the confusion lies in the case of the emission that occurs normally as a result of excitation. But also when we study eg- a four level laser there is a term called spontaneous emission which actually alludes to the exponential decrease in the no of atoms present in the excited state. What is the difference between the two emission processes?
If an atom is in an excited state, it may spontaneously decay into a lower energy level as per QM, and emit a photon in a random direction. This is called spontaneous emission.
It is possible too, that the emission is stimulated by incoming photon, this is called stimulated emission. The emission in this case goes into the same direction as the incoming photon. The effect is that the incoming radiation is amplified. This is how it works to make amplified light in amplifiers and lasers.
Stimulated emission can only happen if the incoming photon has an energy level close to the energy of the laser transition. This laser gain only occurs for optical frequencies within a certain bandwidth. Based on the correct comments, I mean here that a given laser can only amplify light that's close to the fundamental frequency (or frequencies) of that laser.
Based on the correct comments, I will use here ground state as the lower energy level (as opposed to the excited state being the higher energy level), but not the lowest available level. In this case, there might be certain conditions where the lowest energy level is lower then this designated ground state.
Since the atoms in the laser have ground states (lower energy level) and excited states (higher energy level), the atoms in excited states will amplify light, and the atoms in ground state (lower energy level) will absorb them (and get to a excited state). Now light amplification is only possible if more then 50% of the atoms is in excited state. How can they do that? By using methods that remove the atoms from the lower energy levels. usually by moving them to an even lower energy level (which is of course only possible if the electron's mentioned ground state is not the lowest available state).