Plasmonic Bi-Cavity Nanostructure for Efficient Light Collection and Localization

TL;DR

This paper introduces a plasmonic bi-cavity nanostructure, the PTTP, that enhances light collection and localization in TERS using low-NA optics, enabling more practical and versatile nano-Raman measurements.

Contribution

The study presents a novel plasmonic bi-cavity tip design supporting hybrid modes that improve TERS efficiency with low-NA objectives, expanding practical applications.

Findings

Supports hybrid antenna-cavity mode for efficient energy funneling.

Achieves near-field enhancement with low-NA objective.

Reproduces characteristic TERS PSF on graphene.

Abstract

Tip-enhanced Raman spectroscopy (TERS) typically relies on high-NA excitation to generate a strong axial field at the tip apex, which shortens the working distance and constrains sample geometries. We show that a plasmonic bi-cavity tip, the plasmon-tunable tip pyramid (PTTP), co-tuned in nanopyramid length L and plateau length W, supports a hybrid antenna-cavity mode that funnels energy to the apex under radially polarized, on-axis excitation, even with a dry objective of NA = 0.75. Finite-element simulations identify W as a design-critical parameter that sets an in-plane surface-plasmon-polariton (SPP) Fabry-P\'erot-like resonance; co-tuning (L,W) yields a periodic series of maximal apex |E|^2. Experiments on monolayer graphene confirm near-field enhancement and reproduce the characteristic annular TERS point spread function (PSF) with NA = 0.75. Relaxing the NA requirement increases working distance and compatibility with constrained environments, pointing to practical, deployment-ready nano-Raman instrumentation.