Guohang Hu

Combining wet etching and real-time damage event imaging to reveal the most dangerous laser damage initiator in fused silica

A reliable method, combining a wet etch process and real-time damage event imaging during a raster scan laser damage test, has been developed to directly determine the most dangerous precursor inducing low-density laser damage at 355 nm in fused silica. It is revealed that ~16% of laser damage sites were initiated at the place of the scratches, ~49% initiated at the digs, and ~35% initiated at invisible defects. The morphologies of dangerous scratches and digs were compared with those of moderate ones. It is found that local sharp variation at the edge, twist, or inside of a subsurface defect is the most dangerous laser damage precursor.

Characteristics of plasma scalds in multilayer dielectric films

Plasma scalding is one of the most typical laser damage morphologies induced by a nanosecond laser with a wavelength of 1053 nm in HfO(2)/SiO(2) multilayer films. In this paper, the characteristics of plasma scalds are systematically investigated with multiple methods. The scalding behaves as surface discoloration under a microscope. The shape is nearly circular when the laser incidence angle is close to normal incidence and is elliptical at oblique incidence. The nodular-ejection pit is in the center of the scalding region when the laser irradiates at the incidence angle close to normal incidence and in the right of the scalding region when the laser irradiates from left to right at oblique incidence. The maximum damage size of the scalding increases with laser energy. The edge of the scalding is high compared with the unirradiated film surface, and the region tending to the center is concave. Plasma scald is proved to be surface damage. The maximum depth of a scald increases with its size. Tiny pits of nanometer scale can be seen in the scalding film under a scanning electronic microscope at a higher magnification. The absorptions of the surface plasma scalds tend to be approximately the same as the lower absorptions of test sites without laser irradiation. Scalds do not grow during further illumination pulses until 65 J/cm(2). The formation of surface plasma scalding may be related to the occurrence of the laser-supported detonation wave.

Investigations on the catastrophic damage in multilayer dielectric films

HfO2/SiO2 coatings are always fluence-limited by a class of rare catastrophic failures induced by a nanosecond laser with a wavelength of 1053 nm. The catastrophic damage in HfO2/SiO2 coatings behaves as the damage growth with repeated laser irradiation, and thus eventually limits the mirror performance. Understanding the damage processes and mechanisms associated with the catastrophic damage are important for reducing the occurrence of the catastrophic failure and allowing the HfO2/SiO2 coatings to survive at the high fluence required by high laser systems. The rough damage behavior of the catastrophic failure at the proper critical fluence is present. The pit and delamination in the catastrophic failure are investigated to find the possible reasons leading to the catastrophic failure. The experimental results indicate that nodular defect originated from the substrate easily incurs the catastrophic damage. The electric field enhancements of the pit and the substrate impurities may contribute to this phenomenon. The delamination is always present on the left of the pit when laser irradiates from left to right at oblique incidence, which may be related to the plasma plume toward the laser incidence.

Transmittance increase after laser conditioning reveals absorption properties variation in DKDP crystals

By taking multiple measurements of transmittance before and after laser conditioning in DKDP crystals, we found that the transmittance was increased by 0.05%~0.4% through laser conditioning with maximum fluence 6J/cm(2), and then decreased by about 0.1% after subsequent higher fluence conditioning. Variation of scattering intensity and absorber density, the two major factors leading to transmittance change, was monitored by on-line and off-line detection systems. The transmittance decrease was attributed to laser damage scattering, and the increase was derived from reduction of absorbers. Moreover, the absorption was reduced further at higher conditioning fluence. Based on the above analysis, the heating process during laser exposure was analyzed in the time domain, and a local rapid-rising and slow-cooling process was confirmed to reduce defect concentration, which can improve laser damage resistance and increase the transmittance.

Laser damage dependence on the size and concentration of precursor defects in KDP crystals: view through differently sized filter pores.

We investigate the laser-induced damage performance at 1064 nm of potassium dihydrogen phosphate (KDP) crystals grown using filters of different pore sizes. The aim is to explore a novel method for understanding laser-matter interactions with regard to physical parameters affecting the ability of damage precursors to initiate damage. By reducing the pore size of filters in continuous filtration growth, we can improve laser damage resistance. Furthermore, we develop a model based on a Gaussian distribution of precursor thresholds and heat transfer to obtain a size distribution of the precursor defects. Smaller size and/or lower concentration of precursor defects could lead to better damage resistance.

Effect of raw material and growth method on optical properties of DKDP crystal

Three kinds of KH2PO4 raw material are used to grow deuterated potassium dihydrogen phosphate (DKDP) crystals by traditional and rapid growth methods, respectively. The growth habit dependence on the purity of raw material is described and analyzed. The optical properties including transmission spectra and laser-induced damage threshold of these crystals have been measured. It is found that the growth method affects the optical properties of crystal more obviously than the raw material with the mass content of main metal ions below 1 ppm. Moreover, the morphology of the core in the observed damage sites indicates that an explosion process probably occurs during laser-induced breakdown.

High-precision measurement of optical constants of ultra-thin coating using surface plasmon resonance spectroscopic ellipsometry in Otto-Bliokh configuration

In this paper, a surface plasmon resonance (SPR) spectroscopic ellipsometry, based on Otto-Bliokh configuration, is developed for the measurement of thickness and optical constants of ultra-thin coatings. This technique combines sensitivity of surface plasmon with accessibility of optical constants and other advantages of ellipsometry. Surface plasmons (SP) are generated in the sample under test in total reflectance mode and SP geometric distribution over the sample surface is influenced by the coating thickness and optical properties on one hand, and by the air gap thickness on the other hand. Nanoscale control of the thickness of the air gap between a convex surface and the sample was assured using a micron-size beam spot irradiating the contact zone. The amplitude and phase change induced by SPR in the visible and near-infrared spectral range were obtained to determine the dispersion of optical constants and the thickness of the ultra-thin layer. The extracted optical constants were found to be in excellent agreement with the results obtained using TEM and XRR techniques. Both theoretical analysis and experimental results demonstrated high sensitivity and precision of the proposed technique for the analysis of coatings of both metals and dielectrics on metals.

CO2 laser mitigation of the ultraviolet laser damage site on a fused silica surface

Abstract. The CO2 laser mitigation method has been developed to mitigate the ultraviolet laser damage site on a fused silica surface. The mitigation process was monitored by an on-line white light scattering imaging system in order to ensure that the mitigation is successful. Additionally, a total internal reflection microscope was utilized to analyze the mitigation pit. By optimizing the laser mitigation parameters, the rough damage site can be replaced by a smooth Gaussian-shaped mitigation pit. The chemical composition of the damage sites and the CO2 laser mitigation pits was also measured with energy dispersive x-ray spectroscopy. It reveals that the oxygen deficiency center defect of the ultraviolet laser damage site is removed after CO2 laser mitigation, which helps us better understand the CO2 laser mitigation process.

Growth mechanism of laser-induced damage in fused silica

Growth of laser induced damage on the surface of fused silica plays a major role in determining the optics lifetime in high power laser system. Previous studies proved that the size of the crater increased under successive laser shots, but that of the gray haze and CO2 laser mitigation spot remained constant. In this study, Scanning electron microscopy (SEM), focus ion beam (FIB) and profiler were applied to observe their vertical and horizontal cross sections. Energy dispersive spectrometers (EDS) micro-analysis technique and fluorescent microscopy were used to detect the differences of chemical composition and molecular structure among the three. Results showed that the absorbing defect and crack was found in the crater, which did not exist in the gray haze and mitigation spot. Finite difference time domain (FDTD) was applied to calculate the light intensity distribution. Its found that the peak light intensity around the crater was much higher. Based on the above analysis, a growth mechanism of laser induced damage in fused silica was proposed.

Influence of subsurface defects on 355 nm laser damage resistance of monolayer and multilayer coatings

In this study, fused silica substrates were etched to different depths, ranging from about 200 nm to 600 nm. And then single-layer, anti-reflection (AR) and high reflection (HR) coatings were deposited to the etched substrates using E-beam evaporation. It was found that 355nm laser induced damage thresholds (LIDTs) of single-layer and AR coatings on the etched substrates were much higher than that on un-etched substrates, and the existing of the top 200 nm of the substrate was the most important factor influencing the damage behaviors of single-layer and AR coatings. Step Profiler was employed in measuring the depth of damage sites in the coatings deposited on the un-etched substrates. It showed that laser damage was initiated from the coating-substrate interface for single-layer and AR coatings, while from the inside of coating stack for HR samples. On the basis of above results, we concluded that subsurface defects in the redeposition layer of substrate could be a serious factor that lowered the 355nm LIDTs of single-layer and AR coatings.