Recent developments in superconducting tunnel junctions for ultraviolet, optical & near infrared astronomy
Some recent results associated with the development of tantalum based photon counting superconducting tunnel junctions (STJ) suitable for use as broad-band low resolution spectrometers for optical and ultraviolet astronomy are presented. A 20x 20 mu m square tantalum based STJ, operated at a temperature of 0.3 K, has demonstrated a limiting resolution of ~ 8 nm at 200 nm and ~ 80 nm at 1000 nm. The device is extremely linear in response with photon energy, and covers the waveband from 200 nm to 2 mu m while measuring the individual photon wavelength and arrival time. The short wavelength limit is currently constrained by the current experimental configuration (a fibre optic) as well as to some extent the sapphire substrate. The estimated quantum efficiency for single photons is over ~ 50% between 200 and 700 nm with a maximum of ~ 75% at 550 nm. Such an STJ when packaged into an array could contribute significantly to many fields of near infrared, optical and ultraviolet astronomy being able to provide efficiently and simultaneously the broad band spectrum and photon arrival time history of every single object in the field over a very wide dynamic range.