Lenses, like the ones in glasses or in telescopes, are usually rather big and bulky objects. In the current era of nanotechnology, one may wonder whether optical elements can also be made smaller. This is indeed possible, and has led to a completely new area of study, ‘flat optics’, where optical elements of 100nm thick or less are being produced – smaller than the wavelength of visible light itself!

Now you see me...

Flat optical elements use the nanostructure of the materials that they are made of to redirect light. They are very useful in nanotechnology, but have one major issue: once these lenses and other optical elements are fabricated, their properties and functionality are completely fixed. This leads to a challenge for next-generation applications, for example in augmented and virtual reality, where one would like to manipulate light in a dynamical manner depending on external signals.

Physicists from the universities of Amsterdam and Stanford have now managed to construct tiny lenses that do allow such dynamic control. In particular, their lenses can be ‘turned on and off’ at will, to either focus light to a specific location or let it pass through unaffected. To construct these lenses, the researchers used a semiconducting material, tungsten disulfide, that is only a single layer of atoms thick, giving the lens a height of less than a single nanometer. Under normal circumstances, despite the fact that it is very thin, light interacts very strongly with this material, allowing it to be shaped in such a way that it acts as a lens.

However, when a small electrical voltage is applied to the material, it completely changes its properties, and the interaction with light can effectively be turned off. When this happens, the light that was previously focused now passes through the material essentially unhindered: the lens has been turned off.

Virtual reality

The materials that the researchers used are highly transparent, which is a very useful property for applications such as augmented and virtual reality goggles, eye tracking, or beam tapping, where – similar to a wiretap for electricity – one takes away a very small fraction of a signal to learn something about the encoded information. As a result, the insights from this research enable a new way to design dynamic optical elements and to use them in future applications.

Reference

Exciton resonance tuning of an atomically thin lens, Jorik van de Groep, Jung-Hwan Song, Umberto Celano, Qitong Li, Pieter G. Kik and Mark L. Brongersma, Nature Photonics (2020).