Extended Data Fig. 3: Comparison of the intensity of bulk and surface HHG.
From: Tunable non-integer high-harmonic generation in a topological insulator
a, HH intensity, IHH, for two select driving frequencies. Above-bandgap excitation at νTHz = 37 THz (red curve) allows for strong bulk contributions. For νTHz = 28 THz (blue curve) resonant interband transitions in the bulk are not possible and the peak electric THz field (about 3 MV cm−1) is too low for efficient non-resonant interband excitation. Therefore, the bulk contribution drops by orders of magnitude and the observed spectrum is dominated by HHG from the metallic TSS. In a direct comparison of the two spectra, this contribution is reduced with respect to the above-bandgap bulk HH intensity by only one order of magnitude. b, A direct comparison of the same spectra IHH as a function of the harmonic order, n, instead of the harmonic frequency, even reveals a slight enhancement of HHG in the TSS (νTHz = 28 THz) with respect to the above-bandgap bulk HHG (νTHz = 37 THz). Considering the low effective thickness of the TSS of about 1 nm compared with the optical penetration depth of about 30 nm to 100 nm over which bulk HHG is collected, this comparison attests to the strong nonlinearity of Dirac electrons.
