W. L. Schaich

Narrow-band, tunable infrared emission from arrays of microstrip patches

We demonstrate through a combination of theory and experiment that an array of microstrip patches leads to a surface with sharp and tunable emission bands. The physical mechanisms and locations for various emission peaks are described via both analytical theory and numerical simulations. These predictions agree well with our experimental data, taken on systems designed to emit strongly in the infrared. The main peak, which arises from plasmons trapped under a patch, can be well separated from other spectral structures, narrow in wavelength, but broad in angular distribution.

Measurement of the resonant lengths of infrared dipole antennas

The resonant lengths of infrared dipole antennas at 10.6 and 3.39 lm are experimentally investigated. For this purpose, submicron-sized microbolometers coupled to dipole antennas with lengths between 0.7 and 20 lm were fabricated on a SiO2-on-Si substrate. The response of the detector to 10.6 lm radiation shows a first resonance for an antenna length between 1.0 and 2.5 lm. A subsequent zero and a second attenuated resonance are observed as the antenna length increases. Similar behavior is observed for illumination at 3.39 lm, with a first resonance occurring at a length shorter than 1 lm. The results permit evaluation of an eAective dielectric permittivity and shows the eAect of the surface impedance of the metal on the propagation of current-wave on the antenna. The resonance behavior is further studied by changing the irradiation conditions of the detectors. Air-side and substrate-side illumination exhibit identical resonant antenna lengths, but diAerent eAciencies of power collection. The antenna patterns as a function of incident angle have also been measured at 10.6 lm, showing a transition from a primary broadside lobe to the development of side lobes for longer antennas. Finally, an antenna response is measured at visible frequencies. Our measurements point out similarities, as well as diAerences, between infrared antennas and their counterparts at microwave frequencies, and provide insights useful for the design optimization of planar infrared antennas. ” 2000 Elsevier Science B.V. All rights reserved.

The optical effects of an adsorbate layer: d-parameter description of simple models

We develop several theoretical estimates of how changes in optical properties are related to the electron response of an adsorbate layer on a crystalline substrate. The underlying formalism derives from d-parameter theory, which may be explicitly evaluated for simple models of the adlayer and substrate. Although the basic model treats each as an array of point dipole-polarizable entities, we have also solved approximations that treat both or just the substrate as spread into a spatially local dielectric continuum. Comparisons between these calculations with common parameter values help clarify the physics in each theoretical estimate and show how close various analytic approximations can come to the numerically exact solutions.

Modeling parameters for the spectral behavior of infrared frequency-selective surfaces

Comparisons of experiment and theory are presented for transmission spectra over the range 2-15 mum of a set of frequency-selective surfaces consisting of arrays of simple dipole patches of aluminum on or in silicon. The arrays are fabricated by direct-write electron-beam lithography. Important parameters controlling the spectral shape are identified, such as dipole length, spacing, resistance, and dielectric surroundings. The separate influence of these variables is exhibited. Encouraging agreement between simple model calculations and the measurements is found.

Optical resonances in periodic surface arrays of metallic patches

The transmission of light along the surface normal through an air-quartz-glass interface covered with a periodic array of thin, rectangular gold patches has been studied over the visible to infrared range. The various structures that are observed can be qualitatively understood as arising from standing-wave resonances set by the size and surroundings of the metal patches. A method-of-moments calculational scheme provides simulations in good quantitative agreement with the data. It is shown how the standing-wave picture provides a useful conceptual framework to understand and exploit such systems.

Calculation of EELS at a doped semiconductor surface

Abstract We calculate electron energy loss spectra (EELS) within a simple model of a doped semiconductor surface. The model is designed to mimic an n-type GaAs surface for which experimental results exist. Coupling is only allowed via the dipole mechanism and the external electron is presumed to follow a specular trajectory. Using a cumulant expansion, we can treat multiple losses and gains as well as Debye-Waller factors. The basic theoretical quantity is an effective surface dielectric function that depends on frequency and wavevector parallel to the surface. Its calculation requires in general the specification of an additional boundary condition (ABC) in order to match fields at the surface. We explore the consequences of two possible ABC choices. At early stages of these calculations there are large differences between results from different ABCs; but by the time all the broadening (e.g. due to Ohmic damping, to multiple losses, and to finite spatial and energy resolution) has been included in order to compare with experiment, there survives much less dependence on ABC. We conclude that it is difficult to probe fine theoretical details, such as the influence of spatial dispersion, with EELS in such systems.

The Van der Waals interaction between an atom and a solid

Abstract Measurements of the scattering of neutral atoms by metal cylinders have indicated a serious discrepancy between theory and experiment for the strength of the long-range Van der Waals interaction. To clarify the content of the theory, we present here a formal analysis of the Van der Waals interaction between an atom and a solid, based on the approach of McLachlan. A key quantity in the theory is the response of the solid to an external, time-dependent dipole. This response is amenable to classical analysis, and we calculate it for a variety of model systems. When the solid may be treated as a continuum with a flat surface, the concept of surface impedance allows a convenient parameterization and we may incorporate thereby magnetic effects, non-local dielectric response, and the influence of finite layers in the solid. We also study the modifications induced by a rough surface, finding the average change in the coupling strength in the presence of weak roughness.

Resonant enhancement of emission and absorption using frequency selective surfaces in the infrared

We investigate the infrared properties of frequency selective surfaces consisting of aluminum patches on silicon substrates. Resonant behavior is found not only in the transmission and reflection, but also in the absorption and emission of these surfaces. The resonance location is a controllable function of the surface pattern. Simple model calculations reproduce well the qualitative behavior of our samples.

Model calculation of brownian motion parameters at a metal surface

Abstract We calculate the friction parameter and electronic contribution to the binding energy of a single adparticle just inside a metal surface. The semi-infinite metal is treated as a free electron gas bounded by an infinite repulsive barrier. Closed form expressions are obtained for the relevant Greens function through the application of various image theorems which relate this problem to one in which an adparticle and its image are immersed in a uniform electron gas. Numerical calculations are carried out for a simple treatment of the adparticle scattering properties. A strong variation of the results with the distance of the adparticle from the surface is found and discussed.

Dynamical theory calculations of spin-echo resolved grazing-incidence scattering from a diffraction grating

Neutrons scattered or reflected from a diffraction grating are subject to a periodic potential analogous to the potential experienced by electrons within a crystal. Hence, the wavefunction of the neutrons can be expanded in terms of Bloch waves and a dynamical theory can be applied to interpret the scattering phenomenon. In this paper, a dynamical theory is used to calculate the results of neutron spin-echo resolved grazing-incidence scattering (SERGIS) from a silicon diffraction grating with a rectangular profile. The calculations are compared with SERGIS measurements made on the same grating at two neutron sources: a pulsed source and a continuous wave source. In both cases, the spin-echo polarization, studied as a function of the spin-echo length, peaks at integer multiples of the grating period but there are some differences between the two sets of data. The dynamical theory explains the differences and gives a good account of both sets of results.