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New nonlinear optical (NLO) crystals TmCa4O(BO3)3 (TmCOB) were grown by the Czochralski pulling method, and the anisotropy of second-harmonic-generation (SHG) properties were characterized. Based on the ratio of the peaks of the 2ω signals produced by TmCOB and that of KTP crystal samples at the low fundamental energy, the NLO tensor coefficients d12, d32, d31 and d13 were determined and found to be 0.24, 1.70, −0.55 and −0.32 pm/V, respectively. At 1064 nm, the phase-matching (PM) curves and the effective NLO coefficients (deff) in spatial distribution were evaluated. Efficient SHG was realized on a (32.5°, 180°)-cut TmCOB sample (4 × 4 × 11.8 mm3) in principal plane, by using a 1064 nm Nd:YAG pico-second laser, where the highest conversion efficiency of the single-pass light reached up to 51%, while for a (112.5°, 35.9°)-cut TmCOB sample (4 × 4 × 8 mm3) in spatial PM direction, the single-pass light reached 58%. Meanwhile, the angular noncritical phase matching (A-NCPM) wavelengths along the Y and Z principal axes were calculated and measured, and the limit of type-I PM wavelength of TmCOB was found to be 716 nm.
© 2014 Optical Society of America
1. Introduction
At present, the high power all-solid-state visible laser pulses have been widely used for applications such as laser display, medicine, optical storage, bio-photonics, undersea communication, marking and precision micro fabrications etc [1–3]. The frequency doubling infrared light by nonlinear optical (NLO) crystals is one of the most efficient methods to obtain the laser pulses in visible spectral region. By the time, many excellent NLO crystals such as KH2PO4 (KDP), KTiOPO4 (KTP), BaB2O4 (BBO) and LiB3O5 (LBO) have been commercialized and widely used as NLO frequency conversion devices. However, the growth period of these crystals is relatively long, normally lasting several months, or as long as one year [4, 5].
In the last two decades, rare-earth calcium oxyborate crystals ReCa4O(BO3)3 (ReCOB, Re: rare-earth elements) with monoclinic symmetry were investigated and reported to be excellent multifunctional materials in laser, nonlinear optics and piezoelectric fields [6–17]. In 2006, large YCOB crystals with 3 inches in diameter were obtained in lab, and up to 50% energy conversion at 10 Hz was achieved in high average power frequency conversion of the mercury laser [8]. In recent years, large YCOB crystals with comprehensive excellent properties were also reported to be a good candidate for OPCPA applications [13, 14]. In addition, the birefringence of the GdxY1-xCa4O(BO3)3 (GdYCOB) crystals was found to be adjustable by changing the compositional parameter x during the crystal growth, and the angular noncritical phase matching (A-NCPM) third harmonic generation (THG) of Nd:YAG laser (1064 nm) could be realized along the Y principal axis i.e. (90°, 90°), potential for practical generation of high-peak-power ultra-violet (UV) light at 355 nm [15–17]. Moreover, ReCOB crystals were reported to be excellent laser host materials for Yb3+ in ultrafast femtosecond mode-locked laser and Nd3+ in self-frequency doubling (SFD) applications [10–12]. In ReCOB type NLO crystals, the anionic groups ((BO3)3-) were considered to be the most basic structural units responsible for the optical nonlinearity, and their contributions can be summated to produce the bulk NLO responses [18–20], but it is revealed that the difference trivalent cations might exert an effect on the NLO dielectric and optical properties such as refractive index, phase matching (PM) angle, effective nonlinear coefficients (deff) and A-NCPM wavelengths [7, 9, 15–17]. In order to optimize the NLO properties of ReCOB crystals, TmCOB were designed (among the Re elements, the Tm possess large mass but small ionic radius) and grown by Cz pulling method. Moreover, the anisotropy the NLO properties were investigated in this work.
2. Optical principal axes, refractive index and Sellmeier equation
For TmCOB crystal, the intersection angles β between the crystallographic axes were determined to be βab = 90°, βbc = 90° and βac = 101.11°. As a biaxial crystal, TmCOB possess three different optical principal axes, one of the principal axes Y is collinear with the two-fold axis (the crystallographic axis b), while the other two principle axes X and Z located in (010) face are respectively close to the crystallographic axes c and a, with certain angles. Using a polarized microscopy (Axio Lab.A1 made by ZEISS), the extinction experiment was performed with a b-cut crystal sample (~3 mm in thickness), where the intersection angle between one optical principal axis and a-axis was found to be 23.5°, and 12.4° for other principal axis and c-axis. The definition of X, Y, and Z optical principal axes follows the principle of nX<nY<nZ for biaxial crystal. Using refractive indexes measurement equipment (HR SpectroMaster UV-VIS-IR made by TRIOPTICS), the refractive indexes at 14 discrete wavelengths from 253.6 nm to 2325.4 nm were determined and the dispersion Eqs. were fitted as follows:
Thus, the relationship between the refractive principal axes (X, Y and Z) and the crystallographic axes (a, b and c) can be determined, where the b-axis and Y-axis are collinear but with opposite direction with intersection angles being (a, Z) = 23.5° and (c, X) = 12.4°.3. PM angle and deff
Based on the Sellmeir Eqs. (1), the PM directions in space for TmCOB crystals are calculated, and the type-I and type-II PM curves at 1064 nm are shown in Fig. 1(a) and Fig. 1(b), respectively. It is found that there are only two type-I PM angles at 1064 nm i.e. (32.5°, 0°) and (32.5°, 180°) in the ZX principal plane. The relationships between the effective NLO coefficients deff and the independent NLO coefficients d12 and d32 can be determined by using the following Eqs.:
where Ω is the angle of optical axis. It is noted that there are type-I (90°, 34.5°) and type-II (90°, 71°) PM angles (1064 nm) in the XY principal plane and the effective NLO coefficients deff can be expressed to be Taking advantage of the 1064 nm SHG experiments, the independent NLO coefficients d12, d32, d13 and d31 were determined. In the experiments, the laser source was a PY61 Nd:YAG pico-second laser made by Continuum Corp. USA. The working wavelength, repeat frequency and pulse width were 1064 nm, 10 Hz and 35 ps, respectively. In order to improve the beam quality of the incident light, a diaphragm (ϕ = 3 mm) was set before the crystal sample. The filter placed between crystal and energy meter transmit 0.4% at 1064 nm and 80% at 532 nm. A 10 mm long KTP crystal cut along (90°, 23.6°) orientation was used as the standard sample, of which the deff was 2.45 pm/V. The deff values for the PM directions (32.5°, 0°), (32.5°, 180°), (90°, 34.5°) and (90°, 71°) were measured and found to be on the order of 1.11, 0.67, 0.35 and 0.11 pm/V, respectively. Based on Eqs. (2)-(5), the independent NLO coefficients d12, d32, d13 and d31 were obtained to be on the order of 0.24, 1.70, −0.55 and −0.32 pm/V, respectively. It is found that the magnitudes of d12, d32, d13 and d31 for TmCOB crystals are very close to those of YCOB, GdCOB and LaCOB crystals, as presented in Table 1.
Table 1. The measured NLO coefficients of ReCOB crystals
The macro NLO properties are believed to be associated with the micro structures for ReCOB crystals. In 2001, Adams et al reported that the d12 and d32 coefficients of the NLO tensor for LaCOB, GdCOB and YCOB crystals were equivalent within the experimental uncertainty, and the deff values for these ReCOB (Re = Y, Gd and La) crystals in ZX principal plane were varied primarily because of the differences in their birefringence [7]. According to the anionic group theory, the anionic groups [(BO3)3-] in ReCOB crystals were considered to be the most basic structural units responsible for the optical nonlinearity, and their contributions can be summated to produce the bulk NLO properties [18–20]. Besides the contribution of the [(BO3)3-] groups to the NLO properties, the effects from Re3+ cations in ReCOB crystals were also proposed. It is observed that the different Re3+ ions in ReCOB crystals show negligible impact on the values of dij coefficients of the NLO tensor, as presented in Table 1. However, the deff for ReCOB crystals in ZX principal plane varied dramatically, with the maximum deff value (1.10 pm/V) found for TmCOB. The difference of the deff values obtained in ZX principal plane for ReCOB crystals are concluded to be resulted from variation of the refractive index and PM angles, induced by Re3+ cations. Furthermore, it is reported that the birefringences of the Gd1-xYxCa4O(BO3)3, Lu1-xGdxCa4O(BO3)3 and Sc1-xGdxCa4O(BO3)3 crystals were operational by adjusting the compositional parameter x, thus a series of A-NCPM wavelengths were realized along the Y-axis [15–17, 21, 22]. However, the type-I deff of these crystals along the Y axis were varied slightly and found to be on the order of 0.51~0.60 pm/V, approximate to the calculated values (0.58~0.59 pm/V) based on the NLO tensor of YCOB or GdCOB crystals within the experimental uncertainty [16, 22–24]. Therefore, the independent NLO coefficients dij for ReCOB crystals are approximately equal within the experimental uncertainty. Based on the measured NLO coefficients d12, d32, d13 and d31 for TmCOB crystals and the reported NLO coefficients d11 and d33 for YCOB crystals [23], the spatial deff for TmCOB crystals were calculated, as shown in Fig. 2(a) (type-I) and Fig. 2(b) (type-II), where the PM angle (112.5°, 35.9°) was found to possess the largest deff value (type-I) being on the order of 1.46 pm/V, higher than that of type-II (0.36 pm/V) PM crystal cuts, so the type-I (112.5°, 35.9°) PM crystal cuts should be given priority for frequency conversion investigation.
Fig. 2 (a) Distribution of |deff| (type-I PM) of TmCOB crystals at 1064 nm, (b) Distribution of |deff| (type-II PM) of TmCOB crystals at 1064 nm.
4. The ultra-cavity SHG
4.1 SHG conversion in principal plane
Different TmCOB crystal cuts, (32.5°, 0°), (32.5°, 180°) and (90°, 34.5°) in principal planes, were prepared along the type-I PM directions for SHG experiments (sample dimension: 4 × 4 × 11.8 mm3). Figure 3 shows the SHG output energy and conversion efficiency with the change of fundamental energy along different PM directions in principal planes. As can be seen that the SHG conversion efficiency of the TmCOB sample along (32.5°, 180°) PM direction is obtained to be 51% at the input energy of 2.2 mJ (220 MW/cm2), while the SHG conversion efficiency of the samples along (32.5°, 0°) and (90°, 34.5°) are observed to be 34.5% and 15.7%, respectively. When the input energy reached ≥2.2 mJ, saturation behavior was observed for TmCOB crystal cuts (32.5°, 0°) and (32.5°, 180°), whereas for (90°, 34.5°) crystal cuts, this phenomenon was not appeared until the input energy reached up to 3.7 mJ (370 MW/cm2). This indicates that the crystal cuts (32.5°, 0°) and (32.5°, 180°) are easy to get SHG conversion saturation.
Fig. 3 (a) Output energy of TmCOB crystals at 532 nm (crystal cuts in the principal planes), (b) SHG conversion efficiency of TmCOB crystals at 1064 nm (crystal cuts in the principal planes).
4.2 SHG conversion out of principal plane
The type-I PM direction (112.5°, 35.9°) crystal cuts with dimension of 4 × 4 × 8 mm3 were investigated and found to possess the largest deff value being on the order of 1.46 pm/V. Figure 4 gives the variations of the SHG output energy and conversion efficiency as function of fundamental energy for TmCOB crystals, where the SHG conversion efficiency of the TmCOB (112.5°, 35.9°) crystal cuts reached up to 58% at the input energy 2.8 mJ (280 MW/cm2). The SHG conversion efficiency saturation behaviour was observed for (112.5°, 35.9°) crystal cuts when the input energy reached about 2.8 mJ, 27.3% higher than those of the crystal cuts in the ZX principal plane, associated with the short crystal length (8 mm). In addition, the SHG conversion efficiency was found to decease when the input energy reached 2.8 mJ, proposed to be related to the thermal dephasing of NLO TmCOB crystal samples produced by the heat absorption inside the crystal (under high energy level >2.8mJ), which would deteriorate the SHG conversion efficiency.
Fig. 4 SHG conversion efficiency of TmCOB crystals at 1064 nm (crystal cuts out of principal planes).
5. A-NCPM
The A-NCPM wavelengths along X, Y and Z axes were calculated based on the Sellmeier Eqs. (1), as summarized in Table 2. For A-NCPM properties evaluation, the TmCOB single crystals were prepared into cuboid samples along different optical principal axes, of which the sample dimension was 5 (X) × 9 (Y) × 7 (Z) mm3 with six surfaces fine polished. An Opolette (OPO) HE 355 II tunable laser was used as the pump source (410~2400 nm). Based on the measured NLO coefficients d12, d32, d13 and d31 for TmCOB crystals and the reported NLO coefficients d11 and d33 for YCOB crystals [23], the |deff| values for A-NCPM SHG of TmCOB crystals were obtained (Table 2). For the Type-I A-NCPM SHG along the X axis and type-II A-NCPM SHG along the Z axis, the deff were calculated to be zero. Although the type-II |deff| along X axis was found to be on the order of 1.70 pm/V, the A-NCPM SHG wavelength could not been detected since it go out of the OPO turning range we used. The accurate A-NCPM SHG wavelengths were measured and listed in Table 3.Combining with Table 2, it can be concluded that the experimental A-NCPM wavelength values were agreed well with the calculations, indicating the high accuracy of Sellmeier Eqs. (1) for TmCOB crystals. The limit of type-I PM wavelength of TmCOB (716 nm) is shorter than that of YCOB (725 nm [17]) and GdCOB (832 nm [17]) crystals for SHG, which is believed to be associated with the large magnitude of birefringence for the TmCOB crystals (Δn = 0.043 at 1064 nm), slightly larger than those of YCOB (Δn = 0.041 at 1064 nm [19]) and GdCOB (Δn = 0.033 at 1064nm [21]) crystals.
Table 2. The A-NCPM SHG properties of TmCOB crystal (calculated values)
Table 3. The A-NCPM SHG wavelengths of TmCOB crystal (experimental values)
6. Conclusion
The anisotropy of the new nonlinear optical crystals TmCOB were studied for frequency conversion applications. The NLO coefficients d12, d32, d31 and d13 of TmCOB crystals were determined and found to be on the order of 0.24, 1.70, −0.55 and −0.32 pm/V, respectively. Moreover, the distribution of the spatial deff for TmCOB crystals was determined, where the optimum PM direction was found for (32.5°, 180°) in ZX principal plane with deff and SHG conversion efficiency being on the order of 1.10 pm/V and 51%, respectively, while the spatial crystal cuts (112.5°, 35.9°) were observed to be possess high deff value, being on the order of 1.46 pm/V, with SHG conversion efficiency up to 58% at 1064 nm. Furthermore, the A-NCPM wavelengths along the Y and Z principal axes were calculated and measured, and the limit of the type-I PM wavelength of TmCOB was obtained and found to be 716 nm, which was shorter than those of the YCOB and GdCOB crystals.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant Nos. 51202129, 61178060 and 91022034), Program for New Century Excellent Talents in University (NCET-10-0552), Independent Innovation Foundation of Shandong University (2012TS215, 2011GN056), and Natural Science Foundation for Distinguished Young Scholar of Shandong Province (JQ201218).
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