Population inversion is the process by which a laser medium is excited to create more atoms in higher energy state than in the ground state. This allows stimulated emission to happen and produces a coherent and monochromatic beam of light. It is a fundamental principle in the operation of lasers. To achieve population inversion, atoms must be continuously excited from a lower energy level to a higher energy level and the process by which the atoms are excited to a higher energy level is called pumping. The laser must be pumped at a suitable wavelength to achieve this condition.
Electrons within an atom occupy discrete energy levels. When an atom is in the ground energy state, its electrons jump to higher energy levels. When they return to their original energy levels, they release energy in the form of light. This process is called spontaneous emission. If the atoms are excited by an external photon, the process is known as stimulated emission. The light output then obtained will be coherent and monochromatic. In population inversion, more electrons in the ground state are excited to higher energy levels. This state is necessary for lasing action, as it allows stimulated emission to take place. In other words, a population inversion occurs when the number of atoms in an excited state is greater than the number of atoms in the ground state.
Condition for Population Inversion
Let N1 be the number of atoms in the ground state E1 and N2 be the number of atoms in the higher energy state E2. Generally, the atoms in the ground state will be higher than that of the higher energy state and this is the thermal equilibrium condition given by N1>N2. But, for population inversion to occur, the number of atoms in E2 must be greater than E1. i.e., N2>N1
Figure 1: Energy level diagram
Pumping is provided to achieve the condition of population inversion. Some external factors are used to increase the population of higher energy levels. It can be done by pumping energy into a laser medium, such as a gas, liquid, or solid. The energy can be in the form of heat, light, or electrical current and it is absorbed by the atoms in the medium, causing them to become excited and move to higher energy levels.
In some types of lasers, such as gas lasers, the laser medium is excited by an external electrical discharge. In solid-state lasers, the medium is usually excited by a flashlamp or another laser. In semiconductor lasers, the medium is a p-n junction that is forward-biased to create a population inversion.
Types of Pumping
There are different types of pumping such as:
Figure 2: Optical Pumping
In optical pumping, strong light sources are used to increase the atoms or molecules in the higher energy states than the ground state. By providing sufficient energy, the light source causes electrons in the laser medium to undergo a transition from their lower energy state to the higher energy state E3. However, electrons in the higher energy state are not stable and quickly de-excite to the next lower energy state or metastable state E2 by emitting energy in the form of light. The metastable state E2 has a longer lifespan than the lower energy state or ground state E1, leading to an accumulation of more electrons in the E2 state than in the E1 state resulting in population inversion. This type of pumping is used only in solid-state lasers such as ruby laser, Nd:YAG laser, etc.
Figure 3: Electrical Pumping
In electrical pumping, an external electric field or electric power supplies are used. Generally, electrical pumping is provided for gas lasers such as CO2 lasers, Argon lasers, etc. When the light is switched ON, the current flows through the gases. Then the excited electron will collide with the gaseous molecule and these molecules will absorb the energy and gets excited to a higher energy level. It increases the atoms in the higher energy state than the lower energy achieving population inversion.
Figure 4: Thermal Pumping
Thermal pumping involves the use of heat as an energy source or pump source to achieve population inversion in the laser medium. In this type of pumping, when heat is supplied to the laser medium, the lower energy state electrons gain sufficient energy and jump into the higher energy level resulting in the inversion of the population. It is almost similar to that of optical pumping or electric discharge method, except that in this method heat is used as a pump source instead of light or electric discharge. Examples of lasers that use thermal pumping include carbon dioxide lasers and chemical lasers.
Chemical pumping is a method used to excite atoms or molecules in a laser medium by means of a chemical reaction. When a chemical reaction produces an atom or molecule that remains in an excited state, it can serve as a pump. For example, when hydrogen and fluorine gas combine, the resulting hydrogen fluoride molecule is produced in an excited state. This leads to a higher number of excited atoms or molecules being produced than those in a normal state, resulting in population inversion. It is particularly well-suited for use in liquid lasers.
Excitation by In-elastic collision
This type of pumping also occurs in certain gas lasers such as the He-Ne laser. In this case, the highly accelerated electron (e-) will collide with the first gaseous atom (He) and this molecule will absorb energy from the electron and jump into the excited state. Then, the excited state atom (He*) will collide with another molecule of the gas (Ne) which is present in the ground state and comes back to the ground state.
The excited state gas molecule (Ne*) is the active medium and lasing action is caused by this atom. Excimer lasers and gas lasers are examples of lasers that use this method of pumping.
Some of the examples of lasers that use different pumping methods:
Ruby laser, Nd:YAG laser, Semiconductor laser
Semiconductor laser, Diode-pumped solid-state laser, Helium-Neon laser
Carbon dioxide laser, Chemical lasers
Hydrogen fluoride laser, Chemical oxygen iodine laser
Excimer laser, Gas lasers
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