I'm learning a little about lasers. I vaguely understand the main concepts of population inversion, the pumping needed to achieve it, and stimulated emission.
I'm interested in electrical pumping used in solid state lasers. However, all the material I've read about it basically involves using a pn junction, like in a laser diode, in which there are essentially at least two different materials (the differently doped p and n regions, even if the semiconductor itself is the same compound).
On the flip side, a ruby laser is just a single material that's optically pumped.
Are there any cases where a single material can be electrically pumped to achieve population inversion?