A team of scientists and engineers report that within a nanometer-scale device, visible light can travel infinitely fast. While this won’t lead to instantaneous communication, since it would violate causality, such a device could be used as an element of optical circuitry.
The scientists published their findings in the journal Physical Review Letters. In space, light always travels at 300,000,000 meters per second. In materials like glass, light is slower and it depends upon the material’s index of refraction.
Scientists have begun to manipulate the interactions of light and matter by tuning the refractive index in strange ways, like making it negative. This leads to an unusual bending of light.
In this study, scientists have developed a nanoscale device in which the index of refraction for visible light is zero, implying that light waves of a particular wavelength travel infinitely fast.
The device is made up of a rectangular bar of insulating silicon dioxide, 85 nanometers thick and 2,000 nanometers long, surrounded by conducing silver, which light can’t generally penetrate. This light-conveying chamber is called a waveguide and researchers have fashioned different devices in which the silicon dioxide ranges from 120 to 400 nanometers.
In such a waveguide, light behaves differently because electromagnetic fields must obey certain boundary conditions on the side of the device. Short wavelengths of light are bounced back and forth between the ends of these devices. The peaks and troughs of the counter-propagating light waves overlap to create a pattern of bright and dark bands that looks like pressure patterns with a ringing organ pipe.
At the cutoff wavelength, things get interesting. Instead of producing a banded pattern, the whole waveguide lights up. That means that instead of acting as waves with phase fronts, the wave behaves as if its peaks are moving infinitely fast and everywhere at once and the light oscillates in synchrony along the length of the waveguide.
Light has two speeds. The phase velocity describes how fast waves of a given wavelength move and the group velocity describes how fast light conveys energy or information. Only the group velocity must stay below the speed of light in a vacuum.