Work in the Ginsberg Group is motivated by the need to spatially and temporally resolve the complex dynamics of nanoscale processes, such as photosynthetic light harvesting. We use multiple approaches, separately and in combination, including ultrafast spectroscopy, light microscopy, and cathodoluminescence electron mocroscopy.

Current Projects

I. Mapping spatio-temporal photoexcitation trajectories onto the architecture of photosynthetic light harvesters

In photosynthesis, solar-to-chemical energy conversion begins with chlorophyll photoexcitation and its ultrafast energy transfer through a heterogeneous network of integral membrane pigment-protein complexes. The ultimate destination is the reaction center protein, where the excitation leads to charge separation that triggers transduction to chemical fuels. Remarkably, nearly every photoexcitation leads to charge separation, yet the reason for this high quantum efficiency is unclear.

We seek to uncover the underlying mechanisms of this targeted excitation energy transfer by probing photosynthetic membranes and synthesized mimics such as polymers, molecular crystals, and inorganic nanostructures intended for photovoltaic applications. Photoexcitations can travel many tens of nanometers without being quenched and do so much faster than typical molecular fluorescence lifetimes. By borrowing elements from fluorescence microscopy and ultrafast spectroscopy, we are interested in dynamically mapping distributions of traveling photoexcitations as a function of energy, space, and time.

Specifically in photosynthesis, we have only begun to learn the means by which molecular arrangement, the resulting Coulomb coupling between chlorophylls, and protein-induced energy fluctuations cooperatively yield high quantum efficiency transport. A more profound understanding is critical to unlocking nature's strategies for successful light harvesting. Artificial devices tend to suffer from limited excitation diffusion lengths prior to charge separation but can be more robust to photodamage. By observing dynamics in both naturally occurring and 'man-made' light harvesters, we will be poised to compare and contrast them and to mitigate their weaknesses.

II. Near-field cathodoluminescence microscopy

The demands for and on high-resolution imaging systems continue to increase with our eagerness to visualize ever smaller features of live cells and molecular interactions. Each imaging platform involves tradeoffs between resolution, working distance, sample environment, sensitivity, and molecular specificity. With these in mind, we are interested in imaging aqueous samples at high resolution using the near-field cathodoluminescence of thin-film phosphors. This technique will achieve controllable brightness and spectral selectivity and will approach the resolution and speed of electron microscopy, without the complications of mechanical scanning at close range or sample fixation.

Cathodoluminescence is the electron-beam induced generation of optical excitations in materials, by analogy to photoluminescence (light-induced) or electroluminescence (electronically-induced). It is an attractive alternative to visualizing the underlying mechanisms of complex molecular phenomena. The key idea in our studies is to leverage the focusing ability of electron optics to make nanoscopic scanned optical spots that can be used for a broad range of imaging applications. This will reveal information about the optical properties of nano phosphors alongside the interactions of small molecules and proteins in solution.

By bringing our excited optical spots very close to encapsulated aqueous samples, we are interested in revisiting optical probes of distance, binding, and aggregation such as Forster resonance energy transfer (FRET) and fluorescence correlation spectroscopy (FCS). This makes it possible to explore the effects of heterogeneity that may be washed out with diffraction-limited observation volumes and to work in the single-molecule regime at high concentrations. We are also interested in using this novel light source to perform precise fluorescence recovery after photobleaching (FRAP) diffusion studies of photosynthetic and other membrane structures at unprecedentedly small length scales.

Selected Publications

http://www.cchem.berkeley.edu/nsggrp/publications.html