Photon-matter interaction processes

Course Details

Field of Study: Electronics Engineering
Course Name: Optoelectronics
Subtitle: Photon-matter interaction processes

There are three fundamental processes electrons make transitions between two energy levels upon a photon of energy. A two-level system is a model system that only contains two energy levels with which the photon interacts.


Three basic photon-matter interaction processes are:

  • Absorption
  • Spontaneous emission
  • Stimulated emission>

Absorption takes place when the quantum energy hv equals the energy difference between the two energy levels (a resonant condition); the atom gains a quantum of energy. Absorption leads to the attenuation of an optical signal.

Spontaneous emission
When an atom emits a photon, it loses a quantum of energy in the process. An electron spontaneously falls from a higher energy level E2 to a lower one E1, the emitted photon has a frequency V= (E2 – E1) / h.
This photon is emitted in a random direction with arbitrary polarization. The probability of such a spontaneous jump is given quantitatively by the Einstein coefficient for spontaneous emission (known as the “Einstein A coefficient”) defined as A21 = “probability” per second of a spontaneous jump from level L2 to level L1.
Spontaneously emitted photons are random in phase and polarization and are emitted in all directions, though their frequencies are still dictated by the separation between the two energy levels, subject to a degree of uncertainty determined by the linewidth of the transition.
Therefore, stimulated emission results in the amplification of an optical signal, whereas spontaneous emission adds noise to an optical signal.

Stimulated emission
In this case the emission of a photon is triggered by the arrival of another, resonant photon.
Einstein in 1917 first pointed out that stimulated emission is essential in the overall balance between emission and absorption, about reaching thermal equilibrium for a system of atoms. (Einstein Relations)
A photon emitted by stimulated emission has the same frequency, phase, polarization and propagation direction as the optical radiation that induces the process.

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