High-Q surface-plasmon-polariton whispering-gallery microcavity

Interest in the properties of surface plasmon polaritons is intense because of their relevance to plasmonics and nanophotonics. They are electron density waves excited at the interface between metals and dielectric materials and interact strongly with light at a subwavelength-scale. A good starting point for useful applications would be a plasmonic micro- or nanocavity with a high figure of merit, or Q-value; a high Q factor means that the plasmons are strongly confined and bounce around many times inside the cavity before escaping, resulting in a rich range of physical properties. Until now the Q-factor for plasmonic resonant cavities has been limited to values less than one hundred for visible and near-infrared wavelengths. Now Min et al. demonstrate a high-Q 'whispering gallery' microcavity for surface plasmons that is fabricated by coating the surface of high-Q silica microresonator with a thin layer of noble metal. This structure enables room-temperature operation with a Q-factor of around 1,380 in the near infrared for surface plasmon modes — a nearly ideal value. The work also includes a coupling scheme where a tapered optical fibre is in near-contact with the cavity, which provides a convenient way for selectively exciting and probing confined plasmon modes.