IRFP: Ultrafast Spectroscopy of Urbach Tail Excitations in Semiconductors: Unraveling Dynamics of the Anti-Stokes Cooling Cycle

Seletskiy Denis V, Albuquerque NM

Investigators

Abstract

The International Research Fellowship Program enables U.S. scientists and engineers to conduct nine to twenty-four months of research abroad. The program's awards provide opportunities for joint research, and the use of unique or complementary facilities, expertise and experimental conditions abroad. This award is co-funded by the Office of International Science and Engineering and by the Electronic and Photonic Materials Program in the Division of Materials Research. This award will support a twenty-four-month research fellowship by Dr. Denis V. Seletskiy to work with Prof. Alfred Leitenstorfer at the University of Konstanz, in Germany. Optical refrigeration requires laser excitation of a solid below its mean luminescence energy, naturally in the tail of its absorption spectrum (Urbach tail). In this process, one quantum of lattice vibration is annihilated for every emitted high energy photon, as required by energy conservation. If quantum efficiency of this process is high, the laser light will cool the solid. While rare-earth doped insulators have already been cooled to cryogenic temperatures, no cooling of semiconductors has been observed to date, despite more than a decade of extensive research progress. This research for the first time investigates the dynamics of the excitations in the Urbach tail of a semiconductor on an ultrafast time scale, providing insight into how these excitations couple to the internal degrees of freedom of a solid. More specifically, temporal evolution of both carrier density and crystal lattice temperature is measured simultaneously by techniques of transient absorption spectroscopy and coherent anti-Stokes Raman spectroscopy. Highly sensitive experiments are required due to the small signals in the Urbach tail. Unprecedented sensitivity is achieved by employing highly configurable ultra-stable fiber-based femtosecond spectroscopy systems, pioneered by Prof. Leitenstorfer's group at the University of Konstanz, in Germany. Together with novel detection schemes, these high resolution experiments offer new insights into the dynamics of the Urbach tail states and the subsequent laser cooling cycle. Better understanding of the Urbach tail states will advance our knowledge of the principle interactions of low-energy excitations in condensed matter physics. New knowledge of the ultrafast dynamics of UT states can have implications on the telecommunication industry as well as possibly benefit the development of energy efficient technologies such as photovoltaic elements, high quantum efficiency light emitting diodes and ultra-low voltage transistors. Increased understanding of the laser cooling cycle will propel research towards miniaturized all-solid-state cryocoolers, with potential applications ranging from fast and locally-addressable cooling of electronic and space-borne components to brain imaging using novel medical devices with laser-cooled superconducting quantum interference detectors. Continued development of these technologies is essential to address the future of the growing health, communication and energy demands of our society. Finally, state-of-the-art experimental facilities and expertise in the group of Prof. Leitenstorfer at the University of Konstanz are essential for success of this research as well as for PI's professional development. This award will also help Dr. Seletskiy to broaden his network of potential collaborators as well as his scientific scope, proving invaluable for success of his future career.