Pathway to Optical-Cycle Dynamic Photonics: Extreme Electron Temperatures in Transparent Conducting Oxides

Abstract: We find that transparent conducting oxides (TCOs) exhibit oscillatory (sign-reversing) dynamics on a few optical cycle timescale under extreme electron temperatures. We demonstrate a mechanism for such transient dynamics and present an inverse-designed multilayer cavity incorporating an ultrathin TCO layer that supports the oscillatory behavior. This approach yields transmittance oscillations with a period of ~20 fs, which corresponds to three optical cycles of the probe beam. To achieve a similar oscillatory modulation in the refractive index, we incorporate a TCO electron-acceptor layer on top of the inverse-designed cavity, enabling thermionic carrier injection at the TCO heterojunction. The resulting acceptor layer achieves a striking Δn response time as short as 9 fs, approaching a single optical cycle, and is further tunable to sub-cycle timescales. The findings not only clarify the elusive transient physics in TCOs but also demonstrate, for the first time, the critical role of electron temperatures in driving oscillatory dynamic responses. More broadly, we establish TCO-based thermionic carrier injection as a practical route to novel time-varying photonic media operating on the timescale of an optical cycle, enabling time-reflection, time-refraction, and related dynamic phenomena from the visible to the infrared.