Plasmonic field enhancement by metallic subwave lattices on silicon in the near-infrared range

The enhancement of the electric field in plasmonic nanostructures on a Si substrate in the near-infrared range is studied theoretically. Two-dimensional square arrays of circular holes of various diameters in a gold film serve as metasurfaces allowing the conversion of the external electromagnetic radiation into surface plas-mon modes at the Au-Si interface. It is established that illumination of nanostructures from the side of the Si substrate provides a higher field enhancement compared to the case of frontal illumination, since the incident light wave in the former case reaches the gold interface without passage through subwavelength holes for which the transmittance is extremely low. The plasmonic field enhancement, as a function of the diameter of holes of the array, shows the maximum at which the Bloch plasmon polaritons propagating along the Au-Si interface are transformed to localized surface plasmon modes. Two types of localized plasmons are detected. The short-wavelength plasmon is excited along the array diagonals and exists at any angles of incidence of light on the structure. The long-wavelength plasmon occurs only at nonzero angles of incidence and is located along orthogonal directions parallel to the sides of the square array. These results are directly related to the problem of fabricating effective silicon photodetectors with quantum dots.