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Physical Review A

Spontaneous emission of a cesium atom near a nanofiber: Efficient coupling of light to guided modes

Fam Le Kien, Subhasish Dutta Gupta, V. I. Balykin, K. Hakuta

Published Sep 28, 2005• 251 citations
Publisher

Abstract

We study the spontaneous emission of a cesium atom in the vicinity of a subwavelength-diameter fiber. We show that the confinement of the guided modes and the degeneracy of the excited and ground states substantially affect the spontaneous emission process. We demonstrate that different magnetic sublevels have different decay rates. When the fiber radius is about $200\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, a significant fraction (up to 28%) of spontaneous emission by the atom can be channeled into guided modes. Our results may find applications for developing nanoprobes for atoms and efficient couplers for subwavelength-diameter fibers.

DOI
10.1103/physreva.72.032509

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Publisher's Note: Spontaneous emission of a cesium atom near a nanofiber: Efficient coupling of light to guided modes [Phys. Rev. A72, 032509 (2005)]

Fam Le Kien, S. Dutta Gupta, V. I. Balykin +1 more

Physical Review A•Oct 19, 2005•5 citations
We study the spontaneous emission of a cesium atom in the vicinity of a subwavelength-diameter fiber. We show that the confinement of the guided modes and the degeneracy of the excited and ground states substantially affect the spontaneous emission process. We demonstrate that different magnetic sublevels have different decay rates. When the fiber radius is about 200nm200\phantom{\rule{0.3em}{0ex}}\mathrm{nm}200nm, a significant fraction (up to 28%) of spontaneous emission by the atom can be channeled into guided modes. Our results may find applications for developing nanoprobes for atoms and efficient couplers for subwavelength-diameter fibers.
We investigate the scattering of an evanescent light field by a single cesium atom outside a nanofiber. We show that the confinement of the field, the presence of the longitudinal field component and the tangential Poynting vector component, the enhancement of spontaneous emission, and the degeneracy of the atomic ground state substantially affect the scattering process. We find that, in the close vicinity of the fiber surface, the transmittance of the field in the stationary regime can be substantially reduced to 48% due to scattering into radiation modes (with the efficiency as high as 44%) and backward guided modes (with the efficiency as high as 8%).
The spontaneous decay rate of an excited atom placed near a dielectric cylinder is investigated. Special attention is paid to the case when the cylinder radius is small in comparison with radiation wavelength (nanofiber or photonic wire). In this case, the analytical expressions of the transition rates for different orientations of a dipole are derived. It is shown that the main contribution to decay rates is due to the quasistatic interaction of the atom dipole momentum with the nanofiber, and the contributions of guided modes are exponentially small. On the contrary, in the case when the radius of the fiber is only slightly less than the radiation wavelength, the influence of guided modes can be substantial. The results obtained are compared with the case of a dielectric nanospheroid and an ideally conducting wire.
Optical waveguides in the form of glass fibers are the backbone of global telecommunication networks. In such optical fibers, the light is guided over long distances by continuous total internal reflection which occurs at the interface between the fiber core with a higher refractive index and the lower index cladding. Although this mechanism ensures that no light escapes from the waveguide, it gives rise to an evanescent field in the cladding. While this field is protected from interacting with the environment in standard optical fibers, it is routinely employed in air- or vacuum-clad fibers in order to efficiently couple light fields to optical components or emitters using, e.g., tapered optical fiber couplers. Remarkably, the strong confinement imposed by the latter can lead to significant coupling of the light's spin and orbital angular momentum. Taking advantage of this effect, we demonstrate the controlled directional spontaneous emission of light by quantum emitters into a sub-wavelength-diameter waveguide. The effect is investigated in a paradigmatic setting, comprising cesium atoms which are located in the vicinity of a vacuum-clad silica nanofiber. We experimentally observe an asymmetry higher than 10:1 in the emission rates into the counterpropagating fundamental guided modes of the nanofiber. Moreover, we demonstrate that this asymmetry can be tailored by state preparation and suitable excitation of the quantum emitters. We expect our results to have important implications for research in nanophotonics and quantum optics and for implementations of integrated optical signal processing in the classical as well as in the quantum regime.
In this thesis, I present the realization of a fiber-optical interface using optically trapped cesium atoms, which is an efficient tool for coupling light and atoms. The basic principle of the presented scheme relies on the trapping of neutral cesium atoms in a two-color evanescent field surrounding a nanofiber. The strong confinement of the fiber guided light, which also protrudes outside the nanofiber, provides strong confinement of the atoms as well as efficient coupling to near-resonant light propagating through the fiber. In chapter 1, the necessary physical and mathematical background describing the propagation of light in an optical fiber is presented. The exact solution of Maxwell’s equations allows us to model fiber-guided light fields which give rise to the trapping potentials and the atom-light coupling in the close vicinity of a nanofiber. Chapter 2 gives the theoretical background of light-atom interaction. A quantum mechanical model of the light-induced shifts of the relevant atomic levels is reviewed, which allows us to quantify the perturbation of the atomic states due to the presence of the trapping light-fields. The experimental realization of the fiber-based atom trap is the focus of chapter 3. Here, I analyze the properties of the fiber-based trap in terms of the confinement of the atoms and the impact of several heating mechanisms. Furthermore, I demonstrate the transportation of the trapped atoms, as a first step towards a deterministic delivery of individual atoms. In chapter 4, I present the successful interfacing of the trapped atomic ensemble and fiber-guided light. Three different approaches are discussed, i.e., those involving the measurement of either near-resonant scattering in absorption or the emission into the guided mode of the nanofiber. In the analysis of the spectroscopic properties of the trapped ensemble we find good agreement with the prediction of theoretical model discussed in chapter 2. In addition, I introduce a non-destructive scheme for the interrogation of the atoms states, which is sensitive to phase shifts of far-detuned fiber-guided light interacting with the trapped atoms. The inherent birefringence in our system, induced by the atoms, changes the state of polarization of the probe light and can be thus detected via a Stokes vector measurement.