Broadband and weak-dispersion nonlinear response enhancement in the epsilon-near-zero region of a nano-stepped metasurface†

Optical media with dispersion-free large nonlinearity are highly desired for a broad range of applications, such as spectroscopy, all-optical data processing, and quantum information. Here, we report that a metasurface composed of an indium–tin–oxide nano-step array can exhibit weak-dispersion and enhanced optical nonlinearity theoretically in the region of the spectrum where the real part of its effective permittivity is close to zero. Such nonlinear features are attributed to the offset of the structural dispersion and material dispersion of the metasurface in its epsilon-near-zero region. The nonlinear refractive index of our metasurface remains at around n₂ = 1.5 × 10⁻² cm² GW⁻¹ in a wide wavelength range from 1300 to 1510 nm, and the nonlinear absorption coefficient is greater than 1 × 10⁵ cm GW⁻¹ in the range from 1280 to 1780 nm in simulation. Our results open a novel approach to applications of nonlinear photonic devices requiring high integration density and stable performance.