Terahertz nonlinear photonics based on the ultrafast thermodynamics of quantum materials

Quantum materials with massless Dirac fermions -- such as graphene and topological insulators -- exhibit extremely large terahertz nonlinearities. This is the result of their ultrafast thermodynamics: efficient electron heating-cooling dynamics taking place on femtosecond-picosecond timescales. We have recently developed a grating-graphene metamaterial with local field enhancement, giving rise to strongly enhanced THz nonlinearities in graphene. These grating-graphene metamaterials, however, suffer from saturation effects due to inefficient electronic heat dissipation, and therefore produce limited harmonic output power. Most recently, we have used a topological insulator system with surprisingly efficient surface-to-bulk electronic heat dissipation, which strongly reduces saturation effects. As a result, we demonstrated close to a milliwatt of generated third harmonic signal from a fundamental signal at 500 GHz. These results have interesting prospects for nonlinear terahertz photonic applications.