Thermal lensing-induced soliton molecules in beta-phase Gallium Oxide
In recent years, beta gallium oxide (beta-\ce{Ga2O3}) has become the most investigated isomorph of gallium oxide polymorphs, due to the great potential it represents for applications in optoelectronics and photonics for solar technology, particularly in blind ultraviolet photodetector solar cells (SBUV) designs. To optimize its use in these applications, and to identify possible new features, knowledge of its fundamental properties is relevant. In this respect, optical, thermal and electronic properties of beta-\ce{Ga2O3} have been studied expriementally, providing evidence of a wide-band inorganic and transparent semiconductor with a Kerr nonlinearity. Thermo-optical properties of the material, probed in SBUV sensing experiments, have highlighted a sizable heat diffusion characterized by a temperature gradient along the path of optical beams, quadratic in beam position and promoting a refractive-index change with temperature. The experimentally observed Kerr nonlinearity together with the thermally induced birefringence, point unambiguously to a possible formation of soliton molecules during propagation of high-intensity fields in beta-\ce{Ga2O3}. To put this conjecture on a firm ground we propose a theoretical analysis, based on the cubic nonlinear Schroedinger equation in 1+1 spatial dimension, in which thermal lensing creates an effective potential quadratic in the coordinate of beam position. Using the non-isospectral inverse-scattering transform method, the exact one-soliton solution to the propagation equation is obtained. This solution features a bound state of entangled pulses forming a soliton molecule, in which pulses are more or less entangled depending on characteristic parameters of the system.