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arXiv:physics/0407051 · Journal of Optics B Quantum and Semiclassical Optics

Observation of modified radiative properties of cold atoms in vacuum near a dielectric surface

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

Published Sep 18, 2004• 16 citations
arXivHTMLPDF

Abstract

We have observed a distance-dependent absorption linewidth of cold $^{87}$Rb 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.

DOI
10.1088/1464-4266/6/11/005

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DOI: 10.1140/epjd/e2002-00205-1 THE EUROPEAN PHYSICAL JOURNAL D

Abstract. We demonstrate the in situ detection of cold 87Rb atoms near a dielectric surface using the absorption of a weak, resonant evanescent wave. We have used this technique in time of flight experiments determining the density of atoms falling on the surface. A quantitative understanding of the measured curve was obtained using a detailed calculation of the evanescent intensity distribution. We have also used it to detect atoms trapped near the surface in a standing-wave optical dipole potential. This trap was loaded by inelastic bouncing on a strong, repulsive evanescent potential. We estimate that we trap 1:5 104 atoms at a density 100 times higher than the falling atoms. PACS. 32.80.Pj Optical cooling of atoms; trapping { 42.25.Bs Wave propagation, transmission and absorption 1

•Dec 18, 2014•0 citations
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.
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 studied the absorption of a weak probe beam through cold rubidium atoms in a magneto-optic trap. The absorption spectrum shows two peaks, with the smaller peak having linewidth as small as 28% of the natural linewidth. The modification happens because the laser beams used for trapping also drive the atoms coherently between the ground and excited states. This creates "dressed" states whose energies are shifted depending on the strength of the drive. Linewidth narrowing occurs due to quantum coherence between the dressed states. The separation of the states and the linewidth depends on the trapping-laser intensity and detuning as predicted by this model.
In this thesis, a magneto-optical trap setup is used to laser cool and confine a cloud of 85Rb. The cloud typically contains 108 atoms in a 1 mm3 volume at a temperature in the region of the Doppler Limit (146 _K for 85Rb). To study the cold cloud, a subwavelength optical fibre - a nanofibre, or ONF - is positioned inside the cloud. The ONF can be used in two ways. Firstly, it is an efficient fluorescence collection tool for the cold atoms. Loading times, lifetimes and temperatures can be measured by coupling the atomic fluorescence to the evanescent region of the ONF. Secondly, the ONF is used as a probe beam delivery tool using the evanescent field properties of the device, allowing one to perform spectroscopy on few numbers of near-surface atoms. With improvements in optical density of the cloud, this system is an ideal candidate in which to generate electromagnetically induced transparency and slow light. A theoretical study of the van der Waals and Casimir-Polder interactions between an atom and a dielectric surface is also presented in this work in order to understand their effects in the spectroscopy of near-surface atoms.
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.
Surface-Induced Spectral Broadening during Efficient Two-Photon Excitation of Cold 133Cs^{133}\text{Cs}133Cs Atoms near an Optical Nanofiber
We report the experimental observation of the 133^{\text{133}}133Cs 6S1/2_{\text{1/2}}1/2​ to 6D5/2_{\text{5/2}}5/2​ single-frequency two-photon transition at low excitation power using an optical nanofiber embedded in a cold atom cloud. We investigate the 917 nm fluorescence spectra modified by the surface-induced interaction of the nanofiber, which exhibits a pronounced asymmetric broadening with a distinct red-shifted tail. We present a systematic theoretical description of the two-photon transition process of atoms near a nanofiber surface, considering atomic angular momentum coupling, guided-mode decay rate, and surface-induced van der Waals interactions. Owing to the tight spatial confinement provided by the guided evanescent field, efficient nonlinear excitation is realized at excitation powers down to tens of microwatts, corresponding to a reduction of approximately four orders of magnitude relative to typical free-space implementations. Our work reveals the interplay among waveguide-modified radiative dynamics, surface-induced interactions, and the spatial distribution of atoms around the nanofiber, and provides insights into further studies on nonlinear optical transitions and surface-mediated atomic dynamics in waveguide quantum electrodynamics systems.
Abstract. We demonstrate the in situ detection of cold 87Rb atoms near a dielectric surface using the absorption of a weak, resonant evanescent wave. We have used this technique in time of flight experiments determining the density of atoms falling on the surface. A quantitative understanding of the measured curve was obtained using a detailed calculation of the evanescent intensity distribution. We have also used it to detect atoms trapped near the surface in a standing-wave optical dipole potential. This trap was loaded by inelastic bouncing on a strong, repulsive evanescent potential. We estimate that we trap 1:5 104 atoms at a density 100 times higher than the falling atoms. PACS. 32.80.Pj Optical cooling of atoms; trapping { 42.25.Bs Wave propagation, transmission and absorption 1