three-level laser

A three-level laser operates based on quantum mechanics, where atoms or molecules possess discrete energy levels. The lowest energy state is the ground state, and when atoms absorb energy, they become excited. Excited states are unstable, and as electrons drop to lower energy levels, they emit light.1

In a three-level laser, atoms are first excited to a high-energy state, then spontaneously drop to a long-lived, lower-energy state called a metastable state. This metastable state accumulates energy, creating a population inversion where more atoms are in higher energy states than lower ones. This condition allows stimulated emission to occur, where a photon triggers an excited atom to release a photon of the same energy, phase, and direction, amplifying the light. This process builds intensity as light bounces between mirrors, eventually forming a concentrated beam. A challenge in three-level lasers is that the ground state can absorb stimulated emission, often limiting them to pulsed operation.123