J. M. Gildemeister

Single superconducting quantum interference device multiplexer for arrays of low-temperature sensors

We present the design and experimental evaluation of a superconducting quantum interference device (SQUID) multiplexer for an array of low-temperature sensors. Each sensor is inductively coupled to a superconducting summing loop which, in turn, is inductively coupled to the readout SQUID. The flux-locked loop of the SQUID is used to null the current in the summing loop and thus cancel crosstalk. The sensors are biased with an alternating current, each with a separate frequency, and the individual sensor signals are separated by lock-in detection at the SQUID output. We have fabricated a prototype 8 channel multiplexer and discuss the application to a larger array.

Measurements of thermal transport in low stress silicon nitride films

We have measured the thermal conductance, G, of ≈1 μm thick low stress silicon nitride membranes over the temperature range, 0.06 4 K, G is independent of surface condition indicating that the thermal transport is determined by bulk scattering. For T<4 K, scattering from membrane surfaces becomes significant. Membranes which have submicron sized Ag particles glued to the surface or are micromachined into narrow strips have a G that is reduced by a factor as large as 5 compared with that of clean, solid membranes with the same ratio of cross section to length.

A fully lithographed voltage-biased superconducting spiderweb bolometer

A current limiting spark gap for use in a high voltage valve type lightning or surge arrester includes a plurality of insulating gap plates assembled together in a vertical stack to define a generally horizontally extending arc elongation and cooling chamber between adjacent plates. Series gap electrodes are disposed along opposite sides of each plate and are double electrodes formed by a unitary piece of wire preformed to slide into position onto each plate. A gap spacer is provided in each arc chamber interfitting with the series gap electrodes and the insulating plates for accurately defining and enclosing the series gap and for maintaining the spark gap as an assembly. A new and improved spark initiator with two, resilient, ionizing arms is provided in each arc chamber for consistently achieving low impulse voltage sparkover. One or more auxiliary electrodes may be provided in each arc chamber to enhance power follow current limitation by dividing the arc into two or more arc portions and by rapidly elongating and moving the arc portions to the cooling wall portions of each arc chamber. The air gaps in each arc chamber formed between the gap electrodes converge to first air gaps of closest electrode spacing and then diverge along extended arc surfaces of the electrodes to and beyond where the spacing between extended arc surfaces is at least equal to three times the closest spacing.

Monolithic arrays of absorber-coupled voltage-biased superconducting bolometers

We describe a design for bolometric detectors of infrared and mm-wave radiation produced in large-format filled arrays by standard planar lithography. A square grid of metallized silicon nitride absorbs the radiation. The bolometer suspension, sensor, and wiring occupy a small fraction of the area. We have produced a 1024 element array of fully released and suspended 1.5 mm×1.5 mm bolometer micromeshes with a filling factor of 88%. We describe a voltage-biased superconducting bolometer built as a prototype for one array element. We have measured noise equivalent power =2.3×10−17 W/Hz, τ=24 ms, and G=2.7×10−11 W/K at 304 mK.

Voltage-biased high-T/sub c/ superconducting infrared bolometers with strong electrothermal feedback

In the current generation of high-T/sub c/ bolometers the thermal conductance is often chosen for a short time-constant rather than for optimal sensitivity. We describe a novel bolometer bias and readout scheme that promises to relax this constraint. Voltage bias of the superconductor results in strong negative electrothermal feedback that greatly reduces the time-constant of the bolometer. We estimate that a decrease of more than one order of magnitude in time-constant should be possible with existing high-T/sub c/ thermometers. We give theoretical estimates of the performance gain with voltage bias for several bolometers that have been reported in the literature. We find cases where the sensitivity can be greatly improved (by changing the thermal conductance) while holding the time constant fixed and others where the bolometer can be made much faster while maintaining the sensitivity.

Model for excess noise in voltage-biased superconducting bolometers

We are developing superconducting transition-edge bolometers for far-infrared and millimeter wavelengths. The bolometers described here are suspended by thin legs of silicon nitride for thermal isolation. At frequencies between 200 mHz and 10-50 Hz these devices show white noise at their thermal fluctuation limit (NEP approximately 10(-17) W/ radicalHz). At higher frequencies a broad peak appears in the noise spectrum, which we attribute to a combination of thermal fluctuations in complex thermal circuits and electrothermal feedback. Detailed noise calculations fit the noise measured in three different devices that were specifically designed to test the model. We discuss how changes in bolometer materials can shift the noise peak above the frequency range of interest for most applications.

Superconducting bolometer for far-infrared Fourier transform spectroscopy

A voltage-biased superconducting bolometer with a heat sink temperature of 4.2 K has been developed for Fourier transform spectroscopy in the far infrared. This device is based on a Nb transition edge sensor with Tc=8.1 K. It will operate for absorbed infrared power up to 3×10−6 W and has an absorber area of 7 mm2. The response is inherently linear and the noise equivalent power (NEP)=1.2×10−13 W Hz−1/2 is dominated by thermal fluctuation noise. This NEP is at least a factor 10 better than that expected for a conventional 4.2 K semiconductor bolometer which is optimized for 1% saturation at the same infrared power. The optical response time τ=1.2 ms is dominated by the internal thermalization time. A smaller version of this bolometer could be useful for diffraction-limited spectroscopy of small samples throughout the infrared. Estimates suggest that values of detectivity D*>1011 cm Hz+1/2 W−1 and time constants approaching 270 μs could be achieved.

AC voltage-biased superconducting bolometer for a frequency-domain SQUID multiplexer

We demonstrate that a voltage-biased superconducting bolometer (VSB), read out with superconducting quantum interference device (SQUID), can be biased with a sinusoidal voltage. We show that the load curves taken with AC- and DC-bias are nearly identical indicating that there is no identifiable degradation in the performance of the bolometer due to the AC bias. We discuss the use of a frequency-domain SQUID multiplexer with an array of VSBs that are AC-biased.