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Physical Review A

Intensity-dependent spectral reflectivity of a dense-gas–dielectric interface

Richard M. Friedberg, Sven R. Hartmann, Jamal T. Manassah

Published Nov 1, 1990• 30 citations

Abstract

Intense light incident on a dense-gas--dielectric interface, from the dielectric side, is reflected because of the combined effect of the discontinuities in the index of refraction at the interface and at the transition region in the gas where the degree of atomic saturation changes abruptly. This latter discontinuity gives rise to a self-reflected wave. At low intensities this self-reflected wave is weak, the problem is linear, and the net reflected wave intensity taken as a function of frequency has a simple shape that should allow measurement of the hitherto undetected self-induced frequency shift in a collision-broadened gas at an absorption resonance. Calculations show that in the weak intensity regime, r'', the imaginary part of the reflection coefficient, is quasi-Lorentzian and shifted by an amount comparable to its width. At high light intensity r'' broadens, shifts, and distorts. The distortion, which nonuniformly modulates the Lorentzian shape, comes from an interference of the forward and self-reflected waves. In the wings of the resonance line the self-reflected wave can be neglected, and the shape of r'' although not distorted is shifted from where it would be if there were no local field correction.

DOI: 10.1103/physreva.42.5573

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A theory of selective reflection (SR) of a light beam from the interface of a dielectric medium and a dense atomic vapor is presented. Following another earlier work on this subject [J. Guo et al., Opt. Commun. 110, 732 (1994)], we calculate the atomic density matrix and the transmitted field in the vapor self-consistently. We extend our previous theory to include an attractive atom-dielectric wall interaction and multiple atomic transitions. We examine the limits at which the conventional SR theories, which are based on the assumption of a plane-wave field in the vapor, start to fail. It is shown that at or above vapor densities of order ${10}^{17}$/${\mathrm{cm}}^{3}$, the frequency shifts of the SR line shape due to the local-field correction (Lorentz-Lorenz shift), the atom-wall interaction, and the nonexponential attenuation of the field in the vapor can have comparable orders of magnitude, and the proper determination of all these effects is essential for calculating the correct SR signals. A comparison of the theoretical SR spectra with an experimental result for a Rb vapor with a density of order ${10}^{17}$/${\mathrm{cm}}^{3}$ is also presented. \textcopyright{} 1996 The American Physical Society.

Physical Review A•Feb 1, 1996•46 citations

Intense light incident on a dense-gas--dielectric interface, from the dielectric side, is reflected because of the combined effect of the discontinuities in the index of refraction at the interface and at the transition region in the gas where the degree of atomic saturation changes abruptly. This latter discontinuity gives rise to a self-reflected wave. At low intensities this self-reflected wave is weak, the problem is linear, and the net reflected wave intensity taken as a function of frequency has a simple shape that should allow measurement of the hitherto undetected self-induced frequency shift in a collision-broadened gas at an absorption resonance. Calculations show that in the weak intensity regime, r'', the imaginary part of the reflection coefficient, is quasi-Lorentzian and shifted by an amount comparable to its width. At high light intensity r'' broadens, shifts, and distorts. The distortion, which nonuniformly modulates the Lorentzian shape, comes from an interference of the forward and self-reflected waves. In the wings of the resonance line the self-reflected wave can be neglected, and the shape of r'' although not distorted is shifted from where it would be if there were no local field correction.

The dipole–dipole broadening of the spectrum of the selective reflection of intense resonance light from the interface between a transparent dielectric and a gas of the natural mixture of Rb isotopes has been studied experimentally. The case of a high gas density where the Doppler broadening can be neglected has been investigated. It has been shown that dipole–dipole broadening is reduced with increasing the number density of excited atoms. When the laser beam intensity is much higher than the saturation intensity of a resonance transition, a significant broadening due to the very high laser beam intensity has not been observed in the reflection spectrum from the transparent dielectric/gas interface. The observed intensity dependence of the spectral width has been explained by the quenching collisions of the excited atoms with the interface.

We calculate the reflection coefficient of a dielectric-gas interface including the nonlinear local field, saturation, and self-reflected wave effects. We specialize to collisionally broadened gases with absorption lengths that are necessarily small compared with an optical wavelength and preclude the use of the slowly varying amplitude approximation. The dependence of the reflectivity on the intensity of the incoming field is revealed through the changes that it induces in the transverse profile of a reflected Gaussian beam. These changes derive principally from effective aperturing effects in amplitude and phase.

We calculate the modulation of the reflection coefficient for a frequency-modulated (FM) light beam incident on the interface between a dielectric and an atomic vapor. The vapor is described as a gas of resonant, Doppler-broadened, two-level systems, with transition frequency and linewidth arbitrarily depending on the atom-dielectric distance. The atoms are supposed to get deexcited at collisions with the surface. The transient atomic response is calculated to first order in the incident field, for both incoming and desorbed atoms. The reflection coefficient, evaluated to first order in the vapor dipole polarization, leads to a formal expression of the reflectivity modulation, valid for arbitrary atom-surface interaction potentials. One first discusses the reflection signal in absence of wall interactions, for arbitrary modulation frequencies. At large frequencies, it allows one to monitor both vapor absorption and dispersion. Second, the formal theory is applied to the case of a Van der Waals-London surface attraction exerted on the atomic vapor. Both normal and oblique beam incidences are considered. One shows how the vapor dispersion signal is red-shifted and strongly distorted by the appearance of vapor-surface long-range interactions, and how it can be used to monitor these interactions. At non-normal incidences, the lineshapes get Doppler-broadened.

A theory of selective reflection (SR) of a light beam from the interface of a dielectric medium and a dense atomic vapor is presented. Following another earlier work on this subject [J. Guo et al., Opt. Commun. 110, 732 (1994)], we calculate the atomic density matrix and the transmitted field in the vapor self-consistently. We extend our previous theory to include an attractive atom-dielectric wall interaction and multiple atomic transitions. We examine the limits at which the conventional SR theories, which are based on the assumption of a plane-wave field in the vapor, start to fail. It is shown that at or above vapor densities of order

{10}^{17}

/

{\mathrm{cm}}^{3}

, the frequency shifts of the SR line shape due to the local-field correction (Lorentz-Lorenz shift), the atom-wall interaction, and the nonexponential attenuation of the field in the vapor can have comparable orders of magnitude, and the proper determination of all these effects is essential for calculating the correct SR signals. A comparison of the theoretical SR spectra with an experimental result for a Rb vapor with a density of order

{10}^{17}

/

{\mathrm{cm}}^{3}

is also presented. \textcopyright{} 1996 The American Physical Society.