  Multilevel diffractive lenses (MDLs) have emerged as an alternative to both
conventional diffractive optical elements (DOEs) and metalenses for
applications ranging from imaging to holographic and immersive displays. Recent
work has shown that by harnessing structural parametric optimization of DOEs,
one can design MDLs to enable multiple functionalities like achromaticity,
depth of focus, wide-angle imaging, etc. with great ease in fabrication.
Therefore, it becomes critical to understand how fabrication errors still do
affect the performance of MDLs and numerically evaluate the trade-off between
efficiency and initial parameter selection, right at the onset of designing an
MDL, i.e., even before putting it into fabrication. Here, we perform a
statistical simulation-based study on MDLs (primarily operating in the THz
regime) to analyze the impact of various fabrication imperfections (single and
multiple) on the final structure as a function of the number of ring height
levels. Furthermore, we also evaluate the performance of these same MDLs with
the change in the refractive index of the constitutive material. We use
focusing efficiency as the evaluation criterion in our numerical analysis;
since it is the most fundamental property that can be used to compare and
assess the performance of lenses (and MDLs) in general designed for any
application with any specific functionality.
