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At any time, a particle can only be in a state corresponding to a certain energy level, or simply stated as being at a certain energy level.

When interacting with photons, particles jump from one energy level to another, absorbing or radiating photons accordingly, making them more powerful.

This situation is further divided into stimulated absorption and spontaneous emission.

Particles at a lower energy level are excited by the outside world, that is, they have energy exchange interactions with other particles, such as inelastic collisions with photons, and when they absorb energy, they jump to a higher energy corresponding to this energy. level, this transition is called stimulated absorption.

When a particle is excited and enters an excited state, it is not a stable state of the particle. If there is a lower energy level that can accept the particle, the particle will have a certain probability of spontaneously changing from the high energy level excited state e2 to the lower energy state even without external influence. Level ground state e1 transition, and at the same time radiates photons with energy e2e1 and photon frequency νe2e1h.

This radiation process is called spontaneous radiation.

The light emitted by many atoms through spontaneous radiation does not have the same phase, polarization state, and propagation direction. It is what is called incoherent light in physics.

In 1917, Einstein theoretically pointed out that in addition to spontaneous radiation, particles at high energy level e2 can also jump to lower energy levels in another way. He pointed out that when a photon with a frequency of νe2e1h is incident, it will also cause the particle to rapidly transition from the energy level e2 to the energy level e1 with a certain probability, and simultaneously radiate two external photons with the same frequency, phase, polarization state and propagation direction. photons, this process is called stimulated emission of radiation.

It can be imagined that if a large number of atoms are at the high energy level e2, when a photon of frequency νe2e1h is incident, the atoms on e2 will be excited to produce stimulated radiation, and two photons with exactly the same characteristics will be obtained. These two photons will then excite e2 energy Atoms on the level are then caused to produce stimulated radiation, and four photons with the same characteristics can be obtained, which means that the original light signal is amplified.

Considering this, Zhou Wenwen could only find another way, namely laser.

Although Einstein proposed stimulated radiation in 1917, the laser came out in 1960, 43 years apart. Why is this?

The main reason is that the probability of particles producing stimulated radiation in ordinary light sources is extremely small.

When light of a certain frequency is injected into the working material, the two processes of stimulated emission and stimulated absorption exist at the same time. Stimulated emission increases the number of photons, but stimulated absorption decreases the number of photons.

When a substance is in a thermal equilibrium state, the distribution of particles at each energy level follows the statistical distribution law of particles in the equilibrium state.

According to the law of statistical distribution, the number of particles at the lower energy level e1 must be greater than the number of particles at the higher energy level e2.

In this way, when light passes through the working material, the energy of the light will only weaken but not increase. In order for stimulated emission to dominate, the number of particles at the high energy level e2 must be greater than the number of particles at the low energy level e1.

This distribution is exactly opposite to the particle distribution in the equilibrium state, and is called particle number inversion distribution, or particle number inversion for short.

How to technically achieve particle number inversion is a necessary condition for generating laser light.

Theoretical research shows that any working substance, under appropriate excitation conditions, can achieve particle number inversion between specific high and low energy levels of the particle system.

若原子或分子等微观粒子具有高能级e2和低能级e1，e2和e1能级上的布居数密度为n2和n1，在两能级间存在着自发发射跃迁、受激发射跃迁和受激吸收跃迁等三种过程。

The stimulated emission light generated by the stimulated emission transition has the same frequency, phase, propagation direction and polarization direction as the incident light.

Therefore, the stimulated emission light produced by a large number of particles excited by the same coherent radiation field is coherent. The stimulated emission transition probability and the stimulated absorption transition probability are both proportional to the monochromatic energy density of the incident radiation field.

When the statistical weights of two energy levels are equal, the probability of both processes is equal.

In the case of thermal equilibrium, n2<n1, so spontaneous absorption transitions dominate, and light is usually attenuated due to stimulated absorption when passing through the material.

The excitation of external energy can destroy the thermal balance and make n2>n1. This state is called the particle number inversion state.

In this case, the stimulated emission transition dominates. After the light passes through a length l of the laser working material in the particle number inversion state to activate the material, the light intensity increases egl times.

g is a coefficient proportional to n2n1, called the gain coefficient, and its size is also related to the properties of the laser working material and the frequency of the light wave.

A piece of active material is a laser amplifier.

If a section of active material is placed in an optical resonant cavity consisting of two parallel mirrors, at least one of which is partially transparent, particles at high energy levels will produce spontaneous emission in various directions.

Chapter 74 Preview of Theodore Mayman’s Plan