Metastable State

It is also the excited state but having life time of 10-5 to 10-3 seconds. Population inversion occurs only in between the metastable state and lower state. The two states in between the population inversion occurs and laser is achieved, the upper one is known as upper laser level (ULL) and lower is known as lower laser level (LLL).

Note: Two levels lasers cannot be constructed. Can you explain why? Atleast three levels are required to produce laser.

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