Optical lifting demonstrated for the first time (w/ Video)

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Light has been known for some time to be capable of pushing objects and this is the principle behind thesolar sail, which useslightto push vehicles along in space. Now, a new study by physicist Dr. Grover Swatzlander and colleagues of the Rochester Institute of Technology in Rochester, New York shows light is also capable of creating the more complex force of“lift,” which is the force generated by airfoils that make a plane rise upwards as it travels forward.

In a paper that appeared online inNature Photonicson December 5th, Swartzlander and colleagues describe their demonstration of light providing optical lift to tiny lightfoils. The experiment began as computer models that suggested when light is shone on tiny objects shaped like a wing a stable lift force would be created.

Intrigued, the researchers decided to do physical experiments in the laboratory, and they created tiny, transparent, micrometer-sized rods that were flat on one side and rounded on the other, rather like airplane wings. They immersed the lighfoils in water and bombarded them with 130 mW ultraviolet laser light from underneath the chamber. As predicted, the lightfoils were pushed upwards by the light, but they also moved sideways in a direction perpendicular to the beam of light, in other words they were optically lifted. Symmetrical micro-spheres did not show the optical lift effect.

In aerodynamic lift, which is created by an airfoil, the lift occurs because the wing shape causes air flowing under the wing to move more slowly and at higher pressure than that above the wing. In optical lift, created by a lightfoil, the lift is created within the transparent object as light shines through it and is refracted by its inner surfaces. In the lightfoil rods a greater proportion of light leaves in a direction perpendicular to the beam and this side therefore experiences a larger radiation pressure and hence, lift.

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Unlike aerodynamic lift, which has gradual lift angles, the optical lift angles were around 60 degrees, which Swartzlander said was striking, very powerful, and could be compared to a plane taking off at 60 degrees.“Your stomach would be in your feet,” he said.

Swartzlander described the findings as“almost like the first stages of what the Wright brothers did,” and said the next step would be to test lightfoils in air and experiment with a variety of materials with different refractive properties, and with other wavelengths of light.

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Vulnerability in commercial quantum cryptography

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Countermeasures were also implemented within an ongoing collaboration with leading manufacturer ID Quantique.

Quantum cryptographyis a technology that allows one to distribute a cryptographic key across an optical network and to exploit the laws ofquantum physicsto guarantee its secrecy. It makes use of the Heisenberg uncertainty principle - observation causes perturbation - to reveal eavesdropping on anoptical fiber.

The technology was invented in the mid-eighties, with first demonstration less than a decade later and the launch of commercial products during the first years of the century.

Although the security of quantum cryptography relies in principle only on the laws of quantum physics, it is also dependent on the lack of loopholes in specific implementations, just like any other security technology.

"The security of quantum cryptography relies on quantum physics but not only... It must also be properly implemented. This fact was often overlooked in the past,"explains Prof. Gerd Leuchs of the University of Erlangen-Nurnberg and the Max Planck Institute for the Science of Light.

Recently, NTNU in collaboration with the team in Erlangen has found a technique to remotely control a key component of most of today's quantum cryptography systems, thephoton detector, which is reported today inNature Photonicsadvance online publication.

"Unlike previously published attempts, this attack is imple-mentable with current off-the-shelf components,"says Dr. Vadim Makarov, a researcher in the Quantum Hacking group at NTNU, who adds:"Our eavesdropping method worked both against MagiQ Technology's QPN 5505 and ID Quantique Clavis2 systems."

In the framework of a collaboration initiated with ID Quantique, the researchers shared their results with the company prior to publication. ID Quantique has then, with a help of NTNU, developed and tested a countermeasure.

Academic researchers of the two laboratories will continue testing security aspects of quantum cryptography solutions from ID Quantique."Testing is a necessary step to validate a newsecuritytechnology and the fact that this proc-ess is applied today to quantum cryptography is a sign of maturity for this technology,"ex-plains Grégoire Ribordy, CEO of ID Quantique.

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Creating light sources for nanochips

Zhang is part of a group of scientists at Harvard University and the Georgia Institute of Technology working to develop high-quality nanometer-scale lasers. Their work is described inApplied Physics Letters:“Photonic crystal nanobeam lasers.”

“There are a couple of properties that we are interested in when it comes to creating an on-chip light source,” Zhang explains. “One feature is the threshold power, which represents the amount of energy needed to turn the laser on.” A low threshold is desirable, since it means that there is lesspower needed. “We want a light source that is energy friendly as well as being cheap to make.” Zhang and his colleagues have demonstrated a very low-threshold power on the order of the microwatt. (Conventional laser diodes have a threshold on the order of the milliwatt.)

Another property that is important tophotonic devicesis the modulation speed. The modulation speed represents the amount of information that can be carried by the laser.“Our nanobeam laser design has the potential to achieve very high modulation speed in addition to the low threshold,” Zhang says. “We haven’t demonstrated this high modulation yet, but it is our next step.”

The nanobeamlaserdesigned by the scientists at Harvard and GIT is fabricated with conventional nanofabrication technologies.“The team members in Georgia grew thequantum wellsthat have theelectronic propertiesthat allow the photon generation,” Zhang explains. Once the chip arrived at Harvard, Zhang and his colleagues used electron-beam lithography and inductively-coupled plasma reactive ion etching to create a pattern on the structure. Then, the entire structure was suspended with the use of chemical acid. “The final structure is a 500 nm wide beam with an array of perforated holes on it, suspended like bridges. This geometry gives excellent properties of theopticalnano-cavities,” Zhang says. “After the fabrication, we characterize our devices on the optical set up.”

There are a number of possible applications for a device like this. Interest in nanophotonics is growing, and the need for ultra-small light sources is increasing.“This could provide a cheap, on-chiplight sourcefor photonic integrated circuits,” Zhang points out. “This is the sort of application that would benefit from a lower threshold and higher modulation.”

Other applications include the possibility of use for sensors, says Zhang:“These devices would provide an intriguing platform for chemical or bio-sensing with extremely good spatial resolution, attributed to its small device size. Many of these applications are still demonstrated in scientific labs, and are probably not further away from commercialization, though.”

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Detector blinding attacks on quantum cryptography defeated

Quantum cryptography is a method to distribute digital encryption keys across an optical fibre. The protocol has been proven to be perfectly secure from eavesdropping. However, any differences between the theoretical protocol and its real-world implementation can be exploited to compromise the security of specific systems.

Arecent paper publishedin the September edition ofNature Photonicssuggests a method to blind the Indium Gallium Arsenide (InGaAs) avalanche photo-detectors that are commonly used in quantum cryptography. If successful, this attack could allow an eavesdropper to gain information about the secret key.

Now an investigation by the Cambridge team, to be published in the December edition of Nature Photonics, demonstrates that the detector blinding attack is completely ineffective,providedthat the single photon detectors are operated correctly.

The new study shows that the attack is only successful if a redundant resistor is included in series with the singlephoton detector, or if the discrimination levels are set inappropriately. Furthermore, by monitoring the photocurrent generated by the detector it is possible to prevent all bright light attacks on avalanche photodiodes.

Dr Andrew Shields, Assistant Managing Director, Toshiba Research Europe, comments,“Quantum cryptography is now entering a new phase in which the security of particular implementations is carefully analysed and tested. This is important to uncover any security loopholes and to devise appropriate countermeasures. It will allow real-world devices to approach the perfect security that can be proven for the protocol.”

Toshiba recently implemented its quantum key distribution (QKD) technology in thequantum cryptographynetwork set up in the Tokyo metropolitan area in October 2010. In a series of trials Toshiba demonstrated record average secure bit rates on installed fibre in the network. A secure bit rate of 304 kb/s was demonstrated, averaged over a 24 hour period, on a 45km fibre despite a relatively high loss on the link of 14.5dB. In April 2010 the same team announced an average secure bit rate of 1 Mb/s for a laboratory based demonstration on a 50 km fibre spool.

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In Brief: Quantum dot-Induced transparency

Placing a quantum dot near a metal is known to strongly modify the rate at which the dot emits light.

If the interaction between the dot and the metal is strong enough, scattering and absorption by the metal can be nearly eliminated at the quantum-dotresonance frequency, according to the simulations.

Scattering spectra for the structure when the corners of the metal nanoparticles have a curvature of 5 nm (solid squares) and 2 nm (open squares), calculated using the FDTD method. The lines are fits to a coupled-oscillator model.

This occurs even though the dot by itself simply absorbs light, and even though this absorption is nearly 100,000 times smaller than absorption by the metal nanostructure.

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Marvelous light from conductor paths

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A short push on the light switch– and the whole ceiling lights up in a uniform and pleasant color. This“illuminated sky” is not available as yet, but researchers from all over the world are working on it flat out. The technology behind this marvel is based on organiclight-emitting diodes, or OLEDs for short. These diodes use special molecules to emit light as soon as current passes through them. Although the first OLEDs have only recently become available, they are small and expensive. A flat disk with a diameter of eight centimeters costs around Euro 250. Experts of the Fraunhofer Institute forLaser TechnologyILT in Aachen, Germany are working together with Philips to develop a process for making theselampsdistinctly bigger and cheaper– and thus suitable for mass market.

These new lamps are expensive primarily due to the costlymanufacturing process. An OLED consists of a sandwich layer structure: a flat electrode at the bottom, several intermediate layers on top as well as the actual luminescent layer consisting of organic molecules. The final layer is a second electrode made of a special material called ITO (indium tin oxide). Together with the lower electrode, the ITO layer has the job of supplying the OLED molecules with current and causing them to light up. The problem is, however, that the ITO electrode is not conductive enough to distribute the current uniformly across a larger surface. The consequence: Instead of a homogeneous fluorescent pattern, the brightness visibly decreases in the center of the surface luminaire."In order to compensate, additional conductor paths are attached to the ITO layer,"says Christian Vedder, project manager at the Fraunhofer Institute for Laser Technology."These conductor paths consist of metal and distribute the current uniformly across the surface so that the lamp is lit homogenously."

Normally the conductor paths are applied by energy-intensive evaporation and structuring processes, while only a maximum of ten percent of the luminous area may be covered by conductor paths."The large remainder including the chemical etchant has to be recycled in a complicated process,"explains Christian Vedder. This is different in the new process from the researchers from the Fraunhofer Institute for Laser Technology. Instead of depositing a lot of material by evaporation and removing most of it again, the scientists only apply precisely the amount of metal required. First of all they lay a mask foie on the surface of the ITO electrode. The mask has micrometer slits where later the conductive paths are supposed to be. On this mask the researchers deposit a thin film of metal made of aluminum, copper or silver– the metal the conductor path is supposed to be made of. Subsequently a laser passes over the conductor path pattern at a speed of several meters per second. The metal melts and evaporates while the vapor pressure makes sure that the melt drops are pressed through the fine slits in the masks on to the ITO electrode.

The result are extremely fine conductor paths. At up to 40 micrometers, they are distinctly narrower than the 100 micrometer conductor paths which can be produced with conventional technology."We have already been able to demonstrate that our methods works in the laboratory,"says Christian Vedder."The next step is implementing this method in industrial practice together with our partner Philips and developing a plant technology for inexpensively applying the conductor paths on a large scale."The new laser process could be ready for practical application in two to three years.

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Humidity changes color of birds' feathers, biologists discover

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This discovery by Chad Eliason, a University of Akron integrated bioscience Ph.D. program student, and Dr. Matthew Shawkey, assistant professor of biology and integrated bioscience, is published in the Sept. 27 issue ofOptics Express, the international journal of optics.

The finding has implications ranging from technology (colorand vapor sensors) to biology (mate choice), according to the researchers.

Color in iridescentfeathersis created by light scattered from nanoscale structural components (keratin and melanin) of the plumage. The researchers explain separate research that shows that the protein, keratin, absorbswater vapor, which leads to swelling over a range of humidity. Further, the nanoscale arrangement of keratin and melanin at the outer edge of iridescent feather barbules results in coherent scattering of light, thereby producing brilliant, iridescent colors.

Eliason and Shawkey placed iridescent feathers from tree swallows in a small chamber and exposed them to various levels of humidity while measuring their color via spectrometry. This process involveds directing a beam of pure white light at the feather and measuring the amount of light at different wavelengths reflected back. A long wavelength, for instance, indicates a red feather while a blue feather reflects a short wavelength.

“We exposed the feathers to different humidity levels and found that the color had changed very rapidly, within two seconds, (from green to yellow) and reversibly withhumidity,” Eliason says.“Although we don’t know the function yet, this discovery should stimulate some interesting research.”

Eliason predicts that further research to determine if birds detect and respond to the color change, what function it serves, and how technology might mimic this phenomenon in nature are on the horizon.

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Nano-diamond qubits and photonic crystals

A quantum computer operates with quantum bits (qubits) as units of information. Obeying the laws ofquantum mechanics, such a computer would be capable of addressing several of the most difficult computational tasks unsolvable with present technology. In the past few decades, scientists learned to perform room-sized experiments to optically control and read out a small number of qubits.

Now, researchers in Germany have successfully fabricated a rudimentary quantum computing hybrid system using electronic excitations in nano-diamonds as qubits and opticalnanostructures, so-called photoniccrystalswith tailored optical properties. This architecture may allow integration of multi-qubit systems on a single micrometer-sized chip for future quantum computers.

"Our results suggest a strategy for scaling up quantum information to large-scale systems, which has yet to be done,"says Janik Wolters, researcher, at Humboldt Universität in Berlin."We regard our experiment as a milestone on the long road toward on-chip integrated quantum information processing systems, bringing the dream of a quantum computer closer to reality."

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Anti-mirror optical illusion could increase LED luminosity and laser power

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Huanyang Chen, a researcher inmetamaterialsandtransformationoptics from Soochow University in Jiangsu, China, and his colleagues have discovered and modeled the anti-mirror effect. The ability to make multiple objects look like one using“overlapped illusion optics” has not been found in nature before now.

To demonstrate the basic idea, the researchers explained that two identical cylindrical perfect electric conductors (PECs) are placed on opposite sides of a perfect lens made of a negative refractive index material. When viewed from the far field (beyond a certain distance) on either side of the lens, the two PECs look like one. Further, when the scientists replaced one of the circular PECs with an illusion device with an elliptic cylindrical PEC, both PECs look like only one circular cylindrical PEC.

Further elaborating on this effect, the researchers showed that illusion devices with elliptic cylindrical PECs can be used in place of both real cylindrical PECs. Once again, the two illusion devices look like one PEC. As the scientists explained, this effect occurs because the two illusion devices are close enough together so that their virtual illusion spaces overlap. Inside this shared region, both illusion devices form a single PEC image.

“The two PECs on both sides of the perfect lens follow the image-forming principle so that each of them is overlapped with the virtual image of another,” Chen explained toPhysOrg.com.“The anti-mirror effect stems from the evanescent wave amplification of the perfect lens.”

The anti-mirror effect could have applications in both solid-state lighting, such as LEDs, and in coherent light sources, such as lasers. Currently, one of the biggest challenges in LED development is achieving a high enough luminosity for general lighting purposes. One method of increasing LED illuminance is to package many LEDs inside a single bulb; however, the problem is that the lamp's spatial illumination is not uniform. Using the new overlapped illusion optics, the researchers show that the images from multiple LEDs in different places can be overlapped to make the bulb look like a single-LED source with high, uniform illumination.

The proposed overlapped illusion optics method could also increase the power and preserve the spatial uniformity of lasers. Usually, spatial uniformity degrades when two coherent sources are aligned due to interference. Using the same configuration as the LEDs, multiple coherent sources can be operated at the same frequency and phase to double the light amplitude and quadruple the total power of the coherent system. These improvements cannot be achieved using traditional beam-combining techniques.

“Our current work is just a conceptual model,” Chen said.“We have recently realized the first illusionopticaldevice– an“invisible gateway”– by using a transmission-line medium.” See http://arxiv.org/abs/1005.3425 .

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Light on silicon better than copper?

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As good as the metal has been in zipping information from one circuit to another onsiliconinside computers and otherelectronic devices, optical signals can carry much more, according to Duke University electrical engineers. So the engineers have designed and demonstrated microscopically small lasers integrated with thin film-light guides on silicon that could replace the copper in a host of electronic products.

The structures on silicon not only contain tiny light-emitting lasers, but connect these lasers to channels that accurately guide the light to its target, typically another nearby chip or component. This new approach could help engineers who, in their drive to create tinier and faster computers and devices, are studying light as the basis for the next generation information carrier.

The engineers believe they have solved some of the unanswered riddles facing scientists trying to create and control light at such a miniscule scale.

"Getting light onto silicon and controlling it is the first step toward chip scaleoptical systems,"said Sabarni Palit, who this summer received her Ph.D. while working in the laboratory of Nan Marie Jokerst, J.A. Jones Distinguished Professor of Electrical and Computer Engineering at Duke's Pratt School of Engineering.

The results of team's experiments, which were supported by the Army Research Office, were published online in the journalOptics Letters.

"The challenge has been creating light on such a small scale on silicon, and ensuring that it is received by the next component without losing most of the light,"Palit said.

"We came up with a way of creating a thin film integrated structure on silicon that not only contains alight sourcethat can be kept cool, but can also accurately guide the wave onto its next connection,"she said."This integration of components is essential for any such chip-scale, light-based system."

The Duke team developed a method of taking the thick substrate off of alaser, and bonding this thin film laser to silicon. The lasers are about one one-hundreth of the thickness of a human hair. These lasers are connected to other structures by laying down a microscopic layer of polymer that covers one end of the laser and goes off in a channel to other components. Each layer of the laser and light channel is given its specific characteristics, or functions, through nano- and micro-fabrication processes and by selectively removing portions of the substrate with chemicals.

"In the process of producing light, lasers produce heat, which can cause the laser to degrade,"Sabarni said."We found that including a very thin band of metals between the laser and the silicon substrate dissipated the heat, keeping the laser functional."

For Jokerst, the ability to reliably facilitate individual chips or components that"talk"to each other using light is the next big challenge in the continuing process of packing more processing power into smaller and smaller chip-scale packages.

"To use light in chip-scale systems is exciting,"she said."But the amount of power needed to run these systems has to be very small to make them portable, and they should be inexpensive to produce. There are applications for this in consumer electronics, medical diagnostics and environmental sensing."

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'Space-time cloak' to conceal events revealed in new study

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Previously, a team led by Professor Sir John Pendry at Imperial College London showed that metamaterials could be used to make an optical invisibility cloak. Now, a team led by Professor Martin McCall has mathematically extended the idea of a cloak that conceals objects to one that conceals events.

"Light normally slows down as it enters a material, but it is theoretically possible to manipulate the light rays so that some parts speed up and others slow down,"says McCall, from the Department of Physics at Imperial College London. When light is 'opened up' in this way, rather than being curved in space, the leading half of the light speeds up and arrives before an event, whilst the trailing half is made to lag behind and arrives too late. The result is that for a brief period the event is not illuminated, and escapes detection. Once the concealed passage has been used, the cloak can then be 'closed' seamlessly.

Such a space-time cloak would open up a temporary corridor through which energy, information and matter could be manipulated or transported undetected."If you had someone moving along the corridor, it would appear to a distant observer as if they had relocated instantaneously, creating the illusion of a Star-Trek transporter,"says McCall."So, theoretically, this person might be able to do something and you wouldn't notice!"

While using the spacetime cloak to make people move undetected is still science fiction, there are many serious applications for the new research, which was funded by the Engineering and Physical Sciences Research Council (EPSRC) and the Leverhulme Trust. Co-author Dr Paul Kinsler developed a proof of concept design using customised optical fibres, which would enable researchers to use the event cloak in signal processing and computing. A given data channel could for example be interrupted to perform a priority calculation on a parallel channel during the cloak operation. Afterwards, it would appear to external parts of the circuit as though the original channel had processed information continuously, so as to achieve 'interrupt-without-interrupt'.

Alberto Favaro, who also worked on the project, explains:"Imagine computer data moving down a channel to be like a highway full of cars. You want to have a pedestrian crossing without interrupting the traffic, so you slow down the cars that haven't reached the crossing, while the cars that are at or beyond the crossing get sped up, which creates a gap in the middle for the pedestrian to cross. Meanwhile an observer down the road would only see a steady stream of traffic."One issue that cropped up during their calculations was to speed up the transmitted data without violating the laws of relativity. Favaro solved this by devising a clever material whose properties varied in both space and time, allowing the cloak to be formed.

"We're sure that there are many other possibilities opened up by our introduction of the concept of the spacetime cloak,' says McCall,"but as it's still theoretical at this stage we still need to work out the concrete details for our proposed applications."

Metamaterials is an expanding field of science, with a vast array of potential uses, spanning defence, security, medicine, data transfer and computing. Many ordinary household devices that work using electromagnetic fields could be made more cheaply or to work at higher speeds. Metamaterials could also be used to control other types of waves as well as light, such as sound or water waves, opening up potential applications for protecting coastal or offshore installations, or even engineering buildings to withstand earthquake waves.

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Making better biosensors with electron density waves

According to Fainman, tapping the potential of plasmonics for biomolecule detection systems has been a challenge, because localized optical field scales are usually much larger than the molecules in question. In order to make a useful optical biosensor, he says,"We need to increase the interaction cross-section by finding ways to localize optical interrogation fields ideally to the scales comparable to those of biomolecules."Since that is not currently possible, he and his team used an approach of integrating microfluidics and plasmonics on single chips, allowing fluid to ferry the molecules into the cross-section of the optical field.

Fainman expects the system to be particularly useful in studying large arrays of protein-protein interactions for identifying potential drugs that bind to specific target molecules, which may lead to earlier cancer diagnoses and faster discovery of new drugs. Unlike most current methods,optical detectiondoes not require labeling of molecules with fluorescent or radioactive entities -- labels often hinder interaction by covering up or blocking binding surfaces.

The new platform also carries the advantage of being high throughput and multiplexed, offering researchers an opportunity to examine thousands of arrayed compounds simultaneously, which, he says,"biologists and physicians get very excited about."

Fainman will present these results at Frontiers in Optics (FiO) 2010/Laser Science XXVI -- the 94th annual meeting of the Optical Society (OSA).

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World's fastest camera takes a new look at biosensing

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Faster, higher resolution cameras

Since the introduction of solid-stateoptical sensors, like those found in digital cameras, the main trend has been towards increasing the resolution (i.e. number of pixels) while miniaturising the chip.

However, the other factor is the number of frames the chip is capable of recording in a given time. Until recently, fast cameras (i.e. those capturing more than the 24 frames per second required for 'normal' video) were only used in niche markets in science and entertainment.

Ultrafast cameras

Now that higher-than-video speeds are achievable, a whole new range of previously unthinkable applications have emerged– such as: cellular / sub-cellular imaging; neural imaging; biochemical sensors; DNA / protein microarray scanning; automotive collision studies; and high-sensitivity astronomical observations.

The Megaframe Imager uses an extremely sensitive single photon avalanche diode (SPAD) device, and bespoke on-chip intelligence and has shown for the first time that it could potentially be a powerful technology in biosensing.

Reporting in the Optical Society of America's new journalBiomedical Optics Expressthe research team have demonstrated detection of viral DNA binding events using fluoresence lifetime imaging at the very low target concentrations relevant in biosensing applications with acquisition times of less than 30 seconds.

DNA microarrays are important tools for biomolecular detection. Widely used for gene expression profiling, disease screening, mutation and forensic analysis, they also hold much promise for the future development of personalised drugs and point of care testing devices.

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Physicists show that superfluid light is possible

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In their study published in a recent issue ofPhysical Review Letters, Patricio Leboeuf and Simon Moulieras from the University Paris-Sud and CNRS explain thatsuperfluidityis the ability of a fluid to move with zero dissipation or viscosity. A fluid behaves like a superfluid only under a certain critical velocity; above this critical velocity, superfluidity disappears. Most commonly demonstrated in liquid helium, superfluidity occurs when the helium is cooled and some helium atoms have reached their lowest possible energy. At this point, these atoms' quantum wave functions begin to overlap so that they form a Bose-Einstein condensate, in which all the atoms behave as one large atom, and their quantum nature is manifested on the macroscopic scale.

Previously, investigations of the superfluid motion oflighthave not revealed clear evidence of the existence of a superfluid critical velocity. Although some recent experiments have observed superfluidity related to light, these experiments did not use photons, but a composite particle, called a polariton, which is a mixture of a photon and an exciton.

In this study, Leboeuf and Moulieras have shown that a superfluid critical velocity does exist in a nonlinear medium. They explain how superfluid light can be observed in an array of waveguides. From a dynamical point of view, light propagating through a nonlinear medium is formally equivalent to a Bose gas of interacting massive particles. Light can travel straight along the waveguides in the longitudinal direction, or it can tunnel between adjacent guides in the transverse direction. The benefit of this set-up is that it allows the scientists to engineer different characteristics of the array and control the light's flow.

The physicists were specifically interested in what happens to alight pulseas it travels through the array at different velocities in the presence of a defect. If the light is scattered by the defect, it means dissipative processes have occurred. If the light pulse moves through the defect without changing its shape (i.e., without losing collectivity), there is no dissipation and the light has superfluid motion. Through their calculations, the physicists showed that, for certain low velocities, the transverse motion of light is superfluid with zero dissipation. When the velocity increases, dissipative processes occur that destroy the collectivity of the light's oscillations, and superfluidity breaks down.

In the future, the physicists plan to further investigate additional details of superfluid light, such as how it relates to an underlying quantum theory of light and how it is connected to Bose-Einstein condensation. They predict that superfluid motion is a general property of light that exists in a variety of scenarios, and is not limited to the waveguide array proposed here. Superfluid light could also have applications in light transport optimization.

“One straightforward implication is related to transport in the presence of noise,” Leboeuf said.“Such a noise is expected to be present generically, since any material has imperfections and impurities. The impurities are responsible for the scattering of light. In the superfluid regime, we expect a light pulse to be able to propagate through a noisy medium without being affected or scattered (perfect transmission).”

Leboeuf and Moulieras plan to perform their proposed experiment and are discussing the opportunity with experimental groups at the Laboratoire de Photonique et de Nanostructures (LPN) at Marcoussis, France. However, the scientists said that superfluid light is not likely to have any strange effect analogous to a superfluid flowing up a container.

“The most basic 'strange' quantum effect that light shows related to superfluidity is, as shown in our article, dissipationless motion,” Moulieras said.“Another, though more indirect or spectacular, effect is related to quantized vortices, which were observed in laser patterns propagating through nonlinear media. Concerning other possibilities, such as fluid motion up the walls of a container, they are related, for atoms, to the forces between these atoms and a substrate, and the balance between capillary, gravity and viscous forces. We do not see a straightforward application of these concepts to photons, and therefore do not expect them for light.”

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This little light of mine: Changing the color of single photons emitted by quantum dots

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Two important resources forquantum information processingare the transmission of data encoded in thequantum stateof a photon and its storage in long-lived internal states of systems like trapped atoms, ions or solid-state ensembles. Ideally, one envisions devices that are good at both generating and storing photons. However, this is challenging in practice because while typical quantum memories are suited to absorbing and storing near-visible photons, transmission is best accomplished at near-infrared wavelengths where information loss in telecommunications optical fibers is low.

To satisfy these two conflicting requirements, the NIST team combined a fiber-coupled single photon source with a frequency up-conversion single photon detector. Both developed at NIST, the frequency up-conversion detector uses a strong pump laser and a special non-linear crystal to convert longwavelength(low frequency) photons into short wavelength (high frequency) photons with high efficiency and sensitivity (See http://www.physorg.com/news170516085.html).

According to Matthew Rakher and Kartik Srinivasan, two authors of the paper, previous up-conversion experiments looked at the color conversion of highly attenuated laser beams that contained less than one photon on average. However, these light sources still exhibited"classical"photon statistics exactly like that of an unattenuated laser, meaning that the photons are organized in such as way that at most times there are no photons while at other times there are more than one. Secure quantum communications relies upon the use of single photons.

"The quantum dot can act as a true single photon source,"says Srinivasan."Each time we excite the dot, it subsequently releases that energy as a single photon. In the past, we had little control over the wavelength of that photon, but now we can generate a single photon of one color on demand, transmit it over long distances with fiber optics, and convert it to another color."

Converting the photon's wavelength also makes it easier to detect, say co-authors Lijun Ma and Xiao Tang. While commercially available single photon detectors in the near-infrared suffer noise problems, detectors in the near-visible are a comparatively mature and high-performance technology. The paper describes how the wavelength conversion of thephotonsimproved their detection sensitivity by a factor of 25 with respect to what was achieved prior to conversion.

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