Dichroic laser mirrors are usually used as harmonic separators, beam combiners or splitters, and 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. Unfortunately, a traditional dichroic laser mirror (TDLM) composed of alternating high- and low-refractive index (n) pure materials often has difficulty achieving excellent spectral performance and high LIDTs at two wavelengths simultaneously. Therefore, there is a trade-off between the required optical performance and LIDT.
Recently, the research group led by Prof. Jianda Shao and Prof. Meiping Zhu from the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, proposed to use 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 results are published in Photonics Research , Vol. 9, No. 2, 2021 (Tingting Zeng, Meiping Zhu, Yingjie Chai, Jingping Li, Jianda Shao. Dichroic laser mirrors with mixture layers and sandwich-like-structure interfaces[J]. Photonics Research, 2021, 9(2): 02000229).
Schematic diagram of the proposed dichroic laser mirrors with mixture layers and sandwich-like-structure interfaces
To demonstrate the proposed design strategy, dual e-beam co-evaporation technology is employed. A mixture-based dichroic laser mirror (MDLM) is designed and prepared, 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 1064 nm, the LIDTs are almost doubled.
The researchers consider that 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 demonstrates that the performance of a dichroic laser mirror with mixture layers and optimized interfaces is significantly better than that of TDLM. Future work will focus on the further optimizations of the coating structure design and coating layer deposition process to further improve the comprehensive performance of the dichroic laser mirrors, and on the exploration of the potential of using this design strategy to fabricate other types of laser coatings.
