Intraband Exciton Transitions in Photosynthetic Complexes Revealed by Novel Five-Wave-Mixing Spectroscopy.

We calculate the χ(4) optical response of an oriented photosystem II reaction center of purple bacteria described by the Frenkel exciton model using nonlinear exciton equations (NEE). This approach treats each chromophore as an anharmonic oscillator and provides an intuitive quasiparticle picture of nonlinear spectroscopic signals of interacting excitons. It provides a computationally powerful description of nonlinear spectroscopic signals that avoids complete diagonalization of the total Hamiltonian. Expressions for the second- and the fourth-order nonlinear signals are derived. The NEE have been successfully employed in the past to describe even-order-wave-mixing. Here, we extend them to aggregates with broken inversion symmetries. Even-order susceptibilities require the introduction of permanent dipoles, which allow to directly probe low-frequency intraband transitions of excitons.

Quantum Interferometric Pathway Selectivity in Difference-Frequency-Generation Spectroscopy.

Even-order spectroscopies such as sum-frequency generation (SFG) and difference-frequency generation (DFG) can serve as direct probes of molecular chirality. Such signals are usually given by the sum of several interaction pathways that carry different information about matter. Here we focus on DFG, involving impulsive optical-optical-IR interactions, where the last IR pulse probes vibrational transitions in the ground or excited electronic state manifolds, depending on the interaction pathway. Spectroscopy with classical light can use phase matching to select the two pathways. In this theoretical study, we propose a novel quantum interferometric protocol that uses entangled photons to isolate individual pathways. This additional selectivity originates from engineering the state of light using a Zou-Wang-Mandel interferometer combined with coincidence detection.

Quantum interferometry and pathway selectivity in the nonlinear response of photosynthetic excitons.

We propose a time-frequency resolved spectroscopic technique which employs nonlinear interferometers to study exciton-exciton scattering in molecular aggregates. A higher degree of control over the contributing Liouville pathways is obtained as compared to classical light. We show how the nonlinear response can be isolated from the orders-of-magnitude stronger linear background by either phase matching or polarization filtering. Both arise due to averaging the signal over a large number of noninteracting, randomly oriented molecules. We apply our technique to the Frenkel exciton model which excludes charge separation for the photosystem II reaction center. We show how the sum of the entangled photon frequencies can be used to select two-exciton resonances, while their delay times reveal the single-exciton levels involved in the optical process.

Energy, Particle, and Photon Fluxes in Molecular Junctions.

Electroluminescence from a current-carrying molecular junction at steady state is simulated. (Charge) particle conservation and energy conservation are satisfied by a perturbative expansion in the radiation/matter coupling. Our approach makes it possible to adopt standard tools of traditional (equilibrium) spectroscopy to study signals from open systems such as molecular junctions. The nonperturbative analysis of spontaneous light emission signals coincides with the perturbative approach for weak molecule-field coupling.

Spontaneous Light Emission from Molecular Junctions: Theoretical Analysis of Upconversion Signal.

Spontaneous light emission from a current-carrying molecular junction is analyzed. There are two leading processes, fluorescence and electroluminescence, as defined using Liouville space diagrams within the perturbative method, that contribute to the light emission from junctions. This allows us to identify a general mechanism that explains the origin of the so-called upconversion electroluminescence (UCEL) signal, which has been observed in a variety of molecular junctions [Umera et al. Chem. Phys. Lett. 2007, 448, 232; Dong et al. Nat. Photonics 2010, 4, 50]. Here, we show that a double-peak signal, one at energy less than the applied bias and the other at higher energy (UCEL), is generated due to overlap between two processes: one is electron transfer to create the required excited state, and the other is radiative relaxation of the excited state. The lifetimes induced by the lead interactions play a crucial role in determining the required overlap between these processes. Our analysis shows that, unlike the higher-energy signal, the lower-energy peak is sensitive to the applied bias and does not correspond to any optical resonance in the junction. The signal at higher energy is enhanced as the temperature is increased. We demonstrate our findings using nonperturbative analytic results for a model system.