Electric filed dependence of damage thresholds for third harmonic separator

Third harmonic separator has proven to be a critical component in high-power laser systems, which can be used for beam splitting and optical path change of 355 nm laser. It is desirable for the coating to have high spectral efficiency and laser induced damage threshold. Through precise fabrication, the spectrum efficiency can easily meet the requirements with high reflection at 355nm and high transmittance at 532nm and 1064nm. However, the issue is more complicated and quite challenging for obtaining high LIDT, because high refractive index materials tend to have high absorption at ultraviolet, which contribute to a decreased LIDT together with the electric field inside the film. It is significant to investigate the interplay between absorption, electric field intensity and LIDT. In this paper, materials of HfO2 and SiO2 are used to design and fabricate the two third harmonic separator with similar film structure, high spectral efficiency and different electric field distribution by electron beam evaporation. Spectral measurements results show that the prepared samples all have similar spectra with high spectral efficiency. Damage threshold and weak absorption test results show that high intra-film electric field distribution results in higher weak absorption and lower damage threshold. Our research has important reference significance for the design and preparation of high damage threshold third harmonic separator.