Toroidal dipole-modulated dipole-dipole double-resonance in colloidal gold rod-cup nanocrystals for improved SERS and secondharmonic generation

Colloidal metal nanocrystals(NCs)show great potential in plasmon-enhanced spectroscopy owing to their attractive and structure-depended plasmonic properties.Herein,unique Au rod-cup NCs,where Au nanocups are embedded on the one or two ends of Au nanorods(NRs),are successfully prepared for the first time via a controllable wet-chemistry strategy.The Au rod-cup NCs possess multiple plasmon modes including transverse and longitudinal electric dipole(TED and LED),magnetic dipole(MD),and toroidal dipole(TD)modulated LED resonances,producing large extinction cross-section and huge near-field enhancements for plasmon-enhanced spectroscopy.Particularly,Au rod-cup NCs with two embedded cups show excellent surface-enhanced Raman spectroscopy(SERS)performance than Au NRs(75.6-fold enhancement excited at 633 nm)on detecting crystal violet owing to the strong electromagnetic hotspots synergistically induced by MD,LED,and TED-based plasmon coupling between Au cup and rod.Moreover,the strong TD-modulated dipole-dipole double-resonance and MD modes in Au rod-cup NCs bring a 37.3-fold enhancement of second-harmonic generation intensity compared with bare Au NRs,because they can efficiently harvest photoenergy at fundamental frequency and generate large near-field enhancements at second-harmonic wavelength.These findings provide a strategy for designing optical nanoantennas for plasmon-enhanced applications based on multiple plasmon modes.

Anomalous plasmon coupling and Fano resonance under structured light

Structured light carrying orbital angular momentum(OAM)opens up a new physical dimension for studying light–matter interactions.Despite this,the complex fields created by OAM beams still remain largely unexplored in terms of their effects on surface plasmons.This paper presents a revelation of anomalous plasmon excitations in single particles and plasmon couplings of neighboring nanorods under OAM beams,which are forbidden using non-OAM sources.The plasmon excitation of single nanoparticles is determined both by photon spin angular momentum(SAM)and OAM and influenced by the locations of the nanoparticles.Specifically,when SAM and OAM are equal in magnitude and opposite in direction,a pure plasmon excitation along light propagation direction is achieved.Two plasmon dipoles show end-to-end antibonding coupling and side-by-side bounding coupling,which are the opposite of the typical couplings.Furthermore,we observe Fano resonance with a nanorod dimer:one aligned along light propagation direction acting as the bright mode and the other aligned along the global polarization direction of light acting as the dark mode,which is the opposite of the usual plasmonic Fano resonance.By taking advantage of the unique property of the OAM source,this investigation presents a novel way to control and study surface plasmons,and the research of plasmon behavior with OAM would open new avenues for controlling electromagnetic waves and enriching the spectroscopies with more degrees of freedom.