Light squeezed into a 40-nanometre film

Scientists in Poland have built an ultrathin optical structure that traps infrared light inside a layer about 40 nanometres thick, a result that pushes light confinement far below the scale long regarded as practical for conventional photonic components. The work, led by researchers at the University of Warsaw with partners at Łódź University of Technology, Warsaw University of Technology and the Polish Academy of Sciences, uses patterned molybdenum diselenide to hold and intensify light in a film more than 1,000 times thinner than a human hair. Researchers say the advance could help shrink parts used in photonic circuits, sensors and nonlinear optical devices.

The findings were published in ACS Nano on February 26, with the university outlining the results publicly on March 19 and broader science outlets picking them up in early April. At the centre of the work is a subwavelength grating, a tightly spaced set of parallel strips engineered so that light interacts with the structure as though it were both mirror and cavity. In the paper, the team reports an ultrathin grating made from an epitaxially grown MoSe₂ layer roughly 42 nanometres thick, while public-facing summaries describe it as a 40-nanometre platform. The distinction is minor in practical terms, but it underlines that the device operates at a scale measured in only a few dozen nanometres.

What sets the material apart is its refractive strength. The researchers say MoSe₂ has a refractive index of about 4.4 to 4.5 around the relevant near-infrared range, markedly higher than glass and above typical values cited for silicon and gallium arsenide in this context. That means light slows more sharply inside it, allowing the device to confine optical energy in a much thinner layer than older subwavelength gratings made from more familiar photonic materials. The team also says absorption in the near-infrared range is negligibly small, a crucial point because trapping light is far less useful if too much energy is lost as heat. Together, those properties make the material unusually well suited to ultra-compact photonics.

The study is also drawing attention because the grating hosts what physicists call a bound state in the continuum, a type of optical mode that remains tightly confined even though, in principle, it sits within a spectrum where radiation could leak away. This class of resonance has become a major theme in nanophotonics because it can deliver very high quality factors and sharply enhanced light-matter interaction. Nature’s own topic summary on metasurfaces and bound states in the continuum points to their growing role in nonlinear optics, sensing and integrated photonics. In the Warsaw device, that confinement translated into a strong enhancement of third-harmonic generation, a process that converts incoming infrared light into visible blue light.

On that measure, the numbers are striking. The authors report that third-harmonic generation in the patterned MoSe₂ structure was boosted by more than three orders of magnitude relative to an unstructured layer of the same material, while the university press material describes the gain as more than 1,500-fold. The optical effect is straightforward in principle: three infrared photons combine into one photon at triple the frequency, shifting the signal into the blue part of the visible spectrum. That kind of nonlinear conversion matters because it can support functions such as signal processing, wavelength conversion and compact light sources without relying on large bulk crystals or thicker resonator stacks.

Another reason the work stands out is manufacturing. Much earlier research on atomically thin or layered materials relied on exfoliation, the tape-based peeling method that can yield high-quality flakes but only across tiny, inconsistent areas. The Warsaw team instead used molecular beam epitaxy, a more established semiconductor growth technique, to produce uniform MoSe₂ layers across surfaces spanning several square inches with thickness controlled at sub-nanometre precision. That shift matters for commercial relevance. Reviews of integrated photonics published over the past year have underlined that the field’s next phase depends not only on better optical performance, but on scalable fabrication, lower loss and tighter integration with electronics and control software.