Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. Laser is a highly “monochromatic coherent beam of light of very high intensity”. In 1960 Mainmann built the first “LASER” using Ruby as active medium.
Interaction of Radiation with matter
1. Stimulated Absorption:
When an atom in the ground state say E1 absorbs a photon of energy (E2 − E1) it makes transition into exited state E2. This is called Stimulated or Induced absorption. It is represented as follows,
Photon + Atom = Atom*.
2. Spontaneous Emission:
Spontaneous emission is one in which atom in the excited state emits a photon when it returns to its lower energy state without the influence of any external energy.
Consider an atom in the excited state E2. Excited state of an atom is highly unstable. With in a short interval of time, of the order of 10−8 sec,atom returns to its lower energy state say E1 and emits difference in energy in the form of photon of energy hν = E2 - E1 spontaneously.
If the two atoms are in the same excited state and returns to some lower energy states two photons of having same energy are emitted. These two photons may not travel in the same direction. They produce in-coherent beam of light. Spontaneous emission is represented as follows,
Atom* = Atom + Photon.
3. Stimulated Emission:
Consider an atom in the excited state E2. If a photon of energy E2 - E1 is made to incident on the atom in the excited state E2.
The incident photon forces (stimulates) the atom in the excited state to make transition in to ground state E1 by emitting difference in energy in the form of a photon. This type of emission in which atom in the excited state is forced to emit a photon by the influence of another photon of right energy is called stimulated emission. Stimulated emission can be represented as follows.
Photon + atom* = Atom + (photon + photon).
When stimulated emission takes place, incident photon and the emitted photon are in phase with each other and travel along the same direction. Therefore they are coherent.