Laser's 50th birthday

On May 16, 1960, Theodore Meimann made a sensational discovery at Hughes Laboratories in Malibu: he fired the first functioning laser.

In honor of the laser's 50th birthday... The USA Today It came out under the title "Albert Einstein, not Dart Vader, first thought of laser technology". On May 16, 1960, Theodore Meimann made a sensational discovery at Hughes Laboratories in Malibu: he fired the first functioning laser. This is one of the most sensational discoveries ever to come out of a scientific laboratory. But of course Einstein is behind the genius invention, as usually happens in physics. He is the one who wrote the equations behind the laser. In 1917 - while perfecting his theory of general relativity and working on the static and non-expanding model of the universe, Einstein wrote a paper proposing the idea of ​​forced emission, the property of atoms to emit light at a single frequency when struck by light.

1916-17: Einstein, forced escape

In an article that Einstein published in 1916 in a Zurich journal and a year later in a German journal, he identified three types of interaction between matter and light, absorption, spontaneous emission and forced emission: by absorbing a quantum of light a molecule or an atom makes a transition to a higher energy state. The atom will spontaneously - without being stimulated to do so - return to its lower energy state and in the process it will emit a quantum of light. Finally, it will emit a quantum and go to a lower energy if stimulated to do so by another light quantum. In this case the exciting and the emitted quantum have the same wavelength and the same phase. This feature paved the way for the maser and the laser.

Forced emission is not seen in normal matter that is in thermal equilibrium, because lower energy states tend to be more populated than higher energy states. To create forced emission, you need to create a situation where there are more high energy atoms than low energy ones. This situation is called population inversion and was first demonstrated in 1950 by Purcell and Pound in the context of nuclear magnetic resonance. They used the idea of ​​population reversal and forced emission as an engineering amplifier principle. The idea appeared in other articles in an engineering context at the same time and did not attract much attention.

During the 400s, inventors such as Nicholas Thales claimed to have invented "death rays". Tesla announced that he had invented beams capable of destroying ten thousand enemy planes at a distance of XNUMX kilometers. But only after World War II, with the success of the radar technology demonstration, did scientists return to Einstein's idea of ​​forced ejection.

May 1954: The Meiser

In May 1954, a letter was published in Physique Rabio by Gordon, Seeger, and Charles Townes, a small group at Columbia University in New York led by Townes. They not only published the idea but also described a device based on the idea and presented the first results obtained using the new device.
During a visit by Townes to a meeting of scientists in Washington in 1951, Townes realized that population reversal could be achieved by techniques using a beam of molecules. He did not mention the idea at the scientists' meeting, but when he returned to Columbia he immediately sat down to implement it together with Gordon, a graduate student, and Seeger, a postdoctoral fellow who was on an industry fellowship. In fact - in retrospect - the device that Towns and his group created is a laser that works in microwaves.

After several failed attempts using millimeter waves they decided to use 1.5 cm waves because the technology was available from World War II era radar research. They chose ammonia as the appropriate molecule for the beam of molecules to achieve a population inversion by creating both states. And so they planned to get most of the molecules out of the lower state.

The Mazer of Towns and his group. A beam of ammonia passes through the electrostatic focuser to separate the molecules into the upper quantum state.

The first population inversion was achieved with the ammonia molecule - a nitrogen atom together with three hydrogen atoms. Population inversion in ammonia is obtained by separating molecules in the upper quantum state from those in the lower quantum state. The separation itself was achieved in the maser by projecting a molecular beam, a beam of ammonia molecules through a system of focused electrodes. These created an electrostatic field with the direction of the beam. Molecules in the lower quantum state felt an outward force and very quickly left the beam. whereas those in the higher quantum state felt an inwardly focused force and finally entered into space. When microwave power of appropriate resonant frequency is transmitted through space, amplification occurs due to population inversion.

The three described the device as follows, "The device uses a molecular beam in which molecules are selected in an excited state in a microwave transition. Interaction between these excited molecules and the microwave field creates additional radiation and thus increases the forced emission. We call the device that uses this technique 'maser', which is an acronym for 'microwave amplifier by forced emission of radiation'".

The maizer idea was published in 1954 in a theoretically independent form by another group in the Soviet Union, Basov and Prokurov of the Lebedev Institute in Mokva. In 1956, Nicholas Bloomberg of Harvard University improved the Meiser and added the idea of ​​pumping, the injection of energy into the system to create the state of population reversal.

December 1958

An article titled "Infrared and optical masers" was published. In this article, Arthur Sholov and Townes dealt with the possibilities of extending the Maser principle to much shorter wavelengths. Sholub, who worked in the Bell telephone labs connected with Towns and married one of the latter's sisters. The two scientists worked together, pondering the expansion of the maser to new horizons, which also probably led to new thoughts.

It was originally thought that population inversion, which is necessary for the maser to function, cannot be achieved for an energy level difference corresponding to visible or infrared light. But Townes began to think about the situation more seriously towards the end of 1957 and came to the conclusion that there was no absolute prohibition on such a population reversal. And all this, provided that the right conditions are met.

May 1960: The laser

Microwaves with typical wavelengths of about one centimeter require cavities that are fairly easy to build. However, the question was asked, what effective space can be used for visible light with wavelengths that are ten thousand times shorter? Even if there were ways to make such a tiny box, how could the sample be placed inside it? Townes initially thought of a square box with walls made of mirrors, which would reflect wavelengths of different frequencies. He discussed this idea with Sholub. Sholov then came up with the idea of ​​two mirrors that would separate highly directed waves perpendicular to the mirror surfaces. They thought maybe if they pierced the reflective coating of the semi-silvered mirror in a small hole that would be the solution. But these solutions did not "come out of the box" in terms of thinking.

Towns and Sholov's paper led to research by Gould, a doctoral student who worked down the hall a few doors from Towns. He invented the name laser - "light amplifier by forced emission of radiation" for a light maser.

In the end, the first laser was built by hydrogen from the Hughes Laboratory in Malibu, California. Hydrogen significantly reduced the weight of the maser. In addition, he also moved to work with the ruby ​​ruby. The ruby ​​is a crystal of alumina - aluminum oxide with added chromium. The levels of individual chromium atoms dispersed in the crystal enabled the maser activity. While a pure alumina crystal appears as transparent as glass, the ruby ​​is pink or bright red and is therefore called "ruby", and it depends on the chromium content. The emission of light at the frequency of red light is caused by shining light of a higher frequency on the ruby. Hydrogen decided to reach a population reversal in the energy levels of the chromium corresponding to the emission of this red light.

In September 1959 Mayman participated in the first international conference dealing with quantum electronics. Sholov explained that ruby ​​would probably not be suitable as a material for a laser. But Mayman thought otherwise. He returned to his laboratory and built the first laser that worked for the first time exactly fifty years ago, on May 16, 1960.

The first laser was an amazingly simple device: a block of ruby ​​with two parallel silver ends that served as mirrors. An ordinary spiral-shaped flash tube, such as was used at that time for photography, surrounded the ruby ​​block. This supplied the radiation pump. The device was much simpler than any maser built at the time.