Optical Property Analyze Of 193nm Ultra Violet Fused Silica

With the development of deep ultra violet（DUV）laser source,193nm excimer laser is widely used in a variety of areas such as today’s most advanced lithography system and eye surgery.Fused silica,as a rare DUV optical material,is a popular material at193nm wavelength due to low optical loss and extremely high laser damage threshold.However,optical performance of fused silica is strongly limited by intrinsic or laser-induced defects in fused silica.Although SiOH and H₂ doping method are applied to reduced strained bonds and 193nm absorption defects in SiO₂ network,stability and lifetime of fused silica optics are strongly influenced by defects.Generation and annealing of defects lead to reversible absorption change during short-time laser irradiation and irreversible transmittance deterioration after long-time or high fluence laser irradiation.The aim of our work is to investigate possible correlation between defects and optical performance in silica network base on optical properties such as ultra violet（UV）exited photo luminescence（PL）,transmittance spectra and 193nm absorption.193nm excited PL signals are measured as a real-time and sensitive method to monitor defects’concentrations.The time-resolved lifetime spectrum method is introduced to resolve highly overlapped PL bands from different defects.Common resolved PL bands for fused silica are obtained base on PL signals from different kinds of UV fused silica samples.Corresponding excitation energies are estimated by excitation analysis at different wavelength.Considering existence of SiOH can reduce strained bonds and improve transmittance of fused silica,influence between SiOH and DUV absorption is analyzed by UV spectrophotometry,Fourier transform infrared spectrum（FTIR）and calorimetry to obtain UV/IR transmittance spectra and 193nm absorptance respectively.Experimental results of 193nm absorptance and PL bands showed strong correlations between apparent nonlinear absorption at 193nm to 650nm PL band originated from non-bridging oxygen hole center（NBOHC）,and between apparent linear absorption at 193nm to 550nm PL band.In addition,SiOH showed strong relationship with¹60nm absorption.Although no direct relationship between SiOH content and 193nm absorption is observed,NBOHC in these DUV fused silica samples are mainly due to two-photon excitation induced breakage of SiOH bond,indicating a strong impact from NBOHC on apparent nonlinear absorption at 193nm especially in fused silica with a high SiOH content.These results give a new sight to the development of high-performance DUV fused silica materials.Generation mechanism of defects are analyzed by damage and heat-treatment test.With the help of PL scanning method,three defects,oxygen deficiency center（ODC）,non-bridging hole center（NBOHC）and 550nm PL defect,are observed to be strongly associated with damage process.Among these defects,only 550nm PL defect is correlated with strained areas in damage sites.This result shows an important formation about generation process of 550nm PL defects.Finally,optical properties of possible structures of defects in fused silica are calculated by quantum chemistry method and compared with unknown defects observed in our experiment.In addition,considering H₂ doping method is widely used to anneal defects in silica network,reaction between defects and H₂ are also analyzed.Assumptions of dominant 193nm absorption defects’generation and annealing are made based on theoretical and experiment results.These results provide certain reference value in the improvement of high optical DUV performance fused silica optics.