Population Inversion

POPULATION INVERSION

It is a situation in which the number of atoms in higher energy state is more than that in lower energy state.

Usually at thermal equilibrium, the number of atoms N, (population of atoms) at higher energy state is much less than the number of atoms (population of atoms) at lower energy state N1 . i.e., N1 > N2 (Fig. 5.6)

The state of achieving more number of atoms in higher energy state than that of lower energy state (i.e., N2 >N1) is known as population inversion (Fig. 5.7).

Conditions for population inversion

• There must be atleast two energy levels (E1 and E2). 

• There must be a source to supply the energy to the medium. 

• The atoms must be continuously raised to the excited state.

Active Medium or Material

A medium in which population inversion above threshold inversion density is achieved is known as active medium. It is also called active material.

The inversion density which is just enough to compensate for the losses in the medium is called thershold inversion density.

Pumping action

The process to achieve population inversion in the medium is called pumping action.

It is an essential requirement for producing a laser beam.

Methods for pumping action

The methods commonly used for pumping action are

(i) Optical pumping (excitation by photons) 

(ii) Electrical discharge (excitation by electrons) 

(iii) Direct conversion 

(iv) Inelastic collision between atoms.

(i) Optical pumping

When the atoms are exposed to light radiation of energy hv, atoms in the lower energy state absorb these radiation and go to an excited state (Fig. 5.8). This is known as optical pumping.

This type of pumping is used in solid state lasers like ruby and Nd-YAG lasers.

(ii) Electrical discharge (excitation by electrons)

In this method, the electrons are produced in an electrical discharge tube. These electrons are accelerated to high velocities by a strong electrical field. Now, accelerated electrons collide with the gas atoms.

During the collision, the energy of the electrons is transferred to gas atoms. Thereby atoms gain energy and go to excited state (Fig. 5.9). This results in population inversion. This is known as electrical discharge.

The energy transfer is represented by the equation

where

A - Gas atom (or molecule) in ground state

A* - Same gas atom in excited energy state

e*- Electron with more kinetic energy

e - Same electron with less energy.

This method of pumping is used in gas lasers like argon and CO2 lasers.

(iii) Direct conversion

In this method, the electrical energy is applied to a direct band gap semiconductor like GaAs. The recombination of electrons and holes takes place. During the recombination process, the electrical energy is directly converted into light energy (Fig. 5.10).

This method of pumping is used in semiconductor diode laser (e.g. GaAs laser).

(iv) Inelastic collision between atoms

In this method, a combination of two gases (say A and B) is used. The excited energy levels of gases of A and B nearly coincides each other. (Fig. 5.11).

During the electrical discharge, atoms of gas A are excited to higher energy states A* due to collision with the electrons.

e* - Electron with more kinetic energy

e - Same electron with less energy.

Now A* atoms at higher energy state collide with B atoms in lower energy states. Due to this inelastic collision, B atoms gain energy and excited to higher state B*. Hence, A atoms lose energy and return to lower state.

Thus, population inversion in the energy states of B is achieved. This method is used in He - Ne laser.