Laser-I

Module 7 : Laser- I

Lecture : Introduction - Basics of stimulated emission

	$\displaystyle \nu = \frac{E^\ast-E}{h}$
	$\includegraphics{laser3.eps}$
	Stimulated Emission:
	In 1917, Einstein showed that under certain conditions, emission of light may be stimulated by radiation incident on an excited atom. This happens when an electron is in an excited state $E^\ast$ and a photon whose energy is equal to the difference between $E^\ast$ and the energy $E$ of a lower lying level (could be the ground state) is incident on the atom. The incident photon induces the electron in the excited state to make a transition to the lower level by emission of a photon. The emitted photon travels in the same direction as the incident photon. Significantly, the new photon has the same energy as that of the incident photon and is perfectly in phase with it. When two waves travel in the same direction with a constant phase relationship, they are said to be coherent .