High-quality-factor microcavities in two-dimensional photonic crystals at optical frequencies have a number of technological applications, such as cavity quantum electrodynamics, optical switching, filtering, and wavelength multiplexing. For such applications, it is useful to have a simple approach to tune the microcavity resonant wavelength. In this letter, we propose a microcavity design by which we can tune the resonant wavelength by changing the cavity geometry while still obtaining a high quality factor.
We have investigated in situ and in real time vapor-phase self-assembly of 1-decene on Si, using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIRS). The adsorption of 1-decene on hydrogenated Si(100) results in a decane-terminated hydrophobic surface, indicated by the sessile-drop water contact angle at 107 {+-} 2. This maximum contact angle is achieved at 160 C under 30 mTorr of vapor-phase 1-decene. The fractional surface coverage of decane, calculated from the IR absorbance of C-H stretching vibrational modes near 2900 cm{sup -1}, follows a Langmuir isotherm. The absolute surface coverage calculated from the IR absorbance saturates at 3.2 x 10{sup 14} cm{sup -2}. On the basis of this isotherm, the empirical rate constant (k{prime}{sub 2}) that governs the rate-limiting step in 1-decene adsorption on HF-treated Si(100) is (3.3 {+-} 0.7) x 10{sup -2} min{sup -1}. The thickness and cant angle of the decane monolayer at the saturation coverage are calculated from angle resolved X-ray photoelectron spectroscopy (AR-XPS). The calculated thickness ranges from 8.4 to 18 {angstrom} due to the uncertainty in the attenuation lengths of C(1s) and Si(2p) photoelectrons through the decane layer. For the same uncertainty, the calculated cant angle ranges from 0 to 55{sup o}. Spectroscopic ellipsometry is independently used to approximate the film thickness at 16 {angstrom}. Monitoring the decane monolayer over a period of 50 days using AR-XPS indicates that the Si surface underneath the decane monolayer gets oxidized with time, leading to the degradation of the decane layer.
We demonstrate that when vertical threading dislocations in (0001) GaN are imaged in plan-view by transmission electron microscopy, a surface-relaxation contrast operates in addition to that due to the strain fields of dislocations passing through the specimen. We show that all three dislocation types (edge, screw, and mixed) can be detected in the same image using g = (11{bar 2}0) and 18{sup o} specimen tilt from [0001], allowing total densities to be assessed properly. The type of an individual dislocation can also be readily identified.
In this paper we consider the problem of automatically determining whether regions in an outdoor environment can be traversed by a mobile robot. We propose a two-level classifier that uses data from a single color image to make this determination. At the low level, we have implemented three classifiers based on color histograms, directional filters and local binary patterns. The outputs of these low level classifiers are combined using a voting scheme that weights the results of each classifier using an estimate of its error probability. We present results from a large number of trials using a database of representative images acquired in real outdoor environments.
Wireless networking using the IEEE 802.11standards is a viable alternative for data communications in safeguards applications. This paper discusses the range of 802.11-based networking applications, along with their advantages and disadvantages. For maximum performance, safety, and security, Wireless networking should be implemented only after a comprehensive site survey has determined detailed requirements, hazards, and threats.
The propagation of electromagnetic waves through dispersive media forms the basis for a wide variety of applications. Rapid advances in materials have produced new products with tailored responses across frequency bands. Many of these new materials, such as radar absorbing material and photonic crystals, are dispersive in nature. This, in turn, has opened up the possibility for the exploitation of these dispersive dielectric properties for a variety of applications. Thus, it is desirable to know the electromagnetic properties of both man-made and natural materials across a wide frequency range. With the advent of transient pulsers with sub-nanosecond risetimes and rates of voltage rise approaching 10**16 V/s, the frequencies of interest in the transient response extend to approximately the 2 GHz range. Although a network analyzer can provide either frequency- or time-domain data (by inverse transform), common TEM cells are only rated to 0.5 to 1.5 GHz--significantly below the maximum frequency of interest. To extend the frequency range to include 2 GHz, a TEM cell was characterized and a deembedding algorithm was applied to account, in part, for the limitations of the cell. The de-embedding technique is described along with such measurement issues such as clear time and sneak around. Measurements of complex permittivity of common drinking water are shown. This frequency extension will lead to more expansive testing of dielectric materials of interest.
The Multispectral Thermal Imager Satellite (MTI) has been used to test a sub-pixel sampling technique in an effort to obtain higher spatial frequency imagery than that of its original design. The MTI instrument is of particular interest because of its infrared detectors. In this spectral region, the detector size is traditionally the limiting factor in determining the satellite's ground sampling distance (GSD). Additionally, many over-sampling techniques require flexible command and control of the sensor and spacecraft. The MTI sensor is well suited for this task, as it is the only imaging system on the MTI satellite bus. In this super-sampling technique, MTI is maneuvered such that the data are collected at sub-pixel intervals on the ground. The data are then processed using a deconvolution algorithm using in-scene measured point spread functions (PSF) to produce an image with synthetically-boosted GSD.
The Digital Elevation Model (DEM) extraction process traditionally uses a stereo pair of aerial photographs that are sequentially captured using an airborne metric camera. Standard DEM extraction techniques have been naturally extended to utilize satellite imagery. However, the particular characteristics of satellite imaging can cause difficulties in the DEM extraction process. The ephemeris of the spacecraft during the collects, with respect to the ground test site, is the most important factor in the elevation extraction process. When the angle of separation between the stereo images is small, the extraction process typically produces measurements with low accuracy. A large angle of separation can cause an excessive number of erroneous points in the output DEM. There is also a possibility of having occluded areas in the images when drastic topographic variation is present, making it impossible to calculate elevation in the blind spots. The use of three or more images registered to the same ground area can potentially reduce these problems and improve the accuracy of the extracted DEM. The pointing capability of the Multispectral Thermal Imager (MTI) allows for multiple collects of the same area to be taken from different perspectives. This functionality of MTI makes it a good candidate for the implementation of DEM extraction using multiple images for improved accuracy. This paper describes a project to evaluate this capability and the algorithms used to extract DEMs from multi-look MTI imagery.
Coupled double quantum well field-effect transistors with a grating gate exhibit a terahertz ({approx}600 GHz) photoconductive response that resonates with standing two dimensional plasma oscillations under the gate and may be the basis for developing a fast, tunable terahertz detector. The application of a precisely aligned in-plane magnetic field produces no detectable change in the device DC conductance but produces a dramatic inversion, growth of the terahertz photoconductive response and frequency shift of the standing plasmon resonances. The frequency shift can be described by a significant mass increase produced by the in-plane field. The mass increase is substantially larger than that calculated from a single well and we presume that a proper treatment of the coupled double quantum well may resolve this discrepancy.
Fe nanoparticles prepared by iron carbonyl decomposition using different methods are compared structurally, chemically, and magnetically. The specific magnetization of the particles was determined from the magnetic moment, the particle size observed by transmission electron microscopy, and the total iron concentration found from calibrated X-ray fluorescence. The volume fraction of oxide is reported for particles of different sizes and for particles made by slightly different techniques.
The conductivity of extremely high mobility dilute two-dimensional holes in GaAs changes linearly with temperature in the insulating side of the metal-insulator transition. Hopping conduction, characterized by an exponentially decreasing conductivity with decreasing temperature, is not observed when the conductivity is smaller than e{sup 2}/h. We suggest that strong interactions in a regime close to the Wigner crystallization must be playing a role in the unusual transport.
Experiments using high-resolution size exclusion chromatography (HRSEC), dynamic light scattering, and transmission electron microscopy are conducted to investigate the effects of aging of Au nanoclusters in the presence of surfactant ligands. It is observed that contrary to the expectation that aging in solution will always broaden the size dispersion and increase the average size (Ostwald ripening), a narrowing of the size dispersion and change in average size can occur with time under ambient conditions.
Transport measurements of high-mobility two-dimensional electron systems at low temperatures have revealed a large resistance anisotropy around half-filling of excited Landau levels. These results have been attributed to electronic stripe-phase formation with spontaneously broken orientational symmetry. Mechanisms which are known to break the orientational symmetry include poorly-understood crystal structure effects and an in-plane magnetic field, B{sub {parallel}}. Here we report that a large B{sub {parallel}} also causes the transport anisotropy to persist up to much higher temperatures. In this regime, we find that the anisotropic resistance scales sublinearly with B{sub {parallel}}/T. These observations support the proposal that the transition from anisotropic to isotropic transport reflects a liquid crystal phase transition where local stripe order persists even in the isotropic regime.
The metallic conductivity of dilute two-dimensional holes in a GaAs HIGFET (Heterojunction Insulated-Gate Field-Effect Transistor) with extremely high mobility and large r{sub s} is found to have a linear dependence on temperature, consistent with the theory of interaction corrections in the ballistic regime. Phonon scattering contributions are negligible in the temperature range of our interest, allowing comparison between our measured data and theory without any phonon subtraction. The magnitude of the Fermi liquid interaction parameter F{sub 0}{sup {sigma}} determined from the experiment, however, decreases with increasing r{sub s} for r{sub s} {approx}> 22, a behavior unexpected from existing theoretical calculations valid for small r{sub s}.
The Multispectral Thermal Imager Satellite (MTI), launched on March 12, 2000, has now surpassed its one-year mission requirement and its three-year mission goal. Primary and secondary program objectives regarding the development and evaluation of space-based multispectral and thermal imaging technology for nonproliferation treaty monitoring and other national security and civilian application have been met. Valuable lessons have also been learned, both from things that worked especially well and from shortcomings and anomalies encountered. This paper addresses lessons associated with the satellite, ground station and system operations, while companion papers address lessons associated with radiometric calibration, band-to-band registration and scientific processes and results. Things addressed in this paper that went especially well include overall satellite design, ground station design, system operations, and integration and test. Anomalies and other problems addressed herein include gyro and mass storage unit failures, battery under-voltage trips, a blown fuse, unexpected effects induced by communication link noise, ground station problems, and anomalies resulting from human error. In spite of MTI's single-string design, the operations team has been successful in working around these problems, and the satellite continues to collect valuable mission data.
Cross-sections for the elastic recoil of hydrogen isotopes, including tritium, have been measured for {sup 4}He{sup 2+} ions in the energy range of 9.0-11.6 MeV. These cross-sections have been measured at a scattering angle of 30{sup o} in the laboratory frame. Cross-sections were measured by allowing a {sup 4}He{sup 2+} beam to fall incident on solid targets of ErH{sub 2}, ErD{sub 2} and ErT{sub 2}, each of 500 nm nominal thickness and known areal densities of H, D, T and Er. The uncertainty in each cross-section is estimated to be {+-}3.2%.
Hydrogen isotope thin film standards have been manufactured at Sandia National Laboratories for use by the materials characterization community. Several considerations were taken into account during the manufacture of the ErHD standards, with accuracy and stability being the most important. The standards were fabricated by e-beam deposition of Er onto a Mo substrate and the film stoichiometrically loaded with hydrogen and deuterium. To determine the loading accuracy of the standards two random samples were measured by thermal desorption mass spectrometry and atomic absorption spectrometry techniques with a stated combined accuracy of {approx}1.6% (1{sigma}). All the standards were then measured by high energy RBS/ERD and RBS/NRA with the accuracy of the techniques {approx}5% (1{sigma}). The standards were then distributed to the IBA materials characterization community for analysis. This paper will discuss the suitability of the standards for use by the IBA community and compare measurement results to highlight the accuracy of the techniques used.
Previous studies of 5,10,15,20-tetraarylporphyrins have shown that the barrier for meso aryl-porphyrin rotation ({Delta}G{sub ROT}) varies as a function of the core substituent M and is lower for a small metal (M = Ni) compared to a large metal (M = Zn) and for a dication (M 4H{sup 2+}) versus a free base porphyrin (M = 2H). This has been attributed to changes in the nonplanar distortion of the porphyrin ring and the deformability of the macrocycle caused by the core substituent. In the present work, X-ray crystallography, molecular mechanics (MM) calculations, and variable temperature (VT) {sup 1}H NMR spectroscopy are used to examine the relationship between the aryl-porphyrin rotational barrier and the core substituent M in some novel 2,3,5,7,8,10,12,13,15,17,18,20-dodecaarylporphyrins (DArPs), and specifically in some 5,10,15,20-tetraaryl-2,3,7,8,12,13,17,18-octaphenylporphyrins (TArOPPs), where steric crowding of the peripheral groups always results in a very nonplanar macrocycle. X-ray structures of DArPs indicate differences in the nonplanar conformation of the macrocycle as a function of M, with saddle conformations being observed for M = Zn, 2H or M = 4H{sup 2+} and saddle and/or ruffle conformations for M = Ni. VT NMR studies show that the effect of protonation in the TArOPPs is to increase {Delta}G{sub ROT}, which is the opposite of the effect seen for the TArPs, and MM calculations also predict a strikingly high barrier for the TArOPPs when M = 4H{sup 2+}. These and other findings suggest that the aryl-porphyrin rotational barriers in the DArPs are closely linked to the deformability of the macrocycle along a nonplanar distortion mode which moves the substituent being rotated out of the porphyrin plane.
This paper explores quantum-coherence phenomena in a semiconductor quantum-dot structure. The calculations predict the occurrence of inversionless gain, electromagnetically induced transparency, and refractive-index enhancement in the transient regime for dephasing rates typical under room temperature and high excitation conditions. They also indicate deviations from atomic systems because of strong many-body effects. Specifically, Coulomb interaction involving states of the quantum dots and the continuum belonging to the surrounding quantum well leads to collision-induced population redistribution and many-body energy and field renormalizations that modify the magnitude, spectral shape, and time dependence of quantum-coherence effects.
In T1, periodic minimal surfaces in a medium with exclusions (voids) are constructed and in this paper we present two algorithms for computing these minimal surfaces. The two algorithms use evolution of level sets by mean curvature. The first algorithm solves the governing nonlinear PDE directly and enforces numerically an orthogonality condition that the surfaces satisfy when they meet the boundaries of the exclusions. The second algorithm involves h-adaptive finite element approximations of a linear convection-diffusion equation, which has been shown to linearize the governing nonlinear PDE for weighted mean curvature flow.
We present a series of electronic structure calculations that demonstrate a mechanism for spontaneous ionization of hydrogen at the Si-SiO{sub 2} interface. Specifically, we show that an isolated neutral hydrogen atom will spontaneously give up its charge and bond to a threefold coordinated oxygen atom. We refer to this entity as a proton. We have calculated the potential surface and found it to be entirely attractive. In contrast, hydrogen molecules will not undergo an analogous reaction. We relate these calculations both to proton generation experiments and to hydrogen plasma experiments.
The effect of the density and in-plane distribution of interfacial interactions on crack initiation in an epoxy-silicon joint was studied in nominally pure shear loading. Well-defined combinations of strong (specific) and weak (nonspecific) interactions were created using self-assembling monolayers. The in-plane distribution of strong and weak interactions was varied by employing two deposition methods: depositing mixtures of molecules with different terminal groups resulting in a nominally random distribution, and depositing methyl-terminated molecules in domains defined lithographically with the remaining area interacting through strong acid-base interactions. The two distributions lead to very different fracture behavior. For the case of the methyl-terminated domains (50 {micro}m on a side) fabricated lithographically, the joint shear strength varies almost linearly with the area fraction of strongly interacting sites. From this we infer that cracks nucleate on or near the interface over nearly the entire range of bonded area fraction and do so at nearly the same value of local stress (load/bonded area). We postulate that the imposed heterogeneity in interfacial interactions results in heterogeneous stress and strain fields within the epoxy in close proximity to the interface. Simply, the bonded areas carry load while the methyl terminated domains carry negligible load. Stress is amplified adjacent to the well-bonded regions (and reduced adjacent to the poorly bonded regions), and this leads to crack initiation by plastic deformation and chain scission within the epoxy near the interface. For the case of mixed monolayers, the dependence is entirely different. At low areal density of strongly interacting sites, the joint shear strength is below the detection limit of our transducer for a significant range of mixed monolayer composition. With increasing density of strongly interacting sites, a sharp increase in joint shear strength occurs at a methyl terminated area fraction of roughly 0.90. We postulate that this coincides with the onset of yielding in the epoxy. For methyl-terminated area fractions less than 0.85, the joint shear strength becomes independent of the interfacial interactions. This indicates that fracture no longer initiates on the interface but away from the interface by a competing mechanism, likely plastic deformation and chain scission within the bulk epoxy. The data demonstrate that the in-plane distribution of interaction sites alone can affect the location of crack nucleation and the far-field stress required.