Numerical Analysis of Ridged-Circular Nanoaperture for Near-Field Optical Disk

A ridged-circular nanoaperture is investigated through three-dimensional (finite-difference time-domain) FDTD method. The motion equations of free electrons are inserted to analyze a metallic material. The electromagnetic field distributions of optical near-field around the aperture are investigated. The phase change disk illuminated by a near-field optical light through a ridged-circular nanoaperture is also analyzed. The far-field scattering patterns from the phase change disk and the crosstalk characteristics between plural marks are studied. 1. Introduction The recording density of conventional optical recording systems has an optical diffraction limit. Recently, there has been a lot of interest in the field of optical storage technologies for the recording methods that are based on near-field optical principles. This is because they have the potential to overcome the limitation by using the localized optical near-field for writing and reading recorded marks. Many types of nanoapertures and nanoantennas have been proposed to achieve the high throughput of the optical near-field [1–4]. This study focuses on a ridged-circular nanoaperture for a near-field optical disk. The analysis is accomplished by the FDTD method into which the motion equations of free electrons are inserted [5–7]. This method can easily deal with Drude dispersion and it can be applied to the analysis of various plasmonic devices. First, the electromagnetic field distributions of optical near-field around the ridged-circular nano-aperture are analyzed. Next, the scattering characteristics from a phase change disk with the aperture are studied. Finally, the crosstalk characteristics between plural marks are discussed. 2. FDTD Formulation In the FDTD method, special handling of the metallic material is required because the permittivity is dispersive and has a negative value in the optical frequency. In this study, the following motion equation is introduced into the FDTD method to evaluate the conducting current: where is the electron velocity, is the electric field, is the elementary electric charge, is the electron mass, and is the collision frequency. The collision frequency is expressed as follows: where is the angular frequency of a light wave and and are the real and imaginary parts, respectively, of the complex refractive index of a metallic material ( ). Maxwell’s equations are expressed as follows by representing the current density using the electron velocity and the electron density : where is the magnetic field and and are the electric permittivity and magnetic