Large-size, high-power laser coatings are key components in inertial confinement fusion (ICF) laser facilities such as Shenguang (SG) series laser facilities in China, National Ignition Facility (NIF) in the USA, and the Laser Megajoule (LMJ) facility in France. The performance of coatings directly affects the beam quality and output power of the laser facilities.

The large-size, high-power laser coatings required for large-scale laser facilities mainly include antireflection coatings, high reflection coatings, polarizer coatings, and beam splitter coatings. Typical high-power laser coatings should exhibit specific spectral characteristics to satisfy beam transmission requirements, low stress to achieve excellent wavefront quality, and high laser-induced damage threshold (LIDT) to ensure the safe and stable operation of these high-power laser facilities. Among the available coating deposition technologies, electron-beam evaporation deposition is the most commonly used method to prepare coatings for nanosecond laser applications. It offers the advantages of high LIDT, good thickness uniformity, and easy production of large-size optics.

This paper briefly introduces the progress of work related to large-size, high-power laser coatings. Furthermore, the main progress of our research group in recent years with respect to key properties, including thickness control, stress control, laser-induced damage mechanism, and methods to improve the LIDT, is presented.

In terms of coating thickness control, an improved optical monitoring strategy using multiple pieces of witness glass was proposed. To reduce thickness errors, some thick layers were split into two layers and monitored with different witness glasses. Each witness glass was monitored via a wavelength selected based on the required thickness tolerance. The proposed monitoring strategy is suitable for quarter-wavelength and non-quarter-wavelength multilayer coatings and can obtain spectral performance close to the theoretical value. A model of coating thickness distribution correction was established, and the uniformity of large-size laser coatings was improved based on shutters correction technology (Fig.2).

In terms of coating stress control, the influence of the deposition process on coating stress was studied. By optimizing the deposition process, the wavefront quality of the coating was improved and the crack problem of thick coatings was solved. Dense SiO₂ layer, prepared via an ion-assisted deposition, could isolate the electron-beam coatings from water vapor in the air. This decreased the compression stress and improved the environmental stability of HfO₂/SiO₂ multilayer coatings.

To improve the LIDT value, the laser-induced damage mechanism was investigated. Experimental results show that under nanosecond laser pulse irradiation, the laser damage is closely related to the electric field distribution inside the coating and different types of defects located at the substrate, coating, and layer interface. Subsequently, our research group focused on coating design, defect suppression, and repair technologies.

A “reflectivity and laser resistance in one” design was proposed by using tunable nanolaminate layers that served as an effective layer with a high refractive index and large optical bandgap. Al₂O₃-HfO₂ nanolaminate-based mirror coatings for ultraviolet laser applications were experimentally demonstrated, with simultaneously improved high-reflectivity bandwidth and LIDT (Fig.17).

To solve the problem of high defect density due to the alternations of two materials at the coating interface, co-evaporated interfaces (CEIs) were proposed to reduce the interfacial defect density and improve the interface adhesion. Coatings with CEIs exhibit better damage performance with respect to high-power lasers than coatings without CEIs (Fig.19).

For laser damage due to nodules and pits, a nodule dome removal strategy (Fig.22) and a pit suturing strategy (Fig.23) were proposed to eliminate the unwanted local electric field enhancement caused by nodules and pits, respectively. Experimental results showed that the proposed strategy can effectively improve the LIDT of laser coatings.

Based on the aforementioned research, large-size laser coatings, such as high-reflection coatings, polarizer coatings, and harmonic beam splitter coatings, have been successfully produced and effectively used in large laser facilities, including SG series laser facilities and the Shanghai Superintense Ultrafast Laser Facility (SULF).

In recent years, deposition technologies, such as atomic layer deposition, glancing angle deposition, and water bath treatment, have also received increasing attention in the research of high-power laser coatings, especially antireflection coatings. These technologies are expected to further enhance the performance of laser coatings in the future.