Optipedia • SPIE Press books opened for your reference.
Spontaneous and Stimulated Emission
Excerpt from Field Guide to Lasers
If an atom is in an excited state, it may spontaneously decay into a lower energy level after some time, releasing energy in the form of a photon, which is emitted in a random direction. This process is called spontaneous emission. It is also possible that the emission is stimulated by incoming photons, which is called stimulated emission. The emission then goes into the same direction as the incoming photon. In effect, the incoming radiation is amplified. This is the physical basis of light amplification in amplifiers and lasers.
Of course, stimulated emission can only occur for incoming photons that have a photon energy close to the energy of the laser transition. Therefore, the laser gain occurs only for optical frequencies (or wavelengths) within a limited gain bandwidth. A laser normally operates at the optical wavelength where the gain medium provides the highest gain.
In an ensemble of atoms having only two energy levels (a ground state and an excited state), the excited atoms can amplify light, while those atoms in the ground state can absorb light, which brings them back to the excited state. Net amplification can then be achieved only when more than 50% of the atoms are in the excited state. This condition is called population inversion. Laser gain is more easily achieved when there is a mechanism that rapidly removes the atoms from the lower energy level after each emission event (e.g., by transfer into an even lower energy level).