55,245 research outputs found
Directivity patterns of laser-generated sound in solids: Effects of optical and thermal parameters
In the present paper, directivity patterns of laser-generated sound in solids
are investigated theoretically. Two main approaches to the calculation of
directivity patterns of laser-generated sound are discussed for the most
important case of thermo-optical regime of generation. The first approach,
which is widely used in practice, is based on the simple modelling of the
equivalent thermo-optical source as a mechanical dipole comprising two
horizontal forces applied to the surface in opposite directions. The second
approach is based on the rigorous theory that takes into account all
acoustical, optical and thermal parameters of a solid material and all
geometrical and physical parameters of a laser beam. Directivity patterns of
laser-generated bulk longitudinal and shear elastic waves, as well as the
amplitudes of generated Rayleigh surface waves, are calculated for different
values of physical and geometrical parameters and compared with the directivity
patterns calculated in case of dipole-source representation. It is demonstrated
that the simple approach using a dipole-source representation of
laser-generated sound is rather limited, especially for description of
generated longitudinal acoustic waves. A practical criterion is established to
define the conditions under which the dipole-source representation gives
predictions with acceptable errors. It is shown that, for radiation in the
normal direction to the surface, the amplitudes of longitudinal waves are
especially sensitive to the values of thermal parameters and of the acoustic
reflection coefficient from a free solid surface. A discussion is given on the
possibility of using such a high sensitivity to the values of the reflection
coefficient for investigation of surface properties of real solids.Comment: 14 pages, 7 figure
Spontaneous Symmetry Breaking of Population between Two Dynamic Attractors in a Driven Atomic Trap: Ising-class Phase Transition
We have observed spontaneous symmetry breaking of atomic populations in the
dynamic phase-space double-potential system, which is produced in the
parametrically driven magneto-optical trap of atoms. We find that the system
exhibits similar characteristics of the Ising-class phase transition and the
critical value of the control parameter, which is the total atomic number, can
be calculated. In particular, the collective effect of the laser shadow becomes
dominant at large atomic number, which is responsible for the population
asymmetry of the dynamic two-state system. This study may be useful for
investigation of dynamic phase transition and temporal behaviour of critical
phenomena.Comment: 4 pages, 4 figure
Coherent phenomena in semiconductors
A review of coherent phenomena in photoexcited semiconductors is presented.
In particular, two classes of phenomena are considered: On the one hand the
role played by optically-induced phase coherence in the ultrafast spectroscopy
of semiconductors; On the other hand the Coulomb-induced effects on the
coherent optical response of low-dimensional structures.
All the phenomena discussed in the paper are analyzed in terms of a
theoretical framework based on the density-matrix formalism. Due to its
generality, this quantum-kinetic approach allows a realistic description of
coherent as well as incoherent, i.e. phase-breaking, processes, thus providing
quantitative information on the coupled ---coherent vs. incoherent--- carrier
dynamics in photoexcited semiconductors.
The primary goal of the paper is to discuss the concept of quantum-mechanical
phase coherence as well as its relevance and implications on semiconductor
physics and technology. In particular, we will discuss the dominant role played
by optically induced phase coherence on the process of carrier photogeneration
and relaxation in bulk systems. We will then review typical field-induced
coherent phenomena in semiconductor superlattices such as Bloch oscillations
and Wannier-Stark localization. Finally, we will discuss the dominant role
played by Coulomb correlation on the linear and non-linear optical spectra of
realistic quantum-wire structures.Comment: Topical review in Semiconductor Science and Technology (in press)
(Some of the figures are not available in electronic form
A Coherent Ising Machine Based On Degenerate Optical Parametric Oscillators
A degenerate optical parametric oscillator network is proposed to solve the
NP-hard problem of finding a ground state of the Ising model. The underlying
operating mechanism originates from the bistable output phase of each
oscillator and the inherent preference of the network in selecting oscillation
modes with the minimum photon decay rate. Computational experiments are
performed on all instances reducible to the NP-hard MAX-CUT problems on cubic
graphs of order up to 20. The numerical results reasonably suggest the
effectiveness of the proposed network.Comment: 18 pages, 6 figure
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