Probing Broken Time-Reversal Symmetry in 2D Materials with Tailored-Light Photocurrent Generation.

The bulk photogalvanic effect represents a powerful tool for generating photocurrents without external bias in light-matter systems that lack inversion symmetry. While these photocurrents are used in electronic applications such as current sources, switches, and photovoltaics, their presence can also be employed to probe material properties in and out of equilibrium. Here we advance this path of bulk photogalvanic photocurrent spectroscopy by utilizing tailored laser fields for ultrafast photocurrent generation to study time-reversal-symmetry (TRS) broken phases of matter in 2D systems. Combinations of bichromatic linearly polarized laser beams that separately respect mirror and time-reversal symmetries, individually precluding photocurrents, can break symmetries and generate photocurrents when combined. We show in graphene, both theoretically and experimentally, that specific choices of the relative polarization angle and two-color phase impose a forbidden photocurrent selection rule in TRS-invariant systems, as the tailored light maintains TRS while breaking all other symmetries. We then employ state-of-the-art ab initio simulations to validate this physical mechanism and, crucially, predict a broken photocurrent selection rule in materials with intrinsically broken TRS, such as the 2D magnet CrI