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Physical Review A

Effect of spontaneous emission on the atomic modes in a stable gravitational cavity

G. J. Liston, S. M. Tan, Daniel F. Walls

Published Oct 1, 1995• 4 citations

Abstract

We calculate the modes in a stable gravitational cavity where the atomic mirror at the bottom of the bounce is caused by an internally reflected intense Gaussian-mode laser beam. In contrast to Wallis, Dalibard, and Cohen-Tannoudji [Appl. Phys. B 54, 407 (1992)], who treated the bottom bouncing mechanism as due to an infinite potential, we include the effect of the exponentially decaying potential caused by the evanescent wave. Nevertheless, we find that above this evanescent wave potential the mode structure corresponds exactly to that obtained using an infinite potential. The allowed energies differ slightly due to the phase change through the evanescent wave. The broadening of the modes due to the damping effects of spontaneous emission was calculated and because of the intrinsic transverse width of the modes was found to depend strongly on energy and transverse excitation, which differs from a simple one-dimensional classical calculation. The finesse and $Q$ of the cavity due to the damping effects of spontaneous emission were also calculated and in contrast to an optical Fabry-P\'erot cavity were found to depend strongly on energy.

DOI: 10.1103/physreva.52.3057

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We calculate the modes in a stable gravitational cavity where the atomic mirror at the bottom of the bounce is caused by an internally reflected intense Gaussian-mode laser beam. In contrast to Wallis, Dalibard, and Cohen-Tannoudji [Appl. Phys. B 54, 407 (1992)], who treated the bottom bouncing mechanism as due to an infinite potential, we include the effect of the exponentially decaying potential caused by the evanescent wave. Nevertheless, we find that above this evanescent wave potential the mode structure corresponds exactly to that obtained using an infinite potential. The allowed energies differ slightly due to the phase change through the evanescent wave. The broadening of the modes due to the damping effects of spontaneous emission was calculated and because of the intrinsic transverse width of the modes was found to depend strongly on energy and transverse excitation, which differs from a simple one-dimensional classical calculation. The finesse and

Q

of the cavity due to the damping effects of spontaneous emission were also calculated and in contrast to an optical Fabry-P'erot cavity were found to depend strongly on energy.

A curved mirror for atoms was made from an evanescent wave, formed by internal reflection of a quasiresonant laser beam at a curved glass surface. A cold cloud of cesium atoms was dropped onto the mirror and observed to rebound more than 8 times. The mirror size and reflectivity were studied, and reasonable agreement with a simple theory obtained. With 800 mW of laser power and a mirror of 1 mm diameter, we observed up to 73% of the atoms returning after each bounce, the losses being mostly during the free flight between bounces.

We show that the spontaneous emission of a moving atom inside a cavity is modified by two different mechanisms. The first one is a Doppler-induced detuning of the cavity line that leads to observable changes in the spontaneous emission rate for atomic velocities in excess of a characteristic velocity

{\mathit{v}}_{\mathrm{cav}}

=c/Q, Q being the quality factor of the cavity. In addition, spatial mode effects can lead to a temporal modulation of the exponential decay rate. This modulation is observable for velocities that do not exceed another characteristic velocity

{\mathit{v}}_{\mathrm{mod}}

which is typically several orders of magnitude smaller than

{\mathit{v}}_{\mathrm{cav}}

.

In this paper we consider the effects of spontaneous emission on the problem of atomic reflection from an optical evanescent travelling wave. The atoms are assumed to be travelling so slowly that the external degrees of freedom of the atom need to be treated quantum mechanically. Spontaneous emission is modelled using a fully quantum mechanical model based on the quantum Monte Carlo technique which includes the effects of atomic recoil. The results of the simulation are compared with a semi-classical Monte Carlo simulation in which the atomic motion is treated classically and the internal state of the atom is assumed to follow the light-induced quasipotentials adiabatically between spontaneous emission events. The spontaneous emission leads to a broadening of the momentum distribution of the outgoing wave packet and mechanisms for this broadening are discussed.

We revisit the problem of spontaneous emission by a two-level system in a cavity, concentrating in particular on those aspects associated with the finite propagation time of light between the atom and the cavity boundaries. We find that spontaneous emission always occurs first at the free space rate and there is then an abrupt change in these dynamics when light reflected by the cavity mirrors returns to the atom, leading to interference effects that result either in an enhancement or a reduction of this rate or in a periodic exchange of excitation between the atom and the cavity field. From this point of view, reversible spontaneous emission in terms of single-mode ``vacuum Rabi oscillations'' can be interpreted as resulting from the interference between the multimode partial waves reflected at the cavity mirrors and the instantaneously radiated light. \textcopyright{} 1996 The American Physical Society.

Spontaneous emission in a one-dimensional Fabry-P\'erot-type optical cavity is analyzed quantum mechanically. Via a monolayer dielectric cavity model, expressions describing the atomic-location dependence of the emission intensities and the spectra observed on both sides of the cavity are obtained. The probability amplitude for the upper atomic state is analyzed and its time-evolution equation is given. The probability amplitude is shown to be proportional to ``the electric field amplitude for a single emission event'' introduced classically by Huang et al. [Appl. Phys. Lett. 61, 2961 (1992)]. An expression for the emission spectrum applicable to a general one-dimensional cavity structure is obtained.

We present two simple soluble quantum optical models in which an atomic system is coupled to a bath containing an infinite number of reservoir states. The total system is treated conservatively, but the atomic subsystem of interest exhibits decay. The first system consists of a single two-level atom coupled to an infinite number of modes of a one-dimensional cavity which are equally spaced in frequency. The coupling results in spontaneous emission by an excited atom into the modes of the cavity. The model also exhibits the subsequent re-excitation of the atom after the spontaneously emitted radiation is reflected from the ends of the cavity. The second is a single two-level atom coupled to a single mode of a damped optical cavity where the damping is provided by coupling to quantized oscillators in the walls of the cavity. The damped cavity alters the spontaneous emission rate from its free space value. The simplicity of both models makes them attractive as a method for introducing quantum mechanical damping phenomena.