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.

Controlled growth of plasmonic heterostructures and their applications

Plasmonic nanocrystals with unique properties have been extensively studied in the past decades.A combination of plasmonic materials with other characteristic materials of metals and semiconductors leads to properties far beyond single-component materials and excellent performances in many optical-related applications.In this review,we summarize the recent advances in the controlled growth of plasmonic heterostructures with a specific composition,morphology,size,and structural symmetry.Plasmonenhanced properties of the heterostructures and their excellent performances in applications are also discussed.The synthesis strategies and the intriguing properties of the plasmonic heterostructures provide great opportunity for applications in plasmon-enhanced nonlinear optics,optical spectroscopy,photocatalysis,solar energy conversion,and so on.

Enhanced second-harmonic generation of asymmetric Au@CdSe heterorods

The plasmon-enhanced second-harmonic generation(SHG)of Au nanorods(NRs),Au-Ag NRs,and asymmetric Au@CdSe heterorods are comparatively investigated.The Au@CdSe tip-and side-heterorods are synthesized by precisely controlling the concentration of Cd2+precursor,and the plasmon resonances can be well tuned simultaneously.The SHG of asymmetric Au@CdSe heterorods has been greatly enhanced with up to 617.7-fold enhancement compared with the Au NRs because of the asymmetric morphology and local field enhancement.Moreover,the harmonic energy transfer induced by the local interband transition of Au has also been demonstrated by the excitation-wavelengthdependent SHG in the Au NRs and Au@CdSe side-heterorods,while it is negligible in the Au-Ag NRs and Au@CdSe tipheterorods because of the presence of Ag.The plasmon-enhanced SHG of asymmetric Au@CdSe heterorods enables potential applications ranging from sensing to biological spectroscopy.

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.