Shunli Chen

Thermal-dynamical analysis of blister formation in chirped mirror irradiated by single femtosecond lasers

The laser-induced damage behaviors of chirped mirrors (CMs) are studied by single 800 nm, 38 fs lasers. The CMs provide group delay dispersion of around -60  fs² and average reflectivity of about 99.4% with bandwidth range of 200-300 nm at a central wavelength of 800 nm. Interestingly, a circular blister feature appears in the CMs at a wide range of laser fluence. An optical microscope, atomic force microscope, scanning electron microscope, and surface profiler are applied to describe the blister characteristics. An adiabatic expansion model of ideal gas is adopted to illustrate the formation dynamics of blisters. The evolution of blisters can be explained by partial evaporation of the film and a subsequent gas expansion, driving the bulging of the film stack up to the stress limit, where the blister fractures. According to this model, the energy absorption ratio of blisters increases monotonously with increasing laser fluence before the occurrence of the focal spot confinement effect.

Method of mitigation laser-damage growth on fused silica surface

A reliable method, combining femtosecond (fs) laser mitigation and chemical (HF) etching, has been developed to mitigate laser-damage growth sites on a fused silica surface. A rectangular mitigation site was fabricated by an fs laser with a raster scan procedure; HF etching was then used to remove the redeposition material. The results show that the mitigation site exhibits good physical qualities with a smooth bottom and edge. The damage test results show that the growth threshold of the mitigation sites increases. Furthermore, the structural characteristic of samples was measured by a photoluminescence (PL) spectrometer, and the light intensification caused by damage and mitigation sites was numerically modeled by the finite-difference time-domain (FDTD). It revealed that the removal of damaged material and structure optimization contribute to the increase of the damage growth threshold of the mitigation site.

Effect of nanosecond laser pre-irradiation on the femtosecond laser-induced damage of Ta2O5/SiO2 high reflector.

The effect of nanosecond laser pre-irradiation on the femtosecond laser-induced damage behaviors of 800 nm 0° AOI Ta(2)O(5)/SiO(2) high reflectors fabricated by e-beam evaporation was explored. Laser pre-irradiation was carried out by Raster-scanning with scanning mode of 1-on-1 and scanning velocities timed such that there was a beam overlap at 70% of the peak fluence, utilizing 5 Hz 1064 nm 12 ns Nd:YAG fundamental lasers. Femtosecond laser damage was investigated by 1 kHz 800 nm 135 fs Ti: sapphire laser system with 1-on-1 mode test. The results indicated that nanosecond laser pre-irradiation did not promote the femtosecond laser-induced damage threshold of reflectors. Instead, the thresholds of all the samples with various fluence steps for pre-irradiation were reduced by about 20%. Furthermore, the damage morphologies were analyzed by optical microscope, SEM and AFM, which displayed deterministic field induced breakdown characteristics. To explain these phenomena, a theoretical model including photoionization, avalanche ionization, and decays of electrons was built to simulate the evolution of electron density in the conduction band. Field ionization mechanism was considered to dominate the femtosecond laser damage process, while the electronic defects induced by nanosecond laser pre-irradiation accelerated the femtosecond laser damage evolution.

Laser-induced damage of multilayer dielectric for broadband pulse compression grating

The multilayer dielectrics (MLDs) for broad bandwidth 800nm pulse compression gratings were fabricated with optimized design by electron beam evaporation using three different kinds of materials (Ta2O5/SiO2/HfO2), which had more than 99% reflectance with bandwidth larger than 160nm around the center wavelength of 800 nm and high transmission at the exposure wavelength of 413nm. Laser-induced damage behaviors of the mirrors were investigated. It was found that the laser-induced damage threshold (LIDT) of the samples could reach 1.0J/cm2 and 2.0J/cm2 in the normal beam (57 degrees, TE mode) at pulse duration of 50fs and 120fs, respectively. The depth information of the damage sites at these two cases was explored by atomic force microscope (AFM). The reason of the sample having so high LIDT was also discussed in this paper. The MLDs provide a solid base for the high laser threshold 800nm pulse compression gratings and may open a new way for broad bandwidth 800nm reflectance coatings used in the ultrashort pulse laser system.

Ultraviolet laser-induced damage growth characteristic and mechanism on the surface of fused silica

The growth of laser induced damage on the surface of fused silica plays a major role in determining the operation fluence and optics lifetime in high power laser system. In this paper, the damage growth characteristic of fused silica and possible growth mechanisms were investigated. The morphology of damage site was measured by scanning electron microscopy (SEM) and optical microscopy (OM). The finite difference time domain (FDTD) method was used to calculate the electric field distribution around the damage site. Furthermore, energy dispersive spectrometers (EDS) micro-analysis technique, x-ray photoelectron spectrometer (XPS) and Raman spectroscopy were applied to detect the chemical composition, point defect and microstructure of damage site in order to explore the growth mechanism. It’s found that the growth threshold is greatly affected by the size of damage site, and the growth threshold of damage site is much lower than that of undamaged area. Theoretical calculation demonstrated that the rough damage site can strong modulate the distribution of electric field and result in the enhancement of local light field around the damage site. Results also showed that the oxygen defect was generated and the structure was changed after initial laser damage. Based on the above analysis, a mechanism of laser-induced damage growth on fused silica surface was proposed.

Laser-induced damage of coatings on Yb:YAG crystals at cryogenic condition

As large amounts of heat need to be dissipated during laser operation, some diode pumped solid state lasers (DPSSL), especially Yb:YAG laser, operate at cryogenic condition. This work investigated the laser induced damage of coatings (high-reflective and anti-reflective coatings) on Yb:YAG crystals at cryogenic temperature and room temperature. The results show that the damage threshold of coatings at cryogenic temperature is lower than the one at room temperature. Field-emission scanning electron microscopy (FESEM), optical profiler, step profiler and Atomic force microscope (AFM) were used to obtain the damage morphology, size and depth. Taking alteration of physical parameters, microstructure of coatings and the environmental pollution into consideration, we analyzed the key factor of lowering the coating damage threshold at cryogenic conditions. The results are important to understand the mechanisms leading to damage at cryogenic condition.

Mechanism for 355 nm nanosecond-pulse-driven damage in HfO2/SiO2 high reflectivity coating

The effect of oxygen vacancy on the laser-induced damage behaviors of HfO2/SiO2 high reflective coating was investigated by single 355nm-8ns laser. The oxygen vacancy was tuned by controlling the oxygenation of the outmost HfO2 layer during the deposition procedure. The reflectivity of the coating with higher oxygen vacancy drops by 0.2% and the damage threshold drops by 60%, compared with the normal coating. Damage morphologies of samples were obtained by optical microscope, AFM, SEM and FIB technology. Typical morphologies of these coatings show little difference. Average oxygen vacancy of single HfO2 layer prepared on the BK7 substrate measured by XPS is about 43%. Theoretical analysis with a nonlinear thermodynamics model shows that the damage can be attributed to nonlinear thermal process. Moreover, the size of damage crater can be interpreted by a mechanics model.

Incubation effect of laser-induced surface damage of HfO2/SiO2 HR coating in the femto-nanosecond region

This paper is devoted to a long-term investigation into the nature of incubation effect of multilayer dielectric HR mirror coatings. Accumulated damage behaviors of HfO2/SiO2 mirrors for 800nm, 1053nm, and 1064nm, both fabricated by conventional electron beam evaporation (EBE), were investigated by ultra-short pulse (800nm/~100fs), short pulse (1053nm/~1ps), and long pulse (1064nm/~10ns) lasers, respectively. Incubation effect was found to be a universal phenomenon for HfO2/SiO2 mirrors irradiating by the femto-nanosecond lasers. And when the shot number was about 100, the multi-pulse damage threshold of samples decreased to the level of 60~70% of the single-pulse threshold. Typical damage morphologies and depths information of HR samples were characterized by optical microscope and surface profiler. The results revealed that the electric field distribution within the mirrors had significant influence on the initial damage onset of the mirrors. In addition, theoretical simulation was carried out to describe the incubation behaviors of HfO2/SiO2 mirrors in the femto- and nano-second regions. It seemed reasonable that incubation effect was attributed to the accumulation of native or laser-induced electronic trapping states.