Bulk growth and nonlinear optical properties of thulium calcium oxyborate single crystals

Abstract

New nonlinear optical (NLO) TmCa₄O(BO₃)₃ (TmCOB) crystals were grown by the Czochralski method. X-ray powder diffraction results indicated that TmCOB crystals belong to the monoclinic system with space group Cm. The unit cell parameters were calculated to be a = 8.0679 Å, b = 16.0140 Å, c = 3.5224 Å, with α = γ = 90° and β = 101.1115°. Disorder distribution of Ca²⁺ and Tm³⁺ in TmCOB crystals was observed, from which the molecular formula was found to be (Tm_0.78Ca_0.22)(Ca_3.78Tm_0.22)O(BO₃)₃. The relationships between the crystallographic axes and the optical principal axes were determined, and the refractive indices at different wavelengths were measured, from which the phase matching angle at 1064 nm in the ZX principal plane was measured and found to be (32.5°, 180°) and (32.5°, 0°). Furthermore, efficient second harmonic generation in the ZX principal plane was realized by a 1064 nm Nd:YAG picosecond laser, where more than 50% conversion efficiency was achieved for the (32.5°, 180°)-cut sample (4 × 4 × 11.8 mm³), with the effective nonlinear optical coefficient and the acceptance angle determined to be on the order of 1.1 pm V⁻¹ and 3.1 mrad cm, respectively.