Wavepacket Self‐Rotation and Helical Zitterbewegung in Symmetry‐Broken Honeycomb Lattices

Abstract

The toolbox quantities used for manipulating the flow of light include typically amplitude, phase, and polarization. Pseudospins, such as those arising from valley degrees of freedom in photonic structures, have recently emerged as an excellent candidate for this toolbox, in parallel with rapid development of spintronics and valleytronics in condensed-matter physics. Here, by employing symmetry-broken honeycomb photonic lattices, valley-dependent wavepacket self-rotation manifested in spiraling intensity patterns is demonstrated, which occurs without any initial orbital angular momentum. Theoretically, it is shown that such wavepacket self-rotation is induced by the Berry phase and results in Zitterbewegung oscillations. The frequency of Zitterbewegung is proportional to the gap size, while the helicity of self-rotation is valley-dependent, i.e., correlated with the Berry curvature. These results lead to new understanding of the venerable Zitterbewegung phenomenon from the perspective of topology and are readily applicable on other platforms such as 2D Dirac materials and ultracold atoms.

Flat band, tunable chiral anomaly and pitchfork bifurcation in a honeycomb lattice

“…The proposed phenomena, however, could be readily realized in photonic and nanomechanical metamaterials. In photonic crystals the pseudo-electric fields can be created by the site-to-site laser writing [50]. The second extremely promising experimental playground is the platform of "nanomechanical graphene"-a honeycomb lattice of free-standing Si 3 N 4 nanomechanical membranes with a modified lattice structure [51].…”

Mapping and Manipulation of Topological Singularities: From Photonic Graphene to T-Graphene

“…In most previous studies, optical vortices were typically generated with spatial light modulators, q-plates, metasurfaces, or active vortex generators. − On the other hand, a Dirac point in the momentum space similar to that displayed by graphene can be considered a topological singularity (TS) with nontrivial Berry phase winding. In recent experiments, it has been demonstrated that optical vortices can be generated by mapping the TSs from momentum to real space using photonic structures. − This brings about a new way for the generation, manipulation, and transformation of optical vortex beams, along with new features mediated by pseudospin-to-OAM conversion. ,,,, …”

“…In photonics, waveguide arrays are arranged to form HCLs, namely, photonic graphene, where light propagation can mimic electronic transport properties in graphene. Based on such photonic graphene lattices, we have observed several intriguing phenomena in a series of experiments, including unconventional edge states, valley Landau–Zener–Bloch oscillations, valley-dependent vortex states, and wavepacket self-rotation, in addition to pseudospin angular momentum . In particular, under proper alignment of the pseudospin states near the Dirac points, optical vortices of different topological charges can be generated via momentum-to-real-space TS mapping .…”