Toward complete coupling between propagating light (PL) and a single localized-surface-plasmon (LSP) nanostructure, we propose a tapered-fiber-coupled microspherical cavity system combining an Au-coated probe tip. This system possesses the unique characteristic of precise adjustability for the fiber-cavity coupling rate and the cavity-plasmon coupling rate, which is indispensable for achieving the critical coupling conditions. We successfully demonstrate the 93% PL coupling into the LSP antenna with an effective area of a 58 nm circle, exceeding the diffraction limit. Localized surface plasmons (LSPs) of metal nanostructures have attracted considerable attention because of the unique enhancement of the exciton-photon coupling by strong localization of the optical field [1][2][3][4][5][6][7][8][9][10][11]. The LSP polaritons have the ability to confine the optical field into nanometer-scale areas, exceeding the diffraction limit, using the so-called optical antenna effect [1][2][3]. LSP fields with the small mode volumes strongly enhance the interactions between light and matter [4][5][6][7][8]. Recently, vacuum Rabi splitting was observed in exciton-plasmon coupling systems at room temperature [9][10][11]. This effect demonstrates that a single photon harvested by the LSP antenna can couple into a single-exciton state with high efficiency, which will be applicable to highly efficient photonic devices. One of the ultimate goals is a single-photon switching gate for quantum information [12,13].Although the strong-coupling regime can be obtained within the LSP field, the coupling efficiency between a single LSP antenna and propagating light (PL) in free space or in optical waveguides is extremely low. The harvesting antenna size, which corresponds to the extinction cross section of a single metal nanostructure, is typically ß10 3 nm 2 for visible light [14], which is comparable to the actual geometrical cross section of the structure. The desired size of a single antenna structure is on the order of tens of nanometers, which is determined from the LSP resonance wavelength. Conversely, the cross section of the PL mode is ß10 5 nm 2 for a diffraction-limited spot and ß10 7 nm 2 for a single-mode optical fiber. This size mismatch between the LSP antenna and the PL mode results in a low coupling efficiency on the order of ß10 −4 −10 −2 , which limits the total throughput of the exciton-photon coupling via the LSP polariton.The use of microcavities can drastically improve the efficiency of the optical coupling with metal nanostructures. There are some reports on the cavity-LSP coupling systems and their applications to highly sensitive biosensors and efficient photon emitters [15][16][17][18][19]. Those works, however, have not paid attention to ultimate optimization of the coupling between the PL mode and the LSP antenna via the microcavity mode [20,21]. Toward complete PL-LSP coupling, which means that all power of PL can couple into a single LSP antenna * sasaki@es.hokudai.ac.jp without loss, we employ a tapered-fibe...

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