Abstract
We investigate second-harmonic generation (SHG) in a photorefractive photovoltaic medium such as lithium niobate. Our numerical model reveals the complex dynamics of the parametric process during the buildup of the index modification due to the photorefractive (PR) nonlinearity. We investigate a condition in which no external field is applied to the crystal, resulting in a defocusing nonlinearity, as well as the case in which an external bias is applied, producing a self-focusing effect that can enhance the conversion efficiency of the parametric process. We also find the conditions for the initial phase matching and for the background illumination leading to a stable self-confined propagation of the second-harmonic generated light. The developed numerical model shows that as a general case SHG in a self-focusing PR medium results in mode beating inside the generated waveguide, as experimentally observed.
Highlights
Photorefractivity is widely used to induce quasipermanent index structures in materials by means of light [1]
The modification of the index affects the phase matching (PM) conditions via a local change in the phase velocities of both beams, with a process known as the nonlinear self-phase-matching (NLSPM) [17]
The conversion efficiency of the quadratic process is expected to vary due to two different mechanisms driven by the photorefractive photovoltaic (PR-PV) effect: the spatial beam modification, which affects the intensity profile of the fundamental harmonic (FH) and SH beams, and the change in PM conditions, which instead modifies the phase relationship between the interacting waves
Summary
Photorefractivity is widely used to induce quasipermanent index structures in materials by means of light [1]. PR solitons are useful tools to write passive structures, such as matrices [6,7] and junctions [8], inside the material, since they are stable against perturbations and can be permanently fixed under certain conditions in the crystal [9] They can be induced using low intensity visible light and are wavelength sensitive. Interplay between nonlinear effects necessitates the development of a time dependent model in order to explain the observed dynamics and above all to be used as a tool to determine conditions giving an efficient steady-state regime To tackle this problem, we present in this paper a1 + 1͒D time dependent numerical analysis of the SHG in a photorefractive photovoltaic (PR-PV) material. It provides a way to better comprehend the complex dynamics of the process and to determine the parameters to possibly avoid beam distortion and defocusing reaching a stable self-confinement of both interacting beams
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