# Laser physics, Population inversion

I know that for production of laser population inversion is a prerequisite and population inversion can be achieved in three level system(with metastable state). The thing I want to realise is that how is the population inversion achieved in three level system? I want to feel how it is achieved and why it is not possible in two level system?

Two-level system is impossible because the ground state is where all electrons naturally live, it is just too difficult to pump the huge amount of lazy elections to the higher state. It is easier to have population inversion in 2 higher states other than the ground state. We can make the 2nd excited state more enjoyable for the electron to stay than the 1st excited state. For example, we can create some leaking mechanism in the 1st excited state.

Mine is a translation, so I hope to be as clear as possible. (Nac is the active centers density).

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The population inversion is achieved by so called "pumping". In a three level system for example your use light to "pump" your electrons into the highest state. So the process can be described like this:

• Pumping of electrons into highest level
• electrons drop down without laser-emission into the metastable state
• from this point they emit their laser-radiation by spontanious or stimulated emission

So population inversion is achieved between the lowest and mid level.

In a two level system if half of the electrons are excited, the possibility for absorption and stimulated emission are the same. So, you can not achive population inversion from this point on.