Methods for designing frequency selective surface for optical windows are analyzed and experimentally verified. Four methods; ITO coatings, graphene, metallic nano-coatings and metallic meshes; are compared in terms of optical transmission and shielding effectiveness (SE) characteristics. ITO is the most common method for increasing SE performance in the visible band. However, in the infrared band, transmission of ITO is limited. Metallic nano-coating is an alternative for ITO with underperformance from the SE and optical transmission viewpoint. Graphene is an emerging method and needs further development. SE performance of the graphene prevents from being a single material for shielding applications. On the other hand, it might be involved in applications where back reflection is critical because of the electromagnetic absorption characteristic. Metallic mesh is a successful candidate for applications require high level SE and optical transmission. However, dielectric properties of substrate the mesh applied, and diffraction effects should be carefully investigated.

In this study, different Ge IR optical windows are produced and their electromagnetic interference (EMI) shielding performances are investigated. Three Ge single crystals with different doping concentrations are grown by the Czochralski (CZ) technique. Optical windows are obtained after the slicing, lapping, and polishing processes are examined. Formation of (111)-oriented single crystal is observed from the X-ray diffraction (XRD) pattern. In addition, IR transmission of the crystals is measured around 45% in the range of 2-12 µm using Fourier transform infrared spectrometer (FTIR) spectrometer. EMI shielding effectiveness (SE) of the windows is determined in a flanged coaxial SE tester developed for outer diameter as 1 inch. SE results are evaluated by taking into account both resistivity, thickness of the samples and limitations of measurement fixture.

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