Plasmon-Coupled Whispering Gallery Modes on Nanodisk Arrays for Signal Enhancements

Abstract

Metallic nanostructures including single and double nanodisks are successfully used to enhance the localized electric field in vicinity of microcavity in whispering gallery mode (WGM) sensor. We demonstrate numerical calculations of plasmonic coupling of WGMs to single and double nanodisk arrays on a planar substrate. We then experimentally confirmed that the resonance wavelength of WGM sensor was dramatically shifted by adoption of single and double nanodisks on the surface of microcavity in the WGM sensor. Thus, our approach provides the tunable sensitivity of WGM sensor, and has a great potential to be used in numerous areas where the single biomolecule, protein-protein folding and biomolecular interactions are involved.

Figure 1

Metallic nanodisk array for plasmonic coupling in whispering gallery mode sensor. (a) Schematic illustration of the nanodisk-equipped whispering gallery mode sensor for controlled plasmonic coupling. E-beam lithography technique enables the fabrication of precisely defined nanostructure, including Au thinfilm, single nanodisk and double nanodisk. The position of the microsphere on nanodisk array is controlled by piezoelectric actuators. (b,c) Scanning electron microscope images of the single nanodisk array and double nanodisk array. The diameter and thickness of each nanodisk were chosen as 300 nm and 50 nm.

Figure 2

Whispering gallery mode (WGM) resonance of microsphere cavity coupled to the single nanodisk. (a) Schematic illustration of the WGM coupling between microsphere and single nanodisk when we shined the incident light with TM and TE polarization. A photodetector connected to the optical fiber recorded the WGM transmission spectrum in real-time. The changes in spectrum intensity of WGM resonance under the incident light with (b) TM and (c) TE polarization.

Figure 3

Numerical calculation of plasmonic coupling in WGM sensor mounted on the single and double metallic nanodisks (ND). (a) The WGM spectrum intensity of the circulated light in microsphere mounted on thin film and single nanodisk and double nanodisk when the incident light with TM polarization was applied. (b) Electric field distribution |E|² in the vicinity of the (i) thinfilm, (ii) single nanodisk and (iii) double nanodisk.

Figure 4

The wavelength shift in the spectrum of WGM sensor on the metallic nanodisk arrays. (a) Single nanodisk. (b) Double nanodisk.

Figure 5

Experimental measurements of WGM resonance wavelength shifts due to coupling of the ~200 μm microsphere to gold film on BK7 glass substrate with and without nanodisk arrays: (a) single nanodisk and (b) double nanodisks. The WGM resonance wavelengths are near 1310 nm. Arrows indicate optical coupling and decoupling of the microsphere as its vertical position is changed by ~100 nm piezo-stage movements over time.