Stimulated Absorption is the starting point to achieve laser.

It happens when a photon of light having energy E₂ – E₁ = hυ (as shown in the figure) is incident on an atom in the ground state, the atom in the ground state E₁ may absorb the photon and jump to a higher energy state E₂. This process is called stimulated absorption or induced absorption. This is called so because the incident photon has stimulated the atom to absorb the energy.

The above is the diagram to show the stimulated absorption in laser. In the next articles, when will learn about stimulated emission and spontaneous emission. These processes are a must to achieve the laser that is light amplification by stimulated emission of radiation.