Events

Excitons, biexcitons, and higher-order correlations: Direct observations of ultrafast many-body dynamics

April 15, 2009 at 3pm/36-428

Keith A. Nelson
Department of Chemistry Massachusetts Institute of Technology

abstract:
Much is known about the ultrafast dynamics of excitons in semiconductors and organic molecular systems. Far less is understood about higher-order correlations that may lead to bound multi-exciton states, even though biexcitons and triexcitons play key roles in semiconductor optical gain and may play important roles in protection of photosynthetic species from degradation. We have developed multiple-quantum two-dimensional Fourier transform optical (2D FTOPT) spectroscopy methods, directly analogous to multiple-quantum nuclear magnetic resonance (2D FTNMR) techniques, that permit optical generation and direct observation of ultrafast many-body coherences including those involving biexcitons and triexcitons. The experiments require multiple, noncollinear light beams that intersect at the sample and whose fields remain fully phase coherent with each other even as pulses in selected beams are variably delayed. This has been achieved with the magic of spatiotemporal femtosecond pulse shaping, through which all the optical phases, delays, and spectral properties are controlled and the experiment is executed without the use of any delay stages or other moving elements that would normally disrupt phase stability. Many-body dynamics in GaAs quantum wells have been examined in detail, and organic systems are presently under study.