Nanofiber-Based Optical Drag Force and Cavity QED

Recently, it has been shown theoretically that strongly non-paraxial Bessel beams can exert an optical force on a particle that drags the latter towards the light source. In such Bessel beams, the direction of the Poynting vector can be reversed, meaning that it points toward the beam source. However, an experimental verification of the existence of an optical pulling force (OPF) with Bessel beams is very challenging. In the first subproject, we therefore propose to theoretically investigate the possibility of realizing an OPF acting on particles in the evanescent field of highly non-paraxial modes sustained by high index optical nanofibers. Nanofiber modes are promising candidates for such investigations because they can exhibit wormhole regions with negative Poynting vector in their evanescent field. Moreover, a possible OPF could be straightforwardly demonstrated because the mode might provide both the tractor force and a gradient force that automatically traps the particles in the region with a backward force. Beyond the fundamental interest of demonstrating an OPF, its implementation with nanofiber modes may find important applications in both science and technology. The results of this subproject will lay the theoretical foundations for this endeavor. In the second subproject, we propose to theoretically investigate cavity quantum electrodynamical (CQED) effects for an ensemble of fiber-trapped atoms coupled to a nanofiber resonator. So far, experiments on CQED usually employ high-finesse optical microcavities. However, a simple nanofiber or a nanofiber-based cavity (NFC) can also be used to implement many aspects of CQED. In particular, due to the strong lateral confinement of the NFC mode, the CQED effects prevail even if the NFC finesse is moderate (~100) and the resonator is comparatively long (~10 cm). Such nanofiber-based cavities have recently been demonstrated experimentally. Moreover, cold neutral atoms have recently been trapped and optically interfaced in the evanescent field surrounding optical nanofibers. It is therefore very timely highly appealing to consider the combination of NFCs with such nanofiber- based atom traps. The very high optical depth of a nanofiber-trapped atomic ensemble inside a NFC makes this system a prime candidate for advanced (quantum) photonics applications as well as for fundamental studies. We will therefore establish the theoretical framework for the novel system and explore its potential for controlling the flow of light and light light interaction at the single quantum level. The corresponding results will be useful for enhancing light matter interaction at the nanoscale, for boosting the efficiency of channeling of light to nanostructures, as well as for applications in sensing, near-field imaging, waveguiding, quantum information, and quantum communication.

The aim of the project was to theoretically investigate new approaches to fundamental and applied problems of classical and quantum electrodynamics offered by optical nanofibers. The latter have a diameter smaller than the wavelength of the guided light and exhibit a strong lateral confinement of the guided mode in conjunction with a pronounced evanescent field surrounding the fiber. The first sub-project was concerned with studying the optical forces exerted on dielectric particles in the evanescent field near a nanofiber. The central initial question was whether an optical drag force can be realized. Optical drag forces have recently attracted much attention because they are directed towards the light source and prevail in the absence of intensity gradients. The answer to this question turned out negative. However, in the course of their investigation, Dr. Pham Le Kien and his co-applicant, Prof. A. Rauschenbeutel, discovered an unforeseen and intriguing effect: It was known that the Poynting vector of a quasicircularly polarized guided light field of a nanofiber gives rise to an azimuthal component in the energy flow. The energy thus flows along a helical path around the nanofiber. Accordingly, a dielectric spherical particle experiences an azimuthal component of the optical force. Surprisingly, the researchers found that, for an appropriately chosen diameter and refractive index of the particle, the latter is directed oppositely to the circulation direction of the energy flow around the nanofiber. These results open the way to future research on sorting, manipulating, and controlling dielectric particles using nanofibers. In the second sub-project, the researchers studied the propagation of guided light in an array of cesium atoms. Such arrays of atoms have recently been trapped in the evanescent field surrounding an optical nanofiber. They have shown that, when the array period coincides with a half-integer multiple of the wavelength of the guided light and when the atom number is sufficiently large, two intervals of energies form, for which the guided light cannot propagate through the array of atoms. This effect is analogous to the formation of so-called band gaps for electrons in semi-conductors. Furthermore, they have shown that the rate of scattering of guided light from the atom in the steady-state regime into the nanofiber-guided modes is asymmetric with respect to the forward and backward directions and depends on the polarization of the probe field. This asymmetry between the forward and backward scattering can, e.g., be used for realizing so-called optical isolators essential components for integrated optical circuits. These results contributed to state-of-the-art research on the coupling between quantum emitters and optical nanostructures, which constitutes a new and promising interdisciplinary field in quantum physics.