7,270 research outputs found
Predicting the whispering gallery mode spectra of microresonators
The whispering gallery modes (WGMs) of optical resonators have prompted
intensive research efforts due to their usefulness in the field of biological
sensing, and their employment in nonlinear optics. While much information is
available in the literature on numerical modeling of WGMs in microspheres, it
remains a challenging task to be able to predict the emitted spectra of
spherical microresonators. Here, we establish a customizable Finite- Difference
Time-Domain (FDTD)-based approach to investigate the WGM spectrum of
microspheres. The simulations are carried out in the vicinity of a dipole
source rather than a typical plane-wave beam excitation, thus providing an
effective analogue of the fluorescent dye or nanoparticle coatings used in
experiment. The analysis of a single dipole source at different positions on
the surface or inside a microsphere, serves to assess the relative efficiency
of nearby radiating TE and TM modes, characterizing the profile of the
spectrum. By varying the number, positions and alignments of the dipole
sources, different excitation scenarios can be compared to analytic models, and
to experimental results. The energy flux is collected via a nearby disk-shaped
region. The resultant spectral profile shows a dependence on the configuration
of the dipole sources. The power outcoupling can then be optimized for specific
modes and wavelength regions. The development of such a computational tool can
aid the preparation of optical sensors prior to fabrication, by preselecting
desired the optical properties of the resonator.Comment: Approved version for SPIE Photonics West, LASE, Laser Resonators,
Microresonators and Beam Control XV
Comparative study of density functional theories of the exchange-correlation hole and energy in silicon
We present a detailed study of the exchange-correlation hole and
exchange-correlation energy per particle in the Si crystal as calculated by the
Variational Monte Carlo method and predicted by various density functional
models. Nonlocal density averaging methods prove to be successful in correcting
severe errors in the local density approximation (LDA) at low densities where
the density changes dramatically over the correlation length of the LDA hole,
but fail to provide systematic improvements at higher densities where the
effects of density inhomogeneity are more subtle. Exchange and correlation
considered separately show a sensitivity to the nonlocal semiconductor crystal
environment, particularly within the Si bond, which is not predicted by the
nonlocal approaches based on density averaging. The exchange hole is well
described by a bonding orbital picture, while the correlation hole has a
significant component due to the polarization of the nearby bonds, which
partially screens out the anisotropy in the exchange hole.Comment: 16 pages, 5 figures, RevTeX, added conten
Quantum Computing and Communications
This book explains the concepts and basic mathematics of quantum computing and communication. Chapters cover such topics as quantum algorithms, photonic implementations of discrete-time quantum walks, how to build a quantum computer, and quantum key distribution and teleportation, among others
Boundary quasi-orthogonality and sharp inclusion bounds for large Dirichlet eigenvalues
We study eigenfunctions and eigenvalues of the Dirichlet Laplacian on a
bounded domain \Omega\subset\RR^n with piecewise smooth boundary. We bound
the distance between an arbitrary parameter and the spectrum
in terms of the boundary -norm of a normalized trial solution of the
Helmholtz equation . We also bound the -norm of the
error of this trial solution from an eigenfunction. Both of these results are
sharp up to constants, hold for all greater than a small constant, and
improve upon the best-known bounds of Moler--Payne by a factor of the
wavenumber . One application is to the solution of eigenvalue
problems at high frequency, via, for example, the method of particular
solutions. In the case of planar, strictly star-shaped domains we give an
inclusion bound where the constant is also sharp. We give explicit constants in
the theorems, and show a numerical example where an eigenvalue around the
2500th is computed to 14 digits of relative accuracy. The proof makes use of a
new quasi-orthogonality property of the boundary normal derivatives of the
eigenmodes, of interest in its own right.Comment: 18 pages, 3 figure
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