Fabrication and characterization of GaAs-based space-variant inhomogeneous media for polarization control at 10.6μm

Approaches to create radially and azimuthally polarized light beams usually suffer from issues of integration difficulty and system complexity. In this research, we apply a compact design of space-variant inhomogeneous media (SVIM), providing an even more compact solution with relatively higher conversion efficiency. The device we utilize to convert linear polarization at the wavelength of 10.6μm to radial/azimuthal polarization is fabricated on a single GaAs substrate, using space-variant subwavelength periodic structure with locally varying form birefringence. Unlike previous approach, this subwavelength periodic structure is designed to be relatively deep in order to introduce a π phase shift between the TE and the TM components of the input light, and therefore to locally rotate the incident linearly polarized light to the radial/azimuthal direction. To realize the deep space-variant form birefringent structure, we utilize standard photolithography on a GaAs substrate, followed by chemically assisted ion beam etching (CAIBE), rendering an etch profile with high aspect-ratio (6:1) as required by the original design. An optical characterization at 10.6μm shows a close match between the measured and the theoretical polarization distribution. With proper control of the etch profile it shows that the subwavelength structure also serves as an anti-reflection coating at the sample surface.