Nonlinear photoluminescence (N-PL) is a broadband photon emission arising from a nonequilibrium heated electron distribution generated at the surface of metallic nanostructures by ultrafast pulsed laser illumination. N-PL is sensitive to surface morphology, local electromagnetic field strength, and electronic band structure, making it relevant to probe optically excited nanoscale plasmonic systems. It also has been key to accessing the complex multiscale time dynamics ruling electron thermalization. Here, we show that plasmon-mediated N-PL emitted by a gold nanowire can be modified by an electrical architecture featuring a nanogap. Upon voltage activation, we observe that N-PL becomes dependent on the electrical transport dynamics and can thus be locally modulated. This finding brings an electrical leverage to externally control the photoluminescence generated from metal nanostructures and constitutes an asset for the development of emerging nanoscale interface devices managing photons and electrons.

This paper highlights the effectiveness of photo-thermal plasmonic sensors in enabling real-time closed-loop stabilization of photonic devices. To assess the waveguided optical power, the in-line detector leverages the photo-thermal resistance variation of a micrometric plasmonic strip in contact with the waveguide. The generated signal is used to lock to resonance a ring resonator against wavelength and temperature variations, in a silicon-based technology. Thanks to the high sensitivity of the detector and its limited penalty on the waveguide loss, automatic re-tuning of the ring resonator with a recovery time of 20 ms has been successfully achieved. The micrometric dimensions of the sensor, its technological simplicity, and the delivered signal (proportional to the optical power) make the photo-thermal plasmonic sensor an attractive in-line candidate for closed-loop control of optical devices, regardless of the technology employed for the photonic chip fabrication.

Densely integrated photonic integrated circuits (PICs) require efficient solutions for monitoring the light intensity on chip in order to implement control and configuration operations to set and stabilize the working point of the circuit. To this end, waveguides supporting the propagation of surface plasmon polaritons (SPPs) are good candidates to realize small-footprint light detectors. In this work, we report on the realization of an in-line Surface Plasmon Detector (SPD) that exploits the photothermal effect to monitor the optical power in a titanium dioxide (TiO2) optical waveguide. Detailed design guidelines are provided to maximize the responsivity of the SPD, taking into account the effects of the metal geometry on the coupling between the dielectric and plasmonic modes, the power dissipated in the metal, and the equivalent thermal resistance of the structure. Experimental validation of the proposed device is provided demonstrating an ultra-compact 1.6- $\mu \text{m}$ -long SPD operating at a wavelength of 1550 nm with a sensitivity of–20 dBm and a bandwidth higher than 100 kHz. The proposed device concept can be ported to generic dielectric platforms and to other wavelength ranges where SPP propagation is supported.

In recent times the spin angular momentum (SAM) and orbital angular momentum (OAM) of light have gained prominence because of their significance in optical communication systems, micromanipulation, sub-wavelength position sensing. To this end, simultaneous detection of SAM and OAM of light beam is one of the important topics of research from both application and fundamental spin-orbit interaction (SOI) point of view. While interferometry and metasurface based approaches have been able to detect the states, our approach involves elastic scattering from a monocrystalline silver nanowire for the simultaneous detection of SAM and OAM state of a circularly polarized Laguerre-Gaussian (LG) beam. By employing Fourier plane (FP) microscopy, the transmitted scattered light intensity distribution in the FP is analyzed to reconstruct the SAM and OAM state unambiguously. The SAM and OAM induced transverse energy flow as well as the polarization dependent scattering characteristics of the nanowire is investigated to understand the detection mechanism. Our method is devoid of complex nanofabrication techniques required for metasurface based approaches and to our knowledge, is a first example of single nano-object based simultaneous SAM and OAM detection. The study will further the understanding of SOI effects and can be useful for on-chip optical detection and manipulation.

We report the first observation of how a single nano object such as a monocrystalline silver nanowire can be utilized for scattering assisted simultaneous detection of spin and orbital angular momentum of light beams.

Spin-orbit interactions (SOIs) are a set of subwavelength optical phenomena in which spin and spatial degrees of freedom of light are intrinsically coupled. One of the unique examples of a SOI, the spin-Hall effect of light (SHEL), has been an area of extensive research with potential applications in spin controlled photonic devices as well as emerging fields of spinoptics and spintronics. Here, we report our experimental study on SHEL due to forward scattering of focused linearly polarized Gaussian and Hermite-Gaussian (${\text{HG}}_{10}$) beams from a silver nanowire (AgNW). Spin-dependent antisymmetric intensity patterns are obtained when the polarization of the scattered light is analyzed. The corresponding spin-Hall signal is obtained by computing the far-field longitudinal spin density (${s}_{3}$). Furthermore, by comparing the ${s}_{3}$ distributions, significant enhancement of the spin-Hall signal is found for the ${\text{HG}}_{10}$ beam compared to the Gaussian beam. The investigation of the optical fields at the focal plane of the objective lens reveals the generation of longitudinally spinning fields as the primary reason for the effects. The experimental results are corroborated by three-dimensional numerical simulations. The results lead to a better understanding of SOIs and can have direct implications on chip-scale spin assisted photonic devices.

We show that plasmonic nanowire-nanoparticle systems can perform nonlinear wavelength and modal conversions and potentially serve as building blocks for signal multiplexing and novel trafficking modalities. When a surface plasmon excited by a pulsed laser beam propagates in a nanowire, it generates a localized broadband nonlinear continuum at the nanowire surface as well as at active locations defined by sites where nanoparticles are absorbed (enhancement sites). The local response may couple to new sets of propagating modes enabling a complex routing of optical signals through modal and spectral conversions. Different aspects influencing the optical signal conversions are presented, including the parameters defining the local formation of the continuum and the subsequent modal routing in the nanowire.

We report a quantitative analysis of the wavevector diagram emitted by nonlinear photoluminescence generated by a tightly focused pulsed laser beam and distributed along Au nanowire via the mediation of surface plasmon polaritions. The nonlinear photoluminescence is locally excited at key locations along the nanowire in order to understand the different contributions constituting the emission pattern measured in a conjugate Fourier plane of the microscope. Polarization-resolved measurements reveal that the nanowire preferentially emits nonlinear photoluminescence polarized transverse to the long axis at close to the detection limit wavevectors with a small azimuthal spread in comparison to the signal polarized along the long axis. We utilize finite element method to simulate the observed directional scattering by using localized incoherent sources placed on the nanowire. Simulation results faithfully mimic the directional emission of the nonlinear signal emitted by the different portions of the nanowire.

When gold nanowires are excited with a tightly focused femtosecond laser a distributed nonlinear photoluminescence (N-PL) develops throughout the entire structure. A complete spaced-resolved analysis of the spectral signature of the nanowire nonlinear response is carried out to understand the origin of the distributed nonlinear response. We discuss various mechanisms to explain the experimental data and unambiguously demonstrate that the spatial and spectral extension of the N-PL in the nanowire are mainly dictated by the propagation of a surface plasmon excited at the pump wavelength. We also present experimental signature of near-field excitation of a broadband continuum of surface plasmons excited locally throughout the N-PL emission spectrum.

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 review 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.

The generation and momentum space distribution of fluorescence emission from individual SiO2 microsphere on dye‐coated Au mirror are reported here. The molecular fluorescence emission mediated via whispering gallery modes of the sphere is studied using polarization‐resolved optical energy–momentum microspectroscopy. The experiments of this study 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 microcavities 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.

Directional harmonic generation is an important property characterizing the ability of nonlinear optical antennas to diffuse the signal in a well-defined region of space. Herein, we show how sub-wavelength facets of an organic molecular mesowire crystal can be utilized to systematically vary the directionality of second-harmonic generation (SHG) in the forward-scattering geometry. We demonstrate this capability on crystalline diamonoanthraquinone (DAAQ) mesowires with sub-wavelength facets. We observed that the radial angles of the SHG emission can be tuned over a range of 130 deg. This angular variation arises due to spatially distributed nonlinear dipoles in the focal volume of the excitation as well as the geometrical cross section and facet orientation of the mesowire. Numerical simulations of the near-field excitation profile corroborate the role of the mesowire geometry in localizing the electric field. In addition to directional SHG from the mesowire, we experimentally observe optical waveguiding of the nonlinear two-photon excited fluorescence (TPEF). Interestingly, we observed that for a given pump excitation, the TPEF signal is isotropic and delocalized, whereas the SHG emission is directional and localized at the location of excitation. All the observed effects have direct implications not only in active nonlinear optical antennas but also in nonlinear signal processing.

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 have investigated photoconductive properties of single germanium nanowires (NWs) of diameter <100 nm in the spectral range of 300-1100 nm and in the broad band near-infrared spectrum showing peak responsivity (R) approximate to 10(7) A/W at a minimal bias of 2 V. The NWs were grown by the vapor-liquid-solid method using Au nano particles as the 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 photo generated pairs, leaving the holes free to reach the electrodes, (b) Schottky barrier (similar to 0.2-0.3 eV) at the metal/NW interface which gets lowered substantially because of carrier diffusion in the contact region, and (c) photodetector length which is small ( few pm), and there is negligible loss of photogenerated carriers because of 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 the presence of a thin layer of GeOx (as evidenced from the high-resolution transmission electron microscopy study) which provides interface states. It is observed that these states play a crucial role to provide a radial field for separation of the photogenerated electron hole pair which in turn leads to highly effective photoconductive gain that reaches a value >10(7) at an illumination intensity of 10 mu W/cm(2).

Spin-orbit interactions are subwavelength phenomena that can potentially lead to numerous device-related applications in nanophotonics. Here, we report the spin-Hall effect in the forward scattering of Hermite-Gaussian (HG) and Gaussian beams from a plasmonic nanowire. Asymmetric scattered radiation distribution was observed for circularly polarized beams. Asymmetry in the scattered radiation distribution changes the sign when the polarization handedness inverts. We found a significant enhancement in the spin-Hall effect for a HG beam compared to a Gaussian beam for constant input power. The difference between scattered powers perpendicular to the long axis of the plasmonic nanowire was used to quantify the enhancement. In addition, the nodal line of the HG beam acts as the marker for the spin-Hall shift. Numerical calculations corroborate experimental observations and suggest that the spin flow component of the Poynting vector associated with the circular polarization is responsible for the spin-Hall effect and its enhancement.

The optical emission characteristics from individual nanostructures such as organic waveguides, plasmonic nanowires and 2D materials such as MoS2 can vary depending on the nature of interface on which they are grown or deposited. We constructed a dual-channel Fourier-plane microscopy, and studied the directional emission characteristics" of an individual organic mesowires, Ag nanowires and MoS2 nanolayers through the glass substrate or air superstrate. Specifically, we show the ability of our microscope to quantitatively probe the radial and azimuthal angular spread in the waveguided PL from the distal ends of the mesowire across the interface without changing its position or orientation. Furthermore, from the guided PL spectral signatures, we show that the finesse of the waveguided Fabry-Perot resonance depends on whether the measurement was performed through the substrate or superstrate. To reveal the versatility of our microscope, we have quantified angular distribution of directional light scattering from the distal end of Ag nanowire at an interface, and angular distribution of excitonic emission from MoS2 nanolayers through a glass substrate. Our work highlights the capability of dual-channel Fourier microscope in quantifying the angular emission characteristics from individual optical antenna structures at an interface, and can be extrapolated to nonlinear organic nanophotonic regimes.