Burl

The title of this article is ambiguous. Further meanings are listed under Maser (disambiguation) .

The maser [ mɛɪzər , meːzər , maːzər , mɛɪ̯zɐ ] ( acronym for English M icrowave A mplification by S timulated E mission of R adiation , dt. Microwave amplification by stimulated emission of radiation) is a laser in the microwave range.

Basics

The first burl was created before the laser . The latter was only achieved by shortening the wavelength of the radiation used. In the beginning, therefore, one spoke of an optical maser (optical maser).

A maser generates coherent electromagnetic waves that nowadays  cover a frequency range from 10 ⁵  Hz to 10 ¹¹ Hz (corresponding to 100 kHz to 100 GHz), corresponding to a wavelength range from kilometers to millimeters. The smaller wavelengths can be achieved with molecular vibrations or magnetic dipole transitions in atoms.

The basis is the stimulated emission in connection with a population inversion . The latter means that more atoms (their electron energy levels) or molecules (their vibration energy levels) of an active medium must be in the upper excited energy state of the radiation transition in question than in the lower energy state. The inversion is a deviation from the thermal equilibrium and must be achieved through a suitable supply of energy, also known as pumping , and often through emptying the lower energy levels, e.g. B. by sorting out the molecules with low energy levels. An important part of a maser is tuned to the maser frequency resonator (usually a resonant cavity).

The main problem in building laboratory fibers is the generation of population inversion. The basic idea of ​​the measles (and also the laser) was recognized as early as 1951 by the American physicist Charles H. Townes ; Together with his student James P. Gordon and post-doc Herbert Zeiger , he then developed an ammonia maser in 1953. The impetus for the development was the effort to build amplifiers with the lowest possible noise ; The amplifiers used up to that point exhibited considerable noise due to the thermal movement of the charge carriers. For the maser transition, a certain oscillation of the ammonia molecule with a wavelength of 12.7 millimeters is used, in which the nitrogen atom swings through the plane spanned by the three hydrogen atoms. Independently of Townes, Joseph Weber had similar ideas in the early 1950s.

To generate the population inversion, the fact is used that the forces that act on free atoms or molecules in inhomogeneous electric fields are dependent on the dipole moment of the particles: a molecular beam enters a vacuum chamber through a nozzle and becomes there through an inhomogeneous one Field deflected and sorted in such a way that only those molecules that are in the desired upper energy level reach the subsequent cavity resonator , which is tuned to the transition frequency of the molecules. (Note: such microwave resonators are cavities with metal walls, the dimensions of which are comparable to the wavelength of the radiation.) In the cavity resonator, spontaneous and stimulated emission is now possible - a standing electromagnetic wave is formed. Part of the radiation leaves the resonator and represents the output radiation of the maser. The remainder remains in the cavity due to reflection, so that a further, phase-synchronous microwave amplification takes place there through stimulated emission. If a sufficient amount of excited atoms is brought into the resonator, this system can not only amplify a coupled wave, but also work as a microwave oscillator. This created the first burl.

In 1964, Townes received the Nobel Prize in Physics together with the two Soviet physicists Nikolai Gennadijewitsch Bassow and Alexander Michailowitsch Prokhorov , who had independently created the theoretical basis for the maser and laser principle.

Ammonia burls were also at the heart of the very first atomic clocks (ammonia molecule clocks).

Hydrogen burl

A hydrogen maser.

The hydrogen maser was developed in 1960 by the American physicist Norman Ramsey and his colleagues; in 1989 Ramsey received the Nobel Prize in Physics for his work. The hyperfine structure of atomic hydrogen is used as an active maser transition . To the spin of the atomic nucleus - in this case consisting of a single proton - the shell can be aligned electronically parallel or antiparallel. There is an extremely small energy difference between these two possibilities of around 10 ⁻⁵  eV , which corresponds to a frequency of 1.42  gigahertz . This transition is also known from radio astronomy as the 21 cm line as evidence of interstellar hydrogen. Since there are large amounts of hydrogen in the spiral arms of our Milky Way, this spectral line made a major contribution to the elucidation of its structure.

In this gas maser, too, the population reversal is implemented by atomic beam methods in inhomogeneous fields (state selector). The excited, free hydrogen atoms are then placed in a Teflon-lined “storage sphere” made of quartz. The storage chamber is surrounded by a microwave cavity, which is resonantly tuned to the frequency of the transition between the two hyperfine structure states. Exactly this frequency is generated by a microwave transmitter. The radiation intensity of the microwaves is increased by stimulated emission.

Hydrogen masers can work very stably for years. The life of the excited states is about one second, which results in a very high frequency accuracy of the maser; the deviation is only 1 Hz. Hydrogen masers therefore serve as high-precision frequency standards in laboratories and in atomic clocks . Its excellent frequency stability allows the testing of statements of the general relativity theory, in which tiny time differences play the decisive role. Hydrogen masers are used in radio astronomy for long-base interferometry , which depends on the precise timing of the reception signals from various telescopes that are operated far away from each other. Even when evaluating radar measurements to determine continental drift or astronomical distance measurements , in which very small differences in transit time have to be measured, it is no longer possible to imagine the precise time standards of the hydrogen masers.

More burls

Nicolaas Bloembergen had the idea for a solid-state burl in 1956, and Derrick Scovil succeeded in realizing it in the same year. In 1957 Chihiro Kikuchi demonstrated the first ruby burl in Willow Run . It was the starting point for considerations about using the mineral ruby as an active medium for lasers. While Arthur L. Schawlow assumed that ruby ​​was not suitable as a laser-active medium due to its optical properties, Theodore Maiman pursued the idea further and realized the first laser beam source with the ruby laser in May 1960.

The Rydberg maser is one of the more recent developments . In terms of their electronic structure, hydrogen-like atoms (only a single valence electron in the outermost shell, example: rubidium ) are used as the active material. With a variable frequency dye laser, these individual outer electrons can be pumped to very high, long-lasting energy levels close to the ionization limit. The heart of the maser is a cylindrical resonator cooled to the temperature of liquid helium, through which the beam of highly excited atoms is sent. Maser oscillations with only a few chamber atoms can be achieved here; in the extreme case there is only a single excited atom in the chamber.

This case is realized with the “one atom maser”. This is a non-classical radiation source in which the emitted microwaves obey the laws of quantum mechanics . (Note: A normal maser uses the fact that there are discrete, i.e. quantized, energy levels in atoms to amplify its radiation. On the other hand, the emerging radiation field behaves classically, as it is statistically generated by several photons , i.e. the number of Photons in the cavity fluctuates around a mean value. The non-classical radiation of the one-atom maser, on the other hand, consists of photons that come out of the cavity at an even distance.)

This property is particularly interesting for the transmission of messages, so that the one-atom maser can also be used in the research field of quantum computers , especially when controlling individual and correlated ions that are stored in special traps.

With such non-classical radiation sources, the energy exchange between radiation field and atom as well as the properties of pure photon fields can also be researched. In the last few years a further area of ​​research has opened up for the frequency-stable maser, which is related to theories that seek to unite quantum mechanics and gravity ( string theory , loop quantum gravity ). One of the statements to be checked are changes in physical constants over time, such as the speed of light .

In 2012, the research team led by physicist Mark Oxborrow from the British National Physical Laboratory in Teddington, succeeded in developing a maser that emits microwave radiation even at room temperature. A material combination of p-terphenyl doped with pentacene was used for the first time. This organic crystal is embedded in a sapphire ring during the experiment . A yellow, pulsed dye laser with a wavelength of 585 nm serves as the energy source. The organic crystal is excited by means of the TE01δ mode within the microwave resonator. The microwave radiation is coupled out by means of a magnetic coupling loop. The intensity of the output radiation is still low and the frequency range is low.

Natural burl

Masers appear as artificial devices because the population inversion, a condition for the operation of a measles, is a state that is far from thermodynamic equilibrium and can only be achieved through technical effort. It was a surprise when, in the 1960s, radio telescopes discovered objects in the cosmos that emit natural maser radiation. The discovery of such astronomical masers revolutionized many knowledge about our universe. The high-frequency microwave lines - especially the 18 cm line of the OH molecule, but also the 1.35 cm line of the water molecule - provide information about excited molecular gases around emerging stars , about changes in galaxies and about shell processes in Red giants and supergiants. However, the pumping mechanisms of this natural burl have not yet been fully clarified. Excitation processes through collisions with interstellar dust or optical excitation through the radiation energy of neighboring stars or infrared sources come into question.

Applications

• Hydrogen masers are used in atomic clocks (see Hydrogen masers clock ).
• GPS Satellite Block IIF is equipped with a hydrogen maser.
• The Galileo project will also use three hydrogen masers as atomic time clocks.

literature

• JP Gordon, HJ Zeiger, CH Townes: Molecular Microwave Oscillator and New Hyperfine Structure in the Microwave Spectrum of NH ₃ . In: Physical Review . tape 95 , no. 1 , July 1, 1954, p. 282-284 , doi : 10.1103 / PhysRev.95.282 .
• H. Haken: The 1964 Nobel Prize for the burl . In: Physical sheets . tape 21 , no. 3 , 1965, p. 109-114 , doi : 10.1002 / phbl.19650210303 .
• Ronald L. Walsworth: Applied physics. The maser at 50 . In: Science . tape 306 , no. 5694 , August 10, 2004, p. 236-237 , doi : 10.1126 / science.1101354 , PMID 15472067 .
• G. Makhov, C. Kikuchi, J. Lambe, RW Terhune: Maser Action in Ruby . In: Physical Review . tape 109 , no. 4 , February 15, 1958, p. 1399–1400 , doi : 10.1103 / PhysRev.109.1399 .
• HM Goldenberg, D. Kleppner, NF Ramsey: Atomic Hydrogen Maser . In: Physical Review Letters . tape 5 , no. 8 , October 15, 1960, p. 361-362 , doi : 10.1103 / PhysRevLett.5.361 .
• PE Toschek: Nobel Prize 1989: Fields, phases, traps ... Atoms, ions and electrons, looked at closely . In: Physical sheets . tape 45 , no. 12 , 1989, pp. 465-470 , doi : 10.1002 / phbl.19890451204 .
• JM Raimond, M. Brune, S. Haroche: Manipulating quantum entanglement with atoms and photons in a cavity . In: Reviews of Modern Physics . tape 73 , no. 3 , August 28, 2001, p. 565-582 , doi : 10.1103 / RevModPhys.73.565 .
• Andrew W. Clegg: Astrophysical masers. Springer, Berlin 1993, ISBN 0-387-56343-1 .
• Alan H. Cook: Celestial masers. Cambridge Univ. Pr., Cambridge 1977, ISBN 0-521-21344-4 .
• Manfred Brotherton: Maser and Laser - Basics, Functionality, Applications. Umschau Verlag, Frankfurt 1967.

Web links

Commons : Maser  - collection of images, videos and audio files

• What is a cosmic burl? from the alpha-Centauri television series(approx. 15 minutes). First broadcast on June 22, 2003.

Individual evidence

1. ↑ Jeff Hecht: Beam: the race to make the laser , p. 31f.
2. ↑ Sean F. Johnston: Holographic Visions. Oxford University Press, Oxford, New York 2006, p. 170.
3. ↑ Mark Oxborrow, Jonathan D. Breeze, Neil M. Alford: Room-temperature solid-state maser . In: Nature . tape 488 , no. 7411 , August 16, 2012, p. 353-356 , doi : 10.1038 / nature11339 .
4. ↑ Maser at room temperature ( Memento of the original from August 28, 2016 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. top-solar-info.de, August 17, 2012.