Effect of the interface on femtosecond laser damage of a metal-dielectric low dispersion mirror.

Metal-dielectric low dispersion mirrors (MLDM) have a promising application prospect in petawatt (PW) laser systems. We studied the damage characteristics of MLDM and found that the damage source of MLDM (Ag + Al2O3+SiO2) is located at the metal-dielectric interface. We present the effect of the interface on the femtosecond laser damage of MLDM. Finite element analysis shows that thermal stress is distributed at the interface, causing stress damage which is consistent with the damage morphology. After enhancing the interface adhesion and reducing the residual stress, the damage source transfers from the interface to a surface SiO2 layer, and the damage threshold can be increased from 0.60 J/cm2 to 0.73 J/cm2. This work contributes to the search for new techniques to improve the damage threshold of MLDM used in PW laser systems.

Optical properties of thickness-controlled PtSe2 thin films studied via spectroscopic ellipsometry.

Platinum diselenide (PtSe2) has attracted huge attention due to its intriguing physical properties for both fundamental research and promising applications in electronics and optoelectronics. Here, we explored the optical properties of chemical vapor deposition-grown PtSe2 thin films with varied thicknesses via spectroscopic ellipsometry. The dielectric function was extracted by using a Lorentz model over the spectral range of 1.25-6.0 eV. We firstly ascribed the resonant energies, extracted from the Lorentz model, to different interband electronic transitions between valence bands and conduction bands in the Brillouin zone. A predicted exciton is observed at 2.18 eV for the monolayer and the corresponding exciton binding energy is 0.65 eV, in line with previous theoretical calculation and the measured absorption spectra. Additionally, the exciton peak shows a red shift with the increase of thickness, which is the consequence of strong interlayer interaction. These results enrich the fundamental understanding of PtSe2 and are conducive to its potential applications.

Measuring ultrathin metal coatings using SPR spectroscopic ellipsometry with a prism-dielectric-metal-liquid configuration.

A new surface plasmon resonance (SPR) configuration is proposed, which consists of a prism, a dielectric layer, a metal coating, and a matching liquid. The optical constants of each layer in the proposed prism-dielectric-metal-liquid (PDML) configuration have been optimized to match the SPR conditions and reach the strongest intensity. Combining the PDML configuration with spectroscopic ellipsometry, SPR spectroscopic ellipsometry (SPRSE) with a PDML configuration was developed. The SPR wavelength can be adjusted to the desired wavelength by varying the thickness of the dielectric layer. The amplitude and phase change, magnified by the SPR in the visible and near-infrared wavelengths, were obtained to determine the optical constants and thickness of ultrathin metal coatings. The extracted optical constants were found to be in good agreement with the results obtained using transmission electron microscopy (TEM) and X-ray reflectivity (XRR) techniques. These SPRSE measurements show great potential for characterizing the interface between a metal coating and a dielectric layer, and the surface uniformity of ultrathin metal coatings.

Angle-adjustment-based tunable chirped mirrors with continuous dispersion compensation.

We report the feasibility of continuously tunable dispersion control with chirped mirrors (CMs). The concept of tunable second-order and higher-order dispersion is also proposed. Our prototype dispersion-tunable CM makes it possible to provide continuous dispersion support, advancing the CM technique to a new level by overcoming the drawback of the discrete dispersion compensation nature of traditional CMs. This brings extreme convenience and flexibility to the compensation of the dispersion and ensures tailored dispersion compensation in ultrafast laser systems. In our proof-of-concept study, continuously tunable group delay dispersion (GDD) is achieved by altering the angles of incidence on the mirrors. Moreover, continuous duration tunable laser pulses are demonstrated by applying our GDD-tunable CMs in an ultrafast laser system.

Laser resistance dependence of interface for high-reflective coatings studied by capacitance-voltage and absorption measurement.

HfO2/SiO2 bilayer coatings and multilayer high-reflection coatings without and with a modified co-evaporated interface (MCEI) have been prepared. An MCEI is designed to be evaporated at an oxygen-deficient environment to achieve higher absorption than the conventional discrete interface. Capacitance-voltage measurements and absorption measurements demonstrate that an MCEI increases the trap density and leads to higher absorption. The laser-induced damage threshold and nano-indenter test results indicate that the MCEI multilayer coating exhibits better laser resistance and mechanical property, despite the larger absorption. The experimental results suggest that adhesive force between layers plays a more important role in nanosecond laser damage resistance than interface absorption.

Laser-damage characteristics of Nd:glass active mirrors.

The active-mirror architecture is widely used in high-power laser systems. In this study, the laser-damage characteristics of Nd:glass active mirrors are investigated. They are exposed to nanosecond 1064 nm laser incident from the Nd:glass. The laser-induced damage thresholds (LIDTs) of the coated sides are lower than those of the uncoated sides. The LIDT of the active mirror whose substrate is manually scrubbed is lower than that of one whose substrate is ultrasonically cleaned. Analysis shows that the absorbing surface defects on the manually scrubbed Nd:glass surface are responsible for the lower LIDT of the active mirror prepared via manual scrubbing, while the subsurface defects in the ultrasonically cleaned Nd:glass substrate are the main reason for the damage of the active mirror prepared via ultrasonic cleaning. The strong standing-wave electric field near the interface between the coating and the Nd:glass substrate is another factor affecting the damage of the active mirror.

High dispersive mirrors for erbium-doped fiber chirped pulse amplification system.

We report on the development of near-infrared high dispersive mirrors (HDM) with a group delay dispersion (GDD) of -2000 fs2. A HDM pair based on one optimized result at two reference wavelengths (1550 nm and 1560 nm) can reduce the total oscillation of the GDD effectively in the wavelength range of 1530-1575 nm. This HDM pair is designed and fabricated in a single coating run by means of the nonuniformity in film deposition. For the first time, near-infrared HDMs with two different reference wavelengths have been successfully applied in an erbium-doped fiber chirped pulse amplification system for the compression of 4.73 ps laser pulses to 380 fs.

Improving laser damage resistance of 355 nm high-reflective coatings by co-evaporated interfaces.

355 nm high-reflective multilayer coatings with or without coevaporated interfaces (CEIs) were prepared by electron beam evaporation under the same deposition condition. Their transmission spectra, surface roughness, and mechanical stress properties were evaluated. Elemental composition analysis of the multilayer interfaces was performed using x-ray photoelectron spectroscopy, and laser-induced damage thresholds were obtained in both 1-on-1 and 300-on-1 testing modes. The coatings with CEIs reveal a lower mechanical stress and a higher laser damage resistance when irradiated with high laser fluence, and the corresponding damage modeling indicates that CEIs can significantly decrease defect density. The resulting damage morphologies show that CEI coatings can significantly suppress coating delamination and exhibit a "bulk-like" damage behavior, demonstrating better damage performance against high-power lasers.

Multilayer deformation planarization by substrate pit suturing.

In the pursuit of 1064 nm high-power laser resistance dielectric coatings in the nanosecond region, a group of HfO2/SiO2 high reflectors with and without suture layers were prepared on prearranged fused silica substrates with femtosecond laser pits. Surface morphology, global coating stress, and high-resolution cross sections were characterized to determine the effects of substrate pit suturing. Laser-induced damage resistance was investigated for samples with and without suture layers. Our results indicate considerable stability in terms of the nanosecond 1064 nm laser-induced damage threshold for samples having a suture layer, due to decreased electronic field (e-field) deformation with simultaneous elimination of internal cracks. In addition, a suture layer formed by plasma ion-assisted deposition could effectively improve global mechanical stress of the coatings. By effectively reducing the multilayer deformation using a suture layer, electron-beam high-reflective coatings, whose laser-induced damage resistance was not influenced by the substrate pit, can be prepared.

Suppression of nano-absorbing precursors and damage mechanism in optical coatings for 3ω mirrors.

Damage precursors in the 3ω (351 nm) mirror for a high-power laser system are investigated as well as the relevant damage mechanisms. The precursors are classified into two ensembles according to the different laser resistance and damage features. The former is nano-absorbing precursors, which are sensitive to the standing wave electric field and vulnerable to the laser irradiation. The latter is submicrometer nodular defects, which have higher laser resistance and are sensitive to the adhesion strength between the fluoride coatings and oxide coatings. The damage due to nano-absorbing precursors is efficiently suppressed with the double stack design that screens the electric field in the oxides. Currently, the nodular seed is major originating from the Al2O3/SiO2 stack. Even for the same defect type and mirror, the final damage features are dependent on the local mechanical properties at the irradiation location. The investigations of the damage mechanisms provide a direction to further improve the laser-induced damage threshold of the 3ω mirror.

Theoretical and experimental research on laser-induced damage of cylindrical subwavelength grating.

We designed and fabricated two-dimensional cylindrical subwavelength gratings (SWGs) on fused silica for use in 1064 nm laser system. The transmittance and laser-induced damage threshold (LIDT) under the irradiation of 1064 nm pulses were performed, and a lower LIDT compared to blank fused silica was obtained. To understand damage mechanism quantitatively, macro-temperature on the SWGs integrated fused silica and micro-Electric field, micro-thermal-stress distribution on SWGs during the laser irradiation process were investigated by Finite Element Analysis, and the comparison between theoretical and experimental research indicated the presence of absorption centers and the non-uniform thermal mechanical distribution of SWGs contributed to the LIDT reduction on SWGs integrated fused silica.

Experimental demonstration of laser damage caused by interface coupling effects of substrate surface and coating layers.

The laser damage resistance of the coatings for high-power laser systems depends greatly on the surface quality of the substrate. In our work, experimental approaches were employed to understand the interface coupling effect of the substrate surface and coatings on the laser resistance of the coatings. A 1064 nm anti-reflection (AR) coating was deposited by an e-beam coater onto fused silica with and without micro-scale pits (structural defects). The micro-scale pits were precisely fabricated by femtosecond laser processing to prevent the emergence of subsurface cracks. Different deposition temperatures were characterized in order to verify the intensity of the interface coupling effect of the substrate and coating layers. Our experimental results indicate that impurities that are introduced in the finishing process, shifted to the substrate surface, and aggregated during the heating process, play a much more crucial role than structural defects (length: ∼7 µm; width: ∼3 µm; depth: ∼0.8 µm) in the laser-induced damage process. By effectively reducing the intensity of the interface coupling effect, the e-beam AR coatings, whose laser-induced damage resistance was closed to the bare substrate, was prepared.

Impact of substrate pits on laser-induced damage performance of 1064-nm high-reflective coatings.

The laser damage resistance of coatings in high-power laser systems depends significantly on the surface quality of the substrate. In our experiment, pits were precisely fabricated on the surface of fused silica substrate using a femtosecond laser processing bench. The HfO2/SiO2 high-reflective coatings at 1064 nm were deposited by conventional e-beam evaporation onto fused silica substrates with and without pits, respectively. The internal crack that was induced by the substrate geometrical structure was first observed in our experiment. The laser-induced damage threshold test showed negative effects of the substrate pits on the laser resistance of high-reflective coatings. Simulations by the finite element method were carried out, and results demonstrated that the modulation of a high reflector multilayer geometry could lead to electrical-field amplification and reduce laser damage resistance. Combined with its poor mechanical properties, the pits on substrate could contribute to the occurrence of damages.

Comparative Study of Plasma-Enhanced-Atomic-Layer-Deposited Al2O3/HfO2/SiO2 and HfO2/Al2O3/SiO2 Trilayers for Ultraviolet Laser Applications.

High-performance thin films combining large optical bandgap Al2O3 and high refractive index HfO2 are excellent components for constructing the next generation of laser systems with enhanced output power. However, the growth of low-defect plasma-enhanced-atomic-layer-deposited (PEALD) Al2O3 for high-power laser applications and its combination with HfO2 and SiO2 materials commonly used in high-power laser thin films still face challenges, such as how to minimize defects, especially interface defects. In this work, substrate-layer interface defects in Al2O3 single-layer thin films, layer-layer interface defects in Al2O3-based bilayer and trilayer thin films, and their effects on the laser-induced damage threshold (LIDT) were investigated via capacitance-voltage (C-V) measurements. The experimental results show that by optimizing the deposition parameters, specifically the deposition temperature, precursor exposure time, and plasma oxygen exposure time, Al2O3 thin films with low defect density and high LIDT can be obtained. Two trilayer anti-reflection (AR) thin film structures, Al2O3/HfO2/SiO2 and HfO2/Al2O3/SiO2, were then prepared and compared. The trilayer AR thin film with Al2O3/HfO2/SiO2 structure exhibits a lower interface defect density, better interface bonding performance, and an increase in LIDT by approximately 2.8 times. We believe these results provide guidance for the control of interface defects and the design of thin film structures and will benefit many thin film optics for laser applications.

Damage threshold influenced by the high absorption defect at the film-substrate interface under ultraviolet laser irradiation.

The laser-induced damage threshold (LIDT) of a single-layer coating at the nanosecond (ns) regime is obviously lower than an uncoated substrate or a high reflectivity coating coated by the same material. To elucidate this phenomenon, we demonstrate the LIDT of three types of samples at 355 nm with 8 ns. High absorption defects are found at the film-substrate interface by comparing their LIDTs and damage morphologies. These defects originate from the substrate and appear during the coating process. Simulation results show that these defects, coupled to the coating, are mainly responsible for decreasing the damage threshold.

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.

Crystal facet and Na-doping dual engineering ultrathin BiOCl nanosheets with efficient oxygen activation for enhanced photocatalytic performance.

Photocatalytic oxidation (PCO) based on semiconductors offers a sustainable and promising way for environmental remediation. However, the photocatalytic performance currently suffers from weak light-harvesting ability, rapid charge combination and a lack of accessible reactive sites. Ultrathin two-dimensional (2D) materials are ideal candidates to overcome these problems and become hotpots in the research fields. Herein, we demonstrate an ultrathin (<4 nm thick) Na-doped BiOCl nanosheets with {001} facets (Na-BOC-001) fabricated via a facile bottom-up approach. Because of the synergistic effect of highly exposed active facets and optimal Na doping on the electronic and crystal structure, the Na-BOC-001 showed an upshifted conduction band (CB) with stronger reduction potential for O2 activation, more defective surface for enhanced O2 adsorption, as well as the highest visible-light driven charge separation and transfer ability. Compared with the bulk counterparts (BOC-010 and BOC-001), the largest amount of active species and the best photocatalytic performance for the tetracycline hydrochloride (TC) degradation were achieved for the Na-BOC-001 under visible-light irradiation, even though it had slightly weaker visible-light absorption ability. Moreover, the effect of the Na doping and crystal facet on the possible pathways for TC degradation was investigated. This work offers a feasible and economic strategy for the construction of highly efficient ultrathin 2D materials.

Laser-induced layers peeling of sputtering coatings at 1064 nm wavelength.

Large-scale layers peeling after the laser irradiation of dual ion beam sputtering coatings is discovered and a model is established to explain it. The laser damage morphologies relate to the laser fluence, showing thermomechanical coupling failure at low energy and coating layers separation at high energy. High-pressure gradients appear in the interaction between laser and coatings, resulting in large-scale layer separation. A two-step laser damage model including defect-induced damage process and ionized air wave damage process is proposed to explain the two phenomena at different energy. At relatively high energies (higher than 20 J/cm2), ionization of the air can be initiated, leading to a peeling off effect. The peeling effect is related to the thermomechanical properties of the coating materials.

Fast Fabrication of Fishnet Optical Metamaterial Based on Femtosecond Laser Induced Stress Break Technique.

To speed up the fabrication of optical metamaterials by making use of the fast speed advantage of femtosecond laser preparation, a metamaterial appropriate for femtosecond laser processing was designed, and the interaction between femtosecond laser and metal-dielectric-metal fishnet stacks was investigated in detail. Two kinds of processing mechanisms, thermal melting and stress break, were revealed during the fabrication. The thermal melting process, dominated by the interaction of femtosecond laser with metals, makes the upper and lower metal layers adhere to each other, which leads to the magnetic resonance impossible. The stress break process, dominated by the interaction of femtosecond laser with dielectrics, can keep the upper and lower metal coatings isolated. Fishnet optical metamaterial was fabricated by femtosecond laser-induced stress break technique, using back side ablation, high numerical aperture and super-Gaussian beam. The resolution and speed can reach 500 nm, and 100 units/s, respectively. Spectrophotometer measurement results proved that the magnetic resonances were found in the fishnet nanostructure. The theoretical refractive index of the metamaterial on a glass substrate reached -0.12 at the wavelength of 3225 nm. It proved that femtosecond laser-induced stress break was a good and fast tool during the fabrication of optical metamaterials.

A correlational study of Weifuchun and its clinical effect on intestinal flora in precancerous lesions of gastric cancer.

BACKGROUND: Weifuchun (WFC), a Chinese herbal prescription consisting of Red Ginseng, Isodon amethystoides and Fructus Aurantii, is commonly used in China to treat a variety of chronic stomach disorders. The aim of the paper was to determine the effect of WFC on intestinal microbiota changes in precancerous lesions of gastric cancer (PLGC) patients. METHODS: PLGC patients of H. pylori negative were randomly divided into two groups and received either WFC tablets for a dose of 1.44 g three times a day or vitacoenzyme (Vit) tablets for a dose of 0.8 g three times a day. All patients were treated for 6 months consecutively. Gastroscopy and histopathology were used to assess the histopathological changes in gastric tissues before and after treatment. 16S rRNA gene sequencing was carried out to assess the effects WFC on intestinal microbiota changes in PLGC patients. Receiver Operating Characteristics (ROC) analysis was used to assess the sensitivity and specificity of different intestinal microbiota in distinguishing between PLGC patients and healthy control group. RESULTS: Gastroscopy and histopathological results indicated that WFC could improve the pathological condition of PLGC patients, especially in the case of atrophy or intestinal metaplasia. The results of 16S rRNA gene sequencing indicated that WFC could regulate microbial diversity, microbial composition, and abundance of the intestinal microbiota of PLGC patients. Following WFC treatment, the relative abundance of Parabacteroides decreased in WFC group when compared with the Vit group. ROC analysis found that the Parabacteroides could effectively distinguish PLGC patients from healthy individuals with sensitivity of 0.79 and specificity of 0.8. CONCLUSIONS: WFC could slow down the progression of PLGC by regulating intestinal microbiota abundance. Trial registration NCT03814629. Name of registry: Randomized Clinical Trial: Weifuchun Treatment on Precancerous Lesions of Gastric Cancer. Registered 3 August 2018-Retrospectively registered, https://register.clinicaltrials.gov/ NCT03814629.