As discussed before that in a semiconductor, the absorption of photons with energy more than the band gap can transfer electrons from the valence band to the conduction band conserving the energy and momentum. When these electrons can return to the ground state by emitting photons. The emitted radiation is often called the luminescenceLuminescence. Electrons and the holes can radiatively recombine via two fundamentally different pathways, namely the spontaneous and stimulated emission processes. In the stimulated process, the emission of the photon demands the presence of photons with the same energy, momentum and polarization states.