We have investigated photoconductive properties of single Germanium Nanowires (NWs) of diameter < 100 nm in the spectral range of 300-1100 nm and in the broadband Near Infra-red spectrum showing peak Responsivity (ℛ) ~10 7 A/W at a minimal bias of 2V. The NWs were grown by Vapor-Liquid-Solid method using Au nanoparticle as catalyst. In this report we discuss the likely origin of the ultra large ℛ that may arise from a combination of various physical effects which are (a) Ge/GeOx interface states which act as "scavengers" of electrons from the photogenerated pairs, leaving the holes free to reach the electrodes, (b) Schottky barrier (~0.2-0.3eV) at the metal/NW interface which gets lowered substantially due to carrier diffusion in contact region and (c) photodetector length being small (~few μm), negligible loss of photogenerated carriers due to recombination at defect sites. We have observed from power dependence of the optical gain that the gain is controlled by trap states. We find that the surface of the nanowire has presence of a thin layer of GeOx (as evidenced from HRTEM study) which provide interface states. It is observed that these state play a crucial role to provide a radial field for separation of photogenerated electron-hole pair which in turn leads to very high effective photoconductive gain that reaches a value > 10 7 at an illumination intensity of 10 µW/cm 2 .

We report on the experimental observation of differential wavevector distribution of surface-enhanced Raman scattering (SERS) and fluorescence from dye molecules confined to a gap between plasmonic silver nanowire and a thin, gold mirror. The fluorescence was mainly confined to higher values of in-plane wavevectors, whereas SERS signal was uniformly distributed along all the wavevectors. The optical energy-momentum spectra from the distal end of the nanowire revealed strong polarization dependence of this differentiation. All these observations were corroborated by full-wave three-dimensional numerical simulations, which further revealed an interesting connection between out-coupled wavevectors and parameters such as hybridized modes in the gap-plasmon cavity, and orientation and location of molecular dipoles in the geometry. Our results reveal a new prospect of discriminating electronic and vibrational transitions in resonant dye molecules using a subwavelength gap plasmonic cavity in the continuous-wave excitation limit, and can be further harnessed to engineer molecular radiative relaxation processes in momentum space.

We report a new method for label-free, sensitive, and facile detection of lead(II) ions (Pb 2+ ) based on an aptamer−target binding event, which is recognized by orientations of liquid crystals (LCs) at aqueous interfaces. The LC film suspended in the aqueous phase demonstrated a homeotropic orientation in contact with a cationic surfactant cetyltrimethylammonium bromide (CTAB) due to selfassembly of CTAB molecules at the aqueous−LC interface. The ordering of LC subsequently changed to planar in the presence of the spinach RNA aptamer (SRNA) due to interactions between CTAB and SRNA. In the presence of the Pb 2+ ion, the ordering of LC changed to homeotropic caused by reorganization of CTAB at the LC−aqueous interface. This is due to formation of more stable quadruplex structures of SRNA with Pb 2+ ions in comparison to the CTAB-SRNA complex. The sensor exhibited a detection limit of 3 nM, which is well below the permissible limit of Pb 2+ in drinking water. Our experiments establish that addition of Pb 2+ leads to (i) the formation of Pb 2+ -SRNA complexes and (ii) a decrease in density of SRNA on the LC interface, but additional studies are required to determine which of these processes underlie the response of the LCs to the Pb 2+ . We have also demonstrated the potential application of the LC sensor for detection of Pb 2+ in tap water. Unlike current laboratory-based heavy-metal-ion assays, this method is comparatively simple in terms of instrumentation, operation, and optical readout.

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.

Intensity, wavevector, phase, and polarization are the most important parameters of any light beam. Understanding the wavevector distribution has emerged as a very important problem in recent days, especially at nanoscale. It provides unique information about the light–matter interaction. Back focal plane or Fourier plane imaging and spectroscopy techniques help to measure wavevector distribution not only from single molecules and single nanostructures but also from metasurfaces and metamaterials. This article provides a birds‐eye view on the technique of back focal imaging and spectroscopy, different methodologies used in developing the technique, and applications including angular emission patterns of fluorescence and Raman signals from molecules, elastic scattering, etc. We first discuss on the information one can obtain at the back focal plane of the objective lens according to both imaging and spectroscopy viewpoints and then discuss the possible configurations utilized to project back focal plane of the objective lens onto the imaging camera or to the spectroscope. We also discuss the possible sources of error in such measurements and possible ways to overcome it and then elucidate the possible applications.