The low photon energy of driving fields in the multi-terahertz (multi-THz) spectral window has proven particularly suitable to

We rederive the semiconductor Bloch equations emphasizing the close link to the Berry connection. Our rigorous derivation reveals the existence of a third contribution to the (longitudinal) current in addition to the traditional intraband and polarization-related interband terms. The novel term becomes sizable in situations where the dipole-matrix elements are strongly wave-number dependent. We apply the formalism to high-harmonic generation for a Dirac metal. The novel term adds to the frequency-dependent emission intensity (high-harmonic spectrum) significantly at certain frequencies reaching up to 90% of the total signal.

We selectively explore bulk-versus-surface high-harmonic generation in a topological insulator by tuning the driving frequency. Ballistic acceleration of Dirac currents manifests in continuously tunable non-integer harmonic orders and unique polarization fingerprints of the topological invariant.