Harmonic Generations From Aperiodic Optical Superlattices

We first study the second harmonic generation (SHG) and coupled third harmonic generation (CTHG) from quasi-one-dimensional aperiodic optical superlattices (AOSs) with finite lateral width, and then investigate the characteristics of SHG and parametric amplification (PA) generated from the quasi-one-dimensional AOSs in the regimes of large, intermediate, and small pumping signals of the fundamental wave (FW). The AOSs are designed with the use simulated annealing (SA) method and constructed by inverting poled ferroelectric domains.The numerical simulations show that the constructed AOSs can implement multiple wavelength SHG (or CTHG) with an identical effective nonlinear coefficient at the preassigned wavelengths of incident light. We reveal the influences of mode-mode coupling and the lateral width of the superlattices on the SHG (or CTHG) for the incident light beams in both cases of plane wave and Gaussian profiles. As increasing the number of the modes the effective nonlinear coefficients are oscillatorily decreased at the beginning and then tend to a constant for the incidental plane-wave beam; the dependence of the effective nonlinear coefficient on the width of the samples is quite weak, in contrast, for the incident Gaussian profile beam, this dependence exhibits a rapid decrease at the beginning and then tends to a constant. In the large pumping signal levels, we observe the saturation phenonmenon.We demonstrate the variations of the effective nonlinear coefficient with the optical wave propagating distance from the sample surface of incident light impending upon and find that the variations exhibit monotonously increasing behavior. This clearly infers that the contribution of every block to optical SHG (or CTHG) process is coherent accumulation.Finally, we explore the merits of the parametric amplification (PA) by using the designed AOSs that can generate tunable radiation over wide frequency spectral ranges from a single pumping light source. It is anticipated that the proposed designing scheme of AOSs may provide a useful and efficient way for constructingnonlinear optical materials to match various practical applications in nonlinear optical fields..