122 research outputs found
Laser-electron beam interaction applied to optical amplifiers and oscillators
Momentum modulation of a relativistic electron beam by a Nd:YAG laser is demonstrated. The electrons, at 100 MeV energy, interact with the laser light in helium gas at standard temperature and pressure. At an angle of 6.55 mrad between the two wavevectors, corresponding to the Cerenkov angle, a given electron remains in a field of constant phase as it passes through the light beam. The experimental arrangement is illustrated showing the trajectories of the electron and light. The particle momentum is measured by a mass spectrometer, and the angle between the wavevectors is controlled by a rotatable mirror. Experimental results indicate that momentum modulation of an electron beam may be used for amplification. A possible configuration for an optical klystron is illustrated
Driving light pulses with light in two-level media
A two-level medium, described by the Maxwell-Bloch (MB) system, is engraved
by establishing a standing cavity wave with a linearly polarized
electromagnetic field that drives the medium on both ends. A light pulse,
polarized along the other direction, then scatters the medium and couples to
the cavity standing wave by means of the population inversion density
variations. We demonstrate that control of the applied amplitudes of the
grating field allows to stop the light pulse and to make it move backward
(eventually to drive it freely). A simplified limit model of the MB system with
variable boundary driving is obtained as a discrete nonlinear Schroedinger
equation with tunable external potential. It reproduces qualitatively the
dynamics of the driven light pulse
Non-linear emission spectra of quantum dots strongly coupled to photonic mode
A theory of optical emission of quantum dot arrays in quantum microcavities
is developed. The regime of the strong coupling between the quantum dots and
photonic mode of the cavity is considered. The quantum dots are modeled as
two-level systems. In the low pumping (linear) regime the emission spectra are
mainly determined by the superradiant mode where the effective dipoles of the
dots oscillate in phase. In the non-linear regime the superradiant mode is
destroyed and the emission spectra are sensitive to the parity of quantum dot
number. Further increase of the pumping results in the line width narrowing
being an evidence of the lasing regime.Comment: 11 pages, 6 figure
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Nonlinear optics in doped fibers. Final report, May 1, 1992--April 30, 1996
The main objective of this contract was to study a novel scheme to obtain very strong third-order optical nonlinearities in fibers doped with a suitable absorber in order to produce low-power all-optical fiber switches. In these devices, a signal is switched from a first fiber to a second fiber by the application of an optimal pump of wavelength different from that of the signal. The pump acts on the nonlinearity of the fiber, resulting from the dopant present in the fiber core, to modify the fiber index. The switch is made of a fiber interferometer which transforms this index modulation into an amplitude modulation. The signal is switched as long as the pump is applied, and it returns to the first fiber when the pump is turned off. The incentive was to develop switches exhibiting the following properties: (1) low switching power, (2) a short nonlinear fiber to be able to utilize a short and thus environmentally stable interferometer, (3) fast response time, (4) broad range of signal wavelengths, particularly around 1.55 and 1.32 {micro}m, (5) pump wavelengths readily available from diode lasers, and (6) low signal loss. This research also involved the study of various fiber interferometers to determine the best possible switch architectures, in terms of pump power requirement, stability against environmental temperature fluctuations and possible pump-induced heating of the fiber. Switches are strongly needed for a large number of important applications. The rest of this report is a summary of the most important tasks carried out, and of the major discoveries made, under this contract
Radiography and tomography system using refractive lenses
A prototype x-ray imaging system was built and tested for high-resolution x-ray radiography and tomography. The instrument consists of a microspot x-ray tube with a multilayer optic, a parabolic compound refractive lens (CRL) made of a plastic containing only hydrogen and carbon, and an x-ray detector. A rotation stage was added for tomography. Images were acquired of both grid meshes and biological materials, and these are compared to images achieved with spherical lenses. We found the best image quality using the multilayer condenser with a parabolic lens, compared to images with a spherical lens and without the multilayer optics. The resolution was measured using a 155 element parabolic CRL and a multilayer condenser with the microspot tube. The experiment demonstrates about 1.1 µm resolution
Scattering of slow-light gap solitons with charges in a two-level medium
The Maxwell-Bloch system describes a quantum two-level medium interacting
with a classical electromagnetic field by mediation of the the population
density. This population density variation is a purely quantum effect which is
actually at the very origin of nonlinearity. The resulting nonlinear coupling
possesses particularly interesting consequences at the resonance (when the
frequency of the excitation is close to the transition frequency of the
two-level medium) as e.g. slow-light gap solitons that result from the
nonlinear instability of the evanescent wave at the boundary. As nonlinearity
couples the different polarizations of the electromagnetic field, the
slow-light gap soliton is shown to experience effective scattering whith
charges in the medium, allowing it for instance to be trapped or reflected.
This scattering process is understood qualitatively as being governed by a
nonlinear Schroedinger model in an external potential related to the charges
(the electrostatic permanent background component of the field).Comment: RevTex, 14 pages with 5 figures, to appear in J. Phys. A: Math. Theo
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Study of material properties using channeling radiation
A possible application for channeling radiation is for investigating the properties of crystals in which the channeling occurs. In this paper we present some general considerations concerning channeling radiation as a measurement technique, and then we proceed to describe several specific examples
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