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 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.