Entropy mode: Unlocking a wider parameter space for high damage threshold gas optics

Transient gas optics are debris resistant and exhibit high damage thresholds compared to solid-state optics making them ideal for environments relevant to inertial fusion energy. Gas diffraction gratings are formed in an ozone–oxygen mixture by interfering ultraviolet lasers, which induce spatially periodic heating and a corresponding density modulation. Prior experiments have demonstrated damage thresholds above 1 kJ/cm² and diffraction efficiencies above 99%. These experiments operate in a regime where the gas transit time across a grating period exceeds the imprint pulse duration. Heating at the constructive fringes produces counter-propagating pressure waves that form a standing acoustic mode while a non-oscillatory entropy mode arises from slower, thermal diffusion driven density changes. We propose a new approach that isolates the entropy mode by reducing the grating period or extending the imprint pulse duration. We present the theory behind the entropy mode and experimental evidence of its isolated operation. This enables a wider range of imprint pulse durations and grating incidence angles while allowing arbitrary two-dimensional and non-periodic structures such as chirped gratings and beam shapers.