1：Experimental Design and Method Selection:

The study uses an analytical theory developed to model the interaction between free electrons and the plasmonic near-field of metasurfaces, validated with ab initio simulations. The design focuses on optimizing metasurface geometry, electron energy, and laser incidence angle to control high harmonic generation.

2：Sample Selection and Data Sources:

Simulations are performed using hypothetical metasurfaces (e.g., silver metasurface with specific periodicities) and electron beams (e.g., 5 MeV or 7.5 MeV energy, 100 μA current). Data is generated through computational models rather than physical samples.

3：5 MeV energy, 100 μA current). Data is generated through computational models rather than physical samples. List of Experimental Equipment and Materials:

3. List of Experimental Equipment and Materials: No specific physical equipment is listed; the paper relies on theoretical and simulation-based approaches. Mentioned materials include silver for metasurfaces and potential use of graphene or other tunable materials.

4：Experimental Procedures and Operational Workflow:

The process involves exciting plasmons on the metasurface with an optical pulse, launching electrons parallel to the surface, and analyzing the resulting radiation spectrum. Parameters such as laser wavelength (e.g., 370 nm), pulse duration (e.g., 10 fs), and incidence angle are varied to study effects on output intensity and harmonics.

5：Data Analysis Methods:

Analytical formulations (e.g., Eqs. 1-3) are used to predict radiation intensity, compared with ab initio simulation results for validation. Spectrum analysis identifies harmonic orders and their dependencies on experimental parameters.