3,759 research outputs found
Guide to the classics: Don Quixote, the world’s first novel – and one of the best
Completed by Cervantes when he was in prison, Don Quixote is the tale of a man so passionate about reading he leaves home to live the life of his fictional heroes
Mechanisms for photon sorting based on slit-groove arrays
Mechanisms for one-dimensional photon sorting are theoretically studied in
the framework of a couple mode method. The considered system is a nanopatterned
structure composed of two different pixels drilled on the surface of a thin
gold layer. Each pixel consists of a slit-groove array designed to squeeze a
large fraction of the incident light into the central slit. The Double-Pixel is
optimized to resolve two different frequencies in the near infrared. This
system shows a high transmission efficiency and a small crosstalk. Its response
is found to strongly depend on the effective area shared by overlapping pixels.
Three different regimes for the process of photon sorting are identified and
the main physical trends underneath in such regimes are unveiled. Optimal
efficiencies for the photon sorting are obtained for a moderate number of
grooves that overlap with grooves of the neighbor pixel. Results could be
applied to optical and infrared detectors.Comment: 12 pages, 4 figure
Transmittance of a subwavelength aperture flanked by a finite groove array \\ placed near the focus of a conventional lens
One-dimensional light harvesting structures illuminated by a conventional
lens are studied in this paper. Our theoretical study shows that high
transmission efficiencies are obtained when the structure is placed near the
focal plane of the lens. The considered structure is a finite slit-groove array
(SGA) with a given number of grooves that are symmetrically distributed with
respect to a central slit. The SGA is nano-patterned on an opaque metallic
film. It is found that a total transmittance of 80% is achieved even for a
single slit when (i) Fabry-Perot like modes are excited inside the slit and
(ii) the effective cross section of the aperture becomes of the order of the
full width at half maximum of the incident beam. A further enhancement of 8% is
produced by the groove array. The optimal geometry for the groove array
consists of a moderate number of grooves () at either side of the
slit, separated by a distance of half the incident wavelength .
Grooves should be deeper (with depth ) than those typically
reported for plane wave illumination in order to increase their individual
scattering cross section.Comment: 7 pages, 6 figure
Key Production Parameters to Obtain Transparent Nanocellular PMMA
Transparent nanocellular polymethylmethacrylate (PMMA) with relative density around 0.4 is produced for the first time by using the gas dissolution foaming technique. The processing conditions and the typical characteristics of the cellular structure needed to manufacture this novel material are discovered. It is proved that low saturation temperatures (−32 °C) combined with high saturation pressures (6, 10, 20 MPa) allow increasing the solubility of PMMA up to values not reached before. In particular, the highest CO2 uptake ever reported for PMMA, (i.e., 48 wt%) is found for a saturation pressure of 20 MPa and a saturation temperature of −32 °C. Due to these processing conditions, cell nucleation densities of 1016 nuclei cm−3 and cell sizes clearly below 50 nm are achieved. The nanocellular polymers obtained, with cell sizes ten times smaller than the wavelength of visible light and very homogeneous cellular structures, show a significant transparency
Overcoming the Challenge of Producing Large and Flat Nanocellular Polymers: A Study with PMMA
Although nanocellular polymers are interesting materials with improved properties in comparison with conventional or microcellular polymers, the production of large and flat parts of those materials is still challenging. Herein, gas dissolution foaming process is used to produce large and flat nanocellular polymethylmethacrylate samples. In order to do that, the foaming step is performed in a hot press. The methodology is optimized to produce flat samples with dimensions of 100 × 100 × 6 mm3, relative densities in the range 0.25–0.55 and cell sizes around 250 nm. Additionally, foaming parameters are modified to study their influence on the final cellular structure, and the materials produced in this paper are compared with samples produced by using a most conventional approach in which foaming step is conducted in a thermal bath. Results obtained show that an increment in the foaming temperature leads to a reduction in relative density and an increase of cell nucleation density. Moreover, differences in the final cellular structure for materials produced by both foaming routes are studied, proving that although there exist some differences, the mechanisms governing the nucleation and growing are the same in both processes, leading to the production of homogeneous materials with very similar cellular structures
Guide to the classics: Don Quixote, the world’s first novel – and one of the best
Completed by Cervantes when he was in prison, Don Quixote is the tale of a man so passionate about reading he leaves home to live the life of his fictional heroes
Optimal light harvesting structures at optical and infrared frequencies
One-dimensional light harvesting structures with a realistic geometry
nano-patterned on an opaque metallic film are optimized to render high
transmission efficiencies at optical and infrared frequencies. Simple design
rules are developed for the particular case of a slit-groove array with a given
number of grooves that are symmetrically distributed with respect to a central
slit. These rules take advantage of the hybridization of Fabry-Perot modes in
the slit and surface modes of the corrugated metal surface. Same design rules
apply for optical and infrared frequencies. The parameter space of the groove
array is also examined with a conjugate gradient optimization algorithm that
used as a seed the geometries optimized following physical intuition. Both
uniform and nonuniform groove arrays are considered. The largest transmission
enhancement, with respect to a uniform array, is obtained for a chirped groove
profile. Such enhancement is a function of the wavelength. It decreases from
39% in the optical part of the spectrum to 15% at the long wavelength infrared.Comment: 13 pages, 5 figure
Modeling the heat transfer by conduction of nanocellular polymers with bimodal cellular structures
Nanocellular polymers are a new generation of materials with the potential of being used as very efficient thermal insulators. It has been proved experimentally that these materials present the Knudsen effect, which strongly reduces the conductivity of the gas phase. There are theoretical equations to predict the thermal conductivity due to this Knudsen effect, but all the models consider an average cell size. In this work, we propose a model to predict the thermal conductivity due to the conduction mechanisms of nanocellular materials with bimodal cellular structures, that is, with two populations of cells, micro and nanocellular. The novelty of our work is to consider not only the average cell size, but the cell size distribution. The predictions of the model are compared with the experimental conductivity of two real bimodal systems based on poly(methyl methacrylate) (PMMA), and it is proved that this new model provides more accurate estimations of the conductivity than the models that do not consider the bimodality. Furthermore, this model could be applied to monomodal nanocellular polymers. In particular, for monomodal materials presenting a wide cell size distribution and at low densities, the model predicts important variations in comparison with the current models in the literature. This result indicates that the cell size distribution must be included in the estimations of the thermal conductivity of nanocellular polymer
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