Electrically-Substituted Bolometer: Transfer Standard detector over the Wavelength Range from 200 nm to 20 μm

An electrically-substituted bolometer (ESB) has been developed at the National Institute of Standards and Technology (NIST) to serve as a portable transfer-standard detector over the wavelength range 200 nm to 20 µm. The ESB is designed to serve as a reference radiometer for facilities such as the IR-SIRCUS that work in the infrared region, where silicon-photodiode trap detectors cannot be used and where the noise of pyroelectric detectors currently limits the uncertainty. The noise floor of the 8 mm diameter active-area ESB approaches 10 pW/Hz^1/2 at 15 Hz, corresponding to detectivity of the order of 1×10¹¹ cm · Hz^1/2/W. This is an improvement by a factor of almost 1000 on that attainable from a similar-sized room-temperature electrically calibrated pyroelectric radiometer (ECPR). The ESB is linear from the noise floor to 1 mW (the power range of the primary standard radiometers such as POWR), whereas previous helium-cooled bolometers developed at the NIST have a similar noise floor but are non-linear above about 10 µW and so could not be directly calibrated against the cryogenic radiometers. Because of its low noise, linearity over a wide dynamic range and the spectral and spatial flatness of its response, the ESB is finding other applications beyond that which motivated its development.

The ESB detector consists of a receiver element suspended from a copper heat sink. The heat sink serves as a mount for the structure, and incorporates a heater and a calibrated germanium resistance thermometer so that its temperature can be controlled independently of the receiver element. The receiver element is built on a 50 µm thick, 1 cm diameter sapphire substrate. It is composed of an optical absorber/electrical heater and a temperature sensor. The optical absorber is a film of gold-black, about 30 µm thick, deposited on the front of the substrate.

Reference Detector - Electrically Substituted Bolometer (ESB) at IR SIRCUS facility.

Schematic of the detector element of the NIST electrically substituted bolometer. Above: plan view; below: enlarged cross-section of the receiver element.

Two gold-over-chromium contacts, each about 500 µm in diameter, are deposited under the gold-black film and make electrical contact with it. Two NbTi electrical leads are soldered to each contact, allowing the gold-black film to act as a four-wire resistor. Thus the gold-black film can be used as an electrical heater (for electrical substitution) as well as an optical absorber. The NbTi leads become superconducting at a temperature below about 9 K, yet have very low thermal conductance compared with the temperature sensor leads and thermal-shunt wire.

A thin film (20 nm) of aluminum is deposited on the back of the substrate. The gold-black film absorbs most of the incident light over a wide spectral range, from ultraviolet to infrared. The small fraction of incident light transmitted through the gold-black film and the substrate is reflected by the aluminum film for a second pass through the substrate and gold-black film, thereby increasing the overall absorptance. The temperature sensor is a heavily doped silicon chip thermistor, approximate dimensions 300 µm × 300 µm × 300 µm. It is fixed to the back of the substrate with epoxy resin and has two electrical leads. One of these leads is electrically grounded at the heat sink; the other is electrically isolated. Both are thermally grounded to the heat sink, and act as a thermal link between the receiver element and the heat sink. However, the thermal conductance provided by these leads is not sufficient to cool the receiver element to its operating temperature (5 K to 6 K) in the presence of approximately 1 mW of ambient infrared background when the heat sink is at its nominal operating temperature of 4.2 K. Thus an additional copper wire, having diameter 200 µm, was added between the receiver element and the heat sink to act as a thermal shunt. The dimensions of this thermal-shunt wire control the value of the thermal conductance from receiver element to heat sink. This is set at about 1 mW/K, a value determined by considering the trade-off between operating temperature, bolometer sensitivity and time response. The receiver element is supported mechanically from the heat sink by the thermal-shunt wire, the two thermistor leads, and the four NbTi leads.

The ESB detector is mounted in a portable liquid-helium cryostat with a wedged IR window. The window is wedged to prevent interference effects when the ESB is used with lasers, as in calibrating it with respect to POWR. Scattered light is minimized by a set of diffuse black- painted baffles on the liquid-helium-cooled stage and a similar baffle on the liquid-nitrogen-cooled stage. A high-precision aperture, diameter about 5 mm, is mounted at the end of the baffle tube on the helium stage, about 2 mm in front of the ESB receiver.

References

An electrically substituted bolometer as a transfer-standard detector,
Rice J.P.
Metrologia 37, 433-436 (2000).