Study On Excitation Of Photorefractive Surface Waves And Its Applications In Second Harmonic Generation

The photorefractive surface wave (PR SW) is a kind of waves which is associated with the boundary and can propagate along the area near the surface of the photorefractive materials. The density of the light could be highly concentrated at the near surface of the photorefractive materials. As a result, all nonlinear surface optical phenomena could be expected to be strongly enhanced. Therefore, the PR SWs will have high applicable value on optical coupling, phase matching, and integrated optics.Since 1995, G S Garcia Quirino, et. al. have predicted the existence of the PR SW. And in the same year, Mark Cronin Golomb firstly observed thse PR SW with the diffusion nonlinearity in BaTiO3 crystal. With the developments for the fifteen years, people have further recognized the PR SWs. However, further researches on the problems such as excitation mechanism, stability of propagating, and potential application of the PR SWs are still needed. PR SWs with high conversion efficiency could be expected to utilize the coopration of self-bending induced by the nonlocal diffusion nonlinearity and the self-focus induced by the local drift nonlinearity. The PR SWs could also be named PR surface solitons, which are more controllable and have stronger stablility. The reaches on PR SWs will further promote the development of the integrated optics, and offer more charicaristic techniques. The paper will systematically discuss the problems about the excitation of the PR SWs. The main contents and original innovations are shown as follows:1) Theoretical research:firstly, the PR SW with drift and diffusion mechanism with relatively large incident angle is studied. And the influences of the external applied electric field and the uniform background illumination are analyzed. The analytical results show that:An externally applied electric field can change the profiles of the' photorefractive surface waves, especially the spatial frequency etc., and even destructs photorefractive surface waves upon a threshold value. This provides a possible method to control the formation and the shapes of photorefractive surface wave. With the increase of dark intensity Id, the amplitude of PR SW in the deep-seated region increases, and the frequency decreases slightly. Secondly, the existence of the 2D photorefractive surface soliton was predicted for the first time considering the diffusion and drift nonlinearities. The relationship between the shape of the surface soliton and the ratio of the intensity of the incident beam and the background illumination was analyzed. Furthermore, a Beam Propagation Method (BPM) was introduced to simulate the propagation of the surface soliton with the cooperation of the nonlocal diffusion nonlinearity and the local drift nonlinearity. It offers powerful theoretical accordance.2) Experimental research:firstly, the influence of the applied external electric field and the background illumination on the PR SW with drift and diffusion mechanism was experimentally studied. Secondly, the photorefractive surface solitons occurring at the boundary of photorefractive crystal with the cooperation of nonlocal diffusion and local drift nonlinearities was experimentally excited. The experimental results are in good agreement with the theoretical simulations. Furthermore, the characters of multi-times reflection and the near-boundary oscillating of photorefractive screening solitons were also observed for the first time.3) Applications:Based on the previous studies about the second harmonic generation (SHG) induced by the PR SW, surface SHG with higher conversion efficiency was researched taking adavantage of the PR SW with relatively large incident angle. Firstly, through controling the electric field and the background illumination, the surface SHG with diffusion and nonlinearity was excited efficiently. The conversion efficiency of the SHG was greatly improved to 83.4%/W, which make the application of the SHG induced by the photorefractive surface soliton become possible. And then the variations of the conversion efficiency with the applied external electric field and the uniform background illumination were analyzed. The analyses show that to increase the electric field and the background illumination is utilized to improve the conversion efficiency of the second harmonic generation.The original innovations in this paper are listed as follows:1. Theory:2D PR SWs (or surface solitons) with diffusion and drift mechanism are predicted for the first time and their propagation characters are studied. 2. Experiments:The excitation of the PR surface solitons are realized for the first time. The experimental results are in good agreement with the theoretical simulations.3. Applications:Giant enhancement of the second harmonic generation was achieved utilizing the PR SWS with diffusion and drift mechanism for the first time. The conversion efficiency reaches 83.4%/W, which is closed to the applicable level.