Reflectance studies of candidate THz emitters
Semiconductors are efficient emitters of terahertz (THz, 1012 Hz) radiation. Non-contact means of accurately measuring the physical parameters of these materials are of great value. The reflectance of polar crystals yields important information. A dramatic change in reflectance occurs in the frequency range between the transverse-optical (TO) and the longitudinal-optical (LO) phonons. For many materials these frequencies are of the order of a few THz. Analysis of the reflectance in and near this region yields (a) the TO phonon frequency ω T , (b) the LO phonon frequency ω L , (c) the low-frequency or DC reflectance R(0), and thence the DC refractive index, n(0), and dielectric constant, ɛ(0); (d) the high-frequency or optical reflectance R(∞), and thence n(∞) and ɛ(∞) and (e) the phonon damping factor Γ. These constants depend on the lattice itself and may be described within the Lorentz model. If, in addition, the crystal possesses free carriers, reflectance measurements further yield (f) the plasma frequency ω P , and thence the carrier concentration n e/h and (g) the plasma damping factor γ which may be understood in terms of the Drude model. Samples in the form of a parallel plate give rise to interference fringes that yield (h) the sample thickness t. We have examined many polar crystals with a view to understanding THz emission from them with the overall goal of improving the emission efficiency. Measurements have been made in the region 1.5–21 THz (50–700 cm−1) of single and multilayer samples. We use the sum rule to check the internal consistency of the experimental measurements. We have re-examined the relationship between the phonon frequencies and the reduced ion mass. We find the effective spring constant is very similar in all I–VII materials studied and likewise within the II–VI and III–V classes. We use shell theory to account for these results.