Guided-wave terahertz spectroscopy of molecular solids [Invited]

One of the outstanding problems of terahertz spectroscopy is the measurement of the underlying vibrational spectrum of a molecular solid, where individual vibrational transitions are often merged into broad absorption features by line-broadening processes. We address this problem using the technique of waveguide terahertz time-domain spectroscopy (THz-TDS), whereby a thin polycrystalline molecular film is contained within a single-mode metal parallel plate waveguide. Thin films of the molecular solids cyclotrimethylene trinitramine (RDX explosive) and 4-iodo-4'-nitrobiphenyl serve to demonstrate the ability of waveguide THz-TDS to resolve previously unseen complex underlying THz vibrational spectra at cryogenic temperatures, with linewidths as narrow as 7 GHz. With such narrow linewidths we are able to demonstrate the measurement of vibrational line-center frequencies to a precision of 1 GHz. (C) 2009 Optical Society of America