Planar Systems for Optical Transistors

New photonic platform: a three-atom-thick semiconductor layer is placed on a nanophotonic waveguide. (Source: ITMO)

Leading research groups in the field of nano­photonics are working toward developing optical transistors – key components for future optical computers. These devices will process infor­mation with photons instead of electrons, thus reducing the heat and increasing the operation speed. However, photons do not interact with each other well, which creates a big problem for micro­electronics engineers. A group of researchers from ITMO Uni­versity, together with colleagues, have come up with a new solution to this problem by creating a planar system where photons couple to other particles, which enables them to interact with each other. The principle demons­trated in their experiment can provide a platform for developing future optical transistors.

Devices that use photons for infor­mation encoding would produce less heat, require less energy, and work faster. That is exactly why scientists all over the world conduct research in the field of optical computers. However, making an optical transistor is not an easy task, because one can not simply replace the electrons in tran­sistors with photons. “The problem is that photons, unlike electrons, do not interact with each other, so one photon does not control another,” explains Vasily Kravtsov, a leading research fellow at ITMO University. “That is why it is extremely difficult to design a transistor based only on photons.”

Researchers from over the world suggest dif­ferent methods to train photons to interact with each other. The idea of one of these methods is to couple photons with other particles. A group of researchers have demons­trated a new efficient implemen­tation, where photons couple to excitons in single-layer semiconductors. The study was conducted within the framework of a megagrant in collaboration with the University of Sheffield. It seems like polaritons are a straight­forward solution, and now all we need to do is to create a polariton-based transistor. However, it is not that easy: we need to design a system where these particles could exist long enough while still main­taining their high inter­action strength.

In the new approach, polaritons are created with the help of a laser, a waveguide, and an extremely thin molybdenum diselenide semi­conductor layer. A three-atom-thick semiconductor layer is placed on a nano­photonic waveguide, with a precise net of very fine grooves engraved on its surface. After that, it is lit up with a red laser to create excitons in the semi­conductor. These excitons couple with light particles creating polari­tons. The latter are thus temporarily locked in the net of grooves in the waveguide. “The waveguide is structured in a special way so as to create, loosely speaking, a light trap,” expounds Fedor Benimetskiy. “Picture a set of two mirrors facing each other. Under certain conditions, the light of a specific wavelength will be trapped between them. In our case, the waveguide acts like these two mirrors. When light passes through the semi­conductor, it creates polaritons, which are simul­taneously light particles and excitons. Thanks to the waveguide’s geometry, these hybrid particles can live rela­tively long.”

Polaritons created in this way do not only exist for rela­tively long periods of time, but also have extra high non­linearity, meaning that they actively interact with each other. “We were able to demons­trate the highly nonlinear behavior of these particles,” continues Vasily Kravtsov. “In other words, polaritons interact with polaritons, they can scatter off each other. We have measured the highest non­linearity for such systems. It brings us closer to creating an optical transistor, as we now have a planar platform less than 100 nano­meters thick, which could be inte­grated on a chip. As the non­linearity is rather high, we would not need a powerful laser – a small red light source will suffice, which could also be integrated onto the chip.”

At the moment, however, it is too soon to talk about designing a chip – new research efforts are on the way. The scientists now have to demons­trate that their system can operate at a room tempera­ture, as the experi­ments so far have been done at -120 C. Moreover, it is important to further extend the lifetime of polaritons in the system. (Source: ITMO)

Reference: V. Kravtsov et al.: Nonlinear polaritons in a monolayer semiconductor coupled to optical bound states in the continuum, Light: Sci. & Appl. 9, 56 (2020); DOI: 10.1038/s41377-020-0286-z

Link: Electromagnetic states in 2D nanostructures and materials,  

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