Xiaoming Wang (University of Toledo)

Date: Mon. February 2nd, 2026, 12:45 pm-1:45 pm
Location: Rockefeller 221 (Foldy Room) & Zoom
Website: https://scholar.google.com/citations?user=Nsm0df4AAAAJ&hl=en
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Ab Initio Theory of Optical Activity in Crystalline Solids

Xiaoming Wang 

Department of Physics and Astronomy,  University of Toledo

(Host: Walter Lambrecht)

Abstract: Optical activity—the rotation of the polarization plane of linearly polarized light (optical rotation) and the differential absorption of left- and right-circularly polarized light (circular dichroism)—is a symmetry-sensitive probe of electronic structure that becomes especially rich in chiral and non-centrosymmetric systems. It underpins techniques spanning stereochemical analysis, biomolecular sensing, enantioselective spectroscopy, and emerging chiral photonics and spin-optoelectronic concepts. While optical activity has been known since the early 19th century (with α-quartz as a prototypical example), a broadly accurate and practical ab initio framework for predicting optical activity in periodic solids has lagged behind the well-developed molecular theory.

In molecules, optical activity is naturally described via a multipole expansion of the minimal-coupling light–matter Hamiltonian, where magnetic-dipole and electric-quadrupole responses play a central role. In crystals, however, a direct translation runs into a fundamental condensed-matter issue: the position operator is ill-defined in Bloch representations, obscuring the multipole moments that control optical activity and making naive implementations gauge delicate and numerically unstable. This talk will explain how to formulate optical activity in a way that is well-defined under periodic boundary conditions and suitable for first-principles computation. I will present our recent progress toward a predictive ab initio approach to optical activity in crystalline solids. The discussion will proceed from the independent-particle level to the inclusion of microscopic local-field effects, and finally to electron–hole correlations within the Bethe–Salpeter framework. I will emphasize the physical content of each approximation, practical computational considerations (including gauge consistency and convergence), and when many-body effects qualitatively change the response. I will conclude with outlook and open challenges for large, complex chiral materials—such as chiral halide perovskites—where strong spin–orbit coupling, structural complexity, and excitonic effects make optical activity both computationally demanding and scientifically revealing.