Kazimierz Rza̧żewski

Bose-Einstein condensation with magnetic dipole-dipole forces

Since the advent of Bose-Einstein condensation in dilute gases of alkali metals @1# and hydrogen @2#, it has become apparent that the interactions between the condensed atoms govern most of the observed phenomena. In all the experiments so far the interaction can be described by a contact potential that is characterized by the scalar quantity a being the s-wave scattering length. Static properties like the condensate’s ground-state density profile, its instability in the case of negative a, and the mean-field shift in spectroscopic measurements as well as dynamic properties like collective excitations and propagation of sound have been investigated

Generation of attosecond pulse trains during the reflection of a very intense laser on a solid surface

The normal-incidence reflectivity of a plasma generated on a solid surface by a very intense laser field is studied. We demonstrate that the reflected field has the form of trains of subfemtosecond pulses under suitable conditions. Particle-in-cell computations are presented and compared with a naive moving-mirror model. The physical origin of this behavior is discussed in terms of the relativistic motion of the electron plasma.

Quantum anticentrifugal force

In a two-dimensional world, a free quantum particle of vanishing angular momentum experiences an attractive force. This force originates from a modification of the classical centrifugal force due to the wave nature of the particle. For positive energies the quantum anticentrifugal force manifests itself in a bunching of the nodes of the energy wave functions towards the origin. For negative energies this force is sufficient to create a bound state in a two-dimensional \ensuremath{\delta}-function potential. In a counterintuitive way, the attractive force pushes the particle away from the location of the \ensuremath{\delta}-function potential. As a consequence, the particle is localized in a band-shaped domain around the origin.

Semiclassical theory of trapped fermionic dipoles

We investigate the properties of a degenerate dilute gas of neutral fermionic particles in a harmonic trap that interact via dipole-dipole forces. We employ the semiclassical Thomas-Fermi method and discuss the Dirac correction to the interaction energy. A nearly analytic as well as an exact numerical minimization of the Thomas-Fermi-Dirac energy functional are performed in order to obtain the density distribution. We determine the stability of the system as a function of the interaction strength, the particle number, and the trap geometry. We find that there are interaction strengths and particle numbers for which the gas cannot be trapped stably in a spherically symmetric trap, but both prolate and oblate traps will work successfully.

Second harmonic generation and statistical properties of light

Abstract A simple model of second harmonic generation is solved for the fundamental frequency light of various statistical properties. It is found that within the class considered, a field in the state of squeezed vacuum generates the second harmonic most efficiently. In particular, for short times, this efficiency is some three times greater than that for coherent light.

Stimulated Raman scattering of colored chaotic light

The quantum theory of forward Stokes generation by means of stimulated Raman scattering is extended to the case of a colored (finite-bandwidth) chaotic pump, which has both amplitude and phase fluctuations. In both transient and steady-state limits the mean Stokes intensity is found to be enhanced over that resulting from a coherent pump. In the limit that the chaotic-pump bandwidth becomes large, the mean Stokes intensity becomes identical with that resulting from a coherent pump of the same total power.

Thermodynamics of an interacting trapped Bose-Einstein gas in the classical field approximation

We present a convenient technique describing the condensate in dynamical equilibrium with the thermal cloud, at temperatures close to the critical one. We show that the whole isolated system may be viewed as a single classical field undergoing nonlinear dynamics leading to a steady state. In our procedure it is the observation process and the finite detection time that allow for splitting the system into the condensate and the thermal cloud.

Theory of fluorescence spectra induced by short laser pulses

We consider a single-photon bound–bound and bound–free transitions induced by short, coherent laser pulses. Fluorescence energy spectra are calculated. In the case of the fluorescence to a side level, a number of analytic results are presented. For the resonance-fluorescence case the compact Bloch-like equations are derived for the ionization to an autoionization resonance and numerically analyzed. The multipeak structure of the spectra previously found in the simplified models is shown to be a universal feature. The number of maxima of the spectrum increases with the area of the pulse.

Probing the statistical properties of Bose-Einstein condensates with light

A scattering of short, weak, nonresonant laser pulses on a Bose-Einstein condensate is proposed as a tool for studying its statistical properties. We show in particular that the variance of the number of scattered photons may distinguish between the Poisson and microcanonical statistics.

Hydrodynamic excitations of trapped dipolar fermions

A single-component Fermi gas of polarized dipolar particles in a harmonic trap can undergo a mechanical collapse due to the attractive part of the dipole-dipole interaction. This phenomenon can be conveniently manipulated by the shape of the external trapping potential. We investigate the signatures of the instability by studying the spectrum of low-lying collective excitations of the system in the hydrodynamic regime. To this end, we employ a time-dependent variational method as well as exact numerical solutions of the hydrodynamic equations of the system.