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Short Abstract of IMPRS PhD Project

Subwavelength structures are principally diffraction gratings with periods smaller than the wavelength of the light. In this regime the reflected and transmitted light are just propagating in zeroth order and evanescent in all higher orders. The polarization transformation properties of such sub-wavelength gratings have many potential applications in optical design.

I study the polarization properties of subwavelength structures for different applications in visible region. Generating radially polarized light is considered as the main application in my PhD work. I investigate both the metallic (Aluminium) and dielectric (Silicon Nitride) subwavelength structures. I use rigorous calculations to simulate the properties of these structures. The fabrication consists of three main steps:

  • Sample preparation (evaporation, sputtering or PECVD)
  • Pattern generation (electron beam lithography)
  • Pattern transfer (Dry etching)
For the characterizations, I use the space variant Stokes parameters measurement.

Based on rigorous calculation, an efficient Al polarizer with period 200 nm, height 200 nm, and line width 80nm is designed and fabricated in collaboration with University of Eastern Finland. The measured extinction ratio is 1800 and efficiency is 80%. Based on such an efficient polarizer two elements for different applications were fabricated. As the first application, the aluminum subwavelength structure is successfully used in polarization array for shearing interferometry. As the second application "concentric ring metal grating" is fabricated to generate radially polarized light. The characterization of the propagating radially polarized light shows a highly pure polarization mode without any mode cleaning.

Silicon nitride with high form birefringence is selected for subwavelength dielectric structures. Dielectric subwavelength structures are known to introduce artificial birefringence. The figure shows a cross section picture of a subwavelength structures on SiN fabricated by e-beam lithography and RIE in Erlangen. An element with the pattern is fabricated to generate radially polarized light. The generated radially polarized light by this element is in good agreement with theory.

Figure: Example of high aspect ratio subwavelength structures on SiN fabricated with e-beam lithography and RIE etching: period 200nm, line width 80nm, depth 560nm.