Research groups of the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences (CAS), together with Nanjing University, have observed the strong field-driven phenomenon from topological nontrivial states. Scientists demonstrated for the first time the high-harmonic generation (HHG) from topological surface states. This work was published in Nature Physics on November 23, 2020. 

Because of the pioneering research of topological phase transitions and topological phases of matter, three physicists were awarded the 2016 Nobel Prize in Physics. Three-dimensional topological insulators (3D TIs) are a phase of matter that hosts unique spin-polarized gapless surface states protected by the time-reversal symmetry and exhibits unconventional charge and spin transport properties. However, the research of its nonlinear phenomenon driven by strong laser fields is still in its infancy.  

Recently, theoretical studies have predicted that in a low-dimensional topological system HHG is sensitive to topologically non-trivial phases. These findings make the experimental observation of topological-based-HHG more appealing. 

In this study, the researchers showed the observation of HHG from 3D TIs, driven by linearly polarized strong laser pulses delivered from a self-built long-wavelength intense laser system. HHG spectra up to ninth-order were observed (Figure (a)). The polarization components (parallel and perpendicular) as a function of the crystal orientation were also measured (Figure (b)-(e)). The magnitudes and polarizations of the even- and odd-order harmonics, modulated by the azimuth orientation, behaved differently.  

According to the results, the components of the even-order harmonics along the pump polarization stem originate from the spin current in helical surface states, whereas the perpendicular components originate from the out-of-plane spin polarization. 

The finding provides a method for isolating the charge and spin transportation in surface and bulk. The strong field phenomenon provides an effective tool to investigate the interaction between the topological surface states and bulk states in 3D TIs, where both the non-perturbative spin and charge dynamics co-exist with versatile controllability.  

The research fuels broad interests in future studies of the strong-field phenomenon from topological surface states. 

This project was supported by the National Natural Science Foundation of China, the Strategic Priority Research Program of CAS, the Scientific Instrument Developing Project of CAS, and the Shanghai Municipal Development and Reform Commission.