[Enlarge image]Vortex and anti-vortex nanolasers based on topological disclination cavities.
Manipulating vector vortex beams with orbital angular momentum (OAM) has attracted great interest in photonics, as such beams can potentially be used to provide separate spatial channels for data transfer. While generating vortex beams in bulk systems is straightforward, however, achieving them from compact light sources is challenging.
Nanophotonics, which allows vortex beams to be generated via engineered nanostructures, has advanced these efforts. Yet despite such progress, developing ultrasmall photonic nanocavities with optical vortex modes remains difficult, with unavoidable, substantial scattering loss and high energy consumption in compact devices. This year, we proposed a topological technique to create the smallest on-chip vortex nanolaser.1
In general, fractional topological charges can be localized at corners by identifying higher-order topological insulators (HOTIs). Since conventional HOTIs have limitations in compactly localizing multi-corner states,2 we used the disclination of topological crystalline insulators, a class of topological defects that has been shown to trap topological states at the defect boundary.3
In our work, we employed the Volterra process to create disclination geometries with Cn symmetry. For example, a C5 symmetric disclination structure was formed by cutting the lattice along one side, adding a sector, and rejoining the remaining sections with a Frank angle of Ω = π/2, resulting in a nominally pentagonal shape.
We investigated the angular momenta of disclination structures using the tight-binding model and compared them with photonic disclination cavities. Localized bound states of C5 symmetric disclination core showed probability-density distributions with angular momenta l at the interior corners. Similarly, we obtained eigenmodes with l in photonic disclination cavities.
The C5 photonic disclination cavities produced vectorial combinations of OAM light with central singularities in the electric fields, leading to the formation of polarization vortices—opposite handedness OAM coupled to opposite handedness polarization, also known as vector vortex beams. These cavities exhibited mode volumes of less than λ3.
In the experiment, the cavity structure consisted of a 275-nm-thick InGaAsP slab with an air hole radius of ~90 nm and a lattice constant of 500 nm. The vortex-lasing properties were investigated by observing doughnut-shaped intensity profiles in disclination cavities with expanded or shrunken cores. In addition, we characterized the vortex/anti-vortex lasing modes by measuring polarization-resolved mode images, Stokes parameters and self-interference patterns. As a result, the expanded-core cavity has a topological charge of +1 (vortex), while the shrunken-core cavity has a topological charge of −1 (anti-vortex).
Our results show how the physics of one system may be effectively translated to another, highlighting the importance of topology in various systems, from matter to light. Future exploration of photonic disclination cavities can lead to lasers with the symmetry breaking necessary for precise, on-demand OAM emission.
Researchers
Min-Soo Hwang, Hanyang University, Seoul, Republic of Korea
Yuri Kivshar, Australian National University, Canberra, Australia
Hong-Gyu Park, Seoul National University, Seoul, Republic of Korea
References
1. M.-S. Hwang et al. Nat. Photonics 18, 286 (2024).
2. H.-R. Kim et al. Nat. Commun. 11, 5758 (2020).
3. C.W. Peterson et al. Nature 589, 376 (2021).