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# What is Stimulated Emission?

Stimulated Emission is the basic principle of Laser amplification.

To understand Stimulated emission it is important to have knowledge about the Stimulated absorption and Spontaneous emission.

Let us discuss Einstein’s theory of the interaction of electromagnetic radiation with matter. He proposed that electromagnetic radiation interacts with matter in the following three steps.

• Stimulated Absorption
• Spontaneous Emission
• Stimulated Emission

## Stimulated Absorption

Firstly assume E1 and E2 be the energies of the ground and excited states of an atom. Suppose if a photon of energy hν= E1− E2 interacts with an atom present in the ground state, the atom gets excitation from ground state E1 to excited state E2. Thus, this process is called stimulated absorption. In short, Rate of  stimulated absorption depends upon the number of atoms present in the lowest energy state as well as the energy density of photons.

Stimulated absorption rate ∝ Number of atoms in the ground state

∝ The density of photons

## Spontaneous Emission

Let E1 and E2 be the energies of the ground and excited states of an atom. Firstly Suppose, a photon of energy hν= E1− E2 interacts with an atom present in the ground state. Then the atom gets excitation from ground state E1 to excited state E2. However, The excited atom does not stay longer in the excited state. Thus the excited atom gets de-excitation after its lifetime by emitting a photon of energy hν= E1− E2. This process called spontaneous emission. Also Spontaneous means by its own. Here excited atom comes to the ground state on its own. So it is named as spontaneous emission.

In short, the spontaneous emission rate depends upon the number of atoms present in the excited state.

Spontaneous emission ∝ Rate of availability of atoms in the excited state

## Stimulated Emission

This phenomenon is responsible for producing laser light. Let E1and E2 be the energies of the ground and excited states of an atom. Suppose, if a photon of energy hν= E1− E2 interacts with an atom present in the ground state. Hence the atom gets excitation from ground stateE1 to excited state E2. Let, a photon of energy hν= E1− E2 interacts with the excited atom within their lifetime; the atom gets de-excitation to the ground state by emitting another photon. These photons have the same phase and it follows coherence. This phenomenon is called stimulated emission.

In short, Rate of Stimulated emission depends upon the number of atoms present in the excited state as well as the energy density of photons.

Stimulated emission rate ∝ Number of atoms in the excited state

∝ Density of photons

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