Optical metasurfaces based on epsilon-near-zero materials: Towards low power nonlinear optics

Epsilon near zero (ENZ) materials are a promising candidate for nonlinear applications. Specifically, thin-film ENZ materials can demonstrate strong field enhancements and large optically induced refractive index changes, enhanced further through plasmonic antennas in a metasurfaces geometry. In this paper, we investigate the thermo-optic response of such ENZ metasurfaces and present a novel method for characterising the nonlinear phase shift, based on Hilbert transforms. Specifically, the thermo-optic response of the material can be fully characterised by measuring two transmission spectra, at low and high pump power. Through the Hilbert transformations, the phase response associated with each transmission spectrum can be calculated, allowing a characterisation of both the real and imaginary component of the nonlinear transmission function. Using this new approach, we show strong thermo-optic responses for sub-mW optical pumping, opening the way towards low power nonlinear optics.