Ultrafast Imaging of Molecular Polariton Transport: Competition between Coherence and Localization

With support from the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) Program in the Division of Chemistry, Professor Libai Huang and her research group at Purdue University are studying new ways to use light to control the transfer of energy between confined molecules. The project uses light trapped between a pair of very closely spaced mirrors to control the migration of energy between the molecules.

Recent developments demonstrate that millions of molecules can collectively interact with a single mode of light when they are placed in a properly designed cavity. Although the molecules normally act like independently oscillating pendula (representing excited electron-hole pairs called excitons), the interaction with light inside the cavity results in a correlated motion of the pendula as if they are all attached to a single driving rod.

When the energy exchange rate between the photons and the excitons (i.e., the light and the pendula) is faster than other energy loss pathways, new quantum states known as polaritons are formed with mixed light-matter characteristics. Polaritons have a low effective mass that allows the energy to travel at an extremely high speed, many orders of magnitude faster than the excitons alone.

Thus, polaritons have the potential to enhance the speed of energy migration in molecular materials. However, the polariton states are very fragile and short-lived because the synchronization of millions of molecules can be easily disrupted. When the coherence is disrupted, energy becomes localized on a few molecules instead of being shared by millions of molecules.

Understanding the competition between coherence and localization holds the key to harnessing the power of polaritons. The research team led by Prof. Huang is addressing this scientific challenge by developing microscopy techniques to record the propagation of polaritons with a resolution of 15 femtoseconds (a femtosecond is one quadrillionth of a second) and of 50 nanometers (a nanometer is one-billionth of a meter).

The team also images the direction of propagation to differentiate polariton and exciton contributions. The research activities are integrated with K-12, undergraduate, and graduate science education. Specific educational and outreach activities include developing an undergraduate-level quantum chemistry lab module on strong light-matter coupling and partnering with the Superheroes of Science YouTube channel to make short videos for K-12 students and the general public on topics related to the research.

Strong coupling between molecular excitons and cavity photons provides a new paradigm for achieving long-range coherent energy transport. A single mode of light can collectively interact with millions, or even billions, of molecules to form macroscopically coherent light-matter hybrid states known as polaritons. The formation of polaritons can dramatically alter the energy landscape and dynamics of the molecules.

The photonic nature of polaritons enables fast propagation and long-range delocalization, which are beneficial for enhancing exciton transport. However, challenges remain in exploiting strong light-matter interactions for long-range energy transport in molecular systems, due to the inherent inhomogeneities and large vibronic coupling that result in ultrafast dephasing, a high density of dark states, and disorder-induced localization.

This project aims to develop ultrafast microscopy tools in both real- and Fourier-space to provide a comprehensive picture of the competition between coherent polariton transport and decoherence processes in molecular aggregates. The measurements probe the transport of molecular polaritons with simultaneously high spatial (~50 nm) and temporal (~15 fs) resolution as well as momentum selectivity by combining ultrafast pump-probe microscopy with Fourier filtering.

Molecular aggregates in Fabry-Perot microcavities with tunable Rabi splitting are used to investigate the role of vibronic coupling, disorder, and dark states in the transition from coherent to diffusive transport regime, as well as the direct visualization of remote energy transfer on macroscopic length scales resulting from the collective coherence. Beyond the fundamental insights made possible by these measurements, comparison with quantum chemistry calculations in collaboration with theory groups provides valuable information to aid the development of quantum mechanical theories for modeling molecular polaritons.

The broader impacts of the project are further enhanced through educational and outreach activities that make the research accessible to a wide audience.

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.