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La2CaB10O19 (LCB) crystals with size up to 55 × 35 × 25 mm3 have been grown by the top-seeded solution growth (TSSG) method. The refractive indices were accurately measured over the full transmission range, and the second-order nonlinear optical coefficients were determined by the Maker fringe technique. The phase-matching(PM) conditions were calculated for third-harmonic generation (THG) at different wavelengths. The THG experiments for type I and type II LCB crystals were performed. For type I LCB, a 355 nm UV light output of 5.0 mW corresponding to the conversion efficiency of 28.3% was generated under a picosecond Nd:YAG laser, and 16 W with the efficiency of 17.5% was generated under a nanosecond 1064 nm pumping source. For type II LCB, 3.5mW THG output with conversion efficiency of 21.1% was obtained under a picosecond Nd:YAG laser, and 7.6 W with the efficiency of 7.9% was generated under a nanosecond 1064 nm pumping source. The results indicated that the LCB crystal is a promising UV nonlinear optical material because of its good THG performance and nonhygroscopicity.
©2011 Optical Society of America
1. Introduction
355 nm laser sources offer many advantages over 1064 nm and 532 nm for laser processing and scientific research. The performance of solid-state 355 nm laser depends greatly on the reliability of nonlinear optical (NLO) crystals. A good crystal for third harmonic generation (THG) should have these features including high transparency at 355 nm, large NLO coefficient, phase matching at 355 nm, high laser damage threshold, as well as stable physical, chemical and mechanical performance. LiB3O5(LBO) is most widely used in THG effect study after many years research, because it has a good THG conversion efficiency, small spatial walk-off angle and high optical damage threshold. In 2010, The 35 W average power of all-solid-state 355 nm laser has been obtained with the optical efficiency up to 35.8% in LBO [1]. In 2008, the 103 W 355 nm THG output in CsB3O5(CBO) was achieved by D. Rajsh et al. [2]. However, their deliquescence property limits its application although the conversion efficiency is higher than that in LBO. 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 due to a large birefringence [3]. It is worth mentioning, 0.635mJ 355 nm UV light has also been achieved in BaAlBO3F2(BABF) [4] crystals, which has relatively large NLO coefficient and nonhygroscopicity.
La2CaB10O19 (LCB) was discovered in our group in 1998 [5]. The growth and structure of LCB was firstly reported in 2001 by Y.C. Wu et al. [6]. LCB crystal possesses moderate birefringence, a wide transparency range, a relatively large nonlinear optical coefficient, and a high laser damage threshold up to 11.5 GW/cm2 [7]. Meanwhile, it is relatively easier to grow large size crystal and exhibits high chemical stability and superior mechanical properties. In addition, LCB is a nonhygroscopic crystal and can be used in the laboratory environment.
In this paper, the THG properties and hygroscopicity of the LCB with that of LBO under the same experimental conditions were directly compared. The THG experiments for type I and type II LCB were also performed. In addition, we also report the accurate measurements of refractive indices, DUV transmittance spectrum lower to 120 nm, second-order nonlinear optical coefficients, and PM conditions of pure LCB crystals.
2. Crystal Growth and Hygroscopicity
Because of the melting incongruently, LCB could be grown by the top-seeded method from the flux system of Li2O-CaO-B2O3. As previously reported [7], the seed direction is important in high transparent LCB crystal growth. We selected the slightly deviating [110] direction as seed growth direction, and obtained a rhombic-shaped high optical quality LCB with sizes up to 50 × 28 × 28 mm3 shown as Fig. 1(a) . We note that the c axis of the crystal is almost up to 30mm, which is propitious to obtain longer PM devices in xz principal plane.
Fig. 1 (a) LCB crystal grown along deviating [110] direction. (b)Etching photos of LBO and LCB in water.
To compare the hygroscopicity of LCB and LBO crystals, we dipped two polished samples of LCB and LBO into a bottle of water at room temperature for 24 hours. Figure 1(b) shows the etching photos of LBO and LCB. We can see that the LCB has the better nonhygroscopicity performance than LBO.
3. Measurements of the DUV Transmittance Spectrum and Refractive Indices
The transmittance spectrum over the range of 185-3000 nm has been measured previously by F. Jing et al., and they found that it still has a high transmittance at the wavelength of 185 nm [7]. In this paper, we measured the transmittance spectrum in the DUV region from 120 to 380 nm at room temperature by a VUVaS2000 (McPherson) spectrophotometer. The thickness of the sample was 1.78 mm. Figure 2(a) shows the transmission spectrum of the grown LCB crystal. As can be seen from the spectrum, the cut edge is 170 nm in the DUV range, indicating that LCB may be used in THG application.
Fig. 2 (a) Transmission spectrum of LCB crystal in UV-DUV region. (b) The fitted dispersion curves of the LCB prism over the full transmission range.
The refractive indices have been measured previously by X. Guan et al., at visible light range from 404.7 to 700 nm [8]. In this paper, we refined the results by the accurate refractive index measurement system (SpectroMaster UV-VIS-IR, Trioptics, Germany) at 13 different wavelengths from 253.7 to 2325 nm. LCB crystallizes in the monoclinic system and belongs to biaxial crystal. In this experiment, two right-angle prisms with apex angle 30° were cut and kept at 21°C during the measurement. Using the two prisms we can obtained the principal refractive indices nx, ny, nz, as well as the refractive indices na`. With nx, nz, and na`, the angle between the crystallographic axes a and the principal refractive indices nz was calculated to be ~45.5°. The experimental values with a high accuracy of 1 × 10−5 are listed in Table 1 . The refractive indices of nx axis and ny axis are approximate, which is coincides with the former report [8], determines that LCB crystal has a tendency to uniaxial crystal.Figure 2(b) shows the fitted dispersion curves of the LCB prism over the full transmission range, and the Sellmeier's equations fitted by the least squares method were given as following:
Table 1. The Refractive Indices of Biaxial Crystal LCB
4. Measurement of the NLO Coefficients
In 2002, G. Wang et al., derived the deff of LCB to be 1.05 pm/V at the wavelength of 1064 nm using phase-matching method [9]. As above mentioned, LCB is a positive biaxial optical crystal with space group C2 and has four non-zero independent SHG coefficient, i.e. d 21, d 22, d 23, and d 14, which were measured to be 0.433, 0.484, 0.437 and 0.640 pm/V, respectively, by F. Jing et al., in 2008 [7]. However, our laser experiments indicated that the results were not accurate. Here we re-measurement the four coefficients by the Maker fringe technique. In the experiment, four samples were uncoated and cut along different directions: a plate with sizes 8 × 8 × 1.78 mm3 along crystallography directions a-, a plate along optical axis Z with sizes 5.5 × 5.0 × 0.75 mm3, A plate along optical axis Y with sizes 7 × 7 × 2.16 mm3, and one plate along optical axis X with sizes 10 × 12 × 1.90 mm3, respectively. As an example, Fig. 3(a) shows the orientation of the Y-cut LCB sample to measure the Maker fringes of d 14, the E ω is the fundamental light and the E 2ω is the SH light. The type-I Maker fringes of d 14 was shown in Fig. 3(b), where the solid and dashed curves represent the experimental and calculated values, respectively. The d 14 coefficient of LCB crystals relative to d 36 (KDP) was then derived as d 14(LCB) = (1.79 ± 0.12)d 36 (KDP) = (0.70 ± 0.05) pm/V. Another three coefficients were also derived as d 21(LCB) = (1.48 ± 0.14)d 36 (KDP) = (0.58 ± 0.06) pm/V, derived as d 22(LCB) = (−2.66 ± 0.07)d 36 (KDP) = (−1.04 ± 0.03) pm/V, derived as d 23(LCB) = (0.65 ± 0.05)d 36 (KDP) = (0.25 ± 0.02) pm/V, respectively.
Fig. 3 (a) Orientation of the c-cut LCB crystal to measure the Maker fringes of d 22; the (E)ω is the fundamental light and the (E)2ω is the SH light. (b) (Color online) Experimental Maker fringe (type-I) of d 22(solid curve); theoretical fringe and theoretical envelope(dashed curves).
5. Phase-Matching
By using the above measured Sellmeier equations and nonlinear optical coefficients, we calculated PM directions for THG and the corresponding effective nonlinear coefficients. The PM curve for different wavelengths for type I and II is given in Fig. 4(a) and 4(b). From the calculation, we learned that LCB is phase matchable in the region from 790~4530 nm for a PM-I (Fig. 4(a)), 950~4150 nm for a PM-II(eoe) (Fig. 4(b)), and whereas no phase matchable for a PM-II(oee).
6. Optical THG Properties Measurement
A LCB crystal cut for type I PM with sizes of 4 × 4 × 10.01mm3 is used for THG. The PM angle is θ = 49.4° and Φ = 0° for room temperature operation. To examine the LCB crystal, we made a comparison with the LBO crystal in their THG properties. A 4 × 4 × 9.98mm3 LBO crystal cut for type I PM (θ = 90°, Φ = 37.2°) is used as a reference. Both the samples are optically polished and uncoated. The experimental setup is shown as Fig. 5 . 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. P1 is a half wave plate to rotate the polarization of 532nm wave in order to keep fundamental and second-harmonic polarization in the same direction. THG light of 355nm is separated from the fundamental and SHG beams by a prism and detected by a power meter.
The results of the efficiency of the third-harmonic wave on the total power density were shown in Figs. 6(a) and 6(b). In the same experimental condition, we obtain the highest efficiency 28.3% in LCB crystal and 34.3% in LBO crystal. This is the first time obtaining high efficiency of THG in LCB crystal. With an input power of 18.1 mW total in 1064nm and 532nm, an output of 5.0 mW at 355nm generated by using LCB crystal. The highest peak power density is almost 1.5 GW/cm2.
Fig. 6 Efficiency of THG and output power as a function of the total power density of the fundamental plus SH waves. (Type I).
The Type II THG performance of LCB was also been examined. The LCB was cut for type II PM with a size of 4 × 4 × 8.18 mm3 (θ = 65.3°, φ = 66°)and another LBO crystal cut for type II PM with size of 4 × 4 × 8.18mm3 (θ = 42.6°,φ = 90°) were prepared. The results of the efficiency of the third-harmonic wave on the total power density were shown in Fig. 7 . In the same experimental condition, we obtain the highest efficiency of 21.1% in LCB crystal and 22.7% in LBO crystal. With an input power of 18.3 mW total in 1064nm and 532nm, an output of 3.8 mW at 355nm generated by using LCB crystal. The highest peak power density is almost 1.3 GW/cm2. Owing to the high laser damage threshold of LCB, there is no optical damage or gray track in LCB crystal during the Type I and Type II THG experiment.
To obtain the higher average power of third harmonic laser output in LCB, we changed the fundamental light source as a diode-pumped Nd:YAG laser with a pulse duration of 10 ns and a repetition of 10kHz. The maximum average output power of 1064 nm is about 140 W. SHG is realized with a non-critically phase-matched LBO crystal. The second harmonic at 532 nm was mixed with the fundamental light of 1064 nm in a LCB crystal with 16 mm length to generate 355 nm radiation. The beam diameter was minimized by a lens system to 0.5 mm in the LCB crystal.
Figure 8 shows the measured output power at the wavelength of 355 nm as a function of input power at 1064 nm and 532 nm. Both the THG crystals of LCB (type I) and LBO (type I) are used at room temperature. Using the same experimental conditions, the maximum average output power 16 W of THG by using LCB and 19.6 W with LBO is obtained. The maximum conversion efficiency 17.5% and 18.7% is obtained for LCB and LBO, respectively. The energy conversion efficiency P355 nm/P1064+532 is 13.3%. To our knowledge, this output power is the highest one reported to date for LCB. During our experiments, no surface or internal damage was found in the LCB crystal. Since the average output power at 355 nm increased continuously with the input power in the experimental range (from 0 to 133 W), we believe that the output power will further increase if the high optical quality LCB samples can be utilized.
As above, the THG laser performance of LCB was also been performed. One LCB crystal cut for type II PM with a size of 4 × 4 × 8.18 mm3, another LBO crystal cut for type II PM with size of 4 × 4 × 8.18mm3 were prepared. The result of the efficiency of the third-harmonic wave on the total power density is shown as Fig. 9 . In the same experimental condition, we obtain the highest efficiency 7.9% in LCB crystal and 10.8% in LBO crystal. With a pumping energy of 104 W total in 1064nm and 532nm, an output of 7.6 W at 355nm was generated by using LCB crystal. Owing to the high laser damage threshold of LCB, there is no optical damage or gray track in LCB crystal during our experiment.
7. Acceptance Angle and Acceptance Temperature of THG
Using the experimental setup shown as Fig. 5, both of the acceptance angle and acceptance temperature of LCB were measured. Figure 10(a) –10(e) shows the acceptance angle and acceptance temperature for type I (θ = 49.4° and Φ = 0°) and type II (θ = 65.3°,φ = 66°) of LCB, respectively. From the measurement results, we learned that LCB has the moderate angular and temperature acceptance beyond some borate NLO materials.
Fig. 10 (a) The acceptance angle, (b) acceptance temperature for type I of LCB, (c) the acceptance angle θ, (d) the acceptance angle φ, and (e) the acceptance temperature for type II of LCB.
8. Conclusions
In conclusion, we have demonstrated UV generation at 355 nm based on a LCB nonlinear crystal with sum-frequency generation of 1064nm and 532 nm lasers for what we believe to be the first time. For type I LCB, a 355 nm UV light output of 5.0 mW corresponding to the conversion efficiency of 28.3% was generated under a picosecond Nd:YAG laser, and 16 W with a efficiency of 17.5% was generated under the 1064 nm pumping source. For type II LCB, a 355 nm UV light output of 3.5 mW corresponding to the conversion efficiency of 21.1% was generated under a picosecond Nd:YAG laser, and 7.6 W with the efficiency of 7.9% was generated under a nanosecond 1064 nm pumping source. The results indicated that the LCB crystal is a promising UV nonlinear optical material.
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.
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