Second harmonic generation and third harmonic generation in topological insulator-based van der Waals metamaterials

TL;DR

This study demonstrates high-order harmonic generation in topological insulator-based van der Waals metamaterials, achieving frequency up-conversion in the THz range through symmetry-breaking effects.

Contribution

First experimental observation of high harmonic generation in topological insulators using metamaterials with enhanced optical field amplification.

Findings

Achieved THz frequency up-conversion from 6.4 to 9.7 THz.

Observed bulk and surface state contributions to harmonic generation.

Utilized split ring resonators with Bi2Se3-based heterostructures for field enhancement.

Abstract

High-order harmonic generation (HHG) in solids - the frequency up-conversion of an optical signal - is governed by symmetries. At terahertz (THz) frequencies, HHG is a key technology to access high frequency spectral windows that are usually difficult to cover using conventional solid state laser technologies. This effect has been recently exploited in graphene where HHG has been demonstrated, albeit only at odd multiples of the driving frequency owing to its inherent centro-symmetry. In topological insulators (TIs), the combination of spin-orbit interaction and time-reversal symmetry create an insulating bulk state with an inverted band order, inseparably connected with conducting surface states. TIs have been predicted to support unconventional high harmonic generation from the bulk and topological surface, which are usually difficult to be distinguished. However, no experimental results have been provided, so far. Here, we exploit the strong optical field amplification provided by an array of single or double split ring resonators, with embedded Bi2Se3 or (InxBi(1-x))2Se3/ Bi2Se3 van der Waals heterostructures, to achieve up-conversion in the 6.4 (even) - 9.7 (odd) THz frequency range. This results from bulk centro-symmetry (odd states) and symmetry breaking in the topological surface states (odd and even).