Probing topological phase transitions using high-harmonic generation

The prediction and realization of topological insulators have sparked great interest in experimental approaches to the classification of materials(1-3). The phase transition between non-trivial and trivial topological states is important, not only for basic materials science but also for next-generation technology, such as dissipation-free electronics(4). It is therefore crucial to develop advanced probes that are suitable for a wide range of samples and environments. Here we demonstrate that circularly polarized laser-field-driven high-harmonic generation is distinctly sensitive to the non-trivial and trivial topological phases in the prototypical three-dimensional topological insulator bismuth selenide(5). The phase transition is chemically initiated by reducing the spin-orbit interaction strength through the substitution of bismuth with indium atoms(6,7). We find strikingly different high-harmonic responses of trivial and non-trivial topological surface states that manifest themselves as a conversion efficiency and elliptical dichroism that depend both on the driving laser ellipticity and the crystal orientation. The origins of the anomalous high-harmonic response are corroborated by calculations using the semiconductor optical Bloch equations with pairs of surface and bulk bands. As a purely optical approach, this method offers sensitivity to the electronic structure of the material, including its nonlinear response, and is compatible with a wide range of samples and sample environments.

Automated summary

Researchers have found that circularly polarized laser-field-driven high-harmonic generation is highly sensitive to the non-trivial and trivial topological phases in topological insulators. By chemically reducing the spin-orbit interaction strength, the phase transition between non-trivial and trivial topological states can be achieved. This purely optical method offers sensitivity to the electronic structure of the material and is compatible with a wide range of samples and sample environments.