Efficient optical coupling into a single plasmonic nanostructure using a fiber-coupled microspherical cavity

Abstract

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...

Enhancing Coherent Light-Matter Interactions through Microcavity-Engineered Plasmonic Resonances

“…Despite of these efforts, the nonradiative decay from the dipolar plasmonic modes remains serious for small metallic nanostructures which are advantageous for stronger light-matter interaction due to more confined fields [11]. Here we provide the perspective of microcavity-engineered metallic nanostructure system, which is not revealed in previous studies of the hybrid photonic-plasmonic modes [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46]. The microcavity engineers electromagnetic environment of dipolar plasmonic mode, enhancing its radiation rate and further reducing the Ohmic absorption.…”

Optical selection and sorting of nanoparticles according to quantum mechanical properties

“…Thus, we focused on tapered optical fibers, i.e., nanofibers ( 22 , 23 ). We prepared a nanofiber with a diameter of several hundred nanometers and length of several millimeters ( 24 ), which exhibited the characteristics of single-mode propagation, thereby forming an intense evanescent field around the fiber and enabling long-distance propagation while maintaining a tightly focused beam of light. Using these characteristics, a uniform electric field distribution could be generated along the fiber by which the particle motion was restricted to one dimension.…”

“…Figure 1B illustrates the experimental setup. A nanofiber with a diameter of 400 nm was fabricated from a commercial single-mode optical fiber ( 24 ). The diameter is constant in the waist part of the fiber over a length of several hundred micrometers.…”

Fabrication of a nanofiber Bragg cavity with high quality factor using a focused helium ion beam

“…For these properties, nanofibers can realize the efficient input/output of photons to the single mode fiber via the taper waist without a microscope objective. Nanofibers have thus been used as waveguides to couple photons into solid-state microcavities [9,10]. Furthermore, they can efficiently couple photons emitted from single light emitters into the fundamental mode of a single-mode fiber [11,12].…”