A metasurface of split-ring resonances, partially overlayed with 3D colourmaps that show the simulated electric field distribution. High-momentum magnetoplasmons cause the breakdown of polaritons, which are blue spheres with red photon energies. Credit: U. Senica, ETH ZurichThe fascinating interplay of light and matter includes a wide range of phenomena, including photosynthesis and the stunning colors of rainbows or butterfly wings. These manifestations can be as diverse as they are, but they all involve weak light-matter cohesion. In essence, light interacts with the matter system, but it does not alter its fundamental properties. Systems that have been designed to maximize light-matter interaction will produce a distinct set of phenomena. These quantum states are not light or matter but can be intriguing. These states are both of great interest from a fundamental perspective and for creating new functionalities, such as enabling interactions between photons. It has been possible to achieve the strongest couplings with semiconductor materials that are contained within tiny photonic cavities. These devices increase the coupling by shrinking the cavity. Even if the associated fabrication challenges can easily be overcome, the approach will soon face fundamental physical limitations, as Professors Giacomo Scalari (Institute of Quantum Electronics) report in a paper published in Nature Photonics today. They have established quantitative limits for the miniaturization and use of nanophotonic devices.From strength to strengthOver the past 40 years, many platforms have been created to achieve strong coupling between matter and light. Scalari's Faist group pioneered one that has been used experimentally almost continuously since 2011. It is one of the strongest light/matter couplings across all platforms. They also set new records by reaching the "ultrastrong" category, where light-matter coupling is comparable with the relevant energies in the uncoupled material system. This allows for the discovery of a variety of interesting phenomena.The heart of their record-breaking platform is the so-called metallic-split-ring resonators (see figure), which allow electromagnetic fields to be concentrated in very small volumes well below the wavelength of lighttypically terahertz radiationinvolved. These resonators have gaps that are only micrometers wide and are filled with semiconductor quantum wells. They also serve as the matter system. The width of the gap can be decreased to increase the coupling between the excitations in quantum wells and light contained in the resonator (d in figure). It remains to be seen how strong this coupling can still be achieved.But within acceptable limitsShima Rajabali is a Ph.D student in Scalari and Faist. She was inspired by quantum wells developed by Mattias Beck, and a theory study done by Simone De Liberato (UK) to determine if there is a physical limit to subwavelength confinement within such systems. The team discovered that there is indeed a limit to the subwavelength confinement of electromagnetic fields in such systems. If the electromagnetic field becomes concentrated into smaller volumes, then the nature of light-matter hybrid state (in their case, polaritons), starts changing. This fundamental change in the polaritonic features prevents any further increase in coupling strength.SEM image of unit cell with split-ring resonator. Credit: Adapted by S. Rajabali and al. doi: 10.1038/s41566-021-00854-3, Nature Photonics (2021)This is not a distant possibility. This paradigm shift has been observed in nanophotonic devices that are state-of the-art. However, there is still no clear understanding of the reasons. Rajabali and colleagues have now filled this gap. Rajabali et al. have now filled this gap by developing a framework that could be applied to not only the devices they studied but also to other nano-optical system, such as those made of graphene and transition metal dichalcogenides, or for other resonator geometries than split-ring. The new research should also provide quantitative limits on light-matter coupling.Go beyond the localThe team devised a theoretical framework to investigate the limits of increasing light-matter cohesion by decreasing the subwavelength volume in which light is contained. They tested these predictions experimentally and in computer simulations. One of the key findings was that devices with gaps less than 250 nanometres widenon local effects. These results are due to the fact the below-a-critical length scale provides carriers with a large in-plane momentum. The tightly confined lightfield in the resonator also couples to bound electronic state of the quantum and to a continuum high-momentum exccitations that result from a two-dimensional plasmon dipersion in quantum well. This creates new loss channels that eventually change the fundamental nature of light and matter interaction in these nanophotonic devices.Rajabali and his colleagues have shown that the transformation into a regime governed polaritonic not-locality leads to phenomena that cannot possibly be replicated by classical or linear quantum theories that are used to model the interaction between light and matter. We can therefore rest assured that there is still much to explore in the fascinating field of light-matter interaction.Continue reading Researchers create ultra-strong coupling between photons, atomsShima Rajabali and colleagues, Polaritonic Nonlocality in Lightmatter Interaction, Nature Photonics (2021). Information from Nature Photonics Shima Rajabali and colleagues, Polaritonic Nonlocality in Lightmatter Interaction, (2021). DOI: 10.1038/s41566-021-00854-3
