I do not know if they are open to the public per se, but it seemed to be something they do quite regularly. Concerning the material, they say on their site that it is a “polymer”. On this page they go quickly over how the measurements work.

I was actually not visiting sensirion, but lumiphase. A smaller company they are investing in which does optical chips. It’s interesting stuff, they are also depositing unusual materials on the silicon wafer to make their chips work.