Universally polarization-insensitive achromatic metasurfaces
Demir, Hilmi Volkan
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Transparent optical components constitute the key elements of modern electro-optical systems including optical sensors, displays and imaging systems. The working principle of conventional transparent components rely on gradual phase accumulation. As a direct result of their working principle, these components suffer from fundamental limitations on size. Metasurfaces, enabling full wavefront engineering in subwavelength thicknesses, are promising candidates to replace conventional optics and overcome the size limitations. Early examples of this concept include plasmonic metasurfaces containing sub-wavelength metallic structures. However, these plasmonic structures cannot reach practically sufficient efficiency levels in transmission mode due to fundamental ohmic losses. This strongly motivates highefficiency all-dielectric alternatives. These dielectric solutions have thus far been reported to rely on either the resonance tuning or the geometrical (Pancharatnam– Berry) phase. Though remedying the efficiency limitation, unfortunately, these approaches either are impaired with ultra-narrow operation bands or suffer polarization dependency. In this thesis, we propose and demonstrate two new approaches to address these problems. In the first approach of ours, universally polarization-insensitive achromatic wavefront control is achieved using dielectric nanopillars operated as stepindex cylindrical waveguides intentionally away from the scattering resonances. A metalens operating in the mid-wave infrared region of electromagnetic spectrum is shown using these off-resonance waveguiding unit cells as a proof-of-concept demonstration. Polarization-insensitive diffraction-limited focusing over a broad spectral band of operation is verified by full electromagnetic simulations. In our second approach, to further increase the performance and bandwidth of dielectric metasurfaces, a novel architecture of these phase elements is proposed. Full phase control of wavefront is achieved using these unit cells. Such metalenses operating in the mid-wave infrared and visible regions are designed as proof-of-concept demonstrations. Full electromagnetic solutions confirmed entirely polarizationinsensitive achromatic focusing of the proposed metasurfaces with significantly increased operation bandwidth.