Subject： Probing Dynamics in the Far-Infrared with THz Absorption and Emission Spectroscopy (基于太赫兹波吸收和发射光谱的远红外超快动力学研究)

Lecturer：　 Prof. Charles Schmuttenmaer（Yale University, Department of Chemistry, USA）

Time：June 24, 2009 （Wednesday）9: 00am

Place：Transient Optics Department 3F meeting room

Biography
Charles A. Schmuttenmaer was born in Oak Park, IL in the USA.  He received a B.S. degree in chemistry from the University of Illinois, Urbana-Champaign, in 1985, and a Ph.D. degree in chemistry from the University of California, Berkeley, in 1991.  He was a Postdoctoral Fellow at the University of Rochester.  In 1994, he joined Yale University, New Haven, CT, where he is currently a Professor of chemistry.  His current research interests include novel applications of time-resolved terahertz (THz) spectroscopy and THz emission spectroscopy.  In particular, he has exploited the unique features of this recent experimental development to characterize transient photoconductivity in semiconductors, quantum dots, and nanoparticles.  He has also studied THz emission from photoexcited samples such as oriented dye molecules, self-assembled monolayers, ultrafast demagnetization, GaAs, etc.  Prof. Schmuttenmaer is a member of the American Chemical Society, the American Physical Society, the Optical Society of America, and the American Association for the Advancement of Science.
Abstract
In addition to providing a bright source with sensitive detection methods in the far-infrared, terahertz (THz) spectroscopy is the only way to carry out time-resolved studies in the far-infrared region of the spectrum.  Two of the best examples of this are time-resolved THz spectroscopy studies and THz emission studies.
Among other things, time-resolved THz spectroscopy (TRTS) is a non-contact electrical probe capable of measuring the photoconductivity on a sub-ps to nanosecond (ns) timescale.  In essence, materials with high conductivity strongly absorb THz radiation, while those with low conductivity do not.  With THz spectroscopy, not only can the average time-dependent conductivity properties be measured, but also the complete frequency-dependent, complex-valued conductivity (i.e., real and imaginary components), all on a sub-ps timescale, and without attaching any probe wires to the sample.  TRTS has been employed to study the transient photoconductivity in nanocrystalline colloidal TiO2 and ZnO nanoparticles, and ZnO nanowires.
Terahertz Emission Spectroscopy (TES) is a method which has been shown to characterize dynamical processes in physical and chemical systems.  THz emission is the difference-frequency analog of second harmonic generation, and is based on the second order nonlinear susceptibility, χ(2), of the material.  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 current both contribute to χ(2) in GaAs.  Our studies probe these different emission mechanisms, and also reveal the influence of spin-polarized electrons when photoexciting the material with elliptically polarized light.