Nonlinear Optical Properties of Metal-Dielectric Stacks in their Epsilon-Near-Zero Regime

Abstract

In recent years, significant attention has been given to a class of materials referred to as epsilon- near-zero (ENZ) materials which possess a vanishing dielectric permittivity. This ENZ condition occurs naturally in metals, polar dielectrics, doped semiconductors, etc. However, the wavelength range of the ENZ behaviour is dictated by intrinsic material properties and is not necessarily convenient for all applications. The ENZ condition may also be artificially achieved using meta-materials composed of sub-wavelength-scale constituent materials. In this work, we designed and fabricated a sub-wavelength periodic stack using silver (Ag) and silica (SiO2) capable of placing the ENZ wavelength anywhere within the entire visible spectrum by adjusting the respective dimensions of the constituent metal and dielectric. We characterize the nonlinearity of this sample using the Z-scan technique and observed a pronounced enhancement of nonlinear response at the ENZ region. The observed value of the nonlinear refractive index n2 of our metal-dielectric multilayer sample is 1.74 cm^2/GW, which is 10^7 times larger than that of the n2 of fused silica and 20 times larger than that of the n2 of bulk silver. We also find that the nonlinear absorption coefficient β is given by -2.43 ×10^5 cm/GW. The ability to obtain strong nonlinearities at designated optical frequencies makes these metamaterials a flexible platform for the nonlinear applications in the visible spectral range. The results of this work confirm that the enhancement of nonlinear optical properties in the ENZ regime can be generalized to a broader class of materials.