Superconducting detectors become the most prominent technology for radiation sensors with ultimate performance. Typically, they are nanostructures formed from an ultra-thin superconducting film incorporated into an external antenna for efficient radiation coupling. The operation of so-called hot-electron bolometers and photodetectors (HEBs and HEPs) is based on nonequilibrium heating of the electron subsystem by the absorbed radiation and results in the film resistance and a corresponding, easily measurable voltage response when device is current biased [1-2]. A relatively simple, single-layer manufacturing technology made these devices very popular for needs of radioastronomy and remote sensing.
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 system's 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)
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.
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.
The Photon Number-Resolved (PNR) detectors are capable to distinguish between finite numbers n of photons (n = 1, 2, 3, ...) within an ultrafast (femtosecond or picosecond range) week radiation pulse. Recently, a great interest to such devices has been expressed by the quantum information sciences. In the nearest future, an interest to development of such devices can be significantly enhanced by emerging class of new nanophotonic and biological applications, such as single-molecule studies with fluorescent nanoparticles. As it was found recently, the multiexciton state achieved through fast pulse laser irradiation of individual nanoparticle lead to multiphoton character of light emission. The exact knowledge of the number of photons emitted by individual nanoparticle in colloids can provide us with deeper understanding of the interaction between single fluorescent nanoparticle and its environment. However, the development of PNR devices would be necessary to establish such novel photon number-resolved imaging technologies. In this article we briefly review some basic approaches to design and development of the future PNR detectors and electronic readouts.
We present the design and performance of a novel, two-channel single-photon receiver, based on two fiber-coupled NbN superconducting single-photon detectors (SSPDs). The SSPDs are nanostructured superconducting meanders covering an area of 100 μm2 and are known for ultrafast and efficient counting of single, visible-to-infrared photons. Their operation has been explained within a phenomenological hot-electron photoresponse model. Our receiver is intended for fiber-based quantum cryptography and communication systems, operational at near-infrared (NIR) telecommunication wavelengths, λ = 1.3 μm and λ = 1.55 μm. Coupling between the NbN detector and a single-mode optical fiber was achieved using a specially designed, micromechanical photoresist ring, positioned directly over the SSPD active area. The positioning accuracy of the ring was below 1 μm. The receiver with SSPDs was placed (immersed) in a standard liquid-helium transport Dewar and kept without interruption for over two months at 4.2 K. At the same time, the optical fiber inputs and electrical outputs were kept at room temperature. Our best system reached a system quantum efficiency of up to 0.3 % in the NIR radiation range, with the detector coupling efficiency of about 30 %. The response time was measured to be about 250 ps and was limited by our read-out electronics. The measured jitter was close to 35 ps. The presented performance parameters show that our NIR single photon detectors are suitable for practical quantum cryptography and for applications in quantum-correlation experiments.
We present our studies on quantum efficiency (QE), dark counts, and noise equivalent power (NEP) of the latest generation of nanostructured NbN superconducting single-photon detectors (SSPDs) operated at 2.0 K. Our SSPDs are based on 4 nm-thick NbN films, patterned by electron beam lithography as highly-uniform 100÷120-nm-wide meander-shaped stripes, covering the total area of 10x10 μm2 with the meander filling factor of 0.7. Advances in the fabrication process and low-temperature operation lead to QE as high as ~30-40% for visible-light photons (0.56 μm wavelength)-the saturation value, limited by optical absorption of the NbN film. For 1.55 μm photons, QE was ~20% and decreased exponentially with the wavelength reaching ~0.02% at the 5-μm wavelength. Being operated at 2.0-K temperature the SSPDs revealed an exponential decrease of the dark count rate, what along with the high QE, resulted in the NEP as low as 5x10-21 W/Hz-1/2, the lowest value ever reported for near-infrared optical detectors. The SSPD counting rate was measured to be above 1 GHz with the pulse-to-pulse jitter below 20 ps. Our nanostructured NbN SSPDs operated at 2.0 K significantly outperform their semiconducting counterparts and find practical applications ranging from noninvasive testing of CMOS VLSI integrated circuits to ultrafast quantum communications and quantum cryptography.
We present a new class of single-photon devices for counting of both visible and infrared photons. Our superconducting single-photon detectors (SSPDs) are characterized by the intrinsic quantum efficiency (QE) reaching up to 100%, above 10 GHz counting rate, and negligible dark counts. The detection mechanism is based on the photon-induced hotspot formation and subsequent appearance of a transient resistive barrier across an ultrathin and submicron-wide superconducting stripe. The devices are fabricated from 3.5-nm-thick NbN films and operate at 4.2 K, well below the NbN superconducting transition temperature. Various continuous and pulsed laser sources in the wavelength range from 0.4 μm up to >3 μm were implemented in our experiments, enabling us to determine the detector QE in the photon-counting mode, response time, and jitter. For our best 3.5-nm-thick, 10×10 μm2-area devices, QE was found to reach almost 100% for any wavelength shorter than about 800 nm. For longer-wavelength (infrared) radiation, QE decreased exponentially with the photon wavelength increase. Time-resolved measurements of our SSPDs showed that the system-limited detector response pulse width was below 150 ps. The system jitter was measured to be 35 ps. In terms of the counting rate, jitter, and dark counts, the NbN SSPDs significantly outperform their semiconductor counterparts. Already identifeid and implemented applications of our devices range from noninvasive testing of semiconductor VLSI circuits to free-space quantum communications and quantum cryptography.
We have developed a new class of superconducting single-photon detectors (SSPDs) for ultrafast counting of infrared (IR) photons for secure quantum communications. The devices are operated on the quantum detection mechanism, based on the photon-induced hotspot formation and subsequent appearance of a transient resistive barrier across an ultrathin and submicron-wide superconducting stripe. The detectors are fabricated from 3.5-nm-thick NbN films and they operate at 4.2 K inside a closed-cycle refrigerator or liquid helium cryostat. Various continuous and pulsed laser sources have been used in our experiments, enabling us to determine the detector experimental quantum efficiency (QE) in the photon-counting mode, response time, time jitter, and dark counts. Our 3.5-nm-thick SSPDs reached QE above 15% for visible light photons and 5% at 1.3 - 1.5 μm infrared range. The measured real-time counting rate was above 2 GHz and was limited by the read-out electronics (intrinsic response time is <30 ps). The measured jitter was <18 ps, and the dark counting rate was <0.01 per second. The measured noise equivalent power (NEP) is 2 x 10-18 W/Hz1/2 at λ = 1.3 μm. In near-infrared range, in terms of the counting rate, jitter, dark counts, and overall sensitivity, the NbN SSPDs significantly outperform their semiconductor counterparts. An ultrafast quantum cryptography communication technology based on SSPDs is proposed and discussed.
We report our studies on the performance of new NbN ultrathin-film superconducting single-photon detectors (SSPDs). Our SSPDs exhibit experimentally measured quantum efficiencies from ~ 5% at wavelength λ = 1550 nm up to ~10% at λ = 405 nm, with exponential, activation-energy-type spectral sensitivity dependence in the 0.4-μm - 3-μm wavelength range. Using a variable optical delay setup, we have shown that our NbN SSPDs can resolve optical photons with a counting rate up to 10 GHz, presently limited by the read-out electronics. The measured device jitter was below 35 ps under optimum biasing conditions. The extremely high photon counting rate, together with relatively high (especially for λ > 1 μm) quantum efficiency, low jitter, and very low dark counts, make NbN SSPDs very promising for free-space communications and quantum cryptography.
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