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IEEE Journal of Quantum Electronics

Spontaneous excitation of atoms near a phase conjugating mirror

R.J. Cook, Peter W. Milonni

Published Jul 1, 1988• 19 citations
Publisher

Abstract

It is shown by means of a semiclassical theory that the ground state of a localized collection of two-level atoms is unstable when the atomic sample is sufficiently close to a phase-conjugating mirror with high gain. Any small disturbance causes the atoms to become excited and begin fluorescing, the fluorescence taking the form of a sequence of superradiant pulses. Experiments to test this prediction seem possible.>

DOI
10.1109/3.977

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Spontaneous Emission of Radiation in the Presence of a Phase-Conjugate Mirror

Erik J. Bochove

The spontaneous radiation of a classical oscillator and that of a two-level atom both in the vicinity of a phase-conjugate mirror are calculated and compared. In either case the phase-conjugate mirror may be replaced by a phase-conjugate "image" of the source. We study the dependence of the decay constants and the characteristic frequencies on the phase-conjugate reflectivity and the difference between the characteristic frequency of the radiator and the pump frequency of the phase-conjugate mirror.

Physical Review Letters•Nov 30, 1987•18 citations
It is shown by means of a semiclassical theory that the ground state of a localized collection of two-level atoms is unstable when the atomic sample is sufficiently close to a phase-conjugating mirror with high gain. Any small disturbance causes the atoms to become excited and begin fluorescing, the fluorescence taking the form of a sequence of superradiant pulses. Experiments to test this prediction seem possible.>
We present a quantum-electrodynamic theory of spontaneous emission in the presence of a phase-conjugating mirror (PCM) and show that the radiative relaxation rate is increased owing to the amplification of quantum noise by the PCM. For any finite value of the PCM reflectivity an atom in any initial state will relax to a steady state with a finite probability of being excited. In particular, there is a finite probability of spontaneous excitation of a ground-state atom placed near the PCM. The fluorescence spectrum of an atom near a PCM is generally described by two Lorentzian functions.
Fluorescent emission by an atom near a phase conjugator (PC) based on four-wave mixing is studied from first principles. The Maxwell-Heisenberg equations are solved for the radiation field, and with an asymptotic expansion an expression is derived for the field in the far zone. The total emitted power that can be measured by a detector in this region is evaluated, and it is found that this power acquires three distinct contributions. First, there are photons that are emitted by the atom directly towards the detector, and without any interaction with the medium. Second, there are photons that first travel towards the surface of the PC, and they have a certain probability of being reflected in the specular direction and towards the detector. The third kind of radiation consists of phase-conjugated photons, which are emitted independently of the previous ones. It is shown that the first two processes are a result of simple atomic spontaneous decay, but that the emission of a phase-conjugated fluorescent photon involves a three-photon process. The latter process has a probability proportional to the population of the atomic ground state. It is pointed out that an atom in its ground state polarizes the nonlinear medium of the PC, which subsequently can emit spontaneously two photons. An absorption-emission-absorption process by the atom then produces a fluorescent photon, together with a spontaneous excitation of the atom.
The properties of a phase conjugate mirror (PCM) interacting radiatively with other objects are investigated. The spontaneous emission rate of a two-level atom (TLA) is attributed to the dipole emission of the upper energy level into vacuum field modes incident on the front face of the PCM, the emission into vacuum modes transmitted from the rear port of the PCM, enhanced by a factor 1 + R (R = reflectivity), and the emission of a photon while the atom is in the lower level, accompanied by annihilation of two pump photons, weighted by a factor of R per emitted photon. These processes lead to an enhancement factor of 1 + R for the spontaneous decay rate and a steady state upper level occupation probability R/(2R + 2), both in agreement with solution of the Heisenberg equations of TLA operators.1 The momentum transfer is zero, and the steady state emission rate is A12R(R + 2)/(2R + 2) photons/s atom. The stimulated emission and absorption constants are enhanced in a similar way. Contrary to the case of a conventional mirror, the Lippman fringes are present only for coherent radiation states.
In the presence of a high-finesse cavity, the resonance fluorescence spectrum can be altered by stimulated emission and multiwave mixing processes. We show that under some conditions it is possible for spontaneous emission to induce conjugate fields similar to semiclassical phase conjugation, requiring the use of the multiwave quantum theory described in earlier papers in this series. The theory is applicable to laser spectroscopy and optical bistability. We present explicit spectra and suggest possible experimental tests. In particular we find a quantum limit for FM spectroscopy that shows that even if the medium under study leaves the FM phase and amplitude relationships intact according to semiclassical theory, spontaneous emission can create an AM component detectable in a square-law detector.
The spontaneous radiation of a classical oscillator and that of a two-level atom both in the vicinity of a phase-conjugate mirror are calculated and compared. In either case the phase-conjugate mirror may be replaced by a phase-conjugate "image" of the source. We study the dependence of the decay constants and the characteristic frequencies on the phase-conjugate reflectivity and the difference between the characteristic frequency of the radiator and the pump frequency of the phase-conjugate mirror.