Resonant Laser Beam Research Articles

Bragg scattering from atoms in optical lattices.

We observe coherently enhanced reflection (Bragg scattering) of a near resonant laser beam from cesium atoms ordered by a periodic array of optical potential wells known as an optical lattice. The reflected light is enhanced by a factor of $\ensuremath{\sim}{10}^{5}$ over scattering from a disordered atomic sample. We measure the magnitude of the reflectivity and its detuning dependence as functions of the density and the degree of localization of the atoms. The change in lattice constant due to the frequency-dependent effective index of refraction seen by the lattice light field is observed.

Using nuclear spins, radio waves, sodium atoms, and laser light to investigate spatiotemporal nonlinear systems

Within the wide field of nonlinear dynamics we investigate temporal and spatial behavior of electromagnetic systems. A strange type of laser, the nuclear magnetic resonance (NMR) laser, shows truly chaotic behavior and is therefore ideally suited to analyze experimentally and theoretically a variety of temporal nonlinear effects. Of particular interest is the analysis of its strange attractors in terms of unstable periodic orbits. For the extension of our research from the NMR laser (representing a purely temporal system described by the Bloch-Kirchhoff ordinary differential equations) to spatial and spatiotemporal systems, we chose, as a three-dimensional dynamic system, polarized laser beams interacting nonlinearly with sodium atoms. Among other effects, we have observed beam bouncing, beam splitting, and beam switching. This can be well described by partial differential equations for beam propagation derived from the Schrödinger equation and the Maxwell equations. An intuitive explanation is given in terms of intensity and polarization patterns formed by optical-pumping-induced mutual refractive index modifications between polarized resonant laser beams.

Quantum Zeno effect in a double-well potential: A model of a physical measurement.

In this paper we study the quantum Zeno effect in real space due to a position measurement. The motion of a particle is decelerated or comes to a complete stop when its position is observed. Instead of using the von Neumann collapse hypothesis, we treat a real measurement process. The measurement consists of coupling a two-level atom in a double-well potential to a resonant laser beam. Subsequent resonance fluorescence can be used to determine the atom's position within the double well, provided the laser wavelength is short enough to ensure a resolvable scattering pattern of the fluorescence photons. Treating this process in the framework of dissipative quantum mechanics, we derive a master equation which describes the measurement process in all relevant details. Solving the master equation analytically as well as numerically we study the conditions for the decay of the nondiagonal elements. This leads directly to the inhibition of the center-of-mass motion, i.e., to a quantum Zeno effect. Because we treat the measurement process in detail we are able to investigate the conditions for a complete measurement. In particular, we study the role of the intensity and the wavelength of the probing laser field.

Stabilization of transverse solitary waves by a nonlocal response of the nonlinear medium.

Optical pumping of atomic vapors can lead to self-focusing of resonant laser beams. The saturation of the optical pumping process at low intensity allows the study of transverse solitons in the form of self-trapped laser beams, using low-power cw lasers. The long lifetime of the optically pumped atoms allows the atomic diffusion to transport the excitation away from the interaction region. Numerical simulations show that the resulting nonlocal response of the medium represents an important mechanism that stabilizes the self-trapped beam. The theoretical predictions are confirmed by experimental results from sodium vapor.

Excitation of Atomic Hydrogen by Proton Impact in the Presence of a Resonant Laser Field Using the First Order Magnus Approximation

We consider proton collisions from hydrogen atoms in the presence of a laser beam (taken in the electric dipole approximation) that resonantly (or nearly resonantly) excites the hydrogen atoms from the Is to the 2p state. The laser beam is linearly polarised with polarisation either parallel (longitudinal) or perpendicular (transverse) to the direction of incidence of the proton. A non-perturbative quasi-energy approach is used to describe the laser-atom interaction, while the first-order Magnus approximation is used to describe the collision dynamics in the presence of the nearly resonant laser beam. We have calculated the integrated cross section o-(2s} for the excitation of the 2s state. It is found that 0-(2s} is small for longitudinal polarisation, as compared with transverse polarisation. We have also compared our field-free results obtained by using the first-order Magnus approximation to that obtained by the close-coupling approximation. Although both methods give excellent results, the former method is quite demanding in terms of computer time.

Towards laser cooling of fast beams: Acceleration by laser radiation pressure

We report on the acceleration of 100 keV Ne atoms by the light pressure force of a resonant laser beam. An acceleration of 0.12 V/μs or 6 × 10 5 ms −2 has been observed. This experiment verifies the potential application of laser cooling in heavy ion rings.

Optical nutation in direct-gap semiconductors

Using a coherent radiation-semiconductor interaction model based upon the time dependent perturbation technique, the occurrence of optical nutation has been analytically investigated in direct-gap semiconductors such as GaAs, GaSb, InAs, and Hg 1-x Cd x Te, duly irradiated by near resonant laser beams. The present approach is much more simplified and straightforward than the conventional Maxwell-Bloch approach used so far for the study of coherent optical transient effects in molecular and atomic systems. The dispersion and absorption expressed in terms of the real and imaginary parts of the crystal optical susceptibility \chi_{r} and \chi_{i} exhibit temporally damped ringing behavior of Neumann and Bessel types, respectively. These processes have been identified as dispersive and absorptive optical nutation. The theory is developed for arbitrary laser intensity in presence of damping due to nonoptical processes. To understand the mechanisms of dispersive and absorptive optical nutation, we have restricted only to low-power resonant band-to-band transitions neglecting excitonic and damping effects. While studying analytically the transient behavior of \chi_{r} and \chi_{i} for a given pump field strength, it is noticed that the absorptive component exhibit remarkable qualitative agreement with the experimental observations of optical nutation in13CH 3 F.

Stimulated emission in optically pumped atomic-copper vapor

We have observed, for the first time to our knowledge, stimulated emission in atomic-copper vapor that is excited by a resonant tunable laser beam. One of the important and interesting results obtained in this experiment is that excitation of the (2)P(1/2) level of the copper atoms generates strong amplified spontaneous emission (ASE) for both(2)P(1/2)-(2)D(3/2) and (2)P(3/2)-(2)D(5/2) transitions. This is the first reported direct experimental evidence observed for collisional mixing between the (2)P(1/2) and (2)P(3/2) levels in a copper-vapor laser. Excitation of the (2)P(3/2) level induces substantially weaker ASE for the (2)P(1/2)-(2)D(3/2) transition. In addition, we observed collision-induced ASE for both transitions over a wide range of detuning of the pump frequency. The preliminary results of the experiment are presented, and the implications of the results for high-pressure copper-vapor lasers are discussed.

Laser Cooling of an Atomic Beam by Spatial Doppler Tuning of a Resonance Transition

Cooling of an atomic beam by Doppler tuning is proposed. The atomic beam is decelerated by repeatedly crossing a resonant laser beam at a decreasing angle. An example of the experimental configuration to slow down a two-level atomic beam by a factor of 20 using two lasers is given.

Laser cooling and electromagnetic trapping of neutral atoms

Atoms in a thermal beam can be cooled, decelerated, and stopped using the radiation pressure from a nearly resonant laser beam. Several groups have already used this laser-cooling process on an atomic sodium beam. The techniques and results of the various experimental groups are reviewed, and applications of laser-cooled atoms, in particular the possibility of confining them in electromagnetic traps, are discussed.

Stopping atoms with laser light.

We have produced a sample of free sodium atoms at rest in the laboratory by decelerating atoms in an atomic beam using momentum transfer from a counterpropagating, resonant laser beam. These atoms have a density of about ${10}^{5}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and a velocity spread of about 15 m/s full width at half maximum corresponding to a kinetic temperature less than 100 mK.

Application of a very long cavity laser to atom slowing down and optical pumping

A method to slow down an atomic beam with a counter-propagating resonant laser beam is proposed. The laser radiates across the full Doppler profile of the atomic beam with a longitudinal mode separation which is slightly less than the homogeneous linewidth of the atoms. This allows the atoms to come to rest without the need for frequency tuning or laser chirping. The required mode separation can be obtained from a very long-cavity laser. The application to optical pumping will be also discussed.

Laser irradiation of a three-level gas system: Continued-fraction theory and applications

The general steady-state solution of the density matrix equations for a Doppler broadened three-level system irradiated by two resonant standing-wave laser beams of arbritrary intensity is analyzed. The solution is expressed in a matrix continued fraction form, that involves 4×4 matrices in important configurations and is convenient for numerical computations. Some representative cases including the absorption spectra for a probe laser of arbitrary intensity, the Doppler-free multiphoton resonances and the optically pumped lasers are analyzed numerically in connection with previous experimental investigations.

Four-wave mixing resonantly enhanced by ac-Stark-split levels in self-trapped filaments of light

It is observed that when an intense, nearly resonant laser beam propagates through a dilute ($Nl5\ifmmode\times\else\texttimes\fi{}{10}^{14}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) vapor of sodium atoms, a four-wave-mixing process occurs, leading to the generation of strong, coherent radiation in the forward direction at frequencies symmetrically detuned from that of the exciting laser by the generalized Rabi frequency. This phenomenon is explained as resulting from a nearly phase-matched mixing process utilizing the nonlinear response of the sodium atom. Our analysis is based on a solution of the density-matrix equations of motion that includes the effect of level shifts induced by the ac Stark effect, which can be as great as 10 \AA{} for our experimental conditions, and which leads to gain due to the stimulated three-photon effect at one Rabi sideband and to strong coupling due to a parametric mixing process between radiation at the two sideband frequencies. It is also observed that at larger sodium densities ($N\ensuremath{\gtrsim}5\ifmmode\times\else\texttimes\fi{}{10}^{14}$ ${\mathrm{cm}}^{\ensuremath{-}3}$), self-focusing of the laser beam can occur, leading to several complicating effects. We find experimentally that the on-resonance Rabi frequency $\ensuremath{\Omega}$ within a self-trapped filament lies preferentially within the range from 1 to 3 times the laser detuning from resonance $\ensuremath{\Delta}$ and we explain this result as a consequence of strong saturation of the refractive index that occurs for $\ensuremath{\Omega}\ensuremath{\gtrsim}\ensuremath{\Delta}$. Four-wave mixing is found to occur within these self-trapped filaments. The high-frequency sideband thus generated is trapped within the filament, whereas the lower-frequency sideband is ejected as a consequence of Snell's law and forms a cone surrounding the transmitted laser beam.

An atomic fluorescence transit-time velocimeter

We report the development of an atomic laser velocimeter capable of measuring moderate gas flow speeds. It is based on recording the burst of resonance fluorescence emitted by a single atom as it passes through one or two resonant laser beams. Thus far speeds up to 13 m/s have been successfully measured using single atoms crossing one laser beam. If an average is taken over many atoms, higher velocities can easily be realized. With two laser beams, a flow velocity of 88 m/s has been measured with an integration time of 0.5 s.

Conical emission from atomic vapors excited by nearly resonant laser beams

Recent theoretical predictions for the angle of conical emission from atomic vapors excited by nearly resonant laser beams are not in agreement with earlier experimental data.

Quasiclassical theory of laser-induced atomic-beam dispersion

We present a theoretical model for the interaction of a beam of two-level atoms with a resonant laser beam. It is shown that the radiation force, while averaging to zero at resonance, produces a transverse spread of the atomic velocities. If radiative damping is ignored, this spread increases linearly with time and finally saturates towards a steady value. Although the spread has a quantum-mechanical origin, we show that it is possible, in the limit $\ensuremath{\hbar}k\ensuremath{\rightarrow}0$, to describe the phenomenon in terms of differential equations of motion for the atomic translational variables, as in the classical case. In this model it is also possible to describe the effects of radiative damping, as well as other additional effects, which may occur in a real experiment. Finally a comparison of our results with experimental data, recently obtained on an Na atomic beam, has been made. Although an order-of-magnitude fit of the experimental data could be obtained, the weights of the different causes, reducing the spread, were found to be ambiguous and have been left partly open.

Statistical properties of the light scattered by a two-level atom driven by a resonant multimode laser beam

A theory is presented of the near-resonant interaction of two-level atom with strong Gaussian fields. Analytical expressions are given for the fluorescence intensity and for the two-time first- and second-order correlation functions of the intensity in both cases of a chaotic field and a real Gaussian field. The usual three-peak structure in the fluorescence spectrum is found to disappear because the amplitude fluctuations of the driving light prevent the nutation process. The scattered intensity is shown no longer to present a Markovian character when the atom is driven by a Gaussian field. The atom gains a sort of memory because of the intensity fluctuations in the driving field.

Coherent trapping of atomic populations

It is often desirable in laser spectroscopy and isotope separation to extract as much as possible of an atomic or molecular population that is distributed among a number of ground-state sublevels and low-lying metastable levels. We describe a form of coherent trapping that occurs when multiple resonant laser beams are used to couple the various ground states to a common upper level. This effect prevents the extraction of the entire population. We then study the effect with two dye lasers and an atomic beam and suggest possible ways to maximize the pumping efficiency.

Particle kinetics of selective excitation spectroscopy

The role of kinetic velocity distributions on the interaction between resonant laser beams and atoms or ions is shown to affect, and under some conditions dominate, the intensity, line shape, and spatial distribution of diagnostic fluorescence in hot plasmas and gases.