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# Population inversion

ECE
Published on: Mar 4, 2016
Published in: Engineering

#### Transcripts - Population inversion

• 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 jth energy level If Ej increases Nj decreases with constant tempurature     )/exp( )/exp(0 kTEg kTENg N ii jj j jN
• 3. Populations of two level energy systems At thermal equilibrium After population inversion has been produced
• 4. Pumping • If the energy difference between E1 and E2 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 E2. • 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 g1 = g2 .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 E2 form E0 .
• 7. Cont.. • From E2 electrons decay by non-radiative processes to the level E1 and Population inversion created b/w E1 and E0 . Boltzmann distribution before pumping distribution after pumping and the transitions
• 8. Cont.. • E2 to E1 =rapid transition. • E1 to E0 =slow transition.(E1 is metastable state) • This will create the built-up of atoms in E1 then population inversion achieved b/w E1 and E0 . ( N1 > N0 ) • Examples :ruby –requires very high pump powers.
• 9. Four level system Before pumping After pumping
• 10. Cont.. • It has less pumping requirements. • If ( E1 – E0 ) is very large compare to kT then populations at E1 , E2 and E3 are very small at thermal equilibrium. • If atoms pumped to E3 from ground state from rapid transition occur to E2. population inversion achieved b/w E2 and E1 . • If lifetime is short between E3  E2 and E1  E0 are short population inversion maintained between E2 &E1 .
• 11. Other ways to achieve pumping • Optical pumping • Electrical discharge or electron bombardment. • The release of chemical energy. • The passage of current.