CM – Exploring the limits of light-matter coupling on the nanoscale


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August 9, 2021

from ETH Zurich

The interplay of light and matter encompasses an impressive spectrum of phenomena, from photosynthesis to the fascinating colors of rainbows and butterfly wings. As diverse as these manifestations may be, they contain a very weak light-matter coupling – essentially light interacts with the material system, but does not change its fundamental properties. However, a distinctly different set of phenomena occurs in systems artificially engineered to maximize light-matter coupling. Then fascinating quantum states can arise that are neither light nor matter, but a mixture of both. Such states are of great interest both from a fundamental point of view and for the development of novel functionalities, for example to enable interactions between photons. The strongest couplings to date have been realized with semiconductor materials that are limited to tiny photonic cavities. In these devices, coupling is typically increased by making the cavity smaller and smaller. But even if the associated manufacturing challenges can be overcome, the approach will reach fundamental physical limits, as a team led by Professors Giacomo Scalari and Jérôme Faist from the Institute of Quantum Electronics reported in an article published today in Nature Photonics. With this work you set quantitative limits to the miniaturization of such nanophotonic devices.

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In the last four decades different platforms have been developed to achieve a strong coupling between light and matter. Among them is an experimental pioneering work by Scalari in the Faist group, which has been offering one of the strongest light-matter couplings almost continuously since 2011 that has been realized across all platforms. It is important that in the course of ever new records they reached the « ultra-strong » regime in which the light-matter coupling is comparable with the relevant energies of the uncoupled matter system and thus enables access to a wealth of new phenomena.

At the heart of them Record platforms are so-called metallic split-ring resonators (see figure), in which electromagnetic fields can be localized in extremely small volumes far below the wavelength of light – typically terahertz radiation (THz). The micrometer-sized gaps in these resonators are loaded with semiconductor quantum wells with suitable electronic properties in order to serve as a system of matter. A natural way to increase the coupling between excitations in the quantum wells and the light confined in the resonator is then to decrease the width of the gap (d in the figure). How strong a coupling can be constructed in this way, however, remained open.

Shima Rajabali, a Ph.D. Thanks to quantum troughs that their chief scientist Mattias Beck has grown, and a theoretical study by Simone De Liberato and Erika Cortese at the University of Southampton (UK), students in the group of Scalari and Faist have now theoretically and experimentally investigated whether there is a fundamental physical Limit for the restriction to sub-wavelengths in such systems. The team found that as the electromagnetic field is concentrated into smaller and smaller volumes, at some point the nature of light-matter hybrids (known as polaritons in their case) begins to change. This fundamental change in the polaritonic properties in turn prevents a further increase in the coupling strength.

This limitation is not a distant scenario. Signatures of this paradigm shift can already be found in modern nanophotonic devices. Just that there was no sure understanding of the underlying reasons. This loophole is now addressed by Rajabali et al. In addition, their newly developed framework could apply not only to the specific devices they are investigating, but also to other nano-optical systems, for example those based on graphene or transition metal dichalcogenides (TMDs), and for resonator geometries other than split-ring resonators. As such, the new work on light-matter coupling should set general quantitative limits.

To explore the limits of increasing light-matter coupling by decreasing the sub-wavelength volume to which light is confined, the team developed a theoretical framework whose predictions have been tested experimentally and in computer simulations. An important result was that on the smallest length scales considered – they examined devices with gaps of up to 250 nanometers wide – non-local effects occurred. This is due to the fact that below a critical length scale the narrowly limited light field in the resonator not only couples to the bound electronic states of the quantum well, but to a continuum of excitations with high momentum that originate from a known two-dimensional plasmon dispersion in the quantum well. This opens up new channels of loss and ultimately fundamentally changes how light and matter interact in these nanophotonic devices.

Rajabali and colleagues show that this transformation into a regime dominated by polaritonic non-locality leads to phenomena that differ from the classical and linear Quantum theories normally used to model the interplay between light and matter cannot be reproduced. In other words, we can be sure that much remains to be explored in the fascinating arena of light-matter interaction.

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Associated title :
Exploring Limits of Light Matter Coupling on the nanoscale
Investigation of the limits of light-matter coupling on the nanoscale
Exploring the limits of the combination of light and matter on the nanoscale


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