Define what is meant by the term population inversion. Why must this condition be achieved before a system can successfully function as a laser?

Population inversion is a condition where the number of atoms or molecules in an excited energy state (higher energy level) is greater than the number of atoms or molecules in a lower energy state (ground state). In other words, there are more particles with higher energy levels compared to their normal ground state.

A laser (Light Amplification by Stimulated Emission of Radiation) works by emitting photons through a process called stimulated emission. In this process, an incoming photon interacts with an excited molecule or atom which then releases another photon with the same energy, phase, and direction as the original photon. The released photon further stimulates other excited particles, eventually creating an intense and coherent beam of light, which is a laser beam.

For a system to function as a laser, the number of particles in the excited state should be more than the particles in the lower energy state (ground state) to facilitate the stimulated emission process. This requirement is called population inversion. Without population inversion, the chance of stimulated emission occurring is quite low, as more particles would be in their ground state. In the absence of population inversion, particles tend to return to their ground state through a process called spontaneous emission, releasing photons in random directions and phases. This situation would result in more energy loss and less coherent light, which is not desired for a laser to function properly. Therefore, achieving population inversion is crucial for a system to function as a laser, as it allows the stimulated emission process to occur, leading to a coherent and powerful beam of light.