Investigation on near-field optical disk using rectangular nano-aperture with ledges

The recording density of optical storage systems has been increased by using the shorter wavelength of laser light and the higher numerical apertures of the objective lenses. However, the optical diffraction limit restricts these methods to get the higher recording density. Near-filed optical recording is one of the promising techniques because it has the potential to overcome this limitation. Many types of nano-apertures and nano-antennas have been proposed in order to obtain near-field of high emission intensity and a small spot size. In this study, the three-dimensional optical disk structure utilizing a metallic nano-aperture with ledges is analyzed by using FDTD method. The motion equations of free electrons have been installed to analyze the metallic material. First, the nano-aperture with ledges is analyzed and the near-field through the aperture and the far-field scattering patterns are studied. It is demonstrated that the narrow beam width can be achieved by adding the ledges on the aperture. Next, the readout characteristics of the near-field optical disk are investigated. The structure is composed of the rectangular nano-aperture with ledges and the recording layer that is sandwiched between the two protective layers. The dependency of the characteristics on the recorded-mark size is studied and the crosstalk characteristics between adjacent recorded marks are investigated when the plural recorded marks are arranged. It is shown that the crosstalk can be reduced by using the proposed aperture. The readout characteristics of the continuously arranged recorded-marks are also studied and the minimum recorded-mark size that can be distinguished is discussed.