Tuesday, August 9, 2016

Abstract-Revealing hidden optical transitions with tuneable optical-pump THz-probe spectroscopy

While a vast amount of theoretical and experimental approaches can be used to study the band structure of simple solids, the investigation of the electronic properties of high-temperature superconductors and other strongly correlated systems is far less simple. Limitations to both theory and experiments arise from e.g. the many-body nature of the mathematical problem and from the non-trivial surface reconstructions, respectively. Here we propose a novel approach able to reveal energy gaps between band extrema that cannot be identified from the equilibrium optical properties. By combining finely-tunable visible pump pulses with terahertz probe fields, we identify changes to the transient conductivity as the pump wavelength is changed and the density of carriers in different parts of the band structure varies. This approach is demonstrated on a typical semiconductor, undoped silicon, where we identify the band minimum at the L point of the conduction band, corresponding to the second lowest energy indirect gap. When carriers are photo-injected above this secondary energy gap the transmitted terahertz probe fields are dramatically affected. Our results open the possibility of novel, all optical experimental opportunities for the investigation of the ultra-fast electronic dynamics of correlated materials.

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