Metasurfaces with Freely Varying Height in the Visible Using Grayscale Lithography

Metasurfaces have come to the forefront of optics research due to their unique advantages over conventional glass optics. Metasurfaces are often limited to features that are all the same height due to the use of conventional semiconductor fabrication techniques. In this work, we introduce a grayscale electron-beam lithography step to a well-established atomic layer deposition (ALD)-damascene fabrication process to create dielectric metasurfaces with varying heights that operate in visible wavelengths. We show that this degree of freedom allows for more independent control between the applied dispersion and phase. This comes from quasi-periodic oscillations of both phase and dispersion, which arise as the metasurface top becomes aligned with nodes or antinodes of the standing-wave electric field created at a reflective surface. The enhanced control of these metasurface properties is useful for broadband metasurface applications that require unique tailoring of dispersion, such as retardance correction in large telescopes.