High-reflection Coatings Research Articles

Reactive ion-assisted deposition of e-beam evaporated titanium for high refractive index TiO2 layers and laser damage resistant, broad bandwidth, high-reflection coatings

High-reflection coatings with broad bandwidth can be achieved by pairing a low refractive index material, such as SiO2, with a high refractive index material, such as TiO2. To achieve high refractive index, low absorption TiO2 films, we optimized the reactive, ion-assisted deposition process (O2 levels, deposition rate, and ion beam settings) using e-beam evaporated Ti. TiO2 high-index layers were then paired with SiO2 low-index layers in a quarter-wave-type coating to achieve a broader high-reflection bandwidth compared to the same coating composed of HfO2/SiO2 layer pairs. However, the improved bandwidth exhibited by the TiO2/SiO2 coating is associated with lower laser damage threshold. To improve the laser damage resistance of the TiO2/SiO2 coating, we also created four coatings where HfO2 replaced some of the outer TiO2 layers. We present the laser damage results of these coatings to understand the trade-offs between good laser damage resistance and high-reflection bandwidth using TiO2 and HfO2.

The thermal effect of an innovative cool roof on residential buildings in Italy: Results from two years of continuous monitoring

Cool roofs represent an innovative and relatively inexpensive technique to reduce building energy requirements for cooling and to improve indoor thermal comfort conditions. These applications primarily consist of high-reflectance and high-emissivity coatings or membranes commonly applied to the flat roofs of non-residential buildings located in both hot and mild climates. This paper discusses the possibility of applying an innovative “cool roof” solution, consisting of a prototyped cool clay tile, on a traditional residential building in central Italy to improve the thermal conditions of the indoor environment that is adjacent to the roof. In particular, the results of a two-year continuous monitoring campaign are presented. The building was monitored for an entire year in the original configuration and for another entire year in the optimized configuration, with the final objective of quantifying both the summer benefits and the winter penalties of such a solution in residential buildings in temperate climate conditions. The year-round analysis shows that the proposed cool roof solution produces a maximum effect of decreasing summer peak indoor overheating of the attic by up to 4.7°C. The corresponding winter maximum overcooling reduction is 1.2°C. These experimental results show that this innovative cool roof solution can be implemented on traditional sloped roofs with a clay tile covering, producing substantial benefits in summer and relatively small penalties in winter for residential buildings, even in temperate climates.

Investigations on single and multiple pulse laser-induced damages in HfO_2/SiO_2 multilayer dielectric films at 1064 nm

Nanosecond single and multiple pulse laser damage studies on HfO₂/SiO₂ high-reflective coatings were performed at 1064 nm. The evolution of LIDT and 100% damage probability threshold under multiple irradiations revealed that fatigue effects were affected by both laser fluence and shot numbers. And the damage probability curves exhibiting different behaviors confirmed experimentally that this fatigue effect of the dielectric coatings was due to material modification rather than statistical effects. By using a model assuming Gaussian distribution of defect threshold, the fitting results of LID probability curves indicated the turning point appeared in the damage probability curves under large shot number irradiations was just the representation of the existence of newly created defects. The thresholds of these newly created defects were exponential decrease with irradiated shot numbers. Besides, a new kind of damage morphologies under multiple shot irradiations were characterized to further expose the fatigue effect caused by cumulative laser-induced material modifications.

High-reflectivity mirrors by Al2O3, LaF3 and AlF3 for 193nm application

Abstract As important components in deep-ultraviolet (DUV) optics, especially 193 nm lithographic systems, high-reflectivity (HR) mirrors with excellent optical properties and long lifetimes are needed urgently. In this study, we designed and produced three HR coatings for 193 nm on fused quartz substrates: Al2O3/AlF3 coating, LaF3/AlF3 coating and a double stack mirror with combined Al2O3/AlF3 and LaF3/AlF3. The reflectance of the Al2O3/AlF3 coating with 14 layer pairs reached 98.0% at 193 nm. However, the absorption of Al2O3 prevented the reflectance to increase further. The maximum reflectance of the LaF3/AlF3 coating with 15 layer pairs reached 98.1%, with initial micro-cracks formation. The reflectance decreased as the number of layer pairs increased to 16 because of numerous micro-cracks. The mirror with combined Al2O3/AlF3 and LaF3/AlF3 coatings which combined their advantages obtained a reflectance of 98.8% at 193 nm after deposition. This value could still reach 98.5% at 4 months after deposition and remain stable thereafter. Therefore, the combined coating of Al2O3/AlF3 and LaF3/AlF3 is an excellent candidate for practical application.

Laser-induced damage of rugate and quarter-wave stacks high reflectors deposited by ion-beam sputtering

Rugate and high/low quarter-wave stacks high reflector coatings for 1064 nm have been prepared with Ta 2 O 5 and SiO 2 by an ion-beam sputtering technique. A laser-induced damage experiment of the samples has been conducted at 1064 nm with pulse duration of 5 ns [full width at half maximum (FWHM)]. These two samples’ damages both initiate at defects and show almost the same damage threshold within the experimental error. Nevertheless, the damage morphology on rugate is less severe at higher fluences. The thermal shock wave induced by a nanosecond pulsed laser is considered to be the main cause of catastrophic damage.

Development of Clay Tile Coatings for Steep-Sloped Cool Roofs

Most of the pitched roofs of existing buildings in Europe are covered by non-white roofing products, e.g., clay tiles. Typical, cost effective, cool roof solutions are not applicable to these buildings due to important constraints deriving from: (i) the owners of homes with roofs visible from the ground level; (ii) the regulation about the preservation of the historic architecture and the minimization of the visual environment impact, in particular in historic centers. In this perspective, the present paper deals with the development of high reflective coatings with the purpose to elaborate “cool” tiles with the same visual appearance of traditional tiles for application to historic buildings. Integrated experimental analyses of reflectance, emittance, and superficial temperature were carried out. Deep analysis of the reflectance spectra is undertaken to evaluate the effect of different mineral pigments, binders, and an engobe basecoat. Two tile typologies are investigated: substrate-basecoat-topcoat three-layer tile and substrate-topcoat two-layer tile. The main results show that the developed coatings are able to increase the overall solar reflectance by more than 20% with acceptable visual appearance, suitable for application in historic buildings. Additionally, the effect of a substrate engobe layer allows some further contribution to the increase of the overall reflectance characteristics.

Tenfold reduction of Brownian noise in high-reflectivity optical coatings

Thermally induced fluctuations impose a fundamental limit on precision measurement. In optical interferometry, the current bounds of stability and sensitivity are dictated by the excess mechanical damping of the high-reflectivity coatings that comprise the cavity end mirrors. Over the last decade, the dissipation of these amorphous multilayer reflectors has at best been reduced by a factor of two. Here, we demonstrate a new paradigm in optical coating technology based on directbonded monocrystalline multilayers, which exhibit both intrinsically low mechanical loss and high optical quality.

Simulation of Multiple Reflecting Structures Using a Nonparaxial Bidirectional Split-Step Finite Difference Method

We present simulation of nonparaxial waves in multiple reflecting structures, using a recently proposed bidirectional method based on a finite difference split-step nonparaxial scheme. Simulation of finite beams in volume Bragg gratings, through high reflection coatings and tilted fiber Bragg gratings are presented. Comparisons are made with experimental, analytical, and other numerical methods to demonstrate its accuracy and efficiency. The method is noniterative and the propagation matrices are evaluated analytically making it computationally very efficient. The method has very high accuracy for modeling reflection of nonparaxial beams and can simulate large longitudinal index discontinuities efficiently without using special modifications.

Nanosecond pulsed laser damage characteristics of HfO_2/SiO_2 high reflection coatings irradiated from crystal-film interface

The nano-precursors in the subsurface of Nd:YLF crystal were limiting factor that decreased the laser-induced damage threshold (LIDT) of HfO(2)/SiO(2) high reflection (HR) coatings irradiated from crystal-film interface. To investigate the contribution of electric-field (E-field) to laser damage originating from nano-precursors and then to probe the distribution of vulnerable nano-precursors in the direction of subsurface depth, two 1064 nm HfO(2)/SiO(2) HR coatings having different standing-wave (SW) E-field distributions in subsurface of Nd:YLF c5424181043036123rystal were designed and prepared. Artificial gold nano-particles were implanted into the crystal-film interface prior to deposition of HR coatings to study the damage behaviors in a more reliable way. The damage test results revealed that the SW E-field rather than the travelling-wave (TW) E-field contributed to laser damage. By comparing the SW E-field distributions and LIDTs of two HR coating designs, the most vulnerable nano-precursors were determined to be concentrated in a thin redeposition layer that is within 100 nm from the crystal-film interface.

Effects of thickness on the structural, electronic, and optical properties of MgF2 thin films: The first-principles study

First-principles calculations were performed on MgF2 (001) thin films and their structural, electronic, and optical properties were investigated. Results showed that the films are energetically stabilized when the layer number is increased to 25, corresponding to a thickness of about 4nm. The thin films exhibited narrower band gaps compared with their bulk counterparts; gaps increased with increasing thickness of the thin films. Analysis of the optical properties of the films showed that their refractive index is significantly decreased and their extinction coefficient is slightly increased compared with the bulk. With increasing thin film thickness, the refractive index decreased slightly and the extinction coefficient remained unchanged. These results indicate that MgF2 thin films of a certain thickness range have excellent performance as anti-reflection and high-reflection coatings at visible wavelengths.

Improvement of the laser-induced damage threshold of oxide/fluoride double stack high reflective coatings at 355 nm by introducing interlayers

The development of oxide/fluoride double stack coatings presented a good solution for the fabrication of HR coatings with high reflectance and low stress. However, their low laser-induced damage threshold (LIDT) remains to be one problem unsolved. In this paper, SiO2 interlayer was introduced into the optimized double stack coatings and a 50% increase in LIDT was obtained. Characterizations including the residual stress, damage crater depth, damage morphology, electric field distribution, and density of surface defects were conducted in order to analyze the mechanism of such improvement. The decrease in density of sensitive defects and the increase in stability of defects are believed to be the benefits brought by the optimized design.

Fabrication Fluoride High Reflection Mirror by Thermal Evaporation in the ArF Excimer Laser

ArF excimer laser is the main light resource for the microlithography technology. In the laser cavity, the optical components with lowest absorption and scattering loss are necessary. As a consequence only a few materials are promising candidates for 193nm coatings with high transmittance or high reflectance. Fluoride films exhibit relatively low optical loss as well as high laser induced damage thresholds. The potentiality of LaF3 and MgF2 is evaluated in respect of the production of improved optical coatings for applications. For this purpose, single layer is prepared by thermal evaporation at the deposited temperatures of 523K on Fused Silica. A first order bulk inhomogeneity model for extracting the optical constants of weak absorbing film is applied, which is based on spectrophotometry. Refractive index (n) and extinction coefficient (k) of the optimal LaF3 film are 1.678 and 2.24×10-3 at 193nm. In the case of the optimal MgF2 film, n and k are 1.443 and 4.72×10-4 at 193nm. High reflection (HR) LaF3/MgF2 coatings are designed and fabricated for normal incident on CaF2. The experimental results indicate that after coated and fluoride coatings deposit, according to optimal process, can have more suitable optical properties at deep ultraviolet (DUV), the reflectance of HR coatings reaches more than 98% at 193nm.

Green roofs in European climates. Are effective solutions for the energy savings in air-conditioning?

Several studies show the potential benefits achievable by recurring to roof vegetation. Really, little literature investigates the economic feasibility of such solution. The paper verifies utility of green roofs, under environmental and energy point of views, by considering all the aspects that influence their performances. With reference to several climates, intensity of rainfalls, needs of irrigation and kind of building use, a large parametric analysis evaluates the technical and economical feasibility of green roofs applied to a modern office building, considering various vegetations and different external coatings. The scarce amount of rainfall – and thus the irrigation cost – can nullify the savings in energy demand for air-conditioning. Moreover, even if green roofs show satisfactory performance if monthly rainfalls do not imply significant additional watering – the economic investigation shows scarce convenience for well-insulated buildings, above all if the higher initial cost of a green roof, compared to traditional roofing coating, is computed. Finally, cool roofs, by means of high-reflective and high-emissive coatings, are suitable solutions in warm climates, strongly improving the summer performances, with low extra costs for installation and maintenance.

High-Reflectance Coatings at 193nm

As an important component in 193nm laser system, high-reflectance (HR) coatings with excellent optical and mechanical properties are needed urgently. In this study, we designed and produced three different HR coatings at 193nm, Al2O3/ AlF3 coating, LaF3/ AlF3 coating and a double stack mirror with combination of Al2O3/ AlF3 and LaF3/ AlF3. With 14 layer pairs, the reflectance of Al2O3/ AlF3 coating could reach 98% which could not get higher duo to the absorption of Al2O3. The reflectance of LaF3/ AlF3 coatings could achieve 98.13% with 15 layer pairs. When the layer pair increased to 16, the reflectance lessened ascribed to steep rise in scattering loss caused by micro-cracks occurred in the coatings. A mirror with combination of Al2O3/ AlF3 and LaF3/ AlF3 could obtain the reflectance of 98.78% as deposited. Even 4 months later the value could still reach 98.47% which was an excellent candidate for practical application.

Photothermal self-phase-modulation technique for absorption measurements on high-reflective coatings

We propose and demonstrate a new measurement technique for the optical absorption of high-reflection coatings. Our technique is based on photothermal self-phase modulation and exploits the deformation of cavity Airy peaks that occurs due to coating absorption of intracavity light. The mirror whose coating is under investigation needs to be the input mirror of a high-finesse cavity. Our example measurements were performed on a high-reflection SiO2-Ta2O5 coating in a three-mirror ring-cavity setup at a wavelength of 1064 nm. The optical absorption of the coating was determined to be α=(23.9±2.0)·10(-6) per coating. Our result is in excellent agreement with an independently performed laser calorimetry measurement that gave a value of α=(24.4±3.2)·10(-6) per coating. Since the self-phase modulation in our coating-absorption measurement affects mainly the propagation through the cavity input mirror, our measurement result is practically uninfluenced by the optical absorption of the other cavity mirrors.

2.4 W room temperature continuous wave operation of distributed feedback quantum cascade lasers

We demonstrate high power continuous-wave room-temperature operation surface-grating distributed feedback quantum cascade lasers at 4.8 μm. High power single mode operation benefits from a combination of high-reflection and antireflection coatings. Maximum single-facet continuous-wave output power of 2.4 W and peak wall plug efficiency of 10% from one facet is obtained at 298 K. Single mode operation with a side mode suppression ratio of 30 dB and single-lobed far field without beam steering is observed.

Laser induced deflection technique for absolute thin film absorption measurement: optimized concepts and experimental results

Using experimental results and numerical simulations, two measuring concepts of the laser induced deflection (LID) technique are introduced and optimized for absolute thin film absorption measurements from deep ultraviolet to IR wavelengths. For transparent optical coatings, a particular probe beam deflection direction allows the absorption measurement with virtually no influence of the substrate absorption, yielding improved accuracy compared to the common techniques of separating bulk and coating absorption. For high-reflection coatings, where substrate absorption contributions are negligible, a different probe beam deflection is chosen to achieve a better signal-to-noise ratio. Various experimental results for the two different measurement concepts are presented.

Searching for optimal mitigation geometries for laser-resistant multilayer high-reflector coatings

Growing laser damage sites on multilayer high-reflector coatings can limit mirror performance. One of the strategies to improve laser damage resistance is to replace the growing damage sites with predesigned benign mitigation structures. By mitigating the weakest site on the optic, the large-aperture mirror will have a laser resistance comparable to the intrinsic value of the multilayer coating. To determine the optimal mitigation geometry, the finite-difference time-domain method was used to quantify the electric-field intensification within the multilayer, at the presence of different conical pits. We find that the field intensification induced by the mitigation pit is strongly dependent on the polarization and the angle of incidence (AOI) of the incoming wave. Therefore, the optimal mitigation conical pit geometry is application specific. Furthermore, our simulation also illustrates an alternative means to achieve an optimal mitigation structure by matching the cone angle of the structure with the AOI of the incoming wave, except for the p-polarized wave at a range of incident angles between 30° and 45°.

Far-field improvement of quantum cascade lasers through high-reflection coatings

A new approach has been developed for quantum cascade lasers to improve beam quality. Using two carefully designed high-reflection coatings, the fundamental mode was selected to have the lowest threshold current and the horizontal far-field pattern has been greatly improved, in good agreement with simulations.

Ta2O5和Nb2O5单层膜及高反膜激光损伤阈值的对比研究

Ta2O5 and Nb2O5 films are deposited on BK7 glass substrates using an electron beam evaporation method and are annealed at 673 K in the air. In this letter, comparative studies of the optical transmittance, microstructure, chemical composition, optical absorption, and laser-induced damage threshold (LIDT) of the two films are conducted. Findings indicate that the substoichiometric defect is very harmful to the laser damage resistance of Ta2O5 and Nb2O5 films. The decrease of absorption improves the LIDT in films deposited by the same material. However, although the absorption of the Ta2O5 single layer is less than that of the Nb2O5 single layer, the LIDT of the former is lower than that of the latter. High-reflective (HR) coatings have a higher LIDT than single layers due to the thermal dissipation of the SiO2 layers and the decreased electric field intensity (EFI). In addition, the Nb2O5 HR coating achieves the highest LIDT at 25.6 J/cm2 in both single layers and HR coatings.