Population inversion

ECE

1. POPULATION INVERSION Presented by., S .Shanmathee .,

2. What is Meant by Population Inversion?? It is the condition required for light amplification. Non-Equilibrium distribution of atoms among various energy levels of the atomic system. The Boltzmann distribution which applies to the system in thermal equilibrium is., Where., =Population Intensity of the j th energy level If E j increases N j decreases with constant tempurature

3. Populations of two level energy systems At thermal equilibrium After population inversion has been produced

4. Pumping If the energy difference between E 1 and E 2 is equals to kT (nearly 0.025eV at room temp). Then the population inversion of the upper level would be 1/e(0.37) of that of the lower level. For large energy difference then visible radiation will be(2.0eV).then population of the upper level is negligible. So to achieve population inversion we need to supply large amount of energy to excite atoms into E 2. This process named as “PUMPING”.

5. Cont.. This pumping energy can be supplied by using laser technologies. Pumping produces the non-equilibrium situation.

6. Attainment of population inversion Main method used to achieve this population inversion is “stimulated absorption” The energy levels are pumped by intense irradiation of the system. Assume g 1 = g 2 .then further absorption and emissions are equal.(two level) For three level system proposed by bloembergen . Initially it obeys boltzmann’s law. If the collection of atoms is illumination the electrons can be excited( i.e pumped) into E 2 form E .

7. Cont.. From E 2 electrons decay by non- radiative processes to the level E 1 and Population inversion created b/w E 1 and E . Boltzmann distribution before pumping distribution after pumping and the transitions

8. Cont.. E 2 to E 1 =rapid transition. E 1 to E =slow transition.(E 1 is metastable state) This will create the built-up of atoms in E 1 then population inversion achieved b/w E 1 and E . ( N 1 > N ) Examples :ruby –requires very high pump powers.

9. Four level system Before pumping A fter pumping

10. Cont.. It has less pumping requirements. If ( E 1 – E ) is very large compare to kT then populations at E 1 , E 2 and E 3 are very small at thermal equilibrium. If atoms pumped to E 3 from ground state from rapid transition occur to E 2. population inversion achieved b/w E 2 and E 1 . If lifetime is short between E 3  E 2 and E 1  E are short population inversion maintained between E 2 & E 1 .

11. Other ways to achieve pumping Optical pumping Electrical discharge or electron bombardment. The release of chemical energy. The passage of current.

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