High-order harmonic generation (HHG) in solids provides a compact route to attosecond light sources, but its efficiency and tunability remain key challenges. Here, we investigate HHG from $\mathrm{Cu}\left(111\right)\text{/}n\text{−}\mathrm{ML}$ NaCl heterostructures and identify a previously unreported interfacial mechanism that simultaneously enhances the harmonic yield and extends the cutoff. Under fixed laser conditions, the cutoff increases nearly linearly with NaCl thickness, and this scaling remains robust against variations in driving-field parameters. The underlying physics arises from the confinement of Cu surface electrons within the screening length, their enhanced acceleration due to a reduced effective mass, and efficient transfer into NaCl bands during field-driven Bloch oscillations. Furthermore, we demonstrate that applying a bias voltage significantly amplifies the harmonic yield, enabling the synthesis of stronger attosecond pulses. The interplay between the Cu skin effect and the emergent periodicity of NaCl thus provides a controllable platform for engineering strong-field responses, offering opportunities for robust attosecond pulse generation and ultrafast photonic applications.

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• High-order harmonic generation
• Surface states
• Ultrafast optics
• Heterostructures
• Attosecond laser irradiation
• Time-dependent DFT