Applied Physics I: Chapter 10: Applied Optics - Lasers
Population Inversion
Lasers | Optics
The chance for emission (or) the absorption depends only on the number of atoms in the ground state and in the excited state.
POPULATION
INVERSION
Consider
two energy level systems E1 and E2. Suppose a photon of
energy equal to the energy difference between the two energy levels, incident
on the system, then there is equal chances for stimulated emission and
absorption to occur. At this situation, the chance for emission (or) the
absorption depends only on the number of atoms in the ground state and in the
excited state.
Let
N1 be the number of atoms in ground state and N2 be the
number of atoms in excited state. Then,
If
N1>N2 there is more chance for absorption to take
place
and
If
N2>N1 there is more chance for stimulated emission to take
place.
Therefore,
the number of atoms in the excited state should be increased by some means.
Thus the state of achieving more number of atoms in the excited state compared
to the ground state atoms is called population inversion.
We
know from Boltzmann's distribution law N1/N2 = e(E2‒E1)/KBT
Case (i)
If T is +ve N1=N2e+ve
For
example,
If
N2 = 5 and if (E2‒E1) / KBT ≈ 2
Then,
N1
= 5. e+2
N1
= 36.9
N1 > N2 Since 36.9 > 5
Case (ii)
If T is ‒ve N1=N2e‒ve
For
example,
If
N2 = 5 and if (E2‒E1) / KBT ≈ ‒2
Then,
N1
= 5. e‒2
N1
= 0.6766
N2 > N1 Since 5 >0.6766
This
shows that the number of atoms in excited state can be made more than the
number of atoms in the ground state only under negative temperature.
But,
the negative temperature is practically not possible. Therefore population
inversion can be achieved by some other artificial process known as pumping process.
Active medium:
The medium in which the population inversion takes place is called active
medium.
Active centre:
The material in which the atoms are raised to excited state to achieve
population inversion is called active centre.
Applied Physics I: Chapter 10: Applied Optics - Lasers : Tag: Applied Physics : Lasers | Optics - Population Inversion
Applied Physics I: Chapter 10: Applied Optics - Lasers
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