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

We have studied second-harmonic generation (SHG) from quasi-one-dimensional aperiodic optical superlattices (AOSs) of finite lateral width by inverting poled ferroelectric domains. The search for optimal AOS structures corresponds with solving a difficult inverse source problem. We describe the design principle in real-space representation and undertake model designs. The numerical simulations show that the constructed AOSs can implement multiple-wavelength SHG with identical effective nonlinear coefficients at the pre-assigned wavelengths of the incident light. We investigate the effects of mode-mode coupling and the lateral width of the superlattice on the SHG for two cases: incident light beams of plane-wave and Gaussian profiles. When the number of modes increases, the effective nonlinear coefficient decreases in an oscillatory fashion at the beginning and then tends to a constant. For an incident plane-wave beam, the dependence of the effective nonlinear coefficient on the width of the sample is quite weak, while for an incident Gaussian beam this dependence exhibits a rapid decrease at the beginning and then tends to a constant. We display the variation in the effective nonlinear coefficients with the distance of propagation of the optical wave from where the incident light beam impinges on the sample surface and find that this variation exhibits monotonically increasing behaviour. This clearly infers that the contribution of every block to the optical SHG process takes the form of constructive addition. It is expected that this new design method may provide an effective and useful technique for constructing nonlinear optical material to match various practical applications