Epsilon-near-zero time-gate for high-fidelity spatial information transfer through dynamic scattering media.

Transparent conducting oxides such as indium-tin oxide (ITO) exhibit strong optical nonlinearity in the frequency range where their permittivity is near zero. We leverage this nonlinear optical response to realize a sub-picosecond time-gate based on upconversion four-wave mixing (FWM) between two ultrashort pulses centered at the epsilon-near-zero (ENZ) wavelength, in a sub-micron-thick ITO film. By removing the effect of both static and dynamic scattering on the signal pulse, the time gate only retains the photons that are not scattered - the ballistic photons - resulting in high-fidelity transmission of the spatial information encoded in both the intensity and the phase of the signal pulse. Furthermore, in the presence of time-varying scattering, our time-gate can reduce the resulting scintillation by two orders of magnitude. In contrast to traditional bulk nonlinear materials, time gating by sum-FWM in a sub-wavelength-thick ENZ film can produce a scattering-free upconverted signal at a visible wavelength without sacrificing spatial resolution, which is usually limited by the phase matching conditions. Our experiment can have implications for possible applications such as in vivo diagnostic imaging and free-space optical communication.