Fig. 6

Experimental/numerical characterizations on the achromatic reflectionless metasurface with tunable loss. a Top-view SEM image of sample #4 (scale bar: 1 μm) with top-left inset depicting a zoom-in top-view SEM picture (scale bar: 500 nm) and top-right inset depicting a side-view FIB-SEM picture (scale bar: 500 nm), respectively. b Spectra of reflectance and absorbance of sample #4 within the wide frequency range of 0–225 THz, obtained by experiments (stars), simulations (circles) and CMT calculations (solid lines) with parameters computed by the LEM theory, respectively. c-e FEM-simulated spectra of c reflectance/transmittance, d absorbance and e transmission phase for the sample #4, assuming that its absorption damping parameters \({\gamma}_i^{(m)}\) (m = 1,2) take different values (The dotted/ solid/ dashed lines denote cases with \({\gamma}_i^{(m)}=0.1{\gamma}_0^{(m)}\), \({\gamma}_i^{(m)}={\gamma}_0^{(m)}\), and \({\gamma}_i^{(m)}=3{\gamma}_0^{(m)}\), respectively). Here, \({\gamma}_0^{(1)}=3.182\times {10}^{16}{s}^{-1}\) and \({\gamma}_0^{(2)}=3.672\times {10}^{16}{s}^{-1}\) denote the absorption damping parameters adopted in our FEM simulations corresponding to the experimentally realized sample, obtained by fitting with the measured spectra in Fig. 4