Materials Pushing the Application Limits of Wire Grid Polarizers further into the Deep Ultraviolet Spectral Range

Abstract

defined polarization state is a key requirement in numerous photonic applications. For example, linear optical polarizers are frequently utilized in lithography, [7,8] industrial vision, [9] microscopy, ellipsometry [10] or astronomic remote sensing systems. [11] All these applications substantially benefit from efficient nano-optical wire grid polarizers.A wire grid polarizer (WGP) is a grating type metasurface (see Figure 1). The typical operation principle for such elements requires the transmittance of TM polarized light T TM (TM transversal magneticelectrical field orthogonal to the ridges) to be much larger than that of TE polarized light T TE (transversal electric-electrical field parallel to the ridges) to achieve a significant anisotropic filter functionality. Here, the extinction ratio E r = T TM /T TE is used to express the suppression of TE polarized light. [12] WGPs are highly beneficial because of large achievable element sizes (wafer size), compactness (wafer thickness), and large acceptance angles. [13] Furthermore, their nano-optical nature allows an easy integration into other (nano-)optical elements, such as litho graphy masks, [14] enabling local polarization control. Currently, applications advance toward shorter wavelengths in order to benefit from smaller foci and characteristic electronic transitions, which can be utilized for material analysis. While WGPs are well established in the VIS and IR, suitable ones were not available in the deep ultraviolet (DUV) spectral range until very recently. [12,15,16] The lack of applicable DUV WGPs originates from challenging requirements on both structure and material properties.A structural period of the polarizer has to fulfilling the zero order conditionto avoid propagation of diffraction orders greater than the zeroth one.[17] For a normal incidence of light (ϕ = 0°) with a wavelength λ in the DUV and a fused silica substrate with a refractive index n sub a period p in the order of 100 nm is necessary. Additionally, an aspect ratio (see Figure 1: ratio of height and ridge width) larger than five is typically required. [12] The simultaneous realization of large aspect ratio and small periods is technologically extremely challenging. Fortunately however, advances in nanotechnology do allow the fabrication of such structures. [18] Pelletier et al. [15] demonstrated aluminum WGPsWire grid polarizers (WGPs), periodic nano-optical metasurfaces, are convenient polarizing elements for many optical applications. However, they are still inadequate in the deep ultraviolet spectral range. It is shown that to achieve high performance ultraviolet WGPs a material with large absolute value of the complex permittivity and extinction coefficient at the wavelength of interest has to be utilized. This requirement is compared to refractive index models considering intraband and interband absorption processes. It is elucidated why the extinction ratio of metallic WGPs intrinsically humble in the deep ultraviolet, whereas wide bandgap semiconductors are superior materia...

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