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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 a photon, the particle jumps from one energy level to another, and absorbs or radiates the photon accordingly, making it more powerful.

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

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

When the particle is excited and enters the excited state, it is not the stable state of the particle. If there is a lower energy level that can accept the particle, even if there is no external effect, the particle has a certain probability, and spontaneously from the high energy level excited state (E2) Transition to the low-level ground state (E1), while radiating photons with energy (E2-E1), photon frequency ν=(E2-E1)/h.

This radiation process is called spontaneous radiation.

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

In 1917, Einstein theoretically pointed out that apart from spontaneous emission, particles at the high energy level E2 can also transition to lower energy levels in another way. He pointed out that when a photon with a frequency of ν=(E2-E1)/h 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 at the same time radiate two frequencies and phases with the external photon. , photons with the same polarization state and the same direction of propagation, this process is called stimulated emission.

It can be imagined that if a large number of atoms are at the high energy level E2, when a photon with a frequency ν=(E2-E1)/h is incident, the atoms on E2 are excited to generate stimulated radiation, and two photons with exactly the same characteristics are obtained, These two photons re-excite the atoms at the E2 energy level, and make them generate stimulated emission, and four photons with the same characteristics can be obtained, which means that the original optical signal is amplified.

Taking this into account, Zhou Wenwen could only find another way, namely the laser.

Although Einstein proposed stimulated radiation in 1917, the laser came out in 1960, after a gap of 43 years. Why?

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

When light with a certain frequency is injected into the working material, stimulated emission and stimulated absorption exist simultaneously. Stimulated emission increases the number of photons, while stimulated absorption reduces the number of photons.

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

According to the law of statistical distribution, the number of particles in the lower energy level E1 must be greater than the number of particles in the higher energy level E2.

In this way, when the light passes through the working material, the energy of the light will only be weakened and not strengthened. For stimulated radiation 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 kind of distribution is just the opposite of the particle distribution in the equilibrium state, which is called the population inversion distribution, or the population inversion for short.

How to technically realize population inversion is a necessary condition for laser generation.

Theoretical studies have shown that any working substance, under appropriate excitation conditions, can achieve population inversion between specific high and low energy levels of the particle system.

If microscopic particles such as atoms or molecules have high energy level E2 and low energy level E1, the population density on E2 and E1 energy levels is N2 and N1, there are spontaneous emission transitions, stimulated emission transitions and stimulated emission transitions between the two energy levels. Absorption transition and other three processes.

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. Both the stimulated emission transition probability and the stimulated absorption transition probability are proportional to the monochromatic energy density of the incident radiation field.

When the statistical weights of the two energy levels are equal, the probabilities of the two processes are equal.

In thermal equilibrium N2 N1, this state is called the population inversion state.

In this case, stimulated emission transitions dominate. After the light passes through a laser working material (active material) that is in the state of particle number inversion with a length of l, the light intensity increases by eGl times.

G is a coefficient proportional to (N2-N1), called the gain coefficient, and its magnitude is also related to the properties of the laser working material and the frequency of the light wave.

A piece of activated matter is a laser amplifier.

If a piece of activated matter is placed in an optical resonator composed of two parallel mirrors (at least one of which is partially transmissive), particles at high energy levels will produce spontaneous emission in various directions.

Chapter 74 Announcement of Theodore Maiman Project