The Brillouin Zone is a pretty awesome concept. It blew my mind when I was studying semiconductor lasers and left me with a great respect for crystallographers.
Yeah, I have dreams of using crystal field splitting to help design semiconductor lasers in bandgap.io, which would be a nice applied group theory problem. Its about 3 years out on my roadmap!
I'll keep my eye on it! Left the semiconductor industry some time ago, but it still fascinates me.