what is metastable state in laser

A metastable state is an excited state of an atom or other system with a longer lifetime than other excited states. In lasers, metastable states play an important role in achieving population inversion, which is necessary for lasing action. In a three-level laser, the material is first excited to a short-lived high-energy state that spontaneously drops to a somewhat lower-energy state with an unusually long lifetime, called a metastable state. The metastable state traps and holds the excitation energy, building up a population inversion relative to the ground state. This allows for stimulated emission to exceed absorption, leading to the production of coherent photons and lasing action. However, in some lasers, the laser-active atoms or ions can be trapped in certain metastable states, which can lead to a self-terminating laser transition and unsuitability for continuous operation. To avoid this, some solid-state gain media may be doped with additional species that can quench metastable states. In general, metastable levels do not exist in laser gain media, but it is possible to use a short-lived level as the upper laser level in the presence of large emission cross-sections.