Understanding Emission, Absorption and Energy Transfer Involving Classical and Quantum Light Interacting with Molecules

Northwestern University, Evanston IL

Investigators

Abstract

With support from the Chemical Theory, Models and Computational Methods program in the Division of Chemistry, Professor George Schatz of Northwestern University is developing new theory and computational methods for characterizing processes that occur when molecules absorb and emit light. One project is concerned with the rate of transfer of energy in the excited molecule to another molecule, where theory will be developed that describes the process in terms of electrons excited in the donor molecule that transfer their energy to electrons in the acceptor molecule. By using a special model of interactions of electronic states of the molecules, the theory makes it possible to incorporate the influence of other nearby molecules or nanoparticles on the energy transfer process, and in some cases leading to enhanced rates or increased range of transfer, as is important in many kinds of optical devices. Another component of the research is concerned with the influence of quantum light in the form of entangled photons on the light absorption. Here time-dependent quantum theory is used to study the evolution of entanglement as energy transfers from light to electrons (i.e., converting quantum light to quantum electrons). Also being studied are the properties of molecules that are in the entangled states to understand how their properties are different from those in unentangled states. This work is fundamental to technologies involved in photon-based quantum computers and in devices for secure communications. The students and postdocs who work on this project, including a significant number of women and minorities, will be trained in theory development and computational applications that they can use for both academic and industrial jobs related to new device technologies. In addition, there will be outreach to K-12 groups, undergraduate training, numerous seminars and workshops, and communication with the public. Under this award, Professor George Schatz and his team will develop theories and computational methods for (1) describing single photon resonant energy transfer between molecules using real-time electronic structure methods, (2) characterizing the entanglement properties of molecular electronic (and vibronic) states, with emphasis on states that are excited by two-photon absorption, and (3) developing a time-domain description of the excitation of molecules by two-entangled photons that couples quantum electrodynamics to electronic structure theory to study the evolution of entanglement from photons to electrons as characterized using measures of the entanglement known as entanglement witness. The proposed research addresses several knowledge gaps that relate to the interaction of both classical and quantum light with molecules in complex environments. The use of real-time electronic structure methods for energy transfer provides the opportunity to describe energy transfer mediated by nearby molecules or nanoparticles. For quantum light that involves entangled photons, it has recently been demonstrated that photon entanglement can for some problems be converted into electronic entanglement, but the underlying rules for this are not known. The proposed research will use concepts from quantum information science to characterize the electronic entanglement properties of molecules that are important in two photon absorption, including collectivity/Schmidt numbers and Shannon entropy. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.