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Define LASER. What are the transition probabilities of LASER?

LASER

Lasers are the source of a special form of light. The word LASER is an acronym for “Light Amplification by Stimulated Emission of Radiation”. Laser is a device in which we use process of stimulated emission to amplify the infrared, visible or ultraviolet electromagnetic radiation.

Principle of LASER:

To understand the principle of LASER it is essential to understand the three quantum processes through which an electromagnetic radiation interacts with the atoms of matter, viz. (i) Stimulated absorption, (ii) Spontaneous emission and (iii) Stimulated emission.

• Stimulated absorption:

Generally, the atoms of any material reside in ground energy state (E₁) at low temperatures. These atoms can be excited to a higher energy state (E₂) when they absorb a photon of energy ΔE = E₂ – E₁ = hu, where h is the Planck’s constant and u is the frequency of photon. This process is known as stimulated absorption of radiation (Fig.1).

• Spontaneous emission:

At the E₂ level, atoms are unstable and spontaneously return to their ground state E₁, with the emission of a photon of energy ΔE in a very short time of the order of 10^-9 s. This process is referred to as spontaneous emission of radiation (Fig.2).

• Stimulated emission:

If in a system where there exist atoms in excited state, we allow a photon of energy E₂ – E₁, then the incident light photon may force excited atom to go back to its ground state emitting thereby an another light photon of same frequency, phase and same state of polarization. These two photons are perfectly coherent. This process is known as
stimulated emission of radiation (Fig. 3).