16/04/2018

FAPESP magazine highlights study by IMPA and Technion.

Reproduction from FAPESP Research Magazine

Faster than anything else moving in the Universe, light can be completely stopped in its journey through space. This is the result obtained by a theoretical method proposed by three researchers, one of whom is based in Brazil. Using numerical simulations, they claim that it would be possible to stop pulses of light as long as their trajectory is confined by waveguides, physical structures that conduct light (optical fibers or channels), arranged in a way that creates singularities. This mathematical concept refers to exceptional points in a system (in this case, light passing through waveguides) where unusual, undefined, or peculiar properties emerge. When passing through these points, the speed of a pulse of light would be equal to zero, according to calculations by mathematician Alexei Mailybaev, from the Institute of Pure and Applied Mathematics (Impa) in Rio de Janeiro, and physicists Nimrod Moiseyev, from the Israel Institute of Technology (Technion), and Tamar Goldzak, who is doing postdoctoral research at the Massachusetts Institute of Technology (MIT) in the United States.

The new method presents a key difference compared to other approaches pursuing the same objective: the light pulses would decelerate completely without losing their original intensity, according to the article published by the trio of researchers on January 3rd in the scientific journal Physical Review Letters . The light weakens before being completely stopped, a limitation not currently overcome by other techniques. At exceptional points, the various waves that constitute the light pulse would behave as if they were one, an indispensable trick to ensure that, at the same time, the beam stops and maintains its intensity. However, Mailybaev points out that there are technical limitations to putting the idea into practice. “It would be difficult to record whether the light actually stopped,” explains the Russian mathematician, now a Brazilian citizen, who has collaborated with Nimrod Moiseyev's group for eight years. “It's complicated to record where the signal is at each moment within the waveguide and thus calculate the change in speed. But these technical difficulties may be overcome.”

Light results from vibrations of electric and magnetic fields. Physicists mathematically represent, through equations, the properties of these waves, such as frequency, amplitude, energy, and propagation speed. Mailybaev recounts that the idea of working with the question of stopping light arose while the three were discussing physical phenomena that emerge from singularities in mathematical calculations. "Out of curiosity, we considered what would happen to light in these unusual situations," recalls the IMPA researcher. They did the calculations and saw that, when passing through the so-called exceptional points, the speed of the light pulse would be equal to zero. From then on, they began to investigate ways to create exceptional points in structures that direct light – waveguides – and formulated a proposal. If two waveguides are placed close to each other and their configurations are adjusted so that the pulse intensity increases in one while decreasing in the other, these exceptional points would appear – and the light beam would stop moving in these regions. This is because one waveguide dissipates energy at exactly the same rate that the other gains it. "The advantage of our proposal is that it encompasses a large number of parameters within a structure that we can modify," comments Tamar Goldzak.

Slower light
In a vacuum, light travels at a constant speed, reaching its maximum value of approximately 300,000 kilometers per second (km/s). However, when propagating through other mediums, such as air or water, it naturally slows down. The formation of a phenomenon like a rainbow, for example, would not occur if the speed of light in water (approximately 225,000 km/s) and in air (where it moves slightly slower than in a vacuum) were the same. In the last two decades, physicists have been trying to tame light and have obtained surprising results. In 1999, the group of Danish mathematician and physicist Lene Hau, from Harvard University, United States, experimentally reduced the speed of light to 17 meters per second by controlling a laser pulse within an ultracold gas of sodium atoms, a state of matter known as a Bose-Einstein condensate. In 2001, the team went a step further and stopped light for 1 second within a similar system.

Hau's method has since made it possible to turn light upside down; to slow it down, speed it up, or store it. But, until it reaches zero speed, the light signal is extinguished, its intensity is lost, and its shape is almost entirely imprinted on the structure of the atoms; a kind of fingerprint of light. "Slowing down light in ultracold gases is great for fundamental research, but it's unlikely to generate applications," assesses physicist Thomas Krauss of the University of York, UK. Mailybaev, Moiseyev, and Goldzak, however, say that their proposal would have greater applied potential because the exceptional points could be used to control the propagation of any type of wave (light, sound, and others) regardless of the medium in which they move. Even waves in water could be controlled by this method, according to the researchers. "Slower light interacts more with matter," points out Emiliano Martins, a specialist in guided waves at the São Carlos School of Engineering of the University of São Paulo (EESC-USP). "This characteristic is indispensable for the development of telecommunications and optical data processing."