Cross-Phase Modulation via Time-Varying Epsilon-Near-Zero Metasurfaces

Ultrafast all-optical metasurfaces based on epsilon-near-zero (ENZ) materials are particularly interesting due to their ability to induce unity-order change in the refractive index within hundreds of femtoseconds timescales. This strong time-dependent index enhancement can alter the frequency of an optical beam passing through the dynamic medium. Frequency conversion in ENZ metasurfaces has been demonstrated via self-phase modulation when the metasurface is excited at its ENZ wavelength and via adiabatic frequency conversion when both pump and probe beams are tuned to the ENZ wavelength. Here, we report the first observation of cross-phase modulation in a time-varying ENZ-metasurface. An ENZ-metasurface has been designed to exhibit two well-defined absorption bands on both sides of the ENZ region. Under oblique-incidence excitation in these bands, we report large, tunable, and broadband frequency translation of up to 100 nm at lower pump energy, in excellent agreement with numerical simulations. We also demonstrate that exciting this particular metasurface outside the ENZ region induces the time-varying, distinct nonlinear phase shifts by enhancing the nonlinearity in the system. Our results can potentially provide insights into designing an efficient time-varying metasurface for the phase modulation of ultrafast light.