Bumki Min1,2,4, Eric Ostby1, Volker Sorger2, Erick Ulin-Avila2, Lan Yang1,4, Xiang Zhang2,3 & Kerry Vahala1
Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA
Nanoscale Science and Engineering Center, 5130 Etcheverry Hall, University of California, Berkeley, California 94720, USA
Material Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
Present addresses: Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-751, Republic of Korea (B.M.); Department of Electrical and Systems Engineering, Washington University in St Louis, St Louis, Missouri 63130, USA (L.Y.).
Surface plasmon polaritons (SPPs) are electron density waves excited at the interfaces between metals and dielectric materials1. Owing to their highly localized electromagnetic fields, they may be used for the transport and manipulation of photons on subwavelength scales2, 3, 4, 5, 6, 7, 8, 9. In particular, plasmonic resonant cavities represent an application that could exploit this field compression to create ultrasmall-mode-volume devices. A key figure of merit in this regard is the ratio of cavity quality factor, Q (related to the dissipation rate of photons confined to the cavity), to cavity mode volume, V (refs 10, 11). However, plasmonic cavity Q factors have so far been limited to values less than 100 both for visible and near-infrared wavelengths12, 13, 14, 15, 16. Significantly, such values are far below the theoretically achievable Q factors for plasmonic resonant structures. Here we demonstrate a high-Q SPP whispering-gallery microcavity that is made by coating the surface of a high-Q silica microresonator with a thin layer of a noble metal. Using this structure, Q factors of 1,376 65 can be achieved in the near infrared for surface-plasmonic whispering-gallery modes at room temperature. This nearly ideal value, which is close to the theoretical metal-loss-limited Q factor, is attributed to the suppression and minimization of radiation and scattering losses that are made possible by the geometrical structure and the fabrication method. The SPP eigenmodes, as well as the dielectric eigenmodes, are confined within the whispering-gallery microcavity and accessed evanescently using a single strand of low-loss, tapered optical waveguide17, 18. This coupling scheme provides a convenient way of selectively exciting and probing confined SPP eigenmodes. Up to 49.7 per cent of input power is coupled by phase-matching control between the microcavity SPP and the tapered fibre eigenmodes.