132 research outputs found
Polariton-polariton scattering in microcavities: A microscopic theory
We apply the fermion commutation technique for composite bosons to
polariton-polariton scattering in semiconductor planar microcavities.
Derivations are presented in a simple and physically transparent fashion. A
procedure of orthogonolization of the initial and final two-exciton state
wavefunctions is used to calculate the effective scattering matrix elements and
the scattering rates. We show how the bosonic stimulation of the scattering
appears in this full fermionic approach whose equivalence to the bosonization
method is thus demonstrated in the regime of low exciton density. We find an
additional contribution to polariton-polariton scattering due to the exciton
oscillator strength saturation, which we analyze as well. We present a theory
of the polariton-polariton scattering with opposite spin orientations and show
that this scattering process takes place mainly via dark excitonic states.
Analytical estimations of the effective scattering amplitudes are given.Comment: Theoretical paper on polariton-polariton scattering in planar
microcavities. The new version contains a slightly modified abstract and a
revised introduction. Typos have been corrected wherever spotted. 16 page
Influence of fluorine on the fiber performance studied through the NBOHC-related 1.9 eV microluminescence
The distribution of Non Bridging Oxygen Hole Centers (NBOHC) in Fluorine doped optical fibers was investigated by confocal microluminescence spectroscopy monitoring the characteristic 1.9 eV luminescence band. The results show that these defects are generated by the fiber drawing and their concentration further increases after \u3b3 irradiation.
The NBOHC profile along the fiber is anticorrelated to the fluorine content. This finding agrees with the role of
fluorine in the fiber toughness and is discussed from the microscopic point of view on the basis of previous works
v-P 2 O 5 micro-clustering in P-doped silica studied by a first-principles Raman investigation
Synthetic vitreous silica is currently the preferred material for the production of optical fibres because of the several excellent properties of this glass, e.g. high transmission in the visible and IR domains, high mechanical strength, chemical durability, and ease of doping with various materials. For instance, fiber lasers and amplifiers exploit the light amplification properties provided by rare-earth ions employed as dopants in the core of silica-based optical fibers. The structure and composition of the nearest neighbor shell surrounding rare-earth ions in silica-based optical fibers and amplifiers have been intensively debated in the last decade. To reduce aggregation effects between rare-earth ions, co-dopants such as phosphorus and aluminium are added as structural modifiers; phosphorus-doping, in particular, has proved to be very efficient in dissolving rare-earth ions. In this work, we provide further insights concerning the embedding of P atoms into the silica network, which may be relevant for explaining the ease of formation of a phosphorus pentoxide nearest-neighbor shell around a rare-earth dopant. In particular, by means of first-principles calculations, we discuss alternative models for an irradiation (UV, x\u2013, \u3b3-rays) induced paramagnetic center, i.e. the so called room-temperature phosphorus-oxygen-hole center, and its precursors. We report that the most likely precursor of a room-temperature phosphorus-oxygen-hole center comprises of a micro-cluster of a few (at least two) neighboring phosphate tetrahedra, and correspondingly that the occurrence of isolated [(O-) 2 P(=O) 2 ] 12 units is unlikely even at low P-doping concentrations. In fact, this work predicts that the symmetric stretching of P=O bonds in isolated [(O-) 2 P(=O) 2 ] 12 units appears as a Raman band at a frequency of ~1110 cm 121 , and only by including at least another corner-sharing phosphate tetrahedron, it is shown to shift to higher frequencies (up to ~40 cm 121 ) due to the shortening of P=O bonds, thereby leading to an improved agreement with the observed Raman band located at ~1145 cm 121
Scattering states of coupled valence-band holes in point defect potential derived from variable phase theory
In this article we present a method to compute the scattering states of holes
in spherical bands in the strong spin-orbit coupling regime. More precisely, we
calculate scattering phase shifts and amplitudes of holes induced by defects in
a semiconductor crystal. We follow a previous work done on this topic by Ralph
[H. I. Ralph, Philips Res. Rept. 32 160 (1977)] to account for the p-wave
nature and the coupling of valence band states. We extend Ralph's analysis to
incorporate finite-range potentials in the scattering problem. We find that the
variable phase method provides a very convenient framework for our purposes and
show in detail how scattering amplitudes and phase shifts are obtained. The
Green's matrix of the Schroedinger equation, the Lippmann-Schwinger equation
and the Born approximation are also discussed. Examples are provided to
illustrate our calculations with Yukawa type potentials.Comment: 16 pages and 9 figure
p-wave phase shift and scattering length of Li
We have calculated the p-wave phase shifts and scattering length of Li.
For this we solve the partial wave Schr\"odinger equation and analyze the
validity of adopting the semiclassical solution to evaluate the constant
factors in the solution. Unlike in the wave case, the semiclassical
solution does not provide unique value of the constants. We suggest an
approximate analytic solution, which provides reliable results in special
cases. Further more, we also use the variable phase method to evaluate the
phase shifts. The p-wave scattering lengths of Cs and Cs are
calculated to validate the schemes followed. Based on our calculations, the
value of the wave scattering length of Li is .Comment: 10 figure
Radiation hardening techniques for rare-earth based optical fibers and amplifiers
Er/Yb doped fibers and amplifiers have been shown to be very radiation sensitive, limiting their integration in space. We present an approach including successive hardening techniques to enhance their radiation tolerance. The efficiency of our
approach is demonstrated by comparing the radiation responses of optical amplifiers made with same lengths of different rare-earth doped fibers and exposed to gamma-rays. Previous studies indicated that such amplifiers suffered significant degradation for doses exceeding 10 krad. Applying our techniques significantly enhances the amplifier radiation resistance, resulting in a very limited degradation up to 50 krad. Our optimization techniques concern the fiber composition, some possible pre-treatments and the interest of simulation tools used to harden by design the amplifiers.
We showed that adding cerium inside the fiber phosphosilicate-based core strongly decreases the fiber radiation
sensitivity compared to the standard fiber. For both fibers, a pre-treatment with hydrogen permits to enhance again the fiber resistance. Furthermore, simulations tools can also be used to improve the tolerance of the fiber amplifier by
helping identifying the best amplifier configuration for operation in the radiative environment
O2 Loaded Germanosilicate Optical Fibers: Experimental In Situ Investigation and Ab Initio Simulation Study of GLPC Evolution under Irradiation
In this work we present a combined experimental and ab initio simulation investigation concerning the Germanium Lone Pair Center (GLPC), its interaction with molecular oxygen (O2), and evolution under irradiation. First, O2 loading has been applied here to Ge-doped optical fibers to reduce the concentration of GLPC point defects. Next, by means of cathodoluminescence in situ experiments, we found evidence that the 10 keV electron irradiation of the treated optical fibers induces the generation of GLPC centers, while in nonloaded optical fibers, the irradiation causes the bleaching of the pre-existing GLPC. Ab initio calculations were performed to investigate the reaction of the GLPC with molecular oxygen. Such investigations suggested the stability of the dioxagermirane (DIOG) bulk defect, and its back conversion into GLPC with a local release of O2 under irradiation. Furthermore, it is also inferred that a remarkable portion of the O2 passivated GLPC may form Ge tetrahedra connected to peroxy bridges. Such structures may have a larger resistance to the irradiation and not be back converted into GLPC
Coulomb singularities in scattering wave functions of spin-orbit-coupled states
We report on our analysis of the Coulomb singularity problem in the frame of
the coupled channel scattering theory including spin-orbit interaction. We
assume that the coupling between the partial wave components involves orbital
angular momenta such that . In these conditions, the two
radial functions, components of a partial wave associated to two values of the
angular momentum , satisfy a system of two second-order ordinary
differential equations. We examine the difficulties arising in the analysis of
the behavior of the regular solutions near the origin because of this coupling.
First, we demonstrate that for a singularity of the first kind in the
potential, one of the solutions is not amenable to a power series expansion.
The use of the Lippmann-Schwinger equations confirms this fact: a logarithmic
divergence arises at the second iteration. To overcome this difficulty, we
introduce two auxilliary functions which, together with the two radial
functions, satisfy a system of four first-order differential equations. The
reduction of the order of the differential system enables us to use a
matrix-based approach, which generalizes the standard Frobenius method. We
illustrate our analysis with numerical calculations of coupled scattering wave
functions in a solid-state system
Brownian Carnot engine
The Carnot cycle imposes a fundamental upper limit to the efficiency of a
macroscopic motor operating between two thermal baths. However, this bound
needs to be reinterpreted at microscopic scales, where molecular bio-motors and
some artificial micro-engines operate. As described by stochastic
thermodynamics, energy transfers in microscopic systems are random and thermal
fluctuations induce transient decreases of entropy, allowing for possible
violations of the Carnot limit. Despite its potential relevance for the
development of a thermodynamics of small systems, an experimental study of
microscopic Carnot engines is still lacking. Here we report on an experimental
realization of a Carnot engine with a single optically trapped Brownian
particle as working substance. We present an exhaustive study of the energetics
of the engine and analyze the fluctuations of the finite-time efficiency,
showing that the Carnot bound can be surpassed for a small number of
non-equilibrium cycles. As its macroscopic counterpart, the energetics of our
Carnot device exhibits basic properties that one would expect to observe in any
microscopic energy transducer operating with baths at different temperatures.
Our results characterize the sources of irreversibility in the engine and the
statistical properties of the efficiency -an insight that could inspire novel
strategies in the design of efficient nano-motors.Comment: 7 pages, 7 figure
Particlization in hybrid models
In hybrid models, which combine hydrodynamical and transport approaches to
describe different stages of heavy-ion collisions, conversion of fluid to
individual particles, particlization, is a non-trivial technical problem. We
describe in detail how to find the particlization hypersurface in a 3+1
dimensional model, and how to sample the particle distributions evaluated using
the Cooper-Frye procedure to create an ensemble of particles as an initial
state for the transport stage. We also discuss the role and magnitude of the
negative contributions in the Cooper-Frye procedure.Comment: 18 pages, 28 figures, EPJA: Topical issue on "Relativistic Hydro- and
Thermodynamics"; version accepted for publication, typos and error in Eq.(1)
corrected, the purpose of sampling and change from UrQMD to fluid clarified,
added discussion why attempts to cancel negative contributions of Cooper-Frye
are not applicable her
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