Research on topological materials using ultrafast spectroscopy

Abstract

Topological materials, characterized by symmetry-protected nontrivial band structures such as Dirac cones and Weyl nodes, host diverse quantum phenomena, with potential applications in quantum transport, spintronics, and nonlinear optics. Ultrafast pump-probe spectroscopy has emerged as a powerful tool for exploring nonequilibrium dynamics in these systems. Its femtosecond resolution allows charge, spin, orbital, and lattice interactions to be tracked on their intrinsic timescales, thereby revealing key coupling mechanisms in topological phases. This review summarizes progress in ultrafast spectroscopic studies of topological insulators, topological semimetals, and magnetic topological materials. We first discuss the relaxation pathways of photoexcited surface and bulk electronic states, emphasizing electron-phonon scattering, surface-bulk charge transfer, and ultrafast spin conversion. We then examine population inversion in Dirac and Weyl semimetals, spin-polarization dynamics associated with tilted Weyl bands, and the effects of magnetic order on topological states, including coherent phonon and magnon excitations, magnetically driven topological transitions, and terahertz emission. We further review photoinduced topological phase transitions driven by electronic correlations, lattice distortions, and magnetic order under intense optical excitation, highlighting routes toward nonthermal control of quantum phases. Finally, we outline future directions that combine multidimensional ultrafast spectroscopy with temporal, energy, momentum, and spin resolution and advanced theoretical modeling to establish a unified picture of nonequilibrium topological states. This review aims to provide a useful reference for ultrafast studies of topological quantum materials and to advance their applications in high-speed, low-power information processing, spintronics, and quantum technologies.