Plasmon-assisted photoresponse in Ge-coated bowtie nanojunctions

TL;DR

This study demonstrates plasmon-enhanced photoconduction in gold bowtie nanojunctions with germanium overlays, revealing polarization-dependent effects and mode shifts, supported by experimental and simulation analyses.

Contribution

It provides new insights into plasmon mode shifts caused by Ge overlays and confirms that these shifts are due to mode redshifting, not material degradation.

Findings

Nanogap polarization rotates 90 degrees with Ge thickness.

Mode shift from transverse plasmon to lightning rod mode observed.

No degradation of plasmonic properties due to Ge overlay confirmed.

Abstract

We demonstrate plasmon-enhanced photoconduction in Au bowtie nanojunctions containing nanogaps overlaid with an amorphous Ge film. The role of plasmons in the production of nanogap photocurrent is verified by studying the unusual polarization dependence of the photoresponse. With increasing Ge thickness, the nanogap polarization of the photoresponse rotates 90 degrees, indicating a change in the dominant relevant plasmon mode, from the resonant transverse plasmon at low thicknesses to the nonresonant "lightning rod" mode at higher thicknesses. To understand the plasmon response in the presence of the Ge overlayer and whether the Ge degrades the Au plasmonic properties, we investigate the photothermal response (from the temperature-dependent Au resistivity) in no-gap nanowire structures, as a function of Ge film thickness and nanowire geometry. The film thickness and geometry dependence are modeled using a cross-sectional, finite element simulation. The no-gap structures and the modeling confirm that the striking change in nanogap polarization response results from redshifting of the resonant transverse mode, rather than degradation in the Au/Ge properties. We note remaining challenges in determining the precise mechanism of photocurrent production in the nanogap structures.