Dielectric cross-shaped-resonator-based metasurface for vortex beam generation at mid-IR and THz wavelengths

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 paper, we report numerical and experimental results of a vortex-beam-generating metasurface fabricated specifically for infrared (IR) and terahertz (THz) wavelengths. The designed metasurface consists of a 2D array of dielectric cross-shaped resonators with spatially varying length, thereby providing the desired spatially varying phase shift to the incident light. The metasurface was found to be insensitive to the polarization of the incident light. The dimensions of the cross-resonators were calculated using rigorous finite-difference time-domain analysis. The spectral scalability via physical scaling of the meta-resonators is demonstrated using two vortex-generating optical elements operating at 8.8 μm (IR) and 0.78 THz. The vortex beam generated in the mid-IR spectral range was imaged using a Fourier transform IR (FTIR) imaging miscroscope equipped with a focal plane array detector. This design could be used for efficient wavefront shaping and various optical imaging applications in the mid-IR spectral range, where polarization insensitivity is desired.