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Bichromatic Structures for Robust propagation of Light

Periodic Reporting for period 1 - BISTRO-LIGHT (Bichromatic Structures for Robust propagation of Light)

Reporting period: 2017-09-01 to 2019-08-31

Topology has established itself as an invaluable tool to elucidate the properties of physical systems and to discover and/or understand novel physical phenomena. Recently, topological ideas have been transferred from the realm of electrons to that of light, to the point that we can currently speak of “topological photonics” as an autonomous field of research. Topological effects allow us to realize photonic circuits that are more robust against disorder and less sensitive to back-scattering, helping to decrease the power requirements for photonic devices and potentially paving the way to fault-tolerant quantum computation.
The Marie Skłodowska Curie action “BISTRO light” aims at strengthening the theoretical knowledge of topological photonic systems, by performing theoretical research that will improve our understanding of the topology of light fields and will allow us to harness topology-inspired phenomena for demonstrating new fundamental effects and designing better photonic devices. Particular focus is put on the concept of helicity for light fields in inhomogeneous optical media. The helicity and the spin of light play a crucial role for the determination of topological invariants in optical systems. This fact is illustrated, for instance, by the spin Hall effect of light, which was an early and inspiring discovery of the topological structure of a light field. Another primary goal of the action is to demonstrate the nontrivial topological properties of a particular class of photonic crystal structures, so-called bichromatic photonic crystals. These structures are based on the coexistence of two different periodicities in the dielectric constant profile and they can be realized with state-of-the-art fabrication techniques. They represent an ideal platform for exploring topological effects in photonic crystal systems.
The work performed during the action followed three main lines of research. In the first part of the action, theoretical research was performed to clarify and properly define the concept of helicity for photons in inhomogeneous dispersive optical media. This work has broad implications for studying topological invariants in photonic systems and for investigating the interaction with chiral molecules and magneto-electric emitters.
Later, attention has been focused on theoretically demonstrating the nontrivial topological properties of bichromatic photonic crystals by means of extensive numerical simulations. The nontrivial topology of the optical spectrum of bichromatic structures is illustrated by the formation of topologically protected, strongly localized boundary states when a finite-size extent of the bichromatic photonic crystal is embedded in a larger structure. Interestingly, these boundary states reveal a deep connection with condensed matter physics, since it can be proved that they are equivalent to the topologically-protected edge states of the integer quantum Hall effect.
Finally, the third line of research aims at creating a collaborative theoretical-experimental environment. In order to do this, the Fellow has been actively involved in several experimental projects with other members of the hosting Institution, resulting in joint theoretical-experimental work on topological systems in photonics. This collaboration led to the demonstration of a chiral interface based on the coupling between the local spin of light and the valley degree of freedom of two-dimensional excitons in transition-metal-dichalcogenide monolayers and the investigation of the properties of topological points (phase and polarization singularities) in random light fields.
In general, the results of the action have been extremely satisfactory and even exceeding the expectations of the research proposal. The findings have been the subject of several scientific articles in top international journals (6 articles in journals with impact factor > 7). Moreover, they have been presented at three international conferences and several workshops and meetings.
The impact of the action on the research community has been significant, with many theoretical and experimental collaborations having been established during the project. The existence of this network of collaborations ensures that the results of the action will be further developed from both the experimental and the theoretical sides.
From a broader perspective, the results of the action bring the study of the topological properties of optical and near-infrared light to the nanoscale, i.e. beyond the smallest length scale allowed by the diffraction limit. Indeed, photonic crystal nanostructures already display some of the smallest confinement volumes for light together with very low losses, leading to a dramatic enhancement of nonlinear effects and of the interaction with quantum emitters. Combining these striking features with topological effects may pave the way for significant advances in the field of photonics with applications for light manipulation and information processing.
The computed spectrum of bichromatic photonic crystals as a function of the period mismatch.
A bichromatic photonic structure compared with a traditional photonic crystal waveguide.