Enhancement of third-order optical nonlinearity in effective epsilon-near-zero metamaterials of dielectric/metal/dielectric sandwich nanostructures

We report the enhancement of third-order optical nonlinearity in dielectric/metal/dielectric sandwiches driven by the epsilon-near-zero (ENZ) effect. The lithium niobate (LN) and Au are chosen as the typical dielectric and metal, respectively. The sandwich nanostructure consists of two layers of LN film (90 nm) and an insertion of Au layer of different thicknesses (9, 13, and 17 nm). The ENZ wavelength of LN/Au/LN (LAL) sandwiches is experimentally obtained with a modulation from 0.96 μm (Au layer 17 nm) to 1.33 μm (Au layer 9 nm). The nonlinear refractive index n₂ and nonlinear absorption coefficient β are determined at variable near infrared wavelengths using the Z-scan method. The maximum $n2=2.31×10−14(6.76×10−15)m2/W$ and $β=−9.20×10−8(−1.94×10−8)m/W$ are obtained in the LAL sandwich with a 13 nm Au layer of ENZ wavelength 1.088 μm at the wavelength 1.064 μm with a pulse duration of 25 ps (120 fs). The n₂ is around 19 and 25 times larger than those in the pure LN film of thickness 180 nm measured at the picosecond and femtosecond time domains, respectively. The enhancement of n₂ in LAL sandwiches follows the numerical results obtained from the ENZ effect. Especially, the LN layer and the Au layer have comparable contributions to the effective third-order susceptibility $χeff(3)$⁠, which leads to the reconfigurable $χeff(3)$ by changing the thickness of each layer and further to modulate the n₂ and β of the samples. The results offer a promising way to attain large and reconfigurable optical nonlinearities for application in all-optical photonic devices at a specified wavelength.