Danveer Singh

Surface enhanced Raman scattering of a surface plasma wave

A surface plasma wave (SPW), propagating along a metal surface, embedded with regularly arranged nanoparticles, undergoes surface enhanced Raman scattering from molecules adsorbed over the particles. The enhancement in the scattered signal depends on the shape and dielectric constant of the metallic particles. The scattered signal can be detected above the metal surface as a space wave if the particles have a periodic arrangement with wave number q such that ωR > (kS − q)c, where ωR is the frequency of the scattered signal and kS is the wave number of SPW.

Highly tapered pentagonal bipyramidal Au microcrystals with high index faceted corrugation: Synthesis and optical properties

Focusing light at sub-wavelength region opens up interesting applications in optical sensing and imaging beyond the diffraction limit. In the past, tapered Au wires with carved gratings have been employed to achieve nanofocusing. The fabrication process however, is expensive and the obtained wires are polycrystalline with high surface roughness. A chemical synthetic method overcoming these hurdles should be an attractive alternative. Here, we report a method to chemically synthesize Au microcrystals (~10 μm) bearing pentagonal bipyramidal morphology with surface corrugations assignable to high index planes. The method is a single step solid state synthesis at a temperature amenable to common substrates. The microcrystals are tapered at both ends forming sharp tips (~55 nm). Individual microcrystals have been used as pick and probe SERS substrates for a dye embedded in a polymer matrix. The unique geometry of the microcrystal also enables light propagation across its length.

Plasmon assisted light propagation and Raman scattering hot-spot in end-to-end coupled silver nanowire pairs

Herein, we report on the experimental observation of light propagation and localization capabilities of end-to-end connected silver nanowire (Ag NW) pairs. By exciting the surface plasmon polaritons at one end of Ag NW pair, we observed relatively intense light emission at the junction and weak light emission at the distal end of the pair. To probe the localization of light at nanowire junction, we captured far-field Raman image of an isolated Ag NW pair adsorbed with rhodamine 6 G and observed enhanced Raman scattering at the nanowire junction. Such nanophotonic modules with light propagation and localization capabilities can be harnessed for multiplexed on-chip plasmonics.

Directional out-coupling of light from a plasmonic nanowire-nanoparticle junction

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.

Propagation of light in serially coupled plasmonic nanowire dimer: Geometry dependence and polarization control

We experimentally studied plasmon-polariton-assisted light propagation in serially coupled silver nanowire (Ag-NW) dimers and probed their dependence on bending-angle between the nanowires and polarization of incident light. From the angle-dependence study, we observed that obtuse angles between the nanowires resulted in better transmission than acute angles. From the polarization studies, we inferred that light emission from junction and distal ends of Ag-NW dimers can be systematically controlled. Further, we applied this property to show light routing and polarization beam splitting in obtuse-angled Ag-NW dimer. The studied geometry can be an excellent test-bed for plasmonic circuitry.

Remote-excitation surface-enhanced Raman scattering with counter-propagating plasmons: silver nanowire-nanoparticle system

Abstract. Surface-enhanced Raman scattering (SERS) has emerged as a powerful tool to probe molecules at nanoscale. By utilizing plasmon polaritons on metallic nanowires, remote-excitation SERS can be achieved. Enhancement and modulation of remote-SERS intensity are vital for nano-optical spectroscopy. Counter-propagating plasmons have been excited in a plasmonic nanowire-nanoparticle (NW-NP) system and further utilized to perform remote-excitation SERS. By using the polarization of counter-propagating fields, remote-SERS intensity from NW-NP hot-spot junction was enhanced and modulated. Such capabilities of counter-propagating plasmons to control optical fields and SERS intensity at NW-NP junction can have implications in nanowire photonics and nano-optical spectroscopy.

Evanescent field-assisted intensity modulation of surface-enhanced Raman scattering from a single plasmonic nanowire

In this paper, we experimentally show how polarization of evanescent optical fields can be used to modulate surface-enhanced Raman scattering (SERS) intensity from a chemically prepared, single silver nanowire. By systematically varying the input polarization between p and s polarization, we observed a modulation depth of 0.88 in SERS intensity. The scattered output light was polarized perpendicularly to the nanowire axis with a modulation depth of 0.90. Our method of SERS excitation of a single nanowire in total internal reflection geometry can be further utilized as an evanescent-mode nano-optical sensor, especially in cases where soft nanomaterials are employed for molecular detection.

Dual-path remote-excitation surface enhanced Raman microscopy with plasmonic nanowire dimer

We demonstrate how a silver-nanowire-dimer can be employed to optically excite dual-path surface-plasmon-polaritons and utilize them to perform remote-excitation surface-enhanced Raman scattering (SERS) microscopy. Interestingly, this unique geometry allows us to perform dual-path remote-excitation SERS. Our experiments show that for the same value of excitation-laser powers, dual-path excitation leads to enhanced-SERS signal compared to single-path excitation, which has been corroborated by 3-D finite-difference time-domain simulations. Furthermore, we show that SERS-enhancement can be remotely modulated in this geometry by varying the polarization of excitation-fields. The results discussed herein can be extrapolated to remote-excitation pump-probe spectroscopy and dual-colour optical interrogation.

Nonlinear absorption of surface plasmons and emission of electrons from metallic targets

A large-amplitude surface plasma wave (SPW) over a metal-vacuum interface Ohmically heats the electrons and undergoes nonlinear absorption. The attenuation rate increases with the local SPW amplitude. The enhanced electron temperature leads to stronger thermionic emission of electrons. At typical Nd:glass laser intensity IL=7GW∕cm2, if one takes the amplitude of the SPW to be ≈6 times the amplitude of the laser, one obtains the thermionic electron emission current density J=200A∕cm2. However, the emission current density decreases with propagation distance at a much faster rate than the SPW amplitude and electron temperature.

Probing of a surface plasma wave by an obliquely incident laser on the metal surface

A surface plasma wave (SPW) of frequency ω1 and wave number k1 propagating along a metal-free space boundary exerts a ponderomotive force on the free electrons, creating an electron density perturbation at frequency 2ω1. When a laser of frequency ω2 and wave number k2 is incident at a suitable angle on the metal surface, it gives rise to the oscillatory velocity of electrons at frequency ω2. This oscillatory velocity couples with the density perturbation to generate a nonlinear current at frequency 2ω1+ω2. The nonlinear current derives a radiating wave under suitable conditions. By measuring the amplitude of the radiating wave, the SPW field can be probed.