Near-Unity-Efficiency Gas Gratings for Ultraviolet, Visible, and Infrared High-Power Lasers

TL;DR

This work tackles optical damage limits in high-energy lasers by introducing gas-phase volume diffraction gratings formed via photochemical imprinting in an ozone-doped flow. The authors demonstrate diffraction across deep ultraviolet to near-infrared wavelengths and from nanoseconds to femtoseconds, achieving near-unity efficiency and preserving beam quality. They show that adding CO$_2$ dramatically enhances index modulation and diffraction efficiency, enable stable long-term operation, and validate a theoretical framework for predicting performance. The results offer a debris-immune, high-damage-threshold alternative to solid optics and provide a roadmap for scaling gas gratings to real-world high-energy laser systems such as inertial confinement fusion facilities.

Abstract

Interfering deep ultraviolet (DUV) lasers can induce substantial density modulations in an ozone-doped gas flow via photochemical reactions, creating volume diffraction gratings. These transient optics are immune to target debris and shrapnel and feature orders-of-magnitude higher damage thresholds than conventional solid optics, providing a promising method for efficiently manipulating high-energy lasers. In this work, we describe gas gratings that can efficiently diffract probe beams across a variety of wavelengths and pulse durations, ranging from deep ultraviolet to near-infrared and from nanosecond to femtosecond, achieving a full beam diffraction efficiency up to 99% while preserving the focusability and wavefront quality. In addition, we present a comprehensive characterization of the performance of the gas gratings under various experimental conditions, including imprint fluence, gas composition, and grating geometries, showing significant enhancement of this process with the addition of carbon dioxide. We also demonstrate stable performance over hours of operation. Our results validate a previously developed theoretical model and suggest optimal parameters to efficiently scale gas gratings to high-energy applications.

Figures (8)

• Figure 1: Experimental schematic. Two coherent $266\,\mathrm{nm}$ lasers cross in an ozone-doped flow, producing a large gas density modulation that matches the interference pattern of the imprint lasers. The modulated gas then acts as a volume diffraction grating and deflects a probe beam. The insets show (a) the interference pattern of the imprints and (b) the phase shift due to the gas density modulations from an interferometry measurement.
• Figure 2: Images of transmitted pulsed probes of various wavelengths with and without the presence of gas gratings. The diffraction efficiency is $57\%$ for the $266\,\mathrm{nm}$ probe ($84\%$ was transmitted and $68\%$ of the transmitted was diffracted to the first order), $99\%$ for the $532\,\mathrm{nm}$ probe, $85\%$ for the $800\,\mathrm{nm}$ probe, and $81\%$ for the $1064\,\mathrm{nm}$ probe. The $800\,\mathrm{nm}$ probe has a pulse duration of $\sim 35\,\mathrm{fs}$ while the rest have pulse durations of $\sim 5-10\,\mathrm{ns}$. The spectrum of the femtosecond probe spans from $795\,\mathrm{nm}$ to $815\,\mathrm{nm}$ (FWHM). The images were averaged of 100 shots.
• Figure 3: (a,b) Comparison of the $532\,\mathrm{nm}$ nanosecond probe delay scans (red dots/curves: single-shot/averaged measurements) with normalized CW probe diffraction traces (blue curves) under two different conditions: the imprint fluence in (b) was roughly double that of (a) while the ozone concentration was around $2.3\%$ in both measurements. (c) First-order diffraction efficiency $\eta_1$ vs. index modulation $n_1$ according to \ref{['eq:eta']}, with $D = 5\,\mathrm{mm}$, $\theta_B = 0.5^\circ$, and $\lambda = 532\,\mathrm{nm}$. (d) CW traces at varying imprint fluences with $3.9\%\pm0.1\%$$\mathrm{O_3}$ (assuming $1\,\mathrm{atm},\,300\,\mathrm{K}$), $5\,\mathrm{mm}$ wide flow tube, and $\sim\!32\,\text{\textmu}\mathrm{m}$ grating period.
• Figure 4: (a) Diffraction efficiency of the $532\,\mathrm{nm}$ nanosecond probe ($\approx\!5\,\mathrm{ns}$ FWHM pulse duration, $\lessapprox\!200\,\text{\textmu}\mathrm{J}$ average pulse energy, cleaned with a spatial filter) and (b) average gas temperature estimated from CW traces versus imprint fluences at different ozone concentrations ($0.9\%\pm0.1\%,\,1.4\%\pm0.1\%,\,1.9\%\pm0.2\%,\,3.0\%\pm0.1\%,\,3.9\%\pm0.1\%$, indicated by colors). The ozone concentration is calculated assuming $1\,\mathrm{atm},\,300\,\mathrm{K}$. Error bars indicate uncertainty about the local imprint fluence and standard deviations of the measured efficiency. The flow tube was $5\,\mathrm{mm}$ wide. Grating period was around $32\,\text{\textmu}\mathrm{m}$.
• Figure 5: (a) Diffraction efficiency of the $532\,\mathrm{nm}$ nanosecond probe ($\approx\!5\,\mathrm{ns}$ FWHM pulse duration, $\lessapprox\!200\,\text{\textmu}\mathrm{J}$ average pulse energy, cleaned with a spatial filter) and (b) average gas temperature rise estimated from CW traces versus imprint fluence at different $\mathrm{CO_2}$ concentrations ($0\%$, $25\%\pm5\%$, $50\%\pm5\%$, and $75\%\pm5\%$, indicated by colors). The $\mathrm{CO_2}$ concentration are estimated as the ratio of $\mathrm{CO_2}$ flow rate to the total $\mathrm{CO_2}$-$\mathrm{O_2}$-$\mathrm{O_3}$ mixture flow rate, with $\mathrm{CO_2}$ and $\mathrm{O_2}$ initially at similar pressure and temperature. Ozone concentration remains constant at $1.0\%\pm0.1\%$ across all measurements (calculated assuming $1\,\mathrm{atm}$, $300\,\mathrm{K}$). Error bars indicate uncertainty about the local imprint fluence and standard deviations of the measured efficiency. The flow tube was $5\,\mathrm{mm}$ wide. Grating period was around $30\,\text{\textmu}\mathrm{m}$.