Dispersive Fourier transform spectroscopy in the far infrared

The technique of dispersive Fourier transform spectroscopy (DFTS) permits the direct measurement of phase spectra, and thus removes a major limitation of conventional spectroscopy where the phase, which is required to determine the dielectric response, must he derived from a Kramers-Kronig analysis of the measured power reflectivity. DFTS studies between 300K and 100K using interferometers equipped with dielectric beam dividers have recently been reported, but the performance of such instruments at frequencies less than about 30 cm is seriously impaired by interference effects in the dividers.The acquisition of accurate data from this region is extremely important when investigating phase transitions in ferroelectric crystals such as KH2PO4 (KDP), characterised by a 'soft' mode whose frequency The development of polarising interferometers employing wire grid beam dividers has made low frequency dispersive studies feasible, although such measurements have been previously made only at ambient temperature.In the present work a dispersive polarising interferometer has been constructed. The instrument has a working range of 20 - 235 cm when used with a Golay detector and this can be extended to ~ 2 cm by using a liquid helium cooled Ge bolometer. The sample temperature may be held to ~ +/-0.1K anywhere between 90K and 300K. Consequently the interferometer permits low frequency temperature dependent DFTS studies of solids for the first time, simultaneously overcoming all the aforementioned problems.The performance of the instrument was evaluated by measuring the complex reflectivity of Csl and CsBr at 100K and 300K. The results agree well with published data. The complex reflectivity of KDP was measured from 300K down towards the ferroelectric transition at 122K. The resultant fcf axis dielectric functions vere analysed in terms of the coupled mode formalism adopted by previous workers, whose work is briefly reviewed. The results are supplemented by transmission measurements performed with a conventional power interferometer on thin KDP crystals, whose !cf axis spectra were found to exhibit a temperature dependent transmission window. Complex reflectivity spectra of antiferroelectric NHHgPO (ADP) arealso presented. The results illustrate the advantages to be obtained by using dispersive Fourier transform spectroscopy when accurate values of the complex dielectric response are required.