We experimentally realize a Fabry-Perot-type optical microresonator near the cesium D2 line wavelength based on a tapered optical fiber, equipped with two fiber Bragg gratings that enclose a subwavelength diameter waist. Owing to the very low taper losses, the finesse of the resonator reaches F=86 while the on-resonance transmission is T=11%. The characteristics of our resonator fulfill the requirements of nonlinear optics and cavity quantum electrodynamics in the strong coupling regime. These characteristics, combined with the demonstrated ease of use and advantageous mode geometry, open a realm of applications.

Modeling and investigating the thermalization of microscopic objects with arbitrary shape from first principles is of fundamental interest and may lead to technical applications. Here, we study, over a large temperature range, the thermalization dynamics due to far-field heat radiation of an individual, deterministically produced silica fiber with a predetermined shape and a diameter smaller than the thermal wavelength. The temperature change of the subwavelength-diameter fiber is determined through a measurement of its optical path length in conjunction with an ab initio thermodynamic model of the fiber structure. Our results show excellent agreement with a theoretical model that considers heat radiation as a volumetric effect and takes the emitter shape and size relative to the emission wavelength into account.

Tapered optical fibers with a nanofiber waist are widely used tools for efficient coupling of light to photonic devices or quantum emitters via the nanofiber's evanescent field. In order to ensure wellcontrolled coupling, the phase and polarization of the nanofiber guided light field have to be stable. Here, we show that in typical tapered optical fibers these quantities exhibit high-frequency thermal fluctuations. They originate from high-Q torsional oscillations that opto-mechanically couple to the nanofiber-guided light. We present a simple ab-initio theoretical model that quantitatively explains the torsional mode spectrum and that can be used to design tapered optical fibers with tailored mechanical properties.PACS numbers: 46.40.Ff, 62.25.Fg, 78.67.Uh Tapered optical fibers with a subwavelength-diameter waist (TOFs) feature a strong evanescent field in the waist region and are widely used to efficiently interface light and matter or to couple light into photonic devices, such as micro-resonators or photonic crystals. These applications rely on the stability of the phase and polarization of the nanofiber guided light field as well as on the position of the nanofiber. Achieving this stability is all the more challenging in a high vacuum environment where the mechanical damping due to the surrounding gas is negligible. Here, we experimentally demonstrate that under these conditions torsional mechanical modes exhibit surprisingly high quality factors and lead to resonantly enhanced vibrations that modulate the phase and polarization of the optical mode via the strain-optic effect. Based on an analytic model as well as on experimental measurements, we show that the commonly used exponential radius profile confines a subset of the torsional mechanical modes to the nanofiber section, leading to the high Q-factors.From a mechanical point of view, a TOF is a slender cylinder with varying cross-section. Torsional waves in such a structures can be described by a one-dimensional wave equation which has been treated comprehensively in the literature [1,2]. In case of the mechanical modes considered here, their wavelength is much larger than the largest cross-section of the TOF so that no higher order angular and radial modes exist (slender rod approximation). Furthermore, the radius variations occurring in the TOF profile are sufficiently shallow to assume plane wavefronts that are perpendicular to the fiber axis. Under these conditions, the torsional motion can be described by a Webster-type wave equation where φ(t, z) is the angular displacement amplitude as function of time t and axial position along the fiber z and ∂ x = ∂/∂x with x = t, z. The torsional wave velocity is radius independent and given by c t = G/ρ = (3680 ± 130) m/s, where G = (30 ± 2) GPa is the shear modulus and ρ = (2210 ± 10) kg/m 3 is the mass density of silica [3][4][5]. The third term of the wave equation takes the radius profile of the cylinder, a(z), into account via the polar angular moment of inertia I p (z) = π ρ a(z) 4 /2.Using a sepa...

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...