the invention of the laser is a great achievement of science and technology in the 2th century. It finally enables people to drive the luminous process of molecules and atoms with extremely small scale, large quantity and chaotic movement, so as to obtain the ability to generate and amplify coherent infrared rays, visible rays and ultraviolet rays (even X rays and γ rays). The rise of laser science and technology has brought human's understanding and utilization of light to a new level.

The birth history of lasers can be roughly divided into several stages, among which the concept of stimulated radiation put forward by Einstein in 1916 is an important theoretical basis. This theory points out that a matter particle in a high-energy state will be transformed into a low-energy state under the action of a photon whose energy is equal to the energy difference between two energy levels, and a second photon will be generated and emitted at the same time as the first photon, which is stimulated radiation. The light output by this radiation is amplified and coherent, that is, the emission direction, frequency, phase and polarization of multiple photons are exactly the same.

since then, the establishment and development of quantum mechanics have made people have a deeper understanding of the microstructure and motion law of matter, and the energy level distribution, transition and photon radiation of microscopic particles have also been more strongly proved, which has objectively improved Einstein's theory of stimulated radiation and further laid a theoretical foundation for the generation of lasers. After the birth of quantum electronics in the late 194s, it was quickly applied to study the interaction between electromagnetic radiation and various micro-particle systems, and many corresponding devices were developed. The rapid development of these scientific theories and technologies has created conditions for the invention of lasers.

if the number of particles in a high-energy state is more than that in a low-energy state in a system, there will be an inversion state of the number of particles. So as long as there is a photon, it will force an atom in a high energy state to emit a photon which is the same as it, and these two photons will cause other atoms to emit stimulated radiation, thus realizing the amplification of light; If the feedback effect of the appropriate resonant cavity is added, optical oscillation will be formed, and laser will be emitted. This is how the laser works. In 1951, American physicists purcell and Pound successfully reversed the number of particles in the experiment, and obtained stimulated radiation of 5 kHz per second. Later, American physicist Charles Towns and Soviet physicists Massoff and ProHohloff successively put forward the design of generating and amplifying microwaves by using the principle of stimulated radiation of atoms and molecules.

However, most of the above theoretical and experimental studies of microwave spectroscopy belong to "pure science", and whether the laser can be successfully developed was still very slim at that time.

but the efforts of scientists have finally paid off. In 1954, the aforementioned American physicist Thomas finally made the first ammonia molecular beam maser, which successfully set a precedent for using molecular and atomic systems as coherent amplifiers or oscillators for microwave radiation.

The maser developed by Towns et al. only produces microwaves with a wavelength of 1.25 cm, and its power is very small. With the development of production and technology, scientists are urged to explore new luminous mechanisms to produce new light sources with excellent performance. In 1958, Towns and his brother-in-law, Arthur Sholow, combined the maser with the theoretical knowledge of optics and spectroscopy, and put forward the key suggestion of adopting an open resonator, and prevented the properties of laser such as coherence, directivity, linewidth and noise. At the same time, Basov, ProHohloff and others also put forward a principle scheme to realize the optical amplification of stimulated radiation.

Since then, many laboratories in the world have been involved in a fierce development competition to see who can successfully manufacture and operate the world's first laser.

In p>196, American physicist Theodore Mayman narrowly won the worldwide development competition in his research laboratory in Miami, Florida. He used a high-strength flash tube to stimulate the chromium atoms in the ruby crystal, thus producing a fairly concentrated slender red light beam, which can reach a higher temperature than the sun when it hits a certain point.

The "Meman Design" has aroused the shock and suspicion of the scientific community, because scientists have been watching and expecting the He-Ne laser.

although maiman was the first scientist to introduce laser into practical fields, the debate about who invented this technology in court once caused great controversy. One of the competitors is Gordon Gould, the inventor of the word "laser" (the abbreviation of "stimulated emission optical frequency amplifier"). He put forward the word when he was studying for a doctorate at Columbia University in 1957. At the same time, the inventors of maser, Towns and Sholo, also developed the concept of laser. After the final judgment of the court, Towns became the winner because the written work of the research was nine months earlier than Gould. However, the invention right of Maiman's laser has not been shaken.

in December, 196, an Iranian-born American scientist, Jia Wan, led people to successfully manufacture and operate the world's first gas laser-He-Ne laser. In 1962, three groups of scientists invented semiconductor lasers almost simultaneously. In 1966, scientists developed an organic dye laser whose wavelength can be continuously adjusted within a range. In addition, there are chemical lasers with large output energy, high power and independent of power grid.

because of its outstanding characteristics, lasers are quickly used in industry, agriculture, precision measurement and detection, communication and information processing, medical treatment, military and other aspects, and have caused revolutionary breakthroughs in many fields. For example, people can use the concentrated and extremely high energy of laser to process various materials and drill 2 holes on a needle; As a means of stimulating, mutating, cauterizing, vaporizing and other effects on the organism, laser has achieved good results in the practical application of medicine and agriculture. In the field of communication, a light-guiding cable that uses a laser column to transmit signals can carry the information equivalent to that carried by 2 thousand telephone copper wires; Laser is not only used in communication, night vision, early warning, ranging and other aspects in the military, but also a variety of laser weapons and laser-guided weapons have been put into practice.

In the future, with the further research and development of laser technology, the performance and cost of laser will be further reduced, but its application scope will continue to expand and it will play an increasingly huge role.

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