Nonreciprocal Epsilon-Near-Zero-Dielectric Bilayers: Enhancement of Nonreciprocity from a Nonlinear Transparent Conducting Oxide Thin Film at Epsilon-Near-Zero Frequency

We envision the use of an indium tin oxide (ITO) thin film as part of a bilayered silicon-photonics subwavelength device to boost nonlinearity-assisted all-passive and ultrafast nonreciprocal behavior. Asymmetric $p$-polarized oblique excitation of a mode near the epsilon-near-zero frequency, with a highly confined and enhanced normal electric field component and large absorption, allows one to harness the strong subpicosecond nonlinear response of ITO for the generation of notable nonreciprocal performance in an ultracompact two-port element. Although the results are limited by loss, we find the optimal operational point and the maximum attainable nonreciprocal transmittance ratio of the metasurface versus light intensity---including an apparent upper bound slightly over 2---and we perform exhaustive numerical simulations considering nonlinear processes of both anharmonic and thermal nature that validate our predictions, including steady-state and pulsed-laser excitation.