PORTLAND, Ore. — Graphene could make the illusive terahertz laser possible, according to a recent report from the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg. Sources of terahertz radiation, such as that used by those security scanners that look underneath your clothing at airport security checkpoints, are expensive and bulky today, and no source exists for a terahertz laser. Now, Max Planck Institute claims to have demonstrated that graphene could be used to produce terahertz lasers, which could be useful in probing high-temperature superconductors.
Artist's rendering of a graphene monolayer being
excited by an external light source.
(Source: Max Planck Institute)
Graphene -- comprising atomically thin sheets of pure carbon -- is already being developed as the successor to silicon, after the end of the International Technology Roadmap for Semiconductors (ITRS) circa 2023. However, graphene was thought to be unsuitable for lasers, since it does not have a bandgap. A bandgap was previously thought to be a necessary condition for population inversion in lasers, where most of a material's electrons are boosted from their ground state (valence band) across the bandgap to their exited state (conduction band) after which they decay back to the ground state, emitting an avalanche of photons in the process.
Despite its lack of a bandgap, Max Planck researchers were nevertheless able to demonstrate population inversion in graphene at terahertz frequencies after exciting it with an infrared laser, resulting in short bursts that theoretically make it suitable for the stimulated emission of a pulsed terahertz laser.
"We use a first laser pulse to excite electrons from the valence into the conduction band and use photoemission to probe how long the electrons stay in the conduction band before they decay back to their ground state in the valence band," Max Planck researcher Isabella Gierz told EE Times.
The result was terahertz wavelength emissions about 100 femtoseconds in length, suitable for a fast pulsed laser. Many applications could benefit from a terahertz laser, including research into high-temperature superconductors, according to Gierz who performed the work with colleagues at the Central Laser Facility in Harwell, England, and the Max Planck Institute for Solid State Research in Stuttgart, Germany.
"High-temperature superconductors -- but also other materials -- exhibit numerous excitations in the terahertz range," said Gierz. "Aside from fundamental research, terahertz light sources have numerous other applications."
So far, Gierz and her research group at Max Planck have surmounted two of the three hurdles to producing a graphene-based terahertz laser, with the last element on their to-do list.
"A laser consists of three parts: an active medium for light amplification, a pump source that supplies the power, and a laser cavity," Gierz told us. "We have demonstrated the first and second. The cavity, however, needs to be designed."
Gierz and colleagues also tried to show that graphene could be used in a manner opposite to a laser, that is, for light harvesting in solar cells. What they found was that one graphene electron can potentially release multiple photons for stimulated emission in a laser, but the opposite process -- one photon releasing multiple electrons as is necessary for solar cells -- was not observed, leading the group to believe that graphene will not work for photovoltaics.
— R. Colin Johnson, Advanced Technology Editor, EE Times