Session S03: Strong Light-matter Coupling and Enhanced Spectroscopy: Theory and Simulation

Theory of Vibronic Polaritons in Optical Microcavities

The interaction of organic molecules and molecular aggregates with electromagnetic fields that are strongly confined inside optical cavities within nanoscale volumes, has allowed the observation of exotic quantum regimes of light-matter interaction at room
temperature for a wide variety of cavity materials and geometries. In this talk, a new theoretical framework is presented that can successfully describe organic cavities under strong light-matter coupling.[1,2] The theory combines standard concepts in chemical physics
and quantum optics to provide a microscopic description of vibronic organic polaritons that is fully consistent with available experiments, and yet is profoundly different from the common view of organic polaritons. A new class of "dark" vibronic polaritons is introduced in which the photonic component is dressed by intramolecular vibrations. Dark polaritons allow consistent solutions to some of the long-standing puzzles in the interpretation of organic cavity photoluminescence.[3.4]

1. F. Herrera and F. C. Spano, Dark vibronic polaritons and the spectroscopy of organic microcavities, Physical Review Letters 118 (22), 6
(2017).
2. F. Herrera, Spano F.C., Theory of nanoscale organic cavities: The essential role of vibration-photon dressed states, ACS Photonics (in press) (2017).
3. P. A. Hobson, W. L. Barnes, D. G. Lidzey, G. A. Gehring, D. M. Whittaker, M. S. Skolnick and S. Walker, Strong exciton-photon coupling in a low-q all-metal mirror microcavity, Appl. Phys. Lett. 81 (19), 3519-3521 (2002).
4. T. Schwartz, J. A. Hutchison, J. Leonard, C. Genet, S. Haacke and T. W. Ebbesen, Polariton dynamics under strong light-molecule coupling, ChemPhysChem 14 (1), 125-131 (2013).   

Theory of Polariton Energy Transfer

Recent experiments have remarkably demonstrated distance-independent energy transfer between molecular excitons. This astonishing phenomenon occurs for spatially separated donors and acceptors coupled strongly to optical modes inside a microcavity. Such coupling results in hybrid states whose delocalization among donors, acceptors, and cavity photons allows the states to exchange energy at separation-invariant rates. However, a theoretical model that elucidates the mechanisms of this exchange has yet to be proposed. To address this need, we have developed the first comprehensive theory of polariton energy transfer. Specifically, our formalism describes the energetics of slabs of donor and acceptor molecules strongly coupled to surface plasmons. Application of this theory to cyanine dye J-aggregates affords experimentally consistent mechanistic insights and establishes strong light-matter coupling as a promising avenue for long-range energy transfer.   

Semiclassical description of microcavity assisted unimolecular chemical reactions

It has recently been observed that the reactivity of a molecule can be drastically affected by subjecting a particular (electronic or vibrational) degree of freedom to strong-coupling with a microcavity mode. Furthermore, QED-Chemistry calculations have shown how the energetic landscape of a molecule can be affected by strong coupling. In this light, we explore semiclassical approaches to the study unimolecular reaction kinetics taking place inside of a resonant cavity.   

Exploiting polaritonic chemistry to manipulate molecular structure and dynamics

Strong coupling is achieved when the coherent energy exchange between a confined electromagnetic field mode and material excitations becomes faster than the decay and decoherence of either constituent. This creates a paradigmatic hybrid quantum system with eigenstates that have mixed light-matter character (polaritons). It has recently been realized that polariton formation in organic molecules also affects their internal nuclear degrees of freedom, raising the possibility to manipulate and control reactions through polaritonic chemistry. I will first discuss our theoretical approach towards modeling such systems, based on extending the well-known Born-Oppenheimer approximation to describe polaritonic potential energy surfaces. I will then show various applications, such as the possibility to completely suppress reactions such as photoisomerization, which surprisingly works more efficiently when many molecules are coupled to a single light mode due to a “collective protection” effect. Finally, I will show how polaritonic chemistry can be exploited to allow many-molecule reactions triggered by a single photon. Here, the collective nature of polaritons and the resulting formation of a "supermolecule", in which a single excitation is distributed over many molecules, can enable a reaction involving the nuclear degrees of freedom of most or even all coupled molecules. This process can overcome the Stark-Einstein law of photochemistry, which states that a single photon will only induce a reaction in a single molecule and holds for most common cases.
Finally, I will discuss future perspectives, open questions, and remaining challenges to fully exploit the potential of polaritonic chemistry.   

Cavity magnetotransport of a two-dimensional electron gas in the ultrastrong coupling regime

We present a theory predicting how the linear magneto-conductivity tensor of a two-dimensional electron gas is changed by the ultrastrong coupling to a fully confined photon mode, proving how a passive cavity resonator can profoundly modify the dc electronic transport even when no real photon is injected. We show that the modification of the transport is due to: (i) dressing of both ground and excited states by the ultrastrong light-matter coupling; (ii) the diamagnetic current contribution due to the cavity mode. We demonstrate that in the regime of Shubnikov-de Haas oscillations the envelope of the longitudinal magnetoresistance as a function of the magnetic field can be significantly modified and becomes anisotropic, depending on the direction with respect to the cavity mode polarization vector.   

When Do Classical Fields Mimic Quantum-Light-Induced Molecular-Dynamics?

By using the Feshbach projection formalism, the problem on wether quantum-cavity chemistry can be mimicked by classical fields is addressed. Specifically, the following theorem is shown:

Consider a molecular system described by the Hamiltonian $H_m$ , with support on the Hilbert space $\mathcal{H}_m$, in interaction with a photon field characterized by the Hamiltonian $H_{bph}$, with support on the Hilbert space $H\mathcal{bph}$. If the state of the photon field is aimed not to be measured, then, up to second order in the interaction, it is not posible to distinguish between the dynamics induced by a classical electric field having the same statistics as the quantum electric field and the actual quantum photon field.

This theorem allows for understanding recent results on quantum-cavity chemistry and provides a solid ground for exploring genuinely quantum dynamics assisted by quantum-cavity states.   

Molecules in cavities: topological phases and polariton chemistry

I’ll showcase some opportunities that molecular polaritons offer within chemical physics. First, I’ll describe theoretical models on how topologically nontrivial phases can be realized in molecular polaritons. Next, I will discuss some intriguing predictions of photophysical and photochemical properties of molecules upon coupling to confined electromagnetic modes. These include changes in photoluminescence, energy transfer, and reaction dynamics. We conclude with lessons learned about the relevant time and energy scales to develop general strategies to harness molecular polaritons to control molecular processes.   

Correlated Light-Matter Interactions in Picocavities and Cavity QED

In recent years, research at the interface of chemistry, material science, and quantum optics has surged and opened new possibilities to study ultra-strong light-matter interactions. Besides collective strong coupling of many molecules for cavity-QED systems, recent experiments have demonstrated strong light-matter coupling in the single molecule regime for plasmonic picocavities [1]. In this talk, we show how such cavity optomechanical systems can alter chemical reactions such as photochemistry [2] using the methods of cavity Born-Oppenheimer approximation (CBOA) [3] and quantum-electrodynamical density functional theory (QEDFT) [4]. We will also discuss how phonons are formally included in QEDFT to predict new single molecule - cavity optomechanical effects.

[1] F. Benz et al., Science, 354 6313, 726-729 (2016).
[2] J. Flick, P. Narang, work in progress.
[3] J. Flick, et al. J. Chem. Theory Comput., 13 4, 1616-1625 (2017).
[4] M. Ruggenthaler et al., Phys. Rev. A 90, 012508 (2014).   

Theory for Nonlinear Spectroscopy of Vibrational Polaritons

Vibrational polaritons are hybrid excitations of optical resonators and molecules which offer new ways to control chemical processes in the electronic ground-state by introducing long-range coherence into molecular vibrational excited-states. In this talk, I will provide an overview of a recently-developed theory of pump-probe spectroscopy of vibrational polaritons based on the quantum Langevin equations. I will give emphasis to aspects of the polariton nonlinear optical response which are unique to the vibrational case, and show how information gathered from nonlinear optical measurements provides guidance for the manipulation of chemical reactivity in optical cavities.