Laser is an acronym for light Amplification by Stimulated Emission of Radiation. Laser are used for producing an intense, monochromatic and unidirectional coherent beam of light.
The working of laser involves two most important terms
- Stimulated Emission
- Population Inversion
Spontaneous and Stimulated Emission
Consider a sample of free atoms, some of which are in ground state with energy E1 and some in the excited state with energy E2. The photons of energy hf = E2 – E1 are incident on this sample.
The incident photon is absorbed by an atom in the ground state E1 and the atom is excited to state E2. This process is called stimulated or induced absorption. Once the atom is excited, then two things can be happen to an atom,
1. Spontaneous Emission
The excited atom may decay by spontaneous emission by emitting a photon of energy hf = E2 – E1 in any arbitrary direction.
2. Stimulated Emission
The excited atom decays by stimulated or induced emission. In this case, the photon of energy hf = E2 – E1 induces the atom to decay by emitting photon of same energy, going in the same directions.
Thus, we obtain an intense, unidirectional beam, by stimulated emission. For proper working there would be more stimulated emission than spontaneous emission.
Population Inversion and Laser Action
Let us consider a simple case of material whose atoms can reside in three different states. E1 is the ground energy state. E2 is the excited state in which the atom can reside only for 10^-8 seconds and E2 is called metastable state in the atom can reside for 10^-3 seconds.
A matastable state is an excited state in which an excited electron is unusually more stable and from which the electron spontaneously fall to lower state after relatively longer time.
The transition to the metastable state are difficult as compared to other excited States. So instead of direct excitation to this state, the electrons are excited to higher levels for spontaneous fall to metastable state.
Let the incident photons of energy hf = E3 – E1 raised the atom to excited state E3 from ground state E1. The excited atoms do not decay back to E1 but the atom decay to E2 spontaneously. The atom reaches state E2 much faster than they leave state E2.
In this situation, the state E2 contains more atom than E1. This situation is called population Inversion.
The atom in metastable state E2 are bombarded by a photons of energy hf = E2 – E1. This results into induced or stimulated emission, giving an intense, coherent mono chromatic beam of light in the direction of the incident photons.
The emitted photons must be confined in the assembly long enough to stimulate further emissions from other excited atoms. This is achieved by using mirror at the to ends of the assembly. One end is made totally reflecting and the other end is partially transparent to allow the laser beam to escape. As the photon moves back and forth between the reflecting mirrors, they continue to stimulate other excited atoms to emit the photons. As the process continues the number of photons multiply, and the resulting radiation is, therefore, much more intense and coherent than light from ordinary source.