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Population inversion is a condition in which more atoms, ions, or molecules in a given system are present in an excited energy state rather than in their lower energy (ground) state.

To understand why population inversion is essential for the functioning of a laser, it is important to briefly describe how a laser works. A laser operates by releasing energy in the form of light, through a process called stimulated emission. In stimulated emission, the energy from an external source excites the atoms within the laser medium and raises them to a higher energy level. When the excited atoms interact with incoming photons, they release energy in the form of light, which is further amplified by the laser cavity or resonator. This results in a coherent, monochromatic, and highly collimated beam of light as the laser output.

In a typical system with no population inversion, more atoms are in the lower energy (ground) state than the higher energy state. When photons interact with these atoms, they can either be absorbed (raising the atoms to a higher energy level), or they can cause stimulated emission (lowering the atoms to a lower energy level). However, because there are more atoms in the lower energy state than the higher energy state, absorption is more likely to take place than stimulated emission. This process of absorption hinders the efficiency of photons being released as light and the overall laser output. For a laser to function effectively, the probability of stimulated emission has to be greater than the probability of absorption. This can be achieved by creating a population inversion, which moves more atoms to a higher energy state and lowers the number of atoms in the ground state. When population inversion is present, the probability of stimulated emission is higher than absorption, allowing for the effective amplification of light and the production of a coherent laser beam.

In summary, population inversion, a condition in which more atoms are in a higher energy state than the lower energy state, is essential for the successful functioning of a laser. This situation ensures that the probability of stimulated emission is greater than absorption, which allows the laser to effectively produce and amplify a coherent beam of light.