1.2 - Superconducting Bolometer for Terahertz Detection

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings IRS² 2011
I1 - Photon and Thermal Detectors
M. Kehrt, C. Monte, J. Beyer, J. Hollandt - Physikalisch-Technische Bundesanstalt, Berlin (Germany)
21 - 23


Over decades the spectral range from 0.1 THz to 3 THz (the spectral range of far-infrared (FIR) or THz radiation) was covered by astronomic research. In the last years THz radiation became a growing field of interest in laboratory measurements, industry and homeland security. Supporting these efforts we are developing a composite bolometer for 4.2 K operation that addresses the special requirements of Fourier transform spectroscopy, namely high and uniform absorption over the whole spectral range from 0.1 THz to 3 THz and a linear response of the detector.
In composite bolometers the absorber and the thermistor can be optimized separately. Superconducting thermistors, so called transition edge sensors (TESs), make it possible to build bolometers with highest sensitivity. TESs utilise the narrow width of only a few mK of the resistive transition of a superconductor structure. In oder to stabilise its operation as well as to expand its dynamic range, the TES is voltage-biased and operated in negative electro-thermal feedback. We are using an Al/Nb bilayer thermistor with a transition temperature Tc of about 8 K and a wiring connection made of Nb with Tc of about 9 K. The TES read-out will be realized by SQUID current sensors. For high and uniform absorption, a resistive absorber layer with a sheet resistance of 188 Ohm would be desirable. Typical metals have a resistivity of 10 Ohm nm to 1000 Ohm nm which would lead to a thickness of a single-layer absorber of around 1 nm. More practically, however, normal metal layers with a thickness of 10 nm to 100 nm can be fabricated reproducibly and in good quality. Such metal layers, when micro-structured with characteristic lateral dimensions smaller than the shortest wavelength of interest, can be used as efficient and broad-band THz absorbers. We have designed metallic absorber structures that are based on quadratic grids and rings with a line width of 3 μm and period widths of 10 μm to 50 μm and numerically simulated their THz absorption. In this presentation the design of the TES bolometer and first results of reflectance and transmisttance measurements of our microstructured metallic absorbers will be discussed.