Light-field-driven electron dynamics in topologically protected materials

The prediction and realization of topological insulators have sparked great interest in experimental approaches to material classification [1]. The phase transition between a non-trivial and trivial topological state is important not only for basic materials science but also for next-generation technology, such as dissipation-free electronics with ramifications in coherent light-wave electronics [2, 3]. Therefore it is crucial to develop advanced probes 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 [4,5]. The phase transition is chemically initiated by reducing the spin-orbit interaction strength through the substitution of bismuth with indium atoms. 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 crystal orientation [5]. 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 their nonlinear response, and is compatible with a wide range of samples and sample environments.

Refs. [1] Zhang et al., Nature Physics 5, 438-442 (2009), [2] C. Heide et al., PRL 121, 207401 (2018), [3] T. Boolakee, Nature 605 251-255 (2022), [4] D. Baykusheva et al., Nano Lett. 21, 8970-8978 (2021), [5] C. Heide et al., Nat. Photonics 16, 620-624 (2022)