Novel Nonlinear Optical Effects

The control over electron spin is indispensable for many applications, such as spintronics. We proposed a bulk spin photovoltaic (BSPV) effect, based on which light illumination can be used to generate spin current. Through symmetry analyses, we revealed that the only requirement of the BSPV effect is inversion symmetry breaking, and that mirror symmetry can engender the generation of pure spin current. Besides, we also showcased a nonlinear Edelstein effect (NLEE), by which light can control static magnetism, particularly ultrafast magnetic domain switching. We are currently collaborating with the experimental teams to further study both the BSPV and NLEE.

Related publications:

• Haowei Xu, Hua Wang, Jian Zhou, and Ju Li, Pure Spin Photocurrent in Non-centrosymmetric Crystals: Bulk Spin Photovoltaic Effect. Nature Commun. 12, 4330 (2021)

• Haowei Xu, Jian Zhou, Hua Wang, and Ju Li, Light-Induced Static Magnetization: Nonlinear Edelstein Effect. Phys. Rev. B 103, 205417 (2021).

• Haowei Xu, Ju Li, Linear and nonlinear Edelstein effects in chiral topological semimetals, Materials Today Quantum, 5 100022 (2025)

By now most theoretical and computational works on NLO effects rely on the independent particle approximation (IPA), which assumes that electrons are (nearly) free particles and ignores many-body interactions, such as electron-electron correlations. This is unsatisfactory since many-body interactions widely exist and can play a crucial role in NLO responses.

We formulate a generic Green’s function framework, which can be used to deal with various many-body interactions. We first applied this framework to the surface states of topological materials, where the surface-bulk interactions are incorporated into the Green's function. We reveal that the NLO responses on the surface can be disparate from those in the bulk - even the direction of the BPV current can be opposite on the surface. We clarify that this is mainly a topological effect, and can be absent in topologically trivial materials.

We have also applied the Green's function to incorporate electron-phonon coupling. We find that when phonons are out-of-equilibrium, there can be various interesting phenomena. For example, the phonon dressing can lead to symmetry breaking.

We will continue exploring how many-body effects can affect NLO responses. Besides elucidating the underlying physics, we also have a practical (application) question in mind: can many-body interactions enhance linear and nonlinear optical responses?

Related publications:

• Haowei Xu, Jian Zhou, Hua Wang, and Ju Li, Nonlinear Nonreciprocal Photocurrents under Phonon Dressing. Phys. Rev. B 106, 035102 (2022)

• Haowei Xu, Hua Wang, and Ju Li, Abnormal Nonlinear Optical Responses on the Surface of Topological Materials. npj Computational Materials 8, 111 (2022).