Why don't Five-Level Laser Systems exist? I have seen 3 and 4 level laser systems.
My question, why doesn't a five level (or further) laser system exist? Is it possible?
Can one be built?
 A: A laser, in order to work, has to be a three-level system; otherwise, there would not be a "population inversion".
Essentially, you excite an electron to an upper level, and this one, before going back again to the lowest level, is "trapped" on an intermediate one. So, at a given point, you'll have more electrons in the "trapped" state than in the low-energy one.
A 4-level one adds one energy level above the ground state, with a very short life-time (it depopulates quite fast) so that there is always an inverted population.
To be more clear: from the ground state, you go to the excited state, here the electron falls to a lower energy level where it is trapped for a while. Now you have in this trap more electrons than in the "fast depopulating" level; you have lasing with a very low threshold.
In principle, adding more levels is possible, and perhaps you can lasing with more frequency at once... but I doubt it will work since it is hard to forbid intermixed energy transitions, and for some of them you will not have enough electrons to reach the minimum threshold.
