Laser damage properties of LiB3O5 crystal surface under UV laser irradiation.

LiB3O5 (LBO) crystal has a very high bulk laser damage threshold. Laser damage often occurs on the surfaces with a large number of processing defects during application. In this paper, the surface laser damage threshold, damage growth threshold, and damage growth curve of LBO crystal and fused silica under the same processing process have been comparatively studied by using a 355 nm pulsed laser. The surface laser damage performance of LBO crystal has been comprehensive evaluated. The results show that the laser damage threshold and damage growth threshold of LBO are about twice that of fused silica, and the damage growth coefficient is about 0.7 times that of fused silica. The detection and analysis of impurity defects and photothermal weak absorption defects show that the subsurface defects of LBO crystal are less than that of fused silica. Laser damage morphologies show that the damage process is related to strongly bonded chemical structure and anisotropic physical characteristics of LBO crystal. These characteristics together determine the high threshold damage performance of LBO crystal. The results of this study are of great guidance for the application of LBO crystal in high-power laser systems.

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.

Fabrication of Polymer Composite Fibers Embedding Ultra-Long Micro/Nanowires.

Fabrication of polymer composite fibers embedding ultra-long micro/nanowires via an iterative melt co-drawing and bundling technique is reported in this study. The poly(methyl methacrylate) (PMMA) porous array templates were prepared with section-cutting the PMMA/polystyrene (PS) (shell/core) composite fibers and dissolution of inner PS. The results showed that the PS cores or pores in the PMMA matrix are regularly arranged with hexagonal, and their diameter and spacing exhibits a uniform distribution. Especially, the core diameter can be precisely controlled from millimeter-scale to nanometer-scale by multi-step melt co-drawing. Based on the PMMA porous array templates, the Cu nanowires were successfully prepared by electrochemical deposition. Moreover, to fabricate PMMA ultra-long micro/nanowires, the composite fibers with converse shell/core component of PS/PMMA were initially prepared, and then the outer PS was dissolved. The obtained PMMA micro/nanowires were characterized with smooth complete orientation structure. The study provides an experimental basis for fabricating such polymer composite fibers, micro/nano porous array templates, and micro/nanowires with precise and controllable manner to meet the real application requirements.

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.

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.

Convenient and Efficient Fabrication of Colloidal Crystals Based on Solidification-Induced Colloidal Assembly.

The simple yet efficient and versatile fabrication of colloidal crystals was investigated based on the solidification-induced colloidal crystallization process with particle/water suspension as precursor. The resulting colloidal crystals were constituted by crystal grains with sizes ranging from several tens of micrometers to a few millimeters. Each of the grains had a close-hexagonal array of colloids, which endowed the bulk colloidal crystal powders with some specific optical properties. The freezing of water was shown as the major driving force to form colloidal crystal grains, which supersaturated the solution with nanoparticles and thus induced the formation and growth of colloidal crystal seeds. This process is intrinsically different from those conventional methods based on shearing force, surface tension, columbic interaction or magnetic interaction, revealing a new strategy to fabricate colloidal crystals in a convenient and efficient way.

Capping a glass thin layer on the etched surface via plasma chemical vapor deposition for improving the laser damage performance of fused silica.

Buffered HF-based etching can effectively improve the laser damage resistance of the fused silica, but deep etching would cause the deteriorations in surface roughness and hardness, and decrease the laser-induced damage threshold. Capping a glass thin layer on the etched surface via plasma chemical vapor deposition in one step could overcome those deteriorations. We found that the deposition of the glass thin layer can further reduce the impurity element contamination and the PL intensity while retaining the low subsurface defect density as well as for the deeply etched sample. The surface quality, surface hardness and the laser damage resistance of the fused silica can be significantly improved by the glass thin layer, which reveals the potential application in high power laser facility.

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.

Fabrication and properties of broadband antireflective coatings on inert perfluoropolymer films treated by inductively coupled oxygen plasma.

Fluorinated ethylene propylene (FEP) film is an attractive candidate for mitigating target debris in high-power laser systems due to its remarkable advantages such as low cost, low absorption, and high damage threshold. However, the inert surface with bad wettability presents an enormous challenge to realize optical antireflection. In this Letter, we experimentally demonstrate that broadband antireflection of FEP film can be achieved through combining oxygen plasma treatment and solgel coating techniques. By optimizing the plasma treating time and withdrawal rate during the coating process, the treated FEP film has 4.7% enhanced transmittance compared to the untreated sample. In this case, transmittance over 99% with wide wave bands ranging from 600 to 950 nm is achieved. The mechanism of broadband antireflection was revealed by investigating the fluorocarbon groups on the FEP surface. The applicable wave band of antireflective FEP film can be designed at will by changing plasma-treating conditions, which could open up a new avenue in the field of laser debris mitigation.

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.

Effect of PVA coating on the electric field intensity distribution and laser damage performance of fused silica optics surfaces.

A facile method was proposed to enhance the laser damage performance of the fused silica optics by coating a PVA film on the rear surface of the optics. FDTD simulation result suggests that the PVA coating with suitable thickness can transfer the maximal electric field intensity from the rear surface to the interface between the coating and air, and reduce the electric field intensity of the rear surface remarkably. LIDT tests reveal that the LIDT of fused silica with PVA coating changed periodically with respect to the coating thickness, which agrees well with the tendency predicted by FDTD simulation. Finally, PVA coatings with a thickness of 60 nm and 300 nm can both improve the LIDT of AMP-treated fused silica by ~20%, which provide a potential to be applied in high power laser facility.

Ultraviolet Laser Damage Dependence on Contamination Concentration in Fused Silica Optics during Reactive Ion Etching Process.

The reactive ion etching (RIE) process of fused silica is often accompanied by surface contamination, which seriously degrades the ultraviolet laser damage performance of the optics. In this study, we find that the contamination behavior on the fused silica surface is very sensitive to the RIE process which can be significantly optimized by changing the plasma generating conditions such as discharge mode, etchant gas and electrode material. Additionally, an optimized RIE process is proposed to thoroughly remove polishing-introduced contamination and efficiently prevent the introduction of other contamination during the etching process. The research demonstrates the feasibility of improving the damage performance of fused silica optics by using the RIE technique.

Plasma-Induced, Self-Masking, One-Step Approach to an Ultrabroadband Antireflective and Superhydrophilic Subwavelength Nanostructured Fused Silica Surface.

In this work, antireflective and superhydrophilic subwavelength nanostructured fused silica surfaces have been created by one-step, self-masking reactive ion etching (RIE). Bare fused silica substrates with no mask were placed in a RIE vacuum chamber, and then nanoscale fluorocarbon masks and subwavelength nanostructures (SWSs) automatically formed on these substrate after the appropriate RIE plasma process. The mechanism of plasma-induced self-masking SWS has been proposed in this paper. Plasma parameter effects on the morphology of SWS have been investigated to achieve perfect nanocone-like SWS for excellent antireflection, including process time, reactive gas, and pressure of the chamber. Optical properties, i.e., antireflection and optical scattering, were simulated by the finite difference time domain (FDTD) method. Calculated data agree well with the experiment results. The optimized SWS show ultrabroadband antireflective property (up to 99% from 500 to 1360 nm). An excellent improvement of transmission was achieved for the deep-ultraviolet (DUV) range. The proposed low-cost, highly efficient, and maskless method was applied to achieve ultrabroadband antireflective and superhydrophilic SWSs on a 100 mm optical window, which promises great potential for applications in the automotive industry, goggles, and optical devices.

Growth mechanism of one-step self-masking reactive-ion-etching (RIE) broadband antireflective and superhydrophilic structures induced by metal nanodots on fused silica.

This work presents a low-cost, simple, convenient, advanced technology to prepare large-area defect-free subwavelength structures (SWSs). SWSs were obtained by a metal-induced one-step self-masking RIE process on a fused-silica surface, in which metal-fluoride (mainly ferrous-fluoride) nanodots were used to induce and gather stable fluorocarbon polymer etching inhibitors in the RIE polymers as masks. The SWS growth processes are visible with an increase in etching time and some exhibit prominent broadband antireflective properties from the visible to the near-infrared wavelength range. Transmission in the 600-900-nm range increased from approximately 93% for the polished fused silica to above 99% for the double-side SWSs on fused silica. A theoretical simulation by a finite-difference time-domain method agreed well with the experiments. Moreover, the surface of the SWSs exhibits excellent superhydrophilic properties.

Traceless mitigation of laser damage precursors on a fused silica surface by combining reactive ion beam etching with dynamic chemical etching.

HF-based etching has been successful in mitigating damage precursors on the surface of fused silica optics used in high power lasers. However, wet etching generally leaves an etching trace leading to surface roughness, which seriously degrades laser beam quality (e.g., transmission loss and wave-front degradation). A way of addressing this issue is to apply plasma etching as a preprocessing step before HF etching, but so far very few studies have provided a practical scheme for engineering applications. In this work, we proposed a novel two-step scheme by combining reactive ion beam etching with dynamic chemical etching techniques. We demonstrate the combined scheme is capable of tracelessly mitigating the laser damage precursors on a fused silica surface. The 0% probability damage threshold obtained by combined etching is 1.4 times higher than that obtained by HF-based etching. The study opens a new approach towards high damage-resistant optics manufacturing and provides the potential possibility of exploring extreme interactions between high-power lasers and matter.

Non-destructive evaluation of UV pulse laser-induced damage performance of fused silica optics.

The surface laser damage performance of fused silica optics is related to the distribution of surface defects. In this study, we used chemical etching assisted by ultrasound and magnetorheological finishing to modify defect distribution in a fused silica surface, resulting in fused silica samples with different laser damage performance. Non-destructive test methods such as UV laser-induced fluorescence imaging and photo-thermal deflection were used to characterize the surface defects that contribute to the absorption of UV laser radiation. Our results indicate that the two methods can quantitatively distinguish differences in the distribution of absorptive defects in fused silica samples subjected to different post-processing steps. The percentage of fluorescence defects and the weak absorption coefficient were strongly related to the damage threshold and damage density of fused silica optics, as confirmed by the correlation curves built from statistical analysis of experimental data. The results show that non-destructive evaluation methods such as laser-induced fluorescence and photo-thermal absorption can be effectively applied to estimate the damage performance of fused silica optics at 351 nm pulse laser radiation. This indirect evaluation method is effective for laser damage performance assessment of fused silica optics prior to utilization.

Laser emission from flash ignition of Zr/Al nanoparticles.

We report the first laser emission from flash ignition of Zr/Al nanoparticles with the addition of strong oxidizer KClO4 using Nd: YAG as a laser medium. The mixture Zr/Al/Kp-45 (mass ratio = 33%Zr: 33%Al: 34%KClO4) has the highest brightness temperature Tb = 4615 K and the adiabatic flame temperature Tf = 4194 K with the duration of 20 ms. At 1064 nm we measured a maximum output energy of 702.5 mJ with the duration of nearly 10 ms by using only 100 mg mixture with an output coupler (transmission T = 10%). Further optimizing the concentration cavity and increasing the mixture content will yield much higher efficiency and output energy.

Monolayer Colloidal Crystals by Modified Air-Water Interface Self-Assembly Approach.

Hexagonally ordered arrays of polystyrene (PS) microspheres were prepared by a modified air-water self-assembly method. A detailed analysis of the air-water interface self-assembly process was conducted. Several parameters affect the quality of the monolayer colloidal crystals, i.e., the colloidal microsphere concentration on the latex, the surfactant concentration, the polystyrene microsphere diameter, the microsphere polydispersity, and the degree of sphericity of polystyrene microspheres. An abrupt change in surface tension was used to improve the quality of the monolayer colloidal crystal. Three typical microstructures, i.e., a cone, a pillar, and a binary structure were prepared by reactive-ion etching using a high-quality colloidal crystal mask. This study provides insight into the production of microsphere templates with flexible structures for large-area patterned materials.

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.