Dark-state cooling of atoms in the presence of a uniform external (gravity) field is investigated. The external field leads to a finite lifetime for the dark state and thereby to a lower limit for the temperatures obtainable. The minimum temperature in the presence of a gravity field is found to be ${\mathrm{k}}_{\mathrm{B}}$${\mathrm{T}}_{\mathrm{grav}}$∼Mgλ/2π (where M is the atom mass, λ the wavelength of the cooling laser). This limit is shown to be a universal limit for any laser-cooling scheme.

The recollision model of harmonic generation in a time-varying field has two elements: the emission due to a recolliding wave packet, and the quasiperiodic emission of wave packets in successive cycles. By discussing these issues separately, we explain the origin of blueshifting, and the ``smearing'' of the harmonic spectrum. We find that such a model shares many of the qualitative features of the semiclassical theories of harmonic generation.

We study the effects of driving a two-level atom by two intense field modes that have equal frequencies but are otherwise distinguishable; the intensity of one mode is also assumed to be greater than that of the other. We calculate first the dressed states of the system, and then its resonance fluorescence and Autler-Townes absorption spectra. We find that the energy spectrum of the doubly dressed atom consists of a ladder of doublet continua. These continua manifest themselves in the fluorescence spectrum, where they produce continua at the positions of the Mollow sideband frequencies ${\mathrm{\omega }}_{\mathrm{L}}$ ±2Ω of the strong field, and in the Autler-Townes absorption spectrum, which becomes a two-continuum doublet.

The bound state structure and dynamics for an atom trap formed from the whispering gallery modes (WGMs) of a dielectric microsphere are investigated. The coupling of the quantized internal and external atomic degrees of freedom plays a fundamental role in the quantum dynamics of this atom gallery. The radiative processes for a cold atom near a microsphere are modified due to the special symmetry of the atom gallery, the WGM mode structure, and the finite extent of the center-of-mass (c.m.) wave packet. Finally, interesting implications of the quantized c.m. for atomic matter waves and cavity QED with a quantum field are mentioned.

We apply the tools of adiabatic Floquet theory to study the control of population transfer and tunneling processes by strong laser pulses. We show how tunneling can be enhanced by intermediate resonant levels by choosing appropriate pulse parameters. We obtain complete tunneling in times that are much shorter than the bare tunneling time or the times obtained without intermediate levels.

Natural ${\mathrm{Cd}}^{+}$ ions in a Penning trap are cooled sympathetically below 1 K by a Coulomb interaction with directly laser-cooled ${\mathrm{Be}}^{+}$ ions. Spatial separation of the ${\mathrm{Be}}^{+}$ and ${\mathrm{Cd}}^{+}$ ion clouds due to centrifugal force is observed directly. When the frequency of rotation of the ion clouds is accelerated, the separation between ${\mathrm{Cd}}^{+}$ -ion isotopes (A=110 and 116) can be clearly observed, though their mass difference is very small. The equations of motion for 100 ${\mathrm{Be}}^{+}$ ions and 50 ${\mathrm{Cd}}^{+}$ ions are solved to simulate the experiments, and the result shows the spatial separation between the two ion species. The separation between the ${\mathrm{Cd}}^{+}$ -ion isotopes is investigated by comparing the experimental results with numerical calculations.

We discuss the kinetic theory for evaporative cooling of a dilute collisional gas in a trap. The analysis in O. J. Luiten et al., Phys. Rev. A 53, 381 (1996) is extended to incorporate a time-dependent trap strength and truncation energy. We employ the equations obtained to predict the efficiency of evaporative cooling of sodium atoms in a Ioffe trap and realize that a time-dependent trap strength can be crucial for the cooling. The calculations are classical in the sense that no effects of quantum statistics are taken into account: nevertheless the results of the calculations are useful in order to reveal experimental routes towards a Bose-Einstein condensate of sodium atoms.

In a recent experiment by Eichmann et al. [Phys. Rev. Lett. 70, 2359 (1993)], polarization-sensitive measurements of the fluorescence from two four-level ions driven by a linearly polarized laser were made. Depending on the polarization chosen, different degrees of interference were observed. We carry out a theoretical and numerical study of this system, showing that the results can largely be understood by treating the atoms as independent radiators which are synchronized by the phase of the incident laser field. The interference and its loss may be described in terms of the difference between coherent and incoherent driving of the various atomic transitions in the steady state. In the numerical simulations, which are carried out using the Monte Carlo wave-function method, we remove the assumption that the atoms radiate independently and consider the photodetection process in detail. This allows us to see the total interference pattern build up from individual photodetections and also to see the effects of superfluorescence, which become important when the atomic separation is comparable to an optical wavelength. The results of the calculations are compared with the experiment. We also carry out simulations in the non-steady-state regime and discuss the relationship between the visibility of the interference pattern and which-path considerations.

The ion yields of NO molecules under irradiation of focused, high-intensity laser beams at 532 nm were measured by a reflecting time-of-flight mass spectrometer. The intensity distributions of the laser beams at the focal region were measured by direct imaging through a microscopic objective charge-coupled-device camera assembly. NO does not significantly fragment under high-intensity light field when examined using two laser systems with 6-ns and 35-ps pulse widths, the latter delivering a power density up to ${10}^{14}$ W/${\mathrm{cm}}^2$ . The ion yield of NO at various laser power densities was measured over three orders of magnitude, ranging from the unsaturated region to the complete saturation regime. A multiphoton ionization model was used based on rate equation and perturbation theory. The four-photon nonresonant multiphoton ionization cross section for NO at 532 nm was determined to be (7.1±0.1)×${10}^{\mathrm{-}117}$ ${\mathrm{cm}}^8$ ${\mathrm{s}}^7$ by fitting experimental data to the model.

A direct solution of the time-dependent Schrödinger equation (TDSE) for a two-electron model atom in a strong electromagnetic field is used to extract probabilities for both single and double electron ionization. For three-photon ionization at a moderately strong intensity, the TDSE calculations find single ionization at 62%, single ionization with excitation at 10%, and double ionization at 3%. The sequential process of escape from an already once ionized atom is found to make a substantial contribution to the double-ionization probability. Multiphoton-ionization probabilities are also obtained from a solution of the time-dependent unrestricted Hartree-Fock (TDUHF) equations for the same model atom. Although in qualitative agreement with the TDSE results, the TDUHF results show significantly less single ionization with excitation and have a different angular distribution for double ionization.

We report measurements and calculations of the polarization state of high-order harmonics generated by a 790-nm Ti:sapphire laser. The problem of completely characterizing the polarization state of a partially polarized radiation in the XUV range is discussed in detail. The comparison between several gases, xenon, argon and neon, and different orders, from the 17th to the 33rd, shows that the rotation angle and ellipticity strongly depends on the position of the harmonic in the spectrum, and in particular, whether it is in the cutoff or in the plateau. In the plateau, the rotation angle is quite large, and the ellipticity follows that of the fundamental, remaining, however, smaller. The radiation is only partially polarized. In contrast, in the cutoff, both rotation angle and ellipticity remain small, independently of the laser ellipticity. Our experimental results compare well with theoretical predictions including the single-atom response and propagation effects.

The rate of multiphoton dissociation of the ${\mathrm{BeH}}^{2+}$ molecule in its ground and first two excited vibrational levels has been computed via classical mechanics as a function of laser frequency. There is agreement with earlier quantum-mechanical results as regards the existence and magnitude of an optimal frequency, ω, for which the dissociation rate is maximized. This fact has been analyzed and understood via the application of the theory of chaotic scattering. Indeed, we find fractal singularities in the function ${\mathrm{T}}_{\mathrm{d}}$(x) of the duration of photodissociation, and we compute their dimension to be equal to 1, in agreement with the conjecture of Lau, Finn, and Ott [Phys. Rev. Lett. 66, 978 (1991)] that this must be a characteristic of systems exhibiting nonhyberbolic scattering. Turning to the problem of interpreting the appearance of an optimal ω, we propose the following two mechanisms for the reduction of the multiphoton dissociation rate. First is the increase of fractal singularities when the frequency ω attains values larger than ω. Second is the gradually increasing overlap of the classical initial state with the region of Kolmogorov-Arnold-Moser tori when ω<ω. Finally, as the intensity is increased there is a transition from chaotic to regular photodissociation, where the singularities in ${\mathrm{T}}_{\mathrm{d}}$(x) are finite. It is conjectured that this reflects the emergence of the quantum-mechanical phenomenon of above-threshold dissociation.

We have studied laser collimation of a chromium atomic beam using a transverse polarization gradient cooling scheme. We present detailed measurements of the angular distribution of atoms on the beam axis, over a broad range of laser intensities and detunings, including those that produce significant excitation, and observe collimation angles as small as 0.16±0.01 mrad (50% quantile). We compare our results with existing calculations based on assumptions of steady-state conditions and low excited-state population.

A nonlinear theory of a noninversion laser of an open three-level system based on electromagnetically induced transparency is given. In this noninversion laser model, a nonselective pumping mechanism is used for obtaining lasing gain such that the population in the lower lasing level is always larger than a half of the total population no matter whether the driving field is on or off. In the lasing region, there is no direct pumping to the upper level. The conditions for the existence of a nontrivial and stable laser field are obtained. The dependence of the laser intensity on the intensity of the driving field and the ratio of the two decay rates from the upper level has been obtained, and the ways to obtain a large laser field have been studied.

We apply a recently developed scheme for electromagnetic field quantization in dispersive and absorbing dielectrics to calculate the effects of perpendicular propagation through a finite-temperature dielectric slab on the properties of an incident pulse of light. The theory applies to incident pulses of nonclassical light and also reproduces the results for classical pulses in the appropriate limit. The transmitted pulse is assumed to be measured by a detector that receives radiation only from the direction normal to the slab surfaces. The Poynting vector of the transmitted light includes contributions from the incident pulse, whose shift in peak position and additional broadening or narrowing are determined, and from the thermal emission of the slab, which tends to a black-body form in the appropriate limits.

The spectroscopic detection of the excited-state resonances of a strongly coupled atom and optical cavity mode is complicated by inhomogeneous broadening, due to the variation of the dipole coupling constant with location in the cavity. Photon correlation spectroscopy circumvents the difficulty for slowly moving atoms using frequency-selective two-photon absorption and photon coincidence detection of the two-photon decay.

A semiclassical, nonlinear theory of a single-mode laser is developed so that the transition around threshold can be studied. The field is expressed in the frequency domain which allows us to emphasize the role of the Fabry-Pérot cavity. A generalized Airy function is obtained for the laser line shape: it contains the source line shape and the empty cavity line shape. It describes the laser both above and below threshold, including the transition region where there is an abrupt increase of intensity and decrease of the laser linewidth. The general arguments are illustrated by detailed numerical calculations for the 3.39-μm line of the He-Ne laser. The intensity of the spontaneous emission source which plays a central role is computed from first principles.

We derive approximate analytical expressions for the spectrum of a regularly pumped micromaser. Our procedure is based on ensemble averaging rather than time averaging, and the results are obtained from the solution of a non-Markovian evolution problem. For certain values of the atom-field interaction time the spectrum is split into several equidistant peaks. We show that this line splitting reflects the phase reversal of the entire cavity field caused by the transit of a single atom. This phenomenon is due to the occurrence of quantum Rabi oscillations and it is closely related to the Jaynes-Cummings revival.

The eigenstate spectrum of polarization self-modulated lasers is theoretically and experimentally investigated. The polarizations and frequencies of the eigenstates of such lasers are theoretically derived in the framework of the Jones matrix formalism by assuming the self-consistency of the field after one round trip through the cavity. The resulting predictions are successfully checked experimentally as well in the case of a polarization self-modulated multimode gas laser as in the case of a polarization self-modulated external-cavity semiconductor laser. In particular, it is shown experimentally that the pulses at a frequency equal to half the free spectral range observed in such lasers can be interpreted as a beat between the two eigenstates that oscillate in a single round trip in the cavity. This behavior is shown to be in agreement with a one round trip operation of the laser. Then, the temporal properties of such lasers are suggested to be due to the self-mode-locking between the different eigenstates.

A standing light wave appears to a highly detuned two-level atom as a sinusoidal potential. This causes the atoms to be channeled to the minima of the potentials. At the same time, the atom changes the phase of the light field. Detecting this phase shift yields information about the position of the atom. In this paper we show that this information may be used to control the intensity of the light field so as to modify the effective shape of the potential ``seen'' by the atom. Our numerical simulations show that the effectiveness of the channeling may be significantly enhanced by such quantum-limited feedback. We discuss possible applications to atom lithography.