Subject :- Applied Physics 2.

Topic :- Laser.

Difficulty :- Medium.

: With the external radiation being incident on the medium and under steady state condition, the three processes viz- Absorption, spontaneous emission and stimulated emission occurs simultaneously. Atoms may absorb the incident radiation and make a transition to upper state or atoms may emit radiation and drop to lower state. In order to make N1 and N2 constant, number of upward transition must be equal to number of downward transitions.

Thus, Nab = Nsp + Nst

B12N1Q = A21N2 + B21N2Q

At thermal equilibrium ratio of stimulated to spontaneous transition is very small and stimulated process is negligible

(Stimulated transition)/(spontaneous transition) =B21N2Q/A21N2 =B21Q/A21 ------------------------------(1)

Also,stimulated/absorption =B21N2Q/B12N1Q =N2/N1 ---------------------------------------(2)

From equation (1) and (2) it is observed that to achieve stimulated emission exclusively, radiation density should be high and N2>N1. That means more atoms should be present in higher energy state. So long as N1> N2, absorption process will dominate.

In the metastable state the life time of atoms is around milliseconds, which is much longer than the time required for spontaneous emission to take place. Hence a large number of atoms gets accumulated at this level making the population inversion and stimulated emission possible.

Usually atoms have the tendency to return to ground state releasing the absorbed energy. Hence, the population of atoms in ground state is greater than that of excited state. For laser action, there should be more number of atoms in higher energy state. So, state of population inversion cannot be achieved naturally. It has to be induced artificially by continuously raising a large number of atoms to the higher energy state with continuous supply of external energy. This can be achieved with the pumping mechanism.