Aleksandr Verevkin

Fabrication development for nanowire GHz-counting-rate single-photon detectors

We have developed a fabrication process for GHz-counting-rate, single-photon, high-detection-efficiency, NbN, nanowire detectors. We have demonstrated two processes for the device patterning, one based on the standard polymethylmethacrylate (PMMA) organic positive-tone electron-beam resist, and the other based on the newer hydrogen silsesquioxane (HSQ) negative-tone spin-on-glass resist. The HSQ-based process is simple and robust, providing high resolution and the prospect of high fill-factors. Initial testing results show superconductivity in the films, and suggest that the devices exhibit photosensitivity.

Luminescence of colloidal CdSe/ZnS nanoparticles: high sensitivity to solvent phase transitions

We investigate nanosecond photoluminescence processes in colloidal core/shell CdSe/ZnS nanoparticles dissolved in water and found strong sensitivity of luminescence to the solvent state. Several pronounced changes have been observed in the narrow temperature interval near the water melting point. First of all, the luminescence intensity substantially (approximately 50%) increases near the transition. In a large temperature scale, the energy peak of the photoluminescence decreases with temperature due to temperature dependence of the energy gap. Near the melting point, the peak shows N-type dependence with the maximal changes of approximately 30 meV. The line width increases with temperature and also shows N-type dependence near the melting point. The observed effects are associated with the reconstruction of ligands near the ice/water phase transition.

Aluminum hot-electron bolometer mixers at submillimeter wavelengths

We report on the first mixing experiments at submillimeter wavelengths with superconducting diffusion-cooled hot-electron bolometers (HEBs) made from aluminum.

Noise and conversion efficiency of aluminum superconducting hot-electron bolometer mixer

We report on microwave measurements of superconducting aluminum hot-electron bolometers (Al HEBs). Diffusion-cooled Al HEB mixers are good candidates for space-borne applications in the Terahertz frequency range since they are predicted to have small local oscillator (LO) power requirements, intermediate frequency (IF) bandwidths /spl gsim/10 GHz, and a noise temperature lower than that of Nb and NbN HEB mixers. Mixer measurements were made at an LO frequency /spl sim/30 GHz, with an IF in the range 0.1 to 7.3 GHz. For T<0.8 K, a magnetic field H=0.1-0.3 T was applied to suppress the superconductivity in the contact pads, and partly in the bridge. For a 0.6 /spl mu/m long device, we measure an IF bandwidth of 4 GHz, a conversion efficiency /spl eta/=-8 dB, and a mixer noise temperature T/sub M//spl gsim/4 K, DSB (T/sub mixer/=T/sub output noise//2/spl eta/). These results are shown to be in quantitative agreement with simple theoretical predictions.

LabVIEW Graphical User Interface for a New High Sensitivity, High Resolution Micro-Angio-Fluoroscopic and ROI-CBCT System

A graphical user interface based on LabVIEW software was developed to enable clinical evaluation of a new High-Sensitivity Micro-Angio-Fluoroscopic (HSMAF) system for real-time acquisition, display and rapid frame transfer of high-resolution region-of-interest images. The HSMAF detector consists of a CsI(Tl) phosphor, a light image intensifier (LII), and a fiber-optic taper coupled to a progressive scan, frame-transfer, charged-coupled device (CCD) camera which provides real-time 12 bit, 1k × 1k images capable of greater than 10 lp/mm resolution. Images can be captured in continuous or triggered mode, and the camera can be programmed by a computer using Camera Link serial communication. A graphical user interface was developed to control the camera modes such as gain and pixel binning as well as to acquire, store, display, and process the images. The program, written in LabVIEW, has the following capabilities: camera initialization, synchronized image acquisition with the x-ray pulses, roadmap and digital subtraction angiography acquisition (DSA), flat field correction, brightness and contrast control, last frame hold in fluoroscopy, looped play-back of the acquired images in angiography, recursive temporal filtering and LII gain control. Frame rates can be up to 30 fps in full-resolution mode. The user friendly implementation of the interface along with the high frame-rate acquisition and display for this unique high-resolution detector should provide angiographers and interventionalists with a new capability for visualizing details of small vessels and endovascular devices such as stents and hence enable more accurate diagnoses and image guided interventions.

On the Nature of Resistive Transition in Disordered Superconducting Nanowires

Hot-electron single-photon counters based on long superconducting nanowires are starting to become popular in optical and infrared technologies due to their ultimately high sensitivity and very high response speed. We investigate intrinsic fluctuations in long NbN nanowires in the temperature range of 4.2 K-20 K, i.e. above and below the superconducting transition. These fluctuations are responsible for fluctuation resistivity and also determine the noise in practical devices. Measurements of the fluctuation resistivity were performed at low current densities and also in external magnetic fields up to 5 T. Above the BCS critical temperature Tco the resistivity is well described by the Aslamazov-Larkin (AL) theory for two-dimensional samples. Below Tco the measured resistivity is in excellent agreement with the Langer-Ambegaokar-McCumber-Halperin (LAMH) theory developed for one-dimensional superconductors. Despite that our nanowires of 100 nm width are two-dimensional with respect to the coherence length, our analysis shows that at relatively low current densities the one-dimensional LAMH mechanism based on thermally induced phase slip centers dominates over the two-dimensional mechanism related to unbinding of vortex-antivortex pairs below the Berezinskii-Kosterlitz-Thouless transition.

Distortion, Orientation, and Translation Corrections of Tiled EMCCD Detectors for the New Solid State X-ray Image Intensifier (SSXII)

We report on the technology of imaging corrections for a new solid state x-ray image intensifier (SSXII) with enhanced resolution and fluoroscopic imaging capabilities, made of a mosaic of modules (tiled-array) each consisting of CsI(Tl) phosphor coupled using a fiber-optic taper or minifier to an electron multiplier charge coupled device (EMCCD). Generating high quality images using this EMCCD tiled-array system requires the determination and correction of the individual EMCCD sub-images with respect to relative rotations and translations as well as optical distortions due to the fiber optic tapers. The image corrections procedure is based on comparison of resulting (distorted) images with the known square pattern of a wire mesh phantom. The mesh crossing point positions in each sub-image are automatically identified. With the crossing points identified, the mapping between distorted and an undistorted array is determined. For each pixel in a distorted sub-image, the corresponding location in the corrected sub-image is calculated using bilinear interpolation. For the rotation corrections between sub-images, the orientation of the vectors between respective mesh crossing points in the various sub-images are determined and each sub-image is appropriately rotated with the pixel values again determined using bilinear interpolation. Image translation corrections are performed using reference structures at known locations. According to our estimations, the distortion corrections are accurate to within 1%; the rotations are determined to within 0.1 degree, and translation corrections are accurate to well within 1 pixel. This technology will provide the basis for generating single composite images from tiled-image configurations of the SSXII regardless of how many modules are used to form the images.

Real time implementation of distortion corrections for a tiled EMCCD-based solid state x-ray image intensifier (SSXII)

The new Solid State X-ray Image Intensifier (SSXII) is being designed based on a modular imaging array of Electron Multiplying Charge Couple Devices (EMCCD). Each of the detector modules consists of a CsI(Tl) phosphor coupled to a fiber-optic plate, a fiber-optic taper (FOT), and an EMCCD sensor with its electronics. During the optical coupling and alignment of the modules into an array form, small orientation misalignments, such as rotation and translation of the EMCCD sensors, are expected. In addition, barrel distortion will result from the FOTs. Correction algorithms have been developed by our group for all the above artifacts. However, it is critical for the systems performance to correct these artifacts in real-time (30 fps). To achieve this, we will use two-dimensional Look-Up-Tables (LUT) (each for x and y coordinates), which map the corrected pixel locations to the acquired-image pixel locations. To evaluate the feasibility of this approach, this process is simulated making use of parallel coding techniques to allow real-time distortion corrections for up to sixteen modules when a standard quad processor is used. The results of this simulation confirm that tiled field-of-views (FOV) comparable with those of flat panel detectors can be generated in ~17 ms (>30 fps). The increased FOV enabled through correction of tiled images, combined with the EMCCD characteristics of low noise, negligible lag and high sensitivity, should make possible the practical use of the SSXII with substantial advantages over conventional clinical systems. (Support: NIH Grants R01EB008425, R01NS43924, R01EB002873)

Interaction Effects in Quasi‐Ballistic One‐Dimensional Channels Formed in AlGaAs/GaAs Heterostructures

We investigate corrections to the conductivity in weakly disordered, quasi‐one‐dimensional conductors, where the conductor width w<LT = vF/T, vF is the Fermi velocity. For such conductors with a 2D electron spectra the temperature‐dependent corrections are predicted to be proportional to lnT [A. Sergeev et al., Phys. Rev. B 69, 075310 (2004)]. As expected, due to the interference of electron‐electron scattering and elastic electron scattering from impurities and defects the observed temperature dependent conductance of the channels measured is logarithmic and inversely proportional to the channel width, which is strong evidence in favor of one‐dimensional interaction effects.

Crossover from weakly-disordered Fermi liquid to Luttinger liquid in quasi-1D nanostructures

We investigated the temperature?dependent conductance of a long and narrow high-mobility channel, which is formed from an AlGaAs/GaAs heterostructure using the split-gate technique. When the channel is wider than 100 nm, in the range of 1-10 K we observe logarithmic temperature dependence, which is a key signature of the interaction correction in the quasi-one-dimensional weakly-disordered Fermi liquid. For the narrowest channels with width smaller than 100 nm, we observe power-law dependences, which are typical for the Luttinger liquid. The crossover from the weakly-disordered Fermi liquid to the Luttinger liquid takes place when the channel width corresponds to 2-3 one-dimensional subbands.