The concept of
stimulated emission was first put forward by Albert Einstein in 1917.
Let us consider two
energy levels E1 and E2 in a material. Let us assume that
the atom is initially at energy level E2. Also,
consider that an electromagnetic wave of frequency v is incident on a given
material. This wave has the same frequency as that of the specific material.
Therefore, there is a high degree of probability that this wave will force the
atom to undergo a change from energy level E2 to energy level E1.
In this case, the energy difference E2 - E1 is received in
the form of an electromagnetic wave which adds to the incident wave. Here, it
is to be noted that the atom is already in the excited state E2. Before
it could come to the ground state, due to the spontaneous emission process, if it
is irradiated with a photon, whose energy is exactly equal to E2 - E1.
The incident photon will stimulate the excited atom to emit one photon of
exactly the same energy, as that of the incident photon. Thus, two photons will
be emitted in this process. This phenomenon is known as Stimulated
Emission.
Note: The most remarkable feature of the stimulated emission is both the emitted photons will have the same frequency, phase direction and polarization, as that of the incident photon. So, in this process, we give the input of one incident photon and obtain the two photons identical in all respects as an output. Thus, amplification of radiation takes place by the stimulated process.
You can also look at the difference between spontaneous and stipulated emission.