Andrzej Herczynski

Receiving and transmitting light-like radio waves: Antenna effect in arrays of aligned carbon nanotubes

We present optical measurements of random arrays of aligned carbon nanotubes, and show that the response is consistent with conventional radio antenna theory. We first demonstrate the polarization effect, the suppression of the reflected signal when the electric field of the incoming radiation is polarized perpendicular to the nanotube axis. Next, we observe the interference colors of the reflected light from an array, and show that they result from the length matching antenna effect. This antenna effect could be used in a variety of optoelectronic devices, including THz and IR detectors.

Subwavelength waveguide for visible light

The authors demonstrate transmission of visible light through metallic coaxial nanostructures many wavelengths in length, with coaxial electrode spacing much less than a wavelength. Since the light frequency is well below the plasma resonance in the metal of the electrodes, the propagating mode reduces to the well-known transverse electromagnetic mode of a coaxial waveguide. They have thus achieved a faithful analog of the conventional coaxial cable for visible light.

Bio-inspired networks for optoelectronic applications

Modern optoelectronics needs development of new materials characterized not only by high optical transparency and electrical conductivity, but also by mechanical strength, and flexibility. Recent advances employ grids of metallic micro- and nanowires, but the overall performance of the resulting material composites remains unsatisfactory. In this work, we propose a new strategy: application of natural scaffoldings perfected by evolution. In this context, we study two bio-inspired networks for two specific optoelectronic applications. The first network, intended for solar cells, light sources and similar devices, has a quasi-fractal structure and is derived directly from a chemically extracted leaf venation system. The second network is intended for touch screens and flexible displays, and is obtained by metalizing a spiders silk web. We demonstrate that each of these networks attain an exceptional optoelectonic and mechanical performance for its intended purpose, providing a promising direction in the development of more efficient optoelectronic devices.

Hot electron effect in nanoscopically thin photovoltaic junctions

The open circuit voltage in ultrathin amorphous silicon solar cells is found to increase with light energy (frequency), due to extraction of hot electrons. The ultrathin nature of these junctions also leads to large internal electric fields, yielding reduced recombination and increased current. A simple phenomenological argument provides a qualitative understanding of these effects and gives guidelines for designing future, high-efficiency, hot electron solar cells.

On the inverse Magnus effect in free molecular flow

A Newton-inspired particle interaction model is introduced to compute the sideways force on spinning projectiles translating through a rarefied gas. The simple model reproduces the inverse Magnus force on a sphere reported by Borg, Soderholm and Essen [Phys. Fluids 15, 736 (2003)] using probability theory. Further analyses given for cylinders and parallelepipeds of rectangular and regular polygon section point to a universal law for this class of geometric shapes: when the inverse Magnus force is steady, it is proportional to one-half the mass M of gas displaced by the body.

Two-fluid jets and wakes

Similarity solutions for laminar two-fluid jets and wakes are derived in the boundary-layer approximation. Planar and axisymmetric fan jets as well as classical and momentumless planar wakes are considered. The interface between the immiscible fluids is stabilized by the action of gravity, with the heavier fluid, taken to be a liquid, placed beneath the lighter fluid. Velocity profiles for the jets and the classical wake depend intimately, but differently, on the parameter χ=ρ1μ1/ρ2μ2, where ρi and μi are, respectively, the density and absolute viscosity of the fluid in the upper (i=1) and lower (i=2) fluid domains, while the momentumless wake profile depends on the parameter Ω=ρ1μ23/ρ2μ13. Generally, all interfaces deflect from horizontal except the fan jet. However, while the interface for the classical planar two-fluid wake is never flat, the interfaces for the planar jet and the momentumless wake become flat in the particular case μ1=μ2. Velocity profiles illustrating the strongly asymmetrical jet and...

Optimization of hierarchical structure and nanoscale-enabled plasmonic refraction for window electrodes in photovoltaics.

An ideal network window electrode for photovoltaic applications should provide an optimal surface coverage, a uniform current density into and/or from a substrate, and a minimum of the overall resistance for a given shading ratio. Here we show that metallic networks with quasi-fractal structure provides a near-perfect practical realization of such an ideal electrode. We find that a leaf venation network, which possesses key characteristics of the optimal structure, indeed outperforms other networks. We further show that elements of hierarchal topology, rather than details of the branching geometry, are of primary importance in optimizing the networks, and demonstrate this experimentally on five model artificial hierarchical networks of varied levels of complexity. In addition to these structural effects, networks containing nanowires are shown to acquire transparency exceeding the geometric constraint due to the plasmonic refraction.

The complex optical response of arrays of aligned multiwalled carbon nanotubes

The optical properties of periodic and nonperiodic arrays of aligned multiwalled carbon nanotubes are presented. Experimental analysis indicates a complex optical response that is attributed to both the individual carbon nanotube scatterers and also to the array ensembles. These studies indicate that by controlling the geometry and spacing of the arrays, it is possible to create structures that respond very strongly to specific wavelengths or bands of wavelengths. Structures such as these may form the basis for numerous applications in energy conversion. This would include highly efficient solar energy conversion as well as sensitive, finely tuned detectors that can respond to predetermined wavelength bands ranging from the ultraviolet to the infrared region. Experimental, theoretical and modeled results are discussed as they apply to these applications. Challenges and issues are discussed.

Spectroscopic studies of arrays of multiwalled carbon nanotubes

Spectroscopic observations are presented for carbon nanotubes grown on silicon and quartz substrates in a hexagonal honeycomb configuration using self-assembly nanosphere lithography and plasma enhanced chemical vapor deposition method. A white light source is used as an incident beam and light reflected from the surface of the carbon nanotubes results in a distinctive signature in the reflected spectrum. A comparison of non-periodic arrays and periodic arrays of carbon nanotubes show that the reflectance signature is only observed when the carbon nanotubes are oriented in a periodic array. Further observations regarding the light antenna effect observed in nonperiodic arrays are also reported. Theoretical curves show good agreement to experimentally observed phenomena. The unique optical properties of the arrays combined with the excellent mechanical and electrical properties of carbon nanotubes indicate that these materials may find many uses in the field of optoelectronics.

The Sloshing-Induced Motion of Free Containers

Experiments for the time-periodic liquid sloshing-induced sideways motion of free containers are compared with theory using standard normal mode representations for rectangular boxes, upright cylinders, wedges and cones of \(90^{\circ }\) apex angles, and cylindrical annuli. While the wedge and cone exhibit only one mode of oscillation, the boxes, cylinders and annuli have an infinite number of modes. In some cases we have been able to excite the second mode of oscillation. Frequencies \(\omega \) were acquired as the average of three experimental determinations for every filling of mass \(m\) in the dry containers of mass \(m_0\). Measurements of the dimensionless frequencies \(\omega /\omega _R\) over a range of dimensionless liquid masses \(M = m/m_0\) are found to be in essential agreement with theoretical predictions. The frequencies \(\omega _R\) used for normalization arise naturally in the mathematical analysis, different for each geometry considered.