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four-level laser
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four-level laser
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four-level laser

physics
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four-level laser
four-level laser A sustained laser beam can be achieved by using atoms that have two relatively stable levels between their ground state and a higher-energy excited state. As in a three-level laser, the atoms first drop to a long-lived metastable state where they can be stimulated to emit excess energy. However, instead of dropping to the ground state, they stop at another state above the ground state from which they can more easily be excited back up to the higher metastable state, thereby maintaining the population inversion needed for continuous laser operation.

A four-level laser operates based on quantum mechanics, where atoms or molecules have discrete energy levels. When electrons absorb energy, they move to higher, excited states. As they return to lower energy levels, they emit light. In a four-level laser, there are four energy states. An extra transition state between a long-lived metastable state and the ground state allows for efficient population inversion. This means more atoms are in a higher energy state than a lower one, leading to stimulated emission. This process amplifies light, and the extra level helps maintain a continuous beam by preventing the laser action from being shut down by atoms accumulating in the ground state. This contrasts with three-level lasers, which often produce only pulses.1

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2 Britannica Sources
  1. laser - Energy levels and stimulated emissions
  2. laser - Laser elements

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