Experimental Demonstration of Low Loss Gap Plasmon Waveguides on the Silicon-on-Insulator Platform

Michael Nielsen Physics, Imperial College London, London, UK Lucas Lafone Physics, Imperial College London, London, UK Themistoklis Sidiropoulos Physics, Imperial College London, London, UK Stefan Maier Physics, Imperial College London, London, UK Rupert Oulton Physics, Imperial College London, London, UK

Facing the fundamental speed and integration limits of conventional semiconductor electronics, there has been a growing interest in developing all-optical circuitry capable of high speed broadband operation. Silicon photonics, due to its compatibility, was originally considered the obvious candidate. However, the optical diffraction limit restricts such photonic devices to minimum dimensions of half the operating wavelength, far in excess of those in conventional electronics. Here, silicon nanoplasmonics structures have been shown to be a viable alternative; these can operate in the sub-wavelength regime while still being capable of integration on a conventional electronic platform.

This work presents the first experimental demonstration of a novel silicon hybrid gap plasmon waveguide. Formed of a gap in a thin metal film atop an unetched silicon-on-insulator substrate, a recent theoretical proposal showed that these hybrid plasmonic waveguides had unique advantages for nanofocusing applications compared to other hybrid plasmonic waveguides [1]. At large gap widths, in excess of 100nm, the mode propagates primarily in the underlying silicon away from the metallic films, leading to propagation lengths in the tens or even hundreds of micrometres, with a 115nm Au gap having a demonstrated propagation length of 27μm at 1550nm. At these widths, the waveguides are ideal for long range low loss interconnect applications. When the waveguide width is focused much smaller than 100nm, the mode becomes concentrated in the gap, leading to mode areas far below the diffraction limit allowing for extreme nanofocusing. At extremely small gap widths, upwards of 90% of the modal power can be concentrated in the gap, albeit at the cost of reduced propagation lengths; a 25nm Au gap shows a propagation length of 4.5μm at 1550nm. In addition to measuring the linear propagation lengths of the waveguides, several common passive devices were designed, fabricated, and characterised based on these waveguides. Ideally, using a GaAs substrate, the strong focussing, and larger modal overlap with the substrate can be exploited to build sub-wavelength plasmonic nanolasers.

[1] L. Lafone, et al., "Silicon-based metal-loaded plasmonic waveguides for low-loss nanofocusing.," Opt. Lett. 39, 4356–9 (2014).

t.sidiropoulos10@imperial.ac.uk









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