Spontaneous emission of a cesium atom near a nanofiber: Efficient coupling of light
to guided modes
Fam Le Kien,
1,
*
S. Dutta Gupta,
1,2
V. I. Balykin,
1,3
and K. Hakuta
1
1
Department of Applied Physics and Chemistry, University of Electro-Communications, Chofu, Tokyo 182-8585, Japan
2
School of Physics, University of Hyderabad, Hyderabad, India
3
Institute of Spectroscopy, Troitsk, Moscow Region, 142092, Russia
Received 16 June 2005; published 28 September 2005; publisher error corrected 3 October 2005
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 substan-
tially affect the spontaneous emission process. We demonstrate that different magnetic sublevels have different
decay rates. When the fiber radius is about 200 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 PACS numbers: 32.30.Jc, 32.70.Jz, 32.80.Pj, 03.75.Be
I. INTRODUCTION
Coupling of light to subwavelength structures and its con-
trol pose one of the greatest challenges of recent research 1.
In this paper, we show how such coupling with efficiency of
up to 28% can be achieved in a realistic system of a cesium
atom near a subwavelength-diameter fiber. Note that modifi-
cation of the vacuum near the fiber and its effect on the
spontaneous emission has been studied in the context of two-
level atoms 2–4. Many other studies exist involving other
geometries 5,6. Most of these investigations, to the best of
our knowledge, do not go beyond the two-level approxima-
tion for the atom. The inclusion of hyperfine structure of the
atom can significantly affect the actual rate of spontaneous
decay. Some parameters that describe the decay of cross-
level coherences arise only in the framework of a multilevel-
atom model. The knowledge of both diagonal and off-
diagonal types of decay characteristics is required for the
studies of absorption and emission properties, optical re-
sponse, and dynamical behavior of realistic atoms 7. We
show that the confinement of the guided modes and the de-
generacy of the excited and ground states substantially affect
the spontaneous emission process. We find that different
magnetic sublevels have different decay rates. We demon-
strate that the thin fiber can indeed act as a subwavelength
probe, since about one fourth of the spontaneous emission
from the atom can be channeled into guided modes. The
knowledge of spontaneous emission characteristics is impor-
tant yet from the angle of atom optics 8, in particular, from
design considerations of atom traps 9. A recent proposal for
microscopic trapping of individual atoms involves the use of
a subwavelength-diameter silica fiber with a single red-
detuned10 or two red- and blue-detuned light beams
11 launched into it.
The paper is organized as follows. In Sec. II we describe
the model. In Sec. III we derive the basic characteristics of
spontaneous emission for the model. In Sec. IV we present
numerical results. Our conclusions are given in Sec. V.
II. MODEL
We consider a cesium atom trapped in the vicinity of a
subwavelength-diameter silica fiber see the upper part of
Fig. 1. We use the fiber axis z as the quantization axis for
atomic states. For atoms trapped in a magneto-optical trap,
the quantization axis can be specified and hence controlled
by the direction of the magnetic field in the trap. We study
the cesium D
2
line, which occurs at the wavelength
0
= 852 nm and corresponds to the transition from the excited
state 6P
3/2
to the ground state 6S
1/2
see the lower part of
Fig. 1. We assume that the atom is initially prepared in the
hyperfine-structure hfs level F' =5 of the state 6P
3/2
. We
*Also at Institute of Physics and Electronics, Vietnamese Acad-
emy of Science and Technology, Hanoi, Vietnam.
FIG. 1. a An atom interacting with guided and radiation modes
in the vicinity of a thin optical fiber. b Schematic of the 6P
3/2
F'
=5 and 6S
1/2
F =4 hfs levels of a cesium atom.
PHYSICAL REVIEW A 72, 032509 2005
1050-2947/2005/723/0325097/$23.00 ©2005 The American Physical Society 032509-1