Optical gain is a prerequisite for amplifying a signal in an optical amplifier or laser. Usually a high level of current input is required in conventional semiconductors. Studying the complex balance and conversion of excitons and trions in atomically thin two-dimensional materials, the authors found a new amplification mechanism that requires an input power several orders of magnitude lower than in conventional semiconductors. This new amplification mechanism could potentially allow lasers to be made with extremely low input power.
In a new article published in Light of science and applicationsResearchers from Tsinghua University and Arizona State University report on their results from a study of the fundamental physics of excitons, trions, and related complexes. Excitons are quasiparticles formed by an electron, and a void called a hole remains when the electron is excited in a semiconductor. Such exciton can be charged to form the so-called trion when it further binds to another electron or hole. The team discovered an interesting process that gives optical gaina condition for amplifying or generating a signal in a semiconductor by studying the complex balance and conversion of excitons, electrons, holes and trions. Interesting level required input power The implementation of such optical amplification is extremely small: 4–5 orders of magnitude lower than in a conventional semiconductor, such as GaAs or InP, which are currently working materials for optoelectronic devices.
The distribution of these excitonic complexes and their dynamic mutual transformation have been the basis of solid state physics for many decades. Unresolved questions remain about how these excitons form more complex particles and ultimately turn into the ionized conducting phase of charged particles as we introduce more and more of them into the semiconductor. This process is called the Mott transition, named after Sir Neville Francis Mott, a famous British Nobel physicist. The traditional theory of the occurrence of optical amplification states that free excitons cannot create optical amplification before the Mott transition in a semiconductor with freely moving charges. Optical gain occurs after electron density exceeds the so-called Mott density, as a rule, a very high level of density, of the order of trillions of particles per square centimeter. Such an extremely high density requires a high level of supply of electric current or electricity. Most of our modern semiconductor lasers that power our internet, data centers and many other applications are based on such semiconductors.
Studying the relationship between the occurrence of optical amplification and the Mott transition, especially the search for new mechanisms of optical amplification at low densities before the Mott transition, is thus not only a matter of fundamental importance in solid state physics, but also important in applications of devices in photonics. If optical amplification can be achieved with exciton complexes below the Mott transition at low power levels, future amplifiers and lasers can be manufactured that require a small amount of excitation power. This is obviously of great interest to energy-efficient photonic devices or green photonics. But, unfortunately, such problems cannot be fully and systematically studied in the ordinary semiconductor because excitons themselves are not very stable and the probability of obtaining higher exciton complexes is limited.
The recent advent of atomically thin layered materials has made such a study possible and more significant. These materials contain only a few layers of atoms. Due to the fineness of materials, electrons and holes are attracted to each other hundreds of times stronger than in conventional semiconductors. Such strong charge interactions make excitons and trions very stable even at room temperature. For this reason, the authors could investigate such a complex balance and carefully control their mutual conversion to achieve optical amplification. By creating excitons by optical pumping by a laser, excitons form trions with some electrons, the number of which is controlled by the gate voltage. When there are more electrons in the trion state than in the electron state, a condition arises, called population inversion. More photons can be emitted than absorbed, leading to a process called stimulated emission and optical amplification or amplification.
“Another motivation for this study was the apparent contradiction between several high-profile field experiments in recent years. There have been several experiments reporting laser demonstrations using two-dimensional materials as an amplifying medium. There, lasers required a very low level of pumping, when excitons were the dominant mechanism of light emission. But the only experiment that has proven the existence of optical amplification in such materials requires a much higher level of pumping, ”said Nin, the head of the research team. Ning noted that the densities in laser experiments are 3-5 orders of magnitude lower than the Mott density, while optical amplification was observed only after the Mott transition. Since laser operation requires optical amplification, Ning asked: “Where does optical amplification come from? laser experiments? "Or" What are the mechanisms of optical amplification at such a low level of optical pumping? Or, in a more general sense: "Are there any possible new amplification mechanisms before the Mott transition?" These questions led to their experimental study, which began several years ago.
“We have systematically dealt with this issue experimentally for 2-3 years. "We have reflected a broad spectrum light beam from two-dimensional molybdenum ditelluride and carefully make sure that the reflected signal is larger or smaller than the incident beam to find any sign." about amplification of light, "said Hao San, lead author of this article in charge of optical measurements.
“Of course, a similar experiment to amplify trions was carried out in the 1990s with conventional semiconductors,” Nin said. "But the excitons and trions were so unstable that experimental observation and, especially, the use of this optical amplification for real devices is extremely difficult." “Since excitons and trions are much more stable in two-dimensional materials, new possibilities appear for creating real devices from this observation,” Nin said. “At the moment, this result relates to fundamental physical research, but as for all important observations in semiconductors, they can ultimately be used to create real lasers,” commented Nin.
Zhen Wang et al., Exciton complexes and optical amplification in two-dimensional molybdenum ditelluride are significantly lower than the Mott transition, Light: science and application (2020). DOI: 10.1038 / s41377-020-0278-z
Chinese Academy of Sciences
The new optical amplification mechanism in two-dimensional material requires only extremely low input power (2020, March 20)
retrieved March 20, 2020
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