September 03, 2024 -- Topological systems are a foundational and versatile class of physical systems, ubiquitous across many fields of physics, with far-reaching implications for both fundamental research and applied technological advancements. These systems are characterized by properties that make them resilient to perturbations. In simpler terms, they possess certain qualities that are not easily affected by external factors, like disorder or changes in other physical conditions.

This resilience to perturbations is what makes topological systems so important in physics, as it means their behaviour can be very predictable and reliable under a wide range of conditions. Of particular interest are the topological edge states: states that exist at the boundary of a material and that cannot be suppressed without breaking the material’s symmetry.

Topological Nanophotonics: challenges and opportunities

The scientific community, typically attracted by those challenges that are as complex as they are promising, sees the task of bringing topological properties down to the nanoscale as an appealing challenge. Complex because of the ultimate spatial limits of electromagnetic field manipulation that are required; and promising because of the expected consequences, being both fundamental and applied.

To begin with, miniaturizing topological properties to such small physical scales (in the jargon, the deep subwavelength regime) would allow the scientific community to explore exotic physical phenomena (nonlinearities, non-locality, multimodal interactions…) that are expected to arise under these circumstances. On a more practical side, topological systems’ inherent robustness and protection can be harnessed to develop more resilient deep subwavelength optical components, such as nanocavities or fabrication-disorder tolerant waveguides.

Despite the experimental progress towards achieving topological edge states in the deep subwavelength regime, each proposed platform exhibits some benefits but also some major drawbacks, maintaining the 'hot-topic' status in the search for the ultimate solution.

An international team with ICFO researchers Lorenzo Orsini, Dr. Hanan Herzig Sheinfux, Matteo Ceccanti, Karuppasamy Soundarapandian, led by ICREA Prof. at ICFO Dr. Frank H. L. Koppens, and in collaboration with Cornell University, CNRS, University of Cambridge and Kansas State University, has now reported in a Nature Nanotechnology article a substantial advancement in this regard. For the first time, they have demonstrated a deep subwavelength topological edge state within a nanophotonic system, where the chosen platform -typically not considered by the topological Nanophotonics community- was based on the so-called hyperbolic phonon-polaritons (in short, HPhPs). Not only did they confine light into such small size scales, but they also maintained high quality factors through the whole process.

Why hyperbolic phonon-polaritons?

Hyperbolic phonon-polaritons are a type of collective electromagnetic excitation that occurs in hyperbolic materials, where electromagnetic waves (photons) couple with the quanta of vibrations within the atomic lattice of a material (optical phonons). These HPhPs allow light to be confined and guided in very small volumes or along surfaces.  

Thanks to their special features, HPhPs overcome the challenges faced by previous methods for studying topological properties at the nanoscale. These limitations include, for instance, high optical absorption —which is detrimental to reaching the deep subwavelength regime in the case of plasmon polaritons—, fabrication difficulties and the need for cryogenic temperatures —which hinder the realization of topological states in the case of graphene.

With hyperbolic phonon-polaritons these problems are minimized, since they exhibit low absorption even at room temperature and can be relatively easy to fabricate. These features, together with the fact that they allow for high volume confinement, give HPhPs excellent performance characteristics. As attractive as they may seem, HPhPs have been largely unexplored for topological applications due to their deeply complex nature, which has hindered theoretical development in this area.

Nevertheless, the team saw in the hyperbolic phonon polaritons great promise, and their ambitious goal stir them into action. “At the beginning of the project, it was uncertain how these edge states would manifest and what specific properties they would exhibit”, shares Lorenzo Orsini, first author of the article. “While we anticipated their formation, finally observing them in our experiments was a fascinating confirmation of our expectations and an exciting development in the field”.

The experimental setup that led to success

To enable the emergence of topological edge states, the team constructed a one-dimensional polaritonic lattice platform.

First, they sharply defined rectangular holes periodically milled through a 10-nanometer gold film. On top of that, they placed a tens of nanometers thick hexagonal Boron Nitride (hBN) flake. In there, hyperbolic phonon-polaritons would be hosted. The gold layer structure was engineered such that there were two different distributions of the rectangular holes, defining two distinct regions, one next to the other. The researchers expected that, due to the presence of two different arrangements, topological edge states would form right at the boundary where both meet.

After the fabrication process, the characterization and analysis of the system took place. They used the s-SNOM (Scattering-type Scanning Near-field Optical Microscopy) spectroscopy technique to confirm the existence of a localized edge state and that it was indeed in the deep subwavelength regime.

Because of the scarcity of theoretical models, they had to rely heavily on their own experimental results, which made the need to meticulously prove and double-check each step even more crucial than usual. This rigorous, step-by-step approach allowed them to refine the experimental design and achieve clear, reliable results, finally demonstrating deep subwavelength topological edge states within an HPhPs platform.

The powerful potential of HPhPs platforms

In addition to the relevance inherently present in this milestone by itself, the present study also constitutes a big step forward toward the precise control of light at the nanoscale, offering an alternative platform for the realization and investigation of topological physics in nanophotonic systems. Moreover, the authors claim that their results can be extrapolated to other hyperbolic materials, something that would facilitate a broader coverage of the electromagnetic spectrum.

As Orsini concludes: “In the end, taking a chance on hyperbolic phonon polaritons paid off, and now we have opened new possibilities for robust and precise control of light at the nanoscale. A continued exploration and development in this direction could, in turn, lead to breakthroughs in areas as diverse as telecommunications, sensing technologies or quantum information processing”.