To explain how laser light is generated, we need first to investigate the energy transition phenomena in atoms or molecules. These phenomena include: spontaneous emission, stimulated emission/absorption and nonradiative decay.

According to quantum mechanics, the electrons of atoms can take different energy states, say E1, E2, E3, etc., E1<E2<E3<…. Lower energy level is more stable than higher energy levels, so electrons at high energy levels tend to decay to low energy levels, the energy difference between the two levels can be given out as electromagnetic radiation. This process is called Spontaneous Radiation. The relationship is:

E2 - E1 = hn₀

Where E2 is the upper energy level, E1 is the lower energy level, h is Plank’s constant, n₀ is frequency of the radiated EM wave.

Note also that the energy difference between the two levels can decay in forms other than radiative decay or , spontaneous radiation which is called Nonradiative Decay. The energy difference can change into kinetic energy or internal energy through collisions with surrounding atoms, molecules or walls.

When external EM waves of frequency n₀ are incident on the material whose atoms initially are at energy level E2 and n₀ is very near to the transition frequency between E2 and E1, there is a finite probability that the incident waves will force the atoms to undergo transition E2à E1. Each such transition gives out an EM wave (a photon), while the incident wave (incident photon) still exists. Then we have two photons. The above transition process is caused by external excitation, so it is called Stimulated Radiation. Is stimulated radiation caused by some kind of resonance? Yes, the incident EM should have a frequency very close to that of the atoms, and this resonance makes the fundamental difference between spontaneous radiation and stimulated radiation. In the case of spontaneous emission, the radiation is in all directions and in random phases, while in stimulated radiation, the emitted waves of any atoms are in the same direction and in the same phase with the incident wave.

If the atom is initially at level E1, if this is the ground level, the atom will remain in this level unless got excited. When an EM wave of frequency n₀ is incident on the material, there is a finite probability that the atom will absorb the incident energy and jump to energy level E2. This process is called Stimulated Absorption.

Normally the number of atoms at lower energy levels is larger than atoms at higher levels. Stimulated radiation/absorption, spontaneous emission and nonradiative decay are going on in the same time. Even if we ignore the decay factors, stimulated absorption still dominates over stimulated radiation, the incident EM wave can not be amplified in this case. Amplification of incident wave is possible only when the number of upper level atoms is greater than that of lower level atoms. This case is called Population Inversion. To get more atoms in upper level than in lower level, we have to raise the atoms from lower level to upper level. This process is called Pumping.