QLC: EAGER: Collaborative Research: Developing Experiment and Theory for Entangled Photon Spectroscopy

In this EAGER proposal a quantum optical experimental approach and a quantum electronic computational approach will be applied to investigate new properties of organic molecules with entangled two-photon absorption (ETPA). Both experiment and theory will take advantage of the quantum entanglement of photons created by the process of spontaneous parametric down conversion and the subsequent excitation of electronic states in molecules. The phenomenon of ETPA has been theoretically predicted to exhibit interesting non-classical effects such as linear rather than quadratic dependence of absorption rate on incident photon flux. In prior work, Goodson has developed the experimental methods in his lab which make it possible to measure ETPA cross sections for a wide variety of molecules, and indeed the linear dependence on photon flux was observed. In addition, he has developed a theoretical framework for interpreting the experiments, but a surprise in this earlier work is that some molecules show significant ETPA cross sections (~10-17 cm2) while other nominally similar molecules show no observable ETPA. What is needed to go further is to understand intermediate states that can participate in ETPA, but to do this it is necessary to calculate transition moments that couple excited states, which is an ability that is generally missing from electronic structure theories that can be used for large molecules, such as density functional theory. However Schatz has recently developed a new TDDFT approach for calculating energies and transition moments associated with excited states and for exploring more general properties related to the excitation of coherences. The proposed research is therefore concerned with evaluating ETPA cross sections using the new TDDFT method, along with pump-probe entangled time-resolved measurements using a new apparatus. These, and other excited state experiments, will consider a variety of new molecules that are designed to study sensitivity of ETPA cross sections to molecular properties.