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The knowledge of the optical properties of coating materials at high temperatures is important for understanding the dynamic process of high-power laser-material interactions. In this paper, the variations of refractive index and physical thickness of single layer coatings were studied by ellipsometric spectroscopy at different temperatures. From 23 °C to 320 °C, a decrease and then an increase of the refractive index of SiO 2 , and HfO 2 single layer was observed, while the thickness of these layers increased first and then decreased. The inflection points of different coating materials occurred at different temperatures. Water evaporation processes, densification and hydrophilicity of films were used to explain the temperature dependent properties of the dielectric coatings. Results of the variation of refractive index and thickness of single layer coatings at different temperatures in the vacuum proved the mentioned theory. Moreover, HfO 2 single layer shows better resistance to both temperature change and vacuum change, indicating that it is promising for high-reflective coatings in photoelectric devices. Besides, the formation of transient defects during high power laser irradiation was interpreted considering the optical properties variation with temperature. Three confusing and debated issues concerning laser-coating interaction are interpreted and explained. • Change of coatings' optical constants with temperature were studied. • Mechanism relies on water evaporation, film densification and hydrophilicity. • Three confusing issues concerning laser-coating interaction are interpreted. [ABSTRACT FROM AUTHOR]

• HfO 2 –SiO 2 mixture coatings with different proportions were investigated. • Admixing 10% or higher SiO 2 in the mixture coatings suppresses inhomogeneity of HfO 2. • Short-wave pass dichroic laser mirror coatings were prepared using mixture layers. • Using HfO 2 –SiO 2 mixture layers fundamentally solves the half-wave hole effect. • Using different mixture layers gets further optical improvement in the pass band. Hafnia/silica (HfO 2 /SiO 2) short-wavelength pass dichroic laser mirror (DLM) coatings are widely utilized in high-power laser systems. However, the half-wave hole effect resulting from the refractive index inhomogeneity of HfO 2 severely restricts the spectra and potential applications of these coatings. To address this issue, an investigation was conducted on HfO 2 –SiO 2 mixture coatings with different proportions, which were produced by electron beam co-evaporation. Compared to the inhomogeneous growth characteristics observed in pure HfO 2 coatings, the introduction of 10 % or higher SiO 2 admixing in HfO 2 –SiO 2 mixture coatings results in an amorphous structure characterized by excellent refractive index homogeneity and improved surface smoothness. Leveraging this, different DLM coatings with the objective of achieving high transmittance across the 400 to 900 nm range were designed and prepared, while ensuring high reflectance at 1064 nm. The incorporation of HfO 2 –SiO 2 mixture layers instead of pure HfO 2 layers in the mirror coatings led to a fundamental inhibition of the half-wave hole effect throughout the entire broad transmission band, resulting in a substantial improvement in the optical performance of the coatings. Moreover, by incorporating several types of HfO 2 –SiO 2 mixture layers with different proportions in the coating stacks, the transmittance of DLM coatings in the pass band can be further increased, and the transmission fluctuations at different wavelengths can be further compressed. Additionally, compared to traditional DLM coatings with HfO 2 layers, DLM coatings incorporating HfO 2 –SiO 2 mixture layers exhibit lower roughness, higher laser-induced damage threshold (LIDT) at 532 nm, and similar LIDT at 1064 nm. These results indicate a promising strategy for fabricating DLM coatings tailored for high-power laser systems. [ABSTRACT FROM AUTHOR]

Abstract: Y2O3/SiO2 coatings were deposited on fused silica by electron beam evaporation. A continuous wave CO2 laser was used to condition parts of the prepared samples at different scanning speeds in the air. LAMBDA 900 spectrometer was used to investigate the changes of the transmittance and residual reflection spectrum. A Nomarski microscope under dark field was used to examine the changes of the micro defect density. The changes of the surface roughness and the microstructure of the film before and after conditioning were investigated by AFM and X-ray diffraction, respectively. We found that laser-induced damage threshold (LIDT) of the films conditioning at 30mm/s scanning speed was increased by more than a factor of 3 over the thresholds of the as-deposited films. The conditioning effect was correlated with an irradiation-induced decrease of the defect density and absorption of the films. [Copyright &y& Elsevier]

Antireflection (AR) coatings are deposited on UV grade fused silica substrates, which are cleaned in the dual ion beam sputtering device. Compared to ultrasonic and acid etching cleaning progress, ion beam etching improves the laser-induced damage threshold (LIDT) of substrates and AR coatings significantly at 355 nm. Ion beam etching declines the low LIDT defects drastically and removes lots of the impurity elements (Ce, Fe, K, and Na). Roughness test shows that the AR coatings and substrates with ion beam etching are very flat and of low roughness. Ion beam etching reduces the density of deep defect from substrates greatly. Damage morphologies show double layers delamination, which is explained via calculation of layer stress. This study will be helpful for preparation of high LIDT optical coatings. • The LIDT of the 355 nm AR coatings and substrates are improved greatly with ion beam etching of substrates. • The damage curves show that ion beam etching declined the low LIDT defects drastically. • The impurity elements (Ce, Fe, K, and Na) are removed efficiently, and the surfaces are quite smooth. • Damage morphologies show two different defects in AR coatings and correspond to the laser damage curves. • Laser damage mechanism of AR coatings is explained. [ABSTRACT FROM AUTHOR]

We investigate the influence of a protective layer on the picosecond laser-induced damage in HfO 2 /SiO 2 high-reflective (HR) coatings. Two types of 1064-nm HR coatings, with and without a SiO 2 protective layer, are deposited using electron beam evaporation. Laser-induced damage tests are operated with 1064-nm, 30-ps laser pulses in one-on-one mode. Five different angles of incidence (AOIs) are considered in the tests for both coatings, in order to modulate the electric field ( E -field) intensity in the HR coatings. The damage morphologies and cross-sections of the damage sites are characterized using scanning electron microscope (SEM) and focused ion beam (FIB), respectively. Experimental results show that the SiO 2 protective layer slightly improves the laser-induced damage threshold (LIDT) of the HR coatings for all AOI values. The damage morphology contains high-density micrometer-scale pits and layer delaminations, at low and high fluences, respectively. The cross-sections of the damage sites and LIDTs for different AOIs are compared with corresponding E -field distributions. For both coatings, the damage is initiated near the peak of the E -field. A negative linear relationship is observed between the LIDT and corresponding maximum E -field intensity for both coatings. [ABSTRACT FROM AUTHOR]

Highlights: [•] The peeling-off damages occurred at the InterfaceHfO2→SiO2 or InterfaceTa2O5→SiO2. [•] The interface microstructure was not a major factor for peeling-off damages. [•] Incomplete oxides and Na, K, and Li ions should be the cause of stripping damage. [ABSTRACT FROM AUTHOR]

Abstract: We present a monitoring strategy based on using two pieces of witness glass, which are brought to the measuring position in a specially chosen sequence, each witness glass is monitored by one single wavelength. To reduce the thickness error, some thick layers are divided into two layers and monitored by different witness glasses. Theoretical analysis and experimental results have demonstrated that the proposed monitoring strategy can achieve spectral performance close to the theoretical design. [Copyright &y& Elsevier]

Abstract: Two measures were presented to improve the laser-induced damage resistance of the optical films used in vacuum environments. Firstly, the impact of a few O2, N2 and He2 introduced in vacuum on the laser-induced damage threshold of the dielectric mirror coatings were investigated. Then, laser conditioning effects in atmosphere environments on the laser-induced damage threshold of the dielectric mirror coatings used in vacuum environments were also investigated. It was found that a few O2, N2 introduced in vacuum can effectively improve the laser damage resistance of the optical films in vacuum environment due to the resistance to air-vacuum effect. However, a few He2 have no this effect on laser induced damage threshold (LIDT) of the films. In addition, laser conditioning in atmosphere can also effectively improve the LIDT of the optical thin films used in vacuum environment. [Copyright &y& Elsevier]

Fig.1. (a) SEM images of the fabricated low-oscillation dispersion mirrors (LODM). The surface GLAD SiO 2 layer has an ultra-low refractive index of 1.09, which is used to reduce the partial reflection of the surface layer, thereby reducing the dispersion oscillation of the entire mirror. (b) electric field intensity(EFI) distribution of three mirrors. (c) multi-wavelength Dynamic EFI distribution of LODM. In Brief. The dispersion oscillation and damage threshold of a dispersion mirror (DM) are the key influencing factors for high-quality pulses and outputting high power density. The sculptured structure film prepared by the glance-angle deposition(GLAD) technique has a porous structure, and its shape and porosity can be adjusted. In this study, we fabricated a low-oscillation DM (LODM) with a high damage threshold through a sculptured structure layer. Although the static electric field intensity (EFI) inside the LODM is 8.5 times that of the Bragg mirror, their thresholds are still close. As laser damage is a dynamic process, EFI is redistributed in time. A new EFI analysis model, which combines the influence of action time and different frequency components is employed. As a result, the EFI transfer to deep due to sculptured layer is the root cause of high damage threshold. In the depth of the film, there is a time difference in the formation of EFI peaks of different frequencies, which causes the EFI in the depth of the film to cancel out. The dynamic effect of few-cycle laser pulses or attosecond pulses damage will be more common. It can elucidate the ultrafast laser damage behavior of film components. [Display omitted] • A low-oscillation dispersion mirror (LODM) with a high damage threshold is fabricated through a sculptured structure layer; • A new electric field analysis model including time component is established to analyze ultrafast laser damage; • Introducing the standing wave field peak of the multilayer into the depth of the film may be a good way to increase the damage threshold. The dispersion oscillation and damage threshold of a dispersive mirror (DM) are the key influencing factors for high-quality pulses and outputting high power density. Here, we studied the femtosecond damage behavior of low-oscillation DMs(LODM) with SiO 2 sculptured layer. The monochromatic standing wave field(SWF) distribution widely used in ultrafast laser damage analysis is sufficient in Bragg mirror. In more complex systems, the actual electric field intensity(typically smaller) will dynamically change due to the broad bandwidth and the time difference of pulse propagation. Time-domain dynamic electric field intensity displays that the spatio-temporal evolution of the pulse within the stacks averages the electric field intensity, especially the electric field in the deeper layers. The dynamic effect of few-cycle pulses or attosecond pulses damage will be more meaningful. The time-domain dynamic electric field model can elucidate the ultrafast laser damage behavior. Introducing the time-domain dynamic electric model into the merit function can help us design optical coatings with high damage thresholds. [ABSTRACT FROM AUTHOR]

Abstract: Monolayer ZrO2 and multi-layer ZrO2/SiO2 films were contaminated deliberately with toluene in vacuum environments. Laser-induced damage tests were made on clean and contaminated ZrO2/SiO2 high reflective films. Surface adsorption layer model was presented to evaluate the impact of organic contaminations on the laser-induced damage. Based on this model, the distributions of electric field and temperature field in multi-layer ZrO2/SiO2 films were calculated. Results show that the mere organic contaminations cannot induce the thermal damage of optical films. The inter-coupling between defects and organic contaminations is probably attributed to the decrease of the laser-induced damage threshold in contaminated samples. [Copyright &y& Elsevier]

Abstract: Laser-induced damage is associated with nodular defects in HfO2/SiO2 multilayer films. In order to investigate the damage characteristics of HfO2/SiO2 multilayer mirrors and find the information of improving laser-induced damage threshold, nodular defects are characterized by multiple analytical techniques; the damage morphologies induced by nodular-ejections are presented; the depths of nodular-ejection pits are investigated; the laser-induced damage threshold of zero probability and the stabilities of nodular-ejection pits exposed to repetitive illuminations are studied. Results show that domes in the film surface are nodular defects. Reliable depth information of nodular-ejection pits is obtained by counting layers from the damage edge. The depth statistical result implies nodular defects in these samples are usually originated from deep seeds. Some process optimizations suggestions are given based on the depth information. A simple tractable method is proposed to determine the functional damage threshold of these HfO2/SiO2 multilayer films basing on the damage experiments. [Copyright &y& Elsevier]

Abstract: The damage characteristics of anti-reflection films exposed to 1064nm nanosecond laser pulses in vacuum and air were investigated. The laser-induced damage thresholds (LIDT) in vacuum and air environments were compared. The damage morphologies were locally analyzed using optical microscopy and scanning electron microscopy (SEM). The depth of the damage sites in the two environments was analyzed by Alpha2Step 500m. The differences between the damage observed in the two environments were discussed based on the temperature distribution, absorption and vacuum gaseous composition measurements and gas pressure influence analysis. Increased coupling of the plasma shock wave and adsorbing water in the coating layer may be considered as the primary cause of the low LIDT observed in vacuum. [Copyright &y& Elsevier]

Abstract: The impact of two organic contaminations on the damage characteristics of anti-reflector (AR) at 1064nm is investigated. Contamination experiments were made with toluene and acetone in liquid phase on the surface of AR coatings. Chemical and morphological characterization methods were used to identify and understand the damage process. The possible damage process is analyzed and discussed. It is found that toluene decreases laser-induced damage threshold and acetone seems to be benign and has little influence on laser-induced damage threshold due to its quickly spreading into the coating. Adsorption and droplet micro-lensing effect mechanism are the main cause of enhanced laser-induced damage of toluene. [Copyright &y& Elsevier]

Abstract: The influence of organic contamination in vacuum on the laser-induced damage threshold (LIDT) of coatings is studied. TiO2/SiO2 dielectric mirrors with high reflection at 1064nm are deposited by the electron beam evaporation method. The LIDTs of mirrors are measured in vacuum and atmosphere, respectively. It is found that the contamination in vacuum is easily attracted to optical surfaces because of the low pressure and becomes the source of damage. LIDTs of mirrors have a little change in vacuum compared with in atmosphere when the organic contamination is wiped off. The results indicate that organic contamination is a significant reason to decrease the LIDT. N2 molecules in vacuum can reduce the influence of the organic contaminations and prtectect high reflectance coatings. [Copyright &y& Elsevier]

Abstract: A model for refractive index of stratified dielectric substrate was presented according to inhomogeneous coatings theories. The substrate was divided into surface layer, subsurface layer and bulk layer along the normal direction of its surface. The former two layers were equivalent to inhomogeneous coatings. Theoretical deduction was executed by employing the characteristic matrix method of optical coatings, and one mathematical calculation example was presented. The results indicate that reflectance, reflective phase shift and phase difference of polarized light deviate from ideal conditions. It shows that substrate microdefects can induce volume scattering and change propagation characteristic of light both in coatings and substrate. [Copyright &y& Elsevier]

Abstract: A model for refractive index of stratified dielectric substrate was put forward according to theories of inhomogeneous coatings. The substrate was divided into surface layer, subsurface layer and bulk layer along the normal direction of its surface. Both the surface layer (separated into N 1 sublayers of uniform thickness) and subsurface layer (separated into N 2 sublayers of uniform thickness), whose refractive indices have different statistical distributions, are equivalent to inhomogeneous coatings, respectively. And theoretical deduction was carried out by employing characteristic matrix method of optical coatings. An example of mathematical calculation for optical properties of dielectric coatings had been presented. The computing results indicate that substrate subsurface defects can bring about additional bulk scattering and change propagation characteristic in thin film and substrate. Therefore, reflectance, reflective phase shift and phase difference of an assembly of coatings and substrate deviate from ideal conditions. The model will provide some beneficial theory directions for improving optical properties of dielectric coatings via substrate surface modification. [Copyright &y& Elsevier]