We report on the investigation of free-carrier absorption characteristics for epitaxially grownp-type silicon thin films in the far-infrared region (50–200 &mgr;m), where Si homojunction interfacial workfunction internal photoemission (HIWIP) detectors are employed. Five Si thin films were grown by molecular beam epitaxy on different silicon substrates over a range of carrier concentrations, and the experimental absorption data were compared with calculated results. The free-hole absorption is found to be almost independent of the measured wavelength. A linear regression relationship between the absorption coefficient and the carrier concentration, in agreement with theory, has been obtained and employed to calculate the photon absorption probability in HIWIP detectors. ©1997 American Institute of Physics.