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Physical Review A

Dicke narrowing in strong fields: The atom-as-antenna analogy

James C. Camparo, R. P. Frueholz, H. G. Robinson

Published Sep 1, 1989• 6 citations
Publisher

Abstract

It is well known that when a radiator (or absorber) suffers velocity-changing collisions that do not perturb its internal state, the oscillator's successive Doppler shifts are motionally averaged. This process (Dicke narrowing) gives rise to a homogeneously broadened sub-Doppler resonance, which is often astride a broad, apparently Doppler-width, pedestal. Though it has long been held that this pedestal is an inhomogeneous remnant of Doppler broadening, no evidence has ever been obtained to substantiate this expectation. In the present study we investigate the behavior of Dicke-narrowed line shapes in strong fields in an evacuated wall-coated cell. Our experimental results indicate that the hypothesis of an inhomogeneous Doppler remnant pedestal is invalid. Rather, the pedestal is homogeneously broadened and is best understood in terms of the oscillator's response to the electromagnetic field's power spectral density as observed in its rest frame. This conclusion suggests an intimate relationship between the process of Dicke narrowing and the behavior of quantum systems in the presence of stochastic fields. By envisioning the quantum system as a narrow-band antenna that only absorbs a small portion of the field's energy, we develop a simple intuitive model to account for the interaction of the atom with the nonmonochromatic field; the appeal of this simple theory is that it removes the need for extensive computation. Agreement between this ``atom-as-antenna'' analogy and experiment is very good.

DOI
10.1103/physreva.40.2351

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Dicke Narrowing in Strong Fields: The Atom-as-Antenna Analogy

James C. Camparo, R. P. Frueholz, H. G. Robinson

When a radiator (or absorber) suffers velocity-changing collisions that do not perturb its internal state, the oscillator's successive Doppler shifts are motionally averaged. This process (Dicke narrowing) gives rise to a homogeneously broadened sub-Doppler resonance, which is often astride a broad, apparently Doppler width, pedestal. It has long been held that this pedestal is an inhomogeneous remnant of Doppler broadening, but no evidence has ever been obtained to substantiate this expectation. In the present study we investigate the behavior of Dicke-narrowed lineshapes in strong fields in an evacuated wall- coated cell. Our experimental results indicate that the hypothesis of an inhomogeneous Doppler remnant pedestal is invalid. Rather, the pedestal is homogeneously broadened, and is best understood in terms of the oscillator's response to the electromagnetic field's power spectral density as observed in its rest frame. This conclusion suggests an intimate relationship between the process of Dicke narrowing and the behavior of quantum systems in the presence of stochastic fields. By envisioning the quantum system as a narrow-band antenna that only absorbs a small portion of the field's energy, we develop a simple intuitive model to account for the interaction of the atom with the nonmonochromatic field; the appeal of this simple theory is that it removes the need for extensive computation. Agreement between this atom-as-antenna analogy and experiment is very good.

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On the role of Dicke narrowing in the formation of atomic line shapes in the optical domain

Daniel Lisak, Andrzej Bielski, R Ciury o +3 more

The purpose of this work is to verify the possibility of the observation of the Dicke narrowing of atomic spectral lines in the optical domain. As an example we have chosen experimental results obtained by means of a laser-induced fluorescence method for the 114 Cd 326.1 nm line perturbed by xenon. Experimental results were carefully reanalysed using a line shape model which takes into account the speed dependence of collisional broadening and shifting, the velocity-changing collisions and the collision-time asymmetry. We have found no convincing evidence for the occurrence of Dicke narrowing of the 326.1 cadmium line perturbed by xenon. The Dicke narrowing (if it occurs at all) appears to be smaller than that resulting from the estimation based on the accessible data describing the diffusion of Cd in Xe. As a source of this behaviour the correlation between velocity-changing and dephasing collisions was indicated.

Journal of Physics B Atomic Molecular and Optical Physics•Sep 12, 2003•14 citations
It is well known that when a radiator (or absorber) suffers velocity-changing collisions that do not perturb its internal state, the oscillator's successive Doppler shifts are motionally averaged. This process (Dicke narrowing) gives rise to a homogeneously broadened sub-Doppler resonance, which is often astride a broad, apparently Doppler-width, pedestal. Though it has long been held that this pedestal is an inhomogeneous remnant of Doppler broadening, no evidence has ever been obtained to substantiate this expectation. In the present study we investigate the behavior of Dicke-narrowed line shapes in strong fields in an evacuated wall-coated cell. Our experimental results indicate that the hypothesis of an inhomogeneous Doppler remnant pedestal is invalid. Rather, the pedestal is homogeneously broadened and is best understood in terms of the oscillator's response to the electromagnetic field's power spectral density as observed in its rest frame. This conclusion suggests an intimate relationship between the process of Dicke narrowing and the behavior of quantum systems in the presence of stochastic fields. By envisioning the quantum system as a narrow-band antenna that only absorbs a small portion of the field's energy, we develop a simple intuitive model to account for the interaction of the atom with the nonmonochromatic field; the appeal of this simple theory is that it removes the need for extensive computation. Agreement between this ``atom-as-antenna'' analogy and experiment is very good.
When a radiator (or absorber) suffers velocity-changing collisions that do not perturb its internal state, the oscillator's successive Doppler shifts are motionally averaged. This process (Dicke narrowing) gives rise to a homogeneously broadened sub-Doppler resonance, which is often astride a broad, apparently Doppler width, pedestal. It has long been held that this pedestal is an inhomogeneous remnant of Doppler broadening, but no evidence has ever been obtained to substantiate this expectation. In the present study we investigate the behavior of Dicke-narrowed lineshapes in strong fields in an evacuated wall- coated cell. Our experimental results indicate that the hypothesis of an inhomogeneous Doppler remnant pedestal is invalid. Rather, the pedestal is homogeneously broadened, and is best understood in terms of the oscillator's response to the electromagnetic field's power spectral density as observed in its rest frame. This conclusion suggests an intimate relationship between the process of Dicke narrowing and the behavior of quantum systems in the presence of stochastic fields. By envisioning the quantum system as a narrow-band antenna that only absorbs a small portion of the field's energy, we develop a simple intuitive model to account for the interaction of the atom with the nonmonochromatic field; the appeal of this simple theory is that it removes the need for extensive computation. Agreement between this atom-as-antenna analogy and experiment is very good.
The Doppler effect is one of the dominant broadening mechanisms in thermal vapor spectroscopy. For two-photon transitions one would naively expect the Doppler effect to cause a residual broadening, proportional to the wave-vector difference. In coherent population trapping (CPT), which is a two-photon narrow-band phenomenon, such broadening was not observed experimentally. This has been commonly attributed to frequent velocity-changing collisions, known to narrow Doppler-broadened one-photon absorption lines (Dicke narrowing). Here we show theoretically that such a narrowing mechanism indeed exists for CPT resonances. The narrowing factor is the ratio between the atom's mean free path and the wavelength associated with the wave-vector difference of the two radiation fields. A possible experiment to verify the theory is suggested.
Single light beam transmission spectroscopy in an ultrathin dilute vapor cell (10--100 \ensuremath{\mu}m) reveals sub-Doppler features. Indeed, atoms with fast (normal) velocity undergo a short interaction between the cell walls, and are shown to be less absorbing because they have not reached their steady state of interaction with light. The effect is observed in the linear-response regime with a simple two-level system, as ensured from the ultralow level of light irradiation ($<~1\ensuremath{\mu}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ for the Cs D2{D}_{2}D2​ line). The effect, known for a long time in the microwave domain, is demonstrated here in the optical domain, and is extended to cell thicknesses large relative to the optical wavelength.
The purpose of this work is to verify the possibility of the observation of the Dicke narrowing of atomic spectral lines in the optical domain. As an example we have chosen experimental results obtained by means of a laser-induced fluorescence method for the 114 Cd 326.1 nm line perturbed by xenon. Experimental results were carefully reanalysed using a line shape model which takes into account the speed dependence of collisional broadening and shifting, the velocity-changing collisions and the collision-time asymmetry. We have found no convincing evidence for the occurrence of Dicke narrowing of the 326.1 cadmium line perturbed by xenon. The Dicke narrowing (if it occurs at all) appears to be smaller than that resulting from the estimation based on the accessible data describing the diffusion of Cd in Xe. As a source of this behaviour the correlation between velocity-changing and dephasing collisions was indicated.