Researchers from the Department of Physics at the University of Warsaw, in collaboration with teams from Lodz University of Technology, Warsaw University of Technology and the Polish Academy of Sciences, have created a structure that can confine infrared light in a layer just 40 nanometers thick. Their approach is based on a design known as a subwavelength grating made of a special material called molybdenum diselenide (MoSe2). The survey results were published in a magazine ACS nano.
Manipulating light on a very small scale is key to advancing modern technology. As traditional electronics begins to reach their limits, photonics offers an alternative that uses light instead of electrons to carry information. Because photons travel faster and don’t have mass like electrons, light-based devices could become faster and smaller, opening the door to more powerful and compact technologies.
Challenge to the wavelength of light
Light can behave as both particles and waves, but the nature of waves imposes limitations. Each type of light has a wavelength that determines how small structures can be made while effectively controlling them. The wavelength of visible light is several hundred nanometers, but the wavelength of infrared light is over micrometers. This raises an important question: Can light be trapped in structures smaller than its own wavelength?
The research team demonstrated that this is indeed possible. By designing a subwavelength grating, they were able to confine infrared light within a layer just 40 nanometers thick. This structure consists of closely spaced parallel strips that interact with light similar to a prism. When these strips are placed closer together than the wavelength of the light, the grating can act almost like a perfect mirror while simultaneously holding the light within a very small volume.
Why is molybdenum diselenide so effective?
Early versions of such diffraction gratings were made of materials such as silicon or gallium compounds and required a thickness of several hundred nanometers to work effectively. When you reduce the size, you lose the ability to trap light. The main difference in this new approach is the use of molybdenum diselenide, which has a much higher refractive index. Simply put, light travels slower in this material than in other materials. Light is about 1.5 times slower in glass, about 3.5 times slower in silicon or gallium arsenide, and about 4.5 times slower in MoSe2. This strong slowing effect allows the structure to shrink dramatically while efficiently capturing light, resulting in a layer more than 1,000 times thinner than human hair.
Convert infrared light to blue light
MoSe2 also offers additional benefits. Like graphene, it forms a layered structure, but unlike graphene, it is a semiconductor. They also exhibit nonlinear optical behavior, including a process known as third harmonic generation. In this process, three infrared photons combine into one photon of a higher frequency, effectively converting infrared light into visible blue light. The diffraction grating focuses the infrared light strongly, making this conversion more efficient. The researchers found that the effect was more than 1,500 times stronger compared to a flat layer of the same material.
Another major advance is in the way the materials are manufactured. Previously, thin layers of MoSe2 were created using exfoliation, a method similar to peeling a layer off a crystal with adhesive tape. Although this technique is simple, it is inconsistent and limited to very small areas (typically about 10 square micrometers), making it unsuitable for real devices.
To overcome this, the research team used molecular beam epitaxy (MBE), a well-established method for growing semiconductor layers. This approach allowed us to fabricate large, uniform MoSe2 films spanning several square inches. Despite this large size, the layer remains only 40 nanometers thick, giving it an extreme aspect ratio. For comparison, the thickness-to-size ratio of this layer is approximately 1:1 million, while a typical A4 paper has a ratio closer to 1:2000.
Toward practical photonic applications
These results suggest that molybdenum diselenide produced in this way has the potential to significantly change the way light is controlled in future technologies. Structures no longer need to be thick to effectively manipulate light. Instead, very thin layers can perform the same function, and in some cases even better. Because this manufacturing method is scalable, it is becoming increasingly viable for real-world applications such as photonic integrated circuits.
Funding and support
This research was funded by the National Science Center under projects OPUS 2020/39/B/ST7/03502 and 2021/41/B/ST3/04183, European Union funds under ERC-ADVANCED grant number 101053716, the Polish Science Foundation under project ENG.02.01-IP.05-T004/23, and the University of Warsaw. Based on Excellence Initiative – Research University (IDUB) New Ideas in Priority Research Areas II No. 501-D111-20-2004310, titled “Ultrathin subwavelength gratings based on dichalcogenides”.
