Recently, a research team from the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences (CAS) proposed a new design with mixture layers and novel sandwich-like-structure interfaces to meet the challenging requirements of the ideal dichroic laser mirrors. The research article was published on Photonics Research on Jan. 27, 2021, and was highlighted as an Editor's Pick. 

Dichroic laser mirrors are usually used as harmonic separators, beam combiners or splitters. They play an important role in many laser applications, including inertial confinement fusion laser, petawatt femtosecond laser, high power fiber lasers, compact Q-switched or mode-locked lasers, and other emerging lasers. The requirements for dichroic laser mirrors continue to increase with the development of laser technology. The ideal dichroic laser mirror for high power lasers requires a significantly different reflection or transmission property and a high laser-induced damage threshold (LIDT) at two different wavelengths of interest simultaneously. 

Unfortunately, traditional dichroic laser mirrors (TDLM) composed of alternating high- and low-refractive-index (n) pure materials often have difficulty in achieving excellent spectral performance and high LIDTs at two wavelengths simultaneously. There is a trade-off between the required optical performance and LIDT. 

In this work, the researchers designed and prepared a mixture-based dichroic laser mirror (MDLM), which uses HfO₂-Al₂O₃ mixture material as a high-n layer with adjustable n and optical bandgap, and pure SiO₂ as a low-n material. The interface between the low-n SiO₂ layer and the high-n HfO₂-Al₂O₃ mixture layer is a sandwich-like-structure interface ("SiO₂-HfO₂ gradient material | HfO₂ | HfO₂-Al₂O₃ gradient material"), which replace the traditional discrete interface.  

The MDLM shows excellent spectral performance and improved performance over TDLM with finer mechanical property, lower absorption, and higher LIDT. For both the s-polarized 7.7-ns pulses at a wavelength of 532 nm and the p-polarized 12-ns pulses at a wavelength of 1,064 nm, the LIDTs are almost doubled.  

This MDLM design strategy opens new avenues for improved dichroic mirror coatings and other laser coatings and can benefit many areas of laser technology that rely on high-quality laser coatings. 

This work was supported by the National Natural Science Foundation of China, the Youth Innovation Promotion Association of CAS and the Strategic Priority Research Program of CAS. 

Fig. 1. Schematic diagram of the proposed MDLM design. (Image by SIOM)

Fig. 2. Microstructure and optical property of the TDLM and MDLM coatings. (Image by SIOM)