POPULATION INVERSION

Presented by.,

S.Shanmathee.,

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

kTENgN

ii

jj

j

jN

Populations of two level energy systems

At thermal equilibrium After population inversion has been produced

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”.

Cont..

• This pumping energy can be supplied by

using laser technologies.

• Pumping produces the non-equilibrium

situation.

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 .

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

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.

Four level system

Before pumping After pumping

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 .

Other ways to achieve pumping

• Optical pumping

• Electrical discharge or electron bombardment.

• The release of chemical energy.

• The passage of current.