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Population inversion
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More atoms in excited than ground state, required for lasing.
A laser requires more atoms in the excited state than in the lower state so stimulated emission beats absorption. Achieved with three- or four-level systems and external pumping.
Ruby laser pumped by a flash lamp.
Impossible in a two-level system in equilibrium.
In more detail
Population inversion means more atoms are in an excited state than in a lower state, which is required for lasers. It is achieved by pumping energy into the gain medium.
Real-world uses
- Lasers depend on population inversion to amplify light.
- Masers use population inversion for microwave amplification.
- Engineers building lasers create population inversion by pumping atoms to an upper level, so stimulated emission can dominate.
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Frequently asked questions
Is population inversion normal?
No. In thermal equilibrium, lower states are more populated, so inversion requires pumping.
How is it achieved?
By optical pumping, electrical pumping or chemical reactions, depending on the laser.
Why does a laser need inversion?
Normally more atoms are in the ground state, which absorbs light, so inversion is needed for light to be amplified.
Related topics in Quantum Mechanics
Quantum cryptography · Quantum teleportation · Lasers · Einstein coefficients · Blackbody and Planck's law · Quantum statistics of light