Daniel R. Mason

Enhanced resolution beyond the Abbe diffraction limit with wavelength-scale solid immersion lenses

The solid immersion lens (SIL) is a well-developed near-field optical device for imaging and data storage. Recent experiments have demonstrated high-quality imaging beyond the diffraction limit by nanoscale lenses in an SIL-type implementation [Nature 460, 498 (2009)]; we call these nSIL. A question arises as to what resolution is obtainable with an nSIL. From full three-dimensional, finite-difference time-domain calculations, we demonstrate that the FWHM of the focal spot of an objective-lens-nSIL system can be reduced by greater than 25% compared to a regular macroscopic SIL.

Incorporation of nanovoids into metallic gratings for broadband plasmonic organic solar cells

We present investigation and optimization of a newly proposed plasmonic organic solar cell geometry based on the incorporation of nanovoids into conventional rectangular backplane gratings. Hybridization of strongly localized plasmonic modes of the nanovoids with Fabry-Perot cavity modes originating from surface plasmon reflection at the grating elements is shown to significantly boost performance in the long wavelength regime. This constitutes improved broadband operation while maintaining absorption enhancements at short wavelengths derived from conventional rectangular grating. Our calculations predict a figure of merit enhancement of up to 41% compared to when the nanovoid indented grating is absent. This is a significant improvement over the previously considered rectangular grating structures, which is further shown to be maintained over the entire angular range.

Plasmonic Excitations of 1D Metal-Dielectric Interfaces in 2D Systems: 1D Surface Plasmon Polaritons

Surface plasmon-polariton (SPP) excitations of metal-dielectric interfaces are a fundamental light-matter interaction which has attracted interest as a route to spatial confinement of light far beyond that offered by conventional dielectric optical devices. Conventionally, SPPs have been studied in noble-metal structures, where the SPPs are intrinsically bound to a 2D metal-dielectric interface. Meanwhile, recent advances in the growth of hybrid 2D crystals, which comprise laterally connected domains of distinct atomically thin materials, provide the first realistic platform on which a 2D metal-dielectric system with a truly 1D metal-dielectric interface can be achieved. Here we show for the first time that 1D metal-dielectric interfaces support a fundamental 1D plasmonic mode (1DSPP) which exhibits cutoff behavior that provides dramatically improved light confinement in 2D systems. The 1DSPP constitutes a new basic category of plasmon as the missing 1D member of the plasmon family: 3D bulk plasmon, 2DSPP, 1DSPP, and 0D localized SP.

Wavelength-dependent transmission through sharp 90° bends in sub-wavelength metallic slot waveguides

In this paper, we present a comprehensive numerical study of the wavelength-dependence of transmission through sharp 90 degrees bends in metallic slot waveguides with sub-wavelength localization and varying geometrical parameters. In particular, it is demonstrated that increasing the plasmon wavelength results in a significant increase (up to nearly 100%) of transmission through the bend, combined with a reduction in the mode asymmetry in the second arm of the bend. The mode asymmetry and its relaxation are explained by interference of the transmitted mode with non-propagating and leaky modes generated at the bend. Comparison with the two-dimensional case of a metal-dielectric-metal waveguide is also conducted, showing significant differences for the slot waveguides based on the presence of different non-propagating and leaky modes.

Direct Optical Probing of Transverse Electric Mode in Graphene

Unique electrodynamic response of graphene implies a manifestation of an unusual propagating and localised transverse-electric (TE) mode near the spectral onset of interband transitions. However, excitation and further detection of the TE mode supported by graphene is considered to be a challenge for it is extremely sensitive to excitation environment and phase matching condition adherence. Here for the first time, we experimentally prove an existence of the TE mode by its direct optical probing, demonstrating significant coupling to an incident wave in electrically doped multilayer graphene sheet at room temperature. We believe that proposed technique of careful phase matching and obtained access to graphene’s TE excitation would stimulate further studies of this unique phenomenon, and enable its potential employing in various fields of photonics as well as for characterization of graphene.

Embedding metal electrodes in thick active layers for ITO-free plasmonic organic solar cells with improved performance.

We propose and numerically investigate the optical performance of a novel plasmonic organic solar cell with metallic nanowire electrodes embedded within the active layer. A significant improvement (~15%) in optical absorption over both a conventional ITO organic solar cell and a conventional plasmonic organic solar cell with top-loaded metallic grating is predicted in the proposed structure. Optimal positioning of the embedded metal electrodes (EME) is shown to preserve the condition for their strong plasmonic coupling with the metallic back-plane, meanwhile halving the hole path length to the anode which allows for a thicker active layer that increases the optical path length of propagating modes. With a smaller sheet resistance than a typical 100 nm thick ITO film transparent electrode, and an increased optical absorption and hole collection efficiency, our EME scheme could be an excellent alternative to ITO organic solar cells.

Plasmon nanofocusing in a dielectric hemisphere covered in tapered metal film

We propose and analyze a new type of mechanically robust optical nanofocusing probe with minimized external environmental interference. The probe consists of a dielectric optical fiber terminated by a dielectric hemisphere - both covered in thin gold film whose thickness is reduced (tapered) along the surface of the hemisphere toward its tip. Thus the proposed probe combines the advantages of the diffraction-limited focusing due to annular propagation of the plasmon with its nanofocusing by a tapered metal wedge (i.e. a metal film with reducing local thickness). The numerical finite-element analysis demonstrates strongly subwavelength resolution of the described structure with the achievable size of the focal spot of ~20 nm with up to ~150 times enhancement of the local electric field intensity. Detailed physical interpretations of the obtained results are presented and possible application as a new type of SNOM probe for subwavelength imaging, spectroscopy and sensing are also discussed.

Tuning Molecular Self‐Assembly Toward Intriguing Nanomaterial Architectures

Harnessing the self-assembly of organic molecules for the synthesis of arbitrarily structured nanomaterials with diverse physical and chemical properties is undoubtedly one of the most important goals of nanotechnology research. Although performed with ease in Nature, our ability to artificially shape-engineer self-assembled materials in the laboratory remains limited to a known set of precursor molecules that assemble to a few well-known topologies. Currently lacking are materials from which a diverse range of structural topologies can be chosen and extracted from the synthesis process by careful control of the external parameters of the growth conditions. Self-assembly of organic molecules is largely guided by the interplay of intermolecular noncovalent interactions. The strength of these interactions is comparable across both the solution and solid phase. This results in a dynamic equilibrium that may enable the shapes of self-assembled structures to be controlled and optimized into their thermodynamically or kinetically favored morphologies. Here we show the shape control of electroand photochemically active calix[4]hydroquinone (CHQ) into nanoplates, -polygons and -tubes and their dynamic conversion into nanospheres and -hemispheres through a subsequent anisotropic growth phase to form thermodynamically more stable structures. We propose the growth mechanism of CHQ nanostructures based on thermodynamic and kinetic model studies and theoretical simulations. This understanding provides a route to shape-engineering of new nanomaterials. These CHQ nanostructures can be implemented as nanolenses for high-resolution optical imaging or form dielectric templates for a diverse range of metal-coated highly tunable plasmonic materials without the need for additional reducing agents. The self-assembly of CHQ molecules shows the structural versatility of calixarene motifs capable of forming various intermolecular structures when combined with solvent and guest molecules. CHQ consists of four hydroquinone subunits where there are four inner OH groups to stabilize the cone shape of CHQ through the circular proton-tunneling resonance; the other OH groups and aromatic rings contribute to intermolecular interactions (see the Supporting Information). Thus, a CHQ molecule has eight hydrogen bond donors, eight receptors, and four p–p stacking pairs leading to the formation of self-assembled supramolecular structures. Figure 1 shows the CHQ nanostructures with

A method for the analysis of thermal tweezers for manipulation and trapping of nanoparticles and adatoms on crystalline surfaces

We develop a computationally efficient method for the theoretical analysis of thermophoresis of nanoparticles and adatoms on crystalline surfaces (thermal tweezers) for efficient parallel nanofabrication. The analysis of surface diffusion of particles or adatoms in the presence of strong temperature gradients is conducted through the direct determination of probability distributions for diffusing particles, using the numerical solution of the Smoluchowski diffusion equation with varying (temperature-dependent) diffusion constant. The local values of the diffusion constant are determined from the Fokker–Planck equation for the considered crystalline potential of the substrate and local temperature. Steady-state and nonsteady-state particle distributions on the surface are obtained and analyzed in the presence of optically-induced strong temperature gradients. Detailed comparison of this approach with the previously obtained results from the Monte Carlo simulations of the Langevin equation is conducted, demonstrating high computational efficiency, and accuracy of the new method in the high-friction regime. Applicability conditions for the developed method are also determined and discussed.

Top-down, decoupled control of constitutive parameters in electromagnetic metamaterials with dielectric resonators of internal anisotropy

A meta-atom platform providing decoupled tuning for the constitutive wave parameters remains as a challenging problem, since the proposition of Pendry. Here we propose an electromagnetic meta-atom design of internal anisotropy (εr ≠ εθ), as a pathway for decoupling of the effective- permittivity εeff and permeability μeff. Deriving effective parameters for anisotropic meta-atom from the first principles, and then subsequent inverse-solving the obtained decoupled solution for a target set of εeff and μeff, we also achieve an analytic, top-down determination for the internal structure of a meta-atom. To realize the anisotropy from isotropic materials, a particle of spatial permittivity modulation in r or θ direction is proposed. As an application example, a matched zero index dielectric meta-atom is demonstrated, to enable the super-funneling of a 50λ-wide flux through a sub-λ slit; unharnessing the flux collection limit dictated by the λ-zone.