Second-harmonic generation and multiple mode effects in aperiodic opticalsuperlattices with finite lateral width

Abstract

We have studied second-harmonic generation (SHG) from quasi-one-dimensionalaperiodic optical superlattices (AOSs) of finite lateral width by inverting poledferroelectric domains. The search for optimal AOS structures corresponds withsolving a difficult inverse source problem. We describe the design principle inreal-space representation and undertake model designs. The numerical simulationsshow that the constructed AOSs can implement multiple-wavelength SHG withidentical effective nonlinear coefficients at the pre-assigned wavelengths of theincident light. We investigate the effects of mode–mode coupling and thelateral width of the superlattice on the SHG for two cases: incident lightbeams of plane-wave and Gaussian profiles. When the number of modesincreases, the effective nonlinear coefficient decreases in an oscillatoryfashion at the beginning and then tends to a constant. For an incidentplane-wave beam, the dependence of the effective nonlinear coefficient onthe width of the sample is quite weak, while for an incident Gaussianbeam this dependence exhibits a rapid decrease at the beginning and thentends to a constant. We display the variation in the effective nonlinearcoefficients with the distance of propagation of the optical wave from where theincident light beam impinges on the sample surface and find that thisvariation exhibits monotonically increasing behaviour. This clearly infersthat the contribution of every block to the optical SHG process takesthe form of constructive addition. It is expected that this new designmethod may provide an effective and useful technique for constructingnonlinear optical material to match various practical applications.