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Optics and Spectroscopy

QED-modified radiative properties and dynamics of cold atoms moving through an evanescent wave

R. J. C. Spreeuw, V. V. Ivanov, R. A. Cornelussen, H. B. van Linden van den Heuvell

Published Sep 1, 2005• 2 citations
Publisher

Abstract

Measurements of the radiative properties of cold 87 Rb atoms close to a dielectric/vacuum interface are reported. This is the first observation of a quantum-electrodynamic (QED) modification of radiative properties in vacuum near a dielectric surface. Evanescent wave (EW) spectroscopy on cold atoms that were dropped on a glass surface was used. An increase of the natural linewidth by up to 25% compared to the free space value was found. This was attributed to QED broadening and level shifts, as well as local Stark shifts near the surface. By varying the characteristic EW length, a position dependence characteristic for QED was observed. The role of transient internal dynamics of the atoms as they move through the strongly inhomogeneous EW was investigated.

DOI
10.1134/1.2055944

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Measurements of the radiative properties of cold 87 Rb atoms close to a dielectric/vacuum interface are reported. This is the first observation of a quantum-electrodynamic (QED) modification of radiative properties in vacuum near a dielectric surface. Evanescent wave (EW) spectroscopy on cold atoms that were dropped on a glass surface was used. An increase of the natural linewidth by up to 25% compared to the free space value was found. This was attributed to QED broadening and level shifts, as well as local Stark shifts near the surface. By varying the characteristic EW length, a position dependence characteristic for QED was observed. The role of transient internal dynamics of the atoms as they move through the strongly inhomogeneous EW was investigated.
We have observed a distance-dependent absorption linewidth of cold 87^{87}87Rb atoms close to a dielectric-vacuum interface. This is the first observation of modified radiative properties in vacuum near a dielectric surface. A cloud of cold atoms was created using a magneto-optical trap (MOT) and optical molasses cooling. Evanescent waves (EW) were used to observe the behavior of the atoms near the surface. We observed an increase of the absorption linewidth with up to 25% with respect to the free-space value. Approximately half the broadening can be explained by cavity-quantum electrodynamics (CQED) as an increase of the natural linewidth and inhomogeneous broadening. The remainder we attribute to local Stark shifts near the surface. By varying the characteristic EW length we have observed a distance dependence characteristic for CQED.
The atomic response to resonant light irradiation near vapor-dielectric interfaces is monitored via high-resolution reflection spectroscopy. One demonstrates that this Doppler-free spectroscopic technique yields a novel probe of atomic dynamics and QED atom-surface long- range interaction. It provides for the first time a spectral evidence of van der Waals attraction, and the method is intrinsically applicable to the observation of surface interaction exerted on excited atoms. Effects related with atomic excitation level, nature of the dielectrics, and atomic collisions are discussed.
Cold atomic ensembles offer precise tools for probing near-field interactions, yet experimental data linking atom dynamics to surface-induced forces remains limited. This study investigated the interaction between atoms and a dielectric surface using an atomic fountain measurement technique, in which cold rubidium atoms were released from a moving optical dipole trap. The launched cold atoms were irradiated with an evanescent light detuned from the D2_22​ transition by $-$20.2 to $+$20.2 MHz, after which they were recaptured by reactivating the optical dipole trap. Our measurements revealed that the number of recaptured atoms decreased with increasing flight time, and the decay was suppressed under blue-detuned conditions. We modeled the motion dynamics of the cold atomic ensemble, incorporating Casimir-Polder interactions between the dielectric surface and cold atoms, and observed that the rate of decrease in the number of residual atoms depended on the value of the van der Waals potential coefficient C3C_3C3​. The calculation results demonstrated good agreement with the experimental results, allowing us to estimate C3=5.6−1.9+2.4×10−49C_3 = 5.6^{+2.4}_{-1.9} \times 10^{-49}C3​=5.6−1.9+2.4​×10−49 Jm3^33 by comparing simulations with the experimental results across various C3C_3C3​ values, accounting for experimental errors.
The problem of the momentum of the electromagnetic field near a homogeneous dielectric plane surface is studied. In the particular case of evanescent waves, our calculations are consistent with the experimental result of S. Huard and C. Imbert which points out that, during an interaction between a moving atom and a surface wave of pulsation $\ensuremath{\omega}$, the exchanged momentum is greater than $\frac{\ensuremath{\hbar}\ensuremath{\omega}}{c}$. This work leads at first to a straightforward procedure for quantizing the momentum parallel to the diopter of the field. The authors proceed to investigate the form of the component perpendicular to the interface which is transmitted by the field to an atom during an interaction. (In the case of an evanescent wave, this problem involves the interpretation of the imaginary part of the wave vector.) A general theory shows that the presence of a medium widens and shifts the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$ levels of the field. This result agrees with the uncertainty principle.
We show how the internal dynamics of a multilevel atom are modified in the vicinity of the interface between a vacuum and a simple or multilayered lossless dielectric medium. Optical Bloch equations are derived, which take into account the modifications of spontaneous emission rates and energy levels experienced by the atom. van der Waals level shifts are evaluated using the method of images for dielectrics. Spontaneous emission rates and radiation patterns are calculated in a simple way using the Lorentz reciprocity theorem. \textcopyright{} 1996 The American Physical Society.
Over the past ten years, our group has investigated the effects of confinement on atoms inside metallic micron–sized cavities in order to elucidate some basic phenomena in the field of cavity quantum electrodynamics (QED). The first of these was the inhibition of spontaneous emission from an atom inside a cavity. This was followed by a laser spectroscopic measurement of the van der Waals interaction between a single Rydberg atom and a gold cavity, which showed that a simple electrostatic model of the atom–cavity interaction is correct when the cavity is small enough. More recently, the retarded Casimir–Polder force was measured between a ground state sodium atom and a large cavity, demonstrating that the van der Waals potential fails at long enough range and that the vacuum fluctuations of the field then have an important role in the interaction of the atom with the cavity. Our group is now pushing forward these investigations to study cavities whose walls have losses and dispersion, where the theory of cavity QED is significantly more complicated. With real surfaces, we have to deal with the complex dielectric response ε(ω) of the material, which exhibits frequency–dependent absorption and dispersion. One particularly interesting case is when a downward transition in the atom is resonant with an excitation of the cavity walls. This opens a new branch for the atomic decay: as an alternative to creating a photon within the space surrounded by the cavity walls the atomic decay can now create an electromagnetic excitation of the walls themselves. Another novel feature of our experiments is that the Bohr frequencies of the atom are close to the kT/h, where T is room temperature. We therefore expect to be able to measure effects associated with QED at finite temperature; in other words, to study how the blackbody radiation affects our experiments. By conducting experiments with real surfaces, we hope to elucidate and perhaps simplify the theoretical models used to describe these systems.