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Why is "population inversion" the key to laser magic?
In physics, especially statistical mechanics, population inversion is when there are more atoms or molecules in a system in higher-energy states than in lower-energy states. This concept is crucial in laser science because generating population inversions is a required step for standard laser work.
The concept of population inversion involves the interaction of light and matter, which is related to how lasers operate. Without a mechanism to bring the system into a state of population inversion, laser generation would be impossible.
Thermal equilibrium and Boltzmann distribution
To understand the concept of population inversion, you first need to understand some thermodynamics and how light interacts with matter. Suppose there is a set of N atoms, each atom can exist in two energy states: the ground state E1 and the excited state E2. When these atoms are in thermal equilibrium, according to Maxwell–Boltzmann statistics, the ratio of the number of atoms in the ground state and excited state is determined by the Boltzmann factor.
Therefore, when a system is in thermal equilibrium, there will be more people in low energy states than in high energy states, which is the normal state of the system.
As T increases, the number of atoms in high energy states (N2) will increase, but N2 will never exceed N1. To achieve population inversion, the system must be pushed into a non-equilibrium state, which is key to laser operation.
Interaction between light and matter
The interaction of light with atomic systems can be divided into three main types: absorption, spontaneous emission, and stimulated emission.
Absorption
When light with a frequency of ν12 passes through a group of atoms, it may be absorbed by electrons in the ground state, and then excited to a higher energy state. The speed of absorption is proportional to the radiation density of light and related to the number of atoms in the ground state (N1).
Spontaneous radiation
Atoms in an excited state spontaneously return to their ground state, releasing photons. Spontaneous emission is random and has no fixed phase relationship, so its emission is incoherent.
Stimulated radiation
When an incident photon causes an excited atom to give up its energy and release a photon of frequency ν21, the process is called stimulated emission of radiation. What happens here is the interaction of photons, causing the excited atoms and the incident photon to jointly produce photons of the same frequency and phase. This is the key to laser gain.
If there are more people in high energy states than in low energy states, i.e. N2>N1, then a net enhanced radiation will be achieved.
The process of creating population inversion
One way to achieve population inversion is to use indirect methods to transfer atoms from the ground state to the excited state. Three-level laser systems are an example. In this system, atoms can exist in three energy states. If atoms at high energy levels decay rapidly to intermediate energy levels to achieve a relatively low energy level population, this will lead to the formation of combinatorial states.
Four-level laser
In the four-level laser, the energy level is set more reasonably, so that atoms can remove a large number of ground state populations in a short time, thereby achieving the corresponding laser enhancement effect. This makes four-level lasers more efficient than three-level lasers and more common in practical applications.
The development of laser technology has allowed it to play an irreplaceable role in fields such as science, medicine, and communications, and all this is thanks to the mechanism of population inversion.
With the advancement of technology, how will future laser systems evolve and continue to promote the development of human society?
