10,087 research outputs found
Fabrication and characterization of erbium-doped toroidal microcavity lasers
Erbium-doped SiO2 toroidal microcavity lasers are fabricated on a Si substrate using a combination of optical lithography, etching, Er ion implantation, and CO2 laser reflow. Erbium is either preimplanted in the SiO2 base material or postimplanted into a fully fabricated microtoroid. Three-dimensional infrared confocal photoluminescence spectroscopy imaging is used to determine the spatial distribution of optically active Er ions in the two types of microtoroids, and distinct differences are found. Microprobe Rutherford backscattering spectrometry indicates that no macroscopic Er diffusion occurs during the laser reflow for preimplanted microtoroids. From the measured Er doping profiles and calculated optical mode distributions the overlap factor between the Er distribution and mode profile is calculated: Gamma=0.066 and Gamma=0.02 for postimplanted and preimplanted microtoroids, respectively. Single and multimode lasing around 1.5 µm is observed for both types of microtoroids, with the lowest lasing threshold (4.5 µW) observed for the preimplanted microtoroids, which possess the smallest mode volume. When excited in the proper geometry, a clear mode spectrum is observed superimposed on the Er spontaneous emission spectrum. This result indicates the coupling of Er ions to cavity modes
Implicit Density Functional Theory
A fermion ground state energy functional is set up in terms of particle
density, relative pair density, and kinetic energy tensor density. It satisfies
a minimum principle if constrained by a complete set of compatibility
conditions. A partial set, which thereby results in a lower bound energy under
minimization, is obtained from the solution of model systems, as well as a
small number of exact sum rules. Prototypical application is made to several
one-dimensional spinless non-interacting models. The effectiveness of "atomic"
constraints on model "molecules" is observed, as well as the structure of
systems with only finitely many bound states.Comment: 9 pages, 4 figure
Viscosity calculated in simulations of strongly-coupled dusty plasmas with gas friction
A two-dimensional strongly-coupled dusty plasma is modeled using Langevin and
frictionless molecular dynamical simulations. The static viscosity and
the wave-number-dependent viscosity are calculated from the
microscopic shear in the random motion of particles. A recently developed
method of calculating the wave-number-dependent viscosity is
validated by comparing the results of from the two simulations. It is
also verified that the Green-Kubo relation can still yield an accurate measure
of the static viscosity in the presence of a modest level of friction as
in dusty plasma experiments.Comment: 6 pages, 3 figures, Physics of Plasmas invited pape
The Holographic dark energy reexamined
We have reexamined the holographic dark energy model by considering the
spatial curvature. We have refined the model parameter and observed that the
holographic dark energy model does not behave as phantom model. Comparing the
holographic dark energy model to the supernova observation alone, we found that
the closed universe is favored. Combining with the Wilkinson Microwave
Anisotropy Probe (WMAP) data, we obtained the reasonable value of the spatial
curvature of our universe.Comment: divided into sections, add one figure, some typos corrected,
references added, Accepted for publication in PRD; v3: some typos corrected,
title change
Transverse optical mode in a one-dimensional Yukawa chain
A transverse optical mode was observed in a one-dimensional Yukawa chain. Charged particles, suspended in a strongly coupled dusty plasma, were arranged in a 1D periodic structure. Particle displacement in the direction perpendicular to the chain was restored by the confining potential. The dispersion relation of phonons was measured, verifying that the optical mode has negative dispersion, with phase and group velocities that are oppositely directed. A theoretical dispersion relation is presented and compared to the experiment and a molecular dynamics simulation
Introducing the Fission-Fusion Reaction Process: Using a Laser-Accelerated Th Beam to produce Neutron-Rich Nuclei towards the N=126 Waiting Point of the r Process
We propose to produce neutron-rich nuclei in the range of the astrophysical
r-process around the waiting point N=126 by fissioning a dense
laser-accelerated thorium ion bunch in a thorium target (covered by a CH2
layer), where the light fission fragments of the beam fuse with the light
fission fragments of the target. Via the 'hole-boring' mode of laser Radiation
Pressure Acceleration using a high-intensity, short pulse laser, very
efficiently bunches of 232Th with solid-state density can be generated from a
Th layer, placed beneath a deuterated polyethylene foil, both forming the
production target. Th ions laser-accelerated to about 7 MeV/u will pass through
a thin CH2 layer placed in front of a thicker second Th foil closely behind the
production target and disintegrate into light and heavy fission fragments. In
addition, light ions (d,C) from the CD2 production target will be accelerated
as well to about 7 MeV/u, inducing the fission process of 232Th also in the
second Th layer. The laser-accelerated ion bunches with solid-state density,
which are about 10^14 times more dense than classically accelerated ion
bunches, allow for a high probability that generated fission products can fuse
again. In contrast to classical radioactive beam facilities, where intense but
low-density radioactive beams are merged with stable targets, the novel
fission-fusion process draws on the fusion between neutron-rich, short-lived,
light fission fragments both from beam and target. The high ion beam density
may lead to a strong collective modification of the stopping power in the
target, leading to significant range enhancement. Using a high-intensity laser
as envisaged for the ELI-Nuclear Physics project in Bucharest (ELI-NP),
estimates promise a fusion yield of about 10^3 ions per laser pulse in the mass
range of A=180-190, thus enabling to approach the r-process waiting point at
N=126.Comment: 13 pages, 6 figure
Visualization of defect-induced excitonic properties of the edges and grain boundaries in synthesized monolayer molybdenum disulfide
Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs)
are attractive materials for next generation nanoscale optoelectronic
applications. Understanding nanoscale optical behavior of the edges and grain
boundaries of synthetically grown TMDCs is vital for optimizing their
optoelectronic properties. Elucidating the nanoscale optical properties of 2D
materials through far-field optical microscopy requires a diffraction-limited
optical beam diameter sub-micron in size. Here we present our experimental work
on spatial photoluminescence (PL) scanning of large size ( microns)
monolayer MoS grown by chemical vapor deposition (CVD) using a diffraction
limited blue laser beam spot (wavelength 405 nm) with a beam diameter as small
as 200 nm allowing us to probe nanoscale excitonic phenomena which was not
observed before. We have found several important features: (i) there exists a
sub-micron width strip ( nm) along the edges that fluoresces brighter than the region far inside; (ii) there is another brighter
wide region consisting of parallel fluorescing lines ending at the corners of
the zig-zag peripheral edges; (iii) there is a giant blue shifted A-excitonic
peak, as large as meV, in the PL spectra from the edges. Using
density functional theory calculations, we attribute this giant blue shift to
the adsorption of oxygen dimers at the edges, which reduces the excitonic
binding energy. Our results not only shed light on defect-induced excitonic
properties, but also offer an attractive route to tailor optical properties at
the TMDC edges through defect engineering.Comment: 10 pages, 4 figures in Journal of Physical Chemistry C, 201
Mott-Hubbard Transition of Bosons in Optical Lattices with Three-body Interactions
In this paper, the quantum phase transition between superfluid state and
Mott-insulator state is studied based on an extended Bose-Hubbard model with
two- and three-body on-site interactions. By employing the mean-field
approximation we find the extension of the insulating 'lobes' and the existence
of a fixed point in three dimensional phase space. We investigate the link
between experimental parameters and theoretical variables. The possibility to
obverse our results through some experimental effects in optically trapped
Bose-Einstein Condensates(BEC) is also discussed.Comment: 7 pages, 4 figures; to be appear in Phys. Rev.
Role of Particle Interactions in the Feshbach Conversion of Fermion Atoms to Bosonic Molecules
We investigate the Feshbach conversion of fermion atomic pairs to condensed
boson molecules with a microscopic model that accounts the repulsive
interactions among all the particles involved. We find that the conversion
efficiency is enhanced by the interaction between boson molecules while
suppressed by the interactions between fermion atoms and between atom and
molecule. In certain cases, the combined effect of these interactions leads to
a ceiling of less than 100% on the conversion efficiency even in the adiabatic
limit. Our model predicts a non-monotonic dependence of the efficiency on mean
atomic density. Our theory agrees well with recent experiments on Li and
K.Comment: 5 pages, 4 figure
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