How is a population inversion created in a laser material?

A population inversion occurs when more atoms or molecules in a system exist in higher energy states than in lower ones, a condition necessary for laser action. Because heat alone cannot produce this state, the system must be "pumped" to selectively excite the atoms.12

This is typically achieved by:
* Illumination: Exposing the laser material to a bright light source.23
* Electric Current: Passing an electric current through the medium.23

In multi-level laser systems, the process often involves exciting atoms to a short-lived high-energy state that quickly decays into a metastable state—a state with an unusually long lifetime. This allows the material to trap excitation energy and build up the necessary population inversion. While three-level lasers rely on depopulating the ground state, four-level lasers utilize an additional transition state between the metastable and ground states, which facilitates more efficient and continuous operation.2