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...

We present experimental results on nonlinear, ultra-low power photonics applications based on a silica whispering-gallery-mode microresonator. Our bottle microresonator combines an ultrahigh quality factor of Q > 10(8) with a small mode volume V. The resulting Q(2)/V-ratio is among the highest realized for optical microresonators and allows us to observe bistable behavior at very low powers. We report single-wavelength all-optical switching via the Kerr effect at a record-low threshold of 50 microW. Moreover, an advantageous mode geometry enables the coupling of two tapered fiber waveguides to a bottle mode in an add-drop configuration. This allows us to route a CW optical signal between both fiber outputs with high efficiency by varying its power level. Finally, we demonstrate that the same set-up can also be operated as an optical memory.

Highly prolate-shaped whispering-gallery-mode "bottle microresonators" have recently attracted considerable attention due to their advantageous properties. We experimentally show that such resonators offer ultra-high quality factors, microscopic mode volumes, and near lossless in-and out-coupling of light using ultra-thin optical fibers. Additionally, bottle microresonators have a simple and customizable mode structure. This enables full tunability using mechanical strain and simultaneous coupling of two ultra-thin coupling fibers in an add-drop configuration. We present two applications based on these characteristics: In a cavity quantum electrodynamics experiment, we actively stabilize the frequency of the bottle microresonator to an atomic transition and operate it in an ultra-high vacuum environment in order to couple single laser-cooled atoms to the resonator mode. In a second experiment, we show that the bottle microresonator can be used as a low-loss, narrow-band add-drop filter. Using the Kerr effect of the silica resonator material, we furthermore demonstrate that this device can be used for single-wavelength all-optical signal processing.Recently, strain-tuning of a so-called "microbubble resonator" with a diameter of 200 µm over 690 GHz has been demonstrated. 20 For λ = 1550 nm, this corresponds to 2.2 azimuthal FSRs. Microbubble resonators

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