A laser creates light by special actions involving a material called an optical gain medium.

The gain medium is nitrogen molecules in the gas phase.

The gain medium is usually pumped by a transverse electrical discharge.

Because of this, the gain medium is pumped to a highly excited state.

For the gain medium to amplify light, it needs to be supplied with energy.

The gain medium is excited by an external source of energy into an excited state.

The above-mentioned models yield the polarization of the gain medium.

This allows more energy absorption throughout the gain medium for a better transverse mode.

It has also been used as a gain medium for dye lasers.

First the pump beam pumps a spot in the gain medium.