Mapping Free-Carriers in Multijunction Silicon Nanowires Using Infrared Near-Field Optical Microscopy.

We report the use of infrared (IR) scattering-type scanning near-field optical microscopy (s-SNOM) as a nondestructive method to map free-carriers in axially modulation-doped silicon nanowires (SiNWs) with nanoscale spatial resolution. Using this technique, we can detect local changes in the electrically active doping concentration based on the infrared free-carrier response in SiNWs grown using the vapor-liquid-solid (VLS) method. We demonstrate that IR s-SNOM is sensitive to both p-type and n-type free-carriers for carrier densities above ∼1 × 10 cm.

Biomimetic Optical-Filter Detection System for Discrimination of Infrared Chemical Signatures.

Optical-filter-based chemical sensors have the potential to dramatically alter the field of hazardous materials sensing. Such devices could be constructed using inexpensive components, in a small and lightweight package, for sensing hazardous chemicals in defense, industrial, and environmental applications. Filter-based sensors can be designed to mimic human color vision.

Legacies of the Gerchberg-Saxton algorithm.

To celebrate W. Owen Saxton's 65th birthday, this paper presents some of the impact that the Gerchberg-Saxton algorithm has had over the last 40 years. We explore some of the fundamental concepts underlying the success of the Gerchberg-Saxton algorithm, in the context of how it stimulated many related methods for estimating fields and deepening the understanding of the relationships between complex objects, images and their Fourier transforms.

Selective field localization in random structured media.

We have introduced a method to find optimized structured media exhibiting large internal electric field amplitudes. The method is based on a genetic algorithm in which a spatial fitness function according to the computed field distribution in the interior of media is defined and maximized. The main feature of our method is that it enables localization of light at a desired layer (or more) within the structure.

Characterization of novel microsphere chain fiber optic tips for potential use in ophthalmic laser surgery.

Ophthalmic surgery may benefit from use of more precise fiber delivery systems during laser surgery. Some current ophthalmic surgical techniques rely on tedious mechanical dissection of tissue layers. In this study, chains of sapphire microspheres integrated into a hollow waveguide distal tip are used for erbium:YAG laser ablation studies in contact mode with ophthalmic tissues, ex vivo.

Fabrication and characterization of a biomimetic polarization selective lens.

Incorporating optical structures on curved lens surfaces can improve performance, consolidate functions, and create novel, miniaturized devices. Although commonly found in biological systems, patterning of micro- and nano-optical structures on curved surfaces is challenging for conventional methods. Previous works have demonstrated the ability to pattern curved surfaces but have done little to create functioning devices.

Periodically focused modes in chains of dielectric spheres.

We show that, theoretically, Brewster angle conditions for transverse magnetic polarized rays can be periodically reproduced in chains of spheres with index n = [Formula: see text] giving rise to lossless periodically focused modes with 2D period, where D is the sphere diameter. Using ray tracing for a spherical emitter with the diameter D we show that chains of spheres work as filters of such modes at 1.72 < n < 1.

Power characteristics of homogeneously broadened index-antiguided waveguide lasers.

A model is reported that describes a bidirectional homogeneously broadened index-antiguided (IAG) slab laser having arbitrary single-pass gain and distributed losses. Maximum extraction efficiency and corresponding optimum output coupling are determined for various values of unsaturated gain and loss per pass. A method is proposed to determine the intrinsic laser parameters from output power measurements.

Image continuity at different levels of zoom for fringe patterns.

Fringe patterns are raw output data from many measurement systems including laser interferometers and moiré systems. For instruments with a range of zoom levels to measure the object at different scales, a technique (algorithm) is needed to combine and/or compare data to obtain information at different levels of details. A technique to keep the continuity of output images both at different levels of zoom and within the same level of zoom is developed and demonstrated.

Variable-diameter refractive beam shaping with freeform optical surfaces.

We propose a refractive two-element system that converts the gaussian irradiance of an incident laser beam into a nominally flat-top output spot at a given distance with the capability to vary the spot diameter. The elements are high-order freeform surfaces that, when laterally translated, form a variable composite beam shaper. The general approach for determining the required freeform surfaces is discussed, and example design results are presented.

Two-dimensional guided mode resonance filters fabricated in a uniform low-index material system.

We demonstrate the fabrication, simulation, and experimental results of a buried, homogeneous narrowband spectral filter with a periodic, hexagonal unit cell of air pockets, encapsulated in a fused silica substrate. The leaky waveguide is formed by depositing SiO(x) on an etched fused silica grating via plasma-enhanced chemical vapor deposition. Design principles of guided mode resonance filters were utilized to achieve a resonance with 60% reflectivity at a wavelength of 1.

Fiber coupling to BaTiO3 glass microspheres in an aqueous environment.

Compact microspheres with high-quality (Q) whispering gallery modes are required for many applications involving liquid immersion, such as sensing nanoparticles and studying resonant radiative pressure effects. We show that high-index (1.9 and 2.

Performance of conformal guided mode resonance filters.

Guided mode resonance (GMR) filters are highly functional micro-optics capable of narrowband spectral filtering. GMR devices have previously been demonstrated on flat substrates using a wide range of materials and configurations. In this Letter, we apply a soft lithographic technique followed by the deposition of dielectric layers to generate GMR filters on a concave lens surface.

Contact focusing multimodal microprobes for ultraprecise laser tissue surgery.

Focusing of multimodal beams by chains of dielectric microspheres assembled directly inside the cores of hollow waveguides is studied by using numerical ray tracing. The device designs are optimized for laser surgery in contact mode with strongly absorbing tissue. By analyzing a broad range of parameters it is demonstrated that chains formed by three or five spheres with a refractive index of 1.

Polarization selective, graded-reflectivity resonance filter, using a space-varying guided-mode resonance structure.

We designed, fabricated, and tested, polarization selective, graded-reflectivity resonant filters; based on a radial-gradient spatially-distributed, guided-mode resonance device architecture. The demonstrated filters have polarized spectral-resonance responses, distributed across their aperture extent, in the range between 1535 nm and 1540 nm wavelengths. Spectral sensitivity was observed on device tests, for wavelength changes as low as 0.

Gain guiding in large-core Bragg fibers.

We theoretically analyze gain guiding in large-core Bragg fibers, to be used for large-mode-area laser amplifiers with single-transverse-mode operation. The signal is gain-guided in a low-index core, whereas the pump is guided by the photonic bandgap of the Bragg cladding to achieve good confinement. The high-index layers in the Bragg cladding are half-wave thick at the signal wavelength in order to eliminate Bragg reflection, reducing the Bragg fiber effectively to a step-index fiber for gain guiding.

Gain saturation in gain-guided slab waveguides with large-index antiguiding.

We investigate numerically and analytically the effects of gain saturation on the propagation of the fundamental mode in a gain-guided index-antiguided slab waveguide. The propagating mode adapts to gain saturation by becoming less confined, while at the same time its peak intensity increases more slowly. At steady state, both the mode shape and the power remain constant.

Spectroscopy of coherently coupled whispering-gallery modes in size-matched bispheres assembled on a substrate.

Coupling between whispering-gallery modes is studied using imaging spectroscopy in pairs of size-selected (size deviation of <0.03%) spheres in a touching position. In a special geometrical configuration, peculiar shapes resembling kites are observed in the spectral images of bispheres by coupling of multiple pairs of azimuthal modes.

Gain-guiding in transverse grating waveguides for large modal area laser amplifiers.

A new optically pumped waveguide amplifier with ultra-large mode area is proposed. This amplifier is based on gain guiding in a transverse grating waveguide in which the pump is confined by the photonic bandgap while the signal is guided by optical gain. Characteristics of the propagating modes of the waveguide amplifier are analyzed theoretically using the transfer matrix method, indicating robust single-transverse-mode operation with large modal gain.

Perturbations of whispering gallery modes by nanoparticles embedded in microcavities.

The effects of perturbations of whispering gallery modes (WGMs) in cylindrical microcavities by embedded particles are studied by FDTD modeling. The principal effects are: i) spectral shift of the WGM-related peaks caused by the variation of the average index, ii) broadening of the WGM peaks introduced by the scattering, and iii) splitting of the WGM peaks due to formation of symmetric (SSW) and antisymmetric (ASW) standing waves. The focus of this work is on the last effect.

Deposition of tungsten nanogratings induced by a single femtosecond laser beam.

Tungsten nanogratings with sub-100nm linewidths and subwavelength periods are fabricated by laser-induced chemical vapor deposition using a single 400 nm femtosecond pulsed laser beam without any beam shaping. Combining advantages of parallel and direct-write processing, this method can produce various nanograting structures on a wide range of substrates in a single step.

Observation of nanojet-induced modes with small propagation losses in chains of coupled spherical cavities.

Nanojet-induced modes (NIMs) and their attenuation properties are studied in linear chains consisting of tens of touching polystyrene microspheres with sizes in the 2-10 micro m range. To couple light to NIMs we used locally excited sources of light formed by several dye-doped fluorescent microspheres from the same chain of cavities. We directly observed the formation and propagation of NIMs by means of the scattering imaging technique.

Binding of rigid dendritic ruthenium complexes to carbon nanotubes.

Single-walled carbon nanotubes (SWNTs) bind strongly to rigid ruthenium metallodendrimers. High valence ions effectively coagulate these nanotubes from stable dispersions in N,N-dimethylforamide. While ruthenium salts and small [Ru(diimine)(3)](2+) complexes also coagulate the nanotubes, they require much higher concentrations and are easily extracted from the nanotubes with acetonitrile.

Synthesis and characterization of nanowire-based anisotropic conductors.

We investigated the potential of commercially available porous templates to be used for the fabrication of functional anisotropic conductors. A galvanostatic deposition technique was used to fabricate arrays consisting of 200 nm diameter nanowires inside the pores of polycarbonate membranes. A tape lift-off procedure allowed the complete removal of any residual metal from both sides of the polymer membrane to form an anisotropic conductive film.