Karen J. Long

Tunable split-ring resonator for metamaterials using photocapacitance of semi-insulating GaAs

We report a method for tuning a split-ring resonator (SRR) using infrared light. The SRR unit cells are commonly used in arrays to form a metamaterial that exhibits an effective negative permeability and are often used in negative-refractive-index materials. The region of negative permeability is generally limited to a narrow bandwidth at a fixed frequency. In this work, we use photocapacitance as observed in undoped semi-insulating GaAs to modify the gap capacitance of a SRR. We demonstrate a continuously tunable frequency over the range of 0.1–1.5 GHz using a 975 nm laser diode with a power range of 0–189 mW.

Circuit Analysis of Photosensitive Capacitance in Semi-Insulating GaAs

We describe a circuit model for photosensitive capacitance in bulk semi-insulating GaAs toward tunable resonant applications. Capacitances from two separate regions are considered to interpret experimental results. A smaller valued capacitance exists between the depletion edges within the bulk material. Photodoping in this region progressively shorts out the bulk capacitance, leaving only the higher valued depletion capacitance. The depletion capacitance also increases with illumination, and numerical simulation is used to aid interpretation. Thus, the series combination of capacitance can be optically varied over orders of magnitude. Our results indicate that capacitance is nearly independent of applied voltage over a usable parameter space, making the concept attractive for linear application.

Nanomaterials for sensor applications

There is a lot of interest by society at large for reliable and affordable sensor and detection systems. Recent threats regarding public exposure to both biological and chemical agents have helped focus attention to the development of new sensor and detector technologies. Unfortunately, there is not even consensus in the scientific community on a uniform terminology of the subject. A large body of knowledge in sensor materials is available but most existing sensor materials are very limited in their ability to sense and discriminate small amounts of harmful agents embedded in large amounts of chemically inert but complex background. New materials are needed to meet the challenges ahead. Nanomaterials may be a solution to this problem. In this work we review the current state of the sensor lexicon. This is followed by a proposition of a consistent definition of terms used in the sensor and detection technology. Finally, we review a promising IR sensor system being developed using nanomaterials and several detector systems also developed using nanomaterials for nano solar cells and nano smart materials.

Optically Controllable Composite Dielectric Based on Photo-Conductive Particulates

We report a method to optically control the effective permittivity of a composite dielectric in which the active inclusions are fabricated from pulverized semi-insulating GaAs. The electric dipoles from the inclusions are controlled by photo-generation of charge carriers using infrared light. An infrared transparent binder provides the matrix material. Our primary purpose is to develop a low-cost pigment-based ink, or paint, for optically tuning electromagnetic devices including metamaterials, frequency-selective surfaces, filters, phase delays, and antennas. In bulk form, applications of the optically controllable dielectric are imagined including a gradient index lens in which inhomogeneous light intensity can provide a dynamic response for focusing or steering of beams. An important feature is that the effective permittivity is linear with electric field intensity over a usable parameter space. We present experimental results as well as a simple model to describe the behavior qualitatively.

Formation of a Ba-Te Surface on GaAs

The formation of a Ba-Te surface on GaAs has been investigated. The surface was created using molecular beam epitaxy (MRS). A GaAs (100) surface was first exposed to Te and characterized using x-ray photoelectron spectroscopy (XPS), reflective high energy electron diffraction (RHEED) and low energy electron diffraction (LEED). The Te-reacted surface was then exposed to BaF 2 flux producing a second reaction. In this reaction, the BaF 2 dissociated leaving barium on the surface but no fluorine. This is in contrast to the clean (no tellurium) GaAs (100) surface in which BaF 2 has been shown to grow single crystal. Although high order exists during early stages of the Ba-Te growth, further exposure gives way to a polycrystalline form. This paper discusses the formation and analysis of the Ba-Te surface.