IMAGE: Light emission resulting from a mutual annihilation of electrons and holes is the operating principle of semiconductor lasers. (Image credit: Elena Khavina/MIPT) 

Weyl semimetals are a recently discovered class of materials, in which charge carriers behave the way electrons and positrons do in particle accelerators. Researchers from the Moscow Institute of Physics and Technology and Ioffe Institute in St. Petersburg, Russia have shown that these materials represent perfect gain media for lasers. The research findings were published in Physical Review B. 

The 21st-century physics is marked by the search for phenomena from the world of fundamental particles in tabletop materials. In some crystals, electrons move as high-energy particles in accelerators. In others, particles even have properties somewhat similar to black hole matter. 

MIPT physicists have turned this search inside-out, proving that reactions forbidden for elementary particles can also be forbidden in the crystalline materials known as Weyl semimetals. Specifically, this applies to the forbidden reaction of mutual particle-antiparticle annihilation without light emission. This property suggests that a Weyl semimetal could be the perfect gain medium for lasers. 

In a semiconductor laser, radiation results from the mutual annihilation of electrons and the positive charge carriers called holes. However, light emission is just one possible outcome of an electron-hole pair collision. Alternatively, the energy can build up the oscillations of atoms nearby or heat the neighboring electrons. The latter process is called Auger recombination, in honor of the French physicist Pierre Auger (pronounced oh-ZHAY’). 

Auger recombination limits the efficiency of modern lasers in the visible and infrared range, and severely undermines terahertz lasers. It eats up electron-hole pairs that might have otherwise produced radiation. Moreover, this process heats up the device. 

For almost a century, researchers have sought a "wonder material" in which radiative recombination dominates over Auger recombination. This search was guided by an idea formulated in 1928 by Paul Dirac. He developed a theory that the electron, which had already been discovered, had a positively charged twin particle, the positron. Four years later, the prediction was proved experimentally. In Dirac’s calculations, a mutual annihilation of an electron and positron always produces light and cannot impart energy on other electrons. This is why the quest for a wonder material to be used in lasers was largely seen as a search for analogues of the Dirac electron and positron in semiconductors. 

"In the 1970s, the hopes were largely associated with lead salts, and in the 2000s - with graphene," says Dmitry Svintsov, the head of the ？Laboratory of 2D Materials for Optoelectronics at MIPT. "But the particles in these materials exhibited deviations from Dirac's concept. The graphene case proved quite pathological, because confining electrons and holes to two dimensions actually gives rise to Auger recombination. In the 2D world, there is little space for particles to avoid collisions." 

"Our latest paper shows that Weyl semimetals are the closest we’ve gotten to realizing an analogy with Dirac's electrons and positrons," added Svintsov, who was the principal investigator in the reported study. 

See SOURCE below for details of the findings. 

The team gauged the lifetime of an electron-hole pair in a Weyl semimetal to be about 10 nanoseconds. That timespan looks extremely small by everyday standards, but for laser physics, it is huge. In conventional materials used in laser technology of the far infrared range, the lifetimes of electrons and holes are thousands of times shorter. Extending the lifetime of nonequilibrium electrons and holes in novel materials opens up prospects for using them in new types of long-wavelength lasers. 

(SOURCE: MIPT; https://mipt.ru/english/news/physicists_propose_perfect_material_for_lasers)