Optical imprinting of symmetry breaking in quantum materials with circularly polarized light

Optical manipulation of symmetry and related electronic order provides a powerful route to control correlated quantum materials on ultrafast timescales. In this context, circularly polarized (CP) light is a unique tool as it inherently breaks time-reversal and mirror symmetries, enabling selective coupling to various degrees of freedom therein. In this talk, I will introduce recent experimental results using time-resolved angle-resolved photoemission spectroscopy (tr-ARPES) with circularly polarized light. I will highlight double-helicity dichroism, a new technique we developed to isolate subtle symmetry-breaking responses by fully utilizing the helicity of both pump and probe pulses.

Using this approach, I will first show that in TiSe₂, CP mid-infrared (MIR) excitation optically imprints a long-lived metastable state that breaks vertical mirror symmetry. This light-induced order persists for over 50 ps above the CDW transition temperature and suppresses CDW fluctuations, revealing that the two orders are distinct yet competing. Double-helicity dichroism provides direct spectroscopic evidence for optically induced mirror symmetry breaking in this system.

In the second part of the talk, I will discuss a distinct example, Bi₂Se₃, where CP excitation induces time-reversal symmetry breaking that leads to a transient quantum anomalous Hall state through Floquet engineering. I will present the emergence of a gap together with a pronounced Berry curvature signature at the gap, which can be accessed by double-helicity dichroism ARPES.