We show that microstructures built from nonmagnetic conducting sheets exhibit an effective magnetic permeability /spl mu//sub eff/, which can be tuned to values not accessible in naturally occurring materials, including large imaginary components of /spl mu//sub eff/. The microstructure is on a scale much less than the wavelength of radiation, is not resolved by incident microwaves, and uses a very low density of metal so that structures can be extremely lightweight. Most of the structures are resonant due to internal capacitance and inductance, and resonant enhancement combined with compression of electrical energy into a very small volume greatly enhances the energy density at critical locations in the structure, easily by factors of a million and possibly by much more. Weakly nonlinear materials placed at these critical locations will show greatly enhanced effects raising the possibility of manufacturing active structures whose properties can be switched at will between many states.

/pdf/magnetism-from-conductors-and-enhanced-nonlinear-phenomena-2fpnoamwae.pdf

Magnetism from conductors and enhanced nonlinear phenomena

A new type of metallic electromagnetic structure has been developed that is characterized by having high surface impedance. Although it is made of continuous metal, and conducts dc currents, it does not conduct ac currents within a forbidden frequency band. Unlike normal conductors, this new surface does not support propagating surface waves, and its image currents are not phase reversed. The geometry is analogous to a corrugated metal surface in which the corrugations have been folded up into lumped-circuit elements, and distributed in a two-dimensional lattice. The surface can be described using solid-state band theory concepts, even though the periodicity is much less than the free-space wavelength. This unique material is applicable to a variety of electromagnetic problems, including new kinds of low-profile antennas.

/pdf/high-impedance-electromagnetic-surfaces-with-a-forbidden-3vtu72a5an.pdf

High-impedance electromagnetic surfaces with a forbidden frequency band

Semiconductor heterostructures with modulated composition and/or doping enable passivation of interfaces and the generation of devices with diverse functions. In this regard, the control of interfaces in nanoscale building blocks with high surface area will be increasingly important in the assembly of electronic and photonic devices. Core-shell heterostructures formed by the growth of crystalline overlayers on nanocrystals offer enhanced emission efficiency, important for various applications. Axial heterostructures have also been formed by a one-dimensional modulation of nanowire composition and doping. However, modulation of the radial composition and doping in nanowire structures has received much less attention than planar and nanocrystal systems. Here we synthesize silicon and germanium core-shell and multishell nanowire heterostructures using a chemical vapour deposition method applicable to a variety of nanoscale materials. Our investigations of the growth of boron-doped silicon shells on intrinsic silicon and silicon-silicon oxide core-shell nanowires indicate that homoepitaxy can be achieved at relatively low temperatures on clean silicon. We also demonstrate the possibility of heteroepitaxial growth of crystalline germanium-silicon and silicon-germanium core-shell structures, in which band-offsets drive hole injection into either germanium core or shell regions. Our synthesis of core-multishell structures, including a high-performance coaxially gated field-effect transistor, indicates the general potential of radial heterostructure growth for the development of nanowire-based devices.

https://cml.harvard.edu/assets/nature420_57.pdf

Epitaxial core–shell and core–multishell nanowire heterostructures

A photonic structure consisting of an extended 3D network of thin wires is shown to behave like a low density plasma of very heavy charged particles with a plasma frequency in the GHz range. We show that the analogy with metallic behaviour in the visible is rather complete, and the picture is confirmed by three independent investigations: analytic theory, computer simulation and experiments on a model structure. The fact that the wires are thin is crucial to the validity of the picture. This new composite dielectric, which has the property of negative below the plasma frequency, opens new possibilities for GHz devices.

/pdf/low-frequency-plasmons-in-thin-wire-structures-3kcl5hez6o.pdf

Low frequency plasmons in thin-wire structures

Porous carbons that are three-dimensionally periodic on the scale of optical wavelengths were made by a synthesis route resembling the geological formation of natural opal. Porous silica opal crystals were sintered to form an intersphere interface through which the silica was removed after infiltration with carbon or a carbon precursor. The resulting porous carbons had different structures depending on synthesis conditions. Both diamond and glassy carbon inverse opals resulted from volume filling. Graphite inverse opals, comprising 40-angstrom-thick layers of graphite sheets tiled on spherical surfaces, were produced by surface templating. The carbon inverse opals provide examples of both dielectric and metallic optical photonic crystals. They strongly diffract light and may provide a route toward photonic band-gap materials.

https://science.sciencemag.org/content/sci/282/5390/897.full.pdf

Carbon Structures with Three-Dimensional Periodicity at Optical Wavelengths

We have investigated the electromagnetic properties of a 3D wire mesh in a geometry resembling covalently bonded diamond. The frequency and wave vector dispersion show forbidden bands at those frequencies ${\ensuremath{\nu}}_{0}$, corresponding to the lattice spacing, just as dielectric photonic crystals do. But they have a new forbidden band which commences at zero frequency and extends, in our geometry, to $\ensuremath{\sim}\frac{1}{2}{\ensuremath{\nu}}_{0}$, acting as a type of plasma cutoff frequency. Wire mesh photonic crystals appear to support a longitudinal plane wave, as well as two transverse plane waves. We identify an important new regime for microwave photonic crystals, an effective medium limit, in which electromagnetic waves penetrate deeply into the wire mesh through the aid of an impurity band.

/pdf/3d-wire-mesh-photonic-crystals-2i8ywslyz8.pdf

3D Wire mesh photonic crystals.

http://cmliris.harvard.edu/assets/science291_851.pdf

Functional Nanoscale Electronic Devices Assembled Using Silicon Nanowire Building Blocks

Ultra-High Efficiency Light Emitting Diodes Through Epitaxial Lift-off Packaging

Opto-electronics rely on optical packaging approaches for high optical efficiency. Index of refraction matching, transparent packages, current and voltage control through package design all contribute to the optical efficiency as much as the semiconductor device design itself. Traditionally, a package design would be developed around an existing semiconductor device design. It is becoming more prevalent that an enabling package design results in a new, more efficient device design. Epitaxial lift-off packaging is the packaging of a semiconductor chip with the semiconductor substrate removed. This allows for increased optical, thermal and electrical efficiency. The optically absorbing semiconductor substrate may be replaced with a transparent or reflective substrate integrated into the package. The LED device itself was redesigned to take advantage of the packaging approach. The use of epitaxial lift-off required some process customization since the semiconductor device without its supporting substrate is mechanically weak This paper presents the device design and a general overview of the epitaxial lift-off packaging process for a high-efficiency AlGaInP light emitting diode.

Ultra-high efficiency light emitting diodes through epitaxial lift-off packaging

Summary form only given. Most of the work on photonic bandgap materials to date has focused on dielectric structures because of their low loss at optical wavelengths. While metals are not suitable for use in this range, they find many applications at microwave frequencies. We believe we have constructed the first 3-D metallic photonic bandgap structure in a geometry resembling covalently bonded diamond.

Mike Sickmiller

Papers

3D Wire mesh photonic crystals.

Functional Nanoscale Electronic Devices Assembled Using Silicon Nanowire Building Blocks

Ultra-High Efficiency Light Emitting Diodes Through Epitaxial Lift-off Packaging

Ultra-high efficiency light emitting diodes through epitaxial lift-off packaging

3-D metallic photonic bandgap structures