Zu Xiao-Tao

Two localized CO2 laser treatment methods for mitigation of UV damage growth in fused silica

Two methods: high-power, short-time, single-shot irradiation (Method A) and low-power, long-time, multi-shot irradiation (Method B) are investigated to mitigate the UV damage growth in fused silica by using a 10.6-μm CO2 laser. To verify the mitigation effect of the two methods, the laser induced damage thresholds (LIDTs) of the mitigated sites are tested with a 355-nm, 6.4-ns Nd:YAG laser, and the light modulation of the mitigation sites are tested with a 351-nm continuous Nd:YLF laser. The mitigated damaged sites treated with the two methods have almost the same LIDTs, which can recover to the level of pristine material. Compared with Method A, Method B produces mitigated sites with low crater depth and weak light modulation. In addition, there is no raised rim or re-deposited debris formed around the crater edge for Method B. Theoretical calculation is utilized to evaluate the central temperature of the CO2 laser beam irradiated zone and the radius of the crater. It is indicated that the calculated results are consistent with the experimental results.

The Structure Evolution of Fused Silica Induced by CO2 Laser Irradiation

The structure evolution of fused silica induced by CO2 laser irradiation (with a wavelength of 10.6 μm) is studied in detail. In the non-evaporation mitigation process, the irradiation time should be long enough to completely eliminate damage. However, there is a raised rim around the mitigated site. The rim height is enhanced when the irradiation time increases, and the mitigated site can lead to off-axis and on-axis downstream light intensification. Volume shrinkage occurs during the irradiation and rapid cooling processes, and this may be due to a decrease in the Si—O—Si bond angle. The distribution of debris overlaps with the maximum phase retardance induced by stress. The debris arouses an enhanced light absorption in the region from 220 nm to 800 nm.

Effect of N-Doping on Absorption and Luminescence of Anatase TiO2 Films

Anatase TiO2 films are deposited on glass substrates at different oxygen partial pressures of 0.8–1.6 Pa. Room temperature N ion implantation is conducted in the films at ion fluences up to 5 × 1017 ions/cm2. UV-visible absorption and photoluminescence (PL) are investigated. With the increase of N ion fluences, the band gap of TiO2 decreases and the absorbance increases. X-ray photoelectron spectroscopy (XPS) confirms the formation of O-Ti-N nitride after implantation, resulting in the red shift of the band gap. The PL intensity of the deposited films increases with the increasing oxygen partial pressure and decreases remarkably due to the irradiation defects induced by ion implantation.

Incident laser modulation of a repaired damage site with a rim in fused silica rear subsurface

Local CO2 laser treatment has proved to be an effective method to prevent the 351-nm laser-induced damage sites in a fused silica surface from exponentially growing, which is responsible for limiting the lifetime of optics in high fluence laser systems. However, the CO2 laser induced ablation crater is often surrounded by a raised rim at the edge, which can also result in the intensification of transmitted ultraviolet light that may damage the downstream optics. In this work, the three-dimensional finite-difference time-domain method is developed to simulate the distribution of electrical field intensity in the vicinity of the CO2 laser mitigated damage site located in the exit subsurface of fused silica. The simulated results show that the repaired damage sites with raised rims cause more notable modulation to the incident laser than those without rims. Specifically, we present a theoretical model of using dimpled patterning to control the rim structure around the edge of repaired damage sites to avoid damage to downstream optics. The calculated results accord well with previous experimental results and the underlying physical mechanism is analysed in detail.

Mitigation of laser damage growth in fused silica by using a non-evaporative technique

A non-evaporative technique is used to mitigate damage sites with lateral sizes in a range from 50 μm to 400 μm and depths smaller than 100 μm. The influence of the pulse frequency of a CO2 laser on the mitigation effect is studied. It is found that a more symmetrical and smooth mitigation crater can be obtained by increasing the laser pulse frequency form 0.1 to 20 kHz. Furthermore, the sizes of laser-affected and distorted zones decrease with the increase of the laser pulse frequency, leading to less degradation of the wave-front quality of the conditioned sample. The energy density of the CO2 laser beam is introduced for selecting the mitigation parameters. The damage sites can be successfully mitigated by increasing the energy density in a ramped way. Finally, the laser-induced damage threshold (LIDT) of the mitigated site is tested using 355 nm laser beam with a small spot (0.23 mm2) and a large spot (3.14 mm2), separately. It is shown that the non-evaporative mitigation technique is a successful method to stop damage re-initiation since the average LIDTs of mitigated sites tested with small or large laser spots are higher than that of pristine material.

Rear-surface light intensification caused by a Hertzian-conical crack in 355-nm silica optics

Theoretical studies show that a Hertzian-conical crack can be considered to be composed of double cone faces for simplicity. In the present study, the three-dimensional finite-difference time-domain method is employed to quantify the electric-field distribution within the subsurface in the presence of such a defect under normal incidence irradiation. Both impurities (inside the crack) and the chemical etching have been investigated. The results show that the maximum electric field amplitude |E|max is 9.57374 V/m when the relative dielectric constant of transparent impurity equals 8.5. And the near-field modulation will be improved if the crack is filled with the remainder polishing powders or water vapor/drops. Meanwhile, the laser-induced initial damage moves to the glass—air surface. In the etched section, the magnitude of intensification is strongly dependent on the inclination angle θ. There will be a highest modulation when θ is around π/6, and the maximum value of |E|max is 18.57314 V/m. When θ ranges from π/8 to π/4, the light intensity enhancement factor can easily be larger than 100, and the modulation follows a decreasing trend. On the other hand, the modulation curves become smooth when θ > π/4 or θ < π/8.

High temperature thermal behaviour modeling of large-scale fused silica optics for laser facility

High temperature annealing is often used for the stress control of optical materials. However, weight and viscosity at high temperature may destroy the surface morphology, especially for the large-scale, thin and heavy optics used for large laser facilities. It is necessary to understand the thermal behaviour and design proper support systems for large-scale optics at high temperature. In this work, three support systems for fused silica optics are designed and simulated with the finite element method. After the analysis of the thermal behaviours of different support systems, some advantages and disadvantages can be revealed. The results show that the support with the optical surface vertical is optimal because both pollution and deformation of optics could be well controlled during annealing at high temperature. Annealing process of the optics irradiated by CO2 laser is also simulated. It can be concluded that high temperature annealing can effectively reduce the residual stress. However, the effects of annealing on surface morphology of the optics are complex. Annealing creep is closely related to the residual stress and strain distribution. In the region with large residual stress, the creep is too large and probably increases the deformation gradient which may affect the laser beam propagation.

Magnetism of a Nitrogen-Implanted TiO2 Single Crystal

Single rutile crystal TiO2 was implanted using nitrogen ions with energy of 60 keV. The microstructure, ultraviolet-visible light absorption spectra, conductivity and magnetism are investigated. Except for the nitrogen dopant, no impurity can be detected by x-ray diffraction and x-ray photoelectron spectra. The absorption in the visible light region is enhanced with nitrogen implantation dose increasing. By measuring the temperature dependence of resistance, it is found that the sample implanted with 1 × 1018 ions/cm2 is changes from insulating to semiconducting, and the variable range hopping is the main conducting mechanism. Room-temperature ferromagnetism is also obtained in this sample. The magnetism as a function of temperature can be well fitted using the three-dimensional spin wave model plus the Curie-Weiss model, indicating that there is a mixed phase of ferromagnetism and paramagnetism.

Preparation of p-type ZnO:(Al, N) by a combination of sol--gel and ion-implantation techniques

We report the preparation of p-type ZnO thin films on (0001) sapphire substrates by a combination of sol-gel and ion-implantation techniques. The results of the Hall-effect measurements carried out at room temperature indicate that the N-implanted ZnO:Al films annealed at 600?C have converted to p-type conduction with a hole concentration of 1.6 ? 1018 cm?3, a hole mobility of 3.67cm2/V?s and a minimum resistivity of 4.80 cm.?. Ion-beam induced damage recovery has been investigated by x-ray diffraction (XRD), photoluminescence (PL) and optical transmittance measurements. Results show that diffraction peaks and PL intensities are decreased by N ion implantation, but they nearly recover after annealing at 600?C. Our results demonstrate a promising approach to fabricate p-type ZnO at a low cost.

Role of chelating agent in chemical and fluorescent properties of SnO2 nanoparticles

A modified Polyacrylamide gel route is applied to synthesize SnO2 nanoparticles. High-quality SnO2 nanoparticles with a uniform size are prepared using different chelating agents. The average particle size of the samples is found to depend on the choice of the chelating agent. The photoluminescence spectrum detected at λex = 230 nm shows a new peak located at 740 nm due to the surface defect level distributed at the nanoparticle boundaries.