Ultra-high-Q tunable whispering-gallery-mode microresonator

Abstract

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...

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“…resonator fabrication is extremely fast and versatile and reproducibly generates optical quality factors exceeding 5x10 8 , and up to 1x10 9 , which is comparable to the highest observed optical quality factors in other fused-silica resonators [33][34][35]26]. Moreover, our technique allows control of both the fundamental diameter as well as the shape of the resonator sidewall by controlled ablation of material from the glass preform.…”

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“…The resulting OFRR capillary is slightly bulged with a diameter of 75 μm at the center and of a few microns smaller at the two necking points approximately 1 mm apart. This bottle-like structure provides the optical confinement along the OFRR capillary longitudinal direction (37)(38)(39)(40). The wall thickness of the OFRR is 3-4 μm (9).…”

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“…2(a) -top -are highly advantageous when bottle microresonators are used in cavity QED studies [9]. However, such spectral characteristics may be a serious hindrance if HBMRs were to be used as refractometric optofluidic sensors.…”

“…Among microresonators, microdisks [2], microspheres [3] and microtoroids [4] rely on the efficient excitation of whispering gallery modes (WGM), which are confined predominantly in one azimuthal plane and show well resolved spectral characteristics. A recently proposed truly 3-D optical microresonator, namely the bottle microresonator (BMR) [5]- [10], proves to be very promising for a number of applications such as cavity QED studies [9], strain [11] and optofluidic [12] sensors as well as add-drop multiplexers [13]. BMRs, however, exhibit particularly dense spectral features, which require considerable effort to be identified [10].…”

“…The rich spectral features are a result of the strongly broken degeneracy between WGMs with common azimuthal and differing axial mode numbers, as a result of the highly prolate shape. While dense spectral features are highly advantageous when bottle microresonators are used in cavity QED studies [9], they can potentially be a serious hindrance if BMRs were to be used in other applications such as refractometric optofluidic sensors. In this case, a cleaner spectrum with more easily identifiable and traceable spectral features would be desirable.…”