Simulating near-field effects in high-density optical-disk data storage

15 Designing an optical data storage system involves both macroscopic components, such as lenses, filters, and polarizers, which are typically tens of thousands of wavelengths in size, as well as subwavelength-size data elements, gratings, nearfield antennas, and microcavities. Research and development of new approaches for high-density optical-disk data storage often requires detailed simulation of the light-field interaction with subwavelength structures. Optical disks based on current compact disk (CD) and digital versatile disk (DVD) technologies operate near the wavelength λ = 650 nanometers and have geometric features about 400 nm in size. The ever-increasing demand for higher capacity and more compact storage systems has led to a continuous decrease of the size and increased geometrical complexity of the subwavelength components. We can numerically model propagation of the light field through many elements of the optical system using classical diffraction theory equations and plane-wave expansion solution methods. These global spectral transform methods, which use fast Fourier transforms, provide accurate and efficient solutions for such cases.1,2 However, when the light field interacts with structures on scales comparable to the wavelength, the classical diffraction theory’s assumptions, invoked to simplify the problem and allow for approximate solutions, are not applicable. For such cases, we seek numerical solutions of Maxwell’s equations (in conjunction with the relevant constitutive relations) by approximating the continuous time and space derivatives via the appropriate difference operators. Interfacing and combining the solution procedures applicable for different limiting cases lets us accurately simulate the complete optical-disk datastorage system. Simulating the entire system lets us compute optical system performance characteristics, which can also be measured in the experiment, making computations a more direct design tool.