Exciton-polaritons are hybrid states formed when molecular excitons are strongly coupled to photons trapped in an optical cavity. These systems exhibit many interesting, but not fully understood, phenomena. Here, we utilize ultrafast two-dimensional white-light spectroscopy to study donor-acceptor microcavities made from two different layers of semiconducting carbon nanotubes. We observe the delayed growth of a cross peak between the upper- and lower-polariton bands that is oftentimes obscured by Rabi contraction. We simulate the spectra and use Redfield theory to learn that energy cascades down a manifold of new electronic states created by intermolecular coupling and the two distinct bandgaps of the donor and acceptor. Energy most effectively enters the manifold when light-matter coupling is commensurate with the energy distribution of the manifold, contributing to long-range energy transfer. Our results broaden the understanding of energy transfer dynamics in exciton-polariton systems and provide evidence that long-range energy transfer benefits from moderately-coupled cavities.

Strong light–matter coupling results in eigenstates called polaritons which share the properties of both light and matter and provide a useful way to engineer electronic energies and behaviors. In this work, we study nearly monochiral (6,5) semiconducting carbon nanotubes (CNTs) in a Fabry–Pérot microcavity. Light–matter coupling leads to the formation of three bands of bright polariton states (upper, middle, and lowerresulting from coupling to the bright S11 CNT exciton and the X1 phonon sideband of the K-momentum dark exciton state). The structure also supports many exciton-like subradiant states at the bright S11 and X1 energies. Here, ultrafast transient reflection spectroscopy is used to study the dynamics and spectral signatures of excited subradiant-state polariton populations and the pathways by which they are populated. After a pump pulse, the excited subradiant-state population is revealed by (i) spectral signatures with relaxation times (∼5 ps) similar to those of CNT S11 band gap excitons outside of the cavity and (ii) a Rabi contraction of the lower polariton energy, whose magnitude quantifies the excited subradiant-state population. Data show that, following the excitation of the upper polariton (UP), the excited subradiant-state population is maximized at a sample position with a detuning of 118 meV, light–matter coupling of 336 meV, and UP transition energy of 1.52 eV. The excited subradiant-state population is reduced for other detunings. The X1 Hopfield coefficient of the UP also peaks at the same energy, revealing UP to X1 scattering as a potentially efficient relaxation pathway. These results will be important for understanding and controlling energy relaxation and transport in future CNT polariton devices.

We report on a new broadband, ultrafast twodimensional white-light (2DWL) spectrometer that utilizes a supercontinuum pump and a supercontinuum probe generated with a ytterbium fiber oscillator and an all-normal dispersion photonic crystal fiber (ANDi PCF). We demonstrate compression of the supercontinuum to sub-20 fs and the ability to collect high quality 2D spectra on films of single-walled carbon nanotubes. Two spectrometer designs are investigated. Supercontinuum from ANDi PCF provides a means to generate broadband pulse sequences for multidimensional spectroscopy without the need for an optical parametric amplifier.

When investigating the interaction between proteins and protoporphyrins in aqueous solution, one typically has to contend with the tendency of the latter to form polydispersed aggregates. The interference of aggregated protoporphyrins manifests, at least, at two levels: aggregates sequester the majority of the protoporphyrin molecules in solution and prevent their interaction with the proteins, but also their presence interferes with optical experiments such as absorption and fluorescence spectroscopy. In this study we present a protocol which uses dialysis and centrifugation to eliminate the aggregates and yield solutions dominated by non-covalent complexes of albumin (HSA) and protoporphyrins. The elimination of the aggregates enabled us to observe effects which had not been previously observed such as eliminating the discrepancy between the binding constants obtained through the quenching of HSA fluorescence and the one obtained through the emission of the protoporphyrins. Moreover the elimination of the aggregated protoporphyrins enabled us to reveal the occurrence of fluorescence resonance energy transfer (FRET) between the Trp214 residue of HSA and the porphyrin ligands. FRET data were then used to estimate the location of metal free as well as Zn-protoporphyrin IX relative to the well-known location of Trp214. This information was used to refine docking simulations to find the best binding site for the two protoporphyrins. In addition we observed that the irradiation of the protoporphyrins in the visible region prompts small conformational changes in HSA that appear to be largely due to tertiary modifications.

The vibrational energy transfer between an atom and an atomic oscillator is considered for a Morse potential interaction, from the point of view of thermal accommodation coefficient theory. An expression is obtained for the transition probabilities for the exchange of one quantum of energy. A comparison is made with the semiclassical theory. Inconsistencies in the literature are noted and discussed.