Re: For the Ruby LASER, why are the pumping levels energy bands whereas the las

Hello!

   Pumping a laser is usually done by a bright, non-coherent source that has a mix of photon 
energies in it.  Those photons whose energy is greater than the bandgap can kick  up electrons from 
valence to conduction band, and the electrons may end up anywhere within the conduction band, 
depending on how much more energy the kicking photon has than the bandgap energy.

   Once electrons are up in a conduction band, especially if "hot" (i.e. energy much above the lower 
band edge), they collide with the lattice. losing energy until they occupy the states at or very near 
the bottom of the conduction band.Almost all the transistions from conduction to valence band will 
occur from the band edge to band edge (requires the least energetic photon to be emitted).