Methodology and hardware of terahertz BWO-spectroscopy

Modern terahertz (THz) spectrometers based on coherent sources of radiation, the backward-wave oscillators (BWOs), cover frequencies v = 2 cm^-1 - 45 cm^-1 (60-1350 GHz) and allow for measurements at temperatures 2-1000 K, also in magnetic fields up to 8 Tesla (Koslov and Volkov, 1998). They are of a hybrid type and combine merits of techniques existing in neighboring microwave and infrared ranges: the "high quality" of the working radiation (highly monochromatic and polarized, finely frequency-tunable), on one hand, and the broad-band flexible quasioptical measurement schemes (no waveguides are used, radiation is travelling in free space), on the other hand. The main feature of BWO-spectrometers which is important for spectroscopists is that they allow for a direct determination of the spectra of any optical quantity of the sample under study: real and imaginary parts of refractive index n*(v) = n(v) + ik(v), dielectric constant ε*(v) = ε\´(v) + iε"(v), dynamical conductivity σ*(v) = σ\´(v) + iσ"(v), magnetic permeability μ*(v) = μ\´(v) + iμ"(v), etc.