Best Paper Award

  Numerical simulation for analyzing the scattering of light and radio waves on rough surfaces is a fundamental process for optical-device design and the development of communication, and sensing systems. For example, a diffraction grating, which has a periodic groove structure, is used in systems such as optical communications and spectroscopes. A diffraction lens, a hologram, and a photomask are used for beam shaping and optical-image generation. The simulation is also used to analyze the radio-wave propagation through many artifacts in urban areas, and the estimation of the surface shape of objects from the scattered light or radio waves. These analyses are achieved by solving Maxwell’s equations. However, even if the surface structure is simple, the simulation requires a considerable amount of calculations and memory resources. For this reason, approximate simulations have been performed by limiting the analysis region to a very narrow range of several wavelengths to several tens of wavelengths or by neglecting the polarization characteristics of electromagnetic waves.
  In this paper, the rough surface is separated into the flat substrate and many projections, and the field distribution is computed by repeating the process to update the field distribution by returning a projection to the substrate. This updating process is performed by using the concept of a difference field, and thus even if the structure is decomposed, a rigorous solution that satisfies Maxwell's equations can be obtained. The field data required for the field-updating process is only those on the boundaries around the projection returned to the substrate. This feature provides the advantage that necessary memory resources do not depend on the entire size of the rough surface to be analyzed.
  As an example, a diffraction grating with a size 27000 times the wavelength with 10000 irregular grooves is computed with one desktop computer. The memory consumption was only 111 MB. This indicates that there is a margin for the parallel computation of several simulations in one computer. With this simulation, we achieved quantitative analysis of the relationship between the periodicity of the groove distribution and the diffraction efficiencies for each polarization. These results can be used for a system which can easily evaluate the fabrication error of a diffraction grating (quality of the products) from the intensity of diffracted beams.
  In summary, this numerical approach makes it possible to perform high-precision analysis without large-scale computers. We expect to contribute to the development of next-generation devices, communications, and sensing systems in a variety of fields in electronics and photonics.