UNIT-III

Optoelectronics

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Radiative transitions: Absorption, Spontaneous and Stimulated emission

Radiative recombination:�

• When one electron leaving the energy level creates a hole, the process is called electron- hole pair generation. Similarly, when an electron combines with the hole, the process is called recombination.
• Generation and recombination of electron - hole pair essentially require external energies such as optical, electrical or thermal energies. Such processes are mainly between the valence band and the conduction band.
• Radiative recombination occurs when an electron in conduction band recombines with a hole in the valence band and the excess energy is given out as photon.
• Optical process associated with radiative recombination is: Absorption, Spontaneous emission and Stimulated emission.

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Absorption of radiation or light�

• The process of absorbing energy from photons is called absorption of radiation.
• It is well known that there are different energy levels in an atom. The electrons that are very close to the nucleus have lowest energy level. These electrons are also known as ground state electrons.
• Let us consider that the energy level of ground state electrons or lower energy state electrons is E₁ and the next higher energy level or higher energy state is E₂.
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• When ground state electrons or lower energy state electrons (E₁) absorbs sufficient energy from photons, they jump into the next higher energy level or higher energy state (E₂). In other words, when the ground state electrons absorb energy which is equal to the energy difference between the two energy states (E₂ – E₁), the electrons jumps from ground state (E₁) to the excited state or higher energy level (E₂). The electrons in the higher energy level are called excited electrons.
• The light or photons energy applied to excite the electrons can be mathematically written as
• hv = E₂ – E₁
• Where h = Planck’s constant
• V = Frequency of photon
• E₁ = Lower energy level electrons or ground state electrons
• E₂ = Higher energy level electrons or excited state electrons
• Absorption occurs only if the energy of photon exactly matches the difference in energy between the two electron shells or orbits.

Spontaneous emission�

• The process by which excited electrons emit photons while falling to the ground level or lower energy level is called spontaneous emission.
• Electrons in the atom absorb energy from various sources such as heat, electric field, or light. When the electrons in the ground state or lower energy state (E1) absorb sufficient energy from photons, they jump to the excited state or next higher energy state (E₂).
• The electrons in the excited state do not stay for a long period because the lifetime of electrons in the higher energy state or excited state is very small, of the order of 10^-8 sec. Hence, after a short period, they fall back to the ground state by releasing energy in the form of photons or light.

The energy of the emitted photon is directed proportional to the energy gap of the material. The materials with large energy gap will emit high-energy photons or high-intensity light whereas the materials with small energy gap will emit low energy photons or low-intensity light.

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The energy of released photon is equal to the difference in energies between the two electron shells or orbits.

The energy of the excited electrons can also be released in other forms such as heat. If the excited state electrons release energy in the form of photons or light while falling to the ground state, the process is called spontaneous emission.

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In spontaneous emission, the electrons changing from one state (higher energy state) to another state (lower energy state) occurs naturally. So the photon emission also occurs naturally or spontaneously.

The photons emitted due to spontaneous emission do not flow exactly in the same direction of incident photons. They flow in the random direction.

Stimulated emission�

• The process by which electrons in the excited state are stimulated to emit photons while falling to the ground state or lower energy state is called stimulated emission.
• Unlike the spontaneous emission, in this process, the light energy or photon energy is supplied to the excited electrons instead of supplying energy to the ground state electrons.
• The stimulated emission is not a natural process it is an artificial process. In stimulated emission, the electrons in the excited state need not wait for natural spontaneous emission to occur. An alternative method is used to stimulate excited electron to emit photons and fall back to ground state.

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The incident photon stimulates or forces the excited electron to emit a photon and fall into a lower state or ground state.

The energy of a stimulating or incident photon must be equal to the energy difference between the two electron shells.

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In this process, the excited electron releases an additional photon of same energy (same frequency, same phase, and in the same direction) while falling into the lower energy state. Thus, two photons of same energy are released while electrons falling into the ground state.

In stimulated emission process, each incident photon generates two photons.

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The photons emitted in the stimulated emission process will travel in the same direction of the incident photon.

Many ways exist to produce light, but the stimulated emission is the only method known to produce coherent light (beam of photons with the same frequency).

All the photons in the stimulated emission have the same frequency and travel in the same direction.