Laser characteristics

LASER (Light Amplification by Stimulated Emission and Radiation)

Process involved in LASER:

In an atomic system there are infinite set of discrete energy levels. Let us consider two of these energy levels of an atom as shown below:

The atom can make a transition between these two energy levels by emission or absorption of photon of energy  .

where

 = energy of ground level

= energy of excited state

Energy of photon is 

(1) Absorption:

Let us consider an atom is initially in ground state whose energy is . If a photon whose energy is exactly equal to  is incident on this atom. The atom will absorb energy and get excited to the energy state . This phenomena is known as stimulated absorption. The energy difference   required by atom  to undergo transition is obtained from the energy of incident electromagnetic radiation. It can be shown as:

Rate of transition of absorption:

Negative sign is taken into account because number of atoms are decreasing.

(2) Spontaneous Emission:

After being in the excited state atom return to ground state due to unstability of the atom in excited state.

Let us consider two energy levels 1 and  2 of same gien material  and their energies being  and  such that . Now, let us assume that an atom of the material is initially in level 2. Since  atom will tend to decay to level 1. The corresponding energy difference i.e;   must therefore be released by the atom when this energy is delivered in the form of EM wave, process will be called Spontaneous emission. The frequency of the radiative wave is given by expression    

The spontaneous emission can be shown as:

Rate of transition of spontaneous emission:

Probability of spontaneous emission depends on

1. Type of atom.
2. Type of transition.
3. Independent on outer side circumstances.

(2) Stimulated Emission:

As the name implies the process requires stimuls from outside. In this process, an atom is in excited state return to the ground state and emitting a light photon only when it is forced to grew it. Stimulated emission is that in which the emitted photons are parallel to incident photon and are said to be coherent each other.

Let us again suppose that an atom is found initially in the level 2 and an EM wave of frequency  is incident on the material. Since, this wave has same frequency as that of atomic frequency. There is a finite probability that this wave will force the atom to undergo transition from level 2 to level 1. In this case, energy difference is delivered in the form of an EM wave which adds to the incident one. This is the phenomena of stimulated emission. It can be shown as:

Rate of transition of stimulated emission: