Gate-tunable emergence of exceptional high-order terahertz photocurrent in bilayer graphene  

On femtosecond–terahertz timescales, lightwave acceleration of photocurrents represents one of the most complex yet highly tunable forms of light–matter interaction, where electrons are driven coherently within a single oscillating field cycle. However, the rich excitation regimes involving multi-quantum coherences and high-order nonlinearities have remained largely uncontrolled, as they depend on a delicate balance among bandgap, doping, and strong-field processes such as Rabi flopping and Landau–Zener transitions. Combining single-cycle terahertz (THz) pulses with bilayer graphene (BLG) offers a powerful platform for achieving powerful coherent control, as gate tunability and field strength allow continuous tuning of excitation parameters. Here, we demonstrate gate-tunable emergence of exceptionally high-order THz photocurrents in BLG using nonlinear THz spectroscopy. Our findings identify salient features from the perturbative multiphoton to the lightwave-driven regimes. The results are analyzed using quantum-kinetic simulations of the strongly nonlinear photocurrents and compared with other complex material systems.