Population inversion occurs when more atoms or molecules occupy an excited energy state than a lower energy (usually ground) state—a condition opposite to thermal equilibrium.
Why It Matters for Lasers:
- Stimulated emission dominates: In normal conditions, atoms prefer to absorb photons. But with population inversion, stimulated emission outpaces absorption, enabling light amplification.
- Laser gain medium: The material (gas, crystal, semiconductor, etc.) must be energized (pumped) to achieve this inversion.
- Sustained emission: Once achieved, a single photon can trigger a chain reaction of identical photons, forming a coherent laser beam.
Without population inversion, a laser cannot amplify light, and stimulated emission is overwhelmed by absorption. Achieving and maintaining this state is central to the design and function of all lasers.