What must occur for population inversion to be achieved in a laser?

For population inversion to be achieved in a laser, electrons must transition to higher energy states. This is a critical condition because, in a typical thermal equilibrium situation, electrons naturally populate the lower energy states more than the higher energy ones, resulting in a situation where less than half of the electrons are in higher energy states.

In the context of laser operation, achieving population inversion means that more electrons are in the excited (higher energy) state than in the ground (lower energy) state. This inversion is essential for stimulated emission, which is the process that leads to coherent light generation in lasers. When an external energy source, such as a pump, is applied to excite electrons to higher energy levels, it facilitates the process of stimulating emission when a photon interacts with these excited electrons, allowing for the amplification of light.

The other options mentioned do not contribute to achieving population inversion. Electrons transitioning to low energy states would decrease the number of excited electrons, while having more electrons occupy low energy states does not contribute to inversion. Lastly, if electrons remain at stable energy levels without being excited, it would not lead to the population inversion necessary for laser action.