A Metalens Design for On- and Off-Center Focusing with Amorphous Silicon Hydrogenated (a-Si:H)-Based 1D Array in Visible Spectrum

Abstract

The use of optical systems in medical imaging, computer electronics, large-scale industries, and space exploration is common. The performance of these devices is closely related to the compactness and fast responses of lenses that are used in these optical systems. Typical lenses suffer from several key issues, including limited efficiency, significant size, and the presence of diffraction-induced distortions that compromise their overall performance. Herein these limitations are addressed by designing and simulating an ultra-thin compact metalens also known as a flat lens using a dielectric metasurface. A 1D array of 31 nano-cylinders is placed on a glass substrate that is utilized for focusing the incident wave both on and off center in the focal plane using simulations. The nano-cylinders are comprised of amorphous silicon hydrogenated (a-Si:H), which has a varying radius in a 1D configuration. Amorphous silicon hydrogenated (a-Si:H) nano-cylinders are utilized for the manipulation of the phase of the incident beam working at a frequency of 474 THz. Three metalenses are introduced with focal lengths of 7.46 μm, 10 μm, and 12.99 μm, each having a numerical aperture (NA) of 0.7, 0.6, and 0.5, respectively. The designed single-array metalens showed a transmission efficiency of 73%. The nano-cylinders obtained a full 0–360 phase control that is beneficial in focusing the beam at the center and beyond the center. Symmetric focusing is obtained in the case of off-center focusing on both sides of the optical axis. The design and simulations of the metalens are performed using finite difference time domain (FDTD) simulation tools.