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.
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.
Optical surface structuration is of primary interest for applications such as photovoltaics or photodetectors.
Over last years, periodical patterns allowing antireflective effects with efficient properties have been designed
and fabricated. Some specific issues such as diffraction of undesired high energy orders are a direct consequence
of the periodical nature of this kind of pattern.
Random rough surfaces allow the antireflective effect without these undesired diffraction effects. By tuning
their statistics, random rough surfaces offer new degrees of freedom for antireflection but also for controlling
the scattering (polarization, spatial distribution). The two main parameters of such surfaces are the height
probability density function and the autocorrelation function. The height probability density function carries
information about height of the structures. The autocorrelation function is a representation of the lateral
distribution of the surface.
Our photofabrication method uses a speckle pattern recorded on a photoresist. By controlling the exposure
parameters, such as the number of exposure and the beam intensity distribution, one is able to control the
statistics of the speckle, and so of the photofabricated surfaces. Using a chromatic confocal sensor, height
mapping of these surfaces are performed. From these mappings, the height probability density and the
correlation function are calculated.
The experimental statistics are compared with the predicted theoretical ones showing a good agreement.
Results are presented showing a significant modification of the statistics of the photofabricated surfaces.
Digital Micromirror Devices are mainly known for display in video projectors. More and more, they are used
in industrial or research applications. DMD is a versatile tool for lithography allowing photoresist exposure
with easy-changing masking step in direct-patterning. Any application where light shaping is necessary can
be considered by using DMD. We here show photofabrication of random rough surfaces using an indirect
modified beam exposure. A laser beam is enlarged and scattered by a diffusing element. The scattering from
this diffusing surface allows the creation of a speckle pattern with a random light distribution. The intensity is
then recorded on a photoresist coated substrate. The patterned photoresist is next developed and an etching
step enables the transfer on the silicon. It can be shown that the statistical properties of the speckle pattern
can be controlled. The intensity distribution is modified by the number of exposures and the correlation
function is linked to the spatial distribution of the laser beam. Some examples of such photofabrication can
be found using a Gaussian unmodified beam leading to Gaussian correlation photofabricated surfaces. In
order to design random rough surfaces having a non Gaussian correlation we need to modify the laser beam
shape. This modification is achieved using a DMD. The experimental processes from photoresist deposition
to modified exposure are discussed in this paper.
Random rough surfaces are of primary interest for their optical properties: reducing reflection at the interface
or obtaining specific scattering diagram for example. Thus controlling surface statistics during the fabrication
process paves the way to original and specific behaviors of reflected optical waves. We detail an experimental
method allowing the fabrication of random rough surfaces showing tuned statistical properties. A two-step
photoresist exposure process was developed. In order to initiate photoresist polymerization, an energy threshold
needs to be reached by light exposure. This energy is brought by a uniform exposure equipment comprising
UV-LEDs. This pre-exposure is studied by varying parameters such as optical power and exposure time. The
second step consists in an exposure based on the Gray method.1 The speckle pattern of an enlarged scattered
laser beam is used to insolate the photoresist. A specific photofabrication bench using an argon ion laser was
implemented. Parameters such as exposure time and distances between optical components are discussed.
Then, we describe how we modify the speckle-based exposure bench to include a spatial light modulator
(SLM). The SLM used is a micromirror matrix known as Digital Micromirror Device (DMD) which allows
spatial modulation by displaying binary images. Thus, the spatial beam shape can be tuned and so the speckle
pattern on the photoresist is modified. As the photoresist photofabricated surface is correlated to the speckle
pattern used to insolate, the roughness parameters can be adjusted.
Experimental and numerical results concerning the influence of silver nanoparticles on the optical absorption of organic
devices are presented. The metallic nanoparticles (NPs) are placed inside an interpenetrated poly(3-hexylthiophene):
[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) layer using a physical vapor deposition technique. An
absorption enhancement by comparison to devices without NPs is shown. An increase of the absorption by annealing is
also observed. Moreover, calculations are performed via a numerical analysis based on a Finite Difference Time Domain
(FDTD) method. We demonstrate that the light absorption can mainly occur inside the active layer instead of inside the