Laser-I

Module 7 : Laser- I

Lecture : Introduction - Basics of stimulated emission


1.	Introduction:
	We have learnt that the energy levels in atom, ions and molecules are discrete. The lowest possible energy level is known as the ground state and higher energy levels are called the excited states. As the energy of the excited states increase, the separation between the adjacent energy levels become smaller and smaller until the separation becomes so small that the energy levels appear continuous. Such continuous spread of energy is called the continuum .
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	There are three ways in which an incident radiation can interact with the energy levels of atoms.
	Absorption:
	An electron in one of the lower level (ground state or a lower lying excited state) with energy $E_i$ can make a transition to a higher level having energy $E_f$ by absorbing an incident photon. Absorption can occur only when the frequency of the