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Na3La9O3(BO3)8 (NLBO) single crystals with size up to 35 × 20 × 15 mm3 have been grown by the top-seeded solution growth (TSSG) method. The phase-matching (PM) conditions and the effective nonlinear coefficients were fully calculated for third-harmonic generation (THG) at different wavelengths. The THG experiments for NLBO crystals were performed for the first time. A 355 nm UV light output of 1.9 mW was successfully obtained under a picosecond Nd:YAG laser. Through the calculations of effective nonlinear coefficients, we believe that the output power and conversion efficiency will further increase if the high optical quality NLBO samples can be utilized.
©2012 Optical Society of America
1. Introduction
Nonlinear optical (NLO) crystals have played an important role in developing visible to ultraviolet (UV) light sources [1]. In the past few years, borate NLO crystals have been paid more attention for its excellent ability in frequency conversion since they have considerably excellent performances of suitable NLO coefficient, wide transparency range in UV wave band, high laser damage threshold, and good chemical and thermal stability [2]. Its applications prospect is optimistic. Nowadays research is focus on finding new high quality NLO crystals for UV light generation and their optical properties evaluation. Although a large number of borate crystals with good NLO properties have been developed, there are few borate NLO crystals used for third harmonic generation (THG) of high-power Nd:YAG lasers. For example, β-BaB2O4(BBO) has large NLO coefficient, having favorable condition in THG. However, its application performance is weakened by the correspondingly big walk-off angle and small acceptance angle due to a large birefringence [3]. CsB3O5(CBO) and CsLiB6O10(CLBO) can be applied in THG conversion field, however, their NLO coefficients are relatively small [4, 5]. LiB3O5(LBO) is widely used in THG effect study after many years research because it has a good THG conversion efficiency, high pumping energy, small spatial walk-off angel and large size of crystal [6]. However, its deliquescence property limits its application. Because of hygroscopic deterioration of the crystal, it must be operated at an elevated temperature. Therefore, finding new excellent NLO crystals suitable in THG application is desirable.
Na3La9O3(BO3)8 (NLBO) is a potential candidate for optical nonlinear frequency conversion [7], which possesses moderate birefringence, a wide transparency range, and a relatively large nonlinear optical coefficient. Meanwhile, it is relatively easier to grow at low cost and exhibits high chemical stability and superior mechanical properties. In addition, NLBO is a nonhygroscopic crystal and can be used in the laboratory environment without heating. Since 2005, the crystal growth [8], spectral properties [9, 10], thermal properties [11], and second-harmonic generation [12] performance have been investigated. However, the THG performance has not been reported until now.
In this paper, the THG experiments were performed on NLBO crystal for the first time. The phase-matching (PM) conditions and the effective nonlinear coefficients were fully calculated for third-harmonic generation (THG) at different wavelengths. A 355 nm UV light output of 1.9 mW was successfully generated indicating that NLBO might be a promising UV nonlinear optical material for THG applications.
2. Phase-matching
NLBO crystals with sizes up to 35 × 20 × 15 mm3 shown as in Fig. 1 have been grown by the top-seeded solution growth (TSSG) method. By using the measured Sellmeier equations and nonlinear optical coefficients [12], we calculated PM directions for THG and the corresponding effective nonlinear coefficients. The PM curves for different wavelengths for type I and II are given in Fig. 2(a) . From the calculation, we learned that NLBO is phase matchable in the region from 809.7~6780 nm for a PM-I(ooe) (thick solid line), 960~7540 nm for a PM-II(eoe) (dotted line), and 1402~4350 nm for a PM-II(oee) (dot dashed line). For the wavelength of λ = 1064 nm, the PM angle θTHG = 49.8° for a PM-I(ooe), θTHG = 65.1° for a PM-II(eoe), and whereas no phase matchable for a PM-II(oee). The corresponding effective nonlinear coefficients deff (λ = 1064 nm) for the PM-I(ooe) and PM-II(eoe) were shown in Fig. 2(b). As can be seen that the effective nonlinear coefficients are periodic variation in the azimuthal angle range 0 < ϕ < 90, and the period is 60°. The maximum peaks of deff appears at ϕ = 30° or 90° for PM-I(ooe) (solid line), and ϕ = 0° or 60° for PM-II(eoe) (dotted line). The maximum values of deff are 1.49 pm/V for PM-I(ooe), and 0.41 pm/V for PM-II(eoe), respectively, at the fundamental wavelength 1064 nm. Therefore, the optimal PM conditions can be determined as: (θ, ϕ)THG = (49.8°, 30° or 90°) for PM-I(ooe) and (65.1°, 0° or 60°) for PM-II(eoe) at the fundamental wavelength 1064 nm. The max value of deff for PM-I is about 3.6 times larger than that for PM-II, indicating that the PM-I cut samples would show more excellent THG property than those cut in PM-II conditions.
Fig. 2 (a) The PM curve of THG for different wavelengths for type I (thick solid line) and type II (dotted lines), (b) the calculated the effective nonlinear coefficients for the PM-I(ooe) (solid line) and PM-II(eoe) (dotted line).
3. Optical THG properties measurement
A NLBO crystal cut for type I PM with size of 4 × 4 × 5.5mm3 was used for THG. The cut PM angle was θ = 50° and ϕ = 30° for room temperature operation. To examine the NLBO crystal, we made a comparison with the LBO crystal in their THG properties. A 4 × 4 × 5.46 mm3 LBO crystal cut for type I PM (θ = 90°, ϕ = 37.2°) and a 4 × 4 × 10.01 mm3 LCB crystal cut for type I PM (θ = 49.4°, ϕ = 0°) were used as the references. All the samples were optically polished and uncoated. The experimental setup was shown as Fig. 3 . We used a picosecond mode-locked Nd:YAG laser (PL2140 from Ekspla, Lithuania) operating at a 1064 nm wavelength as a fundamental light source and an LBO crystal to obtain SHG light. The laser has a pulse duration of 25 ps and repetition rate of 10 Hz at the working wavelength 1064 nm. The beam diameter was minimized by a lens system to 3 mm in the measured crystals. P1 is a half wave plate to rotate the polarization of 532 nm wave in order to keep fundamental and second-harmonic polarization in the same direction. THG light of 355 nm was separated from the fundamental and SHG beams by a Brewster prism and detected by a power meter.
Firstly, the 355 nm laser output powers were measured in the three samples under the same experimental conditions, and we obtained the output of 1.9 mW at 355nm in NLBO crystal, 2.9 mW in LCB, and 3.4 mW in LBO crystal, respectively. This is the first time obtaining the 355 nm laser by THG in NLBO crystal. Then we started to measure the conversion efficiency of the samples. At the low input power, we found that the profile of the output laser at 355 nm became inhomogeneity, and the black laser damage track could be observed under an optical microscopy. The results of the efficiency of the third-harmonic wave on the total power density were shown in Fig. 4(a) . The highest efficiency of 9.3% in NLBO crystal was successfully obtained although the crystal has been damaged. The measured output power at the wavelength of 355 nm as a function of input power at 1064 nm and 532 nm was shown in Fig. 4(b). With an input power of 3.8 mW total in 1064nm and 532nm, the maximum average output power 0.26 mW of THG by using the damaged NLBO crystal is obtained. As can also be seen from Fig. 4(a, b), at the maximum fundamental power, there is no evidence of saturation in TH output power, that is to say, the output powers could be further increased by using increased fundamental power and larger the crystal lengths. The value of the laser-induced damage threshold of NLBO was measured to be more than 5 GW/cm2@1064 nm with 10 ns pulses, which is comparable with LBO (5 GW/cm2 reported by Newlight Photonics Inc. Canada) and LCB (11.5 GW/cm2) [13], and much higher than that of other inorganic NLO crystals such as KTP (0.4~2.2 GW/cm2) [14] and LiNO3 (0.1~1.0 GW/cm2). Combining the calculations of the PM and effective nonlinear coefficients, we believe that the output power will further increase if the high optical quality NLBO samples can be utilized.
Fig. 4 (a) efficiency of THG as a function of the total power density of the fundamental plus SH waves, (b) average output power at 355 nm as a function of the input power.
4. Conclusions
In conclusion, we have demonstrated UV generation at 355 nm based on a NLBO nonlinear crystal with sum-frequency generation of 1064nm and 532 nm lasers for what we believe to be the first time. For type I NLBO, a 355 nm UV light output of 1.9 mW was generated under a picosecond Nd:YAG laser, and 0.18 mW with a efficiency of 9.3% was generated under the 1064 nm pumping source using the optical damaged crystal. By the calculations of effective nonlinear coefficients, we believe that the output power and conversion efficiency will further increase if the high optical quality NLBO samples can be utilized.
Acknowledgments
This work was supported by the National Basic Research Project of China (No. 2010CB630701) and the National Natural Science Foundation of China under grant no. 50802100. Jianxiu Zhang thanks the Alexander von Humboldt Foundation for all supports during her research in Germany.
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