Laser-driven inertial confinement fusion (ICF) stands as a pivotal approach in the quest for clean and virtually limitless energy, a concept first proposed in the 1960s. The fundamental principle involves using high-power lasers to symmetrically irradiate a fuel-filled target, compressing and heating the fuel to conditions where nuclear fusion can occur. A critical parameter in this process is the laser wavelength. It has been established that using short-wavelength ultraviolet (UV) lasers, typically generated through frequency conversion of infrared (IR) lasers, offers indispensable advantages. These include more efficient energy absorption by the target, better control over plasma instabilities, and reduced generation of undesirable high-energy electrons, all of which are crucial for achieving fusion ignition.

The significance of this technology was brought to the forefront by the historic achievement at the U.S. National Ignition Facility (NIF) in December 2022, where a net energy gain from a fusion reaction was demonstrated for the first time. This milestone, which relied on converting 2.05 MJ of infrared laser energy into 3.15 MJ of fusion energy using third-harmonic generation (THG) in potassium dihydrogen phosphate (KDP) crystals, has profoundly boosted global confidence in the potential of laser fusion energy. As the field now looks beyond single-shot ignition experiments towards the development of a commercially viable fusion power plant, the requirements for laser drivers are evolving. Future laser fusion energy drivers must not only deliver high energy but also operate at high repetition rates (>10 Hz). This shift introduces a formidable challenge: managing the substantial heat deposited in the frequency conversion crystals due to laser absorption. Consequently, the development of nonlinear optical crystals with superior thermal and optical properties, coupled with advanced thermal management technologies, has become a critical bottleneck and a key research focus for realizing the dream of laser fusion energy.

This paper provides a comprehensive review of the progress in frequency conversion technology for laser fusion drivers, from the foundational developments in single-shot systems to the cutting-edge research for future high-repetition-rate energy drivers.

The development of high-efficiency THG for large-scale ICF facilities has been a gradual process of engineering maturation. The nonlinear optical crystals used, primarily KDP and its deuterated form (DKDP), must meet stringent criteria, including high damage threshold, good optical homogeneity, and the ability to be grown to large size (approaching 400 mm). Early large-scale implementation on the Nova laser in the 1980s pioneered the use of a 3×3 segmented crystal array to overcome limitations in single-crystal growth size (Fig. 1). Employing a “Type-II/Type-II” phase-matching scheme, Nova achieved a THG conversion efficiency of over 60% (Fig. 2), providing the first major scientific validation of large-energy UV laser drivers. However, the segmented design introduced diffraction effects that degraded beam quality. The subsequent Beamlet laser, a prototype for NIF, introduced two key innovations: monolithic (single-piece) large-aperture KDP/DKDP crystals and a more robust “Type-I/Type-II” phase-matching scheme [Fig. 3(a)]. This new approach was less sensitive to polarization and temperature variations, consistently achieving over 70% efficiency and demonstrating superior stability [Fig. 3(b)]. The NIF facility inherited and scaled up this architecture, further refining crystal growth to achieve more than 80% THG efficiency with 42 cm crystals, setting the global standard for modern ICF drivers (Table 1).

China’s Shenguang series of laser facilities have also made significant strides. Initially, these facilities faced a bottleneck where the THG efficiency would plateau and fall away from theoretical predictions at high input intensities (Fig. 4). To address this, researchers at the China Academy of Engineering Physics (CAEP) conducted a systematic analysis, identifying and precisely controlling several critical factors that contribute to phase mismatch. Key breakthroughs included improving the optic axis and deuterium content uniformity in DKDP crystals, optimizing crystal mounting techniques to minimize wavefront distortion, and developing an advanced temperature control system capable of maintaining uniformity to within 0.05 ℃, surpassing the NIF specification (Fig. 5, Table 2). The successful integration of these advancements led to a landmark achievement: a stable THG efficiency of over 80% was demonstrated on a 430 mm aperture system, with a peak efficiency of 82.8%, which was in excellent agreement with theoretical models (Fig. 6).

Research has also extended to higher-order harmonics, as even shorter wavelengths could further enhance laser-plasma coupling. Significant progress has been made in fourth-harmonic generation (FHG). Researchers at CAEP developed an innovative scheme using non-critical phase matching (NCPM) in a 65% deuterated DKDP crystal within a converging beam. This technique achieved a remarkable 82% conversion efficiency from second-harmonic to fourth-harmonic light, generating over 180 J of UV output (Fig. 7). Fifth-harmonic generation remains challenging due to material limitations and higher susceptibility to optical damage and thermal effects, with current efficiencies around 14%?30%.

For future fusion energy drivers, the primary challenge shifts to managing thermal effects in high repetition rate and high average power operation. Comparative studies of different crystals for second-harmonic generation (SHG) at 10 J/10 Hz revealed that while DKDP suffered from thermal dephasing, YCOB’s performance was limited by material quality, and lithium triborate (LBO) showed excellent performance, achieving 82% efficiency at 0.7 GW/cm2 (Table 3, Fig. 8). Building on this, the Bivoj/DiPOLE laser facility set a new world record, producing 50 J of third-harmonic energy at a 10 Hz repetition rate using large-aperture LBO crystals (Fig. 9). However, thermal gradients were still observed to evolve in the crystal over time, affecting the beam profile (Fig. 10). In China, research on a 100 Hz system demonstrated 3 J SHG output using an LBO crystal with a micro-channel cooling system that precisely controlled temperature to within ±0.05 °C (Fig. 11), demonstrating a scalable path towards kilowatt-level average power.

Frequency conversion technology is a cornerstone of mainstream laser fusion research. For single-shot, large-scale facilities like NIF, the technology based on KDP/DKDP crystals is mature, and China has recently demonstrated a breakthrough in achieving over 82% third-harmonic conversion efficiency through meticulous engineering controls. However, for the next generation of fusion energy drivers operating at high repetition rates, significant challenges remain. The thermal effects, optical damage, and long-term stability of nonlinear crystals under high average power are unresolved physical problems that require comprehensive breakthroughs.

Future development should focus on three synergistic directions. First is the development of advanced crystal materials with lower absorption, higher damage thresholds, and larger nonlinear coefficients. This includes exploring alternatives like large-scale LBO and innovative concepts like composite “sandwich” crystal structures or artificially micro-structured materials. Second is the advancement of thermal management engineering. Techniques such as bonding crystals to high-conductivity heat sinks or using a gas-cooled, sliced-crystal-stack architecture must be perfected to handle kilowatt-level average powers. Third, the application of intelligent control systems is crucial. By integrating multi-physics modeling with AI-driven, real-time feedback from distributed sensors, future systems can dynamically predict and compensate for thermal distortions, ensuring stable, efficient, and reliable operation. The successful integration of these advancements will pave the way for a robust frequency conversion solution capable of meeting the demanding requirements of a laser fusion power plant.