Dressed-atom Approach Research Articles

Experimental Observation of the Autler-Townes Splitting in Polyatomic Molecules.

Autler-Townes (AT) splitting has been experimentally observed in the optical transition between the zero-point levels of S1 and S0 for supersonically cooled 2-methoxythiophenol, 2-fluorothiophenol, and 2-chlorothiophenol. This is the first experimental observation of the light-dressed quantum states of polyatomic molecules (N > 3) in the electronic transition. In the resonance-enhanced ionization process involving the optically coupled states, if Rabi cycling is ensured within the nanosecond laser pulse, AT splitting is clearly observed for the open system for which the excited-state lifetime is shorter than hundreds of picoseconds. Semiclassical optical Bloch equations and a dressed-atom approach based on the three-level atomic model describe the experiment quite well, giving deep insights into the light-matter interaction in polyatomic molecular systems.

SU(4) based classification of four-level systems and their semiclassical solution

We present a systematic method to classify the four-level system using SU(4) symmetry as the basis group. It is shown that this symmetry allows three dipole transitions which eventually leads to six possible configurations of the four-level system. Using a dressed atom approach, the semi-classical version of each configuration is exactly solved under rotating wave approximation and the symmetry among the Rabi oscillation among various models is studied.

Cascade splitting of two atomic energy levels due to multiphoton absorption

We have theoretically and experimentally studied the spectroscopic properties of dressed levels in a strong monochromatic field, and propose a model of cascade splitting of two atomic energy levels. In this model two related dressed levels can be split into four levels, and transitions connecting four new levels will constitute spectroscopic structures. Two types of proof-in-principle experiments are performed to verify the model. One experiment measures the probe absorption spectra of a degenerate two-level atomic system with two strong monochromatic coupling fields. The system consists of ${5}^{2}{S}_{1/2},F=2$ and ${5}^{2}{P}_{3/2},{F}^{\ensuremath{'}}=3$ states of $^{87}\mathrm{Rb}$ atoms in a magneto-optical trap (MOT) as well as the cooling beams and an additional coupling field. New spectral features are observed and proven to be due to the transitions of new levels generated by splitting of the dressed levels. The other experiment measures the pump-probe spectra in a degenerate two-level atomic system with one strong monochromatic coupling field. The system consists of ${5}^{2}{S}_{1/2},F=2$ and ${5}^{2}{P}_{3/2},{F}^{\ensuremath{'}}=3$ states of the $^{87}\mathrm{Rb}$ atom in a magneto-optical trap and one coupling field. We have observed spectral features that obviously differ from the prediction that comes from the two-level dressed-atom approach. They cannot be explained by existing theories. The model of cascade splitting of two atomic energy levels is employed to explain the observations in these two types of experiments.

Properties of the spontaneous emission of a driven four-level atom coupled to separate reservoirs

We investigate the spontaneous emission properties of a driven four-level atom coupled to separate reservoirs. Both the effect of the external field and the influence of the photonic bandgap reservoir on the atomic spontaneous emission are discussed. It is shown that a few interesting phenomena such as a laser-induced spectral narrowing dark line can be achieved and its physical understanding can be given by invoking the dressedatom approach.

Atomic (or molecular) guiding using a blue-detuned doughnut mode in a hollow metallic waveguide

We propose a new scheme to guide cold atoms (or molecules) using a blue-detuned TE(01) doughnut mode in a hollow metallic waveguide (HMW), and analyze the electromagnetic field distributions of various modes in the HMW. We calculate the optical potentials of the TE(01) doughnut mode for three-level atoms using dressed-atom approach, and find that the optical potential of the TE(01) mode is high enough to guide cold atoms released from a standard magneto-optical trap. Our study shows that when the input laser power is 0.5W and its detuning is 3GHz, the guiding efficiency of cold atoms in the straight HMW with a hollow radius of 15 microm can reach 98%, and this guiding efficiency will be almost unchanged with the change of curvature radius R of the bent HMW as R > 2cm, which is a desirable scheme to do some atom-optics experiments or realize a computer-controlled atom lithography with an arbitrary pattern. We also analyze the losses of the guided atoms in the HMW due to the spontaneous emission and background thermal collisions and briefly discuss some potential applications of our guiding scheme in atom and molecule optics.

Trapping of Neutral Atoms with a Radio-Frequency Field

Based on the dressed-atom approach, we discuss atwo-dimensional (2D) radio-frequency trap for neutral atoms, in whichthe trap potential derives from the magnetic-dipole transitions amongthe hyperfine Zeeman sublevels. By adjusting the detuning of theradiation from resonance, the trapping states will be changedpredominantly from the bare states of mFgF>0 to otherstates of mFgF<0, where mF and gF are thequantum numbers of Zeeman sublevels and the Landé factor, respectively.This character contrasts finely with that of a static magnetic trapthat can only trap or guide the states of mFgF>0. Incomparison to the optical field, the radio-frequency trap eliminatesthe spontaneous emission heating of the atoms. Unlike other oscillatingtraps reported in the literature, the configuration of theradio-frequency trap is suitable for realization of a miniature magneticguide.

Laser effects on donor states in low-dimensional semiconductor heterostructures

A theoretical study of the effects of intense laser fields on the ground-state binding energies of donor impurities in low-dimensional semiconductor heterostructures is performed. The laser-heterostructure interaction is treated within an extended dressed-atom approach, so that, for a laser tuned far below any resonances, the effects of the laser-semiconductor interaction correspond to a renormalization of the semiconductor energy gap and conduction/valence effective masses. Calculations are performed for donors in $\mathrm{GaAs}\text{\ensuremath{-}}(\mathrm{Ga},\mathrm{Al})\mathrm{As}$ quantum wells, cylindrical quantum-well wires, and spherical quantum dots. The binding energies of donors in low-dimensional systems increase with increasing laser intensity, and for a fixed intensity, the influence of the laser is stronger for small detunings. Results obtained within the extended dressed-atom approach are compared with previous calculations performed by using a simplified high-frequency limit of the Kramers-Henneberger approach.

Laser effects in semiconductor heterostructures within an extended dressed-atom approach

We extend the dressed-atom approach to treat the interaction of a laser field with a semiconductor system. The semiconductor is modeled via a simple Kane band-structure scheme and the interaction with the laser field is incorporate through the renormalization of the semiconductor energy gap and conduction/valence effective masses. Far from resonances, such one-body approach allows the study of the effects of laser fields on a variety of optoelectronic phenomena in semiconductor systems for which the effective-mass approximation provides a good physical description. We calculate the effects originated by the laser-dressing on the donor and exciton peack energies in quantum-well heterostructures, and show that they may be quite considerable and observable.

A Dressed-Atom Approach to Laser Effects in Excitons and Shallow Impurities in Low-Dimensional Semiconductor Heterostructures

The dressed-band approach is used to treat the interaction of a laser field with semiconductor GaAs–(Ga,Al)As quantum wells and quantum dots. We adopt the Kane model and a renormalized-mass approximation. We calculate the effects originated by the laser-dressing on exciton energies in quantum-wells and on donor states in quantum wells and quantum dots. Present results suggest that the strong localization of the electronic and impurity states due to the quantum well and quantum dot and enhanced by laser confinement may result useful for manipulation of electronic and donor states in some proposed solid-state based quantum computers.

Dressed-band approach to laser-field effects in semiconductors and quantum-confined heterostructures

A theoretical study of the effects of a laser field on the electronic and optical properties of GaAs-(Ga,Al)As heterostructures is presented by using a Kane model for the GaAs bulk semiconductor and working within an extended dressed-atom approach. For a laser tuned far below any resonances, the effects of the laser-semiconductor interaction correspond to a renormalization of the semiconductor energy gap and conduction/valence effective masses. This renormalized one-body approach may be used to give a qualitative indication of the laser effects on a variety of optoelectronic phenomena in semiconductor heterostructures for which the effective-mass approximation provides a good physical description. As a test, the exciton Stark shift in quantum wells is calculated and the effects due to the band-structure laser dressing are found to be of the same order of magnitude as those obtained from many-body diagrammatic techniques. We have also analyzed the effects of laser dressing on the shallow-donor peak energies in quantum wells, and found them comparable with those produced by a magnetic field of a few teslas.

Cavity QED analog of the harmonic-oscillator probability distribution function and quantum collapses

We establish a connection between the simple harmonic oscillator and a two-level atom interacting with resonant, quantized cavity and strong driving fields, which suggests an experiment to measure the harmonic-oscillator's probability distribution function. To achieve this, we calculate the Autler-Townes spectrum by coupling the system to a third level. We find that there are two different regions of the atomic dynamics depending on the ratio of the: Rabi frequency Omega (c) of the cavity field to that of the Rabi frequency Omega of the driving field. For Omega (c) and moderate coupling of the transition to the cavity mode the spectral peaks are composed of multiplets. A quantized dressed-atom approach provides a simple explanation of the spectral features and shows that the oscillations in the spectral components arise from the oscillations of the population distribution in the dressed states. The observation of these features would provide evidence for the quantum nature of the cavity field. The distribution is an analog of the harmonic-oscillator probability distribution function, and should be experimentally observable. For Omega (c)greater than or equal to Omega there is no Autler-Townes splitting and the spectrum is composed of a single peak located at the frequency of the probe transition. We show that this effect results from the collapse of the atom to the ground state, which has been predicted by Alsing, Cardimona, and Carmichael [Phys. Rev. A 45, 1793 (1992)] for a two-level atom in a lossless cavity.

Laser effects in semiconductor heterostructures within an extended dressed-atom approach

We extend the dressed-atom approach to treat the interaction of a laser field with a semiconductor system. The semiconductor is modeled via a simple two-band isotropic scheme and the interaction with the laser field is incorporated through the renormalization of the semiconductor energy gap and valence and conduction effective masses. We calculate the effects originated by the laser dressing on the donor and exciton peak energies in quantum-well heterostructures and show that the laser dressing may be quite considerable and readily observable.

Theory of atom guidance in a hollow laser beam: dressed-atom approach

We present a general theory of atom guiding in a blue-detuned hollow laser beam. Using the dressed-atom approach, we obtain the mean dipole gradient force, the radiation pressure force, and the momentum diffusion coefficients for three-level Λ-type cold atoms. Using Monte Carlo simulation, we calculate the guiding efficiencies and the final velocity distributions of atoms for various conditions. We find that the guiding efficiency depends not only on the intensity and detuning of the guiding hollow beam but also on the atom-guiding direction with respect to the propagation direction of the hollow laser beam. Comparing our analyses with recent experimental results, we find that they are mutually consistent. The results that we present can also be applied to atom guiding by hollow optical fibers.

Semiclassical dressed-atom approach

A semiclassical approach to the dressed-atom theory is presentedin which dressed energies and states are determined from thesolutions of the semiclassical coupled equations for the atomicprobability amplitudes. The semiclassical approach has anadvantage over the standard quantum-mechanical approach,especially when an atom interacting with a nonmonochromatic fieldis considered, because the standard approach requiresdiagonalization of an infinite-dimensional Hamiltonian matrix foran evaluation of dressed energies and states. Explicitexpressions for dressed states of a two-level atom interactingwith a resonant monochromatic, bichromatic, and trichromaticfield are given.

Laser dressing of the electronic bands in semiconductors

We discuss the validity of the renormalized effective mass approximation to treat the interaction of a laser field with a semiconductor, in a one-body approximation. The electronic band structure of the semiconductor is modeled by two-parabolic, isotropic bands in the k → · p → approximation. To incorporate the laser field into an effective mass formalism (renormalized effective mass approximation) we apply the dressed-atom approach to the two-band model (dressed-band approximation), the eigenvalue problem for the dressed bands is solved analytically and a k expansion is performed. The validity of the renormalized mass approximation is studied, comparing the results for the quasi-energy spectrum and for the effective mass obtained from the renormalized mass approximation, the dressed-band approximation and the “exact” results derived from the diagonalization of the Floquet matrix. We show that for the laser intensities which will not damage the semiconductor samples in experiments, the agreement is excellent.

Theoretical analysis of resonances in the polarization spectrum of a two-level atom driven by a polychromatic field

Analytic solutions of the optical Bloch equations for a two-level atom interacting with a strong polychromatic field whose frequencies are symmetrically positioned with respect to the atomic frequency are used to obtain the polarization spectrum of the atom. The spectrum is found to consist of a series of discrete peaks or dips superimposed on the continuous part of the spectrum. Physical interpretation of resonances exhibited in the continuous part of the spectrum is given using a semiclassical dressed-atom approach.

Interaction of a Laser Field with a Semiconductor System: Application to Shallow-Impurity Levels of Quantum Wells

We extend the dressed-atom approach to treat the interaction of a laser field with a semiconductor system in which the interaction with the laser is incorporated through the renormalization of the semiconductor effective mass. The semiconductor is modeled via a simple parabolic two-band isotropic scheme in the k · p approximation. We compare the effects of the laser dressing on the intradonor transitions in doped GaAs–(Ga,Al)As quantum wells with those produced by an applied external magnetic field. It is shown that these effects may be comparable and therefore experimentally observable. Such approach allows the study of the effects of laser fields on a variety of phenomena in semiconductor systems for which the effective-mass approximation provides a good physical description.

Bloch oscillations of atoms, adiabatic rapid passage, and monokinetic atomic beams

We study the dynamics of ultracold atoms in a periodic optical potential submitted to a constant external force. Bloch oscillations in the fundamental and first excited bands of the potential are observed. In addition to a solid-state analysis we give a quantum-optics interpretation of this effect in terms of photon exchanges between the atoms and the laser waves. The dynamics of Bloch oscillations are equivalent to a sequence of adiabatic rapid passages between momentum states and can be described using the dressed-atom approach. We demonstrate efficient and dissipation-free acceleration of atoms by coherent transfer of a large number of photon momenta (\ensuremath{\approx}100). This technique produces atomic beams with a very small longitudinal velocity spread that may find applications in atom optics and precision measurements.

Theory of fluorescence and probe absorption in the presence of a driving field using semiclassical dressed states.

A theory of resonance fluorescence in a monochromatic field and pump-probe spectroscopy is developed using semiclassical dressed states. The semiclassical dressed states are obtained via a rotation from the time-dependent state vectors in a field-interaction representation. Although the semiclassical dressed states are not stationary states as are the dressed states in a quantized-field formulation, they still offer all the conceptual advantages of a dressed-atom approach. Moreover, the line-shape formulas can be obtained directly from physically intuitive equations, without having to invoke the quantum regression theorem and without having to worry about cascades and coherences in the ladder of quantized dressed states. \textcopyright{} 1996 The American Physical Society.

Autler-Townes effect for an atom in a 100% amplitude-modulated laser field. I. A dressed-atom approach.

A dressed-state rate-equation model is used to describe the Autler-Townes effect that is produced by a resonant 100% amplitude-modulated field driving a two-level resonance. The purpose of using this model is to give an intuitive interpretation of the rather complex absorption spectra that are produced when a weak probe field couples this strongly driven transition to a third atomic state. We obtain analytic expressions for the positions, strengths, and widths of the components of the absorption spectra, which are in excellent agreement with experimentally obtained spectra presented in the accompanying paper [S. Papademetriou, following paper, Phys. Rev. A 53, 997 (1996)]. \textcopyright{} 1996 The American Physical Society.