Opening the 100-Year Window for Time Domain Astronomy

The large-scale surveys such as PTF, CRTS and Pan-STARRS-1 that have emerged within the past 5 years or so employ digital databases and modern analysis tools to accentuate research into Time Domain Astronomy (TDA). Preparations are underway for LSST which, in another 6 years, will usher in the second decade of modern TDA. By that time the Digital Access to a Sky Century @ Harvard (DASCH) project will have made available to the community the full sky Historical TDA database and digitized images for a century (1890--1990) of coverage. We describe the current DASCH development and some initial results, and outline plans for the "production scanning" phase and data distribution which is to begin in 2012. That will open a 100-year window into temporal astrophysics, revealing rare transients and (especially) astrophysical phenomena that vary on time-scales of a decade. It will also provide context and archival comparisons for the deeper modern surveysComment: 6 pages, 3 figures; invited talk at IAUS 28

Zinc oxide nanophotonics : toward quantum photonic technologies

University of Technology Sydney. Faculty of Science.Zinc oxide (ZnO) is a large bandgap (3.37 eV at room temperature) semiconductor and is a good candidate for short wavelength photonic devices such as laser diodes. A large exciton binding energy (60 meV) at room temperature in addition to the advantages of being able to grow various nanostructure forms have made ZnO suitable for a wide range of applications in optoelectronic devices. Driven by the rapid advance of nanophotonics, it is necessary to develop single photon sources (SPSs) and optical resonators in new class of materials. In particular, SPSs are required for a wide range of applications in quantum information science, quantum cryptography, and quantum communications. ZnO has been investigated for classical light emitting applications such as energy efficient light emitting diodes (LEDs) and ultraviolet (UV) lasers. Significantly ZnO has recently been identified as a promising candidate for quantum photonic technologies. Thus in this thesis the optical properties of ZnO micro- and nano-structures were investigated for ZnO nanophotonic technologies, specifically their applications in single photon emission and optical resonators. Firstly, the formation of radiative point defects in ZnO nanoparticles and their photophysical properties were investigated. In particular, using correlative photoluminescence (PL), cathodoluminescence (CL), electron paramagnetic resonance (EPR), and x-ray absorption near edge spectroscopy (XANES) it is shown that green luminescence (GL) at 2.48 eV and an EPR line at g = 2.00 belong to a surface oxygen vacancy (V⁺o,s) center, while a second green emission at 2.28 eV is associated with zinc vacancy (VZn) centers. It is established that these point defects exhibit nanosecond lifetimes when excited by above bandgap or sub-bandgap (405 nm and 532 nm excitation wavelength) excitation. These results demonstrate that point defects in ZnO nanostructures can be engineered for nanophotonic technologies. ZnO nanoparticles were consequently studied for the investigation of room temperature single photon emission from defect centers in ZnO nanoparticles. Under the optical excitation with 532 nm green laser, the emitters exhibit bright broadband fluorescence in the red spectral range centered at 640 nm. The red fluorescence from SPSs in ZnO defect center is almost fully linearly polarized with high signal-to-noise ratio. The studied emitters showed continuous blinking; however, it was confirmed that bleaching can be suppressed using a polymethyl methacrylate (PMMA) coating. Furthermore, passivation by hydrogen treatment increase the density of single photon emitters by a factor of three. ZnO/Si heterojunctions were fabricated and used to investigate electrically driven light emission from localized defects in ZnO nanostructures at room temperature. It is shown that excellent rectifying behaviors were observed with the threshold voltages at ~ 18 V and ~ 7 V for ZnO nanoparticles and thin film-based devices, respectively. Both devices exhibit electroluminescence (EL) in the red spectral region ranging from ~ 500 nm to 800 nm when 40 V and 15 V were applied to ZnO nanoparticles/Si and ZnO thin film/Si, respectively. The emission is bright and stable for more than 30 minutes, providing an important prerequisite for practical devices. Finally, ZnO optical resonators were fabricated and investigated to enhance the visible light emission. Hexagonal ZnO microdisks with diameter ranging from 3 μm to 15 μm were grown by a carbothermal reduction method. Optical characterization of ZnO microdisks was performed using low temperature (80 K) CL imaging and spectroscopy. The green emission is found to be locally distributed near the hexagonal boundary of the ZnO microdisks. High resolution CL spectra of the ZnO microdisks reveal whispering gallery modes (WGMs) emission. Two different sizes (5 μm and 9 μm) of the ZnO microdisks were simulated to analyze the nature of light confinement in terms of geometrical optics. Respective analysis of the mode spacing and the mode resonances are used to show that the ZnO microdisks support the propagation of WGMs. The results show that the experimentally observed WGMs are in excellent agreement with the predicted theoretical positions calculated using a plane wave model. This work could provide the means for ZnO microdisk devices operating in the green spectral range

Digital cultural data and the "hybrid archive"

published_or_final_versionMedia, Culture and Creative CitiesMasterMaster of Social Sciences in Media, Culture and Creative Citie

Simulation and theory for two-dimensional beam-plasma instability

A comparative study of the dynamics of the electron beam-plasma system in two spatial dimensions is carried out by means of particle-in-cell (PIC) simulation and quasilinear theory. In the literature, the beam-plasma instability is usually studied with one-dimensional assumption. Among the few works that include higher-dimensional effects are two-and three-dimensional quasilinear theory and two-dimensional PIC simulations. However, no efforts were made to compare the theory and simulation side by side. The present paper carries out a detailed comparative study of two-dimensional simulation and quasilinear theory. It is found that the quasilinear theory quite adequately accounts for most important features associated with the simulation result. For instance, the particle diffusion time scale, the maximum wave intensity, dynamical development of the electron distribution function, and the change in the wave spectrum all agree quantitatively. However, certain nonlinear effects such as the Langmuir condensation phenomenon are not reproduced by the quasilinear theory. Nevertheless, the present paper verifies that the simple quasilinear theory is quite effective for the study of beam-plasma instability for the present choice of parameters. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3529359]open1155Nsciescopu

Delineation of high resolution climate regions over the Korean Peninsula using machine learning approaches

In this research, climate classification maps over the Korean Peninsula at 1 km resolution were generated using the satellite-based climatic variables of monthly temperature and precipitation based on machine learning approaches. Random forest (RF), artificial neural networks (ANN), k-nearest neighbor (KNN), logistic regression (LR), and support vector machines (SVM) were used to develop models. Training and validation of these models were conducted using in-situ observations from the Korea Meteorological Administration (KMA) from 2001 to 2016. The rule of the traditional Koppen-Geiger (K-G) climate classification was used to classify climate regions. The input variables were land surface temperature (LST) of the Moderate Resolution Imaging Spectroradiometer (MODIS), monthly precipitation data from the Tropical Rainfall Measuring Mission (TRMM) 3B43 product, and the Digital Elevation Map (DEM) from the Shuttle Radar Topography Mission (SRTM). The overall accuracy (OA) based on validation data from 2001 to 2016 for all models was high over 95%. DEM and minimum winter temperature were two distinct variables over the study area with particularly high relative importance. ANN produced more realistic spatial distribution of the classified climates despite having a slightly lower OA than the others. The accuracy of the models using high altitudinal in-situ data of the Mountain Meteorology Observation System (MMOS) was also assessed. Although the data length of the MMOS data was relatively short (2013 to 2017), it proved that the snowy, dry and cold winter and cool summer class (Dwc) is widely located in the eastern coastal region of South Korea. Temporal shifting of climate was examined through a comparison of climate maps produced by period: from 1950 to 2000, from 1983 to 2000, and from 2001 to 2013. A shrinking trend of snow classes (D) over the Korean Peninsula was clearly observed from the ANN-based climate classification results. Shifting trends of climate with the decrease/increase of snow (D)/temperate (C) classes were clearly shown in the maps produced using the proposed approaches, consistent with the results from the reanalysis data of the Climatic Research Unit (CRU) and Global Precipitation Climatology Centre (GPCC)