A Laser Scanning Device for Cryogenic Beam Steering at 20 Millikelvin

It is often desirable to steer light within a cryostat and have a well focussed laser spot incident upon a detector. We demonstrate the operation of superconducting transitionedge sensor microcalorimeters with a fibre-coupled laser scanning device co-located on the 20mK mixing plate of a dilution refrigerator. The scanner directs a 515nm laser beam with spot size$< 40\mu$m over a range of$\pm$14mm in two orthogonal axes (covering 784mm${}^{2}$). We present the optical elements of the system, show that the detector noise spectrum is unchanged when the scanner is operating or off, and observe less than 340 nW power dissipation in our use-case. This device will allow detector calibration up to 10 keV on a pixel-by-pixel basis with simultaneous X-ray measurements. Further, this device will enable more effective characterisation of certain detector artifacts including cross-talk, position dependences, and drift corrections.