Scientists create the thinnest lens on Earth, powered by excitons

Lenses are used to bend and focus light. Normal lenses rely on their curved shape to achieve this effect, but physicists from the University of Amsterdam and Stanford University have made a flat lens just three atoms thick that relies on quantum effects. This type of lens could be used in future augmented reality glasses.

Curved glass lenses work because light refracts (bends) as it enters the glass and again as it exits, making things appear larger or closer than they actually are. Humans have used curved lenses for more than two millennia to study the motions of distant planets and stars, detect tiny microorganisms, and improve vision.

Ludovico Guarneri, Thomas Bauer and Jorik van de Groep from the University of Amsterdam, along with colleagues from Stanford University in California, took a different approach. Using a single layer of a unique material called tungsten disulfide (WS₂ in short), they built a flat lens that is half a millimeter wide but only 0.0000006 millimeters, or 0.6 nanometers, thick. This makes it the thinnest lens on Earth.

Instead of relying on a curved shape, the lens is made of concentric rings of WS₂ with gaps in between. This is called a “Fresnel lens” or “area plate lens” and focuses light using diffraction instead of refraction. The size and distance between the rings (compared to the wavelength of light hitting it) determines the focal length of the lens. The design used here focuses red light 1mm from the lens.

The work was published in the journal Nano Letters.

Quantum enhancement

A unique feature of this lens is that its focusing efficiency relies on quantum effects within the WS₂. These effects allow the material to efficiently absorb and re-emit light at specific wavelengths, giving the lens the built-in ability to work better for these wavelengths.

This quantum enhancement works as follows. First, WS₂ absorbs light by sending an electron to a higher energy level. Because of the ultra-thin structure of the material, the negatively charged electron and the positively charged “hole” it leaves behind in the atomic lattice are held together by the electrostatic attraction between them, forming what is known as an “exciton.”

These excitations are quickly dissipated again by the electron and hole fusing together and sending out light. This re-emitted light contributes to the efficiency of the lens.

The scientists discovered a clear peak in lens efficiency for specific wavelengths of light sent by excitons. While the effect is already noticeable at room temperature, the lens is even more effective when cooled. This is because excitons do their job better at lower temperatures.

Augmented reality

Another of the unique features of the lens is that, while some of the light passing through it creates a bright focal point, most of the light passes through unaffected. While this may seem like a disadvantage, it actually opens new doors for use in future technology.

“The lens can be used in applications where the view through the lens should not be disturbed, but a small portion of the light can be tapped to collect information. This makes it perfect for wearable glasses and augmented reality,” explains Jorik van. de Groep, one of the authors of the paper.

Researchers are now setting their sights on designing and testing more complex and multifunctional optical coatings whose function (such as focusing light) can be adjusted electrically.

“Excitons are very sensitive to the charge density in the material, and therefore we can change the refractive index of the material by applying a voltage,” says Van de Groep.