Jim Schleicher
From WikidChem
Jim is a graduate student in the Schmuttenmaer Lab.
Research
Members of the Schmuttenmaer Lab measure ultrafast dynamics using Terahertz (THz) Emission Spectroscopy (TES). TES is a technique which has been shown to characterize dynamical processes (in the 0.1-10 picosecond range) in physical and chemical systems. Initially, TES was used exclusively to evaluate the performance of new sources of THz radiation, however, more recently TES has been employed as a method to understand a variety of phenomena. A key feature in these more recent experiments is that no focusing optics are placed in the THz beam path, allowing the THz waveform to be carefully analyzed, thereby yielding quantitative information about the process of interest.
Studies on GaAs and thin magnetic films
High intensity laser fields can produce effects based on nonlinear terms in the material susceptibility. For anisotropic systems (i.e., those that do not posses inversion symmetry), the lowest-order non-zero contribution is the second-order susceptibility, χ^(2) . It describes all field-matter interactions that vary as the second power of the electric field strength.
For semiconductors, the exact form of χ^(2) is inherently complicated as numerous processes can lead to difference-frequency generation. In particular, optical rectification and shift currents both contribute to χ^(2) . For pulses well below bandgap, optical rectification dominates. When the photon energy is tuned above bandgap, carriers in the semiconductor are promoted from the valence band to the conduction band. Due to the spatial localization of the electrons in the conduction and valence bands in GaAs (the electrons around the Gamma-point of the conduction band reside primarily on the Ga atoms, while those around the Gamma-point of the valence band are predominantly located on the As atoms), the photoexcitation of carriers can generate a net shift in the center of charge within the unit cell, which is know as a shift current. Both of these processes are capable of difference-frequency generation in semiconductors. However, due to the differing underlying mechanisms at work, each process produces unique spectral features in the emitted THz radiation, thereby allowing these distinct contributions in the emission to be identified.
