Avoided crossings are common in the quasienergy spectra of strongly driven nonlinear quantum wells. In this paper we examine the sinusoidally driven particle in a square potential well to show that avoided crossings can alter the structure of Floquet states in this system. Two types of avoided crossings are identified: one type leads only to temporary changes (as a function of driving field strength) in Floquet state structure while the second type can lead to permanent delocalization of the Floquet states. Radiation spectra from these latter states show a significant increase in high harmonic generation as the system passes through the avoided crossing.

We study the ellipticity dependence of the high-order harmonic generation (HOHG) when a static electric field is presented. Through investigating the corresponding classical trajectories of the electron for the harmonic generation, it is found that there exists two different mechanisms. For circularly polarized incident light the HOHG originates mainly from the recombination of the electrons to the nucleus after multiple returns. We also find that for the elliptical incidence HOHG, the application of a static electric field can generate harmonics far beyond the usual cutoff.

The technique of pulsed-field-ionization–zero-kinetic-energy photoelectron spectroscopy, typically applied to the investigation of ionic states of atoms and molecules resulting from single electron excitation, has been used to probe all known correlation ionic states of xenon up to 25 eV. The high-resolution (1 meV) spectra that result show the formation of satellite states clearly resolved from their neighbors. Their intensities are comparable to that of the ${5s5p}^6{}^2{S}_{1/2}$ main line ionic state and for a given multiplet are relatively independent of the total angular momentum of the ionic state formed. The Rydberg states converging onto different ionic limits demonstrate effective lifetimes that are related to the excitation of the associated ionic core. These spectra allow the determination of relative intensities, and from them, the partial cross sections for the formation of each state at threshold. This supplements the existing intensity studies from the near-threshold to the x-ray region and increases our understanding of the dominant correlation processes within this atom as a function of photon energy.

We analyze the influence of three pulsed laser fields interacting with a four-state atomic system for which there occur two degenerate population-trapping dressed states. We present a simple expression for the geometrical phase acquired by such an atom during a stimulated Raman adiabatic passage (STIRAP) process using a delay between Stokes and pump pulses, during which the two Rabi frequencies adiabatically traverse a closed path in the parameter space. We describe techniques that can produce (geometrical) state-vector phases that are independent of the longitudial velocity with which atoms move across a laser beam, and that are insensitive to radiative decay from the intermediate excited state. The geometrical phase can be changed by changing the relative delay of the pulses. The geometrical phase can be measured by observing the population in atomic states. We show that when the pump and Stokes pulses are properly timed, high population transfer from the initial state to the target state is achieved. In the adiabatic limit the robustness of the population transfer is equivalent to that of STIRAP.

We propose a procedure that allows one to create arbitrary, discrete superpositions of coherent states of the quantized center-of-mass motion of a trapped ion along a line in phase space. In view of the overcompleteness of the coherent states, such a method can be used for the engineering of motional quantum states.

We present experimental and theoretical results of a detailed study of laser-induced continuum structures (LICS’s) in the photoionization continuum of helium out of the metastable state $2s{}^1{S}_0.$ The continuum dressing, with a 1064-nm laser, couples the same region of the continuum to the $4s{}^1{S}_0$ state. The experimental data, presented for a range of intensities, show pronounced ionization suppression (by as much as 70% with respect to the far-from-resonance value) as well as enhancement, in a Beutler-Fano resonance profile. This ionization suppression is a clear indication of population trapping mediated by coupling to a continuum. We present experimental results demonstrating the effect of pulse delay upon the LICS’s, and for the behavior of LICS’s for both weak and strong probe pulses. Simulations based upon numerical solution of the Schrödinger equation model the experimental results. The atomic parameters (Rabi frequencies and Stark shifts) are calculated using a simple model-potential method for the computation of the needed wave functions. The simulations of the LICS profiles are in excellent agreement with experiment. We also present an analytic formulation of pulsed LICS’s. We show that in the case of a probe pulse shorter than the dressing one, the LICS profile is the convolution of the power spectra of the probe pulse with the usual Fano profile of stationary LICS’s. We discuss some consequences of deviation from steady-state theory.

We measured constant-scaled-energy $M=0\mathrm{}$ spectra of helium Rydberg atoms $(n=55\mathrm{}$ to 80) in an electric field, allowing for a test of closed-orbit theory under conditions where bifurcations from the uphill and downhill orbits are important. Recent advances in the theory (uniform approximation) were implemented for a comparison with experimental data. The observed recurrence spectra could be explained in detail as far as the actions of all hydrogenic orbits and the intensities of most recurrences are concerned. Additional peaks in the recurrence spectra that correspond to orbits with an action equal to the sum of two hydrogenic orbits were attributed to scattering at the ${\mathrm{He}}^{+}$ core. This was confirmed in a closed-orbit calculation including the quantum defects in helium. Small changes in the recurrence strength of the hydrogenic orbits were found both in experimental recurrence spectra and in the closed-orbit calculation for helium, and could be ascribed to the effect of core shadowing.

We compare the semiclassical Hartree-Fock approximation for a trapped Bose gas to a direct path-integral quantum Monte Carlo simulation. The chosen parameters correspond to current ${}^{87}\mathrm{Rb}$ experiments. We observe corrections to the mean-field density profile: the path-integral calculation reveals an increase of the number of condensed particles, which is of the same order as a previously computed result for a homogeneous system. We discuss the experimental observability of the effect and propose a method to analyze data of in situ experiments.

The regions of validity of the Markov approximation for the coupling of atoms out of an atomic trap are determined. We consider radio-frequency output coupling in the presence of gravity and collisional repulsion, and Raman output coupling. The Markov approximation is crucial in most theoretical descriptions of an atom laser that assume a continuous process of output coupling from a trapped Bose-Einstein condensate. In this regime many techniques that have proved to be useful for modeling the optical laser, such as master equations, can be used to describe the dynamics of the damping of the condensate mode undergoing output coupling.

We present a numerical study of mixed boson-fermion systems at zero temperature in isotropic and anisotropic harmonic traps. We investigate the phenomenon of component separation as a function of the strength of the interparticle interaction. While solving a Gross-Pitaevskii mean-field equation for the boson distribution in the trap, we utilize two different methods to extract the density profile of the fermion component; a semiclassical Thomas-Fermi approximation and a quantum-mechanical Slater determinant Schrödinger equation.

We study the interaction of a Bose-Einstein condensate in an optical lattice with an additional largely detuned light field propagating through it. If the condensate is in its ground state it acts as a periodic dielectric, and gives rise to photonic band gaps at optical frequencies. The band structure of the combined system of condensed lattice atoms and photons is studied by using the concept of polaritons. If elementary excitations of the condensate are present, they will produce defect states inside the photonic band gaps. The frequency of localized defect states is calculated using the Koster-Slater model.

We study the Bose-Einstein condensate (BEC) in a three-dimensional toroidal sombrero-shaped trap. By changing the parameters of the potential, or the number of bosons, it is possible to modify strongly the density profile of the BEC. We consider the ground-state properties for positive and negative scattering length and calculate the spectrum elementary excitations. We also discuss the macroscopic phase coherence and superfluidity of the BEC by analyzing vortex states and their stability.

The interaction is considered between a two-level atom and the continuum of electric field modes falling within the profile of one of the resonance modes of a dielectric microsphere (a whispering gallery mode). Subject to minimal assumptions, simple equations are obtained on first principles, which describe the spectral properties of the one-photon continuum and the relaxation processes occurring therein. If the initial state of the atom is excited, a doublet structure is formed in the emitted photon spectrum, provided that the interaction is strong enough. If it is the microsphere that is excited at the initial instant, the excited photon spectrum depends substantially on the excitation method used. When excitation is optimal, the atom is effectively excited, and a Rabi-doublet is then formed in the fluorescence spectrum. When the excitation conditions are other than optimal, the spectrum becomes of triplet character, and when the deviation from the optimal excitation conditions is strong enough, the atom practically fails to get excited, and the fluorescence spectrum is of singlet type. A considerable part of the results obtained is of a general nature and can easily be applied to other cases of strong interaction between atoms and resonators.

Spontaneous emission can create coherences in a multilevel atom having close-lying levels, subject to the condition that the atomic dipole matrix elements are nonorthogonal. This condition is rarely met in atomic systems. We report the possibility of bypassing this condition and thereby creating coherences by letting the atom with orthogonal dipoles interact with the vacuum of a preselected polarized cavity mode rather than the free-space vacuum. We derive a master equation for the reduced density operator of a model four-level atomic system, and obtain its analytical solution to describe the interference effects. We report the quantum beat structure in the populations.

We study the dynamics of a vertical-cavity surface-emitting laser operating near threshold and with isotropic optical feedback, using a model developed by San Miguel, Feng, and Moloney [Phys. Rev. A 52, 1728 (1995)]. The model couples the polarization state of the electric field to the semiconductor medium by including the magnetic sublevels of the conduction and valence bands in the quantum wells. The laser dynamics depend significantly on the value of the relaxation rate, ${\gamma }_s,$ of the material magnetization. For low relaxation rates the time-averaged intensity abruptly drops to zero and then recovers, a phenomenon revealed to be a sequence of picosecond pulses. The dropouts are similar to those occurring in conventional semiconductor lasers, but underlying the dropouts there is an antiphase competition between the time-averaged orthogonal linearly polarized components of the electric field. For large values of the relaxation rate, the dropouts tend to disappear and the time-averaged intensity is nearly constant.

We present a method to solve exactly a one-dimensional model of quantized radiations in a cavity oscillating in the fundamental resonance, using the effective Hamiltonian derived by C. K. Law [Phys. Rev. A 49, 433 (1994)]. With this method, we derived explicit analytical expressions for the diagonalized Hamiltonian, the time-varying annihilation, creation, and photon number operators for the radiation field, which completely specify the dynamics of the system.

We propose an alternative method to Q switching for generating giant pulses of laser light in three-level media. This mehod is based on the presence of an external coherent field driving one transition to allow the accumulation of a large population inversion in the other transition without laser oscillation even in a cavity with high-$Q$ factor. The switching off of the external coherent field causes the development of the giant pulse. Different time profiles for the switching off of the external field have been investigated.

We suggest a scheme to reconstruct a two-mode entangled state in cavity QED by using the interaction of a V-configuration three-level atom and by driving the cavity field. After the atomic interaction with the cavity fields, the probability of the atom being in its initial ground state is found to be directly related to the two-mode Wigner characteristic function. The Wigner function and the four-dimensional density-matrix elements can be obtained for the two-mode entangled field by simple transformations. We consider both the cases where two entangled modes are prepared in one cavity or in two spatially separated cavities.

We study the creation of photons in a one-dimensional oscillating cavity with two perfectly conducting moving walls. By means of a conformal transformation, we derive a set of generalized Moore’s equations whose solution contains the whole information of the radiation field within the cavity. For the case of resonant oscillations we solve these equations using a renormalization-group procedure that appropriately deals with the secular behavior present in a naive perturbative approach. We study the time evolution of the energy density profile and of the number of created photons inside the cavity.

Propagation of a pair of ultrashort coherent pulses in a V-type medium is studied. Both pulses are supposed to be resonant with corresponding atomic transitions. It is shown that the resonant character of the atom-field interaction yields “pulse locking,” in which two pulses propagate with equal group velocities. Additionally, in the presence of Raman inversion, the system exhibits resonantly enhanced gain for the probe pulse. These two effects add together to decrease the pump laser threshold intensity by orders of magnitude in comparison with conventional (far-off-resonant) Raman laser schemes.