Anapole-assisted giant electric field enhancement for surface-enhanced coherent anti-Stokes Raman spectroscopy

Maryam Ghahremani,Carlos J. Zapata-Rodriguez,Mojtaba Karimi Habil

doi:10.1038/s41598-021-90061-5

Maryam Ghahremani, Carlos J. Zapata-Rodriguez + Show 1 more

Open Access

https://doi.org/10.1038/s41598-021-90061-5

Copy DOI

Abstract

The coherent anti-Stokes Raman spectroscopy (CARS) techniques are recognized for their ability to detect and identify vibrational coherent processes down to the single-molecular levels. Plasmonic oligomers supporting full-range Fano-like line profiles in their scattering spectrum are one of the most promising class of substrates in the context of surface-enhanced (SE) CARS application. In this work, an engineered assembly of metallic disk-shaped nanoparticles providing two Fano-like resonance modes is presented as a highly-efficient design of SECARS substrate. We show that the scattering dips corresponding to the double-Fano spectral line shapes are originated from the mutual interaction of electric and toroidal dipole moments, leading to the so-called non-trivial first- and second-order anapole states. The anapole modes, especially the higher-order ones, can result in huge near-field enhancement due to their light-trapping capability into the so-called “hot spots”. In addition, independent spectral tunability of the second Fano line shape is exhibited by modulating the gap distance of the corner particles. This feature is closely related to the electric current loop associated with the corner particles in the second-order anapole state and provides a simple design procedure of an optimum SECARS substrate, where the electric field hot spots corresponding to three involved wavelengths, i.e., anti-Stokes, pump, and Stokes, are localized at the same spatial position. These findings yield valuable insight into the plasmonic substrate design for SECARS applications as well as for other nonlinear optical processes, such as four-wave mixing and multi-photon surface spectroscopy.

Highlights

The coherent anti-Stokes Raman spectroscopy (CARS) techniques are recognized for their ability to detect and identify vibrational coherent processes down to the single-molecular levels
Interaction between the surface charge-density oscillations of noble-metal nanoparticles and the incident photons at the resonant frequency leads to the coherent and collective oscillation of free electrons confined at the metaldielectric interfaces, which refers to localized surface plasmon resonances (LSPRs)[1,2,3]
Let us first propose a simple plasmonic oligomer which serves as a basis for our engineered SECARS substrate

Summary

Introduction

The coherent anti-Stokes Raman spectroscopy (CARS) techniques are recognized for their ability to detect and identify vibrational coherent processes down to the single-molecular levels. Independent spectral tunability of the second Fano line shape is exhibited by modulating the gap distance of the corner particles This feature is closely related to the electric current loop associated with the corner particles in the second-order anapole state and provides a simple design procedure of an optimum SECARS substrate, where the electric field hot spots corresponding to three involved wavelengths, i.e., anti-Stokes, pump, and Stokes, are localized at the same spatial position. These findings yield valuable insight into the plasmonic substrate design for SECARS applications as well as for other nonlinear optical processes, such as four-wave mixing and multi-photon surface spectroscopy. Inter-particle gap distances between the adjacent particles define the coupling strength between LSPR modes of the elementary nano-scatterers

Methods

Results

Conclusion