Key ingredient: Change in material boosts prospects of ultrafast single-photon detector

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Colorized micrograph of an ultrafast single-photon detector made of superconducting nanowires. NIST researchers use electron beam lithography to pattern the nanowires (vertical lines) on a thin film of tungsten-silicon alloy, which produces more reliable signals than the niobium nitride material used previously. Credit: Credit: Baek/NIST

By swapping one superconducting material for another, researchers at the National Institute of Standards and Technology (NIST) have found a practical way to boost the efficiency of the world's fastest single-photon detector, while also extending light sensitivity to longer wavelengths. The new tungsten-silicon alloy could make the ultrafast detectors more practical for use in quantum communications and computing systems, experiments testing the nature of reality, and emerging applications such as remote sensing.

The detector, made of superconducting nanowires, is one of several sensor designs developed or used at NIST to register individual photons (particles of light). The original nanowire detector, invented in Russia, uses wires made of niobium nitride and has a detection or quantum efficiency-ability to generate an electrical signal for each arriving photon-of less than 10 percent in its simplest, most compact model. NIST's tungsten-silicon alloy version has an efficiency of 19 to 40 percent over a broad wavelength range of 1280 to 1650 nanometers, including bands used in telecommunications. The limitations are due mainly to imperfect photon absorption, suggesting that, with further design improvements, detector efficiency could approach 100 percent reliably, researchers say.

Superconducting nanowire detectors have many advantages. They are very fast, able to count nearly a billion photons per second, and they operate over a large range of wavelengths, have low dark (false) counts, and produce strong signals, especially at telecom wavelengths. The detectors produce a signal when a photon breaks apart some of the electron pairs that carry current in the superconducting state, where the material has zero resistance. If the nanowires are narrow enough and the DC current across the device is very close to the transition between ordinary and super conductance, a resistive band temporarily forms across each wire, resulting in a measurable voltage pulse.

Niobium nitride is difficult to make into nanowires that are narrow, long, and sensitive enough to work well. NIST researchers selected the tungsten-silicon alloy mainly because it has higher energy sensitivity, resulting in more reliable signals. A photon breaks more electron pairs in the tungsten-silicon alloy than in niobium nitride. The tungsten alloy also has a more uniform and less granular internal structure, making the nanowires more reliably sensitive throughout. As a result of the higher energy sensitivity, tungsten-silicon nanowires can have larger dimensions (150 nanometers wide versus 100 nanometers or less for niobium nitride), which enlarges the detectors' functional areas to more easily capture all photons.

The NIST team now hopes to raise the efficiency of tungsten alloy detectors by embedding them in optical cavities, which trap light for extremely high absorption. High efficiency may enable the use of nanowire detectors in demanding applications such as linear optical quantum computing, which encodes information in single photons. An equally intriguing application may be an experiment to test quantum mechanics-the so-called "loophole-free Bell test." This test of what Einstein called "spooky action at a distance" depends critically on having a nearly 100-percent efficient photon detector. Tungsten-silicon detectors also are sensitive to longer wavelengths of light, in the mid-infrared range, which could be useful for applications such as laser-based remote sensing of trace gases.

More information: B. Baek, A.E. Lita, V. Verma and S.W. Nam. Superconducting a-WxSi1-x nanowire single-photon detector with saturated internal quantum efficiency from visible to 1850 nm. Applied Physics Letters 98, 251105. Published online June 21, 2011.

Provided by National Institute of Standards and Technology (news : web)

Physics team reports gains in thermoelectric performance for bulk semiconductor material

Physics team reports gains in thermoelectric performance for bulk semiconductor material

Researchers from Boston College, MIT, Clemson and Virginia have used nanotechnology to achieve a 60-90 percent increase in the thermoelectric figure of merit of p-type half-Heusler, a common bulk semiconductor compound, the team reported in the American Chemical Society journal Nano Letters.

The dramatic increase in the figure of merit, used to measure a material's relative thermoelectric performance, could pave the way for a new generation of products – from car exhaust systems and power plants to solar power technology – that that runs cleaner, according to co-author Yan Xiao, a researcher in the Department of Physics at Boston College.

The team registered improvement in half-Heusler, which has been under study for its thermal stability, mechanical sturdiness, non-toxicity and low cost. However, the application of half-Heusler has been limited because of its poor thermoelectric performance: it previously registered a peak figure of merit of approximately 0.5 at 700 degrees Celsius for bulk ingots.

Xiao, working with BC Professor of Physics Zhifeng Ren and MIT's Soderberg Professor of Power Engineering Gang Chen, said the team produced an increase in the figure of merit value of p-type half-Heusler to 0.8 at 700 degrees Celsius. Moreover, the groups' material preparation methods proved to save time and expense compared with conventional methods.

"This method is low cost and can be scaled for mass production," Ren said. "This represents an exciting opportunity to improve the performance of thermoelectric materials in a cost-effective manner."

The researchers obtained their results by first forming alloyed ingots using arc melting technique and then creating nanoscale powders by ball milling the ingots and finally obtaining dense bulk by hot pressing. Transport property measurements together with microstructure studies on the nanostructured samples, in comparison with that of bulk ingots, showed that the thermoelectric performance improves largely because of low thermal conductivity produced by enhanced phonon scattering at grain boundaries and defects in the material. The material was also found to have a high Seebeck coefficient, a measure of thermoelectric power.

Researchers in the BC and MIT labs are still trying to prevent grain growth during press, which accounts for the still large thermal conductivity of half-Heusler.

"Even lower thermal conductivity and improved thermoelectric performance can be expected when average grain sizes are made smaller than 100 nm," said Ren, who was joined on the team by fellow Boston College researchers Giri Joshi, Weishu Liu, Yucheng Lan and Hui Wang, MIT's Sangyeop Lee, Virginia's Rogers Professor of Physics Joe Poons and J.W. Simonson and Clemson Professor of Physics Terry M. Tritt.

New material enables 'information sorting' at the speed of light

New material enables 'information sorting' at the speed of light

Artistic impression of metamaterial structure consisting of gold nanorods illuminated by two interacting light beams. Courtesy of R. McCarron.

(PhysOrg.com) -- An international team of scientists led by King’s College London has taken a step closer towards developing optical components for super-fast computers and high-speed internet services of the future. This has the potential to revolutionise data processing speeds by transmitting information via light beams rather than electric currents.

The researchers are studying the science of ‘nanoplasmonic devices’ whose key components are tiny nanoscale metal structures, more than 1000 times smaller than the size of a human hair, that guide and direct light.

Information is routinely sorted and directed in different directions to allow computing, internet connections or telephone conversations to take place. At present, however, computers process information by encoding it in electric signals.

It would be much faster to process and transmit information in the form of light instead of electric signals, but until now, it has been difficult for the light beams to be ‘changed’, that is to interact with other beams of light, while travelling through a material, and this has held up progress.

The scientists have solved this by designing a new artificial material, which allows light beams to interact efficiently and change intensity, therefore allowing information to be sorted by beams of light at very high speeds. The structure of the tailor-made material is similar to a stack of nanoscale rods, along which light can travel and, most importantly, interact.

Professor Anatoly Zayats, in the Department of Physics at King’s, explains: ‘If we were able to control a flow of light in the same way as we control a flow of electrons in computer chips, a new generation of data processing machines could be built, which would be capable of dealing with huge amounts of information much faster than modern computers.

‘The new material we have developed, often called ‘metamaterial’, could be incorporated into existing electronic chips to improve their performance, or used to build completely new all-optical chips and therefore revolutionise data processing speeds.

‘While there are many challenges to overcome, we would anticipate that in the future this faster technology could be in our PCs, mobile phones, aeroplanes and cars, for example.’

Other members of the team involved in this latest research include Argonne National Laboratory in the USA; University of North Florida; University of Massachusetts at Lowell; and Queen’s University of Belfast in the UK.

The research is published in the journal Nature Nanotechnology.