Near-field and photocatalytic properties of mono-and bimetallic nanostructures monitored by nanocavity surface-enhanced Raman scattering

Localized surface plasmon resonances(LSPR)generated in a particle-film nanocavity enhance electric fields within a nanoscale volume.LSPR can also decay into hot carriers,highly energetic species that catalyze photocatalytic reactions in molecular analytes located in close proximity to metal surfaces.In this study,we examined the intensity of the electric field(near-field)and photocatalytic properties of plasmonic nanocavities formed by single nanoparticles(SNP)on single nanoplates(SNL).Using 4-nitrobenzenethiol(4-NBT)as a molecular reporter,we determined the near-field responses,as well as measured rates of 4-NBT dimerization into 4,4-dimercaptoazobenzene(DMAB)in the gold(Au)SNP on AuSNL nanocavity(Au-Au),as well as in AuSNP on AgSNL(Au-Ag),AgSNP on AuSNL(Ag-Au),and AgSNP on AgSNL(Ag-Ag)nanocavities using 532,660,and 785 nm excitations.We observed the strongest near-field signals of 4-NBT at 660 nm in all examined plasmonic systems that is found to be substantially red-shifted relative to the LSPR of the corresponding nanoparticles.We also found that rates of DMAB formation were significantly greater in heterometal nanocavities(Au-Ag and Ag-Au)compared to their monometallic counterparts(Au-Au and Ag-Ag).These results point to drastic differences in plasmonic and photocatalytic properties of mono and bimetallic nanostructures.

Interfacing photonic crystal fiber with a metallic nanoantenna for enhanced light nanofocusing

The direct interfacing of photonic crystal fiber to a metallic nanoantenna has widespread application in nanoscale imaging, optical lithography, nanoscale lasers, quantum communication, in vivo sensing, and medical surgery.We report on the fabrication of a needle-shaped plasmonic nanoantenna on the end facet of a photonic crystal fiber using electron-beam-induced evaporation of platinum. We demonstrate the coupling of light from the fiber waveguide mode to the subwavelength nanoantenna plasmonic mode focusing down to the apex of the plasmonic needle using a polarization-resolved far-field side-scatter imaging technique. Our work provides an important step toward widespread application of optical fibers in nearfield spectroscopic techniques such as tip-enhanced Raman and fluorescence microscopy, single-photon excitation and quantum sensors, nanoscale optical lithography, and lab-on-fiber devices.

Enhanced four-wave mixing process near the excitonic resonances of bulk MoS_2

Two-dimensional materials are generating great interest due to their unique electrical and optical properties.In particular, transition metal dichalcogenides such as molybdenum disulfide(MoS_2) are attractive materials due to the existence of a direct band gap in the monolayer limit that can be used to enhance nonlinear optical phenomena, such as Raman spectroscopy. Here, we have investigated four-wave mixing processes in bulk MoS_2 using a multiplex coherent anti-Stokes Raman spectroscopy setup. The observed four-wave mixing signal has a resonance at approximately 680 nm, corresponding to the energy of the A excitonic transition of MoS_2. This resonance can be attributed to the increased third-order nonlinear susceptibility at wavelengths near the excitonic transition. This phenomenon could open the path to using MoS_2 as a substrate for enhancing four-wave mixing processes such as coherent anti-Stokes Raman spectroscopy.