Quantum materials are a broad class of materials where collective behaviors of 1023 electrons emerge upon strong interactions and/or reduced dimensionality. Examples include high-Tc superconductivity from strong electron correlations, topological insulators from strong spin orbit coupling, and massless Dirac fermions in two-dimensional (2D) monolayer graphene. Our group works to develop and utilize laser-based spectroscopy and microscopy techniques to reveal, understand, and control quantum states of matter in 3D and 2D quantum materials. In this talk, I will focus on one specific family of phases of matter, multipolar orders, that are widely present in numerous 3D and 2D quantum materials, but at the same time, are extremely hard to study for their little coupling to our familiar vector fields (e.g., electric and magnetic fields). Using the ferro-rotational order, the lowest rank multipolar order, as an example, I will show that we developed static rotation anisotropy (RA) electric quadrupole (EQ) second harmonic generation (SHG) spectroscopy to measure its symmetry properties, built spatially-resolved RA-EQ-SHG microscopy to map its real-space distribution, and designed and constructed time-resolved RA-EQ SHG spectroscopy to resolve its collective excitations and drive it into a new transient state. Our success in developing and using EQ-SHG to study the ferro-rotational order opens new venues for coupling and manipulating these otherwise inaccessible multipolar orders with nonlinear and ultrafast optics.

References

W. Jin, E. Drueke, et al Nature Physics 16, 42 (2020)
X. Luo et al Phys. Rev. Letters in press (2021)
X. Guo, R. Owen, et al manuscript in preparation (2021)