Ultrafast Optical Microscopy of Multi-scale Energy Flow
Libai Huang
Notre Dame Radiation Laboratory
Notre Dame, IN and 46556
The frontier in solar energy conversion utilizing nanoscale materials now lies in learning
how to integrate functional entities across multiple length scales to create optimal devices. To
address this new frontier, my research focuses on
elucidating multi-scale energy transfer, migration, and
dissipation processes with simultaneous femtosecond
temporal resolution and nanometer spatial resolution.
We have combined/correlated ultrafast spectroscopy
with high spatial resolution techniques such as optical
microscopy and X-ray crystallography to achieve highresolution spatial mapping of charge carrier dynamics
in solar energy harvesting systems.
Morphology dependent charge dynamics in organic
photovoltaics Transient absorption microscopy
experiments have been performed on thermally
annealed poly(3-hexylthiophene) (P3HT) and [6,6]phenyl-C61-butyric acid methyl ester (PCBM) blends. Figure 1 Atomic force microscopy (AFM)
By directly comparing spatially resolved charge height image of a P3HT: PCBM blend (a)
dynamics to ensemble dynamics, the results and transient absorption microscopy (TAM)
summarized in Figure 1 demonstrate that the apparent image of the same sample area taken at
lifetimes obtained by ensemble measurements can be times of 5 ps (c). (b) shows line section of
AFM and TAM. (d) shows the decay curves
misleading due to averaging over microscopically taken with different positions along with the
disparate areas.
ensemble kinetics. The positions where the
Cofactor-specific function mapping in single decay curves are obtained are labeled in (c).
photosynthetic reaction center crystals High-resolution mapping of cofactor-specific photochemistry in photosynthetic reaction centers (RCs) from Rhodobacter sphaeroides was achieved
by polarization selective ultrafast spectroscopy in single crystals at cryogenic temperatures. By
exploiting the fixed orientation of
cofactors within crystals, we isolated a
single transition within the multi-cofactor
manifold, and elucidated the site-specific
photochemical functions of the cofactors
associated with the symmetry-related
active A and inactive B branches.
Transient spectra associated with the
initial excited states were found to involve
a set of cofactors that differ depending
upon
whether
the
monomeric
bacteriochlorophylls, BChlA, BChlB, or
the special pair bacteriochlorophyll dimer
(P) was chosen for excitation. Proceeding from these initial excited states, characteristic
photochemical functions were resolved. These experiments demonstrate the opportunity to
resolve the photochemical function of individual cofactors within the multi-cofactor RC
complexes using single crystal spectroscopy.