Which (if any) of the following are essential for laser action to occur between two energy levels of an atom?

(a) There are more atoms in the upper level than in the lower.

(b) The upper level is metastable.

(c) The lower level is metastable.

(d) The lower level is the ground state of the atom.

(e) The lasing medium is a gas.

There is a laser action taking place between two energy levels of an atom.

The laser actions to be complete between two energy levels, the following conditions are needed:

a) There must be an inverted population, i.e. more atoms in the excited state than in the ground state.

b) The excited state must be metastable.

c) The emitted photons must stimulate further emission.

From the given concept, we know that the laser action takes place in three steps. Thus, the essential conditions for laser action require having an inversion to the Boltzmann population index. It allows the higher state to have more excited photons to emit into the lower level, which is the ground state of an atom, and the excited state is the first excited upper level.

The laser action must have the upper metastable state. That is, the state which used to have less stability and less life suddenly has a longer life to produce stimulated emission. Again, the radiation medium does not necessarily need to be a gaseous medium.

Hence, the essential conditions that match the criteria for laser action are (a), (b) and (d).