Zur Navigation | Zum Inhalt
FVCML0208 10
Laser, genannt "Maser" PDF Drucken E-Mail

“Teile und Herrsche” ist die Tagespolitik seit dem Römischen Reich. Es sind die Regierungen, die Krieg führen gegen das eigene Volk.

 

 

Der erste Laser, genannt "Maser"  

In der Öffentlichkeit kaum bekannt, der erste Laser, genannt Maser, funktionierte mit Mikrowellen: "Townes entwickelte eine Apparatur, mit dem sich Mikrowellen erzeugen und verstärken ließen. Sie bestand aus einem Hohlraum, durch den ein Strahl von Ammoniak-Molekülen geleitet wurde. Strahlte ein Molekül zufällig eine Mikrowelle ab, wurde diese im Resonator hin- und her reflektiert. Traf die Welle auf andere Moleküle, brachte sie diese dazu, ebenfalls Mikrowellen identischer Frequenz auszusenden. Ein Lawineneffekt war die Folge, der zur Verstärkung der Mikrowellenstrahlung führte... Der Physiker nannte seine Apparatur deshalb kurz „Maser“, ein Akronym für Microwave Amplification by Stimulated Emission of Radiation. Koppelte man die Mikrowelle aus dem Hohlraum aus, erhielt man einen kontinuierlichen, monochromatischen Mikrowellenstrahl." 

www.faz.net 

Und das neueste vom Maser: (für Nicht-Fachleute nur schwer verständlich): "Tunnelnde Elektronen verstärken kohä­rente Mikro­wellen­strah­lung. So wie Laser kohärentes Licht aussenden, emittieren Maser gebündelte und korrelierte Mikrowellenstrahlung. Bisher werden Maser meistens noch optisch über Laserpulse gepumpt. Doch ein neuer Prototyp, entwickelt von einer Arbeitsgruppe an der Princeton University, lässt sich mit elektrischer Spannung betreiben. Aufgebaut ist der neuartige Maser aus Halbleiter-Quantenpunkten. Die Verstärkung der Mikrowellen-Emission basiert auf dem Tunneln einzelner Elektronen durch diese Quantenpunkte."

www.pro-physik.de

 

Mikrowellenlaser funktioniert erstmals bei Raumtemperatur  

 


 

London (Großbritannien) – Smartphones, Radar und Radioteleskope nutzen heute Wellen mit Gigahertzfrequenzen. Für die Verbesserung dieser Funkkanäle entwickelten Physiker nun eine einzigartige Strahlungsquelle: einen Maser, der erstmals auch bei Raumtemperatur funktioniert. Analog zum bekannten Laser, der eng fokussierte und korrelierte Lichtstrahlen aussendet, emittiert ein Maser Mikrowellen mit ähnlichen Eigenschaften. Wie die Forscher in der Zeitschrift „Nature“ berichten, könnten ihre Maser auch zu präziseren Messungen bei der Untersuchung des Weltraums oder in der Molekularbiologie führen.

„Der Maser funktioniert an der Luft bei Raumtemperatur und verstärkt Mikrowellen bei etwa 1,45 Gigahertz“, schreiben Mark Oxborrow vom britischen National Physical Laboratory in Teddington und seine Kollegen am Imperial College London. Möglich wurde dieser Erfolg mit einem Kristall aus der organischen Substanz p-Terphenyl, den die Wissenschaftler zusätzlich mit Pentazenmolekülen dotierten. Angeregt mit gelbem Laserlicht konnten Elektronen in dem Kristall auf höhere Niveaus gehoben werden. Beim Zurückfallen sendeten sie die gewünschten gebündelten Mikrowellen aus. Bisherige Modelle mussten noch aufwendig auf minus 269 Grad Celsius abgekühlt werden, wodurch ihre Einsatzmöglichkeiten stark eingeschränkt waren.

„Der Stand der Maser-Entwicklung ist vergleichbar mit dem des Laser vor 50 Jahren“, sagt Oxborrow. So ist die Intensität der Mikrowellen nicht sehr hoch und auch das Frequenzspektrum noch relativ schmal. Dennoch dürften von solchen Masern, die an der Luft und bei Raumtemperatur unkompliziert einsetzbar sind, viele Anwendungen profitieren. Astronomen könnten ohne aufwendige Kühlung Detektoren bauen, deren Signale weniger von einem Störrauschen beeinträchtigt werden. Diese Vorteile ließen sich auch bei der Analyse von Quantencomputern oder molekularbiologischen Proben nutzen. Zudem lässt sich nicht ausschließen, dass Maser die Grundlage für eine weitere Optimierung des digitalen Funkverkehrs im Gigahertzbereich liefern. „Aber die exakten Anwendungen sind aus heutiger Sicht noch weitgehend unbekannt“, sagt Neil Alford vom Imperial College London, der an der Maserentwicklung beteiligt ist. 

www.weltderphysik.de/.../mikrowellenlaser-funktioniert-erstmals-bei-rau...  

 

What is a MASER?  

 

MASER stands for Microwave Amplification by Stimulation Emission of Radiation. A LASER is a MASER that works with higher frequency photons in the ultraviolet or visible light spectrum (photons are bundles of electromagnetic energy commonly thought of as "rays of light" which travel in oscillating waves of various wavelengths).

The first papers about the MASER were published in 1954 as a result of investigations carried out simultaneously and independently by Charles Townes and co-workers at Columbia University in New York and by Dr. Basov and Dr. Prochorov at the Lebedev Institute in Moscow. All three of these gentlemen received the Nobel Prize in 1964 for their contributions to science.

[The following was paraphrased in part from Halliday & Resnick's "Fundamentals of Physics", second edition.]

The fundamental physical principle motivating the MASER is the concept of stimulated emission, first introduced by Einstein in 1917. Before defining it we look at two related but more familiar phenomena involving the interplay between matter and radiation, absorption and spontaneous emission.

Absorption. According to quantum mechanics, absorption of photons by atoms occurs only if the wavelength of the photon is just the right size (say, of wavelength l). If it is, the atom will "absorb" it (the photon vanishes) and go to a higher energy state. In physics, this process is called "absorption."

Spontaneous Emission. Atoms don't like to stay in high energy states (this is dictated by the laws of thermodynamics), so after absorbing a photon and going to a higher energy state, they will move of their own accord to a lower energy state, emitting a photon in the process. This is called "spontaneous emission" because no outside influence triggers the emission. Normally the average lifetime for spontaneous emissions by excited atoms is around 10-8 seconds (that is, the atom or molecule will usually take around 10-8 seconds before emitting the photon). Occasionally, however, there are states for which the lifetime is much longer, perhaps around 10-3 seconds. These states are called metastable. Metastable emission levels are essential for a working MASER and will be discussed further in a moment.

Now that we've discussed absorption and spontaneous emission, we can get to stimulated emission (a MASER beam is made up entirely of stimulated emission).  

Stimulated Emission. With stimulated emission, a photon of the absorption wavelength, l , is fired at an atom already in its high energy state from prior absorption. The atom absorbs this photon, and then quickly emits two photons to get back to its lower energy state. Thanks to quantum mechanics, both of these newly emitted photons are of wavelength l! The following figure displays this concept in detail:  

MASER. In each frame, a molecule in the upper level of the MASER transition (that is, in the high energy, excited state) is indicated by a large red circle, while one in the lower level (low energy state) is indicated by a small blue circle. (a) All of the molecules are in the upper state and a photon of wavelength l (shown in green) is incident from the left. (b) The photon l stimulates emission from the first molecule, so there are now two photons of wavelength l, in phase. (c) These photons stimulate emission from the next two molecules, resulting in four photons of wavelength l. (d) The process continues with another doubling of the number of photons.  

[Figure courtesy M. L. Kutner, "Astronomy: A Physical Perspective", John Wiley & Sons, Inc. 1987] 

Basically, a man-made MASER is a device that sets up a series of atoms or molecules and excites them to generate the chain reaction, or amplification, of photons. Metastable emission states make MASERs and LASERs possible. To get the proper wavelengths to generate the chain reaction, first electricity or another energy source is "pumped" into a chamber filled with particular atoms or molecules. Then this "pumping" radiation causes the transition of atoms from the ground state to a high energy excited state higher than that referred to in the above paragraphs. From this short-lived state the atoms come down through non-radiative transition to the long-lived metastable state. Once in the metastable state many atoms can be accumulated in one place and in the same state.

The LASER or MASER beam, stimulated emission, arises when all these accumulated atoms simultaneously make a transition to the ground state, releasing their energy of wavelength l, creating a beam of microwave radiation (or visible light in the case of a LASER) which can be sent on to other atoms to cause the chain reaction described in the above figure. Since all the resulting photons are the same wavelength, MASER beams are extremely focussed and coherent. MASERs and their shorter-wavelength counterparts (LASERs), have many practical applications, especially in science and medicine.  

Naturally occurring MASERs have been discovered in interstellar space. For more information about MASERs in space, check out this site for a discussion of astrophysical MASERs.

einstein.stanford.edu/content/faqs/maser.html

 

Rice-sized laser 

 

powered one electron at a time, bodes well for quantum computing 

Princeton University researchers have built a rice grain-sized laser powered by single electrons tunneling through artificial atoms known as quantum dots. The tiny microwave laser, or "maser," is a demonstration of the fundamental interactions between light and moving electrons.

The researchers built the device — which uses about one-billionth the electric current needed to power a hair dryer — while exploring how to use quantum dots, which are bits of semiconductor material that act like single atoms, as components for quantum computers.

"It is basically as small as you can go with these single-electron devices," said Jason Petta, an associate professor of physics at Princeton who led the study, which was published in the journal Science.

 

Petta rice laser_2

 

Princeton University researchers have built a rice grain-sized microwave laser, or "maser," powered by single electrons that demonstrates the fundamental interactions between light and moving electrons, and is a major step toward building quantum-computing systems out of semiconductor materials. A battery forces electrons to tunnel one by one through two double quantum dots located at each end of a cavity (above), moving from a higher energy level to a lower energy level and in the process giving off microwaves that build into a coherent beam of light. (Photo courtesy of Jason Petta, Department of Physics)

The device demonstrates a major step forward for efforts to build quantum-computing systems out of semiconductor materials, according to co-author and collaborator Jacob Taylor, an adjunct assistant professor at the Joint Quantum Institute, University of Maryland-National Institute of Standards and Technology. "I consider this to be a really important result for our long-term goal, which is entanglement between quantum bits in semiconductor-based devices," Taylor said.

The original aim of the project was not to build a maser, but to explore how to use double quantum dots — which are two quantum dots joined together — as quantum bits, or qubits, the basic units of information in quantum computers.

 

Petta rice laser_1 

Yinyu Liu, first author of the study and a graduate student in Princeton's Department of Physics, holds a prototype of the device. (Photo by Catherine Zandonella, Office of the Dean for Research)  

"The goal was to get the double quantum dots to communicate with each other," said Yinyu Liu, a physics graduate student in Petta's lab. The team also included graduate student Jiri Stehlik and associate research scholar Christopher Eichler in Princeton's Department of Physics, as well as postdoctoral researcher Michael Gullans of the Joint Quantum Institute.

Because quantum dots can communicate through the entanglement of light particles, or photons, the researchers designed dots that emit photons when single electrons leap from a higher energy level to a lower energy level to cross the double dot.

Each double quantum dot can only transfer one electron at a time, Petta explained. "It is like a line of people crossing a wide stream by leaping onto a rock so small that it can only hold one person," he said. "They are forced to cross the stream one at a time. These double quantum dots are zero-dimensional as far as the electrons are concerned — they are trapped in all three spatial dimensions." 

Petta rice laser_diagram1 

When the power (P) is turned on, single electrons (small arrows) begin to flow through the two double quantum dots (Left DQD and Right DQD) from the drain (D) to the source (S). As the electrons move from the higher energy level to the lower energy level, they give off particles of light in the microwave region of the spectrum. These microwaves bounce off mirrors on either side of the cavity (k-in and k-out) to produce the maser's beam. (Photo courtesy of Science/AAAS) 

The researchers fabricated the double quantum dots from extremely thin nanowires (about 50 nanometers, or a billionth of a meter, in diameter) made of a semiconductor material called indium arsenide. They patterned the indium arsenide wires over other even smaller metal wires that act as gate electrodes, which control the energy levels in the dots.

To construct the maser, they placed the two double dots about 6 millimeters apart in a cavity made of a superconducting material, niobium, which requires a temperature near absolute zero, around minus 459 degrees Fahrenheit. "This is the first time that the team at Princeton has demonstrated that there is a connection between two double quantum dots separated by nearly a centimeter, a substantial distance," Taylor said.

When the device was switched on, electrons flowed single-file through each double quantum dot, causing them to emit photons in the microwave region of the spectrum. These photons then bounced off mirrors at each end of the cavity to build into a coherent beam of microwave light.

One advantage of the new maser is that the energy levels inside the dots can be fine-tuned to produce light at other frequencies, which cannot be done with other semiconductor lasers in which the frequency is fixed during manufacturing, Petta said. The larger the energy difference between the two levels, the higher the frequency of light emitted. 

 

Petta rice laser_diagram2 

A double quantum dot as imaged by a scanning electron microscope. Current flows one electron at a time through two quantum dots (red circles) that are formed in an indium arsenide nanowire. (Photo courtesy of Science/AAAS) 

Claire Gmachl, who was not involved in the research and is Princeton's Eugene Higgins Professor of Electrical Engineering and a pioneer in the field of semiconductor lasers, said that because lasers, masers and other forms of coherent light sources are used in communications, sensing, medicine and many other aspects of modern life, the study is an important one.

"In this paper the researchers dig down deep into the fundamental interaction between light and the moving electron," Gmachl said. "The double quantum dot allows them full control over the motion of even a single electron, and in return they show how the coherent microwave field is created and amplified. Learning to control these fundamental light-matter interaction processes will help in the future development of light sources."

The paper, "Semiconductor double quantum dot micromaser," was published in the journal Science on Jan. 16, 2015. The research was supported by the David and Lucile Packard Foundation, the National Science Foundation (DMR-1409556 and DMR-1420541), the Defense Advanced Research Projects Agency QuEST (HR0011-09-1-0007), and the Army Research Office (W911NF-08-1-0189).