Normally the population density of atoms/electrons is more in the ground state than the excited state. But if the process of stimulated emission dominates over the process of spontaneous emission, then it may be possible that N2 > N1.
Where N1 is the number of atoms in the ground state and
N2 is the number of atoms in the excited state.
The process of achieving greater population density of atoms in the higher energy state as compared to lower energy state is called population inversion. The atoms from lower energy states are raised to excited states by external energy.
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