156 research outputs found
A Simple Model for Magnetization Ratios in Doped Nanocrystals
Recent experiments on Mn-doped ZnS nanocrystals have shown unusual
magnetization properties. We describe a nearest-neighbor Heisenberg exchange
model for calculating the magnetization ratios of these antiferromagnetically
doped crystals, in which the dopant atoms are distributed inhomogeneously
within the nanocrystal. This simple inhomogeneous doping model is capable of
reproducing the experimental results, and suggests that interior dopant atoms
are localized within the crystal.Comment: 8 pages, 1 figure, 2 tables. Submitted to J. Appl. Phy
Large K-exciton dynamics in GaN epilayers: the non-thermal and thermal regime
We present a detailed investigation concerning the exciton dynamics in GaN
epilayers grown on c-plane sapphire substrates, focussing on the exciton
formation and the transition from the nonthermal to the thermal regime. The
time-resolved kinetics of LO-phonon replicas is used to address the energy
relaxation in the excitonic band. From ps time-resolved spectra we bring
evidence for a long lasting non-thermal excitonic distribution which accounts
for the rst 50 ps. Such a behavior is con rmed in di erent experimental
conditions, both when non-resonant and resonant excitation are used. At low
excitation power density the exciton formation and their subsequent
thermalization is dominated by impurity scattering rather than by acoustic
phonon scattering. The estimate of the average energy of the excitons as a
function of delay after the excitation pulse provides information on the
relaxation time, which describes the evolution of the exciton population to the
thermal regime.Comment: 9 pages,8 figure
Mapping Polarization Fields in Al0.85In0.15N/AlN/GaN Heterostructures
Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 - July 30, 200
Spontaneous Polarisation Build up in a Room Temperature Polariton Laser
We observe the build up of strong (~50%) spontaneous vector polarisation in
emission from a GaN-based polariton laser excited by short optical pulses at
room temperature. The Stokes vector of emitted light changes its orientation
randomly from one excitation pulse to another, so that the time-integrated
polarisation remains zero. This behaviour is completely different to any
previous laser. We interpret this observation in terms of the spontaneous
symmetry breaking in a Bose-Einstein condensate of exciton-polaritons
Blue lasing at room temperature in high quality factor GaN/AlInN microdisks with InGaN quantum wells
The authors report on the achievement of optically pumped III-V nitride blue microdisk lasers operating at room temperature. Controlled wet chemical etching of an AlInN interlayer lattice matched to GaN allows forming inverted cone pedestals. Whispering gallery modes are observed in the photoluminescence spectra of InGaN∕GaN quantum wells embedded in the GaN microdisks. Typical quality factors of several thousands are found (Q>4000). Laser action at ∼420nm is achieved under pulsed excitation at room temperature for a peak power density of 400kW/cm2. The lasing emission linewidth is down to 0.033nm
Lattice matched GaN/InAlN waveguides at λ = 1.55 μm grown by metalorganic vapor phase epitaxy
We report on the demonstration of low-loss, single-mode GaN-InAlN ridge waveguides (WGs) at fiber-optics telecommunication wavelengths. The structure grown by metal-organic vapor phase epitaxy contains AlInN cladding layers lattice-matched to GaN. For slab-like WGs propagation losses are below 3 dB/mm and independent of light polarization. For 2.6-μm-wide WGs the propagation losses in the 1.5- to 1.58-μm spectral region are as low as 1.8 and 4.9 dB/mm for transverse-electric- and transverse-magnetic-polarization, respectively. The losses are attributed to the sidewall roughness and can be further reduced by the optimization of the etching process
High spatial resolution picosecond cathodoluminescence of InGaN quantum wells
The authors have studied InxGa1-xN/GaN (x approximate to 15%) quantum wells (QWs) using atomic force microscopy (AFM) and picosecond time resolved cathodoluminescence (pTRCL) measurements. They observed a contrast inversion between monochromatic CL maps corresponding to the high energy side (3.13 eV) and the low energy side (3.07 eV) of the QW luminescence peak. In perfect correlation with CL images, AFM images clearly show regions where the QW thickness almost decreases to zero. Pronounced spectral diffusion from high energy thinner regions to low energy thicker regions is observed in pTRCL, providing a possible explanation for the hindering of nonradiative recombination at dislocations. (c) 2006 American Institute of Physics
Towards a quantum representation of the ampere using single electron pumps
Electron pumps generate a macroscopic electric current by controlled
manipulation of single electrons. Despite intensive research towards a quantum
current standard over the last 25 years, making a fast and accurate quantised
electron pump has proved extremely difficult. Here we demonstrate that the
accuracy of a semiconductor quantum dot pump can be dramatically improved by
using specially designed gate drive waveforms. Our pump can generate a current
of up to 150 pA, corresponding to almost a billion electrons per second, with
an experimentally demonstrated current accuracy better than 1.2 parts per
million (ppm) and strong evidence, based on fitting data to a model, that the
true accuracy is approaching 0.01 ppm. This type of pump is a promising
candidate for further development as a realisation of the SI base unit ampere,
following a re-definition of the ampere in terms of a fixed value of the
elementary charge.Comment: 8 pages, 7 figure
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