UC Berkeley

The physical properties of crystalized materials are closely related to the symmetry elements of the point group of the crystal. To the contrary, engineered symmetry breaking would allow the emergence of new physical phenomena which are originally forbidden. With the successful exfoliation of two-dimensional Van der Waals materials, the restricted stacking sequence between layers is relaxed and thus, the symmetry of artificially stacked heterostructure can be readily controlled.

In this dissertation, we present two approaches to engineer the symmetry of solid-state and photonic two-dimensional systems respectively. The resulting optical and topological property changes are explored and related back to the underlying symmetry breaking. Chapter 1 gives an overview of the various symmetry breaking mechanisms and their distinct consequences. The emphasis is given to the inversion-symmetry breaking which can be easily implemented in two-dimensional systems via structural or electrical means. In Chapter 2, we show that introducing a twisting between two layers of graphene, the symmetry of bilayer structure is decisively changed, and new nonlinear optical effects are allowable. We further verify that the observed nonlinear signals are controlled by the composited bilayer instead of the linear combination of two individual layers experimentally and confirmed by DFT calculations. The methodology proposed is generic and can be applied to other composite systems. In Chapter 3, we break the symmetry of two-dimensional photonic lattice by ‘tilting’ the constitutive parameter tensors and demonstrate spin degeneracy lifting via electromagnetic simulations. The approach allows us to enlarge the scale of spin-orbit coupling effect in a photonic platform and at the same time, the controllability of spin-polarized energy bands associated with symmetry is revealed.

To sum up, we propose two methods to engineer the symmetry of the 2D system in a controllable way and show their potential for new physical properties and applications.