Cold-Atom Physics Using Ultrathin Optical Fibers: Light-Induced Dipole Forces and Surface Interactions

The strong evanescent field around ultrathin unclad optical fibers bears a high potential for detecting, trapping, and manipulating cold atoms. Introducing such a fiber into a cold-atom cloud, we investigate the interaction of a small number of cold cesium atoms with the guided fiber mode and with the fiber surface. Using high resolution spectroscopy, we observe and analyze light-induced dipole forces, van der Waals interaction, and a significant enhancement of the spontaneous emission rate of the atoms. The latter can be assigned to the modification of the vacuum modes by the fiber.

Figure 1

(a) Schematic experimental setup. A cloud of laser cooled cesium atoms is spatially overlapped with the 500-nm diameter waist of a tapered optical fiber. The transmission through the fiber is measured using a photodetector. (b) Timing of the experiment.

Figure 2

Measured (line graphs) and simulated (squares) absorbance of the atoms versus the detuning of the probe laser. The effective number of atoms contributing to the spectra [see Eq. (5)] is 107, 14, and 2 for (a) 1 nW, (b) 52 pW, and (c) 6 pW of probe laser power, respectively. The decrease of the peak absorbance with increasing power is due to the saturation of the atoms.

Figure 3

(a) Simulations of the relative density of the MOT for different detunings $\delta$ of the probe laser versus the distance from the fiber surface; solid line $\delta =0\mathrm{MHz}$, dotted line $\delta =-3\mathrm{MHz}$, dashed line $\delta =3\mathrm{MHz}$. The following parameters have been used for the simulations: Fiber diameter 500 nm, probe power 1 nW and a 3D Maxwellian velocity distribution of the Cs atoms at a temperature of $125\mu \mathrm{K}$. (b) and (c) show several atomic trajectories for $\delta =+3\mathrm{MHz}$ and $\delta =-3\mathrm{MHz}$, respectively, with a fixed atom velocity of $10\mathrm{cm}/\mathrm{s}$.

Figure 4

Linewidth of the absorbance profiles versus probe laser power. (a) full power range and (b) low power range. The squares correspond to the experimental data and the open circles are the simulated values. The continuous line is a guide to the eye ($b$-spline) of the simulated values. The dashed line is the reduced model not taking into account light-induced dipole forces and surface interactions.