999 research outputs found
Angular constraint on light-trapping absorption enhancement in solar cells
Light trapping for solar cells can reduce production cost and improve energy
conversion efficiency. Understanding some of the basic theoretical constraints
on light trapping is therefore of fundamental importance. Here, we develop a
general angular constraint on the absorption enhancement in light trapping. We
show that there is an upper limit for the angular integration of absorption
enhancement factors. This limit is determined by the number of accessible
resonances supported by an absorber
Fabrication and Characterization of Electrostatic Quantum Dots in a Si/SiGe 2D Electron Gas, Including an Integrated Read-out Channel
A new fabrication technique is used to produce quantum dots with read-out
channels in silicon/silicon-germanium two-dimensional electron gases. The
technique utilizes Schottky gates, placed on the sides of a shallow etched
quantum dot, to control the electronic transport process. An adjacent quantum
point contact gate is integrated to the side gates to define a read-out channel
and thus allow for noninvasive detection of the electronic occupation of the
quantum dot. Reproducible and stable Coulomb oscillations and the corresponding
jumps in the read-out channel resistance are observed at low temperatures. The
fabricated dot combined with the read-out channel represent a step towards the
spin-based quantum bit in Si/SiGe heterostructures.Comment: 3 pages, 4 fig
Lithographic band gap tuning in photonic band gap crystals
We describe the lithographic control over the spectral response of three-dimensional photonic crystals. By precise microfabrication of the geometry using a reproducible and reliable procedure consisting of electron beam lithography followed by dry etching, we have shifted the conduction band of crystals within the near-infrared. Such microfabrication has enabled us to reproducibly define photonic crystals with lattice parameters ranging from 650 to 730 nm. In GaAs semiconductor wafers, these can serve as high-reflectivity (> 95%) mirrors. Here, we show the procedure used to generate these photonic crystals and describe the geometry dependence of their spectral response
Sputtered Gold as an Effective Schottky Gate for Strained Si/SiGe Nanostructures
Metallization of Schottky surface gates by sputtering Au on strained Si/SiGe
heterojunctions enables the depletion of the two dimensional electron gas
(2DEG) at a relatively small voltage while maintaining an extremely low level
of leakage current. A fabrication process has been developed to enable the
formation of sub-micron Au electrodes sputtered onto Si/SiGe without the need
of a wetting layer.Comment: 3 pages, 3 figure
The Scattering Theory of Oscillator Defects in an Optical Fiber
We examine harmonic oscillator defects coupled to a photon field in the
environs of an optical fiber. Using techniques borrowed or extended from the
theory of two dimensional quantum fields with boundaries and defects, we are
able to compute exactly a number of interesting quantities. We calculate the
scattering S-matrices (i.e. the reflection and transmission amplitudes) of the
photons off a single defect. We determine using techniques derived from
thermodynamic Bethe ansatz (TBA) the thermodynamic potentials of the
interacting photon-defect system. And we compute several correlators of
physical interest. We find the photon occupancy at finite temperature, the
spontaneous emission spectrum from the decay of an excited state, and the
correlation functions of the defect degrees of freedom. In an extension of the
single defect theory, we find the photonic band structure that arises from a
periodic array of harmonic oscillators. In another extension, we examine a
continuous array of defects and exactly derive its dispersion relation. With
some differences, the spectrum is similar to that found for EM wave propagation
in covalent crystals. We then add to this continuum theory isolated defects, so
as to obtain a more realistic model of defects embedded in a frequency
dependent dielectric medium. We do this both with a single isolated defect and
with an array of isolated defects, and so compute how the S-matrices and the
band structure change in a dynamic medium.Comment: 32 pages, TeX with harvmac macros, three postscript figure
30% external quantum efficiency from surface textured, thin-film light-emitting diodes
There is a significant gap between the internal efficiency of light-emitting diodes (LEDs) and their external efficiency. The reason for this shortfall is the narrow escape cone for light in high refractive index semiconductors. We have found that by separating thin-film LEDs from their substrates (by epitaxial lift-off, for example), it is much easier for light to escape from the LED structure and thereby avoid absorption. Moreover, by nanotexturing the thin-film surface using "natural lithography," the light ray dynamics becomes chaotic, and the optical phase-space distribution becomes "ergodic," allowing even more of the light to find the escape cone. We have demonstrated 30% external efficiency in GaAs LEDs employing these principles
Inverse scattering of 2d photonic structures by layer-stripping
Design and reconstruction of 2d and 3d photonic structures are usually
carried out by forward simulations combined with optimization or intuition.
Reconstruction by means of layer-stripping has been applied in seismic
processing as well as in design and characterization of 1d photonic structures
such as fiber Bragg gratings. Layer-stripping is based on causality, where the
earliest scattered light is used to recover the structure layer-by-layer.
Our set-up is a 2d layered nonmagnetic structure probed by plane polarized
harmonic waves entering normal to the layers. It is assumed that the dielectric
permittivity in each layer only varies orthogonal to the polarization. Based on
obtained reflectance data covering a suitable frequency interval,
time-localized pulse data are synthesized and applied to reconstruct the
refractive index profile in the leftmost layer by identifying the local,
time-domain Fresnel reflection at each point. Once the first layer is known,
its impact on the reflectance data is stripped off, and the procedure repeated
for the next layer.
Through numerical simulations it will be demonstrated that it is possible to
reconstruct structures consisting of several layers. The impact of evanescent
modes and limited bandwidth is discussed
Threshold Error Penalty for Fault Tolerant Computation with Nearest Neighbour Communication
The error threshold for fault tolerant quantum computation with concatenated
encoding of qubits is penalized by internal communication overhead. Many
quantum computation proposals rely on nearest-neighbour communication, which
requires excess gate operations. For a qubit stripe with a width of L+1
physical qubits implementing L levels of concatenation, we find that the error
threshold of 2.1x10^-5 without any communication burden is reduced to 1.2x10^-7
when gate errors are the dominant source of error. This ~175X penalty in error
threshold translates to an ~13X penalty in the amplitude and timing of gate
operation control pulses.Comment: minor correctio
Calculating photonic Green's functions using a non-orthogonal finite difference time domain method
In this paper we shall propose a simple scheme for calculating Green's
functions for photons propagating in complex structured dielectrics or other
photonic systems. The method is based on an extension of the finite difference
time domain (FDTD) method, originally proposed by Yee, also known as the
Order-N method, which has recently become a popular way of calculating photonic
band structures. We give a new, transparent derivation of the Order-N method
which, in turn, enables us to give a simple yet rigorous derivation of the
criterion for numerical stability as well as statements of charge and energy
conservation which are exact even on the discrete lattice. We implement this
using a general, non-orthogonal co-ordinate system without incurring the
computational overheads normally associated with non-orthogonal FDTD.
We present results for local densities of states calculated using this method
for a number of systems. Firstly, we consider a simple one dimensional
dielectric multilayer, identifying the suppression in the state density caused
by the photonic band gap and then observing the effect of introducing a defect
layer into the periodic structure. Secondly, we tackle a more realistic example
by treating a defect in a crystal of dielectric spheres on a diamond lattice.
This could have application to the design of super-efficient laser devices
utilising defects in photonic crystals as laser cavities.Comment: RevTex file. 10 pages with 8 postscript figures. Submitted to Phys
Rev
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