Enhanced Nonlinear Optical Responses of Layered Epsilon-near-Zero Metamaterials at Visible Frequencies

Optical materials with vanishing dielectric permittivity, known as epsilon-near-zero (ENZ) materials, have been shown to possess enhanced nonlinear optical responses in their ENZ region. These strong nonlinear optical properties have been firmly established in homogeneous materials; however, it is as of yet unclear whether metamaterials with effective optical parameters can exhibit a similar enhancement. Here, we probe an optical ENZ metamaterial composed of a subwavelength periodic stack of alternating Ag and SiO2 layers and measure a nonlinear refractive index n2 = (1.2 ± 0.1) × 10–12 m2/W and nonlinear absorption coefficient β = (−1.5 ± 0.2) × 10–5 m/W at its effective zero-permittivity wavelength. The measured n2 is 107 times larger than n2 of fused silica and 4 times larger than the n2 of silver. We observe that the nonlinear enhancement in n2 scales as 1/(n0Re[n0]), where n0 is the linear effective refractive index. As opposed to homogeneous ENZ materials, whose optical properties are dictated by their intrinsic material properties and hence are not widely tunable, the zero-permittivity wavelength of the demonstrated metamaterials may be chosen to lie anywhere within the visible spectrum by selecting the right thicknesses of the subwavelength layers. Consequently, our results offer the promise of a means to design metamaterials with large nonlinearities for applications in nanophotonics at any specified optical wavelength.