Trapping Light in Ultrathin Layers: A Leap Forward for Photonics
Scientists have achieved a significant breakthrough in nanophotonics by successfully trapping infrared light within a layer just 40 nanometers thick – thousands of times thinner than a sheet of paper. This innovation, spearheaded by researchers at the University of Warsaw, in collaboration with Łódź University of Technology, Warsaw University of Technology and the Polish Academy of Sciences, promises to reshape the future of photonic devices and accelerate the development of faster, smaller technologies.
The Challenge of Miniaturization in Photonics
As traditional electronics approach their physical limitations, photonics – the manipulation of light for information processing – is emerging as a promising alternative. Photons offer the potential for faster speeds and reduced device sizes compared to electrons. But, a fundamental challenge lies in controlling light at the nanoscale, given its wave-like nature and relatively large wavelengths. For infrared light, the wavelength is a micrometer or more, traditionally requiring components of similar size.
Subwavelength Gratings: A Novel Approach
The research team overcame this hurdle by creating a subwavelength grating made from molybdenum diselenide (MoSe2). These gratings, composed of periodic arrays of materials spaced closer than the wavelength of light, act like prisms, diffracting and controlling light. Unlike previous gratings made from materials like silicon, gallium arsenide, or gallium nitride, which required thicknesses of several hundred nanometers to function effectively, the MoSe2 grating maintains its light-confining ability at a mere 40 nanometers.
Why Molybdenum Diselenide?
The key to this achievement lies in the unique properties of MoSe2. It possesses a significantly higher refractive index than commonly used materials. This means light travels slower within MoSe2 – approximately 4.5 times slower than in air, compared to 1.5 times in glass and 3.5 times in silicon or gallium arsenide . This slower speed allows for the drastic reduction in grating thickness without sacrificing performance.
Beyond Confinement: Enhanced Light Transformation
MoSe2 also exhibits nonlinear optical effects, specifically third harmonic generation. This process allows three infrared photons to combine and create a single photon with three times the frequency, effectively converting infrared light into blue light. The strong confinement of light within the MoSe2 grating enhances this effect by more than 1,500 times compared to a flat layer of the material .
A Scalable Fabrication Method
Traditionally, thin layers of MoSe2 were created through exfoliation – a process similar to peeling layers from a crystal with tape. This method is unreliable and produces slight samples, unsuitable for practical applications. The University of Warsaw team employed molecular beam epitaxy (MBE), a standard semiconductor production technique, to create uniform MoSe2 layers covering several square inches with a consistent thickness of 40 nanometers. This scalable fabrication method paves the way for real-world applications.
Implications for the Future
This research, published in ACS Nano, demonstrates that ultrathin subwavelength gratings can achieve strong light confinement and enhanced optical effects. The ability to manipulate light at this scale opens doors to the development of faster, smaller, and more efficient photonic devices, potentially revolutionizing fields like telecommunications, computing, and sensing. The study was funded by the National Science Centre under projects OPUS 2020/39/B/ST7/03502 and 2021/41/B/ST3/04183, with European Union funds under ERC-ADVANCED grant No. 101053716, the Foundation for Polish Science under project ENG.02.01-IP.05-T004/23, and by the University of Warsaw under the Excellence Initiative – Research University (IDUB) New Ideas in Priority Research Areas II No. 501-D111-20-2004310 titled “Ultrathin subwavelength gratings based on dichalcogenides.”