# Why can't there be a 2-level laser with Non-Optical Pumping

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Q.B.
Hi everyone,

I would like to understand the answer to the question in the title.

I got that when we use optical pumping of a laser, the probability, given an incoming photon of adequate energy, for an atom to absorb it and for an other to produce some stimulated emission out of it are equal. Thus the rate equations for the level population cannot be consistent with a stationary population inversion.

However, I couldn't find any justification of why this also applies to non-optical pumping.

Homework Helper
Provide a detailed example of a (hypothetical) non-optical pumping.
ie. Did you have a mechanism in mind that excited electrons but did not involve light/electromagnetism?

Q.B.

For instance, I thought that for laser diodes, the population inversion was realized thanks to some electric current flowing into a PN junction.

There definitely are two level lasers with non-optical pumping. Specifically, the Ammonia Maser, which was the first laser constructed (though in the microwave range) operates by separating the exited states from the ground state molecules in a magnetic field.

Q.B.
Thanks for your answer. Actually I heard about the maser example, but as you said, it involves a magnetic extraction of the excited state of the two level system. However, I had in mind a "local" population inversion, where one leaves the atoms (excited or not) where they are, but the dynamics of the pumping still creates a population inversion.

Gold Member
For instance, I thought that for laser diodes, the population inversion was realized thanks to some electric current flowing into a PN junction.
Yup, you are correct. diode laser is a two level (actually, two band) system with pumping by carrier injection. As long as you can get population inversion, you'll get lasing.