T. A. Eckhause

Observation of surface-avoiding waves: a new class of extended states in periodic media.

Coherent time-domain optical experiments on GaAs-AlAs superlattices reveal the existence of an unusually long-lived acoustic mode at approximately 0.6 THz which couples weakly to the environment by evading the sample boundaries. Classical as well as quantum states that steer clear of surfaces are generally shown to occur in the spectrum of periodic structures, for most boundary conditions. These surface-avoiding waves are associated with frequencies outside forbidden gaps and wave vectors in the vicinity of the center and edge of the Brillouin zone. Possible consequences for surface science and resonant-cavity applications are discussed.

Electric-field-induced heating and energy relaxation in GaN

Electric-field-induced heating is studied using noise measurements in n-type GaN grown on sapphire substrates. The measured electron temperature is found to be an order of magnitude higher than what is expected based on calculations of electron–phonon coupling via acoustic deformation potential scattering processes in GaN. The discrepancy may be explained by a large thermal boundary resistance between the GaN film and the sapphire substrate.

MI-6: Michigan interferometry with six telescopes

Based on the success of four-telescope imaging with the Michigan Infrared Combiner (MIRC) on the CHARA Array, our Michigan-based group will now upgrade our system to combine all six CHARA telescope simultaneously. In order to make this observationally efficient, we have had to improve a number of subsystems and commission new ones, including the new CHAMP fringe tracker, the introduction of photometric channels, the upgrading of the realtime operating systems, and the obvious hardware and software upgrades of the control system and the data pipeline. Here we will discuss the advantages of six-telescope operation, outline our upgrade plans and discuss our current progress.

Ultrafast optical generation and remote detection of terahertz sound using semiconductor superlattices

The authors introduce an all-optical approach to study the propagation of high frequency acoustic phonons in which the generation and detection involves two spatially separated superlattices ∼1μm apart. Propagating modes of frequencies up to ∼1THz escape from the superlattice where they are generated and reach the second superlattice where they are detected and spectrally resolved using pump-probe techniques. The measured frequency spectrum reveals finite size effects, which can be accounted for by a continuum elastic model.

Automating linear accelerator quality assurance.

PURPOSE The purpose of this study was 2-fold. One purpose was to develop an automated, streamlined quality assurance (QA) program for use by multiple centers. The second purpose was to evaluate machine performance over time for multiple centers using linear accelerator (Linac) log files and electronic portal images. The authors sought to evaluate variations in Linac performance to establish as a reference for other centers. METHODS The authors developed analytical software tools for a QA program using both log files and electronic portal imaging device (EPID) measurements. The first tool is a general analysis tool which can read and visually represent data in the log file. This tool, which can be used to automatically analyze patient treatment or QA log files, examines the files for Linac deviations which exceed thresholds. The second set of tools consists of a test suite of QA fields, a standard phantom, and software to collect information from the log files on deviations from the expected values. The test suite was designed to focus on the mechanical tests of the Linac to include jaw, MLC, and collimator positions during static, IMRT, and volumetric modulated arc therapy delivery. A consortium of eight institutions delivered the test suite at monthly or weekly intervals on each Linac using a standard phantom. The behavior of various components was analyzed for eight TrueBeam Linacs. RESULTS For the EPID and trajectory log file analysis, all observed deviations which exceeded established thresholds for Linac behavior resulted in a beam hold off. In the absence of an interlock-triggering event, the maximum observed log file deviations between the expected and actual component positions (such as MLC leaves) varied from less than 1% to 26% of published tolerance thresholds. The maximum and standard deviations of the variations due to gantry sag, collimator angle, jaw position, and MLC positions are presented. Gantry sag among Linacs was 0.336 ± 0.072 mm. The standard deviation in MLC position, as determined by EPID measurements, across the consortium was 0.33 mm for IMRT fields. With respect to the log files, the deviations between expected and actual positions for parameters were small (<0.12 mm) for all Linacs. Considering both log files and EPID measurements, all parameters were well within published tolerance values. Variations in collimator angle, MLC position, and gantry sag were also evaluated for all Linacs. CONCLUSIONS The performance of the TrueBeam Linac model was shown to be consistent based on automated analysis of trajectory log files and EPID images acquired during delivery of a standardized test suite. The results can be compared directly to tolerance thresholds. In addition, sharing of results from standard tests across institutions can facilitate the identification of QA process and Linac changes. These reference values are presented along with the standard deviation for common tests so that the test suite can be used by other centers to evaluate their Linac performance against those in this consortium.

Integrated optic beam combiners in lithium niobate for stellar interferometer

Integrated optics can provide compact and robust solutions for ground and space-based interferometry by integrating optical devices with different functionalities, such as spatial filters, combiners/nullers, and phase modulators, on a single chip. Lithium niobate (LiNbO3) has two distinct advantages over silica-based technologies, including good transparency further into the near-infrared (covering J, H, K, and L bands) and the ability to support electrically-controlled phase modulation through the linear electro-optic (EO) effect. The design, fabrication and preliminary tests of integrated optic components on LiNbO3 substrates for astronomical beam combiners operating in the H and L bands is reported. The components include single-mode waveguides of sufficient length for spatial filtering, symmetric junctions for wavelength insensitive power splitters/combiners, and electro-optic waveguide modulators for path-length control.

Midinfrared studies of the contact region at superconductor–semiconductor interfaces

InAs quantum wells (QWs) have been used as weak links in many recent studies of novel superconductor-normal metal–superconductor junctions. The degree of coupling between the superconducting electrodes depends sensitively on both the superconductor/InAs interface and the QW material in the weak link, factors that are difficult to separate in dc transport studies. Here we used midinfrared spectroscopy to investigate the superconductor/semiconductor contact region. The remnant intersubband absorption we observe in Nb-clad InAs shows that the superconductor/InAs interface produced some confinement of electrons in the InAs. This confinement is, however, consistent with phase coherent transport in the InAs. We find no evidence for charge transfer from the superconductor to the InAs on cooling below the critical temperature of Nb.

Generation and propagation of coherent THz folded acoustic phonons

We report on the propagation of coherent folded acoustic phonons with frequencies near 1 THz. The folded phonons are generated with femtosecond optical pulses and detected, with little attenuation, after traversing 1.2 /spl mu/m of bulk GaAs.

Temperature dependence of critical currents in Nb/InAs/Nb Josephson junction arrays

We study extreme geometries in the field of 2DEG-coupled Josephson Junctions, at dilution refrigerator temperatures. The 2DEG is a continuous layer of InAs that contacts an overlying array of niobium stripes. The temperature dependence of other S–Semi–S systems has been studied in relatively small geometries. Our junctions are in a new regime with widths to 1mm and lengths of 1.5μm. We find temperature-dependences of the critical current, Ic, that are difficult to fit consistently with theories used for other InAs-based junctions.

Density-dependent critical currents in quantum-well-coupled weak links

We investigate a Josephson field-effect transistor in which the electron gas in an InAs quantum well serves as the weak link between superconducting Nb electrodes. We modulate the density Ns of electrons and the critical current Ic in the weak link by applying a voltage to an insulated gate. Measurements of the dependence of Ic on Ns are in good agreement with a model in which each occupied subband of the InAs quantum well makes an independent contribution to Ic. Inclusion of the nonparabolic band structure in InAs is crucial to this agreement.