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

Miscellaneous Problems

Contains reports on two research projects

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

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

Projection-type X-ray microscope based on a spherical compound refractive X-ray lens

New projection- type X-ray microscope with a compound refractive lens as the optical element is presented. The microscope consists of an X-ray source that is 1-2 mm in diameter, compound X-ray lens and X-ray camera that are placed in-line to satisfy the lens formula. The lens forms an image of the X-ray source at camera sensitive plate. An object is placed between the X-ray source and the lens as close as possible to the source, and the camera shows a shadow image of the object. Spatial resolution of the microscope depends on the lens focal length, lens aperture and the distance from the source to the object. One to two micron resolution may be achieved by placing the object at a distance of 1-5 mm from the source. The X-ray source may be designed with the target deposited on a 200-µm thick Be window, which permits the object to be placed very close to the emitting surface. The tube focal spot is equal to 1-2 mm. Results of imaging experiments with an ordinary copper anode X-ray tube and a 10-cm focal length spherical compound refractive X-ray lens are discussed

From Bloch model to the rate equations II: the case of almost degenerate energy levels

Bloch equations give a quantum description of the coupling between an atom and a driving electric force. In this article, we address the asymptotics of these equations for high frequency electric fields, in a weakly coupled regime. We prove the convergence towards rate equations (i.e. linear Boltzmann equations, describing the transitions between energy levels of the atom). We give an explicit form for the transition rates. This has already been performed in [BFCD03] in the case when the energy levels are fixed, and for different classes of electric fields: quasi or almost periodic, KBM, or with continuous spectrum. Here, we extend the study to the case when energy levels are possibly almost degenerate. However, we need to restrict to quasiperiodic forcings. The techniques used stem from manipulations on the density matrix and the averaging theory for ordinary differential equations. Possibly perturbed small divisor estimates play a key role in the analysis. In the case of a finite number of energy levels, we also precisely analyze the initial time-layer in the rate aquation, as well as the long-time convergence towards equilibrium. We give hints and counterexamples in the infinite dimensional case

Miscellaneous Problems

Contains reports on three research projects

Channel spaser

We show that net amplification of surface plasmons is achieved in channel in a metal plate due to nonradiative excitation by quantum dots. This makes possible lossless plasmon transmission lines in the channel as well as the amplification and generation of coherent surface plasmons. As an example, a ring channel spaser is considered

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