Coherent and Incoherent Control in Material Systems

Northwestern University, Evanston IL

Investigators

Abstract

With this award, the Chemical Theory, Models and Computational Method program in the Chemistry division is supporting Dr. Tamar Seideman of Northwestern University to develop and apply new theoretical and computational approaches for controlling the properties of nanoscale devices and thus enhance their functionalities. This research is focused on molecular or nanoscale electronics. In one study, Seideman and coworkers introduce an approach to drive current through junctions with light, rather than with voltage, in a way that circumvents the earlier experienced light-induced damage. A second research direction introduces a much needed approach to understanding transport junctions, which enlists the sensitivity of spectroscopy to accurately characterize the structure and chemical composition of molecular-scale electronics. A third, more ambitious study, introduces a new control concept, namely, quantum optimal environment engineering. Here the Seideman group aims to develop a theory and a numerical method to optimize reaction outcomes using reagents that are less costly than lasers. An application is planned to manipulate charge transfer reactions with a view to enhancing the efficiency of solar cells. The first of these studies builds on the success of previous NSF-supported research, where Seideman introduced an approach to coherent control of transport via semiconductor-based molecular-scale electronics as a route to circumventing the difficulties associated with conventional, metal-based molecular-scale electronics, which were noted in the previous experimental literature. The current research goes beyond her earlier, fully analytical solution, which was restricted to the 1- and 2-site bridge cases and to Markovian dynamics, by developing a numerical method and applying it to explore memory effects and multiple-site dynamics. The second research direction explores current-induced Raman spectroscopy as a route to enlisting the chemical sensitivity of Raman spectra to accurately characterize the structure and chemical composition of molecular-scale junctions, and the transport and current-driven dynamics they exhibit. Also under development is a theory to determine, within a uniform approach, the transport, current-driven dynamics and Raman spectra first for a simple adsorbed diatomic molecule and next for a reduced dimensionality model of rhodamine 6G/silver. The third project relies on recent research that shows that the environment can be configured to steer the quantum system into entangled quantum states with high accuracy, as well as to rapidly switch on and off multiple decay channels with ultrafast time precision. Moreover, these environmental controls can potentially allow steering the system into regions of the Hilbert space that are out of reach of coherent control. Environmental engineering concepts are applied to efficiently transform optically excited donor states into free charge carriers via intermediate higher-lying bridge states, and to suppress the losses caused by charge recombination in the polaron states via effective singlet-to-triplet spin state conversion. .