The Study of Quantum Interference in Metallic Photonic Crystals Doped with Four-Level Quantum Dots

In this work, the absorption coefficient of a metallic photonic crystal doped with nanoparticles has been obtained using numerical simulation techniques. The effects of quantum interference and the concentration of doped particles on the absorption coefficient of the system have been investigated. The nanoparticles have been considered as semiconductor quantum dots which behave as a four-level quantum system and are driven by a single coherent laser field. The results show that changing the position of the photonic band gap about the resonant energy of the two lower levels directly affects the decay rate, and the system can be switched between transparent and opaque states if the probe laser field is tuned to the resonance frequency. These results provide an application for metallic nanostructures in the fabrication of new optical switches and photonic devices

Viscosity kernel of molecular fluids: Butane and polymer melts

The wave-vector dependent shear viscosities for butane and freely jointed chains have been determined. The transverse momentum density and stress autocorrelation functions have been determined by equilibrium molecular dynamics in both atomic and molecular hydrodynamic formalisms. The density, temperature, and chain length dependencies of the reciprocal and real-space viscosity kernels are presented. We find that the density has a major effect on the shape of the kernel. The temperature range and chain lengths considered here have by contrast less impact on the overall normalized shape. Functional forms that fit the wave-vector-dependent kernel data over a large density and wave-vector range have also been tested. Finally, a structural normalization of the kernels in physical space is considered. Overall, the real-space viscosity kernel has a width of roughly 3¿6 atomic diameters, which means that generalized hydrodynamics must be applied in predicting the flow properties of molecular fluids on length scales where the strain rate varies sufficiently in the order of these dimensions (e.g., nanofluidic flows)

Nonlocal viscosity of polymer melts approaching their glassy state

The nonlocal viscosity kernels of polymer melts have been determined by means of equilibrium molecular dynamics upon cooling toward the glass transition. Previous results for the temperature dependence of the self-diffusion coefficient and the value of the glass transition temperature are confirmed. We find that it is essential to include the attractive part of the interatomic potential in order to observe a strong glass transition. The width of the reciprocal space kernel decreases dramatically near the glass transition, being described by a deltalike function near and below the glass transition, leading to a very broad kernel in physical space. Thus, spatial nonlocality turns out to play an important role in polymeric fluids at temperatures near the glass transition temperature

Nonlocal viscosity kernels of linear molecular chains

Navier-Stokes-Fourier hydrodynamics breaks down on the order of molecular length and time scales. Once the confinement approaches molecular dimensions, classical theory must be generalized to allow for local position dependent coefficients. It has been recently shown that for flow fields with high gradients in the strain rate over the with of the r-space kernels nonlocality plays a significant role [1]. We present an extended analysis of the exact homogeneous nonlocal viscosity kernels [2 ] for simple linear molecular chains at a variety of state points and chain lengths. This allows using nonlocal generalization of the linear constitutive relation with appropriate boundary conditions to predict the flow profiles for highly confined molecular fluids. We compute the k-space and r-space kernels calculated from the stress autocorrelation functions and the transverse momentum density autocorrelation functions in atomic and molecular hydrodynamic representation. Functional forms have been found to fit the kernel data over a large density range and k-vectors and a comparative analysis of fitting is presented. The results show that the kernels have a width of a few molecular diameters which means that the generalized hydrodynamic viscosity must be used in predicting the flow properties of fluids on length scales where the gradient in the strain rate is of the order of molecular dimensions (eg in nanofluidics)

Improving solar cell efficiency using photonic band-gap materials

The potential of using photonic crystal structures for realizing highly efficient and reliable solar-cell devices is presented. We show that due their ability to modify the spectral and angular characteristics of thermal radiation, photonic crystals emerge as one of the leading candidates for frequency- and angular-selective radiating elements in thermophotovoltaic devices. We show that employing photonic crystal-based angle- and frequency-selective absorbers facilitates a strong enhancement of the conversion efficiency of solar cell devices without using concentrators. © 2007 Elsevier B.V. All rights reserved

A disruptive technology for thermal to electrical energy conversion

International audienceA disruptive approach to thermal energy harvesting is presented. The new technique can be used for powering ultra-low power electronics. We propose a two-step conversion of heat into electricity: thermal to mechanical accomplished with thermal bimetals and mechanical to electrical accomplished with piezoelectrics. Devices can work in a wide range of temperatures: from -40 degrees C to 300 degrees C and the available mechanical power density is in the order of 1 mW/cm(2). The first electrical results and the first prototype built on a flexible substrate are presented in this work. We evidenced that one of the keys to improve the generated power density is downscaling of individual devices. To demonstrate this point, laws modeling downscaling have been established and show that the miniaturization of the devices by a factor k increases the generated power density by the same factor, due to the higher heat transfer rate. The path followed in order to establish the laws is given in this paper. (C) 2014 Elsevier Ltd. All rights reserved

Infrared frequency selective surfaces fabricated using optical lithography and phase-shift masks

A frequency selective surface (FSS) structure has been fabricated for use in a thermophotovoltaic system. The FSS provides a means for reflecting the unusable light below the bandgap of the thermophotovoltaic cell while transmitting the usable light above the bandgap. This behavior is relatively independent of the light's incident angle. The fabrication of the FSS was done using optical lithography and a phase-shift mask. The FSS cell consisted of circular slits spaced by 1100 nm. The diameters and widths of the circular slits were 870 nm and 120 nm, respectively. The FSS was predicted to pass wavelengths near 7 {micro}m and reflect wavelengths outside of this pass-band. The FSSs fabricated performed as expected with a pass-band centered near 5 {micro}m