121 research outputs found
Research on nonlinear optical materials: an assessment. IV. Photorefractive and liquid crystal materials
This panel considered two separate subject areas: photorefractive materials used for nonlinear optics and liquid crystal materials used in light valves. Two related subjects were not considered due to lack of expertise on the panel: photorefractive materials used in light valves and liquid crystal materials used in nonlinear optics. Although the inclusion of a discussion of light valves by a panel on nonlinear optical materials at first seems odd, it is logical because light valves and photorefractive materials perform common functions
Mobility of slow electrons in a polar crystal
We have obtained an approximate expression for the impedance function at all frequencies, temperatures, and coupling strengths of an electron coupled to a polar lattice (a system commonly called a polaron). The starting point for the calculation is the quantum mechanical expression for the expected current. The phonon coordinates are eliminated from this expression by well-known field-theory techniques. The resulting exact "influence functional" is then approximated by a corresponding quadratic "influence functional" which, it is hoped, imitates the real polaron. Correction terms are computed to account for the difference between the approximate impedance and the exact polaron impedance in a manner closely analogous to Feynman's treatment of the polaron self-energy. In fact, the analytic evaluation of the expression for the impedance obtained here is carried out using the approximate "influence functional" that was successfully employed in minimizing the binding (and free) energy of the polaron in earlier calculations. However, the accuracy obtained using this approximation, for the impedance calculation, is less satisfactory and its limitations are discussed. Nevertheless, beginning at intermediate coupling strengths, the approximate impedance produces a level structure of increasing complexity and narrowing resonances as the coupling strengthens. This suggests that further refinements may be fruitful. Methods for finding a better quadratic influence functional for use in our impedance expression as well as ways of improving the expression further are suggested. A comparison of our results with those of the Boltzmann equation points up interesting differences which arise from reversing the order of taking limits of zero frequency and coupling
On the Localization of One-Photon States
Single photon states with arbitrarily fast asymptotic power-law fall-off of
energy density and photodetection rate are explicitly constructed. This goes
beyond the recently discovered tenth power-law of the Hellwarth-Nouchi photon
which itself superseded the long-standing seventh power-law of the Amrein
photon.Comment: 7 pages, tex, no figure
Nonlinear lensing mechanisms in a cloud of cold atoms
We present an experimental study of nonlinear lensing of near-resonant light by a cloud of laser-cooled rubidium atoms, specifically aimed at understanding the role of the interaction time between the light and the atomic vapor. We identify four different nonlinear mechanisms, each associated with a different time constant: electronic nonlinearity, Zeeman optical pumping, hyperfine optical pumping and radiation pressure. Our observations can be quite accurately reproduced using a simple rate equation model which allows for a straightforward discussion of the various effects. The results are important for planning more refined experiments on transverse nonlinear optics and self-organization in samples of cold atoms
Optical Absorption Spectra of Bipolarons
The absorption of large bipolarons is investigated using the path-integral
method. The response of a bipolaron to an external electromagnetic field is
derived in the framework of the memory-function approach. The bipolaron optical
absorption spectrum consists of a series of relatively narrow peaks. The
peculiarities of the bipolaron optical absorption as a function of the
frequency of the electromagnetic field may be attributed to the transitions
involving relaxed excited states and scattering states of a bipolaron.Comment: 14 pages, 3 figures, E-mail addresses: [email protected],
[email protected]; to be published in Phys. Rev.
Optical phase conjugation for turbidity suppression in biological samples
Elastic optical scattering, the dominant light-interaction process in biological tissues, prevents tissues from being transparent. Although scattering may appear stochastic, it is in fact deterministic in nature. We show that, despite experimental imperfections, optical phase conjugation (λ = 532 nm) can force a transmitted light field to retrace its trajectory through a biological target and recover the original light field. For a 0.69-mm-thick chicken breast tissue section, we can enhance point-source light return by a factor of ~5 x 10^3 and achieve a light transmission enhancement factor of 3.8 within a collection angle of 29°. Additionally, we find that the reconstruction's quality, measured by the width of the reconstructed point source, is independent of tissue thickness (up to a thickness of 0.69 mm). This phenomenon may be used to enhance light transmission through tissue, enable measurement of small tissue movements, and form the basis of new tissue imaging techniques
Coherent control using adaptive learning algorithms
We have constructed an automated learning apparatus to control quantum
systems. By directing intense shaped ultrafast laser pulses into a variety of
samples and using a measurement of the system as a feedback signal, we are able
to reshape the laser pulses to direct the system into a desired state. The
feedback signal is the input to an adaptive learning algorithm. This algorithm
programs a computer-controlled, acousto-optic modulator pulse shaper. The
learning algorithm generates new shaped laser pulses based on the success of
previous pulses in achieving a predetermined goal.Comment: 19 pages (including 14 figures), REVTeX 3.1, updated conten
Quantum Electronics
Contains reports on eleven research projects.Air Force Rome Air Development Center (in collaboration with C.C. Leiby Jr)U.S. Air Force - Rome Air Development Center (Contract F19628-80-C-0077)National Science Foundation (Grant PHY79-09739)Joint Services Electronics Program (Contract DAAG29-80-C-0104)Air Force Geophysics Laboratory (Contract F 19628-79-C-0082)National Science Foundation (Grant DAR80-08752)National Science Foundation (Grant ECS79-19475)National Science Foundation (Grant ECS80-17705)National Science Foundation (Grant ENG79-09980
Theory of four-wave mixing of matter waves from a Bose-Einstein condensate
A recent experiment [Deng et al., Nature 398, 218(1999)] demonstrated
four-wave mixing of matter wavepackets created from a Bose-Einstein condensate.
The experiment utilized light pulses to create two high-momentum wavepackets
via Bragg diffraction from a stationary Bose-Einstein condensate. The
high-momentum components and the initial low momentum condensate interact to
form a new momentum component due to the nonlinear self-interaction of the
bosonic atoms. We develop a three-dimensional quantum mechanical description,
based on the slowly-varying-envelope approximation, for four-wave mixing in
Bose-Einstein condensates using the time-dependent Gross-Pitaevskii equation.
We apply this description to describe the experimental observations and to make
predictions. We examine the role of phase-modulation, momentum and energy
conservation (i.e., phase-matching), and particle number conservation in
four-wave mixing of matter waves, and develop simple models for understanding
our numerical results.Comment: 18 pages Revtex preprint form, 13 eps figure
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