Study On The Micro-mechanism Of Optical Damage In Infrared Nonlinear Optical Crystals

As the key optical devices for frequency conversion of all-solid-state lasers,infrared nonlinear optical crystals can strongly affect output performance and beam quality of infrared lasers.Nevertheless,with the development of high-power infrared lasers and optical devices based on infrared nonlinear optical crystals,the laser-induced damage has limited the further enhancement of high-power laser output.The key problem for the laser damage is laser-induced absorption through optical crystals.During the laser-induced damage,the damage mechanisms mainly include thermal damage,avalanche ionization damage,and multi-photon ionization damage.Meanwhile,defects on the surface and subsurface of optical crystals can easily absorb laser energies during the irradiation of a femtosecond laser,resulting in the damage to optical crystals under laser irradiation.Therefore,the studies on the morphologies,structures,optical properties,and ultrafast damage processes in optical crystals under the irradiation by femtosecond lasers are crucial for in-depth understanding of the damage mechanism.Besides,these studies can improve the performance and service life of optical crystals,and enhance the output power,as well as reduce the operating cost of high-energy laser systems.In this work,morphologies,structures,and optical properties of infrared nonlinear optical crystals,such as Zn Ge P₂（ZGP）,GaSe,and Ba Ga₄Se₇（BGSe）under the irradiation by femtosecond lasers have been studied.The ultrafast damage processes were further studied through ultrafast pump-probe spectroscopy,and analyze the damage mechanism.This main work is listed as below:1.Native point defects in a ZGP crystal were fully studied by the combination of high angle annular dark field scanning transmission electron microscopy（HAADF-STEM）and optical measurements.The atomic structures of the native point defects,such as Zn vacancy,P vacancy,and Ge-Zn antisite were directly obtained through HAADF-STEM images,and proved by Photoluminescence（PL）spectra at 77 K.The carrier dynamics of these defects were further studied by ultrafast pump-probe spectroscopy,and the decay lifetimes of 180.49,346.73,and 322.82 ps are attributed to the donor V_P⁺→valence band maximum（VBM）recombination,donor Ge_Zn⁺→VBM recombination,and donor-acceptor pair（DAP）recombination of V_P⁺→V_Zn^-,respectively,which further confirms the assignment of the electron transitions.The diagrams for the energy bands and excited electron dynamics are established based on these ultrahigh spatial and temporal results.2.The morphologies and optical properties of a ZGP crystal irradiated with various fluences were systematically investigated under laser irradiation at 530,1064 and 1300nm.Based on the morphologies,microstructures,and element distribution after damage at different wavelengths,we propose two ablation mechanisms under irradiation at different wavelengths.At short wavelengths,the laser-induced damage was dominated by the photochemical damage through the combination of photochemical and photothermal reactions,verified by in-situ TR spectroscopy.At the other two long wavelengths,the laser induced damage was caused by heating from irradiated lasers.Meanwhile,two kinds of interactions between the 530 nm femtosecond laser and ZGP crystals were revealed according to two distinct types of irradiated morphologies and Raman spectra,and further confirmed by ultrafast pump-probe spectroscopy.At lower fluences,the laser induced damage threshold（LIDT）enhancement was realized by the combination of laser cleaning and defect removal through lasers.At higher fluences,the plasma formed after the absorption by surface and subsurface defects heated the surface and formed shock waves,removing the surface through vaporization.3.Ultrafast damage process and ablation mechanism of GaSe under femtosecond laser irradiation were studied.Two types of ablation mechanisms are proposed according to two distinct types of ablation morphologies and damage products,and further confirmed by ultrafast pump-probe spectroscopy.At lower fluences,the laser-induced damage of GaSe can be attributed to the formation of new products by chemical bonds breaking and phase transitions,changing the stoichiometry and spectra while barely destroy surface morphologies.At higher fluences,surface plasma was formed with massive hot free electrons in the conduction band from the multi-photon ionization and avalanche ionization.The plasma heated the surface and formed shock wave,which removed the surface layer by vaporization,causing a damage pit on the surface.4.The ultrafast damage process and ablation mechanism of a BGSe crystal irradiated by a femtosecond laser were studied by combining the morphology characterization and optical measurements.Above the threshold,damaged products were identified by SEM-EDS,XPS,and TR spectra,includingβ-Ga₂Se₃,m-Se,a-Se,Ga₂O₃,and Ba O.The photochemical reactions and phase transitions during irradiation were revealed by in-situ TR spectra.It is proposed that the interaction between the 530nm laser and BGSe crystal was dominated by the photochemical damage through the combination of photochemical and photothermal reactions.In this work,the results can provide an in-depth understanding of the ultrafast damage process and damage mechanisms of infrared nonlinear optical crystals under laser irradiation,which is helpful for a deeper understanding of the interaction between laser and materials.This work may promote the enhancement of the device fabrication of IR nonlinear optical crystals.