Nonlinear Modeling of Photochemically-Induced Gaseous Optical elements

A proof-of-principle experiment [1] recently demonstrated dielectric mirrors made out of neutral gas, operating at fluences above 1.5 kJ/cm2 at 10 Hz repetition rate with diffraction efficiencies above 95%. By increasing the damage threshold by two or three order of magnitude compared to solid elements, such gaseous optics have a transformative potential for high-power laser applications such as Inertial Fusion Energy (IFE). Their operation relies on the modulated energy deposition of a low-energy “imprint” beam (such as a pair of overlapping beams) via absorption by a dopant element in the gas (e.g., ozone, for UV imprint beams). The resulting gas heating can initiate an acoustic/entropy wave which modulates the gas density and hence its refractive index, turning the gas into a grating or other diffractive optics elements. Here, we present results from a comprehensive modeling suite that includes: i) the chemistry of UV absorption by ozone and gas heating from the subsequent chemical reactions; ii) the nonlinear hydrodynamic response of the gas from a 1D hydrodynamic code resolving Euler equations; iii) a 3D Fresnel diffraction code to calculate the diffraction of an external, high-power laser off the resulting index modulation. For small perturbations, the simulations show an excellent agreement with linear theory [2]. For stronger perturbations, nonlinear effects arise due to the depletion of ozone and nonlinear wave excitation. We will present comparisons with recent experiments at Stanford University, and discuss future directions and applications of such gas-optics.

[1] Y. Michine and H. Yoneda, Commun. Phys. 3, 24 (2020).

[2] P.Michel et al., Submitted to Phys. Rev. Applied (2024).