Explain why the production of a laser beam requires atoms in the active medium to experience a “population inversion.”
We know that the basic mechanism for laser light production is when photons are incident through a collision on an energized atom. This collision can send the atom down to a lower energy level in the process emitting a photon, which is essentially identical to the one causing the collision in the first place.
Given a population of atoms though, we know the atoms will naturally tend to occupy the lowest possible energy state available to them. At this level, since there is no further down level to be occupied, the atoms are capable of emitting energy. It’s necessary then to work against the natural distribution of atomic energies in order to create a laser beam.
When more atoms than not are at an elevated energy level, this is called a population inversion — inversion because it goes opposite the way atoms will naturally behave. The energized atoms are available to contribute to the overall laser beam.
We can say then that a population inversion is necessary to increase the probability of stimulated emission and increase the number of photons emitted. That’s what makes this condition necessary.