Population inversion is a non-equilibrium distribution of atoms among energy levels that is required for light amplification. It occurs when more atoms are in an excited, higher energy state than in a lower energy ground state. Various pumping techniques can induce population inversion by exciting atoms to higher energy levels. This includes laser excitation, optical pumping, electrical discharges, and chemical reactions. Once inverted, stimulated emission can occur, enabling applications like lasers that rely on stimulated amplification of light.

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.