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

Area of Science:

• Condensed Matter Physics
• Optics and Photonics
• Materials Science

Background:

• High-order harmonic generation (HHG) is a frequency up-conversion process crucial for accessing high-frequency spectral windows.
• Graphene exhibits HHG only at odd multiples of the driving frequency due to its centrosymmetry.
• Topological insulators (TIs) possess unique electronic properties, including conducting surface states, predicted to support unconventional HHG.

Purpose of the Study:

• To experimentally investigate high-order harmonic generation (HHG) in topological insulators (TIs).
• To explore the potential of TIs for generating terahertz (THz) frequencies via HHG.
• To distinguish HHG contributions from the bulk and topological surface states in TIs.

Main Methods:

• Utilized an array of single or double split ring resonators for optical field amplification.
• Embedded Bi2Se3 or (InxBi(1-x))2Se3/Bi2Se3 Van der Waals heterostructures within the resonators.
• Studied HHG in the 6.4–9.7 THz frequency range.

Main Results:

• Achieved frequency up-conversion in the 6.4–9.7 THz range, covering both even and odd multiples of the driving frequency.
• Observed HHG originating from both the bulk centrosymmetry (odd harmonics) and symmetry breaking in topological surface states (odd and even harmonics).
• Demonstrated the feasibility of THz-HHG in TIs, a phenomenon previously unobserved experimentally.

Conclusions:

• The study provides the first experimental evidence of HHG in topological insulators.
• The results confirm the theoretical predictions of unconventional HHG from TI surface states.
• This work opens new avenues for THz frequency generation and exploring exotic electronic properties of TIs.