The photorefractive surface waves with photorefractive nonlinearity of diffusion, drift, and photovoltaic effect is studied numerically for the first time. We find out that the essential cause of photorefractive surface waves is diffusion mechanism, but not drift and photovoltaic effects. The photovoltaic effect only has strong influence on the profile of photorefractive surface waves; the applied external electric field can also change the profile of the surface waves, and even destroy the mode upon a threshold, which provides a possible method to transform the modes of surface waves between high-frequency modes and low-frequency modes.

Surface waves with photorefractive nonlinearity

Nanopolar structures and local ferroelectricity of Sr0.61Ba0.39Nb2O6 relaxor crystal across Curie temperature have been investigated using piezoresponse force microscopy (PFM). The short-range nanopolar clusters are dominated by long-range ferroelectric order at room temperature, but exhibit themselves at higher temperature through the morphology change in written ferroelectric domains and persisting piezoresponse beyond Curie point. A maximum piezoresponse is observed at 80 °C, which is attributed to the peak in the dielectric permittivity near the ferroelectric phase transition. Substantial asymmetry in PFM hysteresis emerges above Curie temperature, indicating an internal bias field that becomes notable in the absence of long-range ferroelectric order.

Nanopolar structures and local ferroelectricity of Sr0.61Ba0.39Nb2O6 relaxor crystal across Curie temperature by piezoresponse force microscopy

Giant enhancement of second-harmonic generation (SHG) with 83.4%/W coversion efficiency is obtained, taking advantage of photorefractive surface waves with diffusion and drift nonlinearity. In this method, the self-bending induced by diffusion nonlinearity can be utilized at the surface and can solve the phase-mismatch problem in bulk due to beam self-bending. With drift nonlinearity, an applied external electric filed and background illumination can further constringe surface waves to the surface and consequently enhance SHG, which provides the possibility and flexibility of control.

Giant enhancement of surface second-harmonic generation using photorefractive surface waves with diffusion and drift nonlinearities

We introduce what we believe to be a new type of nonlinear surface wave, photorefractive lattice surface wave, which can form at the interface between uniform linear media and periodic optical lattice with nonlocal photorefractive diffusion nonlinearity. This type of surface wave originates not only from the gaps but also from the bands of the optical bandgap structures, which are different from surface lattice solitons and can be classified into three kinds, i.e., band lattice surface wave, gap lattice surface wave, and outside bandgap lattice surface wave. The diffusion nonlinearity ensures the concentration of light energy near the boundary of optical lattice and brings a downward shift of the optical bandgap structure.

Photorefractive lattice surface waves with diffusion nonlinearity

We observed the temporal development of surface waves and investigated their power propagation loss in typical photorefractive polymer films sandwiched between ITO glass substrates. We found that amplified scattered waves generated in a pumped region started to develop into surface waves from a point where they reached the substrate through the self-bending effect. The surface waves propagated over a distance of 1.7 mm, thereby confining the power to a region at a distance of 30 microns from the substrate. Considerable propagation power loss of the surface waves was observed at a low pumping power of the beam; however, the power loss decreased considerably when the beam had high power.

Y. Z. Lu

Papers

Surface second-harmonic generation in Sr 0.6 Ba 0.4 NbO 3 with a nonlinear diffusion mechanism