Surface versus Bulk Doping Effects on Second Harmonic Generation from Bi2Se3

Different effects of the bulk and surface doping on the nonlinear response of topological insulator Bi 2 Se x are studied by comparing the second harmonic generation (SHG) from bulk single crystalline samples of different Se proportion, and using field‐effect transistor devices based on Bi 2 Se 3 ultrathin films. While the SHG signal decreases with the bulk electron density, it increases when more electrons are injected through the gate. The former effect is attributed to the screening of the space charge region near the surface, and the latter is due to the doping of the surface energy band into the non‐parabolic region as well as the enhancement of the electrostatic field across the space charge region.