Kerr nonlinearities of amorphous composite diffraction gratings

Abstract

Light diffraction plays a crucial role in the linear and nonlinear optical response of one-dimensional metal gratings. We study Kerr nonlinearities from amorphous Au/Al2O3 composite fillings using nonlinear effective media described by Maxwell-Garnett and Bruggeman theories together with the Fourier modal method and scattering matrix theory. The investigation of the electromagnetic fields enables us to analyze optical properties in the nonlinear regime and optimize light absorption and higher harmonic generation in the gap filled with the composite. The local electric field and third order susceptibility are contributing to the local modification of the permittivity at the fundamental wavelength and are effective parameters of both the structured geometry and the composition of the amorphous medium. The nonlinear susceptibility of metal nanoparticle inclusions becomes uniform off-resonance, indicating weak nonlinear interaction over a wide spectral range. However, we observe an increase of the absorption in the nonlinear regime at wavelengths corresponding to Rayleigh anomalies where light is trapped inside the structure at much longer wavelengths. The nonlinear local field enhancement shows a unique behavior at Rayleigh anomalies increasing for diffraction gratings with a thickness beyond 60 nm instead of decaying. In regions with a stationary electric field, the permittivity aligns with the linear regime.