Terahertz profilometry at 600 GHz with 0.5 µm depth resolution

Characterization of the topography of materials by interferometry in the visible or near-IR wavelength regime becomes difficult or impossible if the surface is rough on the length scale of a 10th of the wavelength and more. In this case, THz radiation can provide an interesting alternative. We demonstrate heterodyne profilometry at 600 GHz as a method for the accurate determination of surface topography with an achievable expanded standard uncertainty <0.5 µm. As a consequence, one can measure topographic signatures on optically rough surfaces of materials such as rolled steel. Other objects of interest (motivating this specific work) are dish antennae for millimeter- and submillimeter-wave astronomical missions such as the Planck mission aiming at a survey of the entire sky at nine different frequencies with the goal to determine fluctuations of the temperature of the cosmic background radiation in great detail. In order to achieve the desired high accuracy, issues like particulate contamination, surface roughness and accuracy, and micro-cracking of the mirror surface become an issue. Accurate testing of the surface figure of the telescope mirror is mandatory.