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

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

Hideaki Takashima, Kazutaka Kitajima, Yoshito Tanaka, Hideki Fujiwara, and Keiji Sasaki

Phys. Rev. A 89, 021801(R) – Published 5 February 2014

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.

Received 21 August 2012
Revised 20 September 2013

DOI:https://doi.org/10.1103/PhysRevA.89.021801

Authors & Affiliations

Hideaki Takashima, Kazutaka Kitajima, Yoshito Tanaka, Hideki Fujiwara, and Keiji Sasaki^*

Research Institute for Electronic Science, Hokkaido University, Kita-20, Nishi-10, Kita-ku, Sapporo, Hokkaido 001-0020 Japan

^*sasaki@es.hokudai.ac.jp

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Issue

Vol. 89, Iss. 2 — February 2014

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Images

Figure 1
Optical coupling into a metal nanostructure using a tapered-fiber-coupled microspherical cavity. The double-headed arrows show the direction of the electric field (TM mode). Propagating light in a tapered fiber is coupled to a microspherical cavity, and a single metal nanostructure (LSP antenna) placed on or near the microsphere surface can be coupled to the cavity mode. By adjusting the coupling parameters, the critical PL-LSP coupling is realized such that the whole power of incident light can be harvested by the LSP antenna.
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Figure 2
PL-LSP coupling system with a tapered-fiber-coupled microspherical cavity. (a) Schematic of experimental setup. A tunable diode laser is introduced into the tapered fiber through a fiber coupler (FC), and the output intensity is detected by a photodiode (PD). The laser frequency is swept by applying a voltage from a function generator (FG) to measure the transmission spectra. The laser power is adjusted using a half-wave plate (λ/2) and a polarization beam splitter (PBS) and measured by an optical power meter (PM) connected to a balanced fused fiber coupler (FFC). The polarization of the incident light is adjusted using a half-wave plate (λ/2) and two quarter-wave plates (λ/4). (b) A photograph of the experimental setup around a tapered-fiber-coupled microspherical cavity with a metal tip. (c) A photograph of the microspherical cavity coupled with the tapered fiber and the metal tip. (d) A scanning electron micrograph of the Au-coated tip. The circle indicates the curvature radius of the end of the Au-coated tip, which is approximately 50 nm.
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Figure 3
Transmission spectra of the tapered-fiber-coupled microspherical cavity without the metal tip measured at four different distances. From top to bottom, the separation distances between the tapered fiber and the microsphere surface are 350, 280, 70, and 0 nm.
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Figure 4
Transmission spectra of the tapered-fiber-coupled microspherical cavity with the metal tip measured at four different distances. From top to bottom, the separation distances between the microsphere surface and the metal tip are 1140, 330, 50, and 0 nm. The inset is a transmission spectrum with uncoated AFM tip under the contact condition.
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Figure 5
Separation distance dependence of the transmittance and linewidth. (a) The transmittance of the resonant dip as a function of the separation distance between the metal tip and the microsphere surface. (b) The resonant linewidth as a function of the separation distance. The vertical and horizontal solid green lines indicate the separation distance at the critical coupling condition and the linewidth at the long-distance limit, respectively.
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