Gain media for laser fusion energy drivers: Retrospect and prospect

Following the historic achievement of fusion ignition at the National Ignition Facility in December 2022, laser-driven inertial confinement fusion has entered a new era, yet the transition from scientific breakeven to commercial power plants imposes unprecedented challenges on laser drivers—specifically, ≥10 Hz repetition rate, ≥10% wall-plug efficiency, and ≥100× target gain, which are orders of magnitude beyond the current～0.5% efficiency and multi-hour shot intervals. As the core component of laser amplifiers, the gain medium fundamentally governs whether the driver can bridge this gap through its energy storage capacity, thermal conductivity, thermo-mechanical reliability, and long-term radiation resistance. This paper systematically reviews the evaluation framework, research status, and engineering progress of gain media for laser fusion energy drivers. An eight-indicator evaluation system is constructed, encompassing stimulated emission cross-section, gain bandwidth, fluorescence lifetime, thermal conductivity, laser-induced damage threshold, nonlinear refractive index, quantum defect, and thermal shock resistance parameter, with detailed analysis of their coupling relationships and trade-off strategies. Three categories of gain media are comprehensively examined: Nd-doped phosphate laser glasses, uniquely capable of meter-scale aperture fabrication but severely limited by ultralow thermal conductivity (～0.8 W·m⁻¹·K⁻¹); laser crystals including Nd:YAG, Yb:YAG, and co-doped alkaline-earth fluoride crystals, offering high thermal conductivity yet constrained by crystal growth defects and size limitations; and laser ceramics, particularly Yb:YAG and sesquioxide ceramics, which combine crystal-like thermal performance with the fabrication flexibility approaching that of glasses. Key engineering challenges are discussed, including large-aperture fabrication, amplified spontaneous emission suppression, thermal management under 10 Hz quasi-steady-state heat accumulation, and thermal fatigue lifetime over 10⁹ shot cycles. A comprehensive comparison of existing gain media reveals that no single material currently satisfies all requirements, while critical gaps remain in large-aperture manufacturing consistency and multi-factor coupled aging data under fusion-relevant conditions. Future research directions including advanced material synthesis, integrated device design, and lifetime assessment methodologies are outlined.