Typical microresonators exhibit a large frequency spacing between resonances and a limited tunability. This impedes their use in a large class of applications which require a resonance of the microresonator to coincide with a predetermined frequency. Here, we experimentally overcome this limitation with highly prolate-shaped whispering-gallery-mode "bottle microresonators" fabricated from standard optical glass fibers. Our resonators combine an ultrahigh quality factor of 3.6 x 10(8), a small mode volume, and near-lossless fiber coupling, characteristic of whispering-gallery-mode resonators, with a simple and customizable mode structure enabling full tunability.

Typical microresonators exhibit a large frequency spacing between resonances and a limited tunability. This impedes their use in a large class of applications which require a resonance of the microcavity to coincide with a predetermined frequency. Here, we experimentally overcome this limitation with highly prolate-shaped whispering-gallery-mode "bottle microresonators" fabricated from standard optical glass fibers. Our resonators combine an ultra-high quality factor of 360 million, a small mode volume, and near lossless fibre coupling, characteristic of whispering-gallery-mode resonators, with a simple and customizable mode structure enabling full tunability.PACS numbers: 42.60. Da, 42.50.Pq Optical microresonators hold great potential for many fields of research and technology [1]. They are used for filters and switches in optical communications [2][3][4], nonlinear optics [5], bio(chemical) sensing [6], microlasers [7][8][9], as well as for cavity quantum electrodynamics applications such as single photon sources [10-12] and interfaces for quantum communication [13,14]. All these applications rely on the spatial and temporal confinement of light by the microresonator, characterized by its mode volume V and its quality factor Q, respectively [1]. The ratio Q/V thus defines a key figure relating the coupling strength between light and matter in the resonator to the dissipation rates of the coupled system. The highest values of Q/V to date have been reached with whisperinggallery-mode (WGM) microresonators [15]. Standard WGM microresonators, like dielectric microspheres, microdisks, and microtori, typically confine the light in a narrow ring along the equator of the structure by continuous total internal reflection at the resonator surface [16]. While such equatorial WGMs have the advantage of a small mode volume they also exhibit a large frequency spacing between consecutive modes. In conjunction with the limited tuning range due to their monolithic design, tuning of equatorial WGM microresonators to an arbitrary frequency has therefore not been realized to date.For this reason, the WGM "bottle microresonator" has recently received considerable attention [17][18][19][20] because it promises a customizable mode structure while maintaining a favourable Q/V ratio [21,22]. Due to its highly prolate shape, the bottle microresonator gives rise to a class of whispering-gallery-modes (WGMs) with advantageous properties, see Fig. 1(a). The light in these "bottle modes" harmonically oscillates back and forth along the resonator axis between two turning points which are defined by an angular momentum barrier [22]. The resulting axial standing wave structure exhibits a significantly enhanced intensity at the so-called "caustics" of the bottle mode, located at the turning points of the harmonic motion. The bottle microresonator possesses an equidistant spectrum of eigenmodes, labelled by the "azimuthal quantum number" m, which counts the number of wave-FIG. 1: (a) Concept of the bottle microresonator. In addition to the r...

The guided modes of sub-wavelength diameter air-clad optical fibers exhibit a pronounced evanescent field. The absorption of particles on the fiber surface is therefore readily detected via the fiber transmission. We show that the resulting absorption for a given surface coverage can be orders of magnitude higher than for conventional surface spectroscopy. As a demonstration, we present measurements on sub-monolayers of 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) molecules at ambient conditions, revealing the agglomeration dynamics on a second to minutes timescale.PACS numbers: 78.66. Qn, 39.30.+w, 68.43.Jk, 78.66.Jg During the last twenty years, numerous optical tools for surface and interface analysis have been developed [1]. The selective sensitivity to surface effects is often obtained by carrying out spectroscopy with evanescent waves (EW), created by total internal reflection of light at the interface. This is straightforwardly realized by exciting waveguide modes in unclad optical fibers [2,3]. If the EW is resonant with the transition frequency of particles (atoms, molecules, quantum dots, etc.) in the surrounding medium, one can use both the particles' fluorescence [4] or the peak attenuation of the waveguide mode [2,5] to infer the concentration of particles at the interface. Moreover, the line shapes allow to spectroscopically retrieve detailed physical information about the nature and strength of the particle-surface interaction.Fiber-based evanescent wave spectroscopy (EWS) is used in various sensors [6]. The robustness, reliability, and ease of use of an all-fiber-based sensor technology is advantageous for in situ sensing in a remote or isolated location or in a harsh environment, e.g., in industrial applications or environmental studies. Furthermore, such sensors also profit from the multiplexing and miniaturization potential inherent to fiber technology. When measuring a volumetric concentration of particles in the surrounding medium, these sensors yield however a reduced sensitivity compared to conventional free-beam absorption: a significant fraction of the light propagates inside the waveguide and therefore does not interact with the particles of interest. This problem can partially be overcome by increasing the power fraction in the EW through proper choice of the fiber mode or geometry [7,8,9]. Yet, even in the ultimate case of 100 % EW, the sensitivity will not exceed that of free-beam absorption techniques.In this letter, we demonstrate that the situation can be dramatically different when employing fiber-based EWS for the spectroscopic study of surface coverages instead of volumetric concentrations: The ultimate sensitivity of fiber-based surface absorption spectroscopy (SAS) is shown to strongly depend on the fiber diameter and to exceed free-beam SAS by several orders of magnitude in the case of sub-wavelength diameter fibers. Fiber-based surface absorption spectroscopy (SAS) has already been used for a number of applications, e.g., in bio-sensors [10]. However, to our kno...

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The forward scattering of light illuminating a transparent dielectric cylinder, such as a tapered fiber, from the side can be understood as interference of the diffracted, reflected, and transmitted light. Additionally, light can be resonantly coupled into the fiber if a multiple of the wavelength matches the circumference. Using a suitable laser setup with a novel evaluation algorithm allows us to quickly extract the fiber radius from the complex diffraction pattern, obtaining an accuracy of better than 50 nm. We demonstrate experimentally our method, which is noncontact and allows one to simultaneously measure the profile of a several-centimeter-long fiber waist with a diameter near the diffraction limit.