Maser

The maser ( acronym for Microwave Amplification by Stimulated English Emission of Radiation, microwave amplification by stimulated emission of radiation ) is a laser in the microwave range.

Basics

A grain generates coherent electromagnetic waves include nowadays a frequency range of 105 Hz to 1011 Hz (corresponding to 100 kHz to 100 GHz), corresponding to a wavelength range of kilometers to millimeters. The shorter wavelengths are achievable with molecular vibrations or magnetic dipole transitions in atoms.

It is based on the stimulated emission in the context of a population inversion. The latter means that more atoms (the electron energy levels ), or molecules (the vibration energy levels ) must be an active medium in the upper excited energy state of the relevant radiation transfer than in the lower energy state. The inversion is a deviation from thermal equilibrium and must be achieved with low energy levels by suitable power supply, also known as "pumping", and often an emptying of the lower energy levels or by sorting the molecules. An important part of a maser is tuned to the maser frequency resonator (usually a cavity ).

The main problem with the construction of laboratory Measles is the generation of the population inversion. The basic idea of the maser (and also of the laser) was recognized in 1951 by the American physicist Charles H. Townes; then together with his students James P. Gordon and Herbert pointer post-doc in 1953, he developed an ammonia maser. The impetus for the development was the effort to build low-noise amplifier as possible; until then common amplifier reported due to the thermal motion of the charge carriers to a significant noise. For the maser transition a certain vibration of the ammonia molecule is used with a wavelength of 12.7 mm, in which the nitrogen atom through swinging through the plane spanned by the three hydrogen atoms level. Regardless of Townes had Joseph Weber in the early 1950s similar ideas.

For the production of the population inversion, the fact is exploited that the forces which act to free atoms or molecules in non-uniform electric fields, the dipole moment of the particle is dependent: a molecular beam enters this through a nozzle into a vacuum chamber where it is through a non-uniform field deflected and sorted so that only those molecules which are situated in the upper energy level required to reach the adjacent cavity resonator is tuned to the transition frequency of the molecules. (Note: those microwave resonators cavities with metal walls, whose dimensions are comparable to the wavelength of the radiation. ) In the cavity spontaneous and stimulated emission is now possible - it forms a standing electromagnetic wave from. A portion of the radiation leaving the cavity, and, the output radiation of the maser. The rest remains in the cavity by reflection, so that there takes place by means of stimulated emission of a further phase-locked microwave amplification.

If a sufficient amount of excited atoms brought into the resonator, so this system can not only reinforce a coupled- wave, but also work as a microwave oscillator. Thus, the first maser was created.

1964 Townes received together with the two Soviet physicists Nikolai Basov and Aleksandr Mikhailovich Prokhorov Gennadievich who had created independently of him, the theoretical foundations for the maser and laser principle, the Nobel Prize for physics.

Hydrogen maser

The hydrogen maser was developed in 1960 by the American physicist Norman Ramsey and his staff; In 1989 Ramsey was awarded the Nobel Prize in Physics for his work. For him, the hyperfine structure of atomic hydrogen is used as an active maser transition. For spin of the atomic nucleus - consisting in this case of a single proton - the shell electron can align parallel or antiparallel. There is a very small energy difference of about 10-5 eV, which corresponds to a frequency of 1.42 GHz between these two possibilities. This transition is also known from radio astronomy as a 21- cm line as evidence of interstellar hydrogen. Since large amounts of hydrogen are present in the spiral arms of the Milky Way, this spectral contributed significantly to the elucidation of their structure.

Also in this gas maser population inversion is realized by atomic beam methods in inhomogeneous fields ( state - selector ). The excited, free hydrogen atoms then pass into a Teflon-lined " memory ball " of quartz. The storage chamber is surrounded by a microwave cavity that is tuned to the resonant frequency of the transition between the two hyperfine states. This very frequency is generated by a microwave transmitter. The intensity of radiation of the microwaves is amplified by stimulated emission.

Hydrogen maser can work very stable over the years. The lifetime of the excited states is about one second, this has a very large frequency accuracy of the maser result; the difference is only 1 Hz hydrogen maser, therefore, serve as a highly accurate frequency standards in laboratories and in atomic clocks. Their excellent frequency stability allows the verification of statements of general relativity, in which tiny time differences play a crucial role. Hydrogen maser used in radio astronomy for the Langbasisinterferometrie, when it comes to a time- accurate record of receiving signals from different, far away from each other -powered telescopes. Also in the evaluation of radar measurements to determine the continental drift or astronomical distance measurements, where very small time differences need to be measured, the precise timing standards of hydrogen masers are indispensable.

More Maser

The idea for a solid state maser had Nicolaas Bloembergen 1956, the first realization succeeded Derrick Scovil in the same year. 1957, demonstrated Chihiro Kikuchi at Willow Run the first ruby ​​maser. He was the starting point for considerations to use ruby as the active medium for lasers. While Arthur L. Schawlow, however, assumed that Rubin was not suitable as the active laser medium due to its optical properties, Theodore Maiman continued to pursue the idea and realized with the ruby laser in May 1960, the first laser beam source.

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

This case is realized when " one-atom maser ". It is a non-classical radiation source, wherein the emitted microwaves obey the laws of quantum mechanics. (Note: A common Maser While using one hand to reinforce its radiation, the fact that there are discrete, ie, quantized energy levels in atoms other hand, behaves the emergent radiation field, as it is created statistically from several photons, classical, that is, the. number of photons in the cavity fluctuates around an average value. the non-classical radiation of the one-atom maser, however, consists of photons that come out of the cavity at a uniform distance. )

Interestingly, this property is mainly for message transmission, so that the one-atom maser can also use in the research field of quantum computing, particularly in the control of individual and together correlated ions, which are kept in special cases.

With such a non-classical radiation sources can also be the energy exchange between the radiation field and atomic as well as the properties of pure photon fields to explore. In recent years, another area of ​​research has opened up for the frequency-stable maser, which is related to theories to unify quantum mechanics and gravitation Search ( string theory, loop quantum gravity ). One of the statements to be checked are temporal changes in physical constants of nature, such as the speed of light.

Masers appear as artificial devices, since the population inversion, a condition for the operation of a maser, is a state that is far from thermodynamic equilibrium and can only be achieved through technical effort. So it surprised, as with radio telescopes detected objects in the cosmos in the 1960s, which emit natural maser radiation. The discovery of such astronomical maser revolutionized many insights into our universe. The narrow-band microwave lines - especially the 18- cm line of OH molecule, but also the 1.35 - cm line of the water molecule - provide information on excited molecular gases to emerging star on changes in galaxies as well as shell processes in red giants and supergiants. However, the pumping mechanisms of these natural masers are not yet fully understood. In question excitation processes occur by collisions with interstellar dust or optical excitation by the radiation energy of neighboring stars or infrared sources.

In 2012, the research team headed by the physicist Mark Oxborrow by the UK National Physical Laboratory in Teddington, the development of a maser, which emits at room temperature microwave radiation. For the first time using this material was a combination of p - terphenyl doped with pentacene. This organic crystal is introduced during the trial in a sapphire ring. As an energy source is used a yellow dye of pulsed laser having a wavelength of 585 nm within the microwave resonator by means of the organic crystal TE 01 - mode is excited. The coupling of microwave radiation by means of a magnetic loop. Still, the intensity of the output radiation and the low frequency range is low.

Applications

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