Free-space and polarisation maintaining delivery of terahertz radiation from a quantum cascade laser in a dry dilution refrigerator

We demonstrate a free-space terahertz (THz) optical system inside a dry dilution refrigerator that allows us to direct polarized THz radiation from a quantum cascade laser (QCL) to a sample held at a base temperature of 28 mK. The sample temperature increases to 47 mK for an average applied 2.6 THz QCL power of 27 μW at a 10% duty cycle, as determined by calibrated thermometry. We also demonstrate that polarization is maintained using a polarization-sensitive bowtie-antenna-patterned photo-electric tunable step device and, additionally, that the polarization can be adjusted using a set of quartz waveplates in a filter wheel. With this work we introduce the ability to measure materials under polarized THz radiation at previously unreachable millikelvin temperatures.