Illustrative scenario of HHG at Terahertz frequencies, induced by the pumping a 2.5 W power THz QCL on a Topological insulator SRR. Credit: Alessandra Di Gaspare et al. High-order harmonic generation (HHG) is a process that transforms light into much higher frequencies, allowing scientists to explore areas of the electromagnetic spectrum that are otherwise difficult to reach. However, generating terahertz (THz) frequencies using HHG has remained a major obstacle because most materials are too