Near infrared performance of a pile-of-plates polariser based on poly-crystalline Zinc Selenide

Abstract

Zinc Selenide (ZnSe) has wide transparency from 0.6 μm–18 μm making it a desirable substrate for infrared (IR) optical applications. We present experimental results on the NIR transmission of a pile-of-plates polariser at 780 nm and 1.06 μm based on polycrystalline ZnSe plates tilted at Brewster's angle. Due to the high refractive index of ZnSe, n ∼2.5, the degree of polarisation (DOP, V) at λ = 780 nm and 1.06 μm reached V = 0.98 and 0.97 respectively with 4 plates and an extinction ratio E < 10−2 at 780 nm. The calculated DOP for 1–4 plates agree well with theory when allowance is made for internal surface reflections, as in the Airy expressions. While the DOP achieved with 4, 1 mm thick plates is relatively high, the absolute transmission for the P component (TP = 0.83, 0.84 respectively) is much lower than expected, based on the NIR extinction coefficients k < 10−6 (780 nm) and k ∼10−7 (1.06 μm), inferring TP > 0.96. The source of this absolute loss has been considered, including residual surface scatter, diffuse scatter from grain boundaries in the polycrystalline micro-structure and the likely effect of residual birefringence. This loss, we believe is primarily due to residual surface scatter in thin crystal lattice damage layers created during the polishing process and observed when using a hand-held near infrared (NIR) camera. This scatter is polarisation dependent as expected but much higher than scatter theory predicts, consistent with crystalline surface damage. A 6-plate assembly demonstrated an extinction ratio of 1/600 and transmission Tp ∼0.72 at 1.06 μm. The very low absorption and dispersion in ZnSe, particularly at longer wavelengths infer that this polariser would work well in the important MIR region.