Observation of valley Landau-Zener-Bloch oscillations and pseudospin imbalance in photonic graphene

• Main Authors: Sun, Yong, Leykam, Daniel, Nenni, Stephen, Song, Daohong, Chen, Hong, Chong, Yi Dong, Chen, Zhigang
• Other Authors: School of Physical and Mathematical Sciences
• Format: Article
• Language: English
• Published: 2018
• Subjects: DRNTU::Science::Physics; Bloch Oscillation (BO); Landau-Zener Tunneling (LZT)
• Online Access: https://hdl.handle.net/10356/87902; http://hdl.handle.net/10220/45605
• Institution: Nanyang Technological University

We demonstrate intervalley Bloch oscillation (BO) and Landau-Zener tunneling (LZT) in an optically induced honeycomb lattice with a refractive-index gradient. Unlike previously observed BO in a gapped square lattice, we show nonadiabatic beam dynamics that are highly sensitive to the direction of the index gradient and the choice of the Dirac cones. In particular, a symmetry-preserving potential leads to nearly perfect LZT and coherent BO between the inequivalent valleys, whereas a symmetry-breaking potential generates asymmetric scattering, imperfect LZT, and valley-sensitive generation of vortices mediated by a pseudospin imbalance. This clearly indicates that, near the Dirac points, the transverse gradient does not always act as a simple scalar force, as commonly assumed, and the LZT probability is strongly affected by the sublattice symmetry as analyzed from an effective Landau-Zener Hamiltonian. Our results illustrate the anisotropic response of an otherwise isotropic Dirac platform to real-space potentials acting as strong driving fields, which may be useful for manipulation of pseudospin and valley degrees of freedom in graphenelike systems.