Within 10 years, computers, tablets, and cellphones will undergo an invisible transformation, driven by light. While these devices will not look all that different, they will run faster and consume less electric energy thanks to a new set of technologies that manipulate light on a microscopic scale. Novel technologies have already afforded the production of silicon nanophotonic chips, which, like conventional silicon chips, are composed of microelectronic parts. The key difference, however, is that the latest chips do not rely on integrated metal circuits to transmit electric signals but communicate instead through light signals—more specifically, lasers. Light offers an advantageous edge over electric signals because it can carry more data at a faster speed. Information exchange in nanophotonic chips is expected to occur without hardly any conversion of electric energy into heat.
Multinational electronics companies have already made nanophotonic chips part of their research programs and have developed prototypes. When the chips are ready to go to market, the first beneficiaries are likely to be the supercomputers located at the world’s main data centers. “There are some basic physics and engineering issues that still need solving,” says Gustavo Wiederhecker, a physicist at the University of Campinas (Unicamp) who studies how light interacts with nanometric materials. “But production costs will eventually drop and nanophotonics can become part of our everyday lives.”
“While recent strides in nanophotonics have been amazing, none of this is as revolutionary as the laser,” explains Paulo Nussenzveig, University of São Paulo (USP) physicist and quantum optics expert. Nussenzveig has been collaborating since 2012 with a team that explores quantum light phenomena in nanophotonic chips, led by physicist Michal Lipson at Columbia University. In 2014, the group published a paper in the journal Nature Photonics showing how a quantum magnetic field effect can be used to guide light along a microscopic channel in a silicon chip. “The laser was the paradigm shift that laid the groundwork for developing all the technologies that followed,” says Nussenzveig, who addressed recent progress in his field at the workshop “Light: Life & Science,” held in July 2015 in São Carlos, a city in the interior of São Paulo State. The event commemorated the International Year of Light, a United Nations initiative to raise public awareness of the importance of photonics, science, and light-control technology, which paved the way for the invention of both the laser and the fiber optics that link computers around the world today (see timeline).
Concentrated light
The invention of the laser would not have been possible if the historical debate over the true physical nature of light had not been settled in the early 20th century. It was only after quantum mechanics came on the scene that scientists could fully understand new phenomena involving matter (like atoms and electrons) and light. According to quantum theory, a beam of light is made up of trillions of photons, the elementary particles that display uncertain behavior, sometimes acting like waves and other times like particles.
In 1916, Albert Einstein gauged that if a photon is present in the vicinity of an “excited” atom—that is, of an atom about to emit a photon—it would stimulate the atom to emit an identical photon under suitable conditions. In the 1950s, a number of researchers attempted to use this effect to create the laser, an acronym for Light Amplification by Stimulated Emission of Radiation. In 1960, U.S. engineer Theodore Maiman succeeded in making the first working laser.
The power of the laser lies in the synchrony of its photons. While the atoms found in natural and artificial light sources emit photons at different times, frequencies, and directions, the atoms in a laser generator emit photons in synchrony, that is, at the same frequency and in the same direction. When a laser beam is used to weld or cut sheet metal, for example, it releases photons with the same power as an ordinary 100-watt light bulb. The difference is that the laser beam concentrates all of this power into a very small area.
The laser also transformed basic research and allowed physicists to explore nonlinear optics (NLO), the specialty of Cleber Mendonça, of USP São Carlos. “This field deals with optical phenomena that are only observed at very high light intensities,” the physicist explains.
When a very intense laser beam is focused on a specific spot in a material, the laser transforms the optical properties of the material in the vicinity of the focal point, like its ability to reflect or refract light; at the same time, the laser light is transformed by the material, for example, altering its oscillation frequencies.
Mendonça and his research team at USP are investigating these nonlinear effects in order to fabricate micrometric and nanometric structures of organic glass and polymers, which they are trying to make compatible with silicon. At Unicamp, a group led by physicist Paulo Dainese is studying how nonlinear effects can enhance the manipulation of data coded in the light pulses conducted by optical fibers.
Optical fibers are flexible threads composed of very uniform transparent glass that can transmit light signals over long distances with nearly no energy loss. Over one billion kilometers of optical fibers now connect computers around the world. This would be impossible to do with standard electric cables, which propagate electric signals when electrons move down a wire. The vibration of the electrons causes a substantial loss of energy, generally converted into heat; in comparison, photons lose hardly any energy at all during this process.
Dainese explains that a single optical fiber can simultaneously transmit multiple messages coded in the form of light signals, thanks to optical components called multiplexers, which combine light beams at different frequencies; each light frequency transmits one message. “Sometimes in telecommunications, a message transmitted at one frequency has to be switched to another,” says Dainese. “Today this is done by converting the signal coded at one light frequency into an electric signal and then retransmitting it at another frequency, but we’re studying ways of using nonlinear effects to eliminate this electrical stage, which is slow and costly.”
Just as the cost of transmitting data over long distances was drastically reduced when fiber optics replaced electric cables, the time has come to do the same with computer chips. “In the recent past, microchips were very compact and had only one processing core, hundreds of micrometers in size,” explains Wiederhecker. “This changed over the past 15 years with the appearance of multicore processors that work in parallel.” In the case of the new processors, a computational task is broken down into subtasks that can be processed simultaneously by different cores.
To keep parallel processors synchronized, the cores need to communicate with each other. This is currently accomplished by electric signals transmitted along a wire. “Communication through light signals would solve the issues of speed and heat dissipation,” says Wiederhecker. “But to do this, we need to re-scale multiplexers, routers, filters, and other components of fiber optic networks down to a few hundred nanometers.”
Wiederhecker and his colleagues are now working to create nanomechanical oscillators that are driven and synchronized by light. “There is a quartz crystal inside computers that resonates when coupled to an electronic circuit,” he explains. “The oscillator works like a metronome, synchronizing the operations of computer components, like the processor, memory, and video card. We want to build a nanometric structure that will vibrate when it receives a light signal.”
Smaller all the time
Wiederhecker and other researchers calculate that the microscopic integration of electronics and laser technology will permit the miniaturization of equipment that performs medical exams and does chemical analyses utilizing light. Most of these devices are now found only at laboratories, but the use of photonic chips in combination with other technologies may allow for the development of cheaper, portable equipment that can be taken anywhere.
“There are still some roadblocks to making this technology a reality, but they are quickly being surmounted,” observes Vilson R. Almeida, a researcher currently investigating the application of photonics in biological and aerospatial sensing at the Technological Institute of Aeronautics (ITA) and Institute of Advanced Studies (IEAv), both part of the Brazilian Air Force’s Aerospace Science and Technology Department. Almeida was on an international team that developed a device made of nanometric structures on a silicon chip that can transmit light in one direction only. The study was featured on the cover of the journal Nature Materials in 2013.
EDUARDO CESARBright paths: Omar Florez Penaloza sets up an experiment at Unicamp’s Fiber Optics Laboratory to measure the scattering of lightEDUARDO CESAR
One of the roadblocks, Almeida explains, is reliance on silicon as the basis for commercial-grade electronic and photonic chips. Although silicon is a good light transmitter, it does not generate or detect light efficiently. “Solutions have already been shown to exist, like using hybrid materials, which are being perfected and should be on the market within three years,” he predicts.
One of the most promising nanomaterials to be integrated into silicon nanophotonic chips are quantum dots, the specialty of Unicamp physicist Lázaro Padilha. Quantum dots are tiny particles less than 10 nanometers in diameter made from a variety of semiconductor materials. By adjusting the size and properties of the material employed in their production, quantum dots can convert electricity into light and serve as powerful microscopic LED lights. The electronics industry recently launched ultra-high definition flat screen monitors made of quantum dots. Padilha was one of the authors of a paper published in Nature Communications in 2013, where researchers showed how to boost the efficiency at which quantum dots convert electricity to light from 0.2% to roughly 8%.
If other adjustments are made, quantum dots can also reverse the process, transforming light into electric energy and thus acting as tiny solar panels. “I always tell my students that solar cells and LEDs are the same thing, just upside down,” says Padilha. “Twenty or thirty years from now,” he forecasts, “the roofs and windows of our houses, the hoods of our cars, everything will be covered with a layer of material that behaves like a microscopic high-efficiency solar panel, converting sunlight into electric energy.”
Projects
1. Femtosecond pulses applied to nonlinear optics: spectroscopy, pulse shaping and microfabrication (nº 2011/12399-0); Grant Mechanism Thematic Project; Principal Investigator Cleber Renato Mendonça (IFSC-USP); Investment R$ 1,181,820.00 (FAPESP).
2. Quantum optics and quantum information on silicon chips (nº 2011/12140-6); Grant Mechanism Scholarship Abroad – Research; Principal Investigator Paulo Alberto Nussenzveig (IF-USP); Investment R$ 105,116.00 (FAPESP).
3. Advanced spectroscopy of novel nanomaterials (nº 2013/16911-2); Grant Mechanism Young Investigators Award; Principal Investigator Lázaro Aurélio Padilha Junior; Investment R$ 2,658,400.00 (FAPESP).
4. Nanophotonics in Group IV and III-V semiconductors (nº 2012/17765-7); Grant Mechanism Young Investigators Award; Principal Investigator Gustavo Silva Wiederhecker (Unicamp); Investment R$ 1,113,640.00 (FAPESP).
5. Light-scattering processes in photonic microstructures (nº 2013/20180-3); Grant Mechanism Young Investigators Award; Principal Investigator Paulo Clóvis Dainese Júnior (Unicamp); Investment R$ 1,219,080.00 (FAPESP).
Scientific articles
LAWRENCE, D. et al. Non-reciprocal phase shift induced by an effective magnetic flux for light. Nature Photonics. Aug. 3, 2014.
FENG, L. et al. Experimental demonstration of a unidirectional reflectionless parity-time metamaterial at optical frequencies. Nature Materials. Nov. 25, 2012.
BAE, W. K. et al. Controlling the influence of Auger recombination on the performance of quantum-dot light-emitting diodes. Nature Communications. Oct. 25, 2013.
