  Many applications involve the phenomenon of a material absorbing
electromagnetic radiation. By exploiting wave interference, the efficiency of
absorption can be significantly enhanced. Here, we propose Friedrich-Wintgen
bound states in the continuum (F-W BICs) based on borophene metamaterials to
realize coherent perfect absorption with a dual-band absorption peak in
commercially important communication bands. The metamaterials consist of
borophene gratings and a borophene sheet that can simultaneously support a
Fabry-Perot plasmon resonance and a guided plasmon mode. The formation and
dynamic modulation of the F-W BIC can be achieved by adjusting the width or
carrier density of the borophene grating, while the strong coupling leads to
the anti-crossover behavior of the absorption spectrum. Due to the weak angular
dispersion originating from the intrinsic flat-band characteristic of the deep
sub-wavelength periodic structure, the proposed plasmonic system exhibits
almost no change in wavelength and absorption at large incident angles (within
70 degrees). In addition, we employ the temporal coupled-mode theory including
near- and far-field coupling to obtain strong critical coupling, successfully
achieve coherent perfect absorption, and can realize the absorption switch by
changing the phase difference between the two coherent beams. Our findings can
offer theoretical support for absorber design and all-optical tuning.
