Category Archives: Optics and photonics

Optical transistor triggered by a single photon

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Nature: Thanks to their ability to switch on and off and amplify signals, transistors are a key component of electrical devices. Several projects have demonstrated the possibility of creating transistors that are controlled by photons instead of electrical signals. The most recent, created by Wenlan Chen of MIT and her colleagues, relies on only a single photon to achieve this functionality. The proof-of-concept project uses a cooled cloud of cesium atoms and a principle called electromagnetically induced transparency in which a photon with a specific energy switches the cloud of cesium atoms between excited and ground states. When the atoms are in ground states, they allow light to pass through the cloud. Neither Chen’s transistor nor other optical transistors are likely to replace traditional ones any time soon, however, as the size and energy costs are significantly greater.

Large-scale optical cloaking in multiple directions

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MIT Technology Review: Last week researchers revealed an optical invisibility cloak large enough to hide a human, but it was limited to only working in a single direction. That achievement has already been surpassed by Hongsheng Chen of Zhejiang University in China and his colleagues. They realized that with most invisibility cloaks, researchers worried about maintaining light’s phase and polarity, but that it isn’t necessary for visible light because humans aren’t sensitive to changes in those characteristics. That realization allowed Chen and colleagues to use conventional optical components to steer light around a hidden central area. They demonstrated two versions of their cloaking device: The first is square, which hides the central area from four directions; the second is a hexagon, which works in six directions.

New technique measures the strength of individual molecular bonds

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Nature: Raman spectroscopy and scanning tunneling microscopes (STMs) have been used together to produce increasingly more detailed images of molecules. A new advancement in the pairing has produced pictures of individual molecules as well as measurements of the strength of the molecules’ bonds. By itself, Raman spectroscopy uses a laser to cause molecules to vibrate, and the way the light is scattered can be used to determine the molecule’s structure. The technique doesn’t work well with small samples. An STM uses a very fine metal tip held just 1 nm from a surface to allow electrons to quantum tunnel across the gap, and the strength of the electric current is used to map surfaces with atomic resolution. An international team of researchers has succeeded in using an STM to narrow the focus of the Raman laser. They use another laser to create oscillations in the STM tip’s electric field. When the frequency of those oscillations matches the frequency of the Raman laser, the beam becomes significantly stronger. At 0.3 nm, the resolution of the new arrangement is still less detailed than that of other techniques, such as atomic force microscopy, but the Raman–STM pairing allows for measuring the strength of molecular bonds as well.

Temporal cloaking device could prove useful in telecommunications

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Science News: To mask data transferred between a sender and a receiver, researchers have developed a cloaking device that creates holes in time and space. Joseph Lukens of Purdue University in West Lafayette, Indiana, and colleagues—whose paper was published online in Nature—manipulated the flow of photons so that minute gaps occurred in the light wave. They then injected an electrical signal, consisting of binary data, that went undetected because the data bits passed through the gaps. Such a device could advance the field of secure communications by concealing transmissions from potential eavesdroppers.

Glasses-mounted camera helps the visually impaired

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New York Times: An Israeli company has developed a device that can read aloud text and identify objects. Designed for the visually impaired, OrCam consists of a camera mounted on a pair of glasses, a small computer that is worn in a pocket, and a bone-conduction earpiece. According to OrCam’s website, all you have to do is point and OrCam “will understand what you want on its own, whether it’s to read, find an item, catch a bus or cross the road.” It can read newspapers and books, labels on medicine bottles, and text on computers and phones. OrCam can also be taught by the wearer to recognize personal items, credit cards, and currency as well as the faces of family and friends. The device uses computer vision algorithms developed by Amnon Shashua, a computer science professor at Hebrew University; fellow faculty member Shai Shalev-Shwartz; and former grad student Yonatan Wexler. Shashua has also supplied similar camera technology to the automobile industry. OrCam is one of a half-dozen devices currently being developed in the field of computer vision.

Quantum dot technology enhances color displays

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New Scientist: Sony has developed a flat-panel electronic display that is more energy efficient and provides richer color than conventional LCD displays. The new display uses quantum dots—tiny semiconductors that confine electric charge and emit light. Each dot’s color depends on its size and shape. Although each spans just 10% of the visible spectrum, the dots can be mixed to produce 100% of the color range visible to the human eye. The company that developed the technology, QD Vision, claims that its Color IQ technology provides “the most radiant reds, brilliant blues and gorgeous greens you will ever see.” Other benefits of the new technology include lower costs and the potential for extremely thin displays.

Duplicating light beams in fiber optics allows clearer signal

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BBC: Signals sent over fiber-optic cables collect noise as they travel: The farther a signal travels, the noisier it becomes. Now, Xiang Liu of Bell Laboratories and his colleagues have demonstrated a way to increase both the speed and the distance that a fiber-optic signal can travel and still maintain its clarity. They duplicated a signal and sent the pair of light beams down a cable. At the end of the cable, when the two signals were recombined, the parts of one of the signals that were disrupted by noise were overwritten by the unchanged parts of the other signal. The overwriting technique allows for the removal of signal repeaters, which are used in fiber optics to extend the distance a signal can travel but which tend to limit the power and speed of the data transmission. Liu’s test setup used a 12 800-km-long fiber-optic cable—longer than the longest transoceanic cable—and reached speeds of 400 Gb/s, four times the speed of the best commercial cable systems.

Laser sensor watches for the watchers

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Popular Science: In combat, knowing you are being watched is just as important as being able to watch your enemy. San Diego-based defense contractor Torrey Pines Logic is developing a robotic system that can determine when someone is using a viewing device to look toward the system. The Beam 100 Optical Detection System uses laser pulses with a range of just over 1000 m in a 360-degree field of view. When the pulses are reflected back to the system, it examines the light for the signature characteristics of optical glass. Optical glass could be any sort of lens used in binoculars, cameras, or even rifle scopes. If it detects a reflection that it judges to be optical glass, it informs the user and indicates the direction and distance. The device is designed purely as a detection system; the use of lasers or other weapons specifically to blind is forbidden by international law.

Lidar used to find ancient city in Honduras

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Independent: A large, unmapped, densely forested area of eastern Honduras may be the site of an ancient city called Ciudad Blanca, first reported by Hernán Cortés in 1526. Cortés never found the city and neither have any subsequent explorers. Now Steve Elkins, a filmmaker and amateur archaeologist, has teamed with archaeologists from Colorado State University to use lidar to map part of the area. Lidar creates a 3D topological map of the ground and structures on it by firing billions of pulses of laser light that can penetrate the organic forest canopy. From data collected over one week, the researchers mapped a 155-km2 area, which revealed what may be a network of plazas and pyramids. Possibly dating back to 500 CE, the city also appears to have had paved roads, parks, and advanced irrigation systems. To prevent looting, the city’s precise location has not been revealed. In partnership with the Honduran government, Elkins plans to lead a ground expedition to explore the area and make a documentary film of the effort.

New LED design increases display brightness and efficiency

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MIT Technology Review: Most video display systems use light-emitting backplanes covered with filters that create the individual pixels. That system requires that the light source remain on, which drains battery power. In addition, the filters reduce the brightness of the screen’s light. Lumiode, a startup in New York, has developed a display technology that uses an array of LEDs as individual pixels, with each LED covered by a layer of silicon to control the amount of light emitted. The company’s prototype is a 50 x 50 array of LEDs just 1 mm2. Lumiode claims that the display is 30 times brighter and 10 times more efficient than other displays. However, the display is currently limited to just a single color, although the company plans to add a color-controlling layer on top of the LED wafer. Lumiode CEO Vincent Lee says that he expects his company to develop a 320 x 240 pixel prototype within the next year. Lee hopes that his company can partner with electronics makers to incorporate the display in heads-up devices such as Google Glass or to create displays on car windshields.