High-harmonic generation (HHG) in bulk solids permits the exploration of materials in a new regime of strong fields and attosecond timescales [1][2][3][4][5][6] . The generation process has been discussed in the context of strongly driven electron dynamics in single-particle bands [7][8][9][10][11][12][13][14] . Two-dimensional materials exhibit distinctive electronic properties compared to the bulk that could significantly modify the HHG process The recent observation of HHG in bulk solids provides a new approach to attosecond photonics and has opened up exciting opportunities for the study of strong-field and ultrafast electron dynamics in the condensed phase [1][2][3][4][5][6]

The recent observation of high-harmonic generation from solids 1-8 creates a new possibility for engineering fundamental strong-field processes by patterning the solid target with subwavelength nanostructures 9,10 . All-dielectric metasurfaces exhibit high damage thresholds and strong enhancement of the driving field 11-20 , making them attractive platforms to control high-harmonics and other high-field processes at nanoscales. Here we report enhanced non-perturbative high-harmonic emission from a Si metasurface that possesses a sharp Fano resonance resulting from a classical analogue of electromagnetically induced transparency. Harmonic emission is enhanced by more than two orders of magnitude compared to unpatterned samples. The enhanced high harmonics are highly anisotropic with excitation polarization and are selective to excitation wavelength due to its

High-harmonic generation (HHG) from a compact, solid-state medium is highly desirable for applications such as coherent attosecond pulse generation and extreme ultra-violet (EUV) spectroscopy 1 , yet the typically weak conversion of pump light to HHG can largely hinder its applications. Here, we use a material operating in its epsilon-near-zero (ENZ) region 2-11 , where the real part of its permittivity vanishes, to greatly boost the efficiency of the HHG process at the microscopic level. In experiments, we report high-harmonic emission up to the 9 th order directly from a low-loss, solid-state ENZ medium: indium-doped cadmium

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