 4. Schematic diagram of Cherenkov-type THz-DFG. 1: frequency doubled Nd:YAG laser; 2: singly-resonant near-degenerated OPO; DM: dichroic mirror;M1-M3: high-reflection (HR) mirrors at 1064 nm; GP: Glan prism; HWP: half-wave-plate; BP: Brewster polarizer; BC: beam combiner; A: aperture.

Fig. 5. Relationship between energies of THz outputETand high-frequency pumpE0for given low-frequency pump energyE2(=2.88 mJ). Inset: energy ofλ0versus the orientation of fast axis of HWPδ.

To explore the tunability of THz generation, we varied the idler wavelengthλ2by adjusting the PM azimuthal angleϕof KTP-OPO. A tuning range of 0.1 THz to 3.2 THz was obtained (squares in Fig. 6), which was relatively narrow, determined by the tuning characteristic of SR-OPO. First, a large PM angle was needed. As seen in the inset of Fig. 4,ϕshould be 28.8°for 3.2 THz generation, which corresponded to an external incidence angle of 8.1°. Second, the KTP crystal was un-coated. Thus, the Fresnel loss leaded to an uneven OPO tuning curve (circles in Fig. 6). The highest output energies (both THz and idler wave) were obtained at normal incidence (0.78 THz withλ2=1067.4 nm). Further improvement in the generation of high-frequencies could be achieved by AR-coating and using KTP cut at largerϕ.

Fig. 6. Tuning characteristic of THz-DFG (squares) and SR-OPO (circles) measured atESH=18.2 mJ. Inset: angle tuning curve of SR-OPO. Solid curves were calculated with the phase-matching condition. Squares were measured experimentally. The grey region between two dashed lines corresponds to the THz tuning range.

4. Widely Tunable Terahertz Source Based on Organic Crystal DSTMS

Organic nonlinear crystals have attracted considerable interests due to its large nonlinear coefficient and low dielectric constant in the THz DFG sources. Widely tunable and monochromatic terahertz (THz) difference frequency generation with organic crystal DSTMS was achieved experimentally[11]. The THz tuning spectrum covered a range of 0.88 THz to 19.27 THz. With the pump energy of 2.47 mJ, output energy reached 85.3 nJ/pulse at 3.80 THz, which corresponded to a peak power of 17.9 W and photon conversion efficiency of 3.6‰.

A schematic diagram of the experimental setup is presented in Fig. 7. An potassium titanium oxide phosphate (KTP) optical parametric oscillator (OPO), pumped by a frequency-doubled Nd:YAG laser (Quanta-Ray, Spectra Physics), worked as the source of dual-wavelength in the range of 1.3 μm to 1.6 μm. The OPO consisted of two independently controlled KTP crystals (7×7×15 mm3,θ= 65°,ϕ=0°).M1was a high-reflection (HR) mirror for the pump (532 nm), signal (0.8 μm to 0.9 μm), and idler waves (1.3 μm-1.6 μm).M2was anti-reflection (ar) for pump and idler waves and HR for signal waves. DM1and DM2were dichroic mirrors with HR@532 nm and AR@1.3 μm-1.6 μm.

Here, we used a double-pass configuration, which has been proposed previously by Taniuchi[12]et al.. Two idler wavesλ1andλ2(λ1＜λ2) illustrated the DSTMS crystals, grown by the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences[13]. Crystals with an aperture of 5 mm and different thickness were utilized. The THz wave generated via type-0 phase-matched DFG was collected by a white polyethylene lens and detected by a He-cooled Si-Bolometer. A black polyethylene film was used to filter out idler waves. Fig. 8 shows the THz pulse energy against frequency at fixedλ1=1.35 μm and dual-wavelength energy around 1.91 mJ with different crystal thicknessd= 0.5, 1.0 and 1.5 mm. A tuning range of 0.88 THz-19.27 THz was achieved.

Fig. 7. (Color on-line) sketch of experimental setup for THz DFG with DSTMS crystal.

Fig. 8. (Color on-line) output tuning spectra of THz DFG with different crystal thickness.

We also compared the THz tuning curves generated by different pump wavelength at given crystal thickness (0.5 mm) and input energy (1.91 mJ). As shown in Fig. 9, there is no remarkable discrepancy among three curves.

Fig. 9. (Color on-line) output tuning spectra of THz DFG with different pump wavelength.

Output energy versus input dual-wavelength energy was measured at 3.80 THz generation, with crystal thickness of 0.5 mm andλ1=1.35 μm (Fig. 10). Data points are well fitted by a quadratic curve, which exhibits the characteristic of second order nonlinear process. At a pump of 2.47 mJ,THz pulse energy reaches 85.3 nJ. Since the OPO pulse width is 6.74 ns, we can estimate the THz pulse width to be 4.76 ns (6.47/) and THz peak power to be 17.9 W. The energy conversion efficiency fromλ1(1.4 mJ, measured by an optical spectrum analyzer) is 6.09×10-5, which corresponds to a photon conversion efficiency of 3.6‰.

Fig. 10. (Color on-line) input-output characteristics of THz DFG.

5. Conclusions

We review the recent progress made by us on monochromatic and tunable terahertz generation based on nonlinear optics. A high power nanosecond pulsed THz-wave source was realized based on intracavity surface-emitted TPO. The maximum THz-wave output energy of 283 nJ/pulse was obtained at 1.54 THz under an intracavity 1064 nm pump energy of 59 mJ. A continuously tunable range from 0.75 THz to 2.75 THz was realized. The high energy of Cherenkov-type monochromatic THz generation was achieved to be 1.58 nJ/pulse. The tunable range of THz source was 0.1 THz to 3.2 THz. Widely tunable (0.88 THz to 19.27 THz) and monochromatic THz-wave generation via DFG in home-made organic crystal DSTMS was achieved. A linewidth around 42.7 GHz was obtained by using a double-pass OPO. The highest output energy of 85.3 nJ/pulse was achieved at 3.80 THz with pump energy of 2.47 mJ. The corresponding peak power was estimated to be 17.9 W and the photon conversion efficiency was 3.6‰.

Acknowledgment

Authors thank Dr. Yin Li (Technical Institute of Physics and Chemistry, Chinese Academy of Sciences) for the growth of DSTMS crystals.

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De-Gang Xu was born in Shandong, China in 1974. He received the B.S. degree in applied electronic technology and M.S. degree in physical electronics in 1998 and 2001, respectively, both from Qufu Normal University, Qufu, China, and the Ph.D. degree in physical electronics

from

Tianjin University,

Tianjin, China in 2005. In 2005, he joined Tianjin University as an assistant in precision instruments and opto-electronic engineering, Tianjin, China. In 2006, he was appointed as a visiting scholar at Manchester University, UK. From 2007 to now, he is an associate professor. He has published more than 40 refereed journal articles in optoelectronics, nonlinear optics, and high power laser. His current research interests include terahertz generation, amplification and detection, and their applications.

Peng-Xiang Liu was born in Shenyang, China in 1987. He received the double bachelor degree in both electronic science and technology opto-electronic from Tianjin University and finance and bank from Nankai University, Tianjin in 2009. Since September 2009, he has been working towards the Ph.D. degree with the Institute of Laser and Opto-electronic, Tianjin University, Tianjin. His interest is in terahertz source based on nonlinear optics.

Yu-Ye Wang was born in Shanxi, China in 1983. She received the B.S. degree in electronic science and technology from Tianjin University, China in 2004, and the M.S. and Ph.D. degrees in physics electronics from Tianjin University, China in 2006 and 2009, respectively. From 2009 to 2011 she worked in Tera-Photonics Lab of RIKEN, Sendai, as a postdoctoral researcher. In 2011, she joined Tianjin University as a lecturer. Her current research interests include high-power terahertz-wave sources and terahertz-wave imaging applications. She is authored and co-authored more than 30 peer-reviewed papers on journals.

Kai Zhong was born in Shandong, China in 1984. He received the Ph.D. degree in optoelectronic technology from Tianjin University in 2010. He is currently working with the College of Precision Instrument and Optoelectronics Engineering, Tianjin University. His research interests include

diode

pumped lasers, nonlinear optical frequency conversion, and optical terahertz sources.

Wei Shi received the Ph.D. degree in optical materials engineering from the State Key Laboratory of Crystal Materials, Shandong University. During 2001 to 2005, he was a researcher at the University of Arkansas, Lehigh University, and University of Arizona, USA. In 2005, he joined NP Photonics, Inc., as a principal engineer. Since 2011, he became a professor and received China“Thousand Talents Program” in Tianjin University. His current research interests include fiber lasers and amplifiers, THz technology, and laser frequency conversion. He has authored or coauthored more than 80 refereed journal articles.

Jian-Quan Yao was born in Shanghai, China, in 1939. He graduated from Graduate School of Precision Instrument Department of Tianjin University in 1965. Since 1966, he joined Tianjin University as an assistant professor. Now, he is a professor and the Director of “Institute of Laser & Opto-electronics” of Tianjin University. As a visiting scholar, he joined the Department of Appl. Phys., Stanford University, USA from 1980 to 1982. He has long been engaged in the research of all-solid laser, nonlinear optics frequency conversion and Terahertz science & technology. His theory of the precise calculation of optimum phase matching of the biaxial crystal has been called the “Yao and Fahlen Technology”. Prof. Yao has been awarded World Gold Medal of Invention, Eureka, Brussels, and National Invention Award Class II of China. In 1997, he was elected as Academician of Chinese Academy of Science.

Manuscript received October 20, 2014; revised November 29, 2014. This work was supported by the National High Technology Research and Development Program of China (863) under Grant No. 2011AA010205, National Natural Science Foundation of China under Grant No. 61172010 61101058, 61107086, and 61275120, the CAEP THz Science and Technology Foundation under Grant No. CAEPTHZ201201 and CAEPTHZ201304, the Natural Science Foundation of Tianjin under Grant No. 11JCYBJC01100 and 13ZCZDSF02300, and the Specialized Research Fund for the Doctoral Program of Higher Education under Grant No. 20120032110053.

D.-G. Xu is with the College of Precision Instruments and Opto-Electronic Engineering, Tianjin University, Tianjin 300072, China.

P.-X. Liu is with the Institute of Laser & Opto-Electronic, the College of Precision Instruments and Opto-Electronic Engineering, Tianjin University, Tianjin 300072, China. (Corresponding author email: sxtb631@126.com).

Y.-Y. Wang, K. Zhong, W. Shi, and J.-Q. Yao are with the College of Precision Instruments and Opto-Electronic Engineering, Tianjin University, Tianjin 300072, China.

Color versions of one or more of the figures in this paper are available online at http://www.intl-jest.com.

Digital Object Identifier: 10.3969/j.issn.1674-862X.2014.04.008