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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.

High temperature superconducters. --- Iron-based superconductors. --- Iron oxypnictide superconductors --- Iron oxypnictides --- Iron pnictide superconductors --- Iron pnictides --- Oxypnictide superconductors --- Oxypnictides --- Physics. --- Superconductivity. --- Superconductors. --- Nanoscale science. --- Nanoscience. --- Nanostructures. --- Spectroscopy. --- Microscopy. --- Optical materials. --- Electronic materials. --- Nanotechnology. --- Strongly Correlated Systems, Superconductivity. --- Spectroscopy and Microscopy. --- Optical and Electronic Materials. --- Nanoscale Science and Technology. --- High temperature superconductors --- Molecular technology --- Nanoscale technology --- High technology --- Optics --- Materials --- Nanoscience --- Physics --- Nano science --- Nanoscale science --- Nanosciences --- Science --- Electronic materials --- Analysis, Microscopic --- Light microscopy --- Micrographic analysis --- Microscope and microscopy --- Microscopic analysis --- Optical microscopy --- Analysis, Spectrum --- Spectra --- Spectrochemical analysis --- Spectrochemistry --- Spectroscopy --- Chemistry, Analytic --- Interferometry --- Radiation --- Wave-motion, Theory of --- Absorption spectra --- Light --- Spectroscope --- Superconducting materials --- Superconductive devices --- Cryoelectronics --- Electronics --- Solid state electronics --- Electric conductivity --- Critical currents --- Superfluidity --- Qualitative --- Spectrometry --- Analytical chemistry