3,190 research outputs found
Broadband antireflection coatings for visible and infrared ranges
International audienceAntireflection coatings are critical elements for space applications as they will influence the overall performances of optical systems. They are among the most classical elements that are produced with optical coatings but remain a challenge when high performances are required. In this paper, we present some recent results based on thin film technology for the production of antireflection coatings dedicated to visible, near-IR and mid-IR spectral ranges. We first present a theoretical and experimental study of broadband antireflection coatings for [400-1100] nm spectral range. We then show antireflection coatings covering the [1.5-15] µm range. Experimental demonstrations and limitations are presented
Composite Reflective/Absorptive IR-Blocking Filters Embedded in Metamaterial Antireflection Coated Silicon
Infrared (IR) blocking filters are crucial for controlling the radiative
loading on cryogenic systems and for optimizing the sensitivity of bolometric
detectors in the far-IR. We present a new IR filter approach based on a
combination of patterned frequency selective structures on silicon and a thin
(50 thick) absorptive composite based on powdered reststrahlen
absorbing materials. For a 300 K blackbody, this combination reflects
50\% of the incoming light and blocks \textgreater 99.8\% of the total
power with negligible thermal gradients and excellent low frequency
transmission. This allows for a reduction in the IR thermal loading to
negligible levels in a single cold filter. These composite filters are
fabricated on silicon substrates which provide excellent thermal transport
laterally through the filter and ensure that the entire area of the absorptive
filter stays near the bath temperature. A metamaterial antireflection coating
cut into these substrates reduces in-band reflections to below 1\%, and the
in-band absorption of the powder mix is below 1\% for signal bands below 750
GHz. This type of filter can be directly incorporated into silicon refractive
optical elements
Sunrise to sunset optimization of thin film antireflective coatings for encapsulated, planar silicon solar cells
We present an approach for the optimization of thin film antireflective coatings for encapsulated planar silicon solar cells in which the variations in the incident spectra and angle of incidence (AOI) over a typical day are fully considered. Both the angular and wavelength dependences of the reflectance from the surface, absorptance within the coating, and transmittance into the device are calculated for both single- and doublelayer antireflection coatings with and without thin silicon oxide passivation layers. These data are then combined with spectral data as a function of time of day and internal quantum efficiency to estimate the average short-circuit current produced by a fixed solar cell during a day. This is then used as a figure of merit for the optimization of antireflective layer thicknesses for modules placed horizontally at the equator and on a roof in the UK. Our results indicate that only modest gains in average short-circuit current could be obtained by optimizing structures for sunrise to sunset irradiance rather than AM15 at normal incidence, and fabrication tolerances and uniformities are likely to be more significant. However, we believe that this overall approach to optimization will be of increasing significance for new, potentially asymmetric, antireflection schemes such as those based on subwavelength texturing or other photonic or plasmonic technologies currently under development especially when considered in combination with modules fixed at locations and directions that result in asymmetric spectral conditions
Low scatter and ultra-low reflectivity measured in a fused silica window
We investigate the reflectivity and optical scattering characteristics at
1064\,nm of an antireflection coated fused silica window of the type being used
in the Advanced LIGO gravitational-wave detectors. Reflectivity is measured in
the ultra-low range of 5-10\,ppm (by vendor) and 14-30\,ppm (by us). Using an
angle-resolved scatterometer we measure the sample's Bidirectional Scattering
Distribution Function (BSDF) and use this to estimate its transmitted and
reflected scatter at roughly 20-40\,ppm and 1\,ppm, respectively, over the
range of angles measured. We further inspect the sample's low backscatter using
an imaging scatterometer, measuring an angle resolved BSDF below
sr for large angles (10--80 from incidence in the plane
of the beam). We use the associated images to (partially) isolate scatter from
different regions of the sample and find that scattering from the bulk fused
silica is on par with backscatter from the antireflection coated optical
surfaces. To confirm that the bulk scattering is caused by Rayleigh scattering,
we perform a separate experiment, measuring the scattering intensity versus
input polarization angle. We estimate that 0.9--1.3\,ppm of the backscatter can
be accounted for by Rayleigh scattering of the bulk fused silica. These results
indicate that modern antireflection coatings have low enough scatter to not
limit the total backscattering of thick fused silica optics.Comment: 9 pages, 10 figure
Intermediate Mirrors to Reach Theoretical Efficiency Limits of Multi-Bandgap Solar Cells
Creating a single bandgap solar cell that approaches the Shockley-Queisser
limit requires a highly reflective rear mirror. This mirror enhances the
voltage of the solar cell by providing photons with multiple opportunities for
escaping out the front surface. Efficient external luminescence is a
pre-requisite for high voltage. Intermediate mirrors in a multijunction solar
cell can enhance the voltage for each cell in the stack. These intermediate
mirrors need to have the added function of transmitting the below bandgap
photons to the next cell in the stack. In this work, we quantitatively
establish the efficiency increase possible with the use of intermediate
selective reflectors between cells in a tandem stack. The absolute efficiency
increase can be up to ~6% in dual bandgap cells with optimal intermediate and
rear mirrors. A practical implementation of an intermediate selective mirror is
an air gap sandwiched by antireflection coatings. The air gap provides perfect
reflection for angles outside the escape cone, and the antireflection coating
transmits angles inside the escape cone. As the incoming sunlight is within the
escape cone, it is transmitted on to the next cell, while most of the
internally trapped luminescence is reflected
Multipurpose S-shaped solvable profiles of the refractive index: application to modeling of antireflection layers and quasi-crystals
A class of four-parameter solvable profiles of the electromagnetic admittance
has recently been discovered by applying the newly developed Property & Field
Darboux Transformation method (PROFIDT). These profiles are highly flexible. In
addition, the related electromagnetic-field solutions are exact, in closed-form
and involve only elementary functions. In this paper, we focus on those who are
S-shaped and we provide all the tools needed for easy implementation. These
analytical bricks can be used for high-level modeling of lightwave propagation
in photonic devices presenting a piecewise-sigmoidal refractive-index profile
such as, for example, antireflection layers, rugate filters, chirped filters
and photonic crystals. For small amplitude of the index modulation, these
elementary profiles are very close to a cosine profile. They can therefore be
considered as valuable surrogates for computing the scattering properties of
components like Bragg filters and reflectors as well. In this paper we present
an application for antireflection layers and another for 1D quasicrystals (QC).
The proposed S-shaped profiles can be easily manipulated for exploring the
optical properties of smooth QC, a class of photonic devices that adds to the
classical binary-level QC.Comment: 14 pages, 18 fi
A reliable all-silver front contact for silicon solar cells
The feasibility of making an adherent and moisture degradation resistant silver-only front contact to silicon solar cells was demonstrated. Optimum fabrication processes and process sequences were determined for making such contacts. These contacts were found to also have good electrical characteristics. A back contact of aluminum-silver was also developed. This proved very satisfactory for low-temperature applications
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