Sub-Bandgap Photocurrent Enhancement in Silicon Nanodisk Hexagonal Array Induced by Fabry–Pérot Bound States in the Continuum

We develop a silicon (Si) nanodisk array on a gold (Au) mirror structure as a platform for a Si-based narrow-band photodetector operating in the sub-bandgap wavelength range. Numerical simulations reveal that the structure possesses the Fabry–Pérot bound states in the continuum (BIC) arising from the destructive interference between the toroidal dipole resonance of the nanodisk array and the mirror dipole. At the critical coupling conditions, narrow band perfect absorption is achieved even if the extinction coefficient of Si nanodisks is very small. We produce the designed structure by nanosphere lithography and studied the reflectance and photocurrent spectra as a function of the distance between a Si nanodisk array and a Au mirror. We demonstrated that the photocurrent can be enhanced in the sub-bandgap wavelength range at the critical coupling condition, and the enhancement factor is modulated by the Si nanodisk array–mirror distance.