(a) A schematic illustration of the experimental setup for solid-state HHG. The fundamental laser field has a wavelength of 5.0 μm, a pulse duration of ~70 fs, and a peak-field strength of ~1 V/nm. A half-wave plate (HWP) for the MIR pulse and a polarizer for the visible spectrum are mounted on rotation stages, which allow us to obtain orientation dependence and polarization information. (b, c) Optical images of the monolayer (b) WS2 and (c) MoSe2 samples. The white bar at the right bottom corresponds to a scale of 500 μm. (d, e) High-harmonic spectra of the monolayer (d) WS2 and (e) MoSe2. The vertical dashed lines indicate the materials’ optical bandgap. Efficient odd- and even-order harmonics are observed from 5th to 15th harmonics.