Image restoration in fiber-coupled imagers using space-variant impulse response characterization.

Fiber-coupled image sensors have attracted interest in recent years for high-resolution conformal image transfer, including mapping of the spherical image surface of a monocentric wide-angle lens to one or more flat focal plane sensors. However, image resolution is lost due to fiber bundle defects, moiré from lateral fiber-sensor misalignment, and blur due to the nonzero gap between fiber bundle and the image sensor. Here we investigate whether subpixel impulse response characterization of the strongly shift-variant impulse response can be used with existing image-processing techniques to recover the resolution otherwise lost in image transfer. We show that the submicrometer impulse response is experimentally repeatable, and can be used to recover image data and reveal fine features of the input surface structure of a 2.5 µm pitch fiber bundle.

Streamlined approach to mapping the magnetic induction of skyrmionic materials.

Recently, Lorentz transmission electron microscopy (LTEM) has helped researchers advance the emerging field of magnetic skyrmions. These magnetic quasi-particles, composed of topologically non-trivial magnetization textures, have a large potential for application as information carriers in low-power memory and logic devices. LTEM is one of a very few techniques for direct, real-space imaging of magnetic features at the nanoscale. For Fresnel-contrast LTEM, the transport of intensity equation (TIE) is the tool of choice for quantitative reconstruction of the local magnetic induction through the sample thickness. Typically, this analysis requires collection of at least three images. Here, we show that for uniform, thin, magnetic films, which includes many skyrmionic samples, the magnetic induction can be quantitatively determined from a single defocused image using a simplified TIE approach.

Finite Difference Micromagnetic Solvers with the Object Oriented MicroMagnetic Framework (OOMMF) on Graphics Processing Units.

A micromagnetic solver using the Finite Difference method on a Graphics Processing Unit (GPU) and its integration with the Object Oriented MicroMagnetic Framework (OOMMF) are presented. Two approaches for computing the magnetostatic field accelerated by the Fast Fourier Transform (FFT) are implemented. The first approach, referred to as the tensor approach, is based on the tensor spatial convolution to directly compute the magnetostatic field from magnetic moments. The second approach, referred to as the scalar potential approach, uses differential operator evaluation through finite differences (divergence for magnetic charge and gradient for magnetostatic field) and spatial convolution for magnetic scalar potential. Comparisons of implementation details, speed, memory consumption and accuracy are provided. The GPU implementation of OOMMF shows up to 32x GPU-CPU speed-up.

Analysis and characterization of high-resolution and high-aspect-ratio imaging fiber bundles.

High-contrast imaging fiber bundles (FBs) are characterized and modeled for wide-angle and high-resolution imaging applications. Scanning electron microscope images of FB cross sections are taken to measure physical parameters and verify the variations of irregular fibers due to the fabrication process. Modal analysis tools are developed that include irregularities in the fiber core shapes and provide results in agreement with experimental measurements. The modeling demonstrates that the irregular fibers significantly outperform a perfectly regular "ideal" array. Using this method, FBs are designed that can provide high contrast with core pitches of only a few wavelengths of the guided light. Structural modifications of the commercially available FB can reduce the core pitch by 60% for higher resolution image relay.

Efficient analysis of deep high-index-contrast gratings under arbitrary illumination.

An efficient method for computing the problem of an electromagnetic beam transmission through deep periodic dielectric gratings is presented. In this method the beam is decomposed into a spectrum of plane waves, transmission coefficients corresponding to each such plane wave are found via Rigorous Coupled Wave Analysis, and the transmitted beam is calculated via inverse Fourier integral. To make the approach efficient for deep gratings the fast variations of the transmission coefficients versus spatial frequency are accounted for analytically by casting the summations and integrals in a form that has explicit rapidly varying exponential terms. The resulting formulation allows computing the transmitted beam with a small number of samples independent of the grating depth.

Dielectric shielded nanoscale patch laser resonators.

Dielectric shielded nanoscale patch laser resonators are introduced. Low-index dielectric shield layers surrounding a high-index core are shown to significantly reduce both metal and radiation losses. Structures suitable for both optical and electrical pumping and smaller than the vacuum wavelength in all three dimensions are shown to have a low enough threshold gain to lase at room temperature. Shifting the gain medium core provides control over the radiation pattern of the resonator and enables coupling of the laser light into a waveguide, opening opportunities for chipscale integration.

Metamaterials for enhanced polarization conversion in plasmonic excitation.

Surface plasmons efficient excitation is typically expected to be strongly constrained to transverse magnetic (TM) polarized incidence, as demonstrated so far, due to its intrinsic TM polarization. We report a designer plasmonic metamaterial that is engineered in a deep subwavelength scale in visible optical frequencies to overcome this fundamental limitation, and allows transverse electric (TE) polarized incidence to be strongly coupled to surface plasmons. The experimental verification, which is consistent with the analytical and numerical models, demonstrates this enhanced TE-to-plasmon coupling with efficiency close to 100%, which is far from what is possible through naturally available materials. This discovery will help to efficiently utilize the energy fallen into TE polarization and drastically increase overall excitation efficiency of future plasmonic devices.

Polarization weighting of Fano-type transmission through bi-dimensional metallic gratings.

We demonstrate an approach allowing isolating effects of surface plasmon polariton mediated resonant transmission in a periodic grating by means of polarization rotation. The grating comprises a square array of cylindrical holes in an optically thick metallic film. Transmittance data for the co- and cross-polarized cases are described accurately with Fano-type and pure Lorentzian-type line shapes, respectively. This polarization control allows for changing the relative weights of resonant and non-resonant transmission mechanisms, thus controlling the shape and symmetry of the observed Fano-type line shapes.

Nanoscale optical field localization by resonantly focused plasmons.

We experimentally demonstrate use of plasmonic resonant phenomena combined with strong field localization to enhance efficiency of confining optical fields in a Si waveguide. Our approach utilizes a plasmonic resonant nano-focusing-antenna (RNFA), that simultaneously supports several focusing mechanisms in a single nanostructure, integrated with a lossless Si waveguide utilized with silicon-on-insulator (SOI) technology, to achieve a sub-diffraction limited focusing with a nanoscale (deeply subwavelength) spot size. The metallic RNFA effectively converts an incoming propagating waveguide mode to a localized resonant plasmon mode in an ultrasmall volume in all 3 dimensions. The near-field optical measurements of the fabricated RNFA using heterodyne near-field scanning optical microscope (H-NSOM) validate the theoretical predictions showing strong optical field localization.

Low threshold gain metal coated laser nanoresonators.

We introduce a low refractive index layer between the metal and the gain medium in metal-coated laser resonators and demonstrate that it can significantly reduce the dissipation losses. Analysis of a gain medium waveguide shows that for a given waveguide radius, the low index layer has an optimal thickness for which the lasing threshold gain is minimal. The waveguide analysis is used for the design of a novel three-dimensional cylindrical resonator that is smaller than the vacuum wavelength in all three dimensions and exhibits a low enough threshold gain to lase at room temperature.

Extraordinary transmission and left-handed propagation in miniaturized stacks of doubly periodic subwavelength hole arrays.

Metallic plates embedded between dielectric slabs and perforated by rectangular arrays of subwavelength holes with a dense periodicity in one of the directions support extraordinary transmission (ET) phenomena, viz. strong peaks in the transmittance frequency dependence. Stacks of such perforated plates support ET phenomena with propagation along the stack axis that is characterized by the left handed behavior. The incorporation of the dielectric materials and dense periodicity allows significantly reducing the illuminated area of the perforated plate required experimentally to observe the ET phenomena as compared to the areas required in the case of free standing rectangular hole arrays. This facilitates the experimental investigation of ET under excitation in the Fresnel zone of Gaussian beams.

Optical wave properties of nano-particle chains coupled with a metal surface.

Optical phenomena supported by ordered and disordered chains of metal nano-particles on a metal surface are investigated by considering a particular example of gold nano-bumps on a gold surface. The TWs supported by these structures are analyzed by studying the frequency-wavenumber spectra of the fields excited by localized sources placed near the chain. Periodic nano-bump chains support traveling waves (TWs) that propagate without radiation loss along, and are confined to the region near, the chain. These TWs are slow waves with respect to both space fields and surface plasmon polaritons supported by the metal surface. For nearly resonant nano-bumps, the TWs are well confined and can be excited efficiently by a localized source placed near the chain but the TW propagation length is short. For non-resonant nano-bumps, the TWs have large propagation lengths but are not well confined and are excited less efficiently. The TWs supported by nano-bump chains were shown to have larger propagation lengths than free-standing chains of the same dimension/size and cross-sectional confinement. TWs also are supported by disordered chains and chains with sharp bends. Perturbations in nano-bump positions are shown to reduce the TW propagation length much less significantly than perturbations in their sizes. Transmission through sharp chain bends is much stronger for nearly resonant nano-bumps than for nonresonant ones. In addition to their ability to support TWs, nano-bump chains can be used to manipulate (excite/reflect/refract) SPPs on the metal surface.

Doubly negative metamaterials in the near infrared and visible regimes based on thin film nanocomposites.

An optical metamaterial characterized simultaneously by negative permittivity and permeability, viz. doubly negative metamaterial (DNM), that comprises deeply subwavelength unit cells is introduced. The DNM can operate in the near infrared and visible spectra and can be manufactured using standard nanofabrication methods with compatible materials. The DNM's unit cell comprise a continuous optically thin metal film sandwiched between two identical optically thin metal strips separated by a small distance form the film. The incorporation of the middle thin metal film avoids limitations of metamaterials comprised of arrays of paired wires/strips/patches to operate for large wavelength / unit cell ratios. A cavity model, which is a modification of the conventional patch antenna cavity model, is developed to elucidate the structure's electromagnetic properties. A novel procedure for extracting the effective permittivity and permeability is developed for an arbitrary incident angle and those parameters were shown to be nearly angle-independent. Extensions of the presented two dimensional structure to three dimensions by using square patches are straightforward and will enable more isotropic DNMs.