Raphael F. Ribeiro

Polariton-Assisted Singlet Fission in Acene Aggregates

Singlet fission is an important candidate to increase energy conversion efficiency in organic photovoltaics by providing a pathway to increase the quantum yield of excitons per photon absorbed in select materials. We investigate the dependence of exciton quantum yield for acenes in the strong light-matter interaction (polariton) regime, where the materials are embedded in optical microcavities. Starting from an open-quantum-systems approach, we build a kinetic model for time-evolution of species of interest in the presence of singlet quenchers and show that polaritons can decrease or increase exciton quantum yields compared to the cavity-free case. In particular, we find that hexacene, under the conditions of our model, can feature a higher yield than cavity-free pentacene when assisted by polaritonic effects. Similarly, we show that pentacene yield can be increased when assisted by polariton states. Finally, we address how various relaxation processes between bright and dark states in lossy microcavities affect polariton photochemistry. Our results also provide insights on how to choose microcavities to enhance similarly related chemical processes.

Corrections to Thomas-Fermi Densities at Turning Points and Beyond

Uniform semiclassical approximations for the number and kinetic-energy densities are derived for many noninteracting fermions in one-dimensional potentials with two turning points. The resulting simple, closed-form expressions contain the leading corrections to Thomas-Fermi theory, involve neither sums nor derivatives, are spatially uniform approximations, and are exceedingly accurate.

Can Ultrastrong Coupling Change Ground-State Chemical Reactions?

Recent advancements on the fabrication of organic micro- and nanostructures have permitted the strong collective light-matter coupling regime to be reached with molecular materials. Pioneering works in this direction have shown the effects of this regime in the excited state reactivity of molecular systems and at the same time have opened up the question of whether it is possible to introduce any modifications in the electronic ground energy landscape which could affect chemical thermodynamics and/or kinetics. In this work, we use a model system of many molecules coupled to a surface-plasmon field to gain insight on the key parameters which govern the modifications of the ground-state Potential Energy Surface. Our findings confirm that the energetic changes per molecule are determined by effects which are essentially on the order of single-molecule light-matter couplings, in contrast with those of the electronically excited states, for which energetic corrections are of a collective nature. Still, we reve...

Two-dimensional infrared spectroscopy of vibrational polaritons

Significance Quantum states of molecular vibrational polaritons, hybrid half-light, half-matter quasi-particles, are studied using ultrafast coherent two-dimensional infrared spectroscopy. Valuable physical insights such as existence of hidden dark states and ultrafast interactions between dark states and bright polariton states are unambiguously revealed. These results not only advance coherent two-dimensional spectroscopy, but also have significant implications in harvesting the creation of molecular vibrational polaritons for infrared photonic devices, lasing, molecular quantum simulation, as well as chemistry by tailoring potential energy landscapes. We report experimental 2D infrared (2D IR) spectra of coherent light–matter excitations––molecular vibrational polaritons. The application of advanced 2D IR spectroscopy to vibrational polaritons challenges and advances our understanding in both fields. First, the 2D IR spectra of polaritons differ drastically from free uncoupled excitations and a new interpretation is needed. Second, 2D IR uniquely resolves excitation of hybrid light–matter polaritons and unexpected dark states in a state-selective manner, revealing otherwise hidden interactions between them. Moreover, 2D IR signals highlight the impact of molecular anharmonicities which are applicable to virtually all molecular systems. A quantum-mechanical model is developed which incorporates both nuclear and electrical anharmonicities and provides the basis for interpreting this class of 2D IR spectra. This work lays the foundation for investigating phenomena of nonlinear photonics and chemistry of molecular vibrational polaritons which cannot be probed with traditional linear spectroscopy.

Theory for Nonlinear Spectroscopy of Vibrational Polaritons

Molecular polaritons have gained considerable attention due to their potential to control nanoscale molecular processes by harnessing electromagnetic coherence. Although recent experiments with liquid-phase vibrational polaritons have shown great promise for exploiting these effects, significant challenges remain in interpreting their spectroscopic signatures. We develop a quantum-mechanical theory of pump-probe spectroscopy for this class of polaritons based on the quantum Langevin equation and the input-output theory. Comparison with recent experimental data shows good agreement upon consideration of the various vibrational anharmonicities that modulate the signals. Finally, a simple and intuitive interpretation of the data based on an effective mode-coupling theory is provided. Our work provides a solid theoretical framework to elucidate nonlinear optical properties of molecular polaritons as well as to analyze further multidimensional spectroscopy experiments on these systems.

Leading corrections to local approximations II : The case with turning points

Quantum corrections to Thomas-Fermi (TF) theory are investigated for noninteracting one-dimensional fermions with known uniform semiclassical approximations to the density and kinetic energy. Their structure is analyzed, and contributions from distinct phase space regions (classically-allowed versus forbidden at the Fermi energy) are derived analytically. Universal formulas are derived for both particle numbers and energy components in each region. For example, in the semiclassical limit, exactly 1/(6\pi3^{1/2}) of a particle leaks into the evanescent region beyond a turning point. The correct normalization of semiclassical densities is proven analytically in the semiclassical limit. Energies and densities are tested numerically in a variety of one-dimensional potentials, especially in the limit where TF theory becomes exact. The subtle relation between the pointwise accuracy of the semiclassical approximation and integrated expectation values is explored. The limitations of the semiclassical formulas are also investigated when the potential varies too rapidly. The approximations are shown to work for multiple wells, except right at the mid-phase point of the evanescent regions. The implications for density functional approximations are discussed.

Vibronic Ground-state Degeneracies and the Berry phase - A Continuous Symmetry Perspective

We develop a geometric construction to prove the inevitability of the electronic ground-state (adiabatic) Berry phase for a class of Jahn-Teller (JT) models with maximal continuous symmetries and N > 2 intersecting electronic states. Given that vibronic ground-state degeneracy in JT models may be seen as a consequence of the electronic Berry phase and that any JT problem may be obtained from the subset that we investigate in this Letter by symmetry-breaking, our arguments reveal the fundamental origin of the vibronic ground-state degeneracy of JT models.

Deriving uniform semiclassical approximations for one-dimensional fermionic systems

A complete derivation is provided of the uniform semiclassical approximations to the particle and kinetic energy densities of N noninteracting bounded fermions in one dimension. The employed methodology allows the inclusion of non-perturbative quantum effects, including tunneling and quantum oscillations, via an infinite resummation of the Poisson summation formula. We explore the analytic behavior, physical meaning, and the relationship between the semiclassical uniform approximations for the fermionic kinetic energy and particle densities.