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A widely tunable, high-energy terahertz wave parametric oscillator based on 1 mol. % MgO-doped near-stoichiometric LiNbO3 crystal has been demonstrated with 1064 nm nanosecond pulsed laser pumping. The tunable range of 1.16 to 4.64 THz was achieved. The maximum THz wave output energy of 17.49 μJ was obtained at 1.88 THz under the pump energy of 165 mJ/pulse, corresponding to the THz wave conversion efficiency of 1.06 × 10−4 and the photon conversion efficiency of 1.59%, respectively. Moreover, under the same experimental conditions, the THz output energy of TPO with MgO:SLN crystal was about 2.75 times larger than that obtained from the MgO:CLN TPO at 1.60 THz. Based on the theoretical analysis, the THz energy enhancement mechanism in the MgO:SLN TPO was clarified to originate from its larger Raman scattering cross section and smaller absorption coefficient.
© 2017 Optical Society of America
1. Introduction
The terahertz wave (THz wave), connected the microwave region and the infrared region, is of great interest for various applications, such as biomedical imaging, material science, molecular analysis, nondestructive evaluation and food inspection. From the perspective of practical applications of THz technologies, high-energy output and wide tunability of THz sources are the particular concerns.
Terahertz parametric oscillator (TPO) based on stimulated polariton scattering in nonlinear crystals has been demonstrated as a promising THz source for its possessing the advantages of widely tunable, high energy output, compactness and room temperature operation. At present, different kinds of nonlinear crystals, such as congruent LiNbO3 (CLN), MgO-doped congruent LiNbO3 (MgO:CLN), KTiOPO4 (KTP), KTiOAsO4 (KTA) and RbTiOPO4 (RTP) crystals have been used in TPOs for obtaining the THz tuning range from 0.6 THz to 3.2 THz and from 3.1 THz to 13.5 THz except several gaps [1–5]. Especially, MgO:CLN crystal is regarded as the most popular gain medium in TPOs for its generated THz frequency range of 1 THz to 3 THz, which contains a large number of molecular fingerprinting. However, owing to the large absorption loss of THz wave within MgO:CLN crystal, several methods such as etched grating coupling, Si prism arrays coupling, cryogenically cooling, different concentration of MgO doping and surface emission have been proposed for improving the performance of THz wave output and tunability [6–10]. Generally, the tunable range is limited below 3 THz. Recently, K. Murate et al. reported a wide tunability up to 5 THz of an injection-seeded THz parametric generator (is-TPG) with MgO:SLN crystal through tilting the crystal. Compared with the conventional is-TPG, where the pump light is perpendicularly incident into the LN crystal, the center core region of the pump source was closer to the exit surface for the inclined crystal configuration. Thus, the THz waves were generated at the very near-surface of the crystal, which can reduce the absorption in the crystal efficiently [11]. The upper limit of the frequency tuning range was increased from 3 to 5 THz. But the optimal crystal inclination angle was different at each THz frequency, it is not suitable for TPO system to realize the Stokes oscillation under noncollinear phase-matching. Moreover, the conversion efficiency was decreased because the pump beam was split into a transmitted beam and a reflected beam, which made the THz output beam deteriorated at the same time. Additionally, such structure is easy to cause damage to crystal owing to the pump pulse direct incidence on the edge of the crystal. In view of the fact that, near-stoichiometric LiNbO3 (SLN) crystal is another stimulated polariton scattering active nonlinear crystal, and the absorption coefficient in SLN is 42.3 cm−1 at 1.81 THz, which is smaller than that of 56.4 cm−1 in CLN crystal [12], it is hopeful to realize widely tunable and high-energy THz output in TPO systems.
In this letter, we proposed a widely tunable, high energy output TPO based on MgO:SLN crystal with 1064 nm nanosecond pulsed laser pumping. Owing to the large Raman scattering cross section and small absorption for THz wave within SLN crystal, the tunable range of 1.16 to 4.64 THz was achieved. The obtained maximum THz wave output energy was 17.49 μJ at 1.88 THz under the pump energy of 165 mJ/pulse, corresponding to the THz wave conversion efficiency of 1.06 × 10−4 and the photon conversion efficiency of 1.59%, respectively. Moreover, under the same experimental conditions, the THz output energy of the MgO:SLN TPO was about 2.75 times larger than that obtained from the MgO:CLN TPO at 1.60 THz. The THz energy enhancement mechanism in the MgO:SLN TPO has been analyzed qualitatively based on the inherent characteristics of SLN crystal.
2. Experimental setup
The schematic diagram of MgO:SLN TPO is shown in Fig. 1(a). The pump source was a multimode Q-switched Nd:YAG laser with the repetition rate of 10 Hz and pulse width of 10 ns. The pump beam from the Nd:YAG laser was first collimated by the telescope lens T1, reducing the spot size in order to increase the power density. The attenuator M3 was used to control the incident pump intensity of the TPO under good beam quality for high conversion efficiency [13]. Then, an aperture with adjustable diameter was utilized to reduce the beam diameter to a suitable value, which was 4 mm in our experiment. The resonant cavity with the length of 220 mm was consisted of a pair of plane-parallel mirrors M1 and M2, which were both coated with high transmission (more than 98%) in the 1063 to 1064.7 nm wavelength range and high reflection in the range of 1067 to 1078 nm (R>70% at 1067 to 1070 nm, R>90% at 1070 to 1078 nm). The nonlinear gain medium was an 1 mol. % MgO-doped near-stoichiometric LiNbO3 (MgO:SLN) crystal with the composition Li:Nb = 49.6:50.4 (mol. %). Figure 2(b) shows the crystal configuration and the cutting angles. The isosceles trapezoid crystal was cut from a rectangle crystal whose dimensions were 40 mm × 20 mm × 10 mm in x, y and z directions, respectively. The angles between the base and the waist of the isosceles trapezoid is 65°, which allows the generated THz wave to be emitted nearly normal to the crystal surface without any coupler and guarantees that the pump wave and the Stokes wave are totally reflected at the crystal surface. The polarization directions of the pump wave and the Stokes wave were both along the z-axis of MgO:SLN crystal. Besides, the resonant cavity mirrors and MgO:SLN crystal were mounted on a rotating stage. Continuous frequency tuning can be obtained by rotating the stage to vary the phase-matching angle between the pump and oscillating Stokes waves inside the crystal. The THz output energy was measured by the calibrated Golay cell detector (TYDEX, Inc.: GC-1P). The calibration of Golay cell detector is specified by the manufacturer Tydex to be 86.95 kV/W at the repetition rates of 10 Hz. In order to block the injection of intense pump and Stokes waves into the detector, transmittance-calibrated black polyethylene sheet (1mm thickness) was used as the THz low-pass filter. The wavelength of the Stokes wave was measured by an optical spectrum analyzer (Agilent: 86142B).
Fig. 1 (a) The schematic diagram of the MgO:SLN TPO. (b) The crystal configuration and the cutting angles.
Fig. 2 The angle-tuning characteristics of the generated THz wave frequency and Stokes wavelength in the MgO:SLN TPO at the pump energy of 165 mJ.
3. Results and discussion
In this section, the detailed characteristics of MgO:SLN TPO was described, including tunability, output energy, energy distribution and stability. Figure 2 shows the angle-tuning characteristics of the generated THz wave and the Stokes wave in the MgO:SLN TPO at the pump energy of 165 mJ/pulse. When the external tuning angle was varied from 0.2° to 5°, the Stokes wavelength was continuously tuned from 1068.8 nm to 1082.2 nm, corresponding to the THz frequency tuned from 1.16 THz to 4.64 THz. Because of the small absorption coefficient for THz wave in SLN crystal, a new kind of wide tunability in THz high frequency region (>3 THz) was achieved in the MgO:SLN TPO. It is deduced that higher THz frequency could be obtained by improving the pump energy at larger phase matching angles. But a higher pump energy was not tried in our experiment in order to avoid damage to the MgO:SLN crystal.
Figure 3(a) shows that the comparison between the calculated THz frequencies with energy conservation law and the detected THz frequencies by a scanning Fabry-Perot etalon consisting of two thin-film THz polarizers. The THz frequencies were calculated based on the pump wavelength of 1064.4 nm and the measured Stokes wavelength of 1068.8 nm, 1071.6 nm, 1074.0 nm, 1077.7 nm, 1079.4 nm and 1080.5 nm at the tuning angles of 0.2°, 1.2°, 2.2°, 3.2°, 3.6° and 4.2°, respectively. Meanwhile, the THz wavelengths were detected as 229.7 μm, 151.2 μm, 116.6 μm, 83.1 μm and 75.3 μm at the tuning angles of 0.2°, 1.2°, 2.2°, 3.2°and 3.6°, respectively. It can be seen that the fitting straight lines for the two kinds of methods were in good agreement with each other. Besides, Figs. 3(b) and 3(c) show the examples of the Stokes wavelength measured by an optical spectrum analyzer and the THz wavelength detected by a scanning Fabry-Perot etalon at the tuning angle of 3.6°, corresponding to the frequency of 4.0 THz.
Fig. 3 (a) Comparison of the two detection methods for THz frequency. (b)The detected THz wavelength using the scanning Fabry–Perot etalon at the tuning angle of 3.6°. (c) The measured Stokes wavelength by an optical spectrum analyzer at the tuning angle of 3.6°.
Figure 4 shows the THz wave tunable output characteristics of the MgO:SLN TPO under a fixed pump energy of 165 mJ/pulse. The maximum THz pulse energy was achieved to be 17.49 μJ at 1.88 THz, corresponding to the maximum conversion efficiency of 1.06 × 10−4, the maximum photon conversion efficiency of 1.59%. And in the range of 1.61 to 2.24 THz, the THz pulse energies, the conversion efficiency and the photon conversion efficiency were larger than 10 μJ, 6.51 × 10−5 and 0.82%, respectively. The minimum THz pulse energy was 243 nJ at 4.64 THz. In addition, it is clearly seen that the THz output energies decreased gradually above 1.88 THz, which were mainly caused by the increase of the absorption coefficient with THz frequency [12]. Besides, the increase of the angle between the pump and Stokes beams leads to the low gain due to the decrease of the interaction area among the pump, Stokes and THz waves.
Moreover, the output performance from the 5 mol. % MgO-doped congruent LiNbO3 (MgO:CLN) TPO was compared with that from the 1 mol. % MgO-doped SLN TPO under the same cavity length and crystal size. Considering the maximum output of MgO:CLN TPO occurred at 1.60 THz [14], the output energies at 1.60 THz for two systems were chosen in order to make a convincing comparison. Figure 5 shows the THz output energies of the two kinds of TPOs under different pump energies at 1.60 THz. It is clearly shown that the MgO:SLN TPO had a better performance at high pump energy. The THz output energy of the MgO:SLN TPO was about 2.75 times larger than that obtained from the MgO:CLN TPO under the pump energy of 155 mJ/pulse. Besides, the threshold of 62 mJ for MgO:SLN TPO was a litter higher than that of 60 mJ for MgO:CLN TPO. It could be attributed to that the nonlinear coefficient (d33 = 23.8 pm/V) of 1 mol. % MgO-doped SLN crystal is relative smaller than that of 5 mol. % MgO-doped CLN crystal (d33 = 25.0 pm/V) [15, 16], which plays a main role in THz wave output under the low pump energy, especially around threshold. Considering the damage threshold of the MgO:CLN crystal is relatively low, the pump energy was just up to 155 mJ/pulse and a higher pump energy has not been tried further.
Fig. 5 The THz output energies of the MgO:SLN TPO and the MgO:CLN TPO under different pump energies at 1.60 THz.
Considering the responsivity of the Goaly cell under high energy incidence, the linear response should be guaranteed in order to validate the detected THz energy. By inserting different number of 0.55 mm-thickness black polyethylene sheets as attenuator in front of the Golay cell, the amplitudes of the THz output signal from two kinds of TPOs were measured under the same pump energy, as shown in Fig. 6. When the number of attenuators was increased from 1 to 5, the detected THz amplitudes for MgO:SLN TPO and MgO:CLN TPO decreased from 2920 mV to 860 mV and from 1660 mV to 490 mV, respectively. The ratio of THz output energy for two kinds of TPOs were about 1.75 times and almost unchanged under different number of attenuators. Moreover, the attenuation rate of each increased polyethylene sheet, indicated as An, was calculated based on (En- En-1)/En-1 (n = 2, 3, 4, 5). The inset in Fig. 6 shows the attenuation rate of each piece of attenuators for two kinds of TPOs were approximately same, and the attenuation rate for THz amplitude under different intensity keeps constant. It means there is a good linear response for Golay cell and the detection of high THz output from MgO:SLN TPO is reliable.
Fig. 6 The detected THz amplitude signal from two kinds of TPOs under different number of attenuators. Inset: the attenuation rate of each pieces of attenuators for two kinds of TPOs.
To explain the mechanism of the enhancement of THz energy in MgO:SLN TPO qualitatively, microscopic physical information is needed. Because THz wave gain is the monotonically increasing function of the Stokes wave gain and monotonically decreasing function of the absorption coefficient of THz wave in the nonlinear crystal [17], measurements of natural Raman scattering and the stimulated polariton scattering are convincing methods. Figure 7 shows the Raman spectra of A1-symmetry mode in 1mol.% MgO:SLN crystal and 5mol.% MgO:SLN crystal, measured by confocal Raman microscope (Renishaw: inVia). It is seen that the Raman scattering intensity of 1mol.% MgO:SLN crystal is higher than that of 5mol.% MgO:SLN crystal for the lowest order A1-symmetry mode (252 cm−1), which is important to the efficient tunable THz generation [8]. This means that 1mol.% MgO:SLN crystal has the larger Raman scattering cross section, which has a positive correlation with the Stokes wave gain. Furthermore, in order to verify the effect of Raman scattering cross section on the Stokes wave gain in MgO:SLN and MgO:CLN crystals, a mirror with 20% reflectance was inserted between the nonlinear crystal and M2 at the angle of 45°. Under the same experimental conditions of TPO cavity length of 220 mm and placements of mirrors and crystal, the reflected Stokes wave intensity from two kinds of TPOs were measured with the input pump energy of 95 mJ/pulse, as shown in Fig. 8. The Stokes wave intensity for MgO:SLN TPO is 3.16 times higher than that for MgO:CLN TPO at the wavelength of 1070.03 nm. It can be deduced that Raman scattering makes a large contribution to the increase of Stokes wave gain. In other words, larger Raman scattering intensity is effective on increasing of THz wave gain for MgO:SLN crystal.
Fig. 7 The Raman spectra of A1-symmetry mode in 1mol.% MgO:SLN crystal and 5mol.% MgO:SLN crystal. Inset: the linewidth of the lowest order A1-symmetry mode (252 cm−1) for two kinds of crystals by Lorentz fitting.
On the other hand, the linewidth of the lowest order A1-symmetry mode (252 cm−1) for two kinds of crystals was analyzed by Lorentz fitting. It can be seen from the inset of Fig. 7 that, the linewidth of the lowest order A1-symmetry mode (252 cm−1) for 1mol.% MgO:SLN crystal is narrower than that for 5mol.% MgO:SLN crystal, which is consistent with the results of Ref [18]. This is because the mode linewidth is effected by homogeneous broadening and inhomogeneous broadening [19]. The temperature-dependent homogeneous broadening is mainly on account of anharmonicities of the interionic potentials, while the inhomogeneous broadening is caused by irregularities of the site symmetry of the lattice. Deviations from stoichiometry are reflected in the inhomogeneous part. Considering the better site symmetry of the lattice in SLN crystal, the inhomogeneous broadening of mode linewidth is relative smaller than that of CLN crystal. Considering the absorption coefficient for THz wave in the LN crystal is proportional to the mode linewidth [15]. Therefore, the absorption coefficient for THz wave in MgO:SLN crystal is smaller than that in the MgO:CLN crystal, which should also enhance the THz wave gain in the MgO:SLN TPO. From the above discussion, the THz energy enhancement in the MgO:SLN TPO can be attributed to its larger Raman scattering cross section and smaller absorption coefficient qualitatively.
Furthermore, in order to evaluate the beam quality of the MgO:SLN TPO, the THz wave beam profile was measured by using two-dimensional scanning with a 1 mm diameter pinhole. Figures 9(a)-9(d) show the THz beam energy distributions at 4 cm, 5 cm, 6 cm and 7 cm far from the output surface. Figure 9(e) shows the beam pattern of the THz wave measured at 4cm from the output surface in the vertical and horizontal directions. The measured THz wave beam profile had a Gaussian distribution both in the vertical and horizontal directions. The measured data fit well to a Gaussian function (red solid lines in Fig. 9(e)). Half maximum (FWHM) beam diameters of 7.42 mm and 6.06 mm were measured in the vertical and horizontal directions, respectively. It is clearly that the measured beam profile did not have a uniform spatial distribution but showed a slightly asymmetric elliptical distribution.
Fig. 9 (a), (b), (c) and (d) THz energy distribution of MgO:SLN TPO measured at 4cm, 5cm, 6cm and 7cm from the output surface, respectively. (e) Beam pattern of the THz wave measured at 4cm in the vertical and horizontal directions.
The THz output energy of the MgO:SLN TPO at 1.88 THz was measured for 65 minutes with a time interval of 1 minute, and the results are presented in Fig. 10. Based on the root mean square (rms) error equation, the calculated output energy fluctuation was 12.5% with the average energies of 12.17 μJ. In the case of high THz wave energy output, the MgO:SLN TPO has a relatively stable performance over a period of time. Considering the multimode pump laser with pulse energy fluctuation of 1.8% and M2 = 4.21, it is deduced that the stability of TPO can be enhanced by improving the pump pulse stability and pump beam quality.
4. Conclusions
In conclusion, a widely tunable, high-energy TPO based on 1 mol. % MgO-doped near-stoichiometric LiNbO3 crystal has been demonstrated in this paper. With the external tuning angle varied from 0.2° to 5°, the THz frequency was continuously tuned from 1.16 THz to 4.64 THz. The maximum THz wave output energy was 17.49 μJ at 1.88 THz under the pump energy of 165 mJ/pulse, corresponding to the THz wave conversion efficiency of 1.06 × 10−4 and the photon conversion efficiency of 1.59%. Moreover, the THz output energy of MgO:SLN TPO was enhanced compared with MgO:CLN TPO. The THz energy enhancement in the MgO:SLN TPO can be attributed to its larger Raman scattering cross section and smaller absorption coefficient qualitatively. The energy fluctuation at around 12.17 μJ was about 12.5% over 1 hour. It is expected that such high output energy THz wave systems with widely tunability can provide good advantages and enlarge its applicable scope.
Funding
National Basic Research Program of China (973) (2015CB755403, 2014CB339802); National Key Research and Development projects (2016YFC0101001); National Key Technology R&D Program of China (2014BAI04B05, 2015BAI01B01); National Natural Science Foundation of China (NSFC) (61107086, 61471257); Natural Science Foundation of Tianjin (14JCQNJC02200); China Postdoctoral Science Foundation (2016M602954); Postdoctoral Science Foundation of Chongqing (Xm2016021); Joint Incubation Project of Southwest Hospital (SWH2016LHJC04, SWH2016LHJC01).
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