Physicists Propose Perfect Material for Lasers
May 9, 2019 | Moscow Institute of Physics and TechnologyEstimated reading time: 4 minutes
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 have shown that these materials represent perfect gain media for lasers.
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 can not 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.
Electrons and holes in a semiconductor do have the same electric charges as Dirac's particles. But it takes more than that to eliminate Auger recombination. Laser engineers seek the kind of particles that would match Dirac's theory in terms of their dispersion relations. The latter tie particle's kinetic energy to its momentum. That equation encodes all the information on particle's motion and the reactions it can undergo.
In classical mechanics, objects such as rocks, planets, or spaceships follow a quadratic dispersion equation. That is, doubling of the momentum results in four-fold increase in kinetic energy. In conventional semiconductors—silicon, germanium, or gallium arsenide—the dispersion relation is also quadratic. For photons, the quanta of light, the dispersion relation is linear. One of the consequences is that a photon always moves at precisely the speed of light.
The electrons and positrons in Dirac's theory occupy a middle ground between rocks and photons: at low energies, their dispersion relation is quadratic, but at higher energies it becomes linear. Until recently, though, it took a particle accelerator to "catapult" an electron into the linear section of the dispersion relation.
Some newly discovered materials can serve as "pocket accelerators" for charged particles. Among them are the "pencil-tip accelerator"—graphene and its three-dimensional analogues, known as Weyl semimetals: tantalum arsenide, niobium phosphate, molybdenum telluride. In these materials, electrons obey a linear dispersion relation starting from the lowest energies. That is, the charge carriers behave like electrically charged photons. These particles may be viewed as analogous to the Dirac electron and positron, except that their mass approaches zero.
The researchers have shown that despite the zero mass, Auger recombination still remains forbidden in Weyl semimetals. Foreseeing the objection that a dispersion relation in an actual crystal is never strictly linear, the team went on to calculate the probability of "residual" Auger recombination due to deviations from the linear law. This probability, which depends on electron concentration, can reach values some 10,000 times lower than in the currently used semiconductors. In other words, the calculations suggest that Dirac's concept is rather faithfully 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," Svintsov explained. "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."
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.
Suggested Items
Groundbreaking Ceremony Marks the Beginning of a New Era for Newccess Industrial; The Construction of the MINGXIN Building
04/12/2024 | Newccess IndustrialOn a clear and sunny day in March, the groundbreaking ceremony for the MINGXIN Building took place in Shenzhen, China. This moment marked the official commencement of construction for a project that will reshape the semiconductor materials industry.
The Need for a Holistic Global Sustainability Standard
04/10/2024 | Michael Ford, Aegis SoftwareNo one can deny that the resources of our fragile planet are finite. The environment seems like a third party, subject to constant degradation. We’re acutely aware of the effects of pollution on our climate, and despite our “throw-away” culture, recycling and recovery of materials has remained relatively expensive, even as we use more energy just to survive.
iNEMI Publishes Four Roadmap Topics
04/04/2024 | iNEMIThe International Electronics Manufacturing Initiative (iNEMI) announces the availability of the first roadmap topics in the new iNEMI Roadmap format. Printed circuit boards, sustainable electronics, smart manufacturing, and mmWave materials and test are now available online.
Insulectro’s 'Storekeepers' Extend Their Welcome to Technology Village at IPC APEX EXPO
04/03/2024 | InsulectroInsulectro, the largest distributor of materials for use in the manufacture of PCBs and printed electronics, welcomes attendees to its TECHNOLOGY VILLAGE during this year’s IPC APEX EXPO at the Anaheim Convention Center, April 9-11, 2024.
Checking In With ICAPE Group
04/03/2024 | Nolan Johnson, I-Connect007ICAPE Group’s field application engineer Erik Pederson drills down on sustainability, supply chain resiliency, and what value engineering really looks like in this exclusive interview. Founded in 1999, European-based ICAPE Group provides 21 million printed circuit boards and over six million technical parts to manufacturers every month. With 30 PCB manufacturing partners globally and 50 partners providing a wide array of technical parts, ICAPE Group has operations in China, Taiwan, Thailand, South Korea, Vietnam, South Africa, Europe, Mexico, and the United States. The company also focuses on the value proposition for its customers.