Orbital angular momentum (OAM) has emerged as an important parameter to store, control, and transport information using light. Recognizing optical beams that carry OAM at the nanoscale and their interaction with subwavelength nanostructures has turned out to be a vital task in nanophotonic signal processing and communication. The current platforms to decode information from different OAM modes are mainly based on bulk optics and requires sophisticated nanofabrication procedures. Motivated by these issues, herein we report on the utility of chemically prepared, individual plasmonic nanowire for OAM read-out. Our method is based on pattern recognition of coherent light scattering from individual nanowires that can be used as direct read-outs of two parameters of an OAM beam: magnitude of topological charge and its sign. All the experimental observations related to pattern formation are corroborated by three-dimensional numerical simulations. Given that pattern formation and recognition are exhaustively utilized in various computational domains, we envisage that our results can be interfaced with machine-learning methods, wherein direct read-out of OAM signals can be performed without human intervention. Such methods may have a direct implication on chip-scale robotics and chiral nanophotonic interfaces.

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 polysterne 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.

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.Directional optical antennas are at the heart of nano-photonics as they influence and provide control on the properties of light for efficient detection and on-chip coupling. 1-3 Thus, there is a continuous endeavor to design structures which can scatter light directionally with a dB, which is an excellent number for a structure prepared using bottom-up approach.

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 WGMmediated 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

Both the in situ measurement of Comet 1P/Halley and the Stardust‐returned samples of Comet Wild 2 showed the presence of a mixture of compact and aggregate particles, with both silicates and organic refractory being in the composition of the cometary dust. Results obtained recently from the Stardust mission suggest that the overall ratio of compact to aggregate particles is 65:35 (or 13:7) for Comet 81P/Wild 2. In the present work, we propose a model that considers cometary dust as a mixture of compact and aggregate particles, with a composition of silicate and organic. We consider compact particles as spheroidal particles and aggregates as both ballistic cluster–cluster aggregate (BCCA) and ballistic agglomeration with two migrations (BAM2) aggregate with a certain size distribution. The mixing ratio of compact to aggregate particles is taken to be 13:7. For modelling Comet 1P/Halley, the power‐law size distribution n(a) ∼a−2.6, obtained from a re‐analysis of the Giotto spacecraft data, for both compact and aggregate particles, is used. We consider a mixture of BAM2 and BCCA aggregates with a lower cut‐off size of about 0.20 μ m and an upper cut‐off of about 1 μ m. We also consider a mixture of prolate, spherical and oblate compact particles with an axial ratio (E) of 0.8–1.2 where a lower cut‐off size of about 0.1 μ m and an upper cut‐off of about 10 μ m are considered. Using a T‐matrix code for polydisperse spheroids (0.1 μ m ≤a≤ 10 μ m) and superposition T‐matrix code for aggregates (0.2 μ m ≤av≤ 1 μ m), the average simulated polarization curves are generated, which can best fit the observed polarization data at the four wavelengths: λ= 0.365, 0.485, 0.670 and 0.684 μ m. The suitable mixing percentages of aggregates obtained from the present modelling are 50 per cent BAM2 and 50 per cent BCCA particles, and the silicate‐to‐organic mixing percentages are 78 per cent silicate and 22 per cent organic, in terms of volume. The present model successfully reproduces the observed polarization data, especially the negative branch, for Comet 1P/Halley at the above four wavelengths, more effectively as compared to other work done in the past. It is found that among the aggregates, the BAM2 aggregate plays a major role in deciding the cross‐over angle and depth of the negative polarization branch.