Multilevel Diffractive Optics for Broadband Applications
Creators: name: Mohammad, Nabil
Publisher: name: University of Utah
Year: 2019
Subjects: Electrical engineering
Optics

description: Diffractive optics, an important part of modern optics, involves the control of optical fields by thin microstructured elements via diffraction and interference. Although the basic theoretical understanding of diffractive optics has been known for a long time, many of its applications have not yet been explored. As a result, the field of diffractive optics is old and young at the same time. The interest in diffractive optics originates from the fact that diffractive optical elements are flat and lightweight. This makes their applications into compact optical systems more feasible compared to bulky refractive optics. Although these elements demonstrate excellent diffraction efficiency for monochromatic light, they fail to generate complex intensity profiles under broadband illumination. This is due to the fact that the degrees-of-freedom in these elements are insufficient to overcome their strong chromatic aberration. As a result, despite their so many advantages over refractive optics, their applications are somewhat limited in broadband systems. In this dissertation, a recently developed diffractive optical element, called a polychromat, is demonstrated for several broadband applications. The polychromat is comprised of linear "grooves" or square "pixels" with feature size in the micrometer scale. The grooves or pixels can have multiple height levels. Such grooved or pixelated structures with multilevel topography provide enormous degrees-of-freedom which in turn facilitates generation of complex intensity distributions with high diffraction efficiency under broadband illumination. Furthermore, the super-wavelength feature size and low aspect ratio of this micro-optic make its fabrication process simpler. Also, this diffractive element is not polarization sensitive. As a result, the polychromat holds the potential to be used in numerous technological applications. Throughout this dissertation, the broadband operation of the polychromat is demonstrated in four different areas, namely, photovoltaics, displays, lenses and holograms. Specifically, we have developed a polychromat-photovoltaic system which facilitates better photon-to-electron conversion via spectrum splitting and concentration, a modified liquid crystal display (LCD) that offers higher luminance compared to a standard LCD, a cylindrical lens that demonstrates super-achromatic focusing over the entire visible band, a planar diffractive lens that images over the visible and near-IR spectrum and broadband transmission holograms that project complex full-color images with high efficiency. In each of these applications, a unique figure of merit was defined and the height topography of the polychromat was optimized to maximize the figure of merit. The optimization was achieved with the aid of scalar diffraction theory and a modified version of direct binary search algorithm. Single step grayscale lithography was developed and optimized to fabricate these devices with the smallest possible fabrication errors. Rigorous characterization of these systems demonstrated broadband performance of the polychromat in all of the applications.