Phase-matching directions and refined Sellmeier equations of the monoclinic acentric crystal BaGa_4Se_7

International audienceWe directly measured the phase-matching directions of second-harmonic and difference frequency generation in the principal planes of the biaxial BaGa 4 Se 7 (BGSe) crystal using the sphere method. The simultaneous fit of the data allowed us to refine the Sellmeier equations of the three principal refractive indices of BGSe, and to calculate the tuning curves for infrared optical parametric generation. Optical parametric generation of coherent radiation in bands II (3–5 μm) and III (8–12 μm) of the transmission of the atmosphere is needed for various applications such as trace gas monitoring and laser surgery, but there is still a lack of suited nonlinear crystals. A real alternative is the newly developed BaGa 4 Se 7 (BGSe) transparent from 0.47 to 18 μm [1–3]. The transmission spectrum is shown in Fig. 1, pointing out a high transparency until 12 μm. BGSe belongs to the monoclinic point group m and, hence, to the optical biaxial class. The principal refractive indices n x < n y < n z have been previously determined as a function of the wavelength between 0.48 and 10.40 μm using three oriented prisms, and the corresponding fitted Sellmeier equations have been used to calculate birefringence phase-matching directions of second-harmonic generation (SHG) in the three principal planes of BGSe [2]. An alternative set of Sellmeier equations obtained in a similar manner between 0.5 and 2.6 μm showed only limited agreement with experimental 1.064 μm pumped optical parametric amplification (OPA) and difference frequency generation (DFG) phase-matching angles [3]. The crystallographic frame of BGSe does not correspond to the orthogonal dielectric frame x; y; z: the x-axis is taken parallel to the b-axis, so that the a-, c-, y-, and z-axes are located in the mirror plane. Furthermore, the z-and c-axes coincide at λ 0.633 μm [2]. We recently studied angle noncritical phase-matched (NCPM) SHG along the x-and y-axes of BGSe after checking that the dielectric frame does not rotate about the special b-axis in the considered wavelengths range [4]. The corresponding NCPM wavelengths and associated spectral and angular acceptances were measured. From our data, we also determined the magnitude of the nonlinear coefficients d 23 , d 32 , and d 31 [4]. In this Letter, we report for the first time, to the best of our knowledge, an exhaustive study of all possible SHG and DFG phase-matching directions of BGSe cut as a sphere. They were directly measured between 2 and 11 μm, in the x; z and y; z principal planes of the spherical crystal, which allowed us to refine more reliable Sellmeier equations in this spectral range. According to the crystal symmetry of BGSe, and by using previously published Sellmeier equations from [2,3], we found that all possible quadratic processes that can be phase matched Fig. 1. Unpolarized transmission of a 11.3 mm thick y-cut BGSe sample. The inset shows a polished part of a BGSe boule. Letter Vol. 41, No. 12 / June 15 2016 / Optics Letters 2731 0146-9592/16/122731-04 Journa