High-harmonic generation (HHG) was firstly, to the best of our knowledge, proved to exist in rare-gas atoms^[1], and nowadays it is one of the backbones of attosecond science^[2–5]. Typical gas-phase HHG experiments need expensive and complex experimental setups. In recent years, solid materials have been extensively used as a driven medium for HHG. One of their advantages is its high density, typically three orders of magnitude larger, allowing the HHG to happen with lower pumping laser fields^[6]. Thus, solid HHG has become a subject of great interest. Investigations have shown that high harmonics generated in bulk crystals could provide new ways to manipulate and tailor light fields and have instrumental prospects for strong field photonics applications^[7–14]. These studies will pave the way to the design of compact ultrafast and short wavelength tunable coherent light sources. Subsequent research has shown that high-order harmonics can be generated in an engineered nanoscale structure, which is able to adequately tailor the local near-field in order to reach the strong field regime. Enhancing the driving electric field by a nanostructure is the most promising way to boost the interaction between the electromagnetic field and the material, allowing it to reach the strong laser–matter interaction, with lower intensity pumping fields.