Physicists Tom Hoekstra and Jorik van de Groep of the UvA-Institute of Physics fabricated an actively tunable metasurface only tens to hundreds of nanometers thick. Metasurfaces are ultrathin optical coatings that can bend, focus, or otherwise control light, but most devices to date have fixed properties once manufactured. For applications that require dynamic control, scientists seek components that can modulate light in real time, similar to how electronic circuits control electrical signals.
In their new work in the journal Light: Science and Applications, Hoekstra and Van de Groep report a metasurface built around a single two-dimensional layer of tungsten disulfide, WS2. This 2D semiconductor enables a mirror for red light at the nanoscale whose reflectivity can be toggled, effectively acting as an optical switch.
The device operates as an optical modulator, changing light transmission and reflection under electrical control. Concepts for using 2D materials in such modulators have existed since these materials were identified in 2004, but achieving strong operation at room temperature has been difficult. The Amsterdam team addressed this by designing a metasurface that traps light in the region containing the WS2 monolayer, greatly strengthening light - matter interaction.
This confinement yields strong coupling between the optical field and electronic states in the material, allowing quantum effects to remain active at room temperature and giving the modulator high efficiency. When WS2 absorbs incoming light, electrons are promoted to higher energy levels and, together with the positively charged holes they leave behind, form bound electron - hole pairs known as excitons.
Excitons underpin the tunability of the mirror. In the on state, excitons cause the device to reflect light at selected wavelengths in the red part of the visible spectrum, so the metasurface behaves like a nanoscale mirror. Because excitons respond sensitively to the charge density in the WS2 layer, applying a voltage suppresses them, switching the device into an off state in which red light is absorbed rather than reflected.
The study shows that excitons in 2D materials can drive compact, active optical components suitable for integrated photonics. The approach could be used wherever fast, precise light control is needed, including free-space optical communication links that transmit data through air and optical computing architectures where photons carry information at high speed and with low energy use. With these potential applications, exciton-based metasurfaces may help launch a new phase in photonics technology.
Research Report:Electrically tunable strong coupling in a hybrid-2D excitonic metasurface for optical modulation
Related Links
Universiteit van Amsterdam
Understanding Time and Space
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