High harmonic generation in solids with an ultra-low threshold of 7 GW/cm.

High harmonic generation (HHG) in solids holds immense potential for probing ultrafast electron dynamics in crystals, with a much lower threshold compared to its molecular counterpart. In this work, we demonstrate an Au-disk array on a cadmium telluride (CdTe) nanofilm, which generates a local electric field on the surface, enhancing harmonic generation. Compared to a bare CdTe nanofilm, the structured sample exhibits a harmonic strength enhancement of ∼50 times, and the harmonic cutoff order extends from the 23rd to the 32nd. Owing to the flatband dispersion near the Fermi level and the high carrier mobility of CdTe, as well as the field enhancement from the design of a metasurface, the 24th harmonic is observed at an ultra-low laser intensity threshold of 7 GW/cm. This demonstrates a new, to the best of our knowledge, HHG threshold and higher order of harmonics. The HHG of a metastructure with ultra-low threshold overcomes the common problem of material damage seen in traditional metastructures, enabling HHG to be driven by compact and cost-effective pump lasers.