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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Videos:Nature Photonics, doi:10.1038/nphoton.2010.266

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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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