Protected Silver Mirrors Research Articles

Unveiling the efficacy of a bulk Raman spectra-based model in predicting single cell Raman spectra of microorganisms

In a previous publication, we trained predictive models based on Raman bulk spectra of microorganisms placed on a silicon dioxide protected silver mirror slide to make predictions for new Raman spectra, unknown to the models, of microorganisms placed on a different substrate, namely stainless steel. Now we have combined large sections of this data and trained a convolutional neural network (CNN) to make predictions for single cell Raman spectra. We show that a database based on microbial bulk material is conditionally suited to make predictions for the same species in terms of single cells. Data of 13 different microorganisms (bacteria and yeasts) were used. Two of the 13 species could be identified 90% correctly and five other species 71%–88%. The six remaining species were correctly predicted by only 0%–49%. Especially stronger fluorescence in bulk material compared to single cells but also photodegradation of carotenoids are some effects that can complicate predictions for single cells based on bulk data. The results could be helpful in assessing universal Raman tools or databases.

Effects of stress, defects, and layer composition on the development of corrosion features on protected silver mirrors.

The long-term environmental durability of protected silver mirror coatings is influenced by numerous factors. Here, accelerated environmental exposure testing of model silver mirror coatings illuminated the effects of stress, defects, and layer composition on the extent and mechanisms of corrosion and degradation. Experiments to reduce stress in the highest-stress layers of the mirror coatings revealed that, while stress may affect the extent of corrosion, coating defects and the composition of the mirror layers have the largest influence on corrosion feature development and growth.

Critical processing temperature for high-performance protected silver mirrors

Silver (Ag) mirrors for astronomical telescopes consist of multiple metallic and dielectric thin films. Furthermore, the topmost surface of such Ag mirrors needs to be covered by a protection coating. While the protection coating is often deposited at room temperature and the entire mirrors are also handled at room temperature, various thin film deposition techniques offer protection coatings with improved characteristics when carried out at elevated temperatures. Thus, in this work, high-performance Ag mirrors were designed and fabricated with a new benchmark. The resulting Ag mirrors were annealed (i.e., post-fabrication annealing) at various temperatures to investigate the viability of introducing thermal processes during and/or after fabrication in improving overall optical performance and durability of protected silver mirrors. In our experiments, Ag mirror samples were deposited by electron-beam evaporation and subsequently annealed at various temperatures in the range from 60 {\deg}C to 300 {\deg}C, and then the mirror samples underwent an environmental stress test at 80 {\deg}C and 80% humidity for 10 days. While all the mirror samples annealed below 200 {\deg}C showed negligible corrosion after undergoing the stress testing, those annealed below 160 {\deg}C presented spectral reflectivity comparable to or higher than that of as-deposited reference samples. In contrast, the mirror samples annealed above 200 {\deg}C exhibited significant degradation after the stress testing. The comprehensive analysis indicated that delamination and voids caused by the growth of Ag grains during the annealing are the primary mechanisms of the degradation.

Correlation of long-duration exposure and accelerated testing of protected silver mirrors.

The stability of two types of protected silver mirrors was studied by long-term exposure in a clean-room laboratory and mixed-flowing-gas (MFG) accelerated environmental test with two different gas concentrations. The two types of mirrors behaved very differently when exposed to the clean-room air for six years. The mirrors subjected to a reduced Battelle Class II MFG test protocol for 10 days exhibited similar corrosion mechanisms to those exposed to the clean-room air. With further testing of silver mirrors in both field exposures and MFG test conditions, it is feasible that the MFG-accelerated test method can be developed to quantitatively assess the durability of protected silver mirrors in ambient applications.

Development and growth of corrosion features on protected silver mirrors during accelerated environmental exposure.

Various layer schemes have been developed to protect thin film silver mirrors from tarnish and corrosion. However, the mechanisms by which these additional layers improve mirror durability are not fully understood. Accelerated environmental exposure testing of protected silver mirrors was used to investigate the effects of layer composition on the mechanisms of corrosion feature development and growth. Two model mirror coatings were analyzed in which the composition of the base layer below the silver and the adhesion layer above the silver were varied. Large circular corrosion features formed preferentially along the silver-chromium interfaces, regardless of where this interface was located within the layered structure of the mirror. The corrosion features originated at coating defects in the dielectric protection layer, and their growth was likely driven by the oxidation and chloridation of both silver and chromium at the Ag-Cr interfaces.

NiCrNx interlayer thickness dependence of spectral performance and environmental durability of protected-silver mirrors

Gemini-style protected-silver mirror (Sub/NiCrNx/Ag/NiCrNx/SiNx/Air) is a suitable choice for optical instruments requiring both long-term environmental durability and high broadband reflectance. Three Gemini-style protected-silver mirrors with NiCrNx interlayer thicknesses between 0.1 and 0.6 nm were prepared by magnetron sputtering, and the dependences of spectral properties and environmental durability of these protected-silver mirrors on the thickness of NiCrNx interlayer between the silver layer and SiNx layer were investigated in-depth. The reflectance, transmittance and total scattering loss measurements, optical microscope, and scanning electron microscope imaging were employed to characterize the spectral properties and surface morphology, and accelerated environmental tests, including humidity test and salt fog test, were applied to investigate the environmental durability. The experimental results showed that both optical and corrosion-resistant properties of protected-silver mirrors were NiCrNx interlayer thickness dependent, and an optimum NiCrNx interlayer thickness should be similar to 0.3 nm for Gemini-style protected-silver mirrors to have reasonably both high reflectance in a broadband spectral range from visible to far infrared and good corrosion resistance for long-lifetime applications in harsh environments. (C) 2018 Society of Photo-Optical Instrumentation Engineers (SPIE)

Local Degradation Mechanisms by Tarnishing of Protected Silver Mirror Layers Studied by Combined Surface Analysis.

In this work, we addressed the local degradation mechanisms limiting the prelaunch environmental durability of thin-layered silver stacks for demanding space mirror applications. Local initiation and propagation of tarnishing were studied by combined surface and interface analysis on model stack samples consisting of thin silver layers supported on lightweight SiC substrates and protected by thin SiO2 overcoats, deposited by cathodic magnetron sputtering and submitted to accelerated aging in gaseous H2S. The results show that tarnishing is locally initiated by the formation of Ag2S columns erupting above the stack surface. Ag2S growth is promoted at high aspect ratio defects (surface pores) of the SiC substrate as a result of an imperfect protection by the SiO2 overcoat. Channels most likely connect the silver layer to its environment through the protection layer, which enables local H2S entry and Ag2S growth. The silver sulfide columns grow in number and size eventually leading to coalescence with increasing H2S exposure. In more advanced stages, tarnishing slows down owing to saturation of all pre-existing imperfectly protected sites of preferential sulfidation. However, it progresses radially at the basis of the Ag2S columns underneath the protection layer, consuming the metallic silver layer and deteriorating the protecting overcoat.

SiNx thickness dependence of spectral properties and durability of protected-silver mirrors

Both long-term environmental durability and high broadband reflectance of protected-silver mirrors are of great importance to the developments of optical instruments for a variety of applications under harsh environments. In this paper, the dependences of spectral properties and environmental durability of protected-silver mirrors with a Sub/NiCrNx/Ag/NiCrNx/SiNx/Air structure on thickness of SiNx protective layer were investigated in details. The reflectance, transmittance, total scattering loss measurements and scanning electron microscope (SEM) imaging were employed to characterize the spectral properties and surface morphology, and accelerated environmental tests, including humidity test and salt fog test, were applied to investigate the environmental durability. Five magnetron-sputtering prepared protected-silver mirrors with SiNx layer thicknesses between 8nm to 16nm were tested. The results showed that both optical and corrosion-resistant properties of protected-silver mirrors were SiNx thickness dependent and the optimum SiNx thickness should be approximately 14nm for Sub/NiCrNx/Ag/NiCrNx/SiNx/Air protected-silver mirrors to have both reasonably high reflectance in the visible spectral range and high corrosion resistance for long lifetime applications in harsh environments.

Silver Film Surface Modification by Ion Bombardment Decreases Surface Plasmon Resonance Absorption

Silver thin films covered with dielectric films serving as protective coatings are desired for telescope mirrors, but durable coatings have proved elusive. As part of an effort to develop long-lived protected-silver mirrors, silver thin films were deposited by electron beam evaporation using a physical vapor deposition system at the University of California Observatories Astronomical Coatings Lab. The silver films were later covered with a stack of dielectric films utilizing silicon nitride and titanium dioxide deposited by ion-assisted electron beam evaporation to fabricate protected mirrors. In-situ argon ion bombardment was introduced after silver deposition and prior to the deposition of dielectric films to assess its effects on the performance of the mirrors. We found that ion bombardment of the silver influenced surface morphology and reflectivity, and these effects correlated with time between silver deposition and ion bombardment. The overall reflectivity at wavelengths in the range of 350-800 nm was found to improve due to ion bombardment, which was qualitatively interpreted as a result of decreased surface plasmon resonance coupling. We suggest that the observed decrease in coupling is caused by silver grain boundary pinning due to ion bombardment suppressing silver surface diffusion, forming smoother silver-dielectric interfaces.

Environmental durability of protected silver mirrors prepared by plasma beam sputtering.

Various overcoat layers have been developed to protect silver mirrors from tarnish and corrosion. However, the mechanisms by which these protective layers improve mirror durability are not fully understood. Mixed flowing gas exposure was used to investigate the corrosion behavior of plasma beam sputtered silver mirrors with different adhesion layer materials. A small amount of nickel in the adhesion layer had a significant impact on the silver-dielectric interface. Additionally, lateral transport of silver was found to be an important factor in the corrosion process. Better stability at all layer interfaces is suggested to improve mirror durability.

Bistatic laser polarimeter calibrated to 1% at visible-SWIR wavelengths.

This paper documents the accuracy and precision of the U. S. Air Force Research Laboratory APCL laser polarimeter in arbitrary bistatic geometries at the three laser wavelengths 633nm, 1064nm, and 1550nm. The difference between measured and theoretical-truth Mueller matrices of calibration components is used as the calibration metric and justified relative to block ellipsometer calibration methods. Calibration of the polarimeter ellipsometry mode is demonstrated first, at quasi-monostatic and large bistatic angles, employing a metallic mirror and a dielectric window as the calibration component, respectively, the latter in order to avoid uncertainty in the retardance of typical metallic mirrors at large incident angles. This uncertainty is demonstrated in measurements of COTS protected-silver mirrors from two vendors, revealing an approximately λ/8 retardance difference, for reflection through 90°, between nominally-identical mirrors from the two vendors. Polarimeter calibration is finally extended beyond ellipsometry by calibrating depolarization measurements using a new technique employing ensembles of polarized states as calibration components.

Optical properties of unprotected and protected sputtered silver films: Surface morphology vs. UV/VIS reflectance

Abstract The optical properties of opaque sputtered silver films are investigated and related to their surface morphology. As reference, an evaporated sample produced by thermal flash evaporation has been studied, too. Standard optical and electrical characterizations as well as scanning electron microscopy (SEM) and X-ray reflectometry have been carried out with unprotected silver films directly after deposition and reveal strong correlations between the corresponding characterization results. The aging behavior of the normal incidence reflectance has been studied over a time period of 36 weeks. Protected silver mirrors have been prepared by sputtering using silica and alumina as the protective coating. It is shown by model calculations that the main spectral features occurring in the reflectance spectra of as-deposited unprotected coatings, unprotected coatings after aging, and protected coatings can be reproduced assuming a rough silver surface covered by a corresponding ultrathin absorbing interface layer or overlayer.

Control of stress in protected silver mirrors prepared by plasma beam sputtering

Plasma beam sputtering was used to deposit dielectric-protected silver mirrors that exhibited excellent durability and controlled stress. The durability of the mirrors was strongly dependent upon the presence of a very thin chromium adhesion layer between the silver layer and the dielectric overcoat. The stress of the five-layer mirror was balanced by controlling the compressive stress in the top dielectric layer, offsetting the net tensile stress of the combination of layers below.

不同类型色散镜的设计和分析

Three types of dispersion mirrors are designed and discussed. The first type is the complementary chirpedmirror pair used for providing smooth group delay dispersion (GDD) in the wavelength range of 550−1050 nm. Such mirrors are obtained by shifting the GDD oscillation period. The second type of mirror combines the characteristics of chirped mirrors and Gires-Tournois interferometer mirrors. It provides a high dispersion compensation of about −800-fs GDD in the range of 780−830 nm and about −1150-fs GDD in the range of 1020−1045 nm. The third type is a protected silver mirror with high reflectivity and low dispersion in the range of 650−1000 nm at 45◦. OCIS codes: 310.1620, 320.5520, 320.7160. doi: 10.3788/COL201008S1.0018.

Field rotator for the GI2T interferometer

A field rotator dedicated to compensate for the celestial field rotation has been designed and built for the Grand Interferometre a 2 Telescopes (GI2T) by the Laboratoire d'Astronomie Spatiale (LAS). In each interferometric arm, we introduce a three-reflection optics rotatable around the axis of the optical train. Since the visibility of the interference fringe pattern is mostly sensitive to the differential instrumental polarization, this device must fulfill not only optomechanical requirements but also polarization constraints. For this purpose, three metallic reflections at small incidences are preferred to classical K-prisms (such as Dove or Kdenig-Abbe prisms) and coatings optimization leads to protected silver mirrors.

The Lick Observatory Hamilton Echelle Spectrometer

The Hamilton Echelle Spectrometer, recently installed at the coudé focus of the Shane 3-m telescope, is a high dispersion spectrograph optimized for use with large format CCD's. It was designed primarily for high resolution (R=50,000) wide bandpass spectroscopy of point-like sources down to a limiting magnitude of about V=16.5, over the 0.34 μm to 1.1 μm spectral region. Its design features a relatively large collimated beam size, the use of prisms rather than gratings for cross dispersion, minimum order separation, the use of protected silver mirror coatings throughout the system, and a fast (f/1.67) folded Schmidt camera with a flat external focal plane. Together, these features yield a very powerful spectrometer for high resolution stellar spectroscopy. This paper gives a brief description of the Hamilton spectrometer and gives several examples of its performance on astronomical objects.

THE LICK OBSERVATORY HAMILTON ECHELLE SPECTROMETER.

ABSTRACT The Hamilton Echelle Spectrometer, installed at the coude focus of the Shane 3-m telescope, is a high-dispersion spectrograph optimized for use with today's largest available CCDs, and for the even larger CCDs expected in the future. It was designed primarily for high-resolution (R = 30,000 to 60,000) wide-bandpass spectroscopy of point-like sources down to a limiting magnitude of about V = 16.5, over the 0.34- to 1.1 -micron spectral region. Its design differs from many conventional echelle spectrographs in several areas. The most important differences are its relatively large collimated beam size, the use of prisms rather than gratings for cross dispersion, minimum order separation, the use of protected silver mirror coatings throughout the system, and a fast (f/1.67) folder Schmidt camera with a flat external focal plane. Together, these features yield a very powerful spectrometer for high-resolution stellar spectroscopy. This paper describes the Hamilton spectrograph and discusses the philosophy behind its design. Several examples of the performance of the system are given.