Re: First-surface mirrors don't polarize, do they?

Louis Boyd wrote:

Many commercial first surface mirrors are overcoated with SiO2 (quartz) to protect the surface. Those will have some weak polarization if the reflection isn't perpendicular to the surface. Likewise "uncoated" aluminized mirrors will form a layer of aluminum oxide just by being in air. You may be able to design your instruments so the effects are equal in both polarization axis and effectively neutral. For example, in a Cassegrain telescope the net polarization close to zero as the angles are small and symmetrical about the optical axis. That assumes the mirror surfaces are uniformly reflective.

Certainly most spectroscope designs aren't polarization neutral. That's difficult to archive in either a grating or prism spectroscope unless the light source is actively rotated and integrated.

It's true that spectrometers are all polarization sensitive at some level. This is hardly avoidable since the p-to-s diffraction efficiency ratio of reflection gratings is massively wavelength-dependent.

Cheap first surface mirrors are usually 'protected aluminum', which is Al covered with a half-wavelength worth of _SiO_, which is a nonstoichiometric, silicon-rich oxide whose index can range anywhere from that of SiO2 (1.46) to about 1.9, depending on the deposition conditions (which change the oxygen content of the film).

Aluminum-air surfaces are much better reflectors than aluminum-glass, so protected aluminum mirrors have quite low efficiencies except where thin film interference in the SiO helps. The thin film interference is polarization sensitive off axis, of course, which makes protected Al mirrors somewhat polarizing.

Better quality aluminum mirrors are usually 'enhanced aluminum', in which the single SiO layer is replaced with a dielectric stack. With more layers, the coating designer has more degrees of freedom, so the off-axis and polarizing performance of enhanced Al is harder to know from first principles.

Cheers,

Phil Hobbs
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