Diameter dependence of polarization resolved reflectance from vertical silicon nanowire arrays: Evidence of tunable absorption

Vertical silicon nanowire arrays of different diameters ranging from 90 nm to 190 nm with a pitch of 400 nm are fabricated. Nanowires are arranged in a square lattice and have a length of 1 μm. Polarization resolved reflectances from these nanowire arrays are measured and significant differences are observed for the reflectance characteristics of the s- and p-polarized beams. A narrow wavelength band of increased absorption is reported by analyzing the reflectance using the finite difference time domain method. The center wavelength of this band shifts toward higher wavelengths as the diameter of the nanowires increases achieving tunable absorption spectra within the nanowires. The wavelength band shifts at a rate of approximately 3.3 nm with every 1 nm increment in diameter. The increased absorption is achieved due to modal excitation and near-field coupling between the nanowires.