A laser amplification occurs over the principle of stimulated emission. Stimulated emission is the process by which an incident photon interacts with the excited electron and forces it to return to the ground state. In stimulated emission, the light energy is supplied directly to the excited electron instead of supplying light energy to the ground state electrons. It is considered to be an artificial process.
In spontaneous emission, the electrons in the excited state will remain there until their life is over. After completing their lifetime, they return to the ground state by releasing energy in the form of light. However, in stimulated emission, the electrons in the excited state need not wait for the completion of their lifetime. An alternative technique is used to forcefully return the excited electron to the ground state before the completion of their lifetime. This technique is known as stimulated emission. When the incident photon interacts with the excited electron, it forces the excited electron to return to the ground state. This excited electron release energy in the form of light while falling to the ground state.
In stimulated emission, two photons are emitted, one is due to the incident photon and another one is due to the energy release of excited electron. Thus, two photons are emitted.
Spontaneous emission is the process by which electrons in the excited state return to the ground state by emitting photons. The electrons in the excited state can stay only for a short period. The time up to which an excited electron can stay at a higher energy state (E2) is known as the lifetime of excited electrons. Thus, after the short lifetime of the excited electrons, they return to the lower energy state or ground state by releasing energy in the form of photons.
In spontaneous emission, the electrons move naturally or spontaneously from one state (higher energy state) to another state (lower energy state) so the emission of photons also occurs naturally. Therefore, we have no control over when an excited electron is going to lose energy in the form of light.
The photons emitted in the spontaneous emission process constitute ordinary incoherent light. Incoherent light is a beam of photons with frequent and random changes of phase between them. In other words, the photons emitted in the spontaneous emission process do not flow exactly in the same direction as incident photons.
Absorption of radiation
Absorption of radiation is the process by which electrons in the ground state absorbs energy from photons to jump into a higher energy level.
Let us consider two energy levels (E1 and E2) of electrons. E1 is the ground state or lower energy state of electrons and E2 is the excited state or higher energy state of electrons. The electrons in the ground state are called lower energy electrons or ground state electrons whereas the electrons in the excited state are called higher energy electrons or excited electrons.
In general, the electrons in the lower energy state can’t jump into the higher energy state. They need sufficient energy to jump into the higher energy state. When the light energy is equal to the energy difference of the two energy levels (E2 – E1) is incident on the atom, the ground state electrons gain sufficient energy and jump from the ground state (E1) to the excited state (E2).
The absorption of radiation or light occurs only if the energy of the incident photon exactly matches the energy difference of the two energy levels (E2 – E1).
Stimulated emission is faster compared to absorption and spontaneous emissions.
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