Quantum Nonlocal Effects on the THz Absorption in Metallic Films

The quantum effects of spatial dispersion on the optical properties of a flat metallic film are studied within the Kubo formalism in the terahertz frequency range. As is shown, the results are qualitatively different from those obtained within either the model of the Boltzmann kinetic equation or the Drude-Lorentz model. Specifically, in sufficiently thin metallic slabs, the quantum resonances emerge and, as a result, significantly alter the absorption spectrum provided by the Landau damping. The latter is an inevitable property of the nonlocal electromagnetic response of conduction electrons. The absorption dependence on the electron relaxation rate, Fermi velocity, thickness of metallic slab and other parameters of the problem have also been analyzed. The predictions of the developed here quantum approach coincide with those of the Boltzmann kinetic equation when the thickness of the film is much greater than the metal skin depth. Within the classical local limit our results correspond to the Drude-Lorentz model.