Scientists at the Mac Planck Institute for the structure and dynamics of matter MPSD and the Massa­chusetts Insitute of Technology have come up with a prediction that trapped light can be used to create a new kind of particle in a solid, consisting of three components at once: Light (photons), electronic exci­tations (excitons) and lattice vibrations (phonons). The use of light trapped in a cavity represents a completely new way to change the behavior of a material by intro­ducing new inter­actions between its micro­scopic components.

The team has discovered that light in a very confined space can be used to control the coupling between electronic exci­tations and the vibration of the nuclei – with remarkable effects. The team’s work demonstrates that controlling this coupling process makes it possible to change the way the material absorbs light. These findings highlight the vast potential of confined light for designing inter­actions among the micro­scopic components of a material. 

The theory group of the MPSD pursues innovative material design approaches, such as engi­neering material properties via the strong coupling between light and matter that can be achieved when a material is placed inside a cavity. The two metal plates of the cavity confine the light in a narrow space and enhance the interaction between photons and the particles in the embedded material. The strong coupling between light and matter particles can dras­tically change the material‘s properties, potentially leading to novel physics. 

In their colla­borative work, the researchers predict from first-principles calcu­lations the emergence of a three-way particle resulting from the mixture between excitons, phonons and photons inside a cavity. This light-matter hybrid particle, dubbed a phonoriton, was predicted more than three decades ago by Ivanov and Keldysh. However, it has thus far eluded detection because it was originally proposed in the context of laser physics for laser intensity regimes that would simply destroy the material.

The use of a cavity to enhance the coupling of light with matter not only solves the problem of the destructive laser inten­sities but also puts the long predicted phonoriton in solids into the range of experi­mentally realizable conditions, thus enabling the observation of the first three-components particle in solids. In addition to demonstrating the existence of the phonoriton, the team provides experi­mentally measurable signatures that can be observed in optical absorption and can help to uniquely identify this novel light-matter particle. 

These findings are significant because they create a new paradigm for the study of excitations in materials, which have thus far only been thought of as involving a maximum of two constituent particles such as polarons, excitons, polaritons, plasma­rons etc. The phonoriton paradigm opens up this zoo of quasi­particles for many novel members arising from three-way combinations. “This work is a neat example of how light can be used as tool to design inter­actions between the microscopic components of matter,“ says Simone Latini, a post-doc and former Humboldt fellow at the MPSD. “An experimental veri­fication of our predictions would be extremely exciting. Our experi­mental colla­borators are already working on it and we cannot wait to see their results!”

"I am very excited about this new paradigm of designing inter­actions in solids by combining known excitations," adds Hannes Hübener, a group leader at the MPSD Theory department. Fellow MPSD group leader Umberto de Gio­vannini, also from the theory department, is thrilled about the findings: “It is great to give new life to a particle proposed a long time ago by Ivanon and Keldysh but never actually observed. This constant work of reviewing and refining known notions as well as the cross-ferti­lization between concepts lies at the very heart of science.” The team believes that this study only scratches the surface of the potential of cavity matter engi­neering. It hopes that its results will motivate the scientific community to dig deeper and unveil new exciting physics. (Source: MPSD)

Reference: S. Latini et al.: Phonoritons as Hybridized Exciton-Photon-Phonon Excitations in a Monolayer h-BN Optical Cavity, Phys. Rev. Lett. 126, 227401 (2021); DOI: 10.1103/PhysRevLett.126.227401

Link: Theory Dept., Max-Planck-Institute for Structure and Dynamics of Matter MPSD, Hamburg, Germany