Intrinsic free carrier absorption limited THz generation from Bi2Te3 and Bi2Se3 topological insulators

We investigate the influence of intrinsic free charge carriers on the ultrashort THz pulse generation efficiency of three-dimensional topological insulators (3dTIs). Wavelength dependence of the optical penetration depth of the femtosecond excitation pulses is exploited to vary the excitation volume within Bi2Te3 and Bi2Se3 crystals and accordingly, the free carrier population, to contribute to THz attenuation. The standard free carrier absorption (FCA) formalism is inadequate to explain wavelength-dependent enhancement in the THz emission as observed in the experiments. Within a modified framework, the THz attenuation by FCA is accounted for accurately, which consistently explains the experimental results for samples having different carrier density and mobility. We conclude that the THz generation efficiency of 3dTIs can be enhanced by engineering samples with high carrier mobility and low intrinsic carrier density and by using excitation wavelengths of minimal penetration depth.