Quantum Control

We have a profound interest in understanding photochemistry that can be manipulated by tailoring the excitation process. Chemists have long sought to control the branching ratios and product yields of photochemical reactions. Such control has became attainable by manipulating the phase properties of excitation pulses, creating quantum mechanical interferences, which ultimately change the reaction outcome. The latest advances in the field show that, although experimental techniques for augmenting a specific outcome are available, the mechanisms for understanding the interference processes are far from being mastered. It is only through the comprehension of these reaction mechanisms and dynamics that we can begin searching for achievable applications. We investigate selective control of excitation and chemical reactivity of chemical bonds in the condensed phase through phase modulation of the excitation pulse. Our research focuses on experiments where carefully shaped excitation pulses drive the system towards a desired product. To attain this goal, ultrafast excitation pulses are required such that the light-matter interaction is completed in times comparable to the vibrational period of the molecule. We use tailored pump pulses to interrogate the dynamics of vibrational excitation and relaxation by selectively populating a limited number of states. Detection of temporally resolved and spectrally dispersed probe beams allows the scrutiny of different transitions and their temporal behavior.

Adaptive laser pulse shaping has enabled impressive control over photophysical processes in complex molecules. However, the optimal pulse shape that emerges rarely offers straightforward insight into the excited-state properties being manipulated. We investigate the emission  quantum yield of a donor-acceptor macromolecule (a phenylene ethynylene dendrimer tethered to perylene)  which can be enhanced by 15% through iterative phase modulation of the excitation pulse.  Analyzing the pulse optimization process and optimal pulse features, we successfully isolated the dominant elements underlying the control mechanism.

Science, 326, 263-267, 2009. DOI: 10.1126/science.1176524