In a relaxed condition, only a specific set of energy states, states of being, are available. However, with continued pumping, or charging up with photons and energy at the proper frequency, the material/substrate will enter into a previously unavailable energy state (population inversion?) causing a cascading amplification and radiation effect.
Edit: Population inversion is when the majority of entities are occupying higher energy states. Hmm. This is when the cascading lasing effect then occurs.
Kendra is the founder of 4Love+Science and works as a Science and Community Consultant