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arXiv:quant-ph/0610067

Optical excitation spectrum of an atom in a surface-induced potential

Fam Le Kien, Subhasish Dutta Gupta, K. Hakuta

Published Oct 10, 2006• 0 citations
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Abstract

We study the optical excitation spectrum of an atom in the vicinity of a dielectric surface. We calculate the rates of the total scattering and the scattering into the evanescent modes. With a proper assessment of the limitations, we demonstrate the portability of the flat-surface results to an experimental situation with a nanofiber. The effect of the surface-induced potential on the excitation spectrum for free-to-bound transitions is shown to be weak. On the contrary, the effect for bound-to-bound transitions is significant leading to a large excitation linewidth, a substantial negative shift of the peak position, and a strong long tail on the negative side and a small short tail on the positive side of the field--atom frequency detuning.

DOI
10.48550/arxiv.quant-ph/0610067

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We study the optical excitation spectrum of an atom in the vicinity of a dielectric surface. We calculate the rates of the total scattering and the scattering into the evanescent modes. With a proper assessment of the limitations, we demonstrate the portability of the flat-surface results to an experimental situation with a nanofiber. The effect of the surface-induced potential on the excitation spectrum for free-to-bound transitions is shown to be weak. On the contrary, the effect for bound-to-bound transitions is significant leading to a large excitation linewidth, a substantial negative shift of the peak position, and a strong long tail on the negative side and a small short tail on the positive side of the field--atom frequency detuning.
We study the optical excitation spectrum of an atom in the vicinity of a dielectric surface. We calculate the rate of the total scattering as well as the rate of the scattering into the evanescent modes. With a proper assessment of the limitations, we demonstrate the portability of the flat-surface results to an experimental situation with a nanofiber. The effect of the surface-induced potential on the excitation spectrum for free-to-bound transitions is shown to be weak. On the contrary, the effect for bound-to-bound transitions is significant leading to a large excitation linewidth, a substantial negative shift of the peak position, and a strong long tail on the negative side and a small short tail on the positive side of the field frequency detuning.
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.
We study spontaneous radiative decay of translational levels of an atom in the vicinity of a semi-infinite dielectric. We systematically derive the microscopic dynamical equations for the spontaneous decay process. We calculate analytically and numerically the radiative linewidths and the spontaneous transition rates for the translational levels. The roles of the interference between the emitted and reflected fields and of the transmission into the evanescent modes are clearly identified. Our numerical calculations for the silica-cesium interaction show that the radiative linewidths of the bound excited levels with large enough but not too large vibrational quantum numbers are moderately enhanced by the emission into the evanescent modes and those for the deep bound levels are substantially reduced by the surface-induced redshift of the transition frequency.
We measure the modification of the transmission spectra of cold 87Rb^{87}\mathrm{Rb}87Rb atoms in the proximity of an optical nanofiber (ONF). Van der Waals interactions between the atoms an the ONF surface decrease the resonance frequency of atoms closer to the surface. An asymmetric spectra of the atoms holds information of their spatial distribution around the ONF. We use a far-detuned laser beam coupled to the ONF to thermally excite atoms at the ONF surface. We study the change of transmission spectrum of these atoms as a function of heating laser power. A semiclassical phenomenological model for the thermal excitation of atoms in the atom-surface van der Waals bound states is in good agreement with the measurements. This result suggests that van der Waals potentials could be used to trap and probe atoms at few nanometers from a dielectric surface, a key tool for hybrid photonic-atomic quantum systems.
Hybrid quantum platforms based on highly excited Rydberg atoms coupled to nanophotonics devices offer a promising route toward scalable quantum networks and integrated quantum technologies. However, the close proximity of Rydberg atoms to dielectric nanostructures makes these systems particularly susceptible to uncontrolled surface electric fields that can lead to a degradation of the excitation process. Here, we experimentally investigate Rydberg excitation of laser-cooled 87^{87}87Rb atoms via the evanescent field of an optical nanofiber in the presence of fiber-guided red- and blue-detuned light fields as used to trap ground state atoms in fiber-based dipole traps. We observe a time evolution of the Rydberg excitation spectrum when both the dipole trapping fields are on and the additional spectral features that appear can be suppressed by applying an external oscillating electric field to the system, strongly indicating that surface charge accumulation is responsible for the observed spectral feature. The experimental results are reproduced qualitatively by a model that incorporates DC energy level shifts arising from electric fields generated by charges deposited on the nanofiber surface. We identify Rydberg-ground state collisional ionization, which is enhanced by the dipole trapping fields, as the dominant mechanism for charge generation. These results provide new insight into charge dynamics at dielectric nanophotonic interfaces and establish practical guidelines for mitigating surface charge-induced electric fields in fiber-integrated Rydberg quantum systems.
Fluorescence of atoms has been the subject of interest in quantum optical measurementsfor many years. The atomic fluorescence is routinely used as a tool to detectatoms, to generate photons and to generate non-classical states of light. One of thewidely used methods to investigate the atomic fluorescence is laser-induced fluorescencespectroscopy. This thesis presents the development of optical nanofibers as atool for the measurement of fluorescence spectrum of atoms.Optical nanofibers are ultrathin optical fibers, with diameters less than the propagationwavelength of the fiber, with a silica core and a vacuum clad. Such opticalnanofibers can support only the single propagating guided-mode. The guided-modesof the nanofiber is confined in the transverse direction of the nanofiber and a partof the propagating field lies outside the fiber, in the evanescent region of the fiber.The mode density of the electromagnetic field is modified by the nanofiber. Due tothe modification of mode density, the spontaneous emission of atoms is modified bythe optical nanofibers and the confinement of the guided-modes results in a strongchanneling of spontaneous emission of atoms into the guided-modes of the nanofiber.Such a property of nanofibers can be used to study fluorescence of atoms around thenanofiber.Optical nanofibers are prepared by the adiabatic tapering of single-mode opticalfibers. Our experiments are performed by overlapping cold Cesium atoms, from amagneto-optical trap (MOT), with the optical nanofiber and observing the fluorescencethrough the guided-modes of the nanofiber. The nanofiber suppresses with highprobability any coupling of scattering light from the excitation laser into the guidedmodesand high coupling efficiency of fluorescence into the guided-modes enables us tomeasure the fluorescence spectrum of atoms present around the nanofiber with a goodsignal-to-noise ratio. In this thesis, we demonstrate the measurement of laser inducedfluorescence (LIF) emission spectra of strongly driven atoms using optical nanofibers.For the fluorescence emission spectrum measurements, we develop and demonstrate anew sensitive and high resolution method consisting of optical heterodyne techniqueand photon correlation spectroscopy. The key point of the method is to extract theinformation of first-order field correlation function from the measured second-orderintensity correlation function. The Fourier transform of the field correlation functionthen gives the fluorescence emission spectrum.The atoms emitting fluorescence in the guided-modes of the nanofiber lie very closeto the nanofiber surface and are subjected to atom-surface interactions. It has beenreported in earlier experiments on nanofiber, that such surface interactions can bemodified by irradiating the MOT-atoms overlapped with the nanofiber with UV lasers.After such irradiations, MOT-atoms are kept from falling into the surface-potentialand behave almost like free-space atoms around the nanofiber. We first perform ourfluorescence spectra measurements under this free-atom conditions. The observedexcitation spectrum lineshape shows a free-atom like Lorentzian behavior, with avery small tail in the red-detuned side (red-tail). The red-tail has been attributedto photoassociation process, due to atoms making transition from free ground statesto bound excited states of the surface potential. Under this conditions, the emissionspectra measured for on-resonant and off-resonant excitations show the well-knownthree-peak Mollow-triplet spectrum of free-space two-level atoms. The observationconfirms that the near surface atoms behave like free-space atoms. Also, the quantitativeagreement between the observed spectra with the theoretical calculationsdemonstrates the validity of the optical nanofiber method for measuring fluorescenceemission spectrum of atoms.As mentioned above, since we are observing near-surface atoms, fluorescence measurementsbased on optical nanofibers may become a unique tool to probe atomsurfaceinteractions. In earlier experiments using nanofiber, signatures of atomsurfaceinteraction have been observed in the excitation spectrum of atoms, in theform of a broad red-tail. The lineshape of the excitation spectrum is understood as aresult of a process where atoms approaching the nanofiber surface fall into the shallowsurface potential, forming long-range atom-surface bound states. In this thesis,we have systematically investigated the evolution of the lineshape of the excitationspectrum from a Lorentzian shape to a broad red-tail condition. The evolution of thespectrum lineshape has been attributed to the changing nanofiber surface condition.The spectrum shape is found to depend on the flux of background Cs-atoms. Wespeculate that this might be due to the adsorption of hot background atoms ontothe nanofiber surface, which changes the surface condition of the nanofiber locallyand helps in loading of cold MOT-atoms into the shallow surface potential. We havefound that the effect of UV laser irradiation is to photoionize the MOT-atoms, andthe generated photoelectrons might help in the desorption of adsorbed atoms on thesurface. We discuss the roles of adsorption and desorption of atoms on the surfaceconditions. We have shown that such changing surface condition can be controlledby adjusting the hot background atom flux and the UV laser intensity. Under suchcontrolled surface condition, we have measured the emission spectrum. The observedemission spectrum under the broad red-tail condition can be explained only by includingthe surface interactions. We also discuss qualitatively the phonon-mode densityfor nanofiber at room temperature and the phonon-mediated decay of atoms in thesurface potential.This work may open up the way for using optical nanofibers as a tool to investigatethe laser-induced fluorescence spectrum of strongly driven atoms, and to study atomsurfaceinteractions. In the future, we hope to create optical dipole traps around thenanofibers by introducing trap laser lights through the guided-modes and performfluorescence measurements of these trapped atoms.