Milliwatt terahertz harmonic generation from topological insulator metamaterials

Achieving efficient, high-power harmonic generation in the terahertz (THz) spectral domain has technological applications, for example in sixth generation (6G) communication networks [1, 2]. Massless Dirac fermions possess extremely large THz nonlinear susceptibilities and harmonic conversion efficiencies [3–7]. However, the observed maximum generated harmonic power is limited, because of saturation effects at increasing incident powers, as shown recently for graphene [8]. Here, we demonstrate room-temperature THz harmonic generation in a Bi2Se3 topological insulator (TI) and TI-grating metamaterial structures with surface-selective THz field enhancement. We obtain a third-harmonic power approaching the milliwatt range for an incident power of 75 mW – an improvement by two orders of magnitude compared to a benchmarked graphene sample. We establish a framework in which this exceptional performance is the result of thermodynamic harmonic generation by the massless topological surface states, benefiting from ultrafast dissipation of electronic heat via surface-bulk Coulomb interactions. These results are an important step towards on-chip THz (opto)electronic applications.