This thesis combines highly accurate optical spectroscopy data on the recently discovered
iron-based high-temperature superconductors with an incisive theoretical analysis. Three
outstanding results are reported: (1) The superconductivity-induced modification of the
far-infrared conductivity of an iron arsenide with minimal chemical disorder is quantitatively
described by means of a strong-coupling theory for spin fluctuation mediated Cooper pairing.
The formalism developed in this thesis also describes prior spectroscopic data on more
disordered compounds. (2) The same materials exhibit a sharp superconductivity-induced anomaly
for photon energies around 2.5 eV two orders of magnitude larger than the superconducting
energy gap. The author provides a qualitative interpretation of this unprecedented observation
which is based on the multiband nature of the superconducting state. (3) The thesis also
develops a comprehensive description of a superconducting yet optically transparent iron
chalcogenide compound. The author shows that this highly unusual behavior can be explained as a
result of the nanoscopic coexistence of insulating and superconducting phases and he uses a
combination of two complementary experimental methods - scanning near-field optical microscopy
and low-energy muon spin rotation - to directly image the phase coexistence and quantitatively
determine the phase composition. These data have important implications for the interpretation
of data from other experimental probes.
