Pathway to optical-cycle dynamic photonics: extreme electron temperatures in transparent conducting oxides

We theoretically demonstrate that transparent conducting oxides (TCOs) exhibit oscillatory and sign-reversing dynamic modulation in transmittance on the order of a few optical cycles under extreme electron temperatures, providing a possible explanation for TCO dynamics observed in earlier experiments. We present an inverse-designed multilayer cavity incorporating an ultrathin TCO layer, which supports an oscillatory optical response more pronounced than those previously observed experimentally in TCOs. This approach yields transmittance oscillations with a characteristic period of  ~ 20 fs, which corresponds to approximately four optical cycles of the 1.431 μm probe beam. To achieve a similar oscillatory modulation in Δn, we incorporate a TCO electron-acceptor layer on top of the inverse-designed cavity, enabling thermionic carrier injection at the TCO junction. The resulting acceptor layer exhibits a striking Δn response as fast as 20 fs, corresponding to only three optical cycles of the 1.8-2.0 μm probe, and can be further tailored into the sub-optical-cycle regime. The findings could both clarify the previously unexplained transient dynamics in TCOs and, for the first time, demonstrate the critical role of electron temperatures in driving oscillatory dynamic responses.