OBSERVATION OF BELOW-GAP PLASMON EXCITATIONS IN SUPERCONDUCTING YBA2CU3O7 FILMS

Finite-wave-vector collective plasmon resonances have been observed in far-infrared transmission measurements on superconducting YBa2Cu3O7 f ilms. An Al grating is used to couple IR radiation to collective two-d imensional plasma modes. The plasmon dispersion, measured by using dif ferent grating periods, spans the frequency range of the energy gap. T he strengths of the plasma resonances weaken as the temperature approa ches T-c from below. The results are analyzed using a grating coupler theory that includes the hybridization of the plasmons with the diffra ction modes.