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Fast quantitative characterization of a rectangular groove with sizes from sub-micrometers to micrometers on a surface has been demanding in quality control of optical elements. We report a numerical procedure for retrieval of the geometrical parameters of a rectangular groove, including width and depth, from the angular distribution of the scattered light generated by launching plane waves on the groove on a metallic surface from different propagation directions. The far-field intensity of scattering light is first obtained by numerically solving the electromagnetic problem with the modal theory. Then, a two-stage inverse modeling approach is developed to retrieve the groove parameters based on the lookup table and repulsive particle swarm optimization from multi-angle scattering light distribution that is highly sensitive to the groove parameters. Noise influences are considered in the simulation. Results suggest that our method can precisely obtain the groove parameters for noisy signals. As the measurement of light scattering can be conducted with high efficiency, it should be a powerful tool to characterize rectangle grooves on metallic surfaces.

An electrically tunable absorber in the visible (Vis) and near-infrared (NIR) regime is theoretically investigated. The absorber is composed of aluminum (Al)-indium tin oxide (ITO) composite square pillars on top of thin dielectric layer and Al substrate. Significant tuning of absorption with modulation ratio of 86% can be achieved. The modulation ratio above 75% can be maintained from 1427 to 1795 nm and over 90% absorbance at the NIR regime from 1500 to 1800 nm can also be achieved. The symmetry of the absorber eliminates the polarization dependence and the near unity absorption efficiency can be maintained by incidence angles up to 70°. The presented method will enhance the functionality of absorber and has the potential for the applications that require active control over light absorption. Keywords: Indium tin oxide (ITO), Perfect absorption, Electrically tunable