  Optical spectroscopy of quantum materials at ultralow temperatures is rarely
explored, yet it may provide critical characterizations of quantum phases not
possible using other approaches. We describe the development of a novel
experimental platform that enables optical spectroscopic studies, together with
standard electronic transport, of materials at millikelvin temperatures inside
a dilution refrigerator. The instrument is capable of measuring both bulk
crystals and micron-sized two-dimensional van der Waals materials and devices.
We demonstrate the performance by implementing photocurrent-based Fourier
transform infrared spectroscopy on a monolayer WTe$_2$ device and a multilayer
1T-TaS$_2$ crystal, with a spectral range available from the near-infrared to
the terahertz regime and in magnetic fields up to 5 T. In the far-infrared
regime, we achieve spectroscopic measurements at a base temperature as low as ~
43 mK and a sample electron temperature of ~ 450 mK. Possible experiments and
potential future upgrades of this versatile instrumental platform are
envisioned.
