Nathan Bluzer

Temporal relaxation measurements of photoinduced nonequilibrium in superconductors

A new technique, transient photoimpedance response (TPR), has been formulated and was used to measure the nonequilibrium relaxation process in YBCO and Niobium. The TPR method dispenses with the need for weak link structures and temporal relaxation measurements are made directly on unpatterned thin films. With a dc current bias, voltage signals are produced in response to photoabsorbed short (300 fs) laser pulses producing a transient impedance change. Nonequilibrium excitation lifetimes are related to the temporal dependence of the voltage signal produced by transient changes in the superconductor’s impedance. With the TPR technique excitation lifetimes in the normal, transition, and superconducting state are observed on each sample. Data interpretation is facilitated by measuring and comparing the TPR signal in all three states. Typically the normal‐ and transition‐state TPR signals in YBCO and Nb are bolometric and correspond to resistive impedance changes. However, the relaxation signal’s time constan...

Current Readout Of Infrared Detectors

A buffered direct-injection (BDI) current readout for infrared detectors is described and analyzed. It is compared with the common direct-injection (DI) circuit with respect to injection efficiency, noise, and tolerance of low RoA product photovoltaic detectors. Power requirements and threshold control are also discussed. Throughout the analysis it is clear that much advantage is gained at relatively little cost by the use of a BDI structure for an integrated circuit focal plane.

Superconducting quantum detectors

The discovery of high-temperature superconductors (HTS) spawned many potential applications, including optical detectors. Realizing viable superconducting detectors requires achieving performance superior to competing and more mature semiconductor detector technologies, and quantum detector technologies in particular. We review why quantum detectors are inherently more sensitive than thermal or bolometric detectors. This sensitivity advantage suggests that for operation at cryogenic temperatures, we should be developing only quantum superconducting detectors. Accordingly, we introduce and describe the structure and the operation of a superconducting quantum detector with a superconducting quantum interference device (SQUID) readout circuit. The superconducting quantum detector, consisting of a superconducting loop, produces a photosignal in response to photoinduced changes in the superconducting condensates kinetic inductance. The superconducting quantum detector is designed to operate only in the superconducting state and not in the resistive or transition states.

Biexponential decay and delay artifact in the photoresponse of superconductors

The photoresponse of superconductor exposed to narrow and wide laser pulses is examined in terms of the superconductors impedance. The impedance helps to identify the causes and the origins of the fast and slow photoresponse decay in YBCO and Nb. The source of the asymmetrical photoresponse signals in Pb is explained in terms of impedance nonlinearities with temperature. Finally, the impedance model is used to explain what has been previously identified incorrectly as delayed photoresponse signals in In and Pb. All these temporal manifestations are produced by the impedance and are not indicative of a delayed response or of intrinsic or sudden changes in the recombination-generation lifetimes in the superconductor.<<ETX>>

Superconducting quantum detectors in YBCO

The discovery of high-temperature superconductors has led to great efforts to find potential applications, including the development of photon detectors. We review the limitations of the different approaches proposed for realizing superconducting photon detectors. The structure and operation of a new quantum superconducting kinetic inductance detector (QSKID) with a SQUID readout circuit is described. The QSKID is made from a superconducting loop where the photosignals are generated in response to photoinduced changes in the condensates kinetic inductance. The QSKID operates in the zero-resistance superconducting state, thereby circumventing Johnson noise.

Charge Partition Noise In Charge-Coupled Devices

In this paper we describe and analyze the internal noise sources of a signal charge partitioning circuit and indicate techniques for minimizing the internal noise of the circuit. Two noise sources are identified and described as a function of the circuit architecture and operating speeds: noise due to the trapping of thermal charge-density fluctuations (Johnson noise) and noise due to electrons scattered by the action of the partitioning gate. The noise due to the trapping of thermal charge-density fluctuations is dependent on the time it takes to divide the charge; at most, it is equal to the thermal Johnson noise of the integration well times the partition ratio. Noise due to the closing of the partitioning gate is dependent on the division ratio, the length of the septum gate, and its closure speed; it is, however, independent of the size of the charge packet being divided. With small division ratios (<10) and optimal operating speeds, the partition noise can be limited to fewer than 100 e-.

Quantum detectors in superconducting YBCO

A superconducting quantum detector structure in YBCO is described with a directly coupled SQUID read out circuit. The detector geometry is optimized for maximum photoresponse with the use of a serpentine pattern. The serpentine pattern reduces quasiparticle diffusion effects and maximizes the photoinduced changes in the kinetic inductance. The operation of this sensor is analyzed in terms of geometry and quasiparticle lifetime to obtain expressions for the signal and noise of this detector. The background limited infrared performance of this detector is calculated to be about 5/spl times/10/sup -17/ NEP. The photoresponse is expected to be limited by the superconducting energy gap, about 30-40 /spl mu/m.<<ETX>>

Sensitivity Limitations On IRFPAS Imposed By Detector Nonuniformities

Advances in IR Focal Plane Technology now make two dimensional detector arrays possible. It is expected that with two dimensional detector arrays the performance of IR Sensors will improve significantly. Ideally, the sensitivity improvements should scale with the square root of the number of detectors in the array. However, with larger IR arrays interdetector variations need to be compensated for. In this paper we consider quantitatively the sensitivity limitations effected by interdetector variations in quantum efficiency and spectral response, and with noise associated with calibration. Correction for the interdetector spectral variations are the most difficult and are the prime cause for sensitivity limitations. The limited sensitivity manifests itself, in scanning arrays as spectral streaking.

CCDs For IR Focal Plane Arrays

It is projected that advanced second generation IR systems will use hybrid focal plane arrays consisting of PV HgCdTe detector arrays and silicon CCD signal processor chips. This choice is in concert with the aim of achieving lower NEAT and finer spatial resolution. PV HgCdTe detector arrays have been selected because of power consumption constraints and CCD processor characteristics. In this paper we report on a CCD signal processor chip intended for coupling with PV detector arrays to form advanced second generation IR focal plane arrays. Measurements reveal excellent (4mV RMS) threshold uniformity and the charge division measured noise was less than 200e-.

High Speed Imager Preprocessor For Real-Time Acousto-Optic (AO) Spectrum Analyzer Systems

An advanced high speed imager preprocessor silicon micro electronic circuit for use with integrated and bulk real time A-0 spectrum analyzers is being developed. The circuits will be capable of analyzing and sorting by amplitude, frequency, and time of arrival, wide dynamic range optical signals presented at its focal plane.