Lasers

Lasers amplify light by stimulated emission.  The presence of one photon of the right energy hf = E₂ - E₁ stimulates the emission of a second photon with the same frequency, phase, direction and polarization, as a quantum system makes a transition from a higher to a lower quantized energy level.

For the amplification to work, we need a population inversion.  To establish this population inversion, we need a pumping mechanism, and a pumping scheme.  Optical pumping is one possible pumping mechanism.  It requires a pumping scheme with more than two levels.

The laser medium can be a solid, liquid, or gas.  A common laser is the He-Ne laser.  It is pumped by maintaining a DC electrical discharge across a narrow tube containing a gas mixture.  Neon gas is the laser medium.