[Image above] Crystal structure of tantalum arsenide (Ta is purple; As is green). Researchers from the Moscow Institute of Physics and Technology propose Weyl semimetals such as this one could be the perfect material for terahertz-range lasers. Credit: Weng et al., Physical Review X (CC BY 3.0)
Imagine a world where you can read a book without once opening its pages. And no, I’m not talking about sleeping with a book under your pillow.
Terahertz-range lasers produce a type of non-ionizing radiation that can penetrate materials like fabric, plastic, and polymers, but the radiation is strongly absorbed by water. These characteristics make lasers that operate in the terahertz range ideal for biomedical imaging and capable of producing high-resolution images of a book’s individual pages while it is closed. However, there is a major obstacle to creating terahertz-range lasers: Auger recombination.
In regular band-to-band (radiative) recombination, an electron in a higher electron shell drops down into a hole in a lower shell (the electron and hole “recombine”) and energy is released in the form of a photon.
In Auger recombination (AR), there is no photon released; it is a nonradiative process. Instead, energy is transferred between two electrons when they collide, causing one electron to drop down into a lower-shell hole and the other electron to jump up to a higher shell.
AR presents an obstacle to achieving terahertz-range lasers because AR uses up electron-hole pairs that would otherwise undergo radiative recombination, meaning there are fewer photons released and the laser has lower light generation efficiency. As such, scientists are searching for ways to suppress AR so they can realize terahertz-range devices.
Most methods for AR suppression have focused on deliberate material manipulations, including strain engineering, modification of wave-function profiles, and exchange effects upon scattering. However, scientists have hoped to find a material that offers natural AR suppression.
“In the 1970s, the hopes were largely associated with lead salts, and in the 2000s – with graphene,” says Dmitry Svintsov, head of the Laboratory of 2D Materials for Optoelectronics at Moscow Institute of Physics and Technology (MIPT), in a MIPT press release. Graphene especially seemed promising at first, as the initial theoretical concept said AR should be prohibited by energy and momentum conservation laws. Unfortunately, MIPT and Tohoku University researchers discovered “prohibited” AR actually does take place.
Even though graphene did not live up to expectations, researchers have faith that another recently discovered material will.
“Weyl semimetals were identified in 2015 in a series of papers published in Nature and Science. These researchers identified TaAs [tantalum arsenide] as a first Weyl semimetal, and the family of such materials is continuously growing,” Svintsov explains in an email.
Weyl semimetals are 3D materials in which electrons behave like an elusive Weyl fermion. Weyl fermions are theorized massless charged quasiparticles with half-integral spin that follow a linear dispersion equation rather than a quadratic one (i.e., doubling of momentum results in a two-fold, rather than four-fold, increase in kinetic energy).
In both graphene and Weyl semimetals, electrons act like Weyl fermions, i.e., as effectively massless particles. This “zero mass” situation allows the electrons to move faster and release more energy, which is helpful for creating powerful terahertz-range lasers. However, these massless electrons run into a problem when confined to 2D space.
“If [a particle’s] mass was non-zero, the AR would be forbidden by energy-momentum conservation laws,” Svintsov says. However, “If the mass is zero, there is a tiny possibility to fulfill the conservation laws for AR: this might happen if colliding electron and hole move strictly face-to-face.”
In the case of graphene, a 2D material, the possibility of colliding strictly face-to-face is quite high. But in 3D space, as in the case of Weyl semimetals, there are plenty of other possibilities to collide at an angle rather than face-to-face. So “the probability of face-to-face collisions is infinitely small, which results in zero AR,” Svintsov explains.
To show that possibility of AR in Weyl semimetals is infinitely small despite the electrons moving as effectively massless particles, Svintsov and his colleagues Aleksandr Afanasiev (PhD student, Ioffe Institute) and Andrey Greshnov (professor, Ioffe Institute) published a recent paper describing multiple experiments they ran to identify all possible loopholes for AR to occur in the Weyl semimetal tantalum arsenide. As Svintsov notes in the press release, “We calculated that the odds of [AR] happening are low.”
In an email, Svintsov says they are currently investigating the possibility of plasmon lasing in Weyl semimetals with population inversion. “This is lasing not with usual electromagnetic waves, but with their ‘compact’ counterparts which are collective oscillations of electrons and electromagnetic field,” he explains. Additionally, they are also planning an experiment with colleagues to study recombination channels in Weyl semimetals.
The paper, published in Physical Review B, is “Relativistic suppression of Auger recombination in Weyl semimetals” (DOI: 10.1103/PhysRevB.99.115202).