Question

What is population inversion and optical pumping? Explain.

Answer 1

Hint: In the band diagram of semiconductors, the population of charge carriers is always more in the valence band as compared to the conduction band in normal conditions. We can invert the population of charge carriers by a procedure called optical pumping.

Detailed step by step solution:
In an intrinsic semiconductor, there are two energy bands which the charge carriers can occupy. The lower energy band called valence band is completely filled with charge carriers while the upper energy band called conduction band is empty which can be occupied by charge carriers. They are separated from each other by an energy gap called band gap which is forbidden and no charge carrier can occupy any state of energy in this gap.

In normal conditions, the conduction band of the semiconductor is empty while the valence band is filled with the charge carriers. The phenomenon in which we invert this configuration is known as population inversion. When population inversion takes place, the conduction band gets filled with charge carriers while the valence band becomes empty.
The population inversion is attained by a process called optical pumping where external energy supplied to the semiconductor through photons. The charge carriers in the valence band absorb the incident photon energy crossing the band gap to occupy the conduction band.

Note: This phenomenon of population inversion and optical pumping to obtain population inversion is very important in constructing lasers. When the population inversion happens, the charge carriers eventually jump back to the valence band releasing energy in the form of photons of light which is how a laser operates.