Screening ineffectiveness and THz emission at bare LO phonon frequencies

Within a hydrodynamic approach we investigate the dynamics of an inhomogeneous electron-hole gas coupled to phonons in Te and the corresponding THz emission. We find that the longitudinal inhomogeneity of the photogenerated electron-hole gas -- due to short absorption lengths in Te -- gives rise to screening ineffectiveness for non-zero wave-vector modes. This allows for THz dynamics and emission at the bare LO phonon frequency nu_{LO} even at high carrier densities. Screening ineffectiveness thus provides an appealingly simple explanation for the existence of bare modes at nu_{LO} in longitudinally inhomogeneous systems such as Te; no lateral inhomogeneity of the excitation spot is needed here.