Mirror, mirror, on and off

A light switch on the nanoscale

In modern laboratory experiments, light can be controlled at small scales in remarkable ways. To achieve this, physicists use ultrathin optical coatings called metasurfaces. These structures are typically just a few tens to hundreds of nanometers thick – about a thousand times thinner than a human hair. Despite their tiny size, these cleverly designed nanostructures can bend light, focus it, or otherwise manipulate it in unprecedented ways.

 

These recent developments make optics at the nanoscale achievable, but the possibilities are still somewhat limited. The main problem is that most metasurfaces are “static”: once they are made, their behavior cannot be changed. For future technologies, scientists need more: they want to have optical components that can be actively tuned – turned up or down, switched on or off – to be able to achieve with light what we can now only achieve in electronic circuits.

 

In new research that was published in the journal Light: Science & Applications this week, physicists Tom Hoekstra and Jorik van de Groep from the UvA-Institute of Physics describe an important breakthrough. Using a new approach, they managed to realize an actively tunable metasurface. At its heart is a novel quantum material: a single “two-dimensional” layer of tungsten disulfide, WS₂ for short. The unique properties of this 2D material allowed the researchers to build a nanoscale mirror for red light that can be turned on and off at will – essentially: a light switch on the nanoscale.

 

Excitons

In technical terms, the device that Hoekstra and Van de Groep managed to construct is known as an optical modulator. That 2D materials might be used for such optical modulation was already proposed shortly after these materials were discovered in 2004, but making the effect work at room temperature proved extremely difficult. The key breakthrough that led the researchers to their success was to construct a metasurface that traps light inside itself – right where the two-dimensional layer of WS₂ layer is located. As a result, the interaction between light and matter becomes unusually strong. So strong, in fact, that quantum effects within the WS₂ layer persist at room temperature, making the device perform with record efficiency.

 

Here's how it works. When WS₂ absorbs light, an electron is excited to a higher energy level. Because of confinement in the atomically thin layer, the negatively charged electron and the positively charged “hole” that the electron leaves behind remain bound together by electrostatic attraction, forming what is known as an exciton.

 

This quantum mechanical phenomenon is at the heart of the device's tunability. Due to the excitons, in the "on" state, the device reflects light at specific wavelengths in the red part of the visible spectrum, like a nanoscale mirror. Since excitons are very sensitive to the charge density in the material, they can be effectively suppressed by applying a voltage. As a result, in the "off" state, all of the red light is absorbed and none of it is reflected anymore.

 

A new era of photonics

The work by Hoekstra and Van de Groep shows that excitons in 2D materials can be harnessed for use in compact, active optical components for all sorts of applications. Looking ahead, their approach offers exciting opportunities to be applied wherever light needs to be controlled quickly and precisely. One can think of optical communication links – where beams of light transmit data wirelessly through the air – or of optical computing – where particles of light, photons, rather than the traditional electrons, carry information at high speeds and low energy cost. With all these potential applications, excitons may well spark a new era of photonics!

---

---