Colloquium

Probing Dynamics in Complex Materials with Ultrafast Optical Spectroscopy

Richard Averitt
Boston University
Wednesday, February 28, 2007
Higgins 310, 4 pm

The allure of ultrafast optical spectroscopy (UOS) in chemistry, biology, and atomic and condensed matter physics is that it provides the ability to temporally resolve phenomena at the fundamental timescales of atomic and electronic motion. UOS is proving invaluable in unraveling the competing degrees of freedom that ultimately determine functional behavior. Driving this utility has been the advances in ultrashort optical pulse technology enabling the generation and detection of subpicosecond pulses from the far-infrared through the visible and into the X-ray region of the electromagnetic spectrum.

From a materials science perspective, subpicosecond temporal resolution combined with spectral selectivity enables studies of electronic, spin, and lattice dynamics, and crucially, the coupling between these degrees of freedom. First, I will provide a brief overview of UOS experiments in condensed matter physics to provide some perspective of what is possible in this emerging field of research.

Secondly, initial attempts to probe dynamics in magnetoelectric materials will be presented. In magnetoelectrics, coupling of coexisting magnetic and electrical ordering results in an electrically controlled magnetic response and vice versa. In certain crystals, the origin of a magnetoelectric (ME) response as related to the appearance of simultaneous ferroelectric order and incommensurate magnetic ordering is under intense scrutiny. Strong dynamical coupling between the phonon and spin degrees of freedom is expected and experiments which aim to probe such dynamics may provide insight into the origin of a ME response. I will present the results of time-resolved optical pump-probe studies on the hexaferrite Ba0.6Sr1.4Zn2Fe12O22. A strong magnetic field dependence of the coherent acoustic phonon response is consistent with a giant magnetoelastic effect in this material.