Juha Javanainen

Effect of State Superpositions Created by Spontaneous Emission on Laser-Driven Transitions

Spontaneous decay of an excited atomic state may give rise to a coherent superposition of two recipient states. Experiments utilizing laser-driven three-level ? systems are devised whose outcomes differ qualitatively depending on the presence or absence of such a spontaneous generation of coherence.

Above-threshold ionization

Abstract Above-Threshold Ionization (ATI) is the name given to the absorption by an atomic electron of more photons than are required for ionization. ATI was discovered in 1979 and is now expected to be a universal phenomenon observable in all atomic species. The effect occurs readily in light pulses short enough to eliminate the influence of collisions, at intensities above 1 TW/cm 2 .

Quantum and semiclassical steady states of a kicked cavity mode

We investigate classically and quantum-mechanically the coherent interaction between a single mode of the electromagnetic field and a stream of two-level atoms under the assumptions that only one atom at a time is coupled to the field and that the interaction times are all equal. This is a quantum-optics analog of a coherently kicked harmonic oscillator. We find that the classical system always evolves toward a marginally stable steady state at the threshold of type-1 intermittency, independently of the initial state of inversion of the atoms. But there are infinitely many such steady states, and which one is reached may depend sensitively on the initial conditions. In contrast, in the case of inverted atoms the quantized system usually does not reach a steady state: The intrinsic quantum fluctuations of the field almost always force it eventually to grow past the classical fixed points. A notable exception occurs under conditions such that the sequence of inverted atoms injected into the cavity leads to the preparation of a highly excited Fock state of the cavity mode.

The microscopic maser

Abstract We present the theory of a truly microscopic maser, and show that it exhibits novel features not associated with masers and lasers generally. In particular, it usually does not produce coherent radiation. The coherence of the field in conventional masers and lasers originates from incoherent averages which smear out quantum details of the field-matter interaction.

Bose-stimulated raman adiabatic passage in photoassociation

We analyze coherent two-color photoassociation of a Bose-Einstein condensate, focusing on stimulated Raman adiabatic passage (STIRAP) in free-bound-bound transitions from atoms to molecules. This problem raises an interest because STIRAP has been predicted to be absent in the nondegenerate case [Javanainen and Mackie, Phys. Rev. A 58, R789 (1998)]. Nevertheless, we find that Bose stimulation enhances the free-bound dipole matrix element for an atomic condensate, and photoassociative STIRAP turns out to be a viable mechanism for converting an atomic condensate to a molecular condensate with near-unit efficiency.

Particle number fractionalization of an atomic Fermi-Dirac gas in an optical lattice

We show that a dilute two-species gas of Fermi-Dirac alkali-metal atoms in a periodic optical lattice may exhibit fractionalization of the particle number when the two components are coupled via a coherent electromagnetic field with a topologically nontrivial phase profile. This results in fractional eigenvalues of the spin operator with vanishing fluctuations. The fractional part can be accurately controlled by modifying the effective detuning of the electromagnetic field.

Measurements of temperature and spring constant in a magneto-optical trap

We have measured the low-density characteristics of a magneto-optical trap for both isotopes of rubidium. Trapped-atom temperatures are measured by the time-of-flight technique and are similar to temperatures in optical molasses. They are below the Doppler limit in most cases and increase linearly with the ratio of laser intensity to laser detuning. A slight difference between the two isotopes is observed. The data agree well with three-dimensional, multistate, semiclassical simulations. Spring constants of the trap are inferred from careful measurements of the size of the trapped-atom cloud. The spring constant is seen to increase with intensity at low intensity, becoming independent of intensity at high intensity. This trend is consistent with sub-Doppler cooling mechanisms.

Broad band resonant light pressure

This paper treats the mechanical pressure of resonant light acting on a two-level system, where the degenerate magnetic sublevels are taken into account. The theory is developed with arbitrary relations between the quantization axis and the propagation and polarization of the light. Rate terms are obtained both for spontaneous and induced transitions; the requirements of incoherence put restrictions on the possible geometries of the experiment. The rate equations are restricted to motion along a light beam only; this one-dimensional case is simpler to handle. For small recoil velocities a Fokker-Planck equation is derived, and an adiabatic elimination procedure enables us to derive an equation for the velocity distribution of the total population. The assumptions and approximations are formulated and discussed.

High-order harmonic production in multiphoton ionization

We report the production of high-order optical harmonics in computer experiments on multiphoton ionization at high laser intensity. We correlate features of the scattered light spectra with above-threshold-ionization electron spectra from the same laser pulse in a number of experiments at different wavelengths and laser intensities.

Shifts of a resonance line in a dense atomic sample

We study the collective response of a dense atomic sample to light essentially exactly using classical-electrodynamics simulations. In a homogeneously broadened atomic sample there is no overt Lorentz-Lorenz local field shift of the resonance, nor a collective Lamb shift. However, the addition of inhomogeneous broadening restores the usual mean-field phenomenology.