Fig. 3: Asymmetric focusing in harmonic generations.

a, b Experimental SHG (a) and THG (b) spectra under forward and backward excitations, as a function of the pump central wavelength, ranging from 1540 to 1600 nm, with 10 nm increments. c Dependence of the nonlinear output power on the input pump power. The shaded gray region indicates the range of pump power where the harmonic power levels off. d Schematic layout of the meta-lens units, whose rotation angle is encoded in the color map. e SEM image of the fabricated sample. The inset shows an enlarged view of a selected region of interest. f Experimental xz- and xy-plane intensity profiles of the focused second harmonics, under forward (top half) and backward (bottom half) excitations and for two output circular polarization states. Xy-plane intensity profiles are extracted at the respective focal planes. g Analogous data to (f), shown for the focused third harmonics.