Optical coherent perfect absorption and amplification in a time-varying medium

TL;DR

This paper demonstrates how a periodically modulated indium tin oxide film can be dynamically tuned to act as an optical amplifier or absorber by controlling probe beam phases, extending coherent perfect absorption concepts into the time domain.

Contribution

It introduces a method to achieve tunable optical amplification and absorption in a time-varying medium using phase control of probe beams in a Floquet-engineered system.

Findings

Achieved up to 80% absorption and 400% amplification.

Demonstrated dynamic switching between gain and loss modes.

Extended coherent perfect absorption to the temporal domain.

Abstract

Time-invariant photonic structures amplify or absorb light based on their intrinsic material gain or loss. The coherent interference of multiple beams in space, e.g., in a resonator, can be exploited to tailor the wave interaction with material gain or loss, respectively maximizing lasing or coherent perfect absorption. By contrast, a time-varying system is not bound to conserve energy, even in the absence of material gain or loss, and can support amplification or absorption of a probe wave through parametric phenomena. Here, we demonstrate theoretically and experimentally how a subwavelength film of indium tin oxide, whose bulk permittivity is homogeneously and periodically modulated via optical pumping, can be dynamically tuned to act both as a non-resonant amplifier and a perfect absorber, by manipulating the relative phase of two counterpropagating probe beams. This extends the concept of coherent perfect absorption to the temporal domain. We interpret this result as selective switching between the gain and loss modes present in the momentum bandgap of a periodically modulated medium. By tailoring the relative intensity of the two probes, high-contrast modulation can be achieved with up to 80% absorption and 400% amplification. Our results demonstrate control of gain and loss in time-varying media at optical frequencies and pave the way towards coherent manipulation of light in Floquet-engineered complex photonic systems.