Simulation of phase modulation using optical metalens for optical coherence tomography

Optical coherence tomography (OCT) has wide applications in diagnosing diseases, benefitting from its non-invasive, high-resolution, and real-time visualization of tissue microstructures. Still, this technology faces the mutual constraint between transverse resolution and depth of focus. Metalens can modulate the light fields at subwavelength scales, effectively enhancing the depth of focus of an OCT system. In this paper, a metalens designed for the OCT system is proposed. The metalens with different structures can generate various phase modulation outcomes. Using the finite-difference time-domain (FDTD) simulation, the impacts of the nanopillar parameters (e.g. height, diameter, and spacing) and the materials (e.g. Si, SiO2, and TiO2) on phase modulation have been analyzed. Based on the simulation results, the phase modulation capability of nanopillars is compromised at reduced heights, while excessively tall structures can adversely decrease transmittance. With a constant height, the radius of the nanopillars can be manipulated to achieve a phase delay of 2π. Furthermore, the propagation efficiency and the typical depth of focus were calculated from the results of the beam intensity distribution, indicating an enhanced imaging performance of the OCT system. The results provide a basis for designing the metalens used in the OCT systems.