  The vortex-state electrodynamics of s-wave superconductors has been studied
by infrared spectroscopy. Far-infrared transmission and reflection spectra of
superconducting NbTiN and NbN thin films were measured in a magnetic field
perpendicular to the film surface, and the optical conductivity was extracted.
The data show clear reduction of superconducting signature. We consider the
vortex state as a two-component effective medium of normal cores embedded in a
BCS superconductor. The spectral features are well explained by the
Maxwell-Garnett theory. Our analysis supports the presence of
magnetic-field-induced pair-breaking effects in the superconducting component
outside of the vortex cores.
