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OPTICS Paper-like mirrors Scientists in Europe have demonstrated thin, flexible, paper-like mirrors whose polarization and peak wavelength of reflection can be tuned Opt.( Express 21, 20821–20830; 2013). As these narrow- band (bandwidth ~100 nm) reflectors are easy and cheap to fabricate, they are potentially well suited for use in low-cost applications such as e-readers, smart credit cards, labels and dosimeters. They offer an attractive alternative to silver- coated reflectors. Gia Petriashvili and co-workers from the Georgian Technical University (Georgia), University of 2013 © OSA Calabria (Italy) and Hewlett Packard Labs (UK) made the reflectors by mixing a change the pitch of the photosensitive the polymer cover film, which also acts a cholesteric liquid crystal with a reactive cholesteric mixture, and thus tune the phase retarder. In this manner, the reflection monomer (RM 257), an optically active spectral response of the mirror’s reflection polarization can be changed (for example, dopant (ZLI-811) and an ultraviolet across the visible range — longer durations between linear and circular polarization). photoinitiator (Irgacure 2100). A thin of ultraviolet exposure give shifts to longer As the reflectors are flexible and layer of the mixture is applied to a wavelengths. Furthermore, by using a mask tunable, they offer considerable more paper substrate, and it is covered with during ultraviolet exposure, an array of opportunities for applications that a protective polymer film. The entire miniature reflectors with different spectral traditionally used silver-coated reflectors. assembly is then irradiated with ultraviolet properties can be realized; for example, For example, their properties can be light to polymerize it and thereby make it a pattern of red, green and blue mirrored tailored to suit any kind of backlight or mechanically robust and flexible. pixels can be realized on the one substrate. reflective display design. Once polymerized, further irradiation The polarizing properties of the reflectors with ultraviolet light can be used to can be controlled by varying the thickness of OLIVER GRAYDON

TRANSFORMATION OPTICS Gravitational lens on a chip Massive objects in space act as gravitational lenses, bending and focusing light. Scientists have now created a photonic analogue of a gravitational lens on a chip, and have shown that it is strong enough to force light into orbits. Ulf Leonhardt

ir Arthur Eddington was once claimed 1919. During the few minutes of such an country who understood Einstein’s theory. to be one of only three people who eclipse, the Moon blocks light from the In desperation, they summarized the story Sunderstood Einstein’s general theory Sun, causing day to turn to night and the along the lines of “Professor Einstein says of relativity. When informed of this by his stars to become visible. Eddington took the stars have moved because space is colleague, Ludwik Silberstein, Eddington photographs of the stars near the eclipsed curved. Nobody understands his theory, paused and murmured, “I am wondering Sun. He found that the stars appeared to but Einstein is right.” who the third one is.” Now, you have a have shifted from their normal positions Now, the experiment demonstrated chance to understand it, too. Writing in by an amount Einstein had predicted based by Genov and colleagues1 shows that Nature Photonics, Dentcho Genov and on his general theory of relativity. When Eddington’s test of Einstein’s theory co-workers report an ingeniously simple Eddington returned to the UK, his findings of general relativity is in fact easy to experiment that recreates Eddington’s and those of a simultaneous expedition understand. For a start, the apparent famous astronomical test of Einstein’s to Sobral in Brazil created a sensation movement of the stars from their usual theory in the laboratory1. at the Royal Society. The press became positions during the eclipse has nothing to The story goes back to 1919 when interested and cables of the news reached do with the eclipse itself; rather it is related Eddington led an expedition to the island the USA, where journalists asked scientists to the fact that their light paths passed close of Príncipe off the west coast of Africa to for explanations and comments. However, to the Sun. Without the Moon blocking observe the total solar eclipse of 29 May they were unable to find anyone in the the Sun, the intense sunlight would have

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© 2013 Macmillan Publishers Limited. All rights reserved. news & views blinded Eddington’s sensitive photographs. With or without an eclipse, the Sun creates a an optical illusion (see Fig. 1a). The Sun’s gravity acts like a gigantic lens that slightly deflects the light from the stars. The closer light rays come to the Sun, the stronger the effect is. This bending of light by gravity is a consequence of general relativity. According to Einstein’s theory, gravity is simply the curvature of space and time. This may initially sound a bit incomprehensible, but there is a good argument for it, as given below. Einstein was deeply puzzled by the O fact that gravity is the most universal force of all. In the absence of other forces, all bodies fall under gravity in the same way, b regardless of what they are made of or how heavy they are; only their velocity dictates their fate. For example, if you throw a stone, it will fall down to Earth, whereas if you were able to throw it at a cosmic speed, it would fall around the Earth, like an orbiting satellite. Everything and everyone is equal under the law of gravity. The question Einstein asked was “how can gravity be so universal?” The most universal concepts one can think of are space and time, Figure 1 | Deflection of starlight in the space curved by the gravity of the Sun.a , Light rays (solid red lines) because all objects reside in space and exist reaching the observer O are bent in the curved space, creating the illusion (dashed lines) that the stars in time. Imagine a distortion of space and have moved away from the Sun. b, Instead of the Sun, a dense star traps light, forcing light into an orbit. time. It would universally deflect all objects Both images are obtained from a visualization of the refractive-index profile of the optical analogue used from their otherwise straight paths. The in the experiment by Genov and co-workers1. only thing that matters for each object is the ratio of how much space it traverses in each instant of time, that is, its velocity. The chip1. On the chip, light propagates in a layer nothing to with its mass. Yet by chance and curvature of space and time explains the of transparent plastic, which acts as a planar clever design, in the experiment by Genov universality of gravity. Light is the fastest waveguide, confining light to the surface and colleagues1, the created refractive- entity in the Universe, but if space–time of the chip. In this way, light can only index profile closely resembles the is curved, gravity will act on light as well, propagate in a ‘flatland’ consisting of two distortion of space caused by a massive star. bending its path in the manner Eddington dimensions. For a curved flatland, which Their device can not only mimic the observed. Collapsed stars with extremely mimics curved space in two dimensions, deflection of starlight that Eddington strong gravitational fields would constrain the team used an ingeniously simple trick observed in 1919, it can also reproduce even light to follow closed orbits around invented by Smolyaninov et al.3 — the much more drastic effects of extreme them (see Fig. 1b). use of a tapered waveguide. They placed gravity that astronomers have yet to In photonics, bending of light is a glass microsphere on the chip while the observe. The refractive-index profile was nothing unusual: it happens all the time plastic was still hot. The plastic clings to the strong enough to force light into orbits, like in transparent materials with a gradually microsphere and rises around it. The thicker a satellite orbiting a collapsed star. When varying refractive index. Not only do such the plastic layer is, the more it influences the light comes too close to the microsphere, gradient-index (GRIN) media bend light, confined light, and the higher the effective it even falls into this analogue of the they also bend it in the same manner as refractive index is. It turns out that the star, where it is absorbed. However, what curved space. This is because light rays index profile Genov and co-workers made Genov and co-workers did not and could propagate along the path with the shortest corresponds, to a good approximation, to not reproduce with GRIN media is the travel time. It takes light longer to traverse the space–time geometry of a star. optical analogue of a black hole. A black a region with a higher refractive index. The refractive-index profile plays the hole swallows all light that crosses its event One can therefore say that the refractive role of curved space. In both photonics and horizon; light that ventures within this index changes the measure of space in the general relativity, space dictates how light horizon is lost forever. In contrast, it is same way that gravity modifies it for light2. propagates. In general relativity, not only always possible in principle to reverse the Consequently, it is easy to understand a light is deflected by gravity; everything path of light in a GRIN medium — if light great deal of general relativity by simply else, all matter, is deflected by it. Space can travel from A to B, then it can also picturing curved space–time as a medium tells matter how to move. However, in travel from B to A. If light can get in, it can with a gradually varying refractive index. general relativity, matter is also the source also get out; it just needs to be reflected Inspired by this connection between of gravity: matter tells space how to curve. back after being captured (but before being GRIN media and general relativity, Genov It is the mass of matter that causes gravity. absorbed), something that is not possible in and collaborators put forward the idea of a This differs from photonics, where the black holes. An optical black hole turns out gravitational lens on an integrated photonic refractive-index profile of a medium has to require moving media4,5.

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Nevertheless, microspheres on chips are Michael Faraday replied in 1850 when References very effective in capturing light. The device asked by her Majesty’s chief tax collector 1. Sheng, C., Liu, H., Wang, Y., Zhu, S. N. & Genov, D. A. Nature Photon. 7, 902–906 (2013). is not only of great pedagogical value, about the practical use of the newly 2. Leonhardt, U. & Philbin, T. Geometry and Light: the Science of illustrating general relativity at work, it discovered laws of electricity, “One day sir, Invisibility (Dover, 2010). may also find applications for the trapping you may tax it”. ❒ 3. Smolyaninov, I. I., Smolyaninov, V. N., Kildishev, A. V. & Shalaev, of light in integrated photonics. Who V. M. Phys. Rev. Lett. 102, 213901 (2009). 4. Philbin, T. G. et al. Science 319, 1367–1370 (2008). would have thought in 1919 that general Ulf Leonhardt is at the Department of Physics of 5. Belgiorno, F. et al. Phys. Rev. Lett. 105, relativity — a theory that supposedly Complex Systems, Weizmann Institute of Science, 203901 (2010). only three people understood — would Rehovot 76100, Israel. one day inspire practical applications? As e-mail: [email protected] Published online: 20 October 2013

VIEW FROM… CLEO-PR & OECC/PS 2013 Cost-effective solutions The integrated optical components used for optical data transmission are technically complex. To keep pace with the exponential growth in communication traffic, researchers are exploring every potential avenue for inexpensively enhancing device performance. Noriaki Horiuchi

his year, the Conference on Lasers and Electro-optics Pacific Rim T(CLEO-PR 2013) was jointly held with the Optoelectronics and Communications Conference (OECC 2013) and Photonics in Switching (PS 2013) at the Kyoto International Conference Center, Japan, from 30 June to 4 July. This joint conference featured about 970 oral and poster presentations, and attracted around 1,400 researchers from 40 countries. The latest breakthroughs in a wide variety of fields were discussed; these fields included laser sources, laser processing, optical metrology, biophotonics, optical devices and optical communications. One aspect that stood out was the efforts of researchers working in the field of optical data transmission to achieve the seemingly conflicting objectives of improving performance and reducing costs. As is well known, communication traffic

is increasing exponentially. In 2011, the NORIAKI HORIUCHI USA backbone network had a staggering traffic volume of about 2 × 104 petabytes Around 1,400 researchers from 40 countries gathered at the Kyoto International Conference Center. per month. In the plenary session, Mark Feuer from AT&T Labs, USA, presented his perspective about expanding the multiple-input—multiple-output digital multifunctional devices, because this would information-carrying capacities of future signal processing, reconfigurable optical permit surplus devices to be removed. networks. “After intense study of time- add/drop multiplexers and optical routing In this respect, Koji Yamada from NTT, and wavelength-division multiplexing, it’s for multicore superchannels. “For cost Japan, proposed a silicon–germanium– time to use space-division multiplexing to benefit, space-division multiplexing needs silica photonic integration platform for a expand the information-carrying capacity,” much denser optical circuits such as optical monolithic device composed of SiOx-based Feuer urged. By using a single optical fibre circuits on silicon,” Feuer said. arrayed-waveguide-grating wavelength with small multicores or a large multimode Indeed, the fabrication cost per unit filters, silicon-based electrically driven core, one can simultaneously send multiple optical device could be reduced if more modulation devices and germanium-based signals in distinct spatial modes. In his optical devices could be integrated and photodetectors. “The silicon–germanium- talk, Feuer explained how space-division produced from a wafer. Further cost silica photonic integration platform would multiplexing can be implemented by using reduction could be achieved by developing be an important step towards marrying

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