Angle noncritical phase-matched second-harmonic generation in the monoclinic crystal BaGa 4 Se 7
Résumé
We performed a complete study of angle noncritical phase-matched second-harmonic generation in BaGa 4 Se 7 in the mid-infrared region. We measured the dielectric frame orientation and showed that it does not rotate as a function of wavelength. We determined the phase-matching wavelengths and the associated spectral and angular acceptances along the x and y axes of the dielectric frame. We also estimated the magnitude of the nonlinear coefficients involved. Coherent light sources emitting in transparency band III of the atmosphere (8–12 μm) can be designed using frequency down-conversion in nonlinear optical crystals. Desirable properties are large quadratic nonlinear coefficients, wide transparency ranges, and high damage thresholds. A potential alternative is the newly developed BaGa 4 Se 7 (BGSe). Compared with other well-known selenide crystals with similar transparency ranges, such as AgGaSe 2 (AGSe) and GaSe, the advantages of BGSe include a wider bandgap leading to a higher damage threshold, improved thermomechanical properties, as well as the capability to be pumped at 1064 nm without two-photon absorption [1]. BGSe is transparent between 0.47 and 18 μm and can be grown in a large size by the Bridgman–Stockbarger method [2]. It belongs to the m monoclinic point group and the special axis perpendicular to the mirror plane m is parallel to the x axis of the dielectric frame [3]. Consequently, the six non-zero and independent components of the second-order electric susceptibility tensor defined in this frame are the following under Kleinman assumption and using the contracted notation: d 23 , d 31 , d 32 , d 21 , d 22 , and d 33. Sellmeier's equations of the three principal refractive indices n x , n y , and n z have been determined using two oriented prisms [3]. They were used to calculate the birefringence phase-matching directions of second-harmonic generation (SHG) [3]. The magnitude and sign of all nonlinear coefficients of BGSe have been calculated from the electronic band structure [4]. Experimental values of d 22 and d 23 have been determined at 0.532 μm using the Maker fringes method and KDP as a reference [5]. In this Letter, we determined the orientation of the dielectric frame x; y; z of BGSe and showed that it does not rotate as a function of wavelength. From this result, we studied type-I and type-II angle noncritical phase-matched (NCPM) SHG along x and y axes. We measured the corresponding fundamental wavelengths and the associated conversion efficiencies as well as spectral and angular acceptances. We also determined the magnitude of the nonlinear coefficients d 31 , d 32 , and d 23 of BGSe relative to d 24 of KTP. By convention, the b axis of the crystallographic frame a; b; c of BGSe is taken as the special (monoclinic) axis. It is then perpendicular to the mirror plane m containing the two other axes, i.e., a and c, making an angle of β 121.2° between them. Consequently, the crystallographic frame does not correspond to the orthogonal dielectric frame x; y; z: x axis is parallel to b, whereas y axis and z axis are located in the mirror plane m. We measured the relative orientation between the dielectric and crystallographic frames by combining x-ray backscattered Laue geometry and conoscopy at 0.633 μm. We used a sample cut as a slab with two polished faces perpendicular to b. We found that z axis is collinear with c and that the angle between a and y axis is α 31.2 0.5°, as shown in the insert in Fig. 1. As for any monoclinic crystal, we had to verify if the relative orientation between the crystallographic and dielectric frames varied as a function of wavelength. For that purpose, the BGSe slab was inserted between two crossed polarizers. It was rotated around its b axis while illuminated by light. We used a tunable 5 ns pulse duration (FWHM) and a 10 Hz repetition rate optical parametric oscillator (OPO) from Continuum. The transmitted intensity follows Malus' law where the positions of the Letter Vol. 40, No. 20 / October 15 2015 / Optics Letters 4591 0146-9592/15/204591-04$15/0$15.00