I'm wondering about achieving population inversion for a laser. I learned that without an active medium, it's not possible to create a laser with only two energy levels, but it would be possible with three. What is the advantage of having four levels then?
When you have a three-level system, the laser transition is between the ground and first excited levels (see figure). In this scheme, it is rather challenging to get population inversion because all atoms tend to stay on the lowest level.
With a four-level scheme, you have an extra level so that the laser transition does not end in a ground state. Thus, if the bottom level gets depopulated faster than the top level in the laser transition the population inversion will be guaranteed independent of how fast or efficiently you pump the system. Many lasers, such as Nd:YAG or HeNe laser to name a few, actually use a four-level scheme exactly for this reason.
The lower level of laser transition is above the ground state,this means that the fast non-radiative transfer from the lower laser level to ground state keeps a reasonable population in the upper laser level(3rd) which is fundamental for lasing.Hence its more effective than 3-level system