Cooled optical filters for Q-band infrared astronomy (15-40 μm)

With a growing interest in mid- and far-infrared astronomy using cooled imaging and spectrometer instruments in high-altitude observatories and spaceflight telescopes, it is becoming increasingly important to characterise and assess the spectral performance of cooled multilayer filters across the Q-band atmospheric window. This region contains spectral features emitted by many astrophysical phenomena and objects fundamental to circumstellar and planetary formation theories. However extending interference filtering to isolate radiation at progressively longer wavelengths and improve photometric accuracy is an area of ongoing and challenging thin-film research. We have successfully fabricated cooled bandpass and edge filters with high durability for operation across the 15-30 µm Q-band region. In this paper we describe the rationale for selection of optical materials and properties of fabricated thin-film coatings for this region, together with FTIR spectral measurements and assessment of environmental durability

Étude et réalisation d'une matrice de détecteurs bolométriques de très haute résolution. Intérêt d'une conception en couches minces

There has been considerable development in the field of bolometric detectors in the last decade, which is evidence of the power and the sensitivity of the thermal measurement of a deposited energy. Our study is aimed at a search for the ultimate resolution in energy and position. We have developed an ultrahigh vacuum system that makes possible the optimisation of the deposition conditions, the adhesion and the thermal treatment of the detectors, due to the ability to transfer the substrate between the different chambers of the system, without leaving the ultrahigh vacuum environment. This system has enabled us to develop an all thin film bolometer, centred on a 10 nm thick thermometric film, of very low heat capacity and very high sensitivity, due to the metal-insulator transition properties. The volume and the shape of each bolometer element are defined by mechanical masking and ionic etching, which enables us to define the heat capacity, the thermal response time and the electrical impedance of each element by controlling the film thickness and shape factor. Two bolometer prototypes have been fabricated using this method. One is dedicated to space astrophysics, and the other one for calorimetric measurements of atomic monolayers. We show that a limit on the size reduction of active volume of the thermometric material appears at low temperature, and that this limit is linked to the rapid decrease of the electron-phonon coupling with temperature. We give a theoretical evaluation of this for a 10µm by 11µm microbolometer. This leads us to show that the reduction of the operating temperature does not give the optimum working conditions in the case of a microbolometer.Le développement considérable des détecteurs bolométriques au cours de la dernière décennie témoigne de la puissance et de la sensibilité de la mesure thermique d'un dépôt d'énergie. Notre étude se situe dans le cadre de la recherche d'une résolution ultime en énergie et en position. Nous avons mis au point un dispositif ultravide qui a rendu possible l'optimisation des conditions de dépôt, d'adhésion et de traitement thermique des détecteurs, grâce à la capacité de transférer l'échantillon entre les différentes enceintes, sans remise à l'air. Cet appareillage nous a permis de développer un bolomètre entièrement constitué de films minces, centré sur un matériau thermométrique de 10 nm d'épaisseur, ayant une très faible capacité calorifique et une très bonne sensibilité, grâce aux propriétés de la transition métal-isolant. Le volume et la forme de chacun des éléments du bolomètre composite sont définis par un masquage mécanique et une gravure ionique, ce qui nous permet de concevoir un détecteur pour lequel la capacité calorifique, le temps de réponse thermique et l'impédance électrique de chaque élément peuvent être définis en jouant sur l'épaisseur des films et leur facteur de forme. Deux bolomètres prototypes ont été fabriqués par cette méthode. L'un est destiné à l'astrophysique spatiale, l'autre est destiné à des mesures calorimétriques de monocouches atomiques. Nous montrons qu'une limite à la réduction de la taille du volume actif du matériau apparaît à basse température, et que cette limite est liée à la décroissance très rapide du couplage électron-phonon avec la température. Elle est évaluée, sur le plan théorique, pour un microbolomètre de 10 µm par 11 µm. Ceci nous conduit à montrer que la réduction de la température de fonctionnement aux plus basses températures ne représente pas un optimum pour un microbolomètre

Mid-infrared filters for astronomical and remote sensing instrumentation

As improvements to the optical design of spectrometer and radiometer instruments evolve with advances in detector sensitivity, use of focal plane detector arrays and innovations in adaptive optics for large high altitude telescopes, interest in mid-infrared astronomy and remote sensing applications have been areas of progressive research in recent years. This research has promoted a number of developments in infrared coating performance, particularly by placing increased demands on the spectral imaging requirements of filters to precisely isolate radiation between discrete wavebands and improve photometric accuracy. The spectral design and construction of multilayer filters to accommodate these developments has subsequently been an area of challenging thin-film research, to achieve high spectral positioning accuracy, environmental durability and aging stability at cryogenic temperatures, whilst maximizing the far-infrared performance. In this paper we examine the design and fabrication of interference filters in instruments that utilize the mid-infrared N-band (6-15 µm) and Q-band (16-28 µm) atmospheric windows, together with a rationale for the selection of materials, deposition process, spectral measurements and assessment of environmental durability performance

Temperature invariant infrared filter

A temperature invariant narrow bandpass filter 5 to transmit mid-infrared radiation that may be used in a mid-infrared sensor for monitoring a species, which may be a component of a fluid or a solid material. The filter has a substrate 14, a first stack deposited on the substrate, a cavity 16 deposited on the first stack and a second stack deposited on the cavity. The first and second stacks are made from alternating high 10 and low 12 refractive index materials, and the cavity is a low refractive index material. By providing a high ratio of low refractive index material in the filter with respective to high refractive index material, the filter is configured so that wavelength transmission remains constant with varying temperature. The high refractive index material has a refractive index that decreases with temperature and the cavity is at least three full wavelengths or three half wavelengths of the transmission wavelength or the narrow band of transmission wavelengths