Electrostatic Control of Terahertz High-Harmonic Generation in Turbostratically Stacked Graphene

Graphene exhibits strong nonlinear optical responses across a broad spectral range, including the terahertz (THz) regime. Although THz high-harmonic generation in graphene has been demonstrated and can be enhanced by electrostatic doping and multilayer stacking, how interlayer carrier redistribution determines the overall nonlinear response remains unclear. Here, we show that the THz nonlinearity of turbostratically stacked multilayer graphene is governed by electrostatically induced interlayer Fermi-level redistribution. We experimentally observe gate-dependent modulation of third- and fifth-harmonic generation in top-gated monolayer, bilayer, and trilayer graphene devices. To account for these results, we combine a thermodynamic model of graphene THz nonlinearity with a nonlinear screening model for interlayer charge redistribution under electrostatic doping. The model quantitatively reproduces the measured layer- and gate-dependent harmonic responses, showing that the total THz nonlinear response of turbostratically stacked multilayer graphene is determined not only by the total carrier density but also by its distribution across the layers. These findings identify control over the layer-resolved carrier distribution as a key design principle for optimizing nonlinear performance.