After a slit, are photons polarized? It seems clear that a photon stream as well as single photons behind a well designed slit - the right slit width for a given wavelength - are polarized. Means, the electric fields of such photons are aligned. What are the theoretical explanations and was this proofed by experiments?
 A: The simplest answer to this is based on Huygens principle
Check the wikipedia link here. According to this principle each wavefront after the slit is made up of small wavelets, each emitted from the wavefront before the slit (see figure).

These wavelets inherit all the properties of the wavefront before the slit, that among them is polarization.
I am not sure if a specific experiment is done to prove this.
A: As John Rennier wrote in his answer to the question about Why does the electric field dominate in light? 

"The reason we tend to concentrate on the electric field (of the light) is that it interacts strongly with charges, e.g. electrons, and there are a lot of electrons around."

More in detail anna v according to Wikipedia pointed out, that

"A wire-grid polarizer converts an unpolarized beam into one with a single linear polarization."

So yes, photons which are influenced by sharp edges are polarised. And the reason for this phenomenon is the interaction between the electric field component of the photon and the surface electrons of the edge.
