Developing Free-space and Polarization Control of THz QCL Radiation Inside a Dry Dilution Refrigerator

Previously we demonstrated integration of a THz quantum cascade laser (QCL) within a dry dilution refrigerator, directing its output using a hollow metal waveguide (HMWG) into the sample space [1]. Here, we show that the waveguide can be replaced successfully by free-space optics within the refrigerator; this allows polarization of the QCL output to be maintained along the way to the sample. We also show active control of polarization using a mechanical filter wheel able to insert different optical components into the beam path during experiments. Our system overcomes waveguide dispersion and losses intrinsic to prior system designs. The results pave the wave to THz polarization control of radiation from THz QCLs at sub-Kelvin temperatures.