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

TL;DR

This paper reports the observation of valley Landau-Zener-Bloch oscillations and pseudospin imbalance in photonic graphene, revealing how symmetry and potential direction influence non-adiabatic beam dynamics near Dirac points.

Contribution

It demonstrates how symmetry-preserving and symmetry-breaking potentials affect inter-valley oscillations, LZT, and pseudospin in a honeycomb photonic lattice, highlighting new control mechanisms.

Findings

Symmetry-preserving potential leads to near-perfect LZT and coherent BO.

Symmetry-breaking potential causes asymmetric scattering and vortex generation.

Transverse gradient effects depend on sublattice symmetry, not just scalar force.

Abstract

We demonstrate inter-valley 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 non-adiabatic 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 graphene-like systems.