Work by MIT researchers reported in Nature Communications this week suggests a potentially new way to turn electricity into light using the carbon compound graphene. The key is actually ‘slowing down light’.
“In a conceptual breakthrough over 80 years old, Cerenkov showed how charged particles emit shockwaves of light when moving faster than the phase velocity of light in a medium. To modern eyes, the Cerenkov effect offers a direct and ultrafast energy conversion scheme from charge particles to photons. The requirement for relativistic particles, however, makes Cerenkov emission inaccessible to most nanoscale electronic and photonic devices,” write the authors of the Nature Paper (Efficient plasmonic emission by the quantum Cerenkov effect from hot carriers in graphene.)
The new work [shows] that graphene plasmons provide the means to overcome this limitation through their low phase velocity and high field confinement. “The interaction between the charge carriers flowing inside graphene and the plasmons enables a highly efficient two-dimensional Cerenkov emission, giving a versatile, tunable and ultrafast conversion mechanism from electrical signal to plasmonic excitation,” write the authors.
Broadly, this new way of converting electricity into visible radiation is highly controllable, fast, and efficient, the researchers say, and could lead to a wide variety of new applications, according to a report on the work posted on the MIT website. Lead author of the paper is post doc Ido Kaminer, working with MIT physics professors Marin Soljačić and John Joannopoulos and colleagues.
When light strikes a sheet of graphene, a two-dimensional form of the element carbon, it can slow down by a factor of a few hundred. That dramatic slowdown presented an interesting coincidence. “The reduced speed of photons (particles of light) moving through the sheet of graphene happened to be very close to the speed of electrons as they moved through the same material.”
“Graphene has this ability to trap light, in modes we call surface plasmons,” explained Kaminer. Plasmons are a kind of virtual particle that represents the oscillations of electrons on the surface. The speed of these plasmons through the graphene is a few hundred times slower than light in free space.
The whole effect is a little like generating a sonic boom explained Soljačić in the MIT news report, “This conversion is made possible because the electronic speed can approach the light speed in graphene, breaking the ‘light barrier.’” Just as breaking the sound barrier generates a shockwave of sound, he says, “In the case of graphene, this leads to the emission of a shockwave of light, trapped in two dimensions,” said Soljačić
Link to the paper: http://www.nature.com/ncomms/2016/160613/ncomms11880/full/ncomms11880.html
Link to the posted report on MIT’s news web site authored by David Chandler: http://news.mit.edu/2016/new-way-turn-electricity-light-using-graphene-0613
Illustration & Caption Source: MIT News