Define population inversion & Meta stable state | construction and working of semiconductor laser

Define the terms population inversion and Meta stable state. Explain the construction
and working of the semiconductor laser.

Answer:-

Population Inversion:- The situation in which the number of atoms in the higher energy state exceeds that in the Lower energy state is known as population inversion.

Meta Stable State:-It is the state where the atoms get excited and remain in the excited state for a longer time than the normal state.

Construction and working of semiconductor laser

Construction:-
Ga − As LASER diode belongs to direct band gap semiconductors. The n-section is derived by doping the substrate with Tellurium and the p-section is derived by doping the substrate with Zinc.

The diode used is in the form of a cube with a dimension of 0.4 mm. The depletion region is a thickness of 0.1 micrometers and lies horizontally as shown in the figure. The current is passed through the ohmic contacts provided to the top and bottom faces.

The front and back faces are polished and made highly parallel to each other to have a LASER cavity. The other two faces are roughened.

Working:-
The Diode is forward biased using an external source. Therefore electrons and holes flow across junction. Hence the current flows through the diode. When a hole meets an electron it recombines with electron emitting a photon.

This could be considered as the transition of electron from conduction band to valance band. When the current is low spontaneous emission is predominant. If the current is sufficiently high population inversion is achieved.

The photons liberated initially due to spontaneous emissions induce further stimulated emissions. The LASER
cavity helps in the amplification of light. Finally this results in an avalanche of photons and hence the LASER action is achieved. If the GaAs semiconductor is used then the wavelength of the LASER emitted is 840nm.