We experimentally realize an optical fiber ring resonator that includes a tapered section with subwavelength-diameter waist. In this section, the guided light exhibits a significant evanescent field which allows for efficient interfacing with optical emitters. A commercial tunable fiber beam splitter provides simple and robust coupling to the resonator. Key parameters of the resonator such as its out-coupling rate, free spectral range, and birefringence can be adjusted. Thanks to the low taperand coupling-losses, the resonator exhibits an unloaded finesse of F = 75 ± 1, sufficient for reaching the regime of strong coupling for emitters placed in the evanescent field. The system is ideally suited for trapping ensembles of laser-cooled atoms along the nanofiber section. Based on measured parameters, we estimate that the system can serve as a platform for optical multimode strong coupling experiments. Finally, we discuss the possibilities of using the resonator for applications based on chiral quantum optics.Over the past years, significant research effort has been devoted to interfacing quantum emitters, such as molecules, quantum dots, color centers, and neutral atoms, with fiber-guided light fields. Suitable lightmatter interfaces are considered to be key elements for future quantum networks [1]. One way to realize such an interface consists in coupling emitters to the evanescent fields surrounding the nanofiber-waist of a tapered optical fiber, i.e., a fiber section with sub-wavelength diameter. Such systems already provide absorption probabilities for single fiber-guided photons of about 25 % for a single emitter located on the nanofiber surface [2] and about 5 % at a distance of 200 nm, typical for cold atoms in nanofiber-based optical dipole traps [3,4].One way to further enhance the light-matter coupling strength is to increase the number of emitters in the evanescent field and to take advantage of their collective coupling. Another option relies on confining the light in an optical resonator, which allows one to even reach the regime of strong coupling in the sense of cavity quantum electrodynamics (CQED) [5]. There, coherent emitterlight interaction strength dominates over the incoherent decay channels. In this context, optical nanofibers are a versatile platform as they allow one to combine both approaches and, in this way, to reach very strong lightmatter coupling in an fiber-integrated environment.Different nanofiber-based Fabry-Pérot resonator schemes have been developed, e.g., based on Bragg structures created using ion beam milling [6] Here, we demonstrate a tapered fiber-based ring resonator with optical characteristics that are compatible with entering the regime of single-atom strong coupling. We experimentally reach a resonator finesse of F = 75 ± 1 which corresponds to a single-atom cooperativity of C ≈ 1. Our implementation offers easy tuning of the resonator eigenpolarizations and out-coupling rate as well as straightforward adjustment of the resonator's free spectral range. Our system is co...

We experimentally determine tensile force-elongation diagrams of tapered optical fibers with a nanofiber waist. The tapered optical fibers are produced from standard silica optical fibers using a heat and pull process. Both, the force-elongation data and scanning electron microscope images of the rupture points indicate a brittle material. Despite the small waist radii of only a few hundred nanometers, our experimental data can be fully explained by a nonlinear stress-strain model that relies on material properties of macroscopic silica optical fibers. This is an important asset when it comes to designing miniaturized optical elements as one can rely on the well-founded material characteristics of standard optical fibers. Based on this understanding, we demonstrate a simple and non-destructive technique that allows us to determine the waist radius of the tapered optical fiber. We find excellent agreement with independent scanning electron microscope measurements of the waist radius.Glass fibers are among the most versatile inventions of the last century with applications reaching from fiberreinforced materials as used in construction [1] to optical data transmission in global telecommunication networks [2]. In recent years, so-called tapered optical fibers (TOFs) have received growing attention [3], both as optical components, e.g., for coupling light into microand nano-optical components, for sensing, as well as for the controlled coupling of light and matter at or near the TOF surface [4][5][6][7]. Beyond their optical properties, which have been extensively studied in the past, it is important to understand and control the mechanical properties of these devices for many of these applications.Here, we study the mechanical response of silica TOFs with a nanofiber waist when exposed to tensile stress. Qualitatively, we observe a brittle stress-strain behavior and no constriction of the two fiber ends when the fiber has been ruptured apart. Previous studies on the mechanical properties of such ultra-thin silica structures are divided over the importance of size effects in the nanoscopic domain, e.g., suggesting a decrease or an increase of Young's modulus compared to bulk values [8][9][10][11]. In our study, even for the smallest investigated waist radius of only 160 nm, the recorded stress-strain diagrams match those of macroscopic optical fibers [12][13][14]. As a consequence, in the parameter range considered here, it is justified to assume the well-studied mechanical properties of standard optical fibers when designing nano-optical elements. Based on this understanding, we provide a practical, non-destructive in-situ method to determine the TOF waist radius. The results obtained in this way are in very good agreement with independent radius measurements using a scanning electron microscope.The TOFs we study in our experiments are produced from commercial optical fibers (SM800, Fibercore) using a heat and pull process [15,16]. A schematic of such a cylindrically symmetric TOF is shown in Fig. 1(a). The TOF con...

No abstract