Nonlinear responses in inversion-breaking 2D lattices

Sensing, detecting and manipulating electromagnetic waves are in general nonlinear processes based on optical or electrical rectification processes. Such processes are conventionally based on semiconductor p-n junctions, in which a strong built-in electric field provides the directionality to rectify signals. Such a mechanism has been challenging in the THz and infrared regimes, calling for new physics and materials. Intriguingly, the intrinsic crystal directionality, set by either the atomic lattice or spin orientation, can be translated into directional properties of electron quantum wavefunctions (Berry phase, Berry curvature, etc.), and rectify electron’s motion in a dramatic way. In this talk, I will show that a dipole moment of Berry curvature can lead to a strong nonlinear (AC excitation, DC output or double-frequency output) Hall effect in non-magnetic bilayer WTe₂ without external magnetic field. I will also show our recent studies of nonlinear electrical and optical responses of bilayer graphene whose inversion symmetry can be broken by an out-of-plane electric field.