Theory of Laser

THEORY OF LASER 

Interaction of light radiation with materials

Consider an assembly of atoms in a material which is exposed to light radiation (a stream of photons with energy hv).

In general, three different processes occur when light radiation interacts with a material. They are

1. Stimulated absorption 

2. Spontaneous emission 

3. Stimulated emission

 Process - 1  

Stimulated absorption

An atom in ground state with energy E1 absorbs an incident photon of energy hv and is excited to higher energy state with energy E2 (Fig. 5.1).

This process is known as stimulated or induced absorption.

It occurs only when the incident photon energy hv is equal to the energy difference between excited state and ground state (E2 - E1).

For each transition, a certain amount of energy (hv) is absorbed from the incident light beam.

The excited atoms do not stay in the higher energy state for a longer time. It is the tendency of atoms in excited state to come to the lower energy state.

Thus, the atoms in excited state quickly return to ground state by emitting a photon of energy hv.

The emission of photons takes place in two ways, namely

(a) spontaneous emission

(b) stimulated emission

Process - 2 

Spontaneous emission

The atom in the excited state E2 (higher energy state) returns to ground state E1 (lower energy state) by emitting a photon of energy hv (ΔE = E2- E1) without the influence of any external agency (Fig. 5.2).

Such emission of light radiation which is not triggered by any external influence is called spontaneous emission.

It is a random and also uncontrollable process.

Process - 3 

Stimulated emission

Einstein suggested that there must be another mechanism by which an atom in excited state can return to ground state.

He found that there is an interaction between the atom in excited state and a photon. During this interaction, the photon triggers the excited atom to make transition to ground state E1 (Fig. 5.3).

This transition produces a second photon which is similar to triggering photon with respect to frequency, phase and propagation direction.

Such kind of forced emission of photons by the incident photons is called stimulated emission. It is also known as induced emission. It plays a key factor