896 research outputs found
Anderson Localization of Polar Eigenmodes in Random Planar Composites
Anderson localization of classical waves in disordered media is a fundamental
physical phenomenon that has attracted attention in the past three decades.
More recently, localization of polar excitations in nanostructured
metal-dielectric films (also known as random planar composite) has been subject
of intense studies. Potential applications of planar composites include local
near-field microscopy and spectroscopy. A number of previous studies have
relied on the quasistatic approximation and a direct analogy with localization
of electrons in disordered solids. Here I consider the localization problem
without the quasistatic approximation. I show that localization of polar
excitations is characterized by algebraic rather than by exponential spatial
confinement. This result is also valid in two and three dimensions. I also show
that the previously used localization criterion based on the gyration radius of
eigenmodes is inconsistent with both exponential and algebraic localization. An
alternative criterion based on the dipole participation number is proposed.
Numerical demonstration of a localization-delocalization transition is given.
Finally, it is shown that, contrary to the previous belief, localized modes can
be effectively coupled to running waves.Comment: 22 pages, 7 figures. Paper was revised and a more precise definition
of the participation number given, data for figures recalculated accordingly.
Accepted to J. Phys.: Cond. Mat
Comments on the nonpharmaceutical interventions in New York City and Chicago during the 1918 flu pandemic
This commentary was originally published in CIDRAP News and it is here reported almost verbatim to allow divulgation through open access. The Editorial summarizes John Barry's concerns about the value of accurate historical reporting and its implications in public policy determination. This short abstract was written by the Editor-in-Chief of the Journal of Translational Medicine to introduce the Editorial
Bound whispering gallery modes in circular arrays of dielectric spherical particles
Low-dimensional ordered arrays of optical elements can possess bound modes
having an extremely high quality factor. Typically, these arrays consist of
metal elements which have significantly high light absorption thus restricting
performance. In this paper we address the following question: can bound modes
be formed in dielectric systems where the absorption of light is negligible?
Our investigation of circular arrays of spherical particles shows that (1) high
quality modes in an array of 10 or more particles can be attained at least for
a refractive index , so optical materials like TiO or GaAs can
be used; (2) the most bound modes have nearly transverse polarization
perpendicular to the circular plane; (3) in a particularly interesting case of
TiO particles (rutile phase, ), the quality factor of the most
bound mode increases almost by an order of magnitude with the addition of 10
extra particles, while for particles made of GaAs the quality factor increases
by almost two orders of magnitude with the addition of ten extra particles. We
hope that this preliminary study will stimulate experimental investigations of
bound modes in low-dimensional arrays of dielectric particles.Comment: Submitted to Physical Review
Phonons in a Nanoparticle Mechanically Coupled to a Substrate
The discrete nature of the vibrational modes of an isolated nanometer-scale
solid dramatically modifies its low-energy electron and phonon dynamics from
that of a bulk crystal. However, nanocrystals are usually coupled--even if only
weakly--to an environment consisting of other nanocrystals, a support matrix,
or a solid substrate, and this environmental interaction will modify the
vibrational properties at low frequencies. In this paper we investigate the
modification of the vibrational modes of an insulating spherical nanoparticle
caused by a weak {\it mechanical} coupling to a semi-infinite substrate. The
phonons of the bulk substrate act as a bath of harmonic oscillators, and the
coupling to this reservoir shifts and broadens the nanoparticle's modes. The
vibrational density of states in the nanoparticle is obtained by solving the
Dyson equation for the phonon propagator, and we show that environmental
interaction is especially important at low frequencies. As a probe of the
modified phonon spectrum, we consider nonradiative energy relaxation of a
localized electronic impurity state in the nanoparticle, for which good
agreement with experiment is found.Comment: 10 pages, Revte
Electron-Phonon Dynamics in an Ensemble of Nearly Isolated Nanoparticles
We investigate the electron population dynamics in an ensemble of nearly
isolated insulating nanoparticles, each nanoparticle modeled as an electronic
two-level system coupled to a single vibrational mode. We find that at short
times the ensemble-averaged excited-state population oscillates but has a
decaying envelope. At long times, the oscillations become purely sinusoidal
about a ``plateau'' population, with a frequency determined by the
electron-phonon interaction strength, and with an envelope that decays
algebraically as t^-{1/2} We use this theory to predict electron-phonon
dynamics in an ensemble of Y_2 O_3 nanoparticles.Comment: 11 pages, 3 figure
Characterization of the inflammatory response to four commercial bone graft substitutes using a murine biocompatibility model
Bone grafting is utilized in nearly all orthopedic subspecialties and in most anatomic regions. Bone graft substitutes have the potential to offer similar efficacy as autogenous grafts without the morbidity of harvest. Several studies have noted the efficacy of new-generation bone substitute products, but few studies have evaluated their safety. This study characterizes and quantifies the inflammatory reaction to four different commercially available bone graft substitutes, which were examined using the in vivo murine air pouch biocompatibility model. One coralline hydroxyapatite product was chosen as an example of a purely osteoconductive material. Three demineralized bone matrix products were chosen to represent products that are both osteoconductive and osteoinductive. Samples were implanted in a murine air pouch and harvested after 14 days in situ. Pouch fluid was extracted, mRNA isolated, and reverse transcription polymerase chain reactions carried out to detect interleukin-1 gene expression as a marker for inflammation. In addition, multiple histological characteristics were examined to quantify cellular responses to the implanted materials. All bone graft substitutes induced a significant inflammatory response compared with negative controls. Histology and polymerase chain reaction data indicated that the level of inflammatory reaction was elevated in materials with a higher demineralized bone matrix to carrier proportion. The hydroxyapatite product generated a low inflammatory reaction. In conclusion, this study used an in vivo model of biocompatibility to demonstrate that a significant inflammatory reaction occurs when using implanted bone graft substitutes. When choosing a bone grafting method, surgeons should consider both the efficacy and safety of methods and materials used. Further studies are necessary to determine the ideal bone graft material to maximize efficacy while minimizing morbidity
- …