Fascination With Technology Ultrathin Lens Halves the Wavelength of Laser Light

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In our "Fascination with Technology" section, we present impressive projects from research and development to engineers each week. Today: how researchers managed to develop a kind of magical meta-lens that halves the wavelength of light, thus transforming infrared light into visible light.

A team of physicists from ETH Zurich has developed tiny meta-lenses that can halve the wavelength of light. They achieved this through the material of the lenses—a metal oxide called lithium niobate—and through tiny structures that the researchers stamped into the material. Such meta-lenses could be used as a security feature on banknotes or to produce extremely thin camera elements.

Lenses are among the most commonly used optics. Camera lenses, for example, focus light to a focal point to capture sharp photos and videos. The rapid development of optics in recent decades is demonstrated by the shift from bulky cameras to compact smartphone cameras. Yet even current smartphone cameras consist of several lenses, often making up the thickest part of the device. This minimum size results from the functioning of classical lenses: light refraction is based on the curvature of the lens or the change in the thickness of the lens.

How Meta-Lenses Work

To overcome this limitation, researchers have developed new solutions over the past decade: so-called meta-lenses. They work like normal lenses but are flat. The lenses have a uniform thickness across the entire surface. This allows them to be about 40 times thinner than a human hair—and they are not made of glass.

In meta-lenses, special structures on the surface deflect light. These structures are only a few hundred nanometers in size, allowing the lenses to be heavily miniaturized. When researchers combine the nanostructures with special materials, they can also study extraordinary properties of light. An example is nonlinear optics, where light is converted from one color to another. Green laser pointers operate on this principle: infrared light passes through a high-quality crystal material and generates light of half the wavelength—in this case, green light.

A common material for such effects is lithium niobate. It is used in telecommunications for coupling electronics and optical fibers.

Comparable to Gutenberg's Printing Press

Rachel Grange, a professor at the Institute for Quantum Electronics at ETH Zurich, researches the production of nanostructures from this material. She and her team have now developed a new process for this. It allows them to work with lithium niobate so that it functions as a meta-lens.

The lithium niobate solution can be stamped directly in the liquid state with a mold negative—similar to Gutenberg's printing press.

Ülle-Linda Talts

For their new method, the physicists combined chemical processes with high-precision nanofabrication technology: "The lithium niobate solution can be stamped in its liquid state directly with a mold negative—similar to Gutenberg's printing press," explains co-first author Ülle-Linda Talts, a doctoral student with Rachel Grange. When the researchers heat the material to 1110 °F, the stamped material acquires the crystalline properties that enable light conversion akin to the green laser pointer.

Mass Production Possible

The new method has several advantages: creating nanostructures in lithium niobate using conventional techniques is difficult because this material is very stable and hard. The new method, however, is even suitable for mass production, as the print template can be used multiple times to produce the desired number of meta-lenses. This saves time and costs compared to previous methods.

Ultrathin Lenses Generate New Light

Thus, for the first time, the ETH researchers around Grange have succeeded in manufacturing meta-lenses from lithium niobate with precisely crafted nanostructures using this technique. These meta-lenses function like normal lenses but can additionally generate and control laser light of half the wavelength. When the researchers pass infrared light with a wavelength of 800 nanometers through the meta-lens, visible light with a wavelength of 400 nanometers emerges on the other side, focused on a defined focal point.

This magic of light conversion, as Rachel Grange calls it, is only possible because the ultrathin meta-lens is specially structured and the material enables this nonlinear optical effect in the first place. The effect is not limited to a specific laser wavelength and is thus versatile across different fields.

From Counterfeit-Proof Banknotes to New Microscopy

Meta-lenses or similar nanostructures that generate holograms could serve as security features on banknotes or for the authenticity verification of artworks. The structures are too small to be seen with visible light, and the nonlinear material properties allow for reliable verification.

Researchers can also use the conversion and control of laser light to detect infrared light as visible light with simple and inexpensive camera sensors.

However, it may still be some time before such meta-lenses reach the market. The research field of these ultrathin optical elements at the intersection of physics, materials science, and chemistry is still young.

We have only begun to grasp the potential of this technological field and are eager to see the impact this cost-effective method will have in the future.

Rachel Grange

The study was partially funded by an SNF Consolidator Grant to Rachel Grange.

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