Phases of quantum matter are classified by the symmetries their collective state breaks. Magnetic, ferroelectric, and charge-ordered phases each lower the symmetry of the underlying lattice. Nonlinear optical techniques, in particular rotational-anisotropy second-harmonic generation (RA-SHG), are sensitive to inversion and rotational symmetry breaking and can determine the point group of an ordered state. Extending these measurements into the time domain reveals how order parameters melt, reorient, and recover after excitation, disentangling the electronic, spin, and lattice contributions that are inseparable in equilibrium.
B. T. Fichera, A. Kumar, B. Lv, Z. Shen, et al., “Higgs-mode electromagnon in the spin-spiral multiferroic CuBr$_2$”, arXiv:2504.18716 (2025).
Light is not only a probe but also a means of control. Intense optical and terahertz fields can renormalize exchange interactions, drive lattice modes far into the anharmonic regime, and dress the electronic structure, allowing a material to be steered into configurations that have no counterpart on its equilibrium phase diagram. Because the drive acts selectively on a chosen degree of freedom, competing orders that are nearly degenerate in equilibrium can be tipped in favor of one another, producing transient or metastable phases with distinct symmetry and topology. Understanding when such states arise, how long they persist, and which couplings stabilize them is central to turning light-induced control into a route toward new material functionality.
M. Mogi, D. Choi, K. H. Oh, …, Z. Shen, et al., “Discovery of transient topological crystalline order in optically driven SnSe”, arXiv:2502.14800 (2025).
B. T. Fichera, B. Lv, K. Morey, Z. Shen, et al., “Light-induced reorientation transition in an antiferromagnetic semiconductor”, Physical Review X 15, 011044 (2025).
工欲善其事，必先利其器。
A craftsman who wishes to do his work well must first sharpen his tools.
— Confucius, Analects, Book XV
K. A. Morey, B. T. Fichera, B. Lv, Z. Shen, N. Gedik, “Automated polarization rotation for multi-axis rotational-anisotropy second harmonic generation experiments”, Review of Scientific Instruments 96, 043002 (2025).