Active carbon based supercapacitors with Au colloids: the case of placing the colloids in close proximity to the electrode interface.

Supercapacitors (SCs) are short-term energy storage elements that find many applications, e.g., electronic charging devices and suppressors of power fluctuations in grids that are interfaced with sustainable sources. The capacitance of an ordinary capacitor increases when dispersing metallic colloids in its dielectric. A similar strategy for SCs means deployment of nano-scale metal colloids (in our case, Au nanoparticles, or AuNPs) at the very narrow interface between an electrolyte and a porous electrode (here, active carbon film, AC, on a grafoil current collector). Unlike previous studies, here we placed AuNPs at a small distance from the electrode. This was achieved by coating the AuNPs with a negatively charged ligand that also enables strong adhesion to the electrode. A very large specific capacitance amplification was demonstrated: for example, C-V data at a scan rate of 20 mV s-1 indicated a specific capacitance amplification of more than 10 when 30 µg of AuNPs was incorporated with 200 mg of active carbon while using a 1 M Na2SO4 electrolyte and a 5% cellulose acetate butyrate binder. Upon replacing the 1 M Na2SO4 electrolyte with 1 M KOH, and keeping the same set of electrodes, the amplification factor decreased but remained large, ∼3, as determined using C-V traces at the same scan rate. This proves that the AuNPs adhered well to the AC electrodes. Simulations indicated the importance of keeping the AuNPs in close proximity to the electrodes, but not in direct contact with them, in order to maintain a substantial amplified polarization effect. Unlike semiconductor embedded electrodes, optical effects were found to be minimal.

Optical cages made of graphitic frameworks.

In pursuit of perfect infrared (IR) radiation absorbers, we examined quasi-crystal structures made of graphite wires. Simulations on an array of subwavelength graphitic cages and cage-within-cage frameworks indicate a flat absorption coefficient between 10-30 µm. The concept could be scaled up through the 50-120 µm [far-IR, terahertz (THz)] region by a proper structural design. For cage-within-cage, the IR radiation energy is funneled toward the inner cage, resulting in a rather hot structure. At longer wavelengths (microwave region), the electrical conductivity dominates the negative dielectric effect, and experiments with copper cages indicate scattering resonances. Graphitic structures allude to some absorption even at microwave frequencies. Applications are envisioned as anti-fogging surfaces, adaptable electromagnetic shields, energy harvesting, and efficient absorbers in the far-IR (THz frequencies).

Curved infrared screens.

We address curved IR screens for multiwavelength systems. To first-order of the approximation, a curved screen may be viewed as composed of many local flat screens. On the other hand, the validity of such an approximation is not clear a priori. We provide experiments and simulations to show that such an approximation works well for cylindrically curved IR screens while monitoring their peak transmission as a function of the screen curvature.

Microfluidic Channels on Nanopatterned Substrates: Monitoring Protein Binding to Lipid Bilayers with Surface-Enhanced Raman Spectroscopy.

We used Surface Enhanced Raman Spectroscopy (SERS) to detect binding events between streptavidin and biotinylated lipid bilayers. The binding events took place at the surface between microfluidic channels and anodized aluminum oxide (AAO) with the latter serving as substrates. The bilayers were incorporated in the substrate pores. It was revealed that non-bound molecules were easily washed away and that large suspended cells (Salmonella enterica) are less likely to interfere with the monitoring process: when focusing to the lower surface of the channel, one may resolve mostly the bound molecules.

Negative differential resistance: gate controlled and photoconductance enhancement in carbon nanotube intraconnects.

Intraconnects, as-grown single-walled carbon nanotubes bridging two metal electrodes, were investigated as gated structures. We show that even with a seemingly "ohmic" contact at zero gate voltage one observes negative differential resistance (NDR) at nonzero gate bias. Large differential photo conductance (DPC) was associated with the NDR effect raising hopes for the fabrication of novel high-speed optoelectronic devices.

The possibility for surface plasmons lasers.

We have demonstrated for the first time, attributes of a surface plasmons' laser: threshold, gain, spectral line narrowing and feedback in the visible range. The surface metallic waveguides were consisted of a nano-scale hole-array in a 50 nm thick layer of aluminum oxide on top of aluminum substrate (anodized aluminum oxide or, AAO). In some cases, two-layer graphene was added on top of the perforated oxide layer, as well. The sub-wavelength array of holes enabled coupling to and from the waveguides as well as, providing feedback to the surface modes. The gain media molecules (fluorescein) were imbedded in the structure's pores. Threshold and spectral line narrowing of 30% were clearly demonstrated when pumped with a pulsed laser.

Polarization-dependent fluorescence of proteins bound to nanopore-confined lipid bilayers.

Lipid bilayers are essential structural component of biological membranes of all the living species: from viruses and bacteria to plants and humans. Biophysical and biochemical properties of such membranes are important for understanding physical mechanisms responsible for drug targeting. Binding events between proteins and the membrane may be ascertained by introducing fluorescence markers (chromophores) to the proteins. Here we describe a novel biosensing platform designed to enhance signals of these fluorescence markers. Nanoporous aluminum oxide membranes with and without gold (Au) surface coating have been employed for optical detection of bound conjugated streptavidin to biotinylated lipid bilayers-a model system that mimics protein docking to the membrane surface. Unexpectedly, it was found that fluorescence signals from such structures vary when pumped with E-polarized and H-polarized incident optical beams. The origin of the observed polarization-dependent effects and the implications for enhanced fluorescence detection in a biochip format are being discussed.

Carbon nanotubes bridges spanning across metal electrode tips.

Systematically growing carbon nanotubes at exact locations within an optoelectronic circuit is a long sought goal. Here we electrically and optically characterize individual CNT bridges spanning across pre-fabricated and addressable metal tips.

Depositing graphene films on solid and perforated substrates.

Graphene-a monolayer of graphite-has attracted vast interest recently owing to its perfect two-dimensional crystallographic nature and its potential use in a new generation of microelectronic devices. Yet, a deposition method, which results in a large coverage of monolayer thick graphite, is still lacking. By using a chemical mechanical polishing (CMP) method we were able to deposit stress-free graphene on solid and perforated substrates alike, achieving area coverage of hundreds of microns squared.

Surface enhanced Raman with anodized aluminum oxide films.

Aluminum is not a platform of choice for surface enhanced Raman scattering (SERS) experiments despite its large negative permittivity value (larger than gold or silver at optical wavelengths). It is also widely believed that an oxide layer on top of any platform substantially impedes SERS signals. Yet, anodized aluminum oxide may be perforated in an organized fashion and we have used it to examine SERS of single walled carbon nanotubes (SWCNTs) at micron length and fullerene (C60) at the nanoscale. The signal-to-noise ratio of the corresponding Raman signals exhibited a large signal enhancement for SWCNTs but not for C60. We attributed the SERS to the formation of standing surface charge waves in this subwavelength environment.

Three-dimensional metallo-dielectric photonic crystals with cubic symmetry as stacks of two-dimensional screens.

Metallo-dielectric photonic crystals with cubic symmetries have been studied here both experimentally and theoretically in the millimeter wavelength region (15-60 mm). In a direct analogy to linear systems, we considered the three-dimensional lattices as a stack of two-dimensional resonating screens. The overall three-dimensional structure was introduced in the calculation through a structural phase. Such an approach proved useful in understanding the related mode propagation and guided us in a study of the transition between cubic and centered body cubic symmetries.

Self-imaging in photonic crystals in a subwavelength range.

Self-imaging was observed in simulations and in microwave experiments that were performed on a face-centered cubic photonic crystal structure. The structure was composed of dielectric spheres that generally were smaller than the incident wavelength. Such self-imaging phenomena occurred along the direction of propagation and at distances smaller than the propagating wavelength.

Self-imaging chirped holographic optical waveguides.

To manipulate light propagation in optical waveguides, we have studied holographic, chirped structures within the waveguide's core. The holographic structures were embedded along the wave propagation direction and extended throughout the entire guide. Various self-imaging guides have been analyzed and realized to demonstrate the effect of different structures.

Transverse holographic optical interconnect design.

The design of A new type of planar optical interconnect, the transverse holographic waveguide, is described. With this type of interconnect a one-dimensional input light distribution is converted into a one-dimensional output light distribution by holographic patterning along the direction of the optical wave propagation.

Artificial dielectric polymeric waveguides: semiconductor-embedded films.

Thin (less than 2 nm) layers of semiconductor clusters (the guest material) are sandwiched between two polymeric films (the host material), and the light-induced electric and magnetic dipoles form a conditional artificial dielectric.

Mode conversion in optically active and isotropic waveguides.

Mode conversion in optically active slab waveguides is studied both theoretically and experimentally. The waveguide is fabricated by homogeneously embedding isotropic and randomly oriented optical active material into a passive polymeric waveguide. The polarization rotation is measured for several configurations of a slab waveguide.

Effect of periodic strain on the leaky modes in single-mode fiber.

The effect of periodic strain on the beat wavelength between the guided LP(01) and the leaky LP(11) modes is approximated around the LP(11) mode cutoff. An abrupt change of the beat wavelength as a function of optical wavelength signifies the onset of the cutoff region. An order parameter is suggested to characterize the transition of the LP(11) mode from a guided to a cladding mode.

Phase delay of coupled modes in single-mode optical fibers.

The phase delay of the fundamental LP(01) mode in a single-mode optical fiber is studied using periodic perturbation of the fiber axis. A large phase shift may be obtained through distortion-induced birefringence, with less than 1.5- dB coupling loss to the radiative modes.

Effect of strain in periodically deformed single-mode optical fibers.

The effect of periodic strains in single-mode fibers, which lead to periodic changes of the relative index of refraction, is studied theoretically and experimentally. The main effect of the periodic strain is to downshift the beat length between the optical modes in the fiber.