Shizhen Xu

Localized CO2 laser treatment and post-heating process to reduce the growth coef f icient of fused silica surface damage

The lifetime of optical components in high-fluence ultraviolet (UV) laser applications is typically limited by laser-initiated damage and its subsequent growth. Using 10.6-μm CO2 laser pulses, we successfully mitigate 355-nm laser induced damage sites on fused silica surface with dimensions less than 200 μm. The damage threshold increases and the damage growth mitigates. However, the growth coefficients of new damage on the CO2 laser processed area are higher than those of the original sample. The damage grows with crack propagation for residual stress after CO2 laser irradiation. Furthermore, post-heating is beneficial to the release of residual stress and slows down the damage growth.

Evolution of Oxygen Deficiency Center on Fused Silica Surface Irradiated by Ultraviolet Laser and Posttreatment

Evolution of oxygen deficiency centers (ODCs) on a fused silica surface irradiated using a 355 nm ultraviolet (UV) laser beam in both vacuum and atmospheric conditions was quantitatively studied using photoluminescence and X-ray photoelectron spectroscopy. When the fusedsilica surface was exposed to the UV laser in vacuum, the laser damage threshold was decreased whereas the concentration of the ODCs was increased. For the fuse silica operated under the high power lasers, creation of ODCs on their surface resulted from the UV laser irradiation, and this is more severe in a high vacuum. The laser fluence and/or laser intensity have significant effects on the increase of the ODCs concentration. The ODCs can be effectively repaired using postoxygen plasma treatment and UV laser irradiation in an excessive oxygen environment. Results also demonstrated that the “gain” and “loss” of oxygen at the silica surface is a reversible and dynamic process.

Scratch Types and Damage Thresholds of Fused Silica

Atom force microscopy (AFM) is used to investigate micro-morphology of various types of scratches in the fused silica surface and sub-surface. Based on the shape, scratches can be classified as lateral scratch, radial scratch, Hertizan cone scratch, and trailing indent scratch. From forming mechanism, scratches can be classified as plastic scratch, brittle scratch and mixed scratch. The statistical damage thresholds of three kinds of common scratches are obtained through raster scanning at different laser fluences. The results show that the damage threshold of radial scratches is the highest and that of the trailing indent scratches is the lowest. In addition, for the samples etched in different concentration buffered hydrofluoric acid solutions for different etching time, the damage thresholds are also obtained. The high concentration and long-time etching can effectively enhance the damage threshold of scratches. Brittle scratches are easier to damage than plastic scratches. Two kinds of mechanisms are responsible for the laser damage: local electrical intensification and mechanical weakness induced by scratches. This work is a reference for further investigation on damage threshold enhancement and damage mechanism of scratches in fused silica.

Time-resolved dynamics of 355-nm laser-irradiated surface damage on fused silica

The dynamics of 355-nm laser ablation on fused silica were studied by instantaneous scattering pulse measurement and a time-resolved shadowgraph imaging. The sharp increase of scattered light of pumped pulse is assumed to be the damage precursor, therefore, the damage start nearly at the peak of the pumped pulse. The plasmas flash due to ion-electron recombination occurred about 21ns after the peak of pumped pulses. The propagating shock wave and ejected material to the air were imaged by shadowgraphic technology. The damage process of fused silica under UV laser ablation was also discussed.

The formation of periodic micro/nano structured on stainless steel by femtosecond laser irradiation

Stainless steel surface was irradiated by linear polarized laser (800 nm, 35 fs, 4 Hz and 0.7 J/cm2) with different pulse numbers. Environmental scanning electron microscope (ESEM/EDS) was used for detailed morphology, microstructure and composition studies. The wettability of irradiated steel surface was tested by Interface Tensiometer JC-2000X and compared with untreated stainless steel. Results showed that micro/nanostripes with different periods were formed. The period increased with the increasing pulse numbers from 450 nm for 90 pulses to 500 nm for 180 pulses. The orientation of those stripes was parallel with the laser beam polarization. Nanoparticles were observed on those periodic structures. EDS indicated that the atomic ratio of Cr increased and the atomic ratios of Fe and Ni decreased after laser irradiation, which may enhance the corrosion resistance due to the Cr-rich layer. The prepared structure exhibited hydrophobic property without further treatment. The formation mechanism of micro/nanoperiodic structures was also explored.