Replacement for optical calcite (Glan Thompson prism)?
I have a pair of Glan Thompson polarizers (Karl Lambrecht) that cost a pretty penny many years ago. I also have several other (less expensive) pairs of calcite polarizers. Nothing to date can match high quality calcite in terms of low scattering, low fluorescence and low extinction ratio. In some cases, with the highest quality calcite devices, it is possible to find 'sweet spots' having $10^{-10}$ extinction ratio. (The best I ever got, after hours in a dark room, was about $8 x 10^{-9}$). In contrast, the plastic sheet polarizers are nice and convenient, but nowhere near good enough for serious polarimetry work. Sodium nitrate crystals might have excellent optical properties (I do not know anything about them), but would they need to be protected from water vapor in the air? After all, potassium nitrate is used in gunpowder and I suspect one reason is that it 'keeps the powder dry'!