Highly reflective optical coatings for high-power applications of micro scanning mirrors in the UV-VIS-NIR spectral region
Thilo SandnerJ. SchmidtHarald SchenkHubert LaknerMinghong YangAlexandre GattoNorbert KaiserStefan BraunThomas FoltynAndreas Leson
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This paper addresses different highly reflective optical coatings on micro scanning mirrors (MSM) for applications in the NIR-VIS-UV- spectral region to enable new applications at high optical power density like laser marking and material treatment. In the common case of MSM with an unprotected Al coating, the absorption limits the maximal power density because of induced heating. In contrast to macroscopic optics HR-micro mirror coatings have to guarantee additional demands like low-stress and CMOS compatibility. Hence, to enable novel high power applications of MSM in the NIR-VIS-UV spectral region highly reflective low-stress coatings have been developed according to a triple strategy: (a) broadband metallic reflectors, (b) dielectric multilayers and (c) enhanced hybrid coatings. For Au and Ag based NIR-coatings an excellent mirror planarity and a reflectance around 99 % (@ 1064 nm) have been achieved, whereas dielectric coatings reached 99.7 % for a (LH)4 design and thinner low-stress hybrid NIR-coatings reached up to 99.8 % enabling an improved mirror planarity and excellent laser damage threshold. For the VIS and UV spectral region enhanced hybrid HR-coatings have been favored, because they enable high reflectance of up to 99.7 % @ 633 nm or 98.8 % @ 308 nm in combination with low stress, high mirror planarity and CMOS compatibility.Keywords:
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The laser-induced damage thresholds for λ=1.06 μm of commercially available (produced at Institute of Optics) dielectric optical coatings, both antireflective and high reflectance, have been determined. The dependence of the optical coatings stability on design and selection of materials has been investigated. An improvement of the coatings durability by using nonquarterwave layers in addition to the basic design of the mirrors has been obtained. The choice of the coating materials is also discussed.
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An extreme ultraviolet multilayer mirror with an integrated spectral filter for the IR range is presented and experimentally evaluated. The system consists of an IR-transparent B4C/Si multilayer stack which is used both as EUV-reflective coating and as a phase shift layer of the resonant IR antireflective (AR) coating. The AR coating is optimized in our particular case to suppress CO2 laser radiation at a wavelength of 10.6 μm, and a suppression of more than two orders of magnitude is demonstrated. The method allows high suppression over a large angular acceptance range, relevant for application in lithography systems.
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Laser 3D nanolithography enables the fabrication of complex shape micro-optical elements. The freedom of multi-surface designs of such components has cost them to suffer from reflection losses. This work presents the deposition of an Antireflective (AR) coating, using Atomic Layer Deposition (ALD), on hybrid organic-inorganic polymer SZ2080™ microstructures and micro-lenses fabricated using Laser Direct Writing (LDW). The single-wavelength AR coating produced using ALD successfully reduced reflection from 3.3 % to 0.1 % at 633 nm for one surface of SZ2080™.
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In recent years, there has been a growing interest in further development of sol-gel method which can produce ceramics and glasses using chemical precursors at relative low-temperatures. The applications for sol-gel derived products are numerous. Department of General and Inorganic Chemistry with Laser Research Center of Vilnius University and Institute of Physics continues an ongoing research effort on the synthesis, deposition and characterization of porous solgel. Our target is highly optically resistant anti-reflective (AR) coatings for general optics and nonlinear optical crystals. In order to produce AR coatings a silica (SiO2) sol-gel has been dip coated on the set of fused silica substrates. The optical properties and structure of AR-coatings deposited from hydrolysed tetraethylorthosilicate (TEOS) sol were characterized in detail in this study. The influence of different parameters on the formation of colloidal silica antireflective coatings by dip-coating technique has been investigated. All samples were characterized performing, transmission electron microscopy, UV-visible spectroscopy, atomic force microscopy, ellipsometric, total scattering and laser-induced damage threshold measurements. Herewith we present our recent results on synthesis of sol-gel solvents, coating fabrication and characterization of their optical properties.
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A type of two-layer SiO2-TiO2 antireflective(AR)coatings used in solar glass was introduced in this paper to improve the complicated two-step catalytic process and promote the mechanical strength of the coatings.Based on the coating design program,the reflectance of the two-layer(λ/4-λ/2 type)AR coatings and the optical parameters of the coating were designed and optimized.Meanwhile,the two-layer SiO2-TiO2 AR coatings was successfully coated on the glass substrate by sol-gel method.The performance of the coatings was analyzed with instruments such as Ellipsometer,UV-VIS-NIR Spectrophotometer,Atomic Force Microscopy.The results show that the average transmittance of glass coated in this way was increased by more than 6% at the wave band of 400 nm~800 nm.And the mechanical strength of the coatings is very well.
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Optical sol-gel materials have been of interest for many years. The reason is that through the preparation of sold with nanoparticulate liquid structures, transparent coatings of many inorganic oxides can be produced. By using oxides for example, with different refractive indices, reflective or antireflective coatings can be fabricated. To obtain stable layers, the gel coating have to be densified at higher temperatures, in general between 400 and 600 degrees C. This may be suitable for glass surfaces, but not for temperature sensitive substrates like plastics. In addition to this, if multilayer coatings have to be produced, between each step a densification process has to be carried out before the net coating step takes place. This leads to an unsatisfying situation if industrial low cost processing is required. In addition to this, the dip coating process is not suitable for high speed or large area coating techniques. This is one of the reasons whey the sol-gel process never has gained a real high significance for industrial coatings on glass and is limited to special products so far.
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This paper discusses the conditions for obtaining the structure of a five-layer antireflection interference coating that contains layers of different thicknesses. Regions of allowable values are given for the refractive index of the layers that form this coating and make it possible to obtain three or five zero values of the reflectance. The resulting structures of five-layer coatings create the possibility of reducing the reflectance of optical materials with either high or low refractive indices. The given coatings are capable of reducing the reflectance in several spectral regions simultaneously.
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Although metal halide anti-reflective (AR) coatings are widely used by manufacturers of electronics equipment, high application temperatures mean that they can only easily be applied to glass substrates. Screens made from plastics materials can be coated but the process requires additional steps to prevent damaging the substrate. A new, easily applied anti-reflective coating has been designed which can be applied to both plastic and glass substrates. The single layer coating applied to an acrylic substrate has proven to be better performing than current commercial single layer anti-reflective coatings. This performance has been achieved from an amorphous fluoropolymer solution which is dip coated onto the substrate.
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In this study, reflective and antireflective coatings were designed and simulated. Optical transmission and reflection values were deduced with a matrix formulation via a personal computer. It was found that the number of layers affects the optical performance. The width of the highreflectance region in the reflectance curves decreases, while its height increases with the increasing number of layers for the reflective coating design. The antireflection coatings transmit about 99.89% in a broad high-pass band at the central wavelength of λ0 = 450 nm. In addition, simulated Fabry-Perot filters result in a single sharp transmittance peak at the desired central wavelength. The half-width of the transmission band at central wavelength decreases and its peak height increases with the increasing number of the coated layers. To compare with theory, both sides of a glass substrate were deposited a two-layer coating of MgO/MgF2 via electron beam evaporation, to produce an antireflective coatings in the visible and near infrared regions. The optical properties of prepared films were studied through optical transmission measurements. The peak transmittance was 98.2% at the central wavelength λ◦ = 450 nm.
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