Condensed matter physics knows of a “magic” angle between graphene layers, wherein the material’s properties drastically change, creating superconductivity. Turns out, photonics can boast its own counterpart – the invisibility angle. If two waveguides are placed at this angle, they turn invisible to each other. This means that a signal sent into an array of such waveguides propagates without losing its intensity, remaining localized in a small group of channels.
Earlier, such effects were observed in one-dimensional photonic structures, but attempts to extend this behavior into two-dimensional ones had been unsuccessful for a long time. In the experiment conducted by ITMO and the University of Chile, the researchers succeeded in attaining the invisibility effect in a two-dimensional waveguide system.
First, the collaborators experimentally demonstrated the existence of an invisibility angle. The entire waveguide system was written by a laser in glass: when subjected to radiation, glass changes its refraction index and by moving the laser, it’s possible to draw a single waveguide or even an array of them. As demonstrated by the experiment, if the waveguides are placed at the angle of about 55° to each other, the light transition between them comes down to almost zero.
Credit: Nano Letters
After that, the researchers engineered a two-dimensional waveguide lattice in which certain angles are equal to the invisibility angle. The previously observed effect was recreated: the light that was passed down a central pair of channels exhibited nearly no scattering, instead remaining localized within a specific area.
Credit: Nano Letters
There was also another interesting effect: “If the light is aimed not at the center of the lattice, but at the top pair of waveguides, the signal is localized better. This is explained not only by the invisibility angle, but also by the lattice’s topology: it has a special geometry and symmetry that create this particular topological state at the angle. In this case, light is ‘locked’ at the edge of the structure and cannot propagate deeper into the lattice,” shares Maxim Mazanov, one of the paper’s authors and a PhD student at ITMO.
Credit: Nano Letters
This discovery has significant practical implications. “When a light beam propagates in nearly any structure, it scatters laterally. This is an effect known as diffraction, which limits the resolution of lenses, telescopes, and other optical devices. With the photon structure we suggested, it’s possible to overcome this diffraction effect – and overcome it, most importantly, for the visible light,” explains Maxim Gorlach, one of the paper’s authors and the head of a frontier laboratory at ITMO.
This research project was supported by ITMO University’s Development Strategy and the Russian Science Foundation grant No. 24-72-10069.
