Subwavelength structures can be described as an effective medium, whose refractive index depends on the geometrical parameters of the designed element. This is also possible for the 2 nd or higher diffraction order. and has been theoretically investigated further e.g. This has been shown for instance by Bartelt et al. Under this specific angle it is possible to choose a parameter set (period, wavelength, fill factor, which is the ratio between the structure width and the period) in such a way, that either the 0 th or 1 st order will be transmitted for the different states of polarization. The grating period p in this case is ≈ λ. Several examples have been suggested for grating beamsplitters working under Bragg (or Littrow) incidence angles. wire grid polarizers) have a lower overall transmission efficiency than dielectric phase gratings. In this paper we just consider dielectric media, as we want to design an efficient transmission grating. Following this definition, resonant structures are of the same magnitude as the wavelength, but allow more than just the 0 th order to propagate (although the functionality of subwavelength structures can of course be explained through resonance effects). There is a large variety of how to realize subwavelength or resonant structure based beamsplitters. Here, we focus on a different approach: The use of gratings for achieving a polarizing beamsplitter functionality. Classical methods use naturally birefringent material or dielectric coatings. There are different approaches for designing a polarizing beamsplitter. Afterwards we will present our in-house manufacturing process including the Soft-UV-Nanoimprint-Lithography before we discuss the experimental verification. The general method can be applied to other diffractive elements with smaller structures. Then we will get to the design process of our hybrid subwavelength-structured diffraction grating, which includes both the effective medium approximation (EMA) and rigorous coupled-wave analysis (RCWA). In the following we will first give an overview over the existing structures which have been used, both with subwavelength features and larger structures. Various approaches have been suggested for the design of diffractive structures, which act as polarizing beamsplitters. The tolerance to a large range of incidence angles is necessary in order to enable the measurement of surface profiles, which reflect the light not solely perpendicularly. Manufacturability and reproducibility (cost-effectiveness) As we continue with the design of such a grating, we keep the following requirements in mind:ĭiffraction efficiency η T E,0> η T M,0 ( η T M,1> η T E,1)įunctionality over a large range of incidence angles θ i This effectively makes the grating a compact polarizing beamsplitter, possibly consisting of only one material.
Thus, the light is diffracted solely into higher orders. For this TM polarization in the return path, the diffraction efficiency is η T M,0→0. The light interacts with the specimen between two passes of the quarter-wave plate, resulting in a rotation of the polarization plane by 90 ∘. For TE-polarization the 0 th order carries all the transmitted intensity, so η T E,0→1. The transversal electric (TE) component of the light wave vector is oriented parallelly to our grating structure and the transversal magnetic component (TM) perpendicularly to our structure. Throughout this paper the diffraction efficiency η is defined as the intensity of the respective diffraction order relative to the intensity of the incoming light.