Lasers

Stimulated Emission.
Population Inversion.
Amplification.
Resonance.

source: http://imedea.uib-csic.es/~salvador/docencia/coms_optiques/addicional/ibm/ch03/03-06.html

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.