arXiv subjects

Sunny Tiwari

Publications and source records attributed to Sunny Tiwari.

17 recordsLinked to original sources

arXiv2024

Fluorescence enhancement in topologically optimized gallium phosphide all-dielectric nanoantennas

Nanoantennas capable of large fluorescence enhancement with minimal absorption are crucial for future optical technologies from single-photon sources to biosensing. Efficient dielectric nanoantennas have been designed, however, evaluating their performance at the individual emitter level is challenging due to the complexity of combining high-resolution nanofabrication, spectroscopy and nanoscale positioning of the emitter. Here, we study the fluorescence enhancement in infinity-shaped gallium phosphide (GaP) nanoantennas based on a topologically optimized design. Using fluorescence correlation spectroscopy (FCS), we probe the nanoantennas enhancement factor and observed an average of 63-fold fluorescence brightness enhancement with a maximum of 93-fold for dye molecules in nanogaps between 20 nm and 50 nm. The experimentally determined fluorescence enhancement of the nanoantennas was confirmed by numerical simulations of the local density of optical states (LDOS). Furthermore, we show that beyond design optimisation of dielectric nanoantennas, increased performances can be achieved via tailoring of nanoantenna fabrication.

physics.optics↗

arXiv2023

Opto-thermoelectric trapping of Fluorescent Nanodiamonds on Plasmonic Nanostructures

Deterministic optical manipulation of fluorescent nanodiamonds (FNDs) in fluids has emerged as an experimental challenge in multimodal biological imaging. Designing and developing nano-optical trapping strategies to serve this purpose is an important task. In this letter, we show how chemically-prepared gold nanoparticles and silver nanowires can facilitate Opto-thermoelectric force to trap individual entities of FNDs using a long working distance lens, low power-density illumination (532 nm laser, 12 $μW/μm^2$). Our trapping configuration combines the thermoplasmonic fields generated by individual plasmonic nanoparticles and the opto-thermoelectric effect facilitated by the surfactant to realise a nano-optical trap down to a single FND 120 nm in diameter. We utilise the same trapping excitation source to capture the spectral signatures of single FNDs and track their position. By tracking the FND, we observe the differences in the dynamics of FND around different plasmonic structures. We envisage that our drop-casting platform can be extrapolated to perform targeted, low-power trapping, manipulation, and multimodal imaging of FNDs inside biological systems such as cells.

physics.optics↗

arXiv2023

Achieving High Temporal Resolution in Single-Molecule Fluorescence Techniques using Plasmonic Nanoantennas

Single-molecule fluorescence techniques are essential for investigating the molecular mechanisms in biological processes. However, achieving sub-millisecond temporal resolution to monitor fast molecular dynamics remains a significant challenge. The fluorescence brightness is the key parameter that generally defines the temporal resolution for these techniques. Conventional microscopes and standard fluorescent emitters fall short in achieving the high brightness required for sub-millisecond monitoring. Plasmonic nanoantennas have been proposed as a solution, but despite huge fluorescence enhancement have been obtained with these structures, the brightness generally remains below 1 million photons/s/molecule. Therefore, the improvement of temporal resolution has been overlooked. In this article, we present a method for achieving high temporal resolution in single-molecule fluorescence techniques using plasmonic nanoantennas, specifically optical horn antennas. We demonstrate about 90% collection efficiency of the total emitted light, reaching a high fluorescence brightness of 2 million photons/s/molecule in the saturation regime. This enables observations of single molecules with microsecond binning time and fast fluorescence correlation spectroscopy (FCS) measurements. This work expands the applications of plasmonic antennas and zero-mode waveguides in the fluorescence saturation regime towards brighter single-molecule signal, faster temporal resolutions and improved detection rates to advance fluorescence sensing, DNA sequencing and dynamic studies of molecular interactions.

physics.optics↗

arXiv2023

Ultraviolet Nanophotonics Enables Autofluorescence Correlation Spectroscopy on Label-Free Proteins With a Single Tryptophan

Using the ultraviolet autofluorescence of tryptophan aminoacids offers fascinating perspectives to study single proteins without the drawbacks of fluorescence labelling. However, the low autofluorescence signals have so far limited the UV detection to large proteins containing several tens of tryptophan residues. This limit is not compatible with the vast majority of proteins which contain only a few tryptophans. Here we push the sensitivity of label-free ultraviolet fluorescence correlation spectroscopy (UV-FCS) down to the single tryptophan level. Our results show how the combination of nanophotonic plasmonic antennas, antioxidants and background reduction techniques can improve the signal-to-background ratio by over an order of magnitude and enable UV-FCS on thermonuclease proteins with a single tryptophan residue. This sensitivity breakthrough unlocks the applicability of UV-FCS technique to a broad library of label-free proteins.

physics.optics↗

arXiv2022

Mirror-Coupled Microsphere can narrow the Angular distribution of Photoluminescence from WS2 Monolayers

Engineering optical emission from two dimensional, transition metal dichalcogenides (TMDs) materials such as Tungsten disulphide (WS2) has implications in creating and understanding nanophotonic sources. One of the challenges in controlling the optical emission from 2D materials is to achieve narrow angular spread using a simple photonic geometry. In this paper, we study how the photoluminescence of a monolayer WS2 can be controlled when coupled to film coupled microsphere dielectric antenna. Specifically, by employing Fourier plane microscopy and spectroscopic techniques, we quantify the wavevector distribution in the momentum space. As a result, we show beaming of the WS2 photoluminescence with angular divergence of θ1/2 = 4.6°. Furthermore, the experimental measurements have been supported by three-dimensional numerical simulations. We envisage that the discussed results can be generalized to a variety of nanophotonic 2D materials, and can be harnessed in nonlinear and quantum technology.

physics.optics↗

arXiv2022

Directing Monolayer Tungsten Disulfide Photoluminescence using a Bent Plasmonic Nanowire on a Mirror Cavity

Designing directional optical antennas without compromising the field enhancement requires specially designed optical cavities. Herein, we report on the experimental observations of directional photoluminescence emission from a monolayer Tungsten Disulfide using a bent-plasmonic nanowire on a mirror cavity. The geometry provides field enhancement and directivity to photoluminescence by sandwiching the monolayer between an extended cavity formed by dropcasting bent silver nanowire and a gold mirror. We image the photoluminescence emission wavevectors by using the Fourier plane imaging technique. The cavity out-couples the emission in a narrow range of wavevectors with a radial and azimuthal spreading of only 11.0° and 25.1°, respectively. Furthermore, we performed three dimensional finite difference time domain based numerical calculations to corroborate and understand the experimental results. We envisage that the results presented here will be readily harnessed for on-chip coupling applications and in designing inelastic optical antennas.

physics.optics↗

arXiv2021

Optothermal pulling, trapping, and assembly of colloids using nanowire plasmons

Optical excitation of colloids can be harnessed to realize soft matter systems that are out of equilibrium. In this paper, we present our experimental studies on the dynamics of silica colloids in the vicinity of a silver nanowire propagating surface plasmon polaritons (SPPs). Due to the optothermal interaction, the colloids are directionally pulled towards the excitation point of the nanowire. Having reached this point, they are spatio-temporally trapped around the excitation location. By increasing the concentration of colloids in the system, we observe multi-particle assembly around the nanowire. This process is thermophoretically driven and assisted by SPPs. Furthermore, we find such an assembly to be sensitive to the excitation polarization at input of the nanowire. Numerically-simulated temperature distribution around an illuminated nanowire corroborates sensitivity to the excitation polarization. Our study will find relevance in exploration of SPPs-assisted optothermal pulling, trapping and assembly of colloids, and can serve as test-beds of plasmon-driven active matter.

cond-mat.soft↗

arXiv2021

Single Molecule SERS in a Single Gold Nanoparticle-driven Thermoplasmonic Tweezer

Surface enhanced Raman scattering (SERS) is optically sensitive and chemically specific to detect single molecule spectroscopic signatures. Facilitating this capability in optically-trapped nanoparticles at low laser power remains a significant challenge. In this letter, we show single molecule SERS signatures in reversible assemblies of trapped plasmonic nanoparticles using a single laser excitation (633 nm). Importantly, this trap is facilitated by the thermoplasmonic field of a single gold nanoparticle dropcasted on a glass surface. We employ bi-analyte SERS technique to ascertain the single molecule statistical signatures, and identify the critical parameters of the thermoplasmonic tweezer that provide this sensitivity. Furthermore, we show the utility of this low power ($\approx$0.1 mW/$μ$m^2) tweezer platform to trap single gold nanoparticle and transport assembly of nanoparticles. Given that our configuration is based on a dropcasted gold nanoparticle, we envisage its utility to create reconfigurable plasmonic metafluids in physiological and catalytic environments, and can be potentially adapted as an in-vivo plasmonic tweezer.

physics.optics↗

arXiv2021

Beaming Elastic and SERS Emission from Bent-Plasmonic Nanowire on a Mirror Cavity

We report on the experimental observation of beaming elastic and surface enhanced Raman scattering (SERS) emission from a bent-nanowire on a mirror (B-NWoM) cavity. The system was probed with polarization resolved Fourier plane and energy-momentum imaging to study the spectral and angular signature of the emission wavevectors. The out-coupled elastically scattered light from the kink occupies a narrow angular spread. We used a self-assembled monolayer of molecules with a well-defined molecular orientation to utilize the out-of-plane electric field in the cavity for enhancing Raman emission from the molecules and in achieving beaming SERS emission. Calculated directionality for elastic scattering and SERS emission were found to be 16.2 and 12.5 dB respectively. The experimental data were corroborated with three-dimensional numerical finite element and finite difference time domain based numerical simulations. The results presented here may find relevance in understanding coupling of emitters with elongated plasmonic cavities and in designing on-chip optical antennas.

physics.optics↗

arXiv2021

Sub-Wavelength Plasmon Polaritons Channeling of Whispering Gallery Modes of Fluorescent Silica Microresonator

Herein, we report sub-wavelength propagation and directional out-coupling of whispering gallery modes (WGMs) of a fluorescent silica microsphere resonator mediated via plasmon polaritons in a single crystalline silver nanowire. Fluorescent spectral signatures of emission from the nanowire ends confirm efficient guiding of WGMs via nanowire plasmons. By employing Fourier plane optical microscopy, we reveal and quantify the directional fluorescence emission of WGMs from the ends of the nanowire. Given that the discussed geometry is self-assembled from a solution-phase, our results will find relevance in WGM-based soft-photonic platforms including miniaturized optical resonators and couplers.

physics.optics↗

arXiv2021

Mirror enhanced directional out-coupling of SERS by remote excitation of a nanowire-nanoparticle cavity

We report on the experimental observation of mirror enhanced directional surface enhanced Raman scattering (SERS) from a self-assembled monolayer of molecules coupled to a nanowire-nanoparticle (NW-NP) junction on a mirror in remote excitation configuration. Placing NW-NP junction on a metallic mirror generates multiple gap plasmon modes which have unique momentum space scattering signatures. We perform Fourier plane imaging of SERS from NW-NP on a mirror to understand the effect of multiple hotspots on molecular emission. We systematically study the effect of ground plane on the directionality of emission from NW-NP junction and show that the presence of a mirror drastically reduces angular spread of emission. The effect of multiple hotspots in the geometry on directionality of molecular emission is studied using 3D numerical simulations. The results presented here will have implications in understanding plasmon hybridization in the momentum space and its effects on molecular emission.

physics.optics↗

arXiv2021

Directional emission from WS2 monolayer coupled to plasmonic Nanowire-on-Mirror Cavity

Influencing spectral and directional features of exciton emission characteristics from 2D transition metal dichalcogenides by coupling it to plasmonic nano-cavities has emerged as an important prospect in nanophotonics of 2D materials. In this paper we experimentally study the directional photoluminescence emission from Tungsten disulfide (WS2) monolayer sandwiched between a single-crystalline plasmonic silver nanowire (AgNW) waveguide and a gold (Au) mirror, thus forming an AgNW-WS2-Au cavity. By employing polarization-resolved Fourier plane optical microscopy, we quantify the directional emission characteristics from the distal end of the AgNW-WS2-Au cavity. Given that our geometry simultaneously facilitates local field enhancement and waveguiding capability, we envisage its utility in 2D material-based, on-chip nanophotonic signal processing, including nonlinear and quantum optical regimes.

physics.optics↗

arXiv2020

Large-scale optothermal assembly of colloids mediated by a gold microplate

Light-activated colloidal assembly and swarming can act as model systems to explore non-equilibrium state of matter. In this context, creating new experimental platforms to facilitate and control two-dimensional assembly of colloidal crystals are of contemporary interest. In this paper, we present an experimental study of assembly of colloidal silica microparticles in the vicinity of a single-crystalline gold microplate evanescently excited by a 532 nm laser beam. The gold microplate acts as a source of heat and establishes a thermal gradient in the system. The created optothermal potential assembles colloids to form a two-dimensional poly-crystal, and we quantify the coordination number and hexagonal packing order of the assembly in such a driven system. Interestingly, we observe variation in assembly-size as a function of excitation-polarization. Furthermore, we observe that the assembly is colloidal-material dependent. Specifically, silica colloids assemble but polystyrene colloids do not, indicating an intricate behaviour of the forces under play. Our work highlights a promising direction in utilizing metallic, single crystalline microstructures that can be harnessed for optothermal colloidal crystal assembly and swarming studies. Our experimental system can be utilized to explore optically driven matter and photophoretic interactions in soft-matter including biological systems such as cells and micro organisms.

physics.optics↗

arXiv2020

Plasmonic nanowire coupled to zero-dimensional nanostructures: A brief review

Metal nanowires and nanoparticles that facilitate surface plasmons are of contemporary interest in nanophotonics, thermoplasmonics and optoelectronics. They facilitate not only subwavelength light propagation and localization capabilities, but also provide an excellent platform for opto-thermal effects confined to volumes down to the nanoscale. This brief review article aims to provide an overview of a specific nanophotonic geometry: a plasmonic nanowire coupled to a zero-dimensional nanostructure. We discuss the methods to prepare such nano-architectures and review some interesting nanophotonic applications that arise out of it. We conclude with a discussion on some emerging research directions that can be facilitated by employing the coupled nanostructures.

physics.optics↗

arXiv2019

Dielectric microsphere coupled to a plasmonic nanowire: A self-assembled hybrid optical antenna

Hybrid mesoscale-structures that can combine dielectric optical resonances with plasmon-polaritons are of interest in chip-scale nano-optical communication and sensing. This experimental study shows how a fluorescent microsphere coupled to a silver nanowire can act as a remotely-excited optical antenna. To realize this architecture, self-assembly methodology is used to couple a fluorescent silica microsphere to a single silver nanowire. By exciting propagating surface plasmon polaritons at one end of the nanowire, remote excitation of the Stokes-shifted whispering gallery modes (WGMs) of the microsphere is achieved. The WGM-mediated fluorescence emission from the system is studied using Fourier plane optical microscopy, and the polar and azimuthal emission angles of the antenna are quantified. Interestingly, the thickness of the silver nanowires is shown to have direct ramifications on the angular emission pattern, thus providing a design parameter to tune antenna characteristics. Furthermore, by employing three-dimensional numerical simulations, electric near-fields of the gap-junction between the microsphere and the nanowire is mapped, and the modes of nanowire that couple to the microsphere is identified. This work provides a self-assembled optical antenna that combines dielectric optical resonances with propagating-plasmons and can be harnessed in hybrid nonlinear-nanophotonics and single-molecule remote sensing.

physics.optics↗

arXiv2019

Vectorial Fluorescence Emission from Microsphere Coupled to Gold Mirror

We report on the generation, and momentum space distribution of fluorescence emission from individual SiO2 microsphere on dye coated Au mirror. The molecular fluorescence emission mediated via whispering gallery modes of the sphere is studied using polarization resolved optical energy-momentum micro-spectroscopy. Our experiments reveal intensity dependence of split modes of the cavity as a function of in-plane wavevector and emission polarization in the far field. The exotic far-field distribution can be understood by sphere-image sphere model that further reveals the polarization dependence of the split modes. The presented results reveal the potential of metallo-dielectric soft micro-cavities to engineer molecular emission that can encode spin and orbital angular momentum states and can be further extrapolated to realize dye-loaded active meta-atoms and meta-surfaces.

physics.optics↗

arXiv2018

Doughnut shaped emission from vertical organic nanowire coupled to thin plasmonic film

Vertical nanowires facilitate an innovative mechanism to channel the optical field in the orthogonal direction and act as a nanoscale light source. Subwavelength, vertically oriented nanowire platforms, both of plasmonic and semiconducting variety can facilitate interesting far field emission profiles and potentially carry orbital angular momentum states. Motivated by these prospects, in this letter, we show how a hybrid plasmonic - organic platform can be harnessed to engineer far field radiation. The system that we have employed is an organic nanowire made of diaminoanthroquinone grown on a plasmonic gold film. We experimentally and numerically studied angular distribution of surface plasmon polariton mediated emission from a single, vertical organic nanowire by utilising evanescent excitation and Fourier plane microscopy. Photoluminescence and elastic scattering from single nanowire was analysed individually in terms of in plane momentum states of the outcoupled photons. We found that the emission is doughnut shaped in both photoluminescence and elastic scattering regimes. We anticipate that the discussed results can be relevant in designing efficient, polariton-mediated nanoscale photon sources which can carry orbital angular momentum states.

physics.optics↗