news feature Let there be light A silicon laser would revolutionize telecommunications electronics and computing Squeezing light out of Silicon is no easy task, but Philip Ball discovers that esearchers are becoming more optimisti about its light-emitting abilities information age suffers from a and diligence, they say, the world s favourite negative charge carriers move into thin split personality. Deep beneath the semiconductor can be coaxed into emitting layers within the laser known as quantum ocean's surface, photons of light light. If an all-silicon laser couldbe created, it wells. Here the carriers recombine, releasing stream through optical fibres, carrying would revolutionize the design of supercom- their energy as a photon of light voice and Internet traffic between conti- puters and lead to new types of optoelectror But it is difficult to incorporate III-V nents. But before routing devices, comput- ic devices "says Leigh Canham of biomateri- alloys into silicon circuits. The two do not fit ers and telephones can use the data, this als company pSiMedica, a spin-off from the together because the spacing between the light-borne information must be converted UK Defence Evaluation and Research Agency atoms in the two materials, known as the into electronic signals With entirely optical (DERA). And if recent progress continues, lattice constant, is different. The ideal laser computers unlikely to replace electronics in that revolution may not be far off. diode for optical telecommunications-a he near future, this uneasy marriage of blend of indium, gallium, arsenic and phos- electrons and photons is likely to persist for Communication breakdown phorus, denoted In, Ga -As, PI-y- illus- ome time At the moment, laser diodes are used to turn trates the problem. This material emits light Improving the interface between silicon electronic signals into light pulses. These at a wavelength of about 1.5 micrometres, electronics and photonics is high on the miniature lasers are built from layers of dif- which is the optimum for transmission t Senda in the field of optoelectronics. Some ferent semiconductors, named Ill-V alloys through glass optical fibres, but has a lattice researchers believe that the solution lies in after the columns of the periodic table from constant thatis 8%bigger than that of silicon a/ring silicon's character Given patience which their constituents come. Positive and This means that atoms at the interface between the two the semiconductor “ Dislocations form near the interface and then thread through the Ill-v layer explain Vincent Crespi, a physicist at Pennsylvania State University. Because Glowing future: this layer is thinner Leigh Canham(left) than the silicon sub- and vincent Crespi strate, the light-emit ope to make silicon ting ill-v material st of the deformation, and the resulting defects 974 A@2001 Macmillan Magazines Ltd NaturEvoL40922febRuary2001www.nature.com
WILL & DENI MCINTYRE/SPL DERA PENN STATE UNIV. The information age suffers from a split personality. Deep beneath the ocean’s surface, photons of light stream through optical fibres, carrying voice and Internet traffic between continents. But before routing devices, computers and telephones can use the data, this light-borne information must be converted into electronic signals. With entirely optical computers unlikely to replace electronics in the near future, this uneasy marriage of electrons and photons is likely to persist for some time. Improving the interface between silicon electronics and photonics is high on the agenda in the field of optoelectronics. Some researchers believe that the solution lies in reforming silicon’s character. Given patience negative charge carriers move into thin layers within the laser known as ‘quantum wells’. Here the carriers recombine, releasing their energy as a photon of light. But it is difficult to incorporate III–V alloys into silicon circuits. The two do not fit together because the spacing between the atoms in the two materials, known as the lattice constant, is different. The ideal laser diode for optical telecommunications — a blend of indium, gallium, arsenic and phosphorus, denoted InxGa11xAsyP11y — illustrates the problem. This material emits light at a wavelength of about 1.5 micrometres, which is the optimum for transmission through glass optical fibres, but has a lattice constant that is 8% bigger than that of silicon. This means that atoms at the interface between the two materials do not match up, and linelike distortions form in the semiconductor. “Dislocations form near the interface and then thread through the III–V layer,” explains Vincent Crespi, a physicist at Pennsylvania State University. Because this layer is thinner than the silicon substrate, the light-emitting III–V material incurs most of the deformation, and the resulting defects and diligence, they say, the world’s favourite semiconductor can be coaxed into emitting light.“If an all-silicon laser could be created,it would revolutionize the design of supercomputers and lead to new types of optoelectronic devices,” says Leigh Canham of biomaterials company pSiMedica, a spin-off from the UK Defence Evaluation and Research Agency (DERA). And if recent progress continues, that revolution may not be far off. Communication breakdown At the moment, laser diodes are used to turn electronic signals into light pulses. These miniature lasers are built from layers of different semiconductors, named III–V alloys after the columns of the periodic table from which their constituents come. Positive and 974 |wwwNATURE|VOL 409 | 22 FEBRUARY 2001 |www.nature.com news feature Let there be light A silicon laser would revolutionize telecommunications, electronics and computing. Squeezing light out of silicon is no easy task, but Philip Ball discovers that researchers are becoming more optimistic about its light-emitting abilities. Glowing future: Leigh Canham (left) and Vincent Crespi hope to make silicon optoelectronics a reality. © 2001 Macmillan Magazines Ltd
news feature verely degrade the layer's electrical cor a,Aa result, semiconductor laser diodes Searching for silicon's perfect partner with silicon circuits, they are using a which hinders the constant drive within the computational methods to devise electronics industry to reduce the size of its semiconductors with the desired rcuitry. Separate lasers introduce further problems when chips are connected together, lattice constant and the band gap because achieving and maintaining precise (see main text needed to produce alignment between the chips and the light light of useful wavelengths sources becomes more complicated. It would John Joannopoulos and be far better if light-emitting devices could b colleagues at the Massachusetts integrated directly with silicon chips That would be simple if silicon itself calculated that a specific blend of emitted light. But normal bulk silicon does zinc, silicon, phosphorus and not, for a subtle reason. Semiconductors arsenic should have a lattice emit light when their electrons jump constant only 0.08% smaller than between energy levels. The size of the energ. that of silicon and should emit gap between these states- known as the light at the ideal frequency for band gap-determines the wavelength of telecommunications. The only the photon emitted This modelled blend of carbon, silicon and tin should emit light snag is that it is likely to be Silicon's band gap corresponds to light and match up with silicon's lattice spacing. extremely hard to make. with a wavelength of 1 um, which would be Others have been searching fine for fibre-optic transmission. But silicon As squeezing light out of silicon silicon substrate pass through the for alloys that should be more has what solid-state physicists call an in- has tended to bring the words first organic layer to the erbium- amenable to synthesis. Vincent direct 'band gap, meaning that any electron 'blood and stone to mind, containing layer, where they meet Crespi and his colleagues at moving between the two energy states must physicists are always on the negative charge carriers provided Pennsylvania State University change its momentum.This makes the tran- lookout for new ways to integrate by a top contact of aluminium As have teamed up with Arizona sition less likely to occur. ptical devices with silicon chips. the charge carriers recombine, State University's John is search has led William Gillin they emit infrared radiation. Kouvetakis, a materials scientist Porous potential of Queen Mary College in London But these OLEDs have a long who specializes in synthesizing Despite these problems, researchers trying down an unusual path-he has way to go before they are mplex inorganic compounds. to persuade silicon to glow are currently in gone organic. practical. The voltage needed to Crespi's team"has modelled an upbeat mood. Silicon, they have found, switch the volts-is blends of carbon, tin, germanium Des emit light if chopped down into tiny based) materials sit happily on a massive by industry standards, and silicon that have lattice pieces just a few nanometres wide. silicon surface because they are and their efficiency, just 0.01%, constants within 1% of silicons, The first evidence for this remarkable amorphous, rather than leaves a lot to be desired Gillin is and optimal wavelengths for change in silicon's behaviour emerged in crystalline-there is no lattice optimistic that it will be possible fibre-optic transmission 990from Canhami's lab, then based at the mismatch because there is no to get his devices to work at as ouvetakis is now trying to Defence Research Agency(DERAs earlier crystal lattice. But the trick is to little as 3 volts. They also have make these materials.So far, he incarnation)at Malvern in the English Mid-find such light-emitting the advantage of being relatively has produced alloys of tin and lands. Canham used hydrofluoric acid to substances that are sufficiently easy to make and can emit light germanium that are stable up to dissolve silicon, etching away much of the conducting to function in of different wavelengths if th 400C and in which, crucially, the materialtoleavea poroussubstancemadeup electronic devices. Gillin and his erbium is replaced with different tin atoms seem to occupy the of a network of silicon nanowires". These colleagues have created tiny threads confine mobile electrons to silicon-based organic light- Other physicists are hoping to separates out into clusters in narrowchannels.As thespaceavailable tothe emitting diode(OLED)using a sidestep the problem of lattice these materials, destroying the electrons shrinks, quantum confinement,layer of erbium tris(8 mismatch by rational design. crystal structure in the process. effects come into play. These affect the band hydroxyquinoline) deposited on Rather than conducting trial-and-"The next challenge,"says Crespi, gap, making it much easier for the electrons another organic compound error searches for the right is to get the proper ratios of tin, o move between energy states. Positive charge carriers from a The benefits are impressive. Illuminate silicon nanowires with laser light to create pairs of negative and positive charge carriers yellow light is reasonably straightforward to chips. Before porous silicon came along, and suddenly porous silicon glows with visi- make. After this, things get tricky because the researchers trying to integrate LEDs into ble light. n as photoluminescence, this tiny wires become increasingly fragile. But chips had been hampered by the same prob process is 10,000 times more efficient in the porous silicon that emits green and blue light lems affecting lasers: the incompatibility of nanowires than in normal silicon has been reported Ordinarily, silicon would emit infrared The prospects for silicon-based opto Fauchet's silicon-based LED is a proof of light, but Canham reasoned that the band electronic devices were boosted in 1996, concept, but it is not yet technologically gap is increased by the quantum confine- when Philippe Fauchet of the University of viable. The energy efficiency and switching menteffect. Asa result, the wavelength of the Rochester in New York created a porous-sili- speeds-the rate at which the LED can be emitted light gets shorter as the nanowires con light-emitting diode(LED)integrated turned on and off still need to be get thinner. Because increasing the etching onto a chip. LEDs are not bright enough for improved But progress is being made. time produces narrower wires, the system long-distance optical telecommunications, Nobuyoshi Koshida and his colleagues can be tuned to produce light of different but they can be used for communicatingover the Tokyo University of Agriculture and Tech colours. Silicon that emits red, orange or the short distances between and within nology have greatly increased the efficiency of NaturEvoL40922FeBruAry2001www.nature.com Macmillan Magazines L 975
severely degrade the layer’s electrical conductivity. As a result, semiconductor laser diodes must be kept separate from silicon circuits, which hinders the constant drive within the electronics industry to reduce the size of its circuitry. Separate lasers introduce further problems when chips are connected together, because achieving and maintaining precise alignment between the chips and the light sources becomes more complicated. It would be far better if light-emitting devices could be integrated directly with silicon chips. That would be simple if silicon itself emitted light. But normal ‘bulk’ silicon does not, for a subtle reason. Semiconductors emit light when their electrons jump between energy levels. The size of the energy gap between these states — known as the band gap — determines the wavelength of the photon emitted. Silicon’s band gap corresponds to light with a wavelength of 1 mm, which would be fine for fibre-optic transmission. But silicon has what solid-state physicists call an ‘indirect’ band gap, meaning that any electron moving between the two energy states must change its momentum. This makes the transition less likely to occur. Porous potential Despite these problems, researchers trying to persuade silicon to glow are currently in an upbeat mood. Silicon, they have found, does emit light if chopped down into tiny pieces just a few nanometres wide. The first evidence for this remarkable change in silicon’s behaviour emerged in 1990 from Canham’s lab, then based at the Defence Research Agency (DERA’s earlier incarnation) at Malvern in the English Midlands. Canham used hydrofluoric acid to dissolve silicon, etching away much of the material to leave a porous substance made up of a network of silicon ‘nanowires’1 . These tiny threads confine mobile electrons to narrow channels.As the space available to the electrons shrinks, quantum ‘confinement’ effects come into play. These affect the band gap, making it much easier for the electrons to move between energy states. The benefits are impressive. Illuminate silicon nanowires with laser light to create pairs of negative and positive charge carriers and suddenly porous silicon glows with visible light. Known as photoluminescence, this process is 10,000 times more efficient in the nanowires than in normal silicon. Ordinarily, silicon would emit infrared light, but Canham reasoned that the band gap is increased by the quantum confinement effect.As a result, the wavelength of the emitted light gets shorter as the nanowires get thinner. Because increasing the etching time produces narrower wires, the system can be tuned to produce light of different colours. Silicon that emits red, orange or yellow light is reasonably straightforward to make.After this,things get tricky because the tiny wires become increasingly fragile. But porous silicon that emits green and blue light has been reported2 . The prospects for silicon-based optoelectronic devices were boosted in 1996, when Philippe Fauchet of the University of Rochester in New York created a porous-silicon light-emitting diode (LED) integrated onto a chip3 . LEDs are not bright enough for long-distance optical telecommunications, but they can be used for communicating over the short distances between and within PEIHONG ZHANG/PENN STATE NATURE|VOL 409 | 22 FEBRUARY 2001 |www.nature.com 975 news feature chips. Before porous silicon came along, researchers trying to integrate LEDs into chips had been hampered by the same problems affecting lasers: the incompatibility of materials. Fauchet’s silicon-based LED is a proof of concept, but it is not yet technologically viable. The energy efficiency and switching speeds — the rate at which the LED can be turned on and off — still need to be improved.But progress is being made. Nobuyoshi Koshida and his colleagues at the Tokyo University of Agriculture and Technology have greatly increased the efficiency of As squeezing light out of silicon has tended to bring the words ‘blood’ and ‘stone’ to mind, physicists are always on the lookout for new ways to integrate optical devices with silicon chips. This search has led William Gillin of Queen Mary College in London down an unusual path — he has gone organic. Gillin’s organic (carbonbased) materials sit happily on a silicon surface because they are amorphous, rather than crystalline — there is no lattice mismatch because there is no crystal lattice. But the trick is to find such light-emitting substances that are sufficiently conducting to function in electronic devices. Gillin and his colleagues have created a silicon-based organic lightemitting diode (OLED) using a layer of erbium tris(8- hydroxyquinoline) deposited on another organic compound9 . Positive charge carriers from a silicon substrate pass through the first organic layer to the erbiumcontaining layer, where they meet negative charge carriers provided by a top contact of aluminium. As the charge carriers recombine, they emit infrared radiation. But these OLEDs have a long way to go before they are practical. The voltage needed to switch them on — 33 volts — is massive by industry standards, and their efficiency, just 0.01%, leaves a lot to be desired. Gillin is optimistic that it will be possible to get his devices to work at as little as 3 volts. They also have the advantage of being relatively easy to make and can emit light of different wavelengths if the erbium is replaced with different metals. Other physicists are hoping to sidestep the problem of lattice mismatch by rational design. Rather than conducting trial-anderror searches for the right crystalline material to integrate with silicon circuits, they are using computational methods to devise semiconductors with the desired properties — both in terms of lattice constant and the band gap (see main text) needed to produce light of useful wavelengths. John Joannopoulos and colleagues at the Massachusetts Institute of Technology have calculated that a specific blend of zinc, silicon, phosphorus and arsenic should have a lattice constant only 0.08% smaller than that of silicon10, and should emit light at the ideal frequency for telecommunications. The only snag is that it is likely to be extremely hard to make. Others have been searching for alloys that should be more amenable to synthesis. Vincent Crespi and his colleagues at Pennsylvania State University have teamed up with Arizona State University’s John Kouvetakis, a materials scientist who specializes in synthesizing complex inorganic compounds. Crespi’s team11 has modelled blends of carbon, tin, germanium and silicon that have lattice constants within 1% of silicon’s, and optimal wavelengths for fibre-optic transmission. Kouvetakis is now trying to make these materials. So far, he has produced alloys of tin and germanium that are stable up to 400 7C and in which, crucially, the tin atoms seem to occupy the correct sites. Normally, tin separates out into clusters in these materials, destroying the crystal structure in the process. “The next challenge,” says Crespi, “is to get the proper ratios of tin, silicon and germanium.” Searching for silicon’s perfect partner This modelled blend of carbon, silicon and tin should emit light and match up with silicon’s lattice spacing. ▲ © 2001 Macmillan Magazines Ltd
news feature these LEDs. They used an oxidation reaction Bagneux, part of the CNRS, France's nation to eliminate much of the bulk-like non-con- al research agency. In addition, the device has fined silicon that remains in the porous net to be cooled with liquid nitrogen. I thinkthe work-which otherwise transports most of largest problem lies in achieving room-tem the charge carriers. Koshida's group has now perature operation, Gennser says. QCLs aised theefficiency to about 1%, an improve also emit too narrow a range of frequen ment of five orders of magnitude over the for use in long-distance optical telecommu earliest results. This is fine for display screens, nications ys Canham. But to provide the photonic It remains unclear which approach will signals needed to transmit information emerge as the best contender for lighting between chips will require a further 10-fold up silicon chips. But given the recent acceler increase And switching speeds are still about ation of progress, those intent on unitin the worlds of photonic and electronic infor mation technology can afford to be opti Dot comms mistic. At the very least, silicon-based Using a network of silicon wires is not the ptoelectronics is starting to seem less of an only way to make the element glow. The oxymoron same helpful quantum effects operate if the Shining example: networks of porous silicon Phillp Ball is a consultant editor of Nature. porous silicon is divided into nanometre- nanowires emit light in the visible spectrum. L.Canham, L. T. ApPL. Phys. Left. 57. 1046-1048(1990). sized particles known as nanocrystals or 2. Mizuno, H, Koyama, H. Koshida, N. Appl. Phys. Lett. quantum dots. Last year, Munir Nayfeh of data so that they can assess these claims. the University of Illinois at Urbana-Cham- But there might be another path to devel-3上x如mBLD甲sE&hh paign and his colleagues used ultrasound to oping a silicon-based laser. When Ulf 4. Gelloz, B, Nakagawa, T& Koshida, NAppl Phys. Lett. The smallest of these particles(about 1 Institute in Villigen, Switzerland, he and his 5. Ackakitr, Oet al Appl Phys. Lett. 76, Ix- O, science nanometre across)emit blue light olleagues described a quantum-cascade 287, 1471-1473(2000 Other teams have reported similar suc- laser(QCL) consisting of alternating laye aesi, L-, Dal Negro, L, Mazzoleni, C, Franzo, G. Priolo, E. cesses. In February last year, Brian Korgel of of silicon and a germanium-silicon alloy. &. belting. ct al a(ne 290.27-2780(2001 Texas at Austin and his col- These devices contain severa eague described how to, grow single blocks of light-emitting units, stacked one"出m)影如m nanowires that emit blue light. Unlike on top of the other. Electrons can tunnel' 10 Wang. T, Moll, N, Cho, K. Joannopoulos, J D. Pis. Rev. porous silicon, which consists of a mesh of between individual layers, emitting a photon nanowires, Korgel's nanowires are discrete in the process I1.Zhang, P, Crespi, V H, Chang, E. Louie, S G& Cohen, M.L threads of silicon just 4-5 nm wide Under the right conditions, this should orenzo Pavesi of the University of Tren- lead to stimulatedemission of coherent light, to in Italy has used an alternativeapproach to but the Swiss team has so far managed to pre create silicon nanocrystals. By firing high- duce only electroluminescence, not laser ergy silicon ions into quartz(silicon diox- emission. "There remain many obstacles de), and then heating the material to 1, 100 before we have a working laser,admits C, Pavesi and his colleagues generated sili- Gennser, who is now at the Laboratory of con particles about 3 nm across that were Microstructures and Microelectronics in embedded in the quartz. Last November, these researchers showed that not only do the nanocrystals emit red light when energized with a laser beam, but they can also amplify a probe beam of the same wavelength as the emission. Known as optical gain, this henomenon is one of the fundamental features flaser emission Although the Italian team has taken the first steps towards creating a silicon laser, light amplification is not the same as laser action. In a laser beam the light is coherent: all the photons are in phase. To achieve this, emitted photons must stimulate the emis- sion of others- the stimulated photons merge in step with those that induced them. This stimulated emission is achieved by acing the emitting material in an optical cavity bounded by mirrors which let phe tons bounce back and forth Tantalizingly, at the Materials Research Society meeting in Boston last December, Nayfeh reported optical gain and stimulated emission from his blue-light-emitting nanocrystals. Others in the field are now aiting for Nayfeh to publish quantitativ Light speed: single silicon nanowires(inset ) might supply photonic signals to fibre-optic cables. 976 Macmillan Magazines L NatuRevOl40922february2001www.nature.com
these LEDs. They used an oxidation reaction to eliminate much of the bulk-like ‘non-confined’ silicon that remains in the porous network — which otherwise transports most of the charge carriers4 . Koshida’s group has now raised the efficiency to about 1%,an improvement of five orders of magnitude over the earliest results. This is fine for display screens, says Canham. But to provide the photonic signals needed to transmit information between chips will require a further 10-fold increase. And switching speeds are still about two orders of magnitude too slow. Dot comms Using a network of silicon wires is not the only way to make the element glow. The same helpful quantum effects operate if the porous silicon is divided into nanometresized particles known as nanocrystals or ‘quantum dots’. Last year, Munir Nayfeh of the University of Illinois at Urbana-Champaign and his colleagues used ultrasound to shatter porous silicon into nanocrystals. The smallest of these particles (about 1 nanometre across) emit blue light5 . Other teams have reported similar successes. In February last year, Brian Korgel of the University of Texas at Austin and his colleagues described how to grow single nanowires that emit blue light6 . Unlike porous silicon, which consists of a mesh of nanowires, Korgel’s nanowires are discrete threads of silicon just 4–5 nm wide. Lorenzo Pavesi of the University of Trento in Italy has used an alternative approach to create silicon nanocrystals. By firing highenergy silicon ions into quartz (silicon dioxide), and then heating the material to 1,100 7C, Pavesi and his colleagues generated silicon particles about 3 nm across that were embedded in the quartz. Last November, these researchers showed that not only do the nanocrystals emit red light when energized with a laser beam, but they can also amplify a ‘probe’ beam of the same wavelength as the emission7 . Known as optical gain, this phenomenon is one of the fundamental features of laser emission. Although the Italian team has taken the first steps towards creating a silicon laser, light amplification is not the same as laser action. In a laser beam the light is coherent: all the photons are in phase. To achieve this, emitted photons must stimulate the emission of others — the stimulated photons emerge in step with those that induced them. This ‘stimulated emission’ is achieved by placing the emitting material in an optical cavity bounded by mirrors which let photons bounce back and forth. Tantalizingly, at the Materials Research Society meeting in Boston last December, Nayfeh reported optical gain and stimulated emission from his blue-light-emitting nanocrystals. Others in the field are now waiting for Nayfeh to publish quantitative data so that they can assess these claims. But there might be another path to developing a silicon-based laser. When Ulf Gennser was working at the Paul Scherrer Institute in Villigen, Switzerland, he and his colleagues described a quantum-cascade laser (QCL) consisting of alternating layers of silicon and a germanium–silicon alloy8 . These devices contain several five-layer blocks of light-emitting units, stacked one on top of the other. Electrons can ‘tunnel’ between individual layers,emitting a photon in the process. Under the right conditions, this should lead to stimulated emission of coherent light, but the Swiss team has so far managed to produce only electroluminescence, not laser emission. “There remain many obstacles before we have a working laser,” admits Gennser, who is now at the Laboratory of Microstructures and Microelectronics in Bagneux, part of the CNRS, France’s national research agency.In addition,the device has to be cooled with liquid nitrogen.“I think the largest problem lies in achieving room-temperature operation,” Gennser says. QCLs also emit too narrow a range of frequencies for use in long-distance optical telecommunications. It remains unclear which approach will emerge as the best contender for lighting up silicon chips. But given the recent acceleration of progress, those intent on uniting the worlds of photonic and electronic information technology can afford to be optimistic. At the very least, silicon-based optoelectronics is starting to seem less of an oxymoron. ■ Philip Ball is a consultant editor of Nature. 1. Canham, L. T. Appl. Phys. Lett. 57, 1046–1048 (1990). 2. Mizuno, H., Koyama, H. & Koshida, N. Appl. Phys. Lett. 69, 3779–3781 (1996). 3. Hirschman, K. D., Tsybekov, L., Duttagupta, S. P. & Fauchet, P. M. Nature 384, 338–341 (1996). 4. Gelloz, B., Nakagawa, T. & Koshida, N. Appl. Phys. Lett. 73, 2021–2023 (1998). 5. Ackakir, O. et al. Appl. Phys. Lett. 76, 1857–1859 (2000). 6. Holmes, J. D., Johnston, K. P., Doty, R. C. & Korgel, B. A. Science 287, 1471–1473 (2000). 7. Pavesi, L., Dal Negro, L., Mazzoleni, C., Franzò, G. & Priolo, F. Nature 408, 440–444 (2000). 8. Dehlinger, G. et al. Science 290, 2277–2280 (2000). 9. Curry, R. J., Gillin, W. P., Knights, A. P. & Gwilliam, R. Appl. Phys. Lett. 77, 2271–2273 (2000). 10.Wang, T., Moll, N., Cho, K. & Joannopoulos, J. D. Phys. Rev. Lett. 82, 3304–3307 (1999). 11.Zhang, P., Crespi, V. H., Chang, E., Louie, S. G. & Cohen, M. L. Nature 409, 69–71 (2001). news feature 976 NATURE|VOL 409 | 22 FEBRUARY 2001 |www.nature.com Shining example: networks of porous silicon nanowires emit light in the visible spectrum. Light speed: single silicon nanowires (inset) might supply photonic signals to fibre-optic cables. ▲ B. KORGEL DERA ALFRED PASIEKA/SPL © 2001 Macmillan Magazines Ltd
