Towards optical intensity interferometry for high angular resolution stellar astrophysics

Most neighboring stars are still detected as point sources and are beyond the angular resolution reach of current observatories. Methods to improve our understanding of stars at high angular resolution are investigated. Air Cherenkov telescopes (ACTs), primarily used for Gamma-ray astronomy, enable us to increase our understanding of the circumstellar environment of a particular system. When used as optical intensity interferometers, future ACT arrays will allow us to detect stars as extended objects and image their surfaces at high angular resolution. Optical stellar intensity interferometry (SII) with ACT arrays, composed of nearly 100 telescopes, will provide means to measure fundamental stellar parameters and also open the possibility of model-independent imaging. A data analysis algorithm is developed and permits the reconstruction of high angular resolution images from simulated SII data. The capabilities and limitations of future ACT arrays used for high angular resolution imaging are investigated via Monte-Carlo simulations. Simple stellar objects as well as stellar surfaces with localized hot or cool regions can be accurately imaged. Finally, experimental efforts to measure intensity correlations are expounded. The functionality of analog and digital correlators is demonstrated. Intensity correlations have been measured for a simulated star emitting pseudo-thermal light, resulting in angular diameter measurements. The StarBase observatory, consisting of a pair of 3 m telescopes separated by 23 m, is described.Comment: PhD dissertatio

Doctor of Philosophy

dissertationMost neighboring stars are still detected as point sources and are beyond the angular resolution reach of current observatories. Methods to improve our understanding of stars at high angular resolution are investigated. Air Cherenkov telescopes (ACTs), primarily used for Gamma-ray astronomy, enable us to increase our understanding of the circumstellar environment of a particular system. When used as optical intensity interferometers, future ACT arrays will allow us to detect stars as extended objects and image their surfaces at high angular resolution. ACTs are used in gamma-ray astronomy to investigate violent phenomena in the universe. However, this technique can also be used for stellar astrophysics on some isolated sources. Such is the case with the X-ray binary LS I +61◦303 which was detected in the TeV range. A gamma-ray attenuation model is developed and applied to this system. This models allows us to place constraints on fundamental properties of the system. However, a much better understanding of this system, and more so of nearby bright stellar systems, could be obtained with high angular resolution techniques. Optical stellar intensity interferometry (SII) with ACT arrays, composed of nearly 100 telescopes, will provide means to measure fundamental stellar parameters and also open the possibility of model-independent imaging. A data analysis algorithm is developed and permits the reconstruction of high angular resolution images from simulated SII data. The capabilities and limitations of future ACT arrays used for high angular resolution imaging are investigated via Monte-Carlo simulations. Simple stellar objects as well as stellar surfaces with localized hot or cool regions can be accurately imaged. Finally, experimental efforts to measure intensity correlations are expounded. The functionality of analog and digital correlators is demonstrated. Intensity correlations have been measured for a simulated star emitting pseudo-thermal light, resulting in angular diameter measurements. The StarBase observatory, consisting of a pair of 3m telescopes separated by 23m, is described

Stellar Intensity Interferometry: Imaging capabilities of air Cherenkov telescope arrays

Sub milli-arcsecond imaging in the visible band will provide a new perspective in stellar astrophysics. Even though stellar intensity interferometry was abandoned more than 40 years ago, it is capable of imaging and thus accomplishing more than the measurement of stellar diameters as was previously thought. Various phase retrieval techniques can be used to reconstruct actual images provided a sufficient coverage of the interferometric plane is available. Planned large arrays of Air Cherenkov telescopes will provide thousands of simultaneously available baselines ranging from a few tens of meters to over a kilometer, thus making imaging possible with unprecedented angular resolution. Here we investigate the imaging capabilities of arrays such as CTA or AGIS used as Stellar Intensity Interferometry receivers. The study makes use of simulated data as could realistically be obtained from these arrays. A Cauchy-Riemann based phase recovery allows the reconstruction of images which can be compared to the pristine image for which the data were simulated. This is first done for uniform disk stars with different radii and corresponding to various exposure times, and we find that the uncertainty in reconstructing radii is a few percent after a few hours of exposure time. Finally, more complex images are considered, showing that imaging at the sub-milli-arc-second scale is possible.Comment: 10 pages, 6 figures; presented at the SPIE conference "Optical and Infrared Interferometry II", San Diego, CA, USA (June 2010

Stellar intensity interferometry: Experimental steps toward long-baseline observations

Experiments are in progress to prepare for intensity interferometry with arrays of air Cherenkov telescopes. At the Bonneville Seabase site, near Salt Lake City, a testbed observatory has been set up with two 3-m air Cherenkov telescopes on a 23-m baseline. Cameras are being constructed, with control electronics for either off- or online analysis of the data. At the Lund Observatory (Sweden), in Technion (Israel) and at the University of Utah (USA), laboratory intensity interferometers simulating stellar observations have been set up and experiments are in progress, using various analog and digital correlators, reaching 1.4 ns time resolution, to analyze signals from pairs of laboratory telescopes.Comment: 12 pages, 3 figur

Failure modes of protection layers produced by atomic layer deposition of amorphous TiO₂ on GaAs anodes

Amorphous titanium dioxide (a-TiO₂) films formed by atomic layer deposition can serve as protective coatings for semiconducting photoanodes in water-splitting cells using strongly alkaline aqueous electrolytes. Herein, we experimentally examine the mechanisms of failure for p⁺-GaAs anodes coated with a-TiO₂ films (GaAs/a-TiO₂). Galvanic displacement of exposed GaAs by Au allowed imaging of pinholes in the a-TiO₂ coatings, and enabled collection of quantitative and statistical data associated with pinhole defects. A combination of imaging, electrochemical measurements, and quantitative analyses of corrosion products indicated that extrinsic pinholes were present in the a-TiO₂ films before electrochemical operation. During electrochemical operation these pinholes led to pitting corrosion of the underlying GaAs substrate. The dominant source of pinholes was the presence of atmospheric particulate matter on the GaAs surface during deposition of the a-TiO₂ layer. The pinhole density decreased substantially when the thickness of the a-TiO₂ coating increased beyond 45 nm, and approached zero when the thickness of the film exceeded 112 nm. The density of pinholes in films thinner than 45 nm decreased when the a-TiO₂ coating was deposited in an environmentally controlled cleanroom. Pinhole-free GaAs/a-TiO₂ devices were also tested via chronoamperometry to quantify the rate of pinhole formation during electrochemistry. The time-to-failure increased with thickness, suggesting that the failure mechanism may involve diffusion or migration through the film. However, other mechanisms may also contribute to the degradation of thicker films (>112 nm). Nevertheless, as previously hypothesized, extrinsic pinhole defects formed during deposition and testing control the short-term protective performance of the a-TiO₂ film for GaAs anodes evolving O₂ from water

Characterization of Electronic Transport through Amorphous TiO_2 Produced by Atomic-Layer Deposition

Electrical transport in amorphous titanium dioxide (a-TiO_2) thin films, deposited by atomic layer deposition (ALD), and across heterojunctions of p+-Si|a-TiO_2|metal substrates that had various top metal contacts has been characterized by ac conductivity, temperature-dependent dc conductivity, space-charge-limited current spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, X-ray photoelectron spectroscopy, and current density versus voltage (J–V) characteristics. Amorphous TiO_2 films were fabricated using either tetrakis(dimethylamido)-titanium with a substrate temperature of 150 °C or TiCl_4 with a substrate temperature of 50, 100, or 150 °C. EPR spectroscopy of the films showed that the Ti^(3+) concentration varied with the deposition conditions and increases in the concentration of Ti^(3+) in the films correlated with increases in film conductivity. Valence band spectra for the a-TiO_2 films exhibited a defect-state peak below the conduction band minimum (CBM) and increases in the intensity of this peak correlated with increases in the Ti^(3+) concentration measured by EPR as well as with increases in film conductivity. The temperature-dependent conduction data showed Arrhenius behavior at room temperature with an activation energy that decreased with decreasing temperature, suggesting that conduction did not occur primarily through either the valence or conduction bands. The data from all of the measurements are consistent with a Ti^(3+) defect-mediated transport mode involving a hopping mechanism with a defect density of 10^(19) cm^(–3), a 0.83 wide defect band centered 1.47 eV below the CBM, and a free-electron concentration of 10^(16) cm^(–3). The data are consistent with substantial room-temperature anodic conductivity resulting from the introduction of defect states during the ALD fabrication process as opposed to charge transport intrinsically associated with the conduction band of TiO_2

Sensory determinants of behavioral dynamics in Drosophila thermotaxis

Complex animal behaviors are built from dynamical relationships between sensory inputs, neuronal activity, and motor outputs in patterns with strategic value. Connecting these patterns illuminates how nervous systems compute behavior. Here, we study Drosophila larva navigation up temperature gradients toward preferred temperatures (positive thermotaxis). By tracking the movements of animals responding to fixed spatial temperature gradients or random temperature fluctuations, we calculate the sensitivity and dynamics of the conversion of thermosensory inputs into motor responses. We discover three thermosensory neurons in each dorsal organ ganglion (DOG) that are required for positive thermotaxis. Random optogenetic stimulation of the DOG thermosensory neurons evokes behavioral patterns that mimic the response to temperature variations. In vivo calcium and voltage imaging reveals that the DOG thermosensory neurons exhibit activity patterns with sensitivity and dynamics matched to the behavioral response. Temporal processing of temperature variations carried out by the DOG thermosensory neurons emerges in distinct motor responses during thermotaxis

Characterization of Electronic Transport through Amorphous TiO_2 Produced by Atomic-Layer Deposition

Electrical transport in amorphous titanium dioxide (a-TiO_2) thin films, deposited by atomic layer deposition (ALD), and across heterojunctions of p+-Si|a-TiO_2|metal substrates that had various top metal contacts has been characterized by ac conductivity, temperature-dependent dc conductivity, space-charge-limited current spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, X-ray photoelectron spectroscopy, and current density versus voltage (J–V) characteristics. Amorphous TiO_2 films were fabricated using either tetrakis(dimethylamido)-titanium with a substrate temperature of 150 °C or TiCl_4 with a substrate temperature of 50, 100, or 150 °C. EPR spectroscopy of the films showed that the Ti^(3+) concentration varied with the deposition conditions and increases in the concentration of Ti^(3+) in the films correlated with increases in film conductivity. Valence band spectra for the a-TiO_2 films exhibited a defect-state peak below the conduction band minimum (CBM) and increases in the intensity of this peak correlated with increases in the Ti^(3+) concentration measured by EPR as well as with increases in film conductivity. The temperature-dependent conduction data showed Arrhenius behavior at room temperature with an activation energy that decreased with decreasing temperature, suggesting that conduction did not occur primarily through either the valence or conduction bands. The data from all of the measurements are consistent with a Ti^(3+) defect-mediated transport mode involving a hopping mechanism with a defect density of 10^(19) cm^(–3), a 0.83 wide defect band centered 1.47 eV below the CBM, and a free-electron concentration of 10^(16) cm^(–3). The data are consistent with substantial room-temperature anodic conductivity resulting from the introduction of defect states during the ALD fabrication process as opposed to charge transport intrinsically associated with the conduction band of TiO_2