Captivating cavities
Laser technology is a familiar example of how confining light between two mirrors can tune its properties. Quantum mechanics also dictates that even without extraneous light, matter confined in a cavity resonant with its electronic or vibrational transitions can couple with vacuum electromagnetic field fluctuations. Garcia-Vidal et al. review the remarkable and still somewhat mysterious implications of this “strong-coupling” regime, with manifestations ranging from enhanced charge transport to site-selective chemical reactivity across a range of molecular and solid-state materials.
Science, abd0336, this issue p. eabd0336
Structured Abstract
BACKGROUND
One of the most important phenomena in cavity quantum electrodynamics (cQED) is the so-called strong coupling regime, which appears when the interaction between a photon tightly confined in an optical cavity and a matter excitation creates hybrid light-matter states. When the latter are populated, hybrid particles called polaritons are formed. These particles are very attractive because they combine properties of their constituents, which enables applications ranging from low-threshold lasing in semiconductors to photon quantum information. Since its discovery, most of the investigations on strong coupling have been aimed mainly toward the modification of optical properties. During the past decade, an alternative area of research has emerged that takes advantage of collective strong coupling to take chemistry and materials science into new directions. For this purpose, no external light source is necessary as the hybrid light-matter states are formed even in the dark because the coupling occurs through the zero-point energy of the optical mode (i.e., the vacuum field). The mere presence of the hybrid states has a substantial effect on material properties, as reviewed here.
ADVANCES
Both experimental and theoretical studies have shown changes to photochemical reaction rates under strong coupling between the electronic excitations of molecules and cavity electromagnetic modes. Strong coupling modifies the shape of the potential energy surfaces associated with the excited states of the molecule, allowing for a manipulation of its photophysical properties. Moreover, ground-state chemical reactivity can also be completely modified when molecular vibrations are strongly coupled to infrared cavity modes. Although a detailed picture of the mechanism is still missing, symmetry seems to play a key role. Material properties can also be changed by strong coupling. Charge and energy transport in organic materials and magneto-conductivity in two-dimensional electron gases have been shown to be altered. Thanks to the intrinsic delocalized character of the polaritonic modes, transport properties can be then tuned at a macroscopic scale. It is also feasible to manipulate phases of matter by means of strong coupling. It has been reported that the critical temperature of a superconductor can be substantially enhanced by judiciously exploiting vibrational strong coupling and that the ferromagnetism of nanoparticles can be boosted by orders of magnitude. These examples illustrate the potential of using vacuum fields instead of intense laser fields to induce modification of material properties.
OUTLOOK
There are many classes of organic reactions that are currently being explored under strong coupling. As more results are collected, the underlying physical chemistry will be further clarified and should lead to some general principles to guide chemists and physicists in their use of vibrational strong coupling. The recent demonstrations that water, under vibrational strong coupling, modifies enzyme activity illustrates the potential for manipulating biological activity under strong coupling—an avenue that remains unexplored. Regarding solid-state material properties, the influence of strong coupling in phonon-based phase transitions should also be fully explored, aiming at inducing new condensed phases. Moreover, cavity-controlled magneto-transport might reach the quantum Hall regime. In general, two-dimensional materials are very well suited to be integrated in cavity resonators with deeply subwavelength photon confinement, which provides an intriguing platform to modify electronic properties through vacuum fields.
(Left) Charge transfer complexation between mesitylene and iodide (courtesy of K. Nagarajan). (Right) Energy transfer between donor and acceptor molecules (courtesy of J. Galego).
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(Left) Charge transfer complexation between mesitylene and iodide (courtesy of K. Nagarajan). (Right) Energy transfer between donor and acceptor molecules (courtesy of J. Galego).
Abstract
Over the past decade, there has been a surge of interest in the ability of hybrid light-matter states to control the properties of matter and chemical reactivity. Such hybrid states can be generated by simply placing a material in the spatially confined electromagnetic field of an optical resonator, such as that provided by two parallel mirrors. This occurs even in the dark because it is electromagnetic fluctuations of the cavity (the vacuum field) that strongly couple with the material. Experimental and theoretical studies have shown that the mere presence of these hybrid states can enhance properties such as transport, magnetism, and superconductivity and modify (bio)chemical reactivity. This emerging field is highly multidisciplinary, and much of its potential has yet to be explored.