Toroidal Dipole-Induced Photocurrent Enhancement in Si Nanodisk Hexagonal Array below the Band Gap

A hexagonal array of low-aspect-ratio silicon nanodisks is formed on a silicon thin film and the optical absorption and photocurrent properties are studied. Numerical simulations reveal that the nanodisk array possesses the toroidal dipole modes that tightly confine incoming light in a silicon region below the nanodisks. The field confinement brings about narrowband absorption when the extinction coefficient (kappa) is very small, for example, kappa = 10(-2)similar to 10(-3). This suggests that defect-related sub-band gap absorption of silicon can be enhanced by utilizing the modes. Transmittance spectra of fabricated devices reveal that narrow dips assigned to the toroidal dipole resonances appear in the sub-band gap region. At the resonance wavelengths, the photocurrent is substantially enhanced; the enhancement factor reaches 30-fold. The observed narrow-band photodetection can be used as a current-detection-type refractive index sensor operating in the near-infrared range.