Far-infrared transmittance and reflectance studies of oriented<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">YBa</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Cu</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mrow><mml:mn>7</mml:mn><mml:mi mathvariant="normal">−</mml:mi><mml:mi mathvariant="normal">δ</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>thin films

We have measured both the transmittance (scrT) and reflectance (scrR) for several ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ thin films over the frequency range from 20 to 360 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at temperatures between 20 and 300 K. The films, grown on MgO substrates by pulsed laser ablation, are highly a-b-plane oriented. We use scrT and scrR to extract the optical conductivity, and find good agreement with results from Kramers-Kronig analysis of reflectance measurements alone. The transmittance is consistent with a two-component dielectric function consisting of Drude (free-carrier) and midinfrared (bound-carrier) absorption. The Drude relaxation rate varies linearly with temperature, and compares well with values obtained from the measured dc resistivity. We use a two-fluid model for the superconducting state and obtain a BCS-like temperature dependence for the superfluid density. A superconducting gap is not observed in our spectra.