Observation of below-gap plasmon excitations in superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ films

Finite-wave-vector collective plasmon resonances have been observed in far-infrared transmission measurements on superconducting ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ films. An Al grating is used to couple IR radiation to collective two-dimensional plasma modes. The plasmon dispersion, measured by using different grating periods, spans the frequency range of the energy gap. The strengths of the plasma resonances weaken as the temperature approaches ${\mathit{T}}_{\mathit{c}}$ from below. The results are analyzed using a grating coupler theory that includes the hybridization of the plasmons with the diffraction modes.