Physicists Suggest Ideal Material for Lasers

Weyl semimetals are a newly discovered group of materials, wherein charge carriers act in the same way as electrons and positrons do in particle accelerators. Scientists from the Moscow Institute of Physics and Technology and Ioffe Institute in St. Petersburg have demonstrated that these materials signify impeccable gain media for lasers. The research findings have been published in Physical Review B.

The 21^st-century physics is denoted by the search for phenomena from the realm of fundamental particles in tabletop materials. In certain crystals, electrons travel as high-energy particles in accelerators. In others, particles even possess properties slightly akin to black hole matter.

MIPT physicists have spun this hunt inside-out, showing that reactions prohibited for elementary particles can also be prohibited in the crystalline materials called Weyl semimetals. Specifically, this applies to the prohibited reaction of mutual particle-antiparticle annihilation without light release. This property recommends that a Weyl semimetal could be the impeccable gain medium for lasers.

In a semiconductor laser, radiation results from the mutual obliteration of electrons and the positive charge carriers called holes. However, light emission is simply one likely outcome of an electron-hole pair collision. Alternatively, the energy can increase the oscillations of atoms nearby or heat the adjacent electrons. The latter process is referred to as Auger recombination, in recognition of the French physicist Pierre Auger.

Auger recombination restricts the efficiency of contemporary lasers in the infrared and visible range, and relentlessly undermines terahertz lasers. It consumes electron-hole pairs that might have otherwise formed radiation. Furthermore, this process heats up the device.

For virtually a century, scientists have pursued a “wonder material” in which radiative recombination rules over Auger recombination. This search was guided by an idea put forth by Paul Dirac in 1928. He came up with a theory that the electron, which had already been found, 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 at all times creates light and cannot convey energy on other electrons. This is why the hunt for a wonder material to be used in lasers was mainly seen as a search for analogs 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. 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.

Dmitry Svintsov, Head of Laboratory of 2D Materials for Optoelectronics, MIPT

“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 chief investigator in the reported research.

Electrons and holes in a semiconductor do possess the same electric charges as Dirac’s particles. But it requires more than that to remove Auger recombination. Laser engineers aim to find the kind of particles that would match Dirac’s theory with regards to their dispersion relations. The latter links particle’s kinetic energy to its momentum. That equation encrypts all the information on particle’s motion and the reactions it can experience.

In classical mechanics, objects such as planets, rocks, or spaceships are based on a quadratic dispersion equation. That is, replication the momentum results in four-fold increase in kinetic energy. In conventional semiconductors — germanium, silicon, or gallium arsenide — the dispersion relation is also quadratic. The dispersion relation for photons, the quanta of light, is linear. One of the concerns is that a photon at all times travels at precisely the speed of light.

The positrons and electrons in Dirac’s theory inhabit a middle ground between photons and rocks: at low energies, their dispersion relation is quadratic, but at higher energies, it turns linear. Until lately, though, it took a particle accelerator to “catapult” an electron into the linear area of the dispersion relation.

Some recently discovered materials can act as “pocket accelerators” for charged particles. Among them are the “pencil-tip accelerator” — graphene and its 3D analogs, called Weyl semimetals: niobium phosphate, tantalum arsenide, and molybdenum telluride. In these materials, electrons follow a linear dispersion relation beginning from the lowest energies. That is, the charge carriers act like electrically charged photons. These particles may be seen as similar to the Dirac electron and positron, except that their mass is close to zero.

The scientists have demonstrated that in spite of the zero mass, Auger recombination still remains forbidden in Weyl semimetals. Predicting the objection that a dispersion relation in an actual crystal is on no occasion strictly linear, the team proceeded in calculating the probability of “residual” Auger recombination because of deviations from the linear law. This probability, which relies on electron concentration, can attain values some 10,000 times lower than in the currently used semiconductors. Simply put, the calculations propose that Dirac’s concept is rather devotedly reproduced in Weyl semimetals.

We were aware of the bitter experience of our predecessors who hoped to reproduce Dirac’s dispersion relation in real crystals to the letter. That is why we did our best to identify every possible loophole for potential Auger recombination in Weyl semimetals. For example, in an actual Weyl semimetal, there exist several sorts of electrons, slow and fast ones. While a slower electron and a slower hole may collapse, the faster ones can pick up energy. That said, we calculated that the odds of that happening are low.

Dmitry Svintsov, Head of Laboratory of 2D Materials for Optoelectronics, MIPT

The researchers evaluated the lifetime of an electron-hole pair in a Weyl semimetal to be approximately 10 nanoseconds. That timespan seems very small by ordinary standards, but for laser physics, it is massive. In conventional materials used in laser technology of the far infrared range, the lifetimes of holes and electrons are thousands of times shorter. Prolonging the lifetime of non-equilibrium electrons and holes in new materials paves the way for them to be used in new varieties of long-wavelength lasers.