Multilayer dielectric gratings(MLDGs)have been widely used in chirped pulse amplification due to their high laser induced damage thresholds(LIDTs). The quest for MLDG LIDT improvement is endless. For MLDGs applied in picosecond(ps) lasers, damage shows the characteristics of both thermal effect and nonlinear effect. The thermal damage of multilayer dielectric films (MLDFs) and MLDGs were investigated using a 1064 nm laser with a duration of 8 ns in our study. Differently from previous 1-on-1 studies, Raster Scan method is adopted to investigate the effect of low-density defects on the laser damage resistance of MLDFs with different top layers and MLDGs. The results show that the LIDTs of MLDGs are half of those MLDFs. For MLDFs with the top layer of HfO2, the damage behaves the ablation of the top layer material due to the surface strong electrical field. For MLDFs with top layer Ta2O5 and SiO2, the typical morphologies are nodule ejections. The initial damage of MLDGs fabricated by etching these three kinds of grating films are similar, and all behave nodule ejections. This indicates that reducing nodule defects can help the MLDGs LIDT improvement in ps pules. These results provide guidance for process optimizations of MLDG fabrication.
Multilayer dielectric gratings (MLDGs) have been widely used as pulse compression grating (PCG) in chirped pulse amplification (CPA) technology due to their high laser induced damage thresholds (LIDTs). The quest for MLDGs LIDTs improvement is endless. As one of the core components of CPA process, MLDGs will encounter laser irradiation of nanosecond, picosecond and femtosecond. Therefore, the damage characteristics of MLDGs should be studied at various pulse widths. We performed the LIDTs test on a Nd:YAG laser system with a wavelength of 1064 nm and a pulse width of 8 ns. Damage characteristics of both MLDFs and MLDGs were investigated. MLDFs were deposited on the substrates cleaned by hand wipe or ultrasonic cleaning. The results show that the LIDTs of MLDGs are approximately 60% of MLDFs. Besides, LIDTs of MLDFs with HfO2 top layer will not be affected by the methods of substrates cleaning due to its surface damage characteristic related to the non-zero EFI on the surface material. However, for the MLDFs with top layer of Ta2O5, LIDTs of MLDFs deposited on substrates cleaned by hand wipe are higher than those deposited on the ultrasonically cleaned substrates.
Since negative photoresist SU-8 has become a common material for multi-photon micro-lithology, it is necessary to study laser conditions adopted in lithology process. Optical transmittance of SU-8 was tested. According to Urbach optical-absorption theory and Gaussian laser lateral spatial intensity envelope, relationship between theory and actual polymerization size of SU-8 was shown. Experimentally, we investigated multi-photon polymerization threshold and laser-induced damage of SU-8 under femtosecond laser irradiation with the pulse width of 45 fs at 800 nm by 1-on-1 tests. The polymerization and damage threshold at 45 fs are 2.7 and 8.9 TW/cm2, respectively. Polymerization and damage morphologies are shown with high contrast and polymerization sizes are measured under SEM. Theoretical polymerization sizes versus laser fluence are calculated by laser-induce multi-photon polymerization size analysis (LMPSA), including Urbach optical-absorption theory and Gaussian laser lateral spatial intensity distribution. The calculated results show that diffusion exists in the femtosecond laser-induced polymerization.
This work is dedicated to the study of fatigue effects upon femtosecond laser-induced damage of Ta2O5/HfO2/SiO2 highreflective coatings irradiated by pulse train at 1Hz (65 fs, 800 nm). Upon on comparative measurements of different pulse numbers involving between 10 and 300 pulses, laser-induced damage threshold (LIDT) decreases and the multipulse LIDT decreased to the level of 70~75% of the single pulse LIDT. In addition, we found that the probability of damage performs an increasing trend with the number of pulse increases when the coating is irradiated with the same fluence. The evolution of LIDT and 100% damage probability threshold under multipulse irradiations revealed that fatigue effects were affected by both laser fluence and shot numbers. The deep defects play an important role in the multi-shot mode. A correlative theory model based on critical conduction band electron density is constructed to elucidate the experimental phenomena.
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