Fig. 4. Control of multi-harmonic output radiation via metasurface geometry and input wavelength.
In the plots of output intensity as a function of input laser wavelength for various metasurface designs, we see that the silver tips (a) and triangular grating structure (b) enhance the negative orders (denoted by dashed lines) compared with the positive orders (denoted by solid lines), as opposed to the rectangular grating structure (c), where positive and negative orders are much more comparable in intensity. d Using the first (second) plasmon mode of the graphene nanoribbon leads to an enhanced first (second) order emission. The silver metasurface with periods of 30 nm and containing 40-nm high structures ((a) to (c)) and 30- nm periods of 20- nm-wide graphene nanoribbons (d). The incident Gaussian beam has an angle of θi = 65°, a spot size of 12 µm FWHM on the metasurface, and an energy of 7.5 MeV. The tips in (a) are 6 nm wide and the blocks in (b) are 15 nm wide. The incident fields in(a)–(c) are E0 = 1 GV/m, and the field in (d) is E0 = 0.1 GV/m