Recently, a research team from Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences (CAS), proposed an on-line measurement technique for high-power laser-induced emission spectra. It revealed the effect of internal molecular vibration on the transient defect formation in KDP/DKDP crystals. The results were published in Optics & Laser Technology in March 2021.

Potassium dihydrogen phosphate (KDP) and potassium diphosphonium phosphate (DKDP) are important nonlinear optical materials widely used for frequency conversion and optoelectronic switching in high-power laser systems.

Currently, some invisible transient defects during high-power laser irradiation are widely recognized. These defects can alter the energy structure of KDP crystals and generate laser damage sites. Exploring the formation mechanism of transient defects in KDP/DKDP crystals during high-power laser irradiation has become one of the most important tasks for researchers.

The stimulated Raman scattering spectra of KDP and DKDP crystals under a high-power nanosecond laser and a low-power CW laser irradiation were compared in detail. The Raman spectra of the damaged and undamaged sites of KDP and DKDP crystals were also thoroughly analyzed. 

Researchers found that the energy delivered by the high-power laser increased the molecule vibration in the crystal, the vibration displacement occurred due to the high electric field, then a new emission peak at 467 nm was generated. These transient defects were the precursors of laser damage, and the energy level was approximately 2.66 eV. Moreover, the stronger intensity of the stimulated Raman scattering corresponded to higher laser damage resistance, and it revealed that laser damage resistance of KDP and DKDP crystals depended on the stability of their internal molecular structure.

This work was supported by the National Key R&D Program of China, the National Natural Science Foundation of China, and the National Special Support Program for Young Top-notch Talent. 

Fig 1. Schematic diagram of high-resolution detection of high-power laser-induced emission spectra. (Image by SIOM)

Fig 2. Emission spectra of (a) KDP, (b) 70% deuterated DKDP sample under high-power laser irradiation at O polarization, Emission spectra of (c) KDP, (d) 70% deuterated DKDP sample under high-power laser irradiation at E polarization. (Image by SIOM)

Article website:
https://doi.org/10.1016/j.optlastec.2020.106681