Single-molecule surface-enhanced Raman scattering (SM-SERS) is one of the vital applications of plasmonic nanoparticles. The SM-SERS sensitivity critically depends on plasmonic hot-spots created at the vicinity of such nanoparticles. In conventional fluid-phase SM-SERS experiments, plasmonic hot-spots are facilitated by chemical aggregation of nanoparticles. Such aggregation is usually irreversible, and hence, nanoparticles cannot be re-dispersed in the fluid for further use. Here, we show how to combine SM-SERS with plasmon polariton-assisted, reversible assembly of plasmonic nanoparticles at an unstructured metal-fluid interface. One of the unique features of our method is that we use a single evanescent-wave optical excitation for nanoparticle assembly, manipulation and SM-SERS measurements. Furthermore, by utilizing dual excitation of plasmons at metal-fluid interface, we create interacting assemblies of metal nanoparticles, which may be further harnessed in dynamic lithography of dispersed nanostructures. Our work will have implications in realizing optically addressable, plasmofluidic, single-molecule detection platforms.

Modulation of photoluminescence of atomically thin transition metal dichalcogenide two-dimensional materials is critical for their integration in optoelectronic and photonic device applications. By coupling with different plasmonic array geometries, we have shown that the photoluminescence intensity can be enhanced and quenched in comparison with pristine monolayer MoS2. The enhanced exciton emission intensity can be further tuned by varying the angle of polarized incident excitation. Through controlled variation of the structural parameters of the plasmonic array in our experiment, we demonstrate modulation of the photoluminescence intensity from nearly fourfold quenching to approximately threefold enhancement. Our data indicates that the plasmonic resonance couples to optical fields at both, excitation and emission bands, and increases the spontaneous emission rate in a double spacing plasmonic array structure as compared with an equal spacing array structure. Furthermore our experimental results are supported by numerical as well as full electromagnetic wave simulations. This study can facilitate the incorporation of plasmon-enhanced transition metal dichalcogenide structures in photodetector, sensor and light emitter applications.

The localized interaction between metallic nanostructures and surrounding fluorescent molecules can influence the emission characteristics of the molecule. With this hindsight, herein, by employing a Fourier optical fluorescence microscope, we experimentally show how fluorescence emission from molecules in the vicinity of a chemically prepared silver nanowire-dimer-junction can be directed in one or two channels. Measured forward-to-backward ratio of the fluorescence emission in a single channel was as high as 4.3 dB, and the observed polar and azimuthal angular spread was as narrow as 15° and 60°, respectively. Interestingly, the angle between the two emission channels mimicked the angle between the nanowires, thus, highlighting the prospect of geometrical control of the emitted light. These observations were further corroborated by three-dimensional finite-difference time-domain simulations. The presented results will have implications in momentum-space engineering of molecular fluorescence emission and can be extrapolated to single-emitter studies.

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We experimentally show how a single Ag nanoparticle (NP) coupled to an Ag nanowire (NW) can convert propagating surface plasmon polaritons to directional photons. By employing dual-excitation Fourier microscopy with spatially filtered collection-optics, we show single-and dual-directional out-coupling of light from NW-NP junction for plasmons excited through glass-substrate and air-superstrate. Furthermore, we show NW-NP junction can influence the directionality of molecular-fluorescence emission, thus functioning as an optical antenna. The results discussed herein may have implications in realizing directional single-photon sources and quantum plasmon circuitry.The ability to control and direct light at nanoscale is an important challenge in nano-optics and nanophotonics. To achieve this, plasmons, which are collective oscillations of light and freeelectrons at metal-dielectric interface, have been employed for sub-wavelength propagation and localization of light [1]. Plasmons can reduce the mismatch between the cross-section of an excitation optical beam and the absorption cross-section of emitters such as atoms, molecules, and quantum dots, thus mediating and enhancing light-matter interaction [2]. A variety of plasmonic geometries have been innovated and studied in the context of enhancing light-matter interactions. One such geometry is chemically prepared silver nanowires (Ag NWs) [3][4][5][6]. These Ag NWs have atomically smooth surfaces that facilitate sub-wavelength propagation of surface plasmon polaritons (SPPs) over a distance of a few microns [7]. They can be used as plasmonic resonators [8], logic gates [9], spontaneous emission amplifiers [10], single photon sources [11][12][13], and photon-to-plasmon convertors [14], and have opened up new opportunities in quantum plasmonic circuits [15] and nanoscale quantum optics [16]. In the context of NW-based SPP waveguiding, one of the issues to be addressed is to deterministically convert propagating SPPs