For any system that is in thermal equilibrium with its environment, there will always be more atoms found at the lower energy level(E1) than at the higher energy level(E2). Hence, number of atoms at E1 is a lot more than those at E2. This is because any excited atoms will de-excite via spontaneous emission.
Conditions for lasing action to occur
- Population Inversion (Number of atoms at E2 more than at E1).
- Placing the lasing medium in an optical cavity consisting of two facing mirrors.
Why is population inversion necessary?
Taking the number of atoms at lower energy level (E1) be N1 and the number of atoms at higher energy level (E2) be N2.
At normal conditions, N1 is a lot greater than N2. Photons of energy equal to E2 – E1 is incident on the medium. Atoms at E1 absorbs the photons and gets excited to E2. Here, the atoms can de-excite via 2 ways: Spontaneous emission or stimulated emission. Since N1 is a lot greater than N2, the probability of the photons being absorbed is a lot greater than the probability of it inducing a stimulated emission. Hence, a lot of the excited atoms will undergo spontaneous emission. Photons emitted by spontaneous emission are in all directions, which is not what we want. We are looking for a giant coherent pulse of light(laser) that can only be achieved by stimulated emission = population inversion.
Note: You might ask if it is possible to excite all/most the atoms at E1 to E2 in a two-levels system, hence achieving population inversion using a two levels system. This is not possible. A two level system is one where the absorption and emission of energy occurs between the same pair of energy levels. The average lifetime of an excited atom is 10-8 seconds. Thus, population inversion is impossible in a two-level system as atoms arrive in the excited level at the same rate as they leave. The best that you can achieve is N1=N2.
With population inversion achieved and the mirrors in place, the photons will interact repeatedly with the atoms(most of them in excited state due to population inversion), hence inducing stimulated emission. The photons will bounce back and forth, building the intensity of the light to a high level, resulting in more stimulated emissions. At this point, all the photons will be coherent and in the same direction, hence achieving light amplification by stimulated emission of radiation(laser). One of the mirrors will be partially reflecting so that some of the light emerges as the useful laser beam
Simulation for lasers to help you understand better: http://phet.colorado.edu/en/simulation/lasers