laser – Baza de Date

Punctele cuantice vizualizează rezonanțele vibraționale minuscule(Quantum dots visualize tiny vibrational resonances)

lumină.
O aplicație posibilă a acestei descoperiri, potrivit lui Sam Carter de la Naval Research Lab, unul dintre autorii lucrării, este de a detecta forțe subtile produse de obiecte dense din apropiere. „Materialele nucleare ascunse ar putea fi detectabile”, a spus el, „din moment ce materiale dense cum ar fi plumbul sunt folosite pentru a proteja dispozitivele.”
Ecranul foarte dens necesar pentru materialele nucleare cauzează mici anomalii gravitaționale și mișcări minore care ar putea fi detectabile de un dispozitiv bazat pe principiul descoperit aici. Cercetătorii intenționează să-și continue activitatea făcând abstracție de spinul electronic. Se speră că tehnicile de măsurare a efectului pe spin vor crește sensibilitatea dispozitivelor.

Posted: Oct 30, 2017
Quantum dots visualize tiny vibrational resonances (Nanowerk News) In the late 18th century, Ernst Chladni, a scientist and musician, discovered that the vibrations of a rigid plate could be visualized by covering it with a thin layer of sand and drawing a bow across its edge. With the bow movement, the sand bounces and shifts, collecting along the nodal lines of the vibration. Chladni’s discovery of these patterns earned him the nickname, „father of acoustics.” His discovery is still used in the design and construction of acoustic instruments, such as guitars and violins.
Recently, investigators have discovered a similar effect with much smaller vibrating objects excited by light waves. When laser light is used to drive the motion of a thin, rigid membrane, it plays the role of the bow in Chladni’s original experiment and the membrane vibrates in resonance with the light. The resulting patterns can be visualized through an array of quantum dots (QDs), where these tiny structures emit light at a frequency that responds to movement. The advance is reported this week in a cover article of Applied Physics Letters („Sensing flexural motion of a photonic crystal membrane with InGaAs quantum dots”).

Background: Image of a Chladni plate’s mode of vibration visualized by grains of sand collected at the nodes. Left-top: Cross-sectional scanning tunneling microscopy image of an indium arsenide quantum dot. Left-bottom: Variation of quantum dot emission line frequencies as a function of time due to vibrations of the photonic crystal membrane. Right: Scanning electron micrograph of a photonic crystal membrane, displaced according to one of the vibrational modes, with red and blue representing positive and negative displacement, respectively. (Image: Sam Carter and co-authors)
In addition to being a modern take on an old phenomenon, the new discovery could lead to the development of sensing devices as well as methods for controlling the emission characteristics of QDs. Since the light frequency emitted by the QDs is correlated with the movement of the underlying membrane, new devices for sensing motion, such as accelerometers, can be envisioned. A reverse application is also possible since the motion of the underlying membrane can be used to control the frequency of light emitted by the QDs.
The tiny devices in the work reported here consist of a 180-nanometer thick slice of semiconductor, suspended like a trampoline above a solid substrate. An array of QDs, analogous to the sand in the acoustic example, are embedded in the slice, whose thickness is less than one-tenth of one percent that of a human hair.
A second probe laser is used to visualize the resulting resonances. The QDs absorb the probe light and emit a second light pulse in response, which is picked up by a detector and routed to a display. The resulting patterns are remarkably like those visualized in Chladni’s original acoustic experiment, even though the new device is driven entirely by light.
One possible application of this discovery, according to Sam Carter of the Naval Research Lab who is one of the paper’s authors, is to sense subtle forces produced by nearby dense objects. „Concealed nuclear materials could be detectable,” he said, „since dense materials like lead are used to shield the devices.”
The highly dense shielding needed for nuclear materials causes small gravitational anomalies and tiny movements that might be detectable by a device based on the principle discovered here. The investigators plan to continue their work by looking at electronic spin. It is hoped that techniques to measure the effect on spin will increase the sensitivity of the devices.

Source: American Institute of Physics

Ghidarea laserului aleatoriu(Guiding the random laser)

Cercetătorii sunt capabili să obțină aceste rezultate prin fabricarea și utilizarea unei fibre optice din fibră optică Anderson. Fibra este fabricată dintr-un „cuart satinos”, un geam artizan extrem de poros, utilizat în mod obișnuit doar pentru calibrarea mașinilor care atrag fibră optică. Când este tras în bucle lungi, materialul poros formează zeci de canale microscopice de aer în fiecare fibră.Echipa de cercetare a fabricat un nou tip de fibră optică care este capabilă să controleze aceste lasere aleatorii.

Posted: Oct 27, 2017
Guiding the random laser (Nanowerk News) At its most basic level, a random laser is precisely what its name implies; random. It’s random in the spectrum of light it produces and in the way that light is emitted, making what could be an extremely versatile laser source, nearly useless for most practical applications.
So, how do you control some of the randomness to make useful devices? It’s a question that’s led a team of researchers at The University of New Mexico to a discovery that’s taking laser technology to the next level.
„It’s been incredible to see how this project has progressed,” said Behnam Abaie, a Ph.D. student at UNM’s Center for High Technology Materials (CHTM). „When I first came to work with Professor [Arash] Mafi, I knew this project had the potential to be very successful but I never expected this.”
Abaie is the first-author on a paper recently published in Nature’s Light: Science & Applications („Random lasing in an Anderson localizing optical fiber”). The article provides a technical analysis of how the research team, led by CHTM Interim Director Arash Mafi, is able to reliably control these extremely powerful, but previously uncontrollable, lasers.

The research team fabricated a new type of optical fiber that is capable of controlling these random lasers. (Image: University of New Mexico)
„Our success in being able to control these random lasers addresses decade-old issues that have prevented these lasers from becoming mainstream devices,” said Mafi, who is also an associate professor in UNM’s Dept. of Physics & Astronomy. „It’s a very exciting contribution.”
Traditional lasers consist of three main components: an energy source, gain medium and optical cavity. The energy source is provided through a process called ‘pumping’ and can be supplied through an electrical current or another light source. That energy then passes through the gain medium which contains properties that amplify the light. The optical cavity - a pair of mirrors on either side of the gain medium – bounce the light back and forth through the medium, amplifying it each time. The result is a directed, intense beam of light called a laser.
Random lasers, by comparison, perform using a pump, a highly-disordered gain medium but no optical cavity. They are extremely useful due to their simplicity and broad spectral features, meaning a single random laser can produce a beam of light containing multiple spectra, a very beneficial property for certain applications like biomedical imaging. However, given their nature, random lasers are difficult to reliably control due to their multi-directional output and chaotic fluctuation.
The UNM team, in collaboration with researchers at Clemson University and the University of California San Diego, has been able to overcome these obstacles in an efficient way – a victory they hope will continue to push the use of random lasers forward.
„Our device has all the great qualities of a random laser, plus spectral stability and it is highly directional,” said Mafi. „It’s a wonderful development.”
Researchers are able to achieve these results through the fabrication and use of a unique glass Anderson localizing optical fiber. The fiber is made of a ‘satin quartz’, an extremely porous artisan glass that is typically only used to calibrate the machinery that draws fiber optics. When pulled into long rods, the porous material forms dozens of microscopic air channels in each fiber.
„The glass that we’re using for these fiber optics is actually material that we would typically throw away because it is very porous,” said Abaie. „But, it’s those holes in the glass that are actually creating the channels that control the laser.”
Once filled with a gain medium and pumped using a single-colored green laser, the random laser becomes less random and highly controllable, thanks to a phenomenon known as Anderson Localization.
„There is still a lot to learn about Anderson Localization but it’s exciting for us to be part of this development,” said Mafi. „To be able to actually make devices that utilize this phenomenon, it’s taking the science to yet another level.”
Mafi and his research team are some of the leading experts in Anderson Localization. In 2014, they published an article on a different device capable of transmitting images using the phenomenon. That research was named one of Physics World’s Top Ten Breakthroughs of the year.
Moving forward, Mafi says they hope to broaden the spectrum of this new device and make it more efficient, creating a broad spectrum illumination source that can be utilized around the world.

Source: University of New Mexico

Spintronics pentru tehnologiile informaționale viitoare

Tehnologiile informaționale viitoare shoulderstand folosesc în mod considerabil mai puțină energie pentru procesarea datelor. O clasă de materiale interesante pentru acest izolatori topologice cuprinde. Izolatori topologice se disting prin electronii lor la suprafata fiind extrem de mobil, în timp ce materialul este în vrac într-un izolator și nu se efectuează. Deci, din moment ce electronii Simultan transporta un moment magnetic (rotire), izolatori topologice s-ar putea face, prin urmare componente „spintronice“ fezabilă. Componentele spintronice nu ar trebui să se bazeze pe mișcarea purtătorilor de sarcină cum ar fi electroni (ca în componente semiconductoare), ci pe transportul sau manipularea lor rotiri. Acest lucru ar necesita considerabil mai puțină energie.

Posted: Apr 29, 2016
Spintronics for future information technologies (Nanowerk News) Researchers have demonstrated that this class of materials is suitable for data processing based on spin. The work has been published in the renowned periodical Physical Review B and was selected as „Editor’s Suggestion” article („Ultrafast spin-polarization control of Dirac fermions in topological insulators”).

The illustration depicts the characteristic spin orientation (arrows) of electrons in a topological insulator (below). Using an initial circular polarised laser pulse, the spins are excited and point up or down. This can be proven by a second linearly polarised laser pulse (above). (Image: HZB)
Future information technologies should employ considerably less energy for processing data. One exciting class of materials for this comprises topological insulators. Topological insulators are distinguished by their electrons at the surface being extremely mobile, while the bulk material within is an insulator and does not conduct. Since electrons also simultaneously carry a magnetic moment (spin), topological insulators might also make „spintronic” components feasible. Spintronic components would not be based on the movement of charge carriers like electrons (as in semiconductor components), but instead on the transport or manipulation of their spins. This would require considerably less energy.
An international team headed by HZB physicist Jaime Sánchez-Barriga has now shown how the spins of the electrons in topological insulators can be controlled. The team investigated samples of antimony-telluride, a topological insulator, using circularly polarised laser light. They were able to initiate and direct currents of electrons whose spins were oriented in parallel (i. e., spin-polarised currents) using the „rotational direction” of the laser light. In addition, they were successful in changing the orientation of the spins as well. The team was made up of experimentalists from the Max Born Institute in Berlin and Lomonossow University Moscow, together with theoreticians from Ludwig-Maximilians-Universität München (LMU).
„If you were to utilise magnetically doped topological insulators, you could also probably store this spin information”, explains Oliver Rader, who heads the research group for green spintronics at HZB. „To investigate this however, and also be able to explore the dynamic behaviour of the magnetic moments in particular, ultra-short light pulses in the soft X-ray region are needed. These kinds of experiments can become standard with the planned upgrade of the BESSY II synchrotron source to BESSY-VSR”, he hopes.

Source: Helmholtz-Zentrum Berlin für Materialien und Energie

Atomii de cupru au fost făcuți pentru a leza la lungimi de undă ultra-scurte cu raze X (Copper atoms made to lase at ultrashort x-ray wavelengths )

laserele atomice generează raze X cu distribuții mult mai clare ale lungimii de undă și o coerență temporală mai mare pe durata impulsului. În consecință, laserul atomic cu carcasă atomică are un potențial interesant pentru o astfel de cercetare. „Ne așteptăm ca această tehnologie poate fi aplicată, de exemplu, pentru a explora interferență cuantică pe un zeptosecond [10 -21 mărfuri de calitatea a două] la scară“

Posted: Feb 05, 2016
Copper atoms made to lase at ultrashort x-ray wavelengths (Nanowerk News) Using RIKEN’s powerful SACLA x-ray free-electron laser (XFEL) facility, researchers have developed the world’s first atomic x-ray laser capable of generating high-energy (‘hard’) x-rays with an ultrashort wavelength of just 1.5 ångströms (or 0.15 nanometers)—almost ten times shorter than the wavelengths of x-rays produced by earlier atomic lasers („Atomic inner-shell laser at 1.5-ångström wavelength pumped by an X-ray free-electron laser”).

Photograph of the experimental setup used to achieve a copper atomic x-ray laser. The copper foil target is mounted in the sample chamber (left) and is bombarded with an x-ray beam (from right) generated by the SACLA x-ray free-electron laser. (Image: Makina Yabashi, RIKEN SPring-8 Center)
Almost half a century ago, scientists theoretically predicted that copper atoms could be made to lase in the x-ray region by exciting them with x-rays of a suitable wavelength. The incident x-rays eject electrons from inner electron shells, resulting in a cascade of electron energy drops that produce light at a very specific wavelength of 1.5 ångströms—a length similar to the diameter of a copper atom. The emitted photons are then amplified by interacting with other excited copper ions, resulting in a process known as amplified spontaneous emission (ASE). Until now, this kind of x-ray amplification had been impossible to achieve in the lab due to the inconceivably high excitation intensities it demands.
Now, Hitoki Yoneda of the University of Electro-Communications and RIKEN SPring-8 Center and Makina Yabashi of RIKEN SPring-8 Center with their co-workers have succeeded in stimulating ASE of hard x-rays and used it to produce laser pulses with a wavelength of 1.5 ångströms. They realized this by shining very short pulses of intense x-rays from SACLA onto a copper foil (Fig. 1). ASE was further enhanced by combining the excitation ‘pump’ pulse with a ‘seed’ pulse from the same XFEL excitation source operating in two-color mode—a unique functionality of SACLA.
“We needed an extremely intense x-ray beam to create the highly excited states of copper atoms,” explains Yabashi. “The SACLA x-ray source has the highest intensity in the world, which we combined with state-of-the-art x-ray optics technology to focus the XFEL beam to a spot just 100 nanometers in diameter.” With this approach, the team achieved an unprecedented power density of about 1019 watts per square centimeter.
Although x-ray free-electron lasers are capable of producing hard x-rays at ångström wavelengths—SACLA itself supplies a 1.4-ångström x-ray pulse to stimulate ASE from the copper foil—they produce ‘fuzzy’ beams at such short wavelengths. This fuzziness poses problems for probing phenomena at very short time scales or spatial scales approaching the laser wavelength.
In contrast, atomic lasers generate x-rays with much sharper wavelength distributions and higher temporal coherence over the pulse duration. Consequently, the atomic inner-shell laser has exciting potential for such research. “We expect this technology may be applied, for example, to explore quantum interference on a zeptosecond [10-21 second] scale,” notes Yabashi.

Source: RIKEN

Quantum dots visualize tiny vibrational resonances

Guiding the random laser

Spintronics for future information technologies

Copper atoms made to lase at ultrashort x-ray wavelengths