By combining the antireflective properties from gradual changes in the effective refractive index and cavity coupling from cone gratings, and the efficient optical behavior of a tungsten film, we have conceived a flexible filter showing very broad antireflective (AR) properties from the visible to short wavelength infrared region (SWIR: 0.7-1.5 μm) and simultaneously a mirror-like behavior in the mid-infrared wavelength region (MWIR: 3-5 μm) and long-infrared wavelength region (LWIR: 8 to 15 μm). Nanoimprint technology has permitted us to replicate inverted cone patterns on a large scale on a flexible polymer, afterwards coated with a thin tungsten film. This optical metafilter is of great interest in the stealth domain where optical signature reduction from the optical to SWIR region is an important matter. As it also acts as selective thermal emitter offering a good solar-absorption/ infrared-emissivity ratio, interests are found as well for solar heating applications.
We propose two distinctive designs of metamaterials demonstrating antireflective properties in the optical and near
infrared region and, simultaneously, a high reflectivity in the mid-infrared.
Since the emissivity is related to the absorption of a material, our structures would then offer a high emissivity in the
visible and near infrared. Beyond those wavelengths, the emissivity would be quite low. Usually, such systems find their applications in the field of thermophotovoltaics, where the goal is to convert radiation from the visible up to 2.5 microns into electrons, while limiting the emissivity for the larger wavelengths. A particular interest in the field of
optoelectronics is found as well, especially for optical detection.
Here, the major difficulty is to offer a metal thick enough to be considered as mirror across the electromagnetic radiation spectrum that possesses at the same time an anti-reflective character within a range of several microns. Thus, we have summoned the exceptional physical properties of the material patterning.
Numerical analysis has been performed on commonly used metamaterial designs: a perforated metallic plate and a
metallic cross grating. Through all these structures, we have demonstrated the various physical phenomena contributing to a reduction in the reflectivity in the optical and near infrared region. By showing realistic geometric parameters, the structures were not only designed to demonstrate a good optical response but were also meant to be feasible on large surfaces by lithographic methods such as micro contact printing or nano-imprint lithography.
Achieving a broadband antireflection property from material surfaces is one of the highest priorities for those who want to improve the efficiency of solar cells or the sensitivity of photo-detectors. To lower the reflectance of a surface, we have decided to study the optical response of a top-flat cone shaped silicon grating, based on previous work exploring pyramid gratings.
Through rigorous numerical methods, such as Finite Different Time Domain or Rigorous Coupled-Wave Analysis, we then designed several structures theoretically demonstrating an antireflective character within the middle infrared region. From the opto-geometrical parameters such as period, depth and shape of the pattern determined by numerical analysis, these structures have been fabricated using controlled slope plasma etching processes. Afterwards, optical characterizations of several samples were carried out. The reflectance of the grating in the near and middle infrared domains has been measured by Fourier Transform Infrared spectrometry and a comparison with numerical analysis has been made.
As expected, those structures offer a fair antireflective character in the region of interest. Further numerical investigations led to the fact that patterning the top of the cone could enlarge the antireflective domain to the visible region. Thus, as with the simple cone grating, a comparison of the numerical analysis with the experimental measurements is made. Finally, diffracted orders are studied and compared between both structures. Those orders are critical and must be limited as one wants to avoid crosstalk phenomena in imaging systems.
The “m-lines” guided mode method has been employed as a new approach to measure the penetration depth of UV light
in partially exposed thin film photoresist layers. This non-destructive method presents the advantage that the penetration
depth can be measured before developing the sample, allowing for fine tuning of exposure parameters. Results are
presented for a positive photoresist (Shipley S1813) deposited by spin coating onto glass slides, forming layers
approximately 2.2μm thick. Such films are exposed to varying degrees with a programmable UV exposure tool. Using
the “m-lines” technique, light is coupled into the photoresist samples using a prism coupler in close proximity to the
sample surface. This coupling occurs for specific incident angles, known as synchronous angles, which depend on the
sample structure. By measuring two such incident angles, one can calculate the thickness and refractive index of a
homogeneous film. We propose a two layer model which allows us to extract the thickness and the refractive index of
the upper exposed layer from the synchronous angles provided by the “m-lines” technique.