What state do atoms of the lasing medium achieve through an external energy source?

Atoms of the lasing medium achieve an excited state through the application of an external energy source, such as electrical discharge or optical pumping. This process involves providing energy to the atoms, allowing them to move from their lower energy levels (ground state) to higher energy levels (excited state). In this excited state, atoms possess more energy than they do in their ground state and can emit photons when returning to the ground state, which is essential for laser operation.

The excited state is critical in the laser process because it allows for stimulated emission of radiation, which is what generates the coherent light characteristic of lasers. The presence of atoms in an excited state ensures that when one photon interacts with an excited atom, it can stimulate the release of additional photons, thereby amplifying the light.

In summary, the correct answer reflects the fundamental principle of how lasers operate, emphasizing the significance of the excited state achieved via external energy input.