Double Circular Dichroism High Harmonic Spectroscopy: An Ultrafast Probe for Topological Photocurrents

Abstract Understanding optical responses of topological matter is a central problem for enabling optoelectronic applications based on topological physics, which is of fundamental concern for photocurrent control and spectroscopy. Currently, schemes for sensing ultrafast photocurrents and separating their bulk/surface contributions are lacking but are crucial for exploring topological ultrafast physics. We introduce here double circular dichroism (DCD) harmonic spectroscopy as an all-optical probe of ultrafast dynamics in topological materials. In this scheme, pump and probe pulses are circular with helicities that are independently controlled, yielding the circular dichroism of the circular dichroism─a time-resolved response evaluating how probe-induced dichroism depends on pump helicity. While DCD vanishes in symmetric systems, it survives in broken time-reversal symmetry materials including Chern insulators. We theoretically demonstrate this concept through simulations in a Haldane nanoflake, where a pump laser manipulates chiral current-carrying states, and intense probe pulses drive harmonic emission. We show that DCD originates from both bulk and edge-localized states, but these have opposite signs, similar magnitudes, and different amplitude scaling. Moreover, edge-associated DCD is connected with the outermost atomic layer of exponentially localized protected modes, and is relatively robust to bulk/edge interference. Hence, DCD could allow efficient separation of bulk/edge contributions to photocurrents. Variation of the electronic structure and laser parameters further reveals anomalies that might be useful for probing topological attributes of photocurrents and material phase in select harmonics. This is especially apparent in the oscillatory behavior of HHG DCD, which varies greatly across the phase transition and becomes stable and nonoscillatory in topologically trivial phases. Further scanning pump–probe delay, we show that DCD oscillates on attosecond time scales, which could serve as an ultrafast probe of topological photocurrents and their associated coherence. Overall, our work introduces DCD as a potentially powerful approach for disentangling bulk/boundary photoresponses in broken-symmetry quantum and topological matter, and could also be implemented in other pump–probe spectroscopies based on photoelectrons and absorption, as well as chiral systems.