Prof. Keith Nelson will be here Tuesday, January 28, 2014 in LPSC 402 at 4pm, presenting a seminar entitled: High-Field Terahertz Pulse Generation and Nonlinear THz Spectroscopy of Molecules and Materials.

Abstract:  In recent years it has become possible to generate single-cycle or few-cycle pulses of light at terahertz (THz) frequencies with electric field strengths in the MV/cm range. This has opened up broad new possibilities for nonlinear spectroscopy and coherent control of previously inaccessible collective processes and low-frequency resonances. Generation of intense pulses in the 1-THz frequency range is itself achieved through collective coherent control over the lattice of a nonlinear optical crystal. If the crystal is a thin slab, it can act as a THz waveguide into which elements such as THz bandgap structures, dipole antennas, and others can be incorporated for a variety of applications. THz pulses generated in a bulk crystal can be projected into free space and used for nonlinear spectroscopy. The THz fields can be exploited through their acceleration of electrons or ions or through the forces they exert on ionic or molecular dipoles. THz-driven electronic responses have resulted in impact and tunneling ionization of semiconductors and in collective electronic/structural phase transitions in correlated electron materials. These dynamical responses have been monitored with THz, optical, and hard x-ray probe pulses. THz-driven lattice responses include “soft” optic phonon vibrations as well as induced orientation of nanometer-size polar regions in crystals near ferroelectric phase transitions. Finally, THz coherent control over multiple rotational states of polar molecules in the gas phase has been demonstrated, including the observation of an unusual form of THz superradiance resulting from the transient collective orientation of the molecular dipoles. THz field generation, manipulation, and applications, and the wide-ranging prospects for nonlinear THz spectroscopy, will be discussed.



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