Anisotropy of nonlinear optical properties in ... - OSA Publishing

Anisotropy of nonlinear optical properties in monoclinic crystal TmCa4O(BO3)3 Yanqing Liu,1,2 Fapeng Yu,1,2,3 Zhengping Wang,1,2* Xinguang Xu,1,2 and Xian Zhao1,2 2

1 State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China Key Laboratory of Functional Crystal Materials and Device (Shandong Universiy), Ministry of Education, Jinan 250100, China 3 [email protected] *[email protected]

Abstract: 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 OCIS codes: (190.4400) Nonlinear optics, materials; (140.3515) Lasers, frequency doubled; (140.3580) Lasers, solid-state.

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Received 13 Jun 2014; revised 3 Aug 2014; accepted 4 Aug 2014; published 6 Aug 2014

(C) 2014 OSA 1 September 2014 | Vol. 4, No. 9 | DOI:10.1364/OME.4.001787 | OPTICAL MATERIALS EXPRESS 1787

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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

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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 nX2500 >3000 715 1008 813 1200, 2480

| deff | (pm/V) Type-I 0 0.55 0.24

Type-II 1.70 0.32 0

Table 3. The A-NCPM SHG wavelengths of TmCOB crystal (experimental values) Optic axis

A-NCPM SHG Wavelength (nm) Type-I Type-II

Y

716

Z

815

1011 -

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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