Integrated on-chip localized surface plasmon resonances in a subwavelength periodic array of gold nanowires

Motivated by the miniaturization of optical devices for lab-on-a-chip applications, in this contribution we present an integrated hybrid plasmonic-photonic structure able to excite the localized surface plasmon resonance... [ view full abstract ]

Motivated by the miniaturization of optical devices for lab-on-a-chip applications, in this contribution we present an integrated hybrid plasmonic-photonic structure able to excite the localized surface plasmon resonance (LSPR) of a sub-diffractive periodic array of metallic nanowires (Fig. 1a). The system, fabricated with e-beam lithography and thermal evaporation processes, consists of a periodic array of 500 gold nanowires of thickness h_x=30 nm, width w_z=75 nm, length L_y=300 µm and period Λ=185 nm (Fig. 1b and c), integrated on top of an ion-exchanged glass waveguide (IExWg).

The device was experimentally characterized in the far- and near-field regimes with the use of transmission spectroscopy with a white-light source, and near-field scanning optical microscopy (NSOM) with a laser source.

Due to the invariance of the nanowires along the y direction, their LSPR is only excited when the fundamental TM0 mode of the IExWg propagates through the structure (Fig. 2a), and not when propagating the fundamental TE0 mode of the IExWg (Fig. 2b). The transmission spectrum of the TM0 mode measured at the output facet of the waveguide, exhibits broad depth with a minimum value around λ=675 nm, corresponding to the excitation of the LSPR of the nanowires (Fig. 2c).

Using the NSOM technique (Fig. 3) and analyzing the resulting images in the Fourier space, we identified two modes radiated into the air and glass substrate with effective indices n_eff,sup=1.058±0.073 and n_eff,rad=1.35±0.073, respectively, and the propagating TM0 mode of the hybrid structure with an effective index n_eff,TM0=1.496±0.073. No other mode was observed in the structure, implying that there is no light propagation along the metallic nanowires.

This interesting result means that even when the structure is a sub-diffractive metallic grating, there is no strong coupling of the plasmonic resonances between adjacent nanowires. Hence, the hybrid system can be regarded as a collection of almost individual localized light sources excited with the photonic TM0 mode of the IExWg.

Due to the ultra-low coupling loss with commercial single-mode optical fibers in the visible range, the proposed system opens new perspectives in the design of optical chips to control light at the nanoscale for biosensing applications.