Population inversion is the process of achieving a greater population of photons in a higher energy state as compared to the lower energy state. The population inversion technique is mainly used for light amplification purposes. In the normal state of matter, the population of photons will be such that most of the photons reside in the ground or lowest levels, leaving the upper levels somewhat depopulated. When photons are excited and fill these upper levels to the extent that there are more photons excited than not excited, then the population is said to be inverted. This concept is of fundamental importance in laser science because the production of population inversion is a necessary step in the workings of a standard laser.
To achieve population inversion, we need to supply energy to the laser medium. The process of supplying energy to the laser medium is called pumping. The source that supplies energy to the laser medium is called the pump source. The type of pump source used depends on the laser medium. Different pump sources are used for different laser mediums to achieve population inversion. Some of the most commonly used pump sources are as follows:
When the light source provides enough energy to the lower energy state electrons in the laser medium, they jump into the higher energy state E3. The electrons in the higher energy state do not stay for a long period. After a very short period, they fall back to the next lower energy state or metastable state E2 by releasing radiationless energy.
The metastable state E2 has a greater life than the lower energy state or ground state E1. Hence, more electrons are accumulated in the energy state E2 than the lower energy state E1. Thus, population inversion is achieved. Optical pumping is used in solid-state lasers such as ruby lasers.
Electric Discharge or Excitation by Electrons
Electric discharge refers to the flow of electrons or electric current through a gas, liquid, or solid.
In this method of pumping, electric discharge acts as the pump source or energy source. A high voltage electric discharge (flow of electrons, electric charge, or electric current) is passed through the laser medium or gas. The intense electric field accelerates the electrons to high speeds and they collide with neutral atoms in the gas. As a result, the electrons in the lower energy state gains sufficient energy from external electrons and jumps into the higher energy state. This method of pumping is used in gas lasers such as argon lasers.
The process of achieving population inversion in the gas laser is almost similar to the solid laser. The only difference is the pump source used for supplying energy and the type of material or medium (solid or gas) used as a laser medium. In solid lasers, an external light source like a xenon flash lamp is used as a pump source whereas, in gas lasers, a high voltage electric discharge is used as a pump source.
An inelastic collision is a collision in which kinetic energy is not conserved due to the action of internal friction between the atoms. Like the electric discharge method, here also a high voltage electric discharge acts as a pump source. However, in this method, a combination of two types of gases, say X and Y are used. The excited state of gas X is represented as X+ whereas gas Y is represented as Y+. Both X and Y gases have the same excited states (X+ and Y+).
When high voltage electric discharge passes through a laser medium having two types of gases X and Y, the lower energy state electrons in gas X will move to the exciting state X+ similarly the lower energy state electrons in gas Y moves to the excited state Y+.
Initially, during electric discharge, the lower energy state electrons in gas X or atom X gets excited to X+ due to continuous collision with electrons. The excited state electrons in gas X+ now collide with the lower energy state electrons in gas Y. As a result, the lower energy state electrons in gas Y gains sufficient energy and jump into the exciting state Y+. This method is used in the Helium-Neon (He-Ne) laser.
In thermal pumping, heat acts as the pump source or energy source. In this method, population inversion is achieved by supplying heat into the laser medium. When heat energy is supplied to the laser medium, the lower energy state electrons gain sufficient energy and jumps into the higher energy level.
The process of achieving population inversion in thermal pumping is almost similar to the optical pumping or electric discharge method, except that in this method heat is used as a pump source instead of light or electric discharge.
If an atom or a molecule is produced through some chemical reaction and remains excited at the time of production, then it can be used for pumping. The hydrogen fluoride molecule is produced in an excited state when hydrogen and fluorine gas chemically combine. The number of produced excited atoms or molecules is greater than the number of normal state atoms or molecules. Thus, population inversion is achieved.
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