Spontaneous and Stimulated Emission
1) Absorption
An atomic system is characterized by discrete energy states, and usually the atoms exist in the lowest energy state, which is normally referred to as the ground state. An atom in a lower energy state may be excited to a higher energy state through a variety of processes. Due to thermal collision particles gain some energy and can excite to higher energy levels. On the other hand, atoms can also be excited to higher levels when electromagnetic radiation of suitable frequency is absorbed by them. Such process are called as stimulated absorption shown in Fig 1.a.
2) Spontaneous emission
If an atom is initially in the upper state E2, it can come down to lower state E1 by emitting a photon of energy hv as shown in Fig. 1.b. This is known as spontaneous emission. This is the natural radiation decay process that is inherent in all excited states of all materials. However, such emission is not always the dominant decay process.
3) Stimulated emission
In stimulated emission (Fig 1.c), an incident photon of frequency triggers an atom in an excited state to emit radiation. The rate of stimulated emission depends both on the intensity of the external field and also on the number of atoms in the upper state.
4) Population inversion
In a system of atoms in thermal equilibrium, the number of atoms in the ground state is generally much greater than in a higher energy state. This is known as normal population of atoms among the available energy states. A state in which the number of atoms in higher energy state is greater than that of lower energy state is known population inversion.
5. Metastable state
In laser system the life time of atoms in ordinary excited states are usually of the order of 10-8 s. In addition, there are also some temporarily stable states, in which the life time is around 10-3 s. These long-lives states are called as metastable states. When there is a transition from these metastable state to lower energy state, laser is formed.