Research on the mitigation of redeposition defects on the fused silica surface during wet etching process.

The laser-induced damage of ultraviolet fused silica optics is a critical factor that limits the performance enhancement of high-power laser facility. Currently, wet etching technology based on hydrofluoric acid (HF) can effectively eliminate absorbing impurities and subsurface defects, thereby significantly enhancing the damage resistance of fused silica optics. However, with an increase in the operating fluence, the redeposition defects generated during wet etching gradually become the primary bottleneck that restricts its performance improvement. The composition and morphology of redeposition defects were initially identified in this study, followed by an elucidation of their formation mechanism. A mitigation strategy was then proposed, which combines a reduction in the generation of precipitation with an acceleration of the precipitation dissolution process. Additionally, we systematically investigated the influence of various process parameters such as extrinsic impurity, etching depth, and megasonic excitation on the mitigation of deposition defects. Furthermore, a novel multiple-step dynamic etching method was developed. Through comprehensive characterization techniques, it has been confirmed that this new etching process not only effectively mitigate redeposition defects under low fluence conditions but also exhibits significant inhibition effects on high fluence precursors. Consequently, it significantly enhances the laser damage resistance performance of fused silica optics.

First-principle study of neutron irradiation induced performance degradation of amorphous porous silica.

Neutron irradiation induced degradation of porous silica film is studied by Molecular Dynamics and Density-Functional theory-based methods. The degradation of microscopic structure, thermal property, and optical property of porous silica film are systematically investigated. Low-energy recoil is used to simulate the neutron irradiation effect. The pair and bond angle distributions, and coordination number distributions reveal that, under neutron irradiation, the microscopic structure of porous silica film is obviously modified, and the coordination defects are induced. We find that the higher recoil energy, the more coordination defects are formed in the film. The increased defects lead to a decrease in thermal conductivity. In addition, neutron irradiation induces additional optical absorption peaks in UV region and increasement in refractive index, resulting in a noticeable reduction in light transmittance. The detailed calculation of density of states reveals that these optical absorption peaks originate from the irradiation induced defect states in band gap. Our work shows that low-energy neutron irradiation can induce obvious defect density and degrade thermal and optical properties of porous silica film, which are responsible for subsequent laser-induced damage.

Assessing the UV-pulse-laser-induced damage density of fused silica optics using photo-thermal absorption distribution probability curves.

A photo-thermal absorption distribution probability curve based on a normal distribution model was proposed to describe the distribution of absorptive defects on fused silica surfaces under different processing conditions. Simultaneously, the maximum distribution probability absorption coefficient (MPA) and absorption distribution deviation (ADD) were used to quantitatively describe the overall absorption level and the uniformity of the absorption distribution on the fused silica surface. Based on this, the MPA (µ) and ADD (δ) were used to establish a statistical numerical relationship with the surface damage density of fused silica. The results showed that when µ ≤ 0.095 ± 0.015 and δ ≤ 0.045 ppm, the fused silica optics met the manufacturing process requirements for high laser-induced damage performance. Thus, a non-destructive approximate evaluation of the laser-induced damage density on the fused silica surface was achieved. This evaluation method provides a new, to the best of our knowledge, technology for evaluating the manufacturing process quality related to the damage performance of fused silica optics in high-power solid-state laser facilities and is an important supplement to popular destructive laser-induced damage testing methods.

Role of each step in the combined treatment of reactive ion etching and dynamic chemical etching for improving the laser-induced damage resistance of fused silica.

We investigate the role of each step in the combined treatment of reactive ion etching (RIE) and dynamic chemical etching (DCE) for improving the laser-induced damage resistance of fused silica optics. We employ various surface analytical methods to identify the possible damage precursors on fused silica surfaces treated with different processes (RIE, DCE, and their combination). The results show that RIE-induced defects, including F contamination, broken Si-O bonds, luminescence defects (i.e., NBOHCs and ODCs), and material densification, are potential factors that limit the improvement of laser-induced damage resistance of the optics. Although being capable of eliminating the above factors, the DCE treatment can achieve rough optical surface with masses of exposed scratches and pits which might serve as reservoirs of the deposits such as inorganic salts, thus limiting the further improvement in damage resistance of fused silica. The study guides us to a deep understanding of the laser-induced damage process in achieving fused silica optics with enhanced resistance to laser-induced damage by the combined treatment of RIE and DCE.

Strong UV laser absorption source near 355 nm in fused silica and its origination.

As a high-performance optical material, fused silica is widely applied in high-power laser and photoelectric systems. However, laser induced damage (LID) of fused silica severely limits the output power and performance of these systems. Due to the values in strong field physics and improving the load capacity and performance of high power systems at UV laser, LID at 355 nm of fused silica has attracted much attention. It has been found that, even be treated by advanced processing technologies, the actual damage threshold of fused silica at 355 nm is far below the intrinsic threshold. It means that there is an absorption source near 355 nm in fused silica. However, to date, the absorption source is still unknown. In this paper, a absorption source near 355 nm is found by first-principles calculations. We find that the absorption source near 355 nm is neutral oxygen-vacancy defect (NOV, ≡Si-Si≡) and this defect originates from the oxygen deficiency of fused silica. Our results indicate that NOV defect can be taken as a damage precursor for 355 nm UV laser, and this precursor can be obviously reduced by increasing the ratio of oxygen to silicon. Present work is valuable for exploring damage mechanisms and methods to improve the damage threshold of fused silica at UV laser.

Understanding the role of fluorine-containing plasma on optical properties of fused silica optics during the combined process of RIE and DCE.

Reactive ion etching (RIE) is crucial for fabricating high-quality fused silica optics since this technique can be used as a first step before dynamic chemical etching (DCE) for tracelessly removing the fractured defects in subsurface layer. The final quality of the optics is dramatically influenced by the plasma etching condition but still lacks sufficient information for practical application. In this work, combination of RIE and DCE was investigated deeply on polished fused silica surface by changing the gas type and flow rate. We show that the proper choice of fluorine-containing plasma condition during the RIE process allows the simultaneous occurrence of high surface quality and a low concentration of etching-introduced defects on fused silica. This leads to an ultrahigh laser-induced damage threshold at 355 nm while substantially keeping the surface roughness unchanged. This study paves the way for designing and developing a next-generation surface modification ability of high-quality fused silica with the great potential for high-power laser application.

Straightforward Approach to Antifogging, Antireflective, Dual-Function, Nanostructured Coatings.

Here, we report a straightforward approach to fabricate antifogging antireflective dual-function nanostructured coatings, where antireflective nanograsses were etched into antifogging polymer coatings by self-masking reactive ion etching (RIE). The transmittance of coatings increases with the etching time, and the maximum transmittance reaches up to 98.9% in 180 s. The effective refractive index of grass-like nanostructure was calculated to be 1.15 and its optical property was simulated via the finite difference time domain (FDTD) model. The antifogging property of polymer coatings remains unchanged after RIE, which results from the hygroscopicity of polymer matrix. This strategy surpasses traditional design concepts of antifogging polymer coatings by combining excellent antireflective and antifogging properties on the same outermost layer, which demonstrates that it is probable to achieve multifunction on a single layer of a single composition.

Effects of combined process of reactive ion etching and dynamic chemical etching on UV laser damage resistance and surface quality of fused silica optics.

We investigate the interest of combined process of reactive ion etching (RIE) and dynamic chemical etching (DCE) as a final step after polishing to improve the laser damage resistance of fused silica optics at the wavelength of 355 nm. The investigation is carried out on the polished fused silica optics by changing the RIE depth while keeping the DCE depth fixed. We evidence that the combined etching process can effectively remove the damage precursors on the fused silica surface and thus improve its laser-induced damage threshold exceeding the level of the deep HF-etched surface. The effects of the combined etching depth on the surface roughness and surface error are also studied systematically. We show that the combined shallow etching can achieve better overall surface quality. Deeper etching will cause surface quality degradation of the fused silica optics, which is believed to be associated with the chemical etching during the combined process. Given that HF acid processing will degrade the surface quality of fused silica optics, the combined shallow etching appears as a pertinent alternative to HF-based deep etching.

Fabrication of concave microlens arrays by local fictive temperature modification of fused silica.

A simple and convenient means of fabricating concave microlens arrays direct on silica glass by using the local fictive temperature modification of fused silica is presented. This method is based on the fact that an increased fictive temperature results in a much higher HF acid etching rate of fused silica. Combining the abrupt local fictive temperature enhancement by the CO2 laser pulse and the subsequent etching by the HF acid solution, concave microlens arrays with high fill factors, excellent smoothness, and optical performance are generated on fused silica.

Thermal conductivity of vitreous silica from molecular dynamics simulations: The effects of force field, heat flux and system size.

The thermal conductivity of vitreous silica is computed using the direct method in molecular dynamics simulations with three sets of empirical force fields, including the BKS, Teter, and ReaxFF, to investigate their performance in thermal characterization. Various heat flux and system sizes are used in the simulations to evaluate the statistical uncertainty and the finite-size effect. While all these potentials can reproduce realistic silica structures, the ReaxFF provides better agreement with experiments at 300 K than the BKS and Teter, which is due to its improved description of low-frequency vibrations. Increasing the heat flux and cross-sectional area tends to reduce the calculated standard deviation induced by thermal fluctuations, thus contributing to more accurate thermal conductivity predictions.

Polarization smoothing for single beam by a nematic liquid crystal scrambler.

Polarization smoothing (PS) is a key approach to suppress laser plasma instabilities (LPI) in inertial confinement fusion (ICF) experiments. Here, we propose a liquid crystal (LC) PS element to realize single beam smoothing and demonstrate its smoothing effect, in principle, with a 2×2 LC polarization checkerboard, which reduces the laser intensity variation in the focal spot to 78.4%. LC PS elements, which have potential applications in high-power ICF laser drivers, have many advantages because they are easy to fabricate, cost effective, flexible, and large.

Reaction ion etching process for improving laser damage resistance of fused silica optical surface.

Laser induced damage of fused silica optics occurs primarily on optical surface or subsurface resulting from various defects produced during polishing/grinding process. Many new kinds of surface treatment processes are explored to remove or control the defects on fused silica surface. In this study, we report a new application of reaction ion etching (RIE)-based surface treatment process for manufacture of high quality fused silica optics. The influence of RIE processes on laser damage resistance as a function of etching depth and the evolution of typical defects which are associated with laser damage performance were investigated. The results show that the impurity element defects and subsurface damage on the samples surface were efficiently removed and prevented. Pure silica surface with relatively single-stable stoichiometry and low carbon atomic concentration was created during the etching. The laser damage resistance of the etched samples increased dramatically. The increase of roughness and ODC point defect with deeper etching are believed to be the main factors to limit further increase of the damage resistance of fused silica. The study is expected to contribute to the development of fused silica optics with high resistance to laser induced degradation in the future.

Combination of reaction ion etching and dynamic chemical etching for improving laser damage resistance of fused silica optical surfaces.

In this Letter, an effective combined process of reaction ion etching (RIE) and dynamic chemical etching (DCE) is applied for significantly improving the damage resistance of fused silica optics, while minimizing the removal amount. By optimizing the combination process and removal depth, a near-perfect optical surface of fused silica with relatively low roughness (<0.7 nm) is created with 1 µm RIE pretreatment and 3 µm DCE retreatment. In this case, the sample has a 2.4 times enhanced 0% probability damage threshold compared to the original sample. We show that the optimized combining process with a low removal amount is superior to a conventional HF-based etching process with a high removal amount in enhancing damage resistance and controlling the surface shape and roughness of fused silica. The results advance our understanding of a key factor influencing the RIE-DCE matching relationship and can lead to further optimization of associated applications, ranging from material processing to high-power laser systems.

Suppression of transverse stimulated Raman scattering with laser-induced damage array in a large-aperture potassium dihydrogen phosphate crystal.

A laser-induced damage array composed of numerous pinpoints is generated in a large-aperture KDP crystal to suppress the transverse stimulated Raman scattering (TSRS). The 36 cm × 8.5 mm × 7 mm damage array is used to block the propagation of the TSRS photons within the crystal and decrease the TSRS gain length. Then several series of experiments were conducted on a large-aperture laser system to test this method and experimental results show that the amplification of TSRS is significantly suppressed by the laser-induced damage array.

Spectral Failsafe System of High-Power Laser Using Dual Fiber Bragg Gratings.

Phase-modulated (PM) spectral failsafe systems are necessary to promptly terminate amplification processes following accidental seeding of a high-power laser chain with a non-PM pulse to prevent optical damage. In this work, we present a reliable spectral failsafe system that can indicate the presence or absence of sufficient PM light. This requirement is met by combining dual temperature-sensitive fiber Bragg gratings detection with high-speed RF amplitude comparisons. The failsafe trigger signal is generated when the spectral power at the peak sideband exceeds that at the center. The spectral failsafe system has the ability to distinguish between adequate and inadequate PM pulses, and it exhibits significant robustness in pulse width, TEC temperature drift, and DFB wavelength drift in experiments, making it valuable for safe high-power laser operations and providing a useful reference for other detection system designs.

Understanding the effect of HF-based wet shallow etching on optical performance of reactive-ion-etched fused silica optics.

The optical performance of fused silica optics used in high-power lasers is known to depend not only on their surface damage resistance, but also on their surface quality. Previous studies have shown that good fused silica damage performance and surface quality can be achieved by the use of reactive ion etching (RIE), followed by HF-based wet shallow etching (3 µm). In this study, two kinds of HF-based etchants (aqueous HF and HF/NH4F solutions) were employed to investigate the effect of HF-based etching on the optical performance of reactive-ion-etched fused silica surfaces at various HF-based shallow etching depths. The results showed that the addition of NH4F to HF solution makes it possible to produce a high-quality optical surface with a high laser-induced damage threshold, which is strongly associated with the surface roughness and fluorescence defect density. Additionally, changing the HF-based etching depth over the range from 1 µm to 3 µm can affect the surface damage resistance and absorption performance of RIE-treated fused silica. The light-scattering results indicate that the point defect density plays an important role in the determination of the HF-based etching depth. Understanding these trends can enable the advantages of the combined technique of RIE and HF-based etching during the fabrication of high-quality fused silica optics.

Quadratic Meta-Reflectors Made of HfO2 Nanopillars with a Large Field of View at Infrared Wavelengths.

Metasurfaces, being composed of subwavelength nanostructures, can achieve peculiar optical manipulations of phase, amplitude, etc. A large field of view (FOV) is always one of the most desirable characteristics of optical systems. In this study, metasurface-based quadratic reflectors (i.e., meta-reflectors) made of HfO2 nanopillars are investigated to realize a large FOV at infrared wavelengths. First, the geometrical dependence of HfO2 nanopillars' phase difference is analyzed to show the general principles of designing infrared HfO2 metasurfaces. Then, two meta-reflectors with a quadratic phase profile are investigated to show their large FOV, subwavelength resolution, and long focal depth. Furthermore, the two quadratic reflectors also show a large FOV when deflecting a laser beam with a deflecting-angle range of approximately ±80°. This study presents a flat optical metamaterial with a large FOV for imaging and deflecting, which can greatly simplify the optical-mechanical complexity of infrared systems, particularly with potential applications in high-power optical systems.

High-Efficiency Metasurfaces with 2π Phase Control Based on Aperiodic Dielectric Nanoarrays.

In this study, the high-efficiency phase control Si metasurfaces are investigated based on aperiodic nanoarrays unlike widely-used period structures, the aperiodicity of which providing additional freedom to improve metasurfaces' performance. Firstly, the phase control mechanism of Huygens nanoblocks is demonstrated, particularly the internal electromagnetic resonances and the manipulation of effective electrical/magnetic polarizabilities. Then, a group of high-transmission Si nanoblocks with 2π phase control is sought by sweeping the geometrical parameters. Finally, several metasurfaces, such as grating and parabolic lens, are numerically realized by the nanostructures with high efficiency. The conversion efficiency of the grating reaches 80%, and the focusing conversion efficiency of the metalens is 99.3%. The results show that the high-efficiency phase control metasurfaces can be realized based on aperiodic nanoarrays, i.e., additional design freedom.

Suppression of FM-to-AM conversion in third-harmonic generation at the retracing point of a crystal.

FM-to-AM conversion can cause many negative effects (e.g., reducing of margin against damage to the optics) on the performance of a third-harmonic conversion system. In this Letter, the FM-to-AM conversion effect in third-harmonic generation is investigated both at and away from the retracing point of Type II potassium dihydrogen phosphate crystal. Obtained results indicate that the FM-to-AM conversion can be suppressed effectively when the crystal works at the retracing point.

Fabrication of Antireflective Nanostructures on a Transmission Grating Surface Using a One-Step Self-Masking Method.

Suppression of Fresnel reflection from diffraction grating surfaces is very important for many optical configurations. In this work, we propose a simple method to fabricate subwavelength structures on fused-silica transmission grating for optical antireflection. The fabrication is a one-step self-masking reaction ion etching (RIE) process without using any masks. According to effective medium theory, random cone-shaped nanopillars which are integrated on the grating surface can act as an antireflective layer. Effects of the nanostructures on the reflection and transmission properties of the grating were investigated through experiments and simulations. The nanostructure surface exhibited excellent antireflection performance, where the reflection of the grating surface was suppressed to zero over a wide range of incident angles. Results also revealed that the etching process can change the duty cycle of the grating, and thus the diffraction orders if there are oblique lateral walls. The simulation results were in good agreement with the experimental ones, which verified our physical comprehension and the corresponding numerical model. The proposed method would offer a low-cost and convenient way to improve the antireflective performance of transmission-diffractive elements.