  Metasurfaces are engineered thin surfaces comprising two dimensional (2D)
arrays of sub-wavelength spaced and sub-wavelength sized resonators.
Metasurfaces can locally manipulate the amplitude, phase and polarization of
light with high spatial resolution. In this study, we report numerical and
experimental results of a vortex-beam-generating metasurface fabricated
specifically for infrared (IR) and terahertz (THz) wavelengths. The designed
metasurface consisted of a 2D array of dielectric cross-shaped resonators with
spatially varying length, thereby providing desired spatially varying phase
shift to the incident light. The metasurface was found to be insensitive to
polarization of incident light. The dimensions of the cross-resonators were
calculated using rigorous finite difference time domain (FDTD) analysis. The
spectral scalability via physical scaling of meta resonators was demonstrated
using two vortex generating optical elements operating at 8.8~$\mu$m (IR) and
0.78~THz (Terahertz). The vortex beam generated in the mid-IR spectral range
was imaged using FTIR imaging miscroscope equipped with a focal plane array
(FPA) detector. This design could be used for efficient wavefront shaping as
well as various optical imaging applications in mid-IR spectral range, where
polarization insensitivity is desired.
