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Characterization of Infrared Metasurface Optics with an Optical Scatterometer
Optical metasurfaces have undergone substantial development over the last decade, and are starting to be implemented into their own scientific instruments. It is expected that they will become the preferred lenses when weight and thickness are a concern in the coming years. With this in mind, it becomes increasingly important to accurately characterize metasurface lenses and to improve on their designs. Optical scattering is one metric that is often overlooked when characterizing metasurfaces, and this thesis addresses that. An optical scatterometer is used in this experiment to create scatter profiles for one particular metasurface lens and two variants of the same design. This particular design uses dielectric pillars of varying radius to create the parabolic phase delay required for lensing. The two variants of this design change the height of the cylindrical pillars from the design height of 4.0 micrometers to 0.9 and 5.2 microns. Optical scatter measurements were conducted at the design wavelength of 4 micrometers and at 3.39 micrometers and 5micromet, away from design. The results of these measurements show that when only a portion of these metasurface lenses are irradiated away from the center of the lens, they behave like blazed diffraction gratings. Where a conventional lens would show a single focal area, a diffraction grating shows multiple diffraction orders, and blazed gratings show preference to (more power within) one particular, non-zero order. The metasurface lenses show diffraction orders with a preference to the positive order direction, but also show a scatter floor several orders of magnitude higher than that of a conventional refractive optic, as well as a large amount of power passing straight through them. This beam pass-through was the largest with longer-wavelength incident light, as well as with shorter pillar height. The notable exception to these trends was with the wafer with 4.0 micrometer pillars at 3.39-micrometer incident light.
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