77,419 research outputs found
Numerical simulations of time resolved quantum electronics
This paper discusses the technical aspects - mathematical and numerical -
associated with the numerical simulations of a mesoscopic system in the time
domain (i.e. beyond the single frequency AC limit). After a short review of the
state of the art, we develop a theoretical framework for the calculation of
time resolved observables in a general multiterminal system subject to an
arbitrary time dependent perturbation (oscillating electrostatic gates, voltage
pulses, time-vaying magnetic fields) The approach is mathematically equivalent
to (i) the time dependent scattering formalism, (ii) the time resolved Non
Equilibrium Green Function (NEGF) formalism and (iii) the partition-free
approach. The central object of our theory is a wave function that obeys a
simple Schrodinger equation with an additional source term that accounts for
the electrons injected from the electrodes. The time resolved observables
(current, density. . .) and the (inelastic) scattering matrix are simply
expressed in term of this wave function. We use our approach to develop a
numerical technique for simulating time resolved quantum transport. We find
that the use of this wave function is advantageous for numerical simulations
resulting in a speed up of many orders of magnitude with respect to the direct
integration of NEGF equations. Our technique allows one to simulate realistic
situations beyond simple models, a subject that was until now beyond the
simulation capabilities of available approaches.Comment: Typographic mistakes in appendix C were correcte
Dielectric nanoantenna as an efficient and ultracompact demultiplexer for surface waves
Nanoantennas for highly efficient excitation and manipulation of surface
waves at nanoscale are key elements of compact photonic circuits. However,
previously implemented designs employ plasmonic nanoantennas with high Ohmic
losses, relatively low spectral resolution, and complicated lithographically
made architectures. Here we propose an ultracompact and simple dielectric
nanoantenna (silicon nanosphere) allowing for both directional launching of
surface plasmon polaritons on a thin gold film and their demultiplexing with a
high spectral resolution. We show experimentally that mutual interference of
magnetic and electric dipole moments supported by the dielectric nanoantenna
results in opposite propagation of the excited surface waves whose wavelengths
differ by less than 50 nm in the optical range. Broadband reconfigurability of
the nanoantennas operational range is achieved simply by varying the diameter
of the silicon sphere. Moreover, despite subwavelength size () of
the proposed nanoantennas, they demonstrate highly efficient and directional
launching of surface waves both in the forward and backward directions with the
measured front-to-back ratio having a contrast of almost two orders of
magnitude within a 50 nm spectral band. Our lithography-free design has great
potential as highly efficient, low-cost, and ultracompact demultiplexer for
advanced photonic circuits.Comment: added relevant references; fixed typos in Supplementary eq. 8,9,1
Enhancement of inherent Raman scattering in dielectric nanostructures with electric and magnetic Mie resonances
Resonantly enhanced Raman scattering in dielectric nanostructures has been
recently proven to be an effcient tool for developing nanothermometry and
experimental determination of their mode- composition. In this paper, we
develop a rigorous analytical theory based on the Green's function approach to
calculate the Raman emission from crystalline high-index dielectric
nanoparticles. As an example, we consider silicon nanoparticles which have a
strong Raman response due to active optical phonon modes. We relate enhancement
of Raman signal emission to Purcell effect due to the excitation of Mie modes
inside the nanoparticles. We also employ the numerical approach to the
calculation of inelastic Raman emission in more sophisticated geometries, which
do not allow a straightforward analytical form of the Green's function
description. The Raman response from a silicon nanodisk has been analyzed
within the proposed method, and the contribution of the various Mie modes has
been revealed
Monte Carlo Simulation of Comptonization in Inhomogeneous Media
Comptonization is the process in which photon spectrum changes due to
multiple Compton scatterings in the electronic plasma. It plays an important
role in the spectral formation of astrophysical X-ray and gamma-ray sources.
There are several intrinsic limitations for the analytical method in dealing
with the Comptonization problem and Monte Carlo simulation is one of the few
alternatives. We describe an efficient Monte Carlo method that can solve the
Comptonization problem in a fully relativistic way. We expanded the method so
that it is capable of simulating Comptonization in the media where electron
density and temperature varies discontinuously from one region to the other and
in the isothermal media where density varies continuously along photon paths.
The algorithms are presented in detail to facilitate computer code
implementation. We also present a few examples of its application to the
astrophysical research.Comment: 12 pages, 4 figures, Postscript file, in press ("Computers in
Physics", Vol. 11, No. 6
Understanding Hot-Electron Generation and Plasmon Relaxation in Metal Nanocrystals: Quantum and Classical Mechanisms
Generation of energetic (hot) electrons is an intrinsic property of any
plasmonic nanostructure under illumination. Simultaneously, a striking
advantage of metal nanocrystals over semiconductors lies in their very large
absorption cross sections. Therefore, metal nanostructures with strong and
tailored plasmonic resonances are very attractive for photocatalytic
applications. However, the central questions regarding plasmonic hot electrons
are how to quantify and extract the optically-excited energetic electrons in a
nanocrystal. We develop a theory describing the generation rates and the
energy-distributions of hot electrons in nanocrystals with various geometries.
In our theory, hot electrons are generated owing to surfaces and hot spots. The
formalism predicts that large optically-excited nanocrystals show the
excitation of mostly low-energy Drude electrons, whereas plasmons in small
nanocrystals involve mostly hot electrons. The energy distributions of
electrons in an optically-excited nanocrystal show how the quantum many-body
state in small particles evolves towards the classical state described by the
Drude model when increasing nanocrystal size. We show that the rate of surface
decay of plasmons in nanocrystals is directly related to the rate of generation
of hot electrons. Based on a detailed many-body theory involving kinetic
coefficients, we formulate a simple scheme describing the plasmon's dephasing.
In most nanocrystals, the main decay mechanism of a plasmon is the Drude
friction-like process and the secondary path comes from generation of hot
electrons due to surfaces and electromagnetic hot spots. This latter path
strongly depends on the size, shape and material of the nanocrystal,
correspondingly affecting its efficiency of hot-electron production. The
results in the paper can be used to guide the design of plasmonic nanomaterials
for photochemistry and photodetectors.Comment: 90 pages, 21 figures, including Supplementary Informatio
Numerical methods for computing Casimir interactions
We review several different approaches for computing Casimir forces and
related fluctuation-induced interactions between bodies of arbitrary shapes and
materials. The relationships between this problem and well known computational
techniques from classical electromagnetism are emphasized. We also review the
basic principles of standard computational methods, categorizing them according
to three criteria---choice of problem, basis, and solution technique---that can
be used to classify proposals for the Casimir problem as well. In this way,
mature classical methods can be exploited to model Casimir physics, with a few
important modifications.Comment: 46 pages, 142 references, 5 figures. To appear in upcoming Lecture
Notes in Physics book on Casimir Physic
Tidal stellar disruptions by massive black hole pairs: II. Decaying binaries
Tidal stellar disruptions have traditionally been discussed as a probe of the
single, massive black holes (MBHs) that are dormant in the nuclei of galaxies.
In Chen et al. (2009), we used numerical scattering experiments to show that
three-body interactions between bound stars in a stellar cusp and a
non-evolving "hard" MBH binary will also produce a burst of tidal disruptions,
caused by a combination of the secular "Kozai effect" and by close resonant
encounters with the secondary hole. Here we derive basic analytical scalings of
the stellar disruption rates with the system parameters, assess the relative
importance of the Kozai and resonant encounter mechanisms as a function of
time, discuss the impact of general relativistic (GR) and extended stellar cusp
effects, and develop a hybrid model to self-consistently follow the shrinking
of an MBH binary in a stellar background, including slingshot ejections and
tidal disruptions. In the case of a fiducial binary with primary hole mass
M_1=10^7\msun and mass ratio q=M_2/M_1=1/81, embedded in an isothermal cusp, we
derive a stellar disruption rate \dot{N_*} ~ 0.2/yr lasting ~ 3X10^5 yr. This
rate is 3 orders of magnitude larger than the corresponding value for a single
MBH fed by two-body relaxation, confirming our previous findings. For q<<0.01,
the Kozai/chaotic effect could be quenched due to GR/cusp effects by an order
of magnitude, but even in this case the stellar-disruption rate is still two
orders of magnitude larger than that given by standard relaxation processes
around a single MBH. Our results suggest that >~10% of the tidal-disruption
events may originate in MBH binaries.Comment: 16 pages, 20 figures, accepted for publication in the Astrophysical
Journa
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