Coupled third harmonic generations and multiple mode effects in aperiodic optical superlattices with a finite lateral width

Abstract

We study the coupled third harmonic generations (CTHGs) from quasi-one-dimensional aperiodic optical superlattices (AOSs) with a finite lateral width. We design the special AOS that can implement multiple wavelength CTHG with identical effective nonlinear coefficient at the preassigned wavelengths of incident light with the use of simulated annealing method. We reveal the effects of mode-mode coupling and the lateral width of the AOSs on the CTHG for the two cases: One case corresponds to the incident light beam of a plane wave profile and other case to the incident light beam of Gaussian profile. The results show that the characteristics of the CTHG from the AOSs are closely related to the profiles of the incident fundamental frequency light beam. For the incidental plane wave beam, the dependence of the effective nonlinear coefficient on the width of the AOS is quite weak. On the contrary, for the incident Gaussian profile beam, this dependence exhibits a rapid decrease at the beginning and then tends to a constant. We also display the variation of the effective nonlinear coefficients with the optical wave propagating distance from the impinging surface of incident light beam in the sample illuminating by the plane wave light beam and find that this variation exhibits basically increasing behavior. This clearly infers that the contribution of every individual block to optical CTHG process is a constructive addition. When increasing the number of the modes, the effective nonlinear coefficient of CTHG is rapidly decreased at the beginning and it then tends to a saturation value for the incident plane wave light beam. However, for the incident light beam of Gaussian profile, similar behavior is observed only when the lateral width of the AOS is smaller than that of the Gaussian profile beam; in contrast, the Nc dependence of the effective nonlinear coefficient of the CTHG is quite weak.