476 research outputs found
Gain without population inversion in V-type systems driven by a frequency-modulated field
We obtain gain of the probe field at multiple frequencies in a closed
three-level V-type system using frequency modulated pump field. There is no
associated population inversion among the atomic states of the probe
transition. We describe both the steady-state and transient dynamics of this
system. Under suitable conditions, the system exhibits large gain
simultaneously at series of frequencies far removed from resonance. Moreover,
the system can be tailored to exhibit multiple frequency regimes where the
probe experiences anomalous dispersion accompanied by negligible
gain-absorption over a large bandwidth, a desirable feature for obtaining
superluminal propagation of pulses with negligible distortion.Comment: 10 pages + 8 figures; To appear in Physical Review
Lasing without inversion in three-level systems : self-pulsing in the cascade schemes
Lasing without inversion (LWI) in specific models of closed three-level systems is analyzed in terms of nonlinear dynamics. From a linear stability analysis of the trivial nonlasing solution of the homogeneously broadened systems with on-resonance driving and laser fields, we find that, near lasing threshold, resonant closed Λ and V schemes yield continuous-wave LWI while resonant cascade schemes can give rise to self-pulsing LWI. The origin of this different behavior is discussed. For parameters of a real cascade system in atomic 138Ba we check numerically that the self-pulsing solution is stable in a broad range of nonzero detunings. It is shown that the self-pulsing emission can still be observed when the typical residual Doppler broadening of an atomic beam is taken into account
Electric-octupole and pure-electric-quadrupole effects in soft-x-ray photoemission
Second-order [O(k^2), k=omega/c] nondipole effects in soft-x-ray
photoemission are demonstrated via an experimental and theoretical study of
angular distributions of neon valence photoelectrons in the 100--1200 eV
photon-energy range. A newly derived theoretical expression for nondipolar
angular distributions characterizes the second-order effects using four new
parameters with primary contributions from pure-quadrupole and octupole-dipole
interference terms. Independent-particle calculations of these parameters
account for a significant portion of the existing discrepancy between
experiment and theory for Ne 2p first-order nondipole parameters.Comment: 4 pages, 3 figure
Vertical-external-cavity surface-emitting lasers and quantum dot lasers
The use of cavity to manipulate photon emission of quantum dots (QDs) has
been opening unprecedented opportunities for realizing quantum functional
nanophotonic devices and also quantum information devices. In particular, in
the field of semiconductor lasers, QDs were introduced as a superior
alternative to quantum wells to suppress the temperature dependence of the
threshold current in vertical-external-cavity surface-emitting lasers
(VECSELs). In this work, a review of properties and development of
semiconductor VECSEL devices and QD laser devices is given. Based on the
features of VECSEL devices, the main emphasis is put on the recent development
of technological approach on semiconductor QD VECSELs. Then, from the viewpoint
of both single QD nanolaser and cavity quantum electrodynamics (QED), a
single-QD-cavity system resulting from the strong coupling of QD cavity is
presented. A difference of this review from the other existing works on
semiconductor VECSEL devices is that we will cover both the fundamental aspects
and technological approaches of QD VECSEL devices. And lastly, the presented
review here has provided a deep insight into useful guideline for the
development of QD VECSEL technology and future quantum functional nanophotonic
devices and monolithic photonic integrated circuits (MPhICs).Comment: 21 pages, 4 figures. arXiv admin note: text overlap with
arXiv:0904.369
Lasing without inversion in three-level systems without external coherent driving
We have studied an incoherently pumped laser operating with a Doppler-broadened three-level system placed in a doubly resonant cavity. This system generates two laser fields, one of them without population inversion. Both ladder and V-type three-level schemes are considered with a ratio R=ωα/ωβ of inversionless laser frequency ωα to ordinary laser frequency ωβ of R=0.67 and R=1.88, respectively. Dual-wavelength lasing extends up to Doppler-broadening values for optical transitions of atoms in a vapor cell. Some considerations for the practical realization of this dual-wavelength laser are discussed
Molecular Dynamics Studies of Dislocations in CdTe Crystals from a New Bond Order Potential
Cd1-xZnxTe (CZT) crystals are the leading semiconductors for radiation
detection, but their application is limited by the high cost of detector-grade
materials. High crystal costs primarily result from property non-uniformity
that causes low manufacturing yield. While tremendous efforts have been made in
the past to reduce Te inclusions / precipitates in CZT, this has not resulted
in an anticipated improvement in material property uniformity. Moreover, it is
recognized that in addition to Te particles, dislocation cells can also cause
electric field perturbation and the associated property non-uniformity. Further
improvement of the material, therefore, requires that dislocations in CZT
crystals be understood and controlled. Here we use a recently developed CZT
bond order potential to perform representative molecular dynamics simulations
to study configurations, energies, and mobilities of 29 different types of
possible dislocations in CdTe (i.e., x = 1) crystals. An efficient method to
derive activation free energies and activation volumes of thermally activated
dislocation motion will be explored. Our focus gives insight into understanding
important dislocations in the material, and gives guidance toward experimental
efforts for improving dislocation network structures in CZT crystals
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