RUI: Ultrafast THz Spectroscopy of Spin Dynamics in Semiconductors

This project will use ultrafast THz spectroscopy to study spin excitations in narrow-gap semiconductor quantum wells. These systems exhibit a large spin-orbit coupling, permitting the spin-state of carriers to be controlled with an electric field such as that from a gate contact. These systems will be investigated with a time-resolved spectroscopic probe tuned to the energy scale of the spin-orbit interaction (1-5meV). The lifetimes and energy spectra of spin excitations will be determined for a number of quantum well structures. Such measurements are made possible by the spin-orbit interaction, which will allow optical generation of carriers in a well-defined spin-state, and optical probes of the evolution of spins. Ultrafast THz spectroscopy will also be used to perform pulsed-EPR measurements at terahertz frequencies on donor electrons in InAs. The successful performance of these experiments will demonstrate the applicability of this technique to a wide range of physical systems. Terahertz frequency generation on patterned semiconductor surfaces will also be investigated. It is expected that surface patterning will boost the efficiency of generation of optically pumped THz pulses from semiconductor surfaces by about one order of magnitude. Lastly, carrier lifetimes will be investigated in picosecond carrier-lifetime materials such as low-temperature grown GaAs and radiation damaged semiconductors. Undergraduate students will participate in this research which will be performed at Macalester College and the University of Minnesota. The project will also benefit from collaboration with an industrial partner. The students will thus also acquire research experience in a setting of a major research enterprise.

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As advances in technology push the size of transistors towards atomic dimensions, their properties will be increasingly influenced by quantum mechanics. For this reason, scientists are exploring devices that rely on quantum phenomena for their function. One such promising area involves semiconductor devices in which an electron's spin, rather than its charge, is used to control the flow of electric current. At the present time the most promising systems for realizing such semiconductor spin-transport devices are thin layers of materials such as indium arsenide and indium antimonide. Realizing these new technologies requires, however, a detailed understanding of the behavior of electron spins in these systems at ultra-short time intervals. This research is devoted to an experimental investigation of the optical properties of indium arsenide and indium antimonide by means of infrared spectroscopy at a time resolution of one trillionth of a second. Such measurements will reveal how much energy is required to change the direction of the spin in these systems, and how long the spin remains in the newly oriented state. Additional experiments will be performed to demonstrate the feasibility of pulsed magnetic resonance spectroscopy at these ultra-short time scales. This research will be conducted at Macalester College as well as at the University of Minnesota. The project will also benefit from the participation of an industrial collaborator. Undergraduate students will be engaged in this research. They will thereby acquire skills and knowledge in a forefront area of condensed matter physics and materials science. They will be prepared for advanced studies with an appreciation for the needs of advanced technology and for entry into the scientific/technological workforce.