Researchers from ITMO’s Faculty of Physics and their collaborators from POSTECH were the first to discover an effect that amplifies exciton current by 80 times. Additionally, the researchers developed an electroplasmonic nanoresonator, a special structure to control the exciton current at the nanoscale. In the future, it can lay the foundation for energy-efficient, fast, and compact exciton integrated circuits for data transfer and AI computations in addition to existing electron and optical circuits. The corresponding paper has been published in Nature Materials.

^{Vasily Kravtsov, Ivan Kalantaevskii, and Artem Abramov (left to right). Photo by Dmitry Grigoryev / ITMO NEWS}

At data centers, information is transferred, processed, and stored using integrated circuits – either electronic (that use the electric current) or optical (based on photons). Both technologies, however, have their limitations. When large amounts of data need to be computed, for instance, with AI, electronic integrated circuits consume a lot of energy and can overheat. Optical ones, on the other hand, are hard to miniaturize due to fundamental limitations connected to the light’s wavelength.

Alternative integrated circuits can be based on excitons, neutral quasiatoms “born” in a semiconductor as a result of interaction of the negatively charged electron and the positively charged “hole,” a quasiparticle. As they are neutral, excitons can act as “intermediaries” between electronic and optical processes, i.e. help the material effectively turn light into electronic excitation and back.

Thus, excitons possess the advantages of both types of integrated circuits. Just like optical circuits, they can absorb photons and radiate light; this means that light can be used to transfer information to an exciton and then read without complex transformations in between. At the same time, excitons have the electronic circuit’s capacity to miniaturize: quasiatoms can be implemented in a thin semiconductor layer, just one-nanometer-thick compared to hundreds of nanometers in an optical circuit.

Information is transferred similarly with excitons and electrons; but in exciton devices exciton current is used instead of the electric one. In order to make information transfer faster and more efficient, scientists are looking for ways to amplify exciton current, for example, by increasing electron density in integrated circuits. However, as it turned out, there is another way to solve this problem.

A team from ITMO’s Faculty of Physics and Pohang University of Science and Technology (POSTECH), South Korea, have discovered a new exciton-control mechanism at the nanoscale and developed an electroplasmonic nanoresonator, a structure that increases exciton current by 80 times. The electroplasmonic nanoresonator is just 25-nanometers-thick, consisting of two ultra-thin semiconductor layers on a gold substrate. The layers are stacked on top of each other at a specific angle: a “hole” forms in the top layer made of tungsten diselenide, and an electron forms in the bottom layer made of an alloy of molybdenum, tungsten, and selenium. Under laser illumination, excitons arise in the bilayer structure. 

In order to control exciton movement, the researchers shaped a sharp gold electrode into a resonator. Localized there is the optical field of the laser that generates quasiatoms; simultaneously, a local electric field is also formed. The influence of two fields creates a difference in exciton concentration: there is a lot of them in the nanoresonator under the electrode, but nearly none in the rest of the semiconductor. This drop in concentration creates an exciton density gradient that pushes quasiatoms out of the nanoresonator and makes them spread to more vacant positions in the semiconductor.

“Earlier, physicists believed that exciton current can be amplified by creating more quasiparticles in a single location and moving them along the semiconductor with a changing electric field. Having studied the effect of exciton density gradient, we discovered a new mechanism of exciton current amplification. It demonstrates that what matters is not only the number of created quasiatoms, but also the difference in concentrations between the excitons remaining in the nanoresonator and those that left it. In practice, it means that we can generate less excitons and still get a good amplification. In our study, we were already able to demonstrate an 80-fold current amplification on the scale of several dozen nanometers. Thanks to the exciton density gradient, we can create energy-efficient, robust, faster, and more compact exciton integrated circuits that don’t require cooling that will supplement electronic and optical circuits used for AI computations and data transfer,” saus Vasily Kravtsov, the head of the study, a senior researcher at ITMO’s Faculty of Physics.

In the future, ITMO researchers are planning to see if it’s possible to amplify exciton current even further and how this can be achieved, study the ways to control quasiparticles, and outline the limitations of their solution. It’s also necessary to find a photon source for a nanoscale exciton integrated circuit. One possible candidate for the role is the world’s smallest laser developed by scientists from ITMO and MIPT. Its volume is less than 0.005 cubic microns, 13 times smaller than the wavelength cube of the emitted light. This is a record for the blue spectrum (400-500 nm).

^{PhD student Artem Abramov setting up the experimental installation to assemble heterostructures based on 2D semiconductors. Photo by Dmitry Grigoryev / ITMO NEWS}

^{PhD student Artem Abramov setting up the experimental installation to assemble heterostructures based on 2D semiconductors. Photo by Dmitry Grigoryev / ITMO NEWS}

This study is supported by national program Priority 2030 and the Russian Science Foundation grant No. 25-42-01019.