Silicon Waveguide Integrated Nanoplasmonics for Optoelectronic and Sensing Applications

Abstract

Silicon photonics, utilizing silicon and other semiconductors for on-chip light manipulation, has scaled the conventional optoelectronic devices (e.g. waveguide, photodetector and modulators) down to sub-micrometer footprint. This enables rack-to-rack high-bandwidth optical communication in data centers. Moreover, silicon photonics provides an on-chip platform for fundamental research including optomechanics, cavity electrodynamics and chemical sensing. On the other hand, surface plasmon polariton (SPP), can confine light into hotspot below optical diffraction limit and significantly enhance local electromagnetic field. This boosts the light-matter interaction significantly and enable biosensing applications including SERS and SEIRA. However, the metallic structure introduces high optical absorption, which is detrimental for long distance light propagation. In this dissertation, we focus on the on-chip integration of silicon photonics and plasmonics: using the dielectric waveguide for light delivery and plasmonics for light focusing. This integration approach combines the advantages of the low propagation loss of silicon waveguides, high-field confinement of a plasmonic structure for enhanced light-matter interaction. In the first project, we show the integration of a black phosphorus photodetector in a three-dimensional architecture of silicon photonics and metallic nanoplasmonics structures. By vertically integrating plasmonic grating on top of a silicon waveguide grating, a nanoscale optic hotspot is created. Adding another layer of 2D material, black phosphorus, an efficient telecom-band photodetector is fabricated. The short-channel (∼60 nm) BP FET shows an on-off ratio up to 1000. Moreover, benefiting from the ultrashort channel and near-field enhancement enabled by the nanogap structure, the photodetector shows an intrinsic responsivity as high as 10 A/W afforded by internal gain mechanisms, and a 3-dB roll-off frequency of 150 MHz. Such a hybrid integration also obviates the need for a bulky free-space optics setup and can lead to fully integrated, on-chip optical sensing systems. In the second project, we directly pattern an ultra-compact plasmonic resonator atop a mid-infrared silicon waveguide for spectroscopic chemical sensing. The footprint of the plasmonic nanorod resonators is as small as 2 µm2, yet they can couple with the mid-infrared waveguide mode efficiently. The plasmonic resonance is verified by measuring the transmission spectrum of the waveguide, with a coupling efficiency greater than 70% and a field intensity enhancement factor of over 3600 relative to the evanescent waveguide field intensity. Using this hybrid device and a tunable mid-infrared laser source, surface-enhanced infrared absorption spectroscopy of both a thin PMMA film and an octadecanethiol monolayer are successfully demonstrated.