Fig. 3. Quasi-relativistic quantum quasiparticle response in Hg_0.81Cd_0.19Te.

(A and B) Classical parabolic (A) and quasi-relativistic Kane (B) low-energy electronic band structure in a semiconductor. Quasi-linear ultrarelativistic segments of the Kane band structure result from a large spin-orbit coupling. The strong itinerant response is dominated by charge-carriers photoexcited predominantly from the HH band to the C band, as indicated by a blue arrow. (C) Dependence of the unscreened plasma frequency (blue symbols) on the number of photons per pump pulse (photon energy is 1.38 eV in this case). The latter is proportional to the number of photoexcited charge-carriers well below the saturated-absorption regime. Solid black as well as dashed green and red lines indicate the best fit to the experimental data obtained using the power-law dependence with exponent p of the plasma frequency on itinerant electron density in the case of classical parabolic (dashed red; p = 1/2) as well as 3D (dashed green; p = 1/3) and 2D (solid black; p = 1/4) quasi-linear Kane dispersion of the C band (see text). The latter two dependencies are purely quantum and have no classical analog (13).