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  • Discovering a new mechanism of optical gain

     Japo_Japo updated 2 months, 4 weeks ago 1 Member · 1 Post
  • Japo_Japo

    September 9, 2021 at 4:12 pm

    Optical gain — the ability of a material to amplify light or photons — is the fundamental concept that drives all lasers. To produce optical gain, electrons are injected into a semiconductor material.

    Semiconductors convert energy to power for electronics. Injecting an electrical current into a semiconductor material, such as silicon or gallium nitride, produces negatively charged electrons and positively charged particles called holes. In conventional semiconductors, when the electrons and holes reach a high enough density, they form an electron-hole gas and optical gain occurs.

    But the new 2D materials Ning and his research team studied several years ago achieved optical gain before the required density appeared to be reached.

    To understand why this may have occurred, in a new experiment, Ning and researchers from ASU and Tsinghua University discovered a process that creates optical gain in 2D semiconductor materials.

    The properties of 2D materials cause electrons and holes to form tightly bound pairs called excitons, which can bind to another electron or hole to form units called trions.

    In his latest line of research, Ning and his peers explored the intricate balance of physics that governs how electrons, holes, excitons and trions coexist and mutually convert into each other to produce optical gain.

    “While studying the fundamental optical processes of how a trion can emit a photon [a particle of light] or absorb a photon, we discovered that optical gain can exist when we have sufficient trion population,” Ning said. “Furthermore, the threshold value for the existence of such optical gain can be arbitrarily small, only limited by our measurement system.”

    In Ning’s experiment, the team measured optical gain at density levels four to five orders of magnitude — 10,000 to 100,000 times — smaller than those in conventional semiconductors that power optoelectronic devices, like barcode scanners and lasers used in telecommunications tools.

    Ning has been driven to make such a discovery by his interest in a phenomenon called the Mott transition, an unresolved mystery in physics about how excitons form trions and conduct electricity in semiconductor materials to the point that they reach the Mott density (the point at which a semiconductor changes from an insulator to a conductor and optical gain first occurs).

    But the electrical power needed to achieve Mott transition and density is far more than what is desirable for the future of efficient computing. Without new low-power nanolaser capabilities like the ones he is researching, Ning says it would take a small power station to operate one supercomputer.

    “If optical gain can be achieved with excitonic complexes below the Mott transition, at low levels of power input, future amplifiers and lasers could be made that would require a small amount of driving power,” Ning said.

    This development would be game-changing for energy-efficient photonics, or light-based devices, and provide an alternative to conventional semiconductors, which are limited in their ability to create and maintain enough excitons.

    As Ning observed in previous experiments with 2D materials, it is possible to achieve optical gain earlier than previously believed. Now they have uncovered a mechanism that could make it work.

    “Because of the thinness of the materials, electrons and holes attract each other hundreds of times stronger than in conventional semiconductors,” Ning said. “Such strong charge interactions make excitons and trions very stable even at room temperatures.”

    This means the research team could explore the balance of the electrons, holes, excitons and trions as well as control their conversion to achieve optical gain at very low levels of density.

    “When more electrons are in the trion state than their original electron state, a condition called population inversion occurs,” Ning said. “More photons can be emitted than absorbed, leading to a process called stimulated emission and optical amplification or gain.”

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