When light is incident on a semiconductor material, some photons get absorbed in the material which then transfer their energy to the electrons in the ground state and cause them to migrate to the excited state. This phenomenon is called absorption.

The transition of an electron from the excited state to the ground state can happen as a result of the natural tendency of the electron without the action of any external agent. The radiation produced as a result of such transitions is called as spontaneous radiation.

When electron is in an excited state in a semiconductor material, and we illuminated this semiconductor with external photons, the externally incident photons may cause the electron in the excited state to jump down to the ground state by releasing the excess energy in the form of a photon.

The externally incident photon does not get absorbed in the process; it just initiates the generation of this new photon by causing the electron in the excited state to release its energy and come down to the ground state.

Stimulated emission of radiation is the process whereby photons are used to generate other photons that have exact phase and wavelength as that of parent photon. Thus stimulated emission is caused by external stimuli.