Question

Discuss the physical principles behind lasers.

Answer 1

LASER literally means Light Amplification by Stimulated Emission of Radiation. The electrons in amy atom can only aquire certain specific energies (Quantization of enrgies) and transition of electrons can take place between different orbits of fixed enrgies known as shells. In general when energy is supplied from outside to any atom the electron from ground state absorbs the energy and moves to an excited state. As the electron is unstable in the excited state so it jumps back to it's initial state thereby releasing energy given by:

E₁ - E₂ = ℏω ; where ω is the frequency af released photon.

This process in general is spontaneous in nature. However if one perturbs the electron by the electric field of a photon with frequency ω, it may release a second photon which is identical to the first one (same frequency and same phase) and then the atom will decay back to it's ground state thereafter. This phenomenon is termed as stimulated emmission.

Now this process of stimulated emmission can be amplified by keeping a number of excited atoms between two mirrors and applying a perturabation of some electric field of particular frequency which then triggers a chain reaction of photons with the help of these atoms thereby producing a huge amount of identical photons. These identical photons are completely coherent and produces no diffraction when released.

One important thing is to maintain the atoms in their excited state which is acheived by pumping energy from outside continuously. In reality the number of electron are not kept in the actual excited level but between the excited and the ground state, known as meta-stable state. This phenomenon of maintaing a large number of electrons in the meta-stable state as compared to the ground state is known as Population Inversion.