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

Monochromatic and Tunable Terahertz Source Based on Nonlinear Optics

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

—Recent progresses made by authors on monochromatic and tunable terahertz (THz) generation based on nonlinear optics are reviewed, including THz parametric oscillation (TPO) and difference frequency generation (DFG). From the technical point of view, we develop extra- and intra-cavity surface-emitted TPO, as well as DFG with QPM-GaAs crystal. From the point of view of mechanism, Cherenkov phase-matching is comprehensively investigated in both bulk crystal and planar waveguide. A novel scheme for cascading enhanced Cherenkov DFG in waveguide is proposed. From the point of view of material, organic crystal 4-N,N-dimethylamino-4’-N’-methyl-stibazolium tosylate (DAST) is utilized as the nonlinear medium.

Index Terms—Difference frequency generation; nonlinear optics, parametric oscillation, terahertz source.

1. Introduction

Terahertz (THz) technology has various applications[1]. One of its unique properties is that the transition frequencies between vibrational and rotational states of molecules (fingerprint spectrum) are mainly in THz regions. Monochromatic and tunable coherent THz source is much needed, especially for the testing of low-pressure gases, because of its narrow linewidth with high spectral resolution. A proven approach for monochromatic THz generation is based on second-order nonlinear optical effect, e.g. THz parametric oscillation (TPO) and difference frequency generation (DFG), due to following advantages: availability of both 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 (eg. 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 monochromatic and tunable THz generation based on nonlinear optics, including extra- and intra-cavity surface-emitted TPO, DFG with QPM-GaAs crystal, Cherenkov phase-matching in both bulk crystal and planar waveguide, and also DFG with DAST crystal.

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

High power monochromatic THz-wave generation was achieved by surface-emitted TPO with MgO doped LiNbO3[7], due to the high coupling efficiency of the geometry and the high damage threshold of the nonlinear medium.

The schematic diagram of experimental setup was shown in Fig. 1: an extracavity TPO pumped by a Q-switched Nd:YAG laser. In our experiment, the pump beam was controlled using an aperture with diameters of 3 mm, 4.5 mm, and 5 mm. A pentagonal 5 mol% MgO doped LiNbO3crystal was used as nonlinear gain medium. THz-wave was emitted perpendicularly to the surface of crystal via noncollinear phase matching. The cavity mirrors and the crystal were mounted on a rotating stage. Frequency tuning was achieved by rotating this stage. THz output energy was detected by a Golay Cell (with a calibration to be 137.71 kV/W).

Fig. 2 shows the THz output energy varied with pump intensity under different sized pump beam at 1.56 THz generation. The threshold decreased with a larger pumpbeam size, because the interaction region increased. The maximum energies of THz-wave were about 365 nJ, 438 nJ, and 328 nJ with the diameters of 3 mm, 4.5 mm, and 5 mm, respectively. When the diameter of pump beam was 4.5 mm, the maximum energy conversion efficiency was 1.94×10?6at the pump intensity of 1.4 J/cm2. Considering the THz pulse width (estimated to be 7 ns), the peak power could be about 62.6 W. For the 5 mm-diameter pump beam, the maximum pump intensity was limited by the damage threshold of the nonlinear crystal. So we did not increase the pump intensity over 1.1 J/cm2for the 5 mm-diameter pump source. But with the data we achieved, larger sized pump source had a higher output of THz-wave, at the same pump intensity can be concluded.

Fig. 1. Schematic diagram of extracavity surface-emitted TPO.

Fig. 2 THz-wave output characteristics with different diameter of pump at 1.56 THz generation

Fig. 3. THz tuning curve with a diameter pump of 4.5 mm and pump intensity of 1.03 J/cm2.

The tuning characteristics of TPO were presented in Fig. 3. By varying the phase matching angle, a tuning range of 1 THz to 3 THz was obtained. The maximum output was about 195 nJ/pulse at 1.56 THz when the pump intensity was 1.03 J/cm2.

3. Intracavity THz Parametric Oscillation with Surface-Emitted Geometry

We also developed an intracavity TPO with surfaceemitted geometry inside Nd:YAG laser[8]. The schematic diagram is shown in Fig. 4. An aperture with diameters of 3 mm and 3.5 mm was used to limit the pump beam size. The nonlinear gain medium was a 5 mol% MgO doped LiNbO3crystal, which was cut at a dimension of 70×46×5 mm3in thex,y,zdirections, respectively. Continuous frequency tuning can be obtained by rotating the stage to vary the phase-matching angleθbetween the pump and oscillated Stokes lights inside the crystal. The THz-wave was focused into a calibrated Golay cell detector by a white polyethylene lens.

Fig. 4. Schematic diagram of the intracavity TPO with surface-emitted configuration.

Fig. 5 (a) shows the THz-wave output energy at 1.54 THz under different pump beam size. The THz-wave energy increases with the pump energy. The maximum output energies were 201 nJ/pulse and 283 nJ/pulse under the diode pump energy of 595 mJ when the pump beam sizes were 3 mm and 3.5 mm in diameters, respectively. Higher pump power density did not try to avoid the damage to MgO:LiNbO3crystal. The threshold 1064 nm pump energies in the cavity were about 13.7 mJ/pulse and 12.9 mJ/pulse with the pump beam diameters of 3 mm and 3.5 mm, respectively, which are much lower than that of extracavity surface emitted TPO[5]. Moreover, we consider the threshold can be further reduced with coated MgO:LiNbO3crystal and shorter cavity length due to the low loss in the cavity.

Fig. 5 (b) shows the optical conversion efficiency for THz-wave generation increased 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 the diode pump energy of 595 mJ for 3 mm and 3.5 mm pump beam spots, respectively, which corresponded to the intracavity 1064 nm pump energies of 52.8 mJ and 59 mJ. The corresponding photon conversion efficiencies were 0.07% and 0.088% for two cases, respectively.

Fig. 5. Under different pump beam size for the intracavity surface emitted TPO: (a) THz-wave output energy and (b) conversion efficiency.

Fig. 6. THz tuning characteristic under the 1064 nm pump energy of 48 mJ.

The tunable output of THz-wave was obtained through rotating the TPO cavity mirrors ofM2andM3simultaneously. Fig. 6 shows the tuning characteristic under the intracavity 1064 nm pump energy of 48 mJ. We set the pump beam size of 3 mm and the repetition rate of 10 Hz. Wide range tunability from 0.75 THz to 2.75 THz was observed. The maximum output was achieved at 1.54 THz. The frequency of the THz-wave was calculated using the law of energy conservation with a pump wavelength of 1.0644 μm and the measured Stokes wavelength of 1.0673 μm to 1.0749 μm.

4. Difference Frequency Generation with Quasi-Phase-Matched GaAs

Zinc-blende crystal (e.g. GaAs and GaP with low absorption in THz region) can be used for THz difference frequency generation via quasi-phase-matching (QPM). In the experiment, we used QPM-GaAs crystal pumped by dual-wavelength pulses around 2 μm[9].

The schematic diagram is shown in Fig. 7. The dual-wavelength source is the intracavity optical parametric oscillator (OPO) pumped by a Q-switched Nd:YAG laser. The OPO works near the degenerate point of 2.128 μm with the repetition rate of 10 Hz. It consists of two identical uncoated KTiOPO4(KTP) crystals (7×8×15 mm3, cut at θ = 49.5°, ? = 0°) to reduce the walk-off effect and improve the beam overlap area of the output signal and idler waves (type-II phase-matching).

We used a QPM-GaAs sample made of 6 individual GaAs plates with type-II QPM for DFG. The QPM-GaAs sample had an aperture of 10×10 mm2, a length of 4.8 mm, and QPM period of 650 μm. QPM-GaAs fabrication also involves separate [110] wafers of GaAs which are brought together with a 180°-rotation about [110] between neighboring wafers. The wafers were contacted with each other directly without heating after optical polishing. The generated THz-wave was detected with a 4 K Si-bolometer after focusing with a white polyethylene lens (L) and filtered by a germanium wafer coated for high reflection (HR) at 1.8 μm to 2.5 μm.

Fig. 7. Schematic diagram of the THz source with QPM-GaAs: (a) experimental setup and (b) the twin-crystal walk-off compensated arrangement.

Fig. 8 (a) shows THz-wave tuning curves with external angle of KTP crystals. Tunable and coherent THz radiation from 0.06 THz to 3.34 THz is achieved when the two pump waves are in the range of 2.101 μm to 2.127 μm (o-wave) and 2.1557 μm to 2.129 μm (e-wave), respectively.

Fig. 8. THz tuning characteristic with QPM-GaAs: (a) output frequency versus PM angle and (b) energy versus THz frequency.

Fig. 8 (b) shows the measured THz-wave output energy versus frequency. Considering the transmittance of the focusing lens, germanium filter and THz attenuators (Microtech Instruments Inc.), the maximum output energy is 45 nJ at the frequency of 1.68 THz when the pump energy of double wavelength laser is 9 mJ with pulse width of 4.2 ns and repetition rate of 10 Hz. The energy conversion efficiency is 5×10?6with the photon conversion of about 0.08%. The maximum peak power is about 10 W referring to the pulse-width of the pump wavelength.

5. Cherenkov Phase-Matched Difference Frequency Generation with Bulk LiNbO3Crystal

Cherenkov phase-matching in LiNbO3[10]is a promising configuration, which can provide a wide tuning range without inherent dip[11], as the PM condition is automatically satisfied. We did further investigation of this phenomenon both theoretically[12]and experimentally.

A sketch of Cherenkov phase-matched THz DFG was shown in Fig. 9. To develop a theoretical model of this phenomenon, we consider an experimental situation: dual-wavelength ns-pulses tightly focused by a cylindrical lens into LiNbO3crystal.

THz electric fieldETshould obey this wave equation:

where ε is the relative dielectric function of ωTinside the crystal, andPNLis the nonlinear polarization at difference frequency ωT.

Fig. 9. Schematic diagram of Cherenkov phase-matched THz DFG: (a) wave-front and (b) wave-vector.

With undepleted pump approximation at low conversion efficiency, the source termPNLhas constant amplitude. To take the diffraction of THz radiation into account, we assume the THz field is with a form of Fourier expansion:

here ～ denotes quantities in spatial Fourier domain,βTandgare longitudinal and transverse components of THz-wave vector, respectively. As the base of Fourier space is an orthogonal function system, we finally derived the expression of Cherenkov-type THz electric field:

wheredeffis the effective nonlinear coefficient of LiNbO3,E0is the amplitude of the pump electric field,kg= ωTng/cis the group wave vector,ngis the group refractive index of the dual-wavelength pump pulse, factorG~(g)= [r/(8π)1/2]exp(?g2r2/8) is the Fourier transformation of beam intensity profile: exp(?2x2/r2), andg*=ωT(ε?ng2)1/2/c.

Such information can be provided by (3). First, the radiation pattern consists of two plane waves in two directions around pump laser path, described by the argument of the sine factor. Second, the transverse phase mismatch, due to the size of pump beam[13], is described by factorG(g?), which falls with THz frequency and pump beam size.

Calculated tuning curves for different pump beam width and spatial distribution of THz electric field were presented in Fig. 10. Theoretical result reasonably agrees with the experiment[13]. The shape of curves and dependence on pump beam width were well reflected by our model.

Fig. 10. Calculated tuning curves for different pump beam width. Inset: calculated spatial distribution of THz electric field.

Fig. 11. With angled-cut (squares) and Si-prism (circles) geometry: (a) sketch of experimental set up for Cherenkov-type THz DFG and (b) output spectrum of Cherenkov-type THz DFG.

Cherenkov-type THz DFG was achieved experimentally, with angled-cut and Si-prism coupled LiNbO3crystals, as shown in Fig. 11 (a). A walk-off compensated near-degenerated OPO (similar to Fig. 7 (a)) was utilized as the source of dual-wavelengths pump wave, which can provide high pump beam quality.

THz output spectrum with pump energy of 7.8 mJ is illustrated in Fig. 11 (b). The tuning range, achieved with Si-prism geometry, is 0.15 THz to 5.5 THz, which is 0.1 THz to 4.8 THz with angled-cut crystal. Due to the low absorption and dispersion of Si, the tuning range with Si-prism geometry is broadened and the peak value moves toward higher frequency. At 1.65 THz, the maximal output energy is 0.56 nJ, which is 77% enhanced.

6. Widely Tunable Cherenkov-Type THz Generation in a Planar Waveguide

In the case of bulk-crystal-based Cherenkov DFG, there is a contradiction between transverse phase mismatch due to the size of pump beam (restricts the generation at high frequencies) and severe divergence of tightly focused pump beam (greatly limits the effective radiation length). It can be resolved by adopting sandwich-like waveguide structure, which was first proposed by Bakunovet al.[14]. Later, it was applied to Cherenkov-type THz DFG and achieved an extremely wide tuning range of 0.1 THz to 7.2 THz[15]. Wefurther developed a theory of investigating waveguidebased Cherenkov THz DFG[16].

A sketch of Cherenkov-type THz generation in a sandwich-like waveguide is shown in Fig. 12. Dualwavelength pump laser was coupled into the nonlinear core (with a thickness of 2dinxdirection) by a cylindrical lens and propagates inzdirection as a guided mode.

To obtain an analytical expression, we also utilized the undepleted pump approximation. Based on the planar waveguide theory, THz field should be in the form of:

whereκandβTare transverse and longitude propagation constant, subscriptsc,s, andpdenote core, substrate and clad, respectively,F(x) is the forced-wave responds, and the other four terms are free-wave responds. The coefficients in (4),C1toC4, can be derived from boundary condition atx=±d.

With (4), we can perform numerical calculation and analyze the characteristic of this kind of THz source. Spatial distribution of THz electric field with three typical substrate materials: non-doped LN, BK7 glass, and quartz are presented in Fig. 13 (a), (b), and (c), respectively. As seen in Fig. 13 (a) and (b), the attenuation in non-doped LN and BK7 substrates is caused by the absorption of materials. In quartz substrate (Fig. 13 (c)), THz electric field is in the form of evanescent waves.

Tuning curves with non-doped LN and quartz substrate for different core thickness are shown in Fig. 14 (a) and (b), respectively. In Fig. 14 (a), the tuning curve for 2d= 3.8 μm is free from structure dips in the whole frequency band, and has a shape reasonably agree with the experimental result (Fig. 3 in [15]). As the dielectric function of substrate εsdecreases, tuning curves becomes more uneven. More dips and sharp peaks arise in Fig. 14 (b), because reflection at core-substrate interface is enhanced and interference (both destructive and constructive) becomes important. It can be seen that non-doped LN substrate can provide flat tuning spectrum, and quartz substrate is suitable for target frequency generation with total reflection enhancement.

7. Cascading Enhanced Cherenkov Difference Frequency Generation with a Waveguide

We also proposed a new scheme that combines the cascaded DFG, Cherenkov phase matching, and waveguide structure[17]. In this configuration, THz leaky mode (radiated mode) is with the same longitudinal phase velocity as that of the pump laser (phase matching condition is satisfied). Meanwhile, the effective length of structure is not influenced by the high absorption of LN-core, owing to its surface-emitting geometry. Thus, the long interaction length and high pump intensity in the core make it possible for cascaded frequency downconversion[18], which will overcome the quantum-defect related limitations and finally improve the efficiency of THz DFG.

Fig. 14. Tuning curves at different core thickness with (a) non-doped LN (a) and (b) quartz substrates.

Fig. 15. Schematic diagram of the guided-wave Cherenkov-type THz DFG and the cascaded frequency down-conversion process.

As illustrated in Fig. 15, the THz-wave is generated in the interaction process between two incident optical waves, which consumes the high-frequency pump (ωj) and amplifies the low-frequency component (ωj+1). According to Cherenkov phase-matching condition, the THz-wave propagates in the structure as a radiated (leaky) mode, partly coupled out by the Si-clad (dashed arrow). Inside the LN-core, however, the low-frequency beam (ωj+1) can also work as a high-frequency pump, which amplifies the THz leaky mode and produces a new optical frequency (ωj+2), because of their close longitudinal phase velocities. Then, a cascading process will sequentially occur (shown at the bottom of Fig. 15).

The energy conversion dynamics in this configuration can be described by the following coupled-wave equations:

where subscriptsj(=1, 2, …) denote different order pump waves,A(z) is the slowly varying complex amplitude,G(x) is the normalized modal distribution,n=βc/ωis the effective refractive index in the waveguide,αis THz absorption coefficient, andδis the leakage coefficient. The operator is defined as:

By solving (5) numerically with the Runge-Kutta method, we can get the variation of the pump spectrum during the process of the cascaded DFG, as shown in Fig. 16 with original pump wavelengths of 1060.7 nm and 1068.2 nm (two dashed lines). The main branch on the right of the dashed lines reflects a cascaded red shift.

The dynamics of the THz generation for both the cascaded (solid) and non-cascaded DFG (dashed) is presented in Fig. 17. Two conclusions can be drawn by comparison between two curves. The cascading effect not only enhances the THz amplitude but also increases the effective interaction length. It should be noted that the total output power in the Cherenkov configuration is proportional to the area between the curve and the horizontal axis. Total THz power could be nearly 8 times enhanced by cascading effect.

Fig. 16. Energy fluence among pump waves during the process of cascaded DFG.

Fig. 17.z-dependence of generated THz intensities inside LN-core for cascaded (solid) and non-cascaded DFG (dashed).

8. Difference Frequency Generation with Organic Crystal

An organic salt: 4-N,N-dimethylamino-4’-N’-methylstibazolium tosylate (DAST) is a promising candidate for widely tunable THz-wave generation, due to its large nonlinear coefficient (d11=260 pm/V) and small dielectric constant. In our experiment, we used DAST for THz DFG between two wavelengths in the range of 1.32 μm to 1.46 μm, generated by an OPO.

The experimental setup is shown in Fig. 18. A dual-wavelength OPO, pumped by a frequency doubled Nd:YAG laser, works as the pump source of DFG. It consists of two identical KTP crystals (7×7×15 mm3, θ=65°, ?=0°), which are tuned independently. DAST crystals are nonpolished, grown by the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences[19]. The THz-wave is generated collinearly with the pump light, collected by a white polyethylene lens and detected by a He-cooled Si-Bolometer. THz tuning curves with different pump wavelengths are presented in Fig. 19 (a). The shapes of three curves are almost the same. The highest amplitude was obtained at λ1=1359.4 nm, because KTP-I is normal incident and pump energy is higher (Fig. 19 (b)).

Fig. 18. Experimental setup of THz-DFG with DAST crystal.

Fig. 19. Tuning curves with different pump wavelengths: (a) THz and (b) dual-wavelength.

Tuning curves with different thicknesses of DAST are shown in Fig. 20 (λ1=1359.4 nm). The highest output energy is 10.9 nJ (3.55 THz) and the tuning range is 1.37 THz to 19.34 THz.

9. 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 extracavity surface-emitted TPO. The maximal THz output energy of 438 nJ (peak power of 62.6 W) was achieved. A low-threshold compact intracavity TPO was developed, with the highest output energy of 283 nJ/pulse and a tuning range of 0.75 TH to 2.75 THz. With 2 μm pumped QPMGaAs, we obtained THz pulse energy of 45 nJ with a tuning range of 0.06 THz to 3.34 THz. Theoretical models of Cherenkov phase-matched DFG in bulk crystal and planarwaveguide were developed, which can well explain the previous experimental results. Experimentally, Cherenkovtype THz DFG was achieved in bulk LiNbO3crystal, with a tuning range of 0.1 THz to 5.5 THz. A scheme for THz generation via cascading enhanced Cherenkov-type DFG in a waveguide is proposed, which was elucidated by developing a coupled-mode theory. The novel scheme has the potential to overcome the quantum-defect limit. An extremely wide tuning range of 1.37 THz to 19.34 THz was obtained with DAST crystal.

Fig. 20. Tuning curves with different thicknesses of DAST.

Acknowledgment

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

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De-Gang Xuwas 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, Shandong, 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 of precision instruments and opto-electronic engineering, Tianjin, China. In 2006, he was appointed as visiting scholar at Manchester University, UK. Since 2007, he has been an associate professor at Tianjin University. 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 Liuwas born in Liaoning, 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 Wangwas 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 source and terahertz-wave imaging application. She is authored and co-authored more than 30 peer-reviewed journals.

Kai Zhong wasborn 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.

Jian-Quan Yaowas 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 and lecture. 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 precise calculation of optimum phase matching of 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.

ceived 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 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.

Manuscript

October 25, 2013; revised November 24, 2013. 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 Institute of Laser & Opto-Electronic, 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 Institute of Laser & Opto-Electronic, 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.2013.04.002