Electric near-field enhancing properties of a finite-size metal conical nano-tip

Nanometer-scaled landscape of polymer: fullerene blends mapped with visibles-SNOM

Bulk heterojunction is one key concept leading to breakthrough in organic photovoltaics. The active layer is expectantly formed of distinct morphologies that carry out their respective roles in photovoltaic performance. The morphology-performance relationship however remains stymied, because unequivocal morphology at the nanoscale is not available. We used scattering-type scanning near-field optical microscopy operating with a visible light source (visibles-SNOM) to disclose the nanomorphology of P3HT:PCBM and pBCN:PCBM blends. Donor and acceptor domain as well as intermixed phase were identified and their intertwined distributions were mapped. We proposed energy landscapes of the BHJ active layer to shed light on the roles played by these morphologies in charge separation, transport and recombination. This study shows that visibles-SNOM is capable of profiling the morphological backdrop pertaining to the operation of high performance organic solar cells.

Design of Surface Enhanced Raman Scattering (SERS) Nanosensor Array

An advanced Surface-Enhanced Raman Scattering (SERS) Nanosensor Array, dedicated to serve in the future as a pH imager for the real-time detection of chemical reaction, is presented. The full flow of elementary steps—architecture, design, simulations, fabrication, and preliminary experimental results of structural characterization (Focused Ion Beam (FIB), TEM and SEM)—show an advanced SERS pixel array that is capable of providing spatially resolved measurements of chemical pH in a fluid target that became more than desirable in this period. Ultimately, the goal will be to provide real-time monitoring of a chemical reaction. The pixels consist of a nanostructured substrate composed of an array of projections or cavities. The shape of the nanostructures and the thickness of the metallic (Ag or Au) layer can be tuned to give maximal enhancement at the desired wavelength. The number and arrangement of nanostructures is optimized to obtain maximal responsivity.

Refractive twisted microaxicons

Complex-shaped light fields with specially designed intensity, phase, and polarization distributions are highly demanded for various applications including optical tweezers, laser material processing, and lithography. Here, we propose a novel (to the best of our knowledge) optical element formed by the twisting of a conic surface, a twisted microaxicon, allowing us to controllably generate high-quality spiral-shaped intensity patterns. Performance of the proposed element was analyzed both analytically and numerically using ray approximation and the rigorous finite difference time domain (FDTD) solution of Maxwell's equation. The main geometric parameters, an apex cone angle and a degree of twisting, were considered to control and optimize the generated spiral-shaped intensity patterns. The three-dimensional structure of such a microaxicon cannot be described by an unambiguous height function; therefore, it has no diffraction analogue in the form of a thin optical element. Such an element can be produced via direct laser ablation of transparent targets with structured laser beams or direct laser writing via two-photon photopolymerization and can be used in various micro- and nano-optical applications.

Quantitative comparison of plasmon resonances and field enhancements of near-field optical antennae using FDTD simulations

Plasmon resonances and electric field enhancements of several near-field optical antennae with plasmonic nanostructures engineered at their apices were quantitatively compared using finite difference time domain simulations. Although many probe designs have been tested experimentally, a systematic comparison of field enhancements has not been possible, due to differences in instrument configuration, reporter mechanism, excitation energy, and plasmonic materials used. For plasmonic nanostructures attached to a non-plasmonic support (e.g., a nanoparticle functionalized AFM tip), we find that the complex refractive index of the support material is critical in controlling the overall plasmonic behavior of the antenna. Supports with strong absorption at optical energies (Pt, W) dampen plasmon resonances and lead to lower enhancements, while those with low absorption (SiO₂, Si₃N₄, Si) boost enhancement by increasing the extinction cross-section of the apex nanostructure. Using a set of physically realistic constraints, probes were optimized for peak plasmonic enhancement at common near-field optical wavelengths (633-647 nm) and those with focused ion-beam milled grooves near the apex were found to give the largest local field enhancements (~30x). Compared to unstructured metal cones, grooved probes gave a 300% improvement in field strength, which can boost tip-enhanced Raman spectroscopy (TERS) signals by 1-2 orders of magnitude. Moreover, grooved probe resonances can be easily tuned over visible and near-infrared energies by varying the plasmonic metal (Ag or Au) and groove location. Overall, this work shows that probes with strong localized surface plasmon resonances at their apices can be engineered to provide large field enhancements and boost signals in near-field optical experiments.

Focused, evanescent, hollow, and collimated beams formed by microaxicons with different conical angles

Diffraction patterns formed by axicons with different tip (vertex) angles are analytically and numerically investigated. Results show that the axicon (or tapered dielectric probe) can form an extended axial light beam, a compact evanescent field, a hollow beam, and a collimated beam, depending on the vertex angle. Two-dimensional and three-dimensional models of a tapered dielectric probe show that, with small changes to the vertex angle, light transmitted by the probe is scattered rather than focused, and vice versa. Angle meanings corresponded to boundary transitions have a quantum character and densify as the angle approaches zero. These features should be taken into consideration when manufacturing microaxicons intended for various applications.

Broadband converging plasmon resonance at a conical nanotip

We propose an analytical theory which predicts that Converging Plasmon Resonance (CPR) at conical nanotips exhibits a red-shifted and continuous band of resonant frequencies and suggests potential application of conical nanotips in various fields, such as plasmonic solar cells, photothermal therapy, tip-enhanced Raman and other spectroscopies. The CPR modes exhibit superior confinement and ten times broader scattering bandwidth over the entire solar spectrum than smooth nano-structures. The theory also explicitly connects the optimal angles and resonant optical frequencies to the material permittivities, with a specific optimum half angle that depends only on the real permittivity for high-permittivity and low-loss materials.

Optical antennas as nanoscale resonators

Recent progress in nanotechnology has enabled us to fabricate sub-wavelength architectures that function as antennas for improving the exchange of optical energy with nanoscale matter. We describe the main features of optical antennas for enhancing quantum emitters and review the designs that increase the spontaneous emission rate by orders of magnitude from the ultraviolet up to the near-infrared spectral range. To further explore how optical antennas may lead to unprecedented regimes of light-matter interactions, we draw a relationship between metal nanoparticles, radio-wave antennas and optical resonators. Our analysis points out how optical antennas may function as nanoscale resonators and how these may offer unique opportunities with respect to state-of-the-art microcavities.

Nanofocusing radially-polarized beams for high-throughput funneling of optical energy to the near field

We theoretically show that a weakly-focused radially polarized beam can excite surface-plasmon-polaritons in metal nanowires and nanocones with efficiencies of the order of 90% and large bandwidths. The coupling mechanism relies on the formation of a standing wave on the nanowire facet, which imposes a relationship between the operating wavelength and the nanowire radius. An immediate application of this finding is nanofocusing of optical energy for implementations of ultra-fast and high-throughput linear and nonlinear nanoscopies, optical nanolithographies, quantum nano-optics and photochemistry at the nanoscale.

Nanofocusing of surface plasmon polaritons by a pyramidal structure on an aperture

A numerical study of the nanofocusing of surface plasmon polaritons (SPPs) by a pyramidal structure on a rectangular aperture is performed by the volume integral equation method. It is possible to perform nanofocusing using this structure by using a linearly polarized wave as the incident wave. The focusing process of SPPs by the tip of the pyramidal structure has been demonstrated numerically. The characteristics of the focused optical field near the tip have been investigated in detail. It was found to be similar to that of monopole rather than that of a tiny dipole. The optical field at the tip is sensitive to the local shape of the tip. The enhanced intensity on the tip increases with an increase in the aperture width.