Metalens manages the whole spectrum

The future of optics is flat. That’s the prediction of a range of scientists working on ways to create better and, crucially, smaller lenses for cameras and virtual reality systems.

The field is called metalens research, and its key design change is the elimination of traditional curved lenses and replacing them with tiny flat ones. 

The flat lenses, constructed using nano-engineering, are achromatic – meaning they transmit light without first separating it into its constituent rainbow colours. That’s the goal – although until now reaching it has proved challenging.

The reason is that different parts of the visible light spectrum, characterised by different size wavelengths, pass through lenses at different speeds. Red wavelengths, for instance, move more rapidly than blue ones, meaning that when both pass through a single traditional lens the red will reach a target surface first. 

For precision applications, different colour wavelengths arriving at different times have a significant disruptive effect on focus. To get around this problem, instrument-makers deploy a range of curved lenses with varying thicknesses – a solution that works, but which also adds bulk and expense.

Flat metalenses, by deploying nano-size surface structures called fins, potentially resolve the wavelength issue without adding mass. Constructing a lens that can transmit the entire visible spectrum has proved challenging.

Now, however, a team from the John A. Paulson School of Engineering and Applied Sciences at Harvard University in the US has found a way through the challenge.

The researchers, led by Wei Ting Chen, report the construction of a metalens that uses nanofins made from titanium oxide to produce an equal focus of wavelengths across the spectrum.

The key to the design is the use of nanofins in pairs, enabling the speed of various wavelengths to be controlled.

“One of the biggest challenges in designing an achromatic broadband lens is making sure that the outgoing wavelengths from all the different points of the metalens arrive at the focal point at the same time,” says Chen.

“By combining two nanofins into one element, we can tune the speed of light in the nanostructured material, to ensure that all wavelengths in the visible spectrum are focused in the same spot, using a single metalens. This dramatically reduces thickness and design complexity compared to composite standard achromatic lenses.”

The research is published in the journal Nature Nanotechnology.

It describes a lens that is only about 500 nanometres in diameter. Chen and his team now intend to scale the lens up to around one centimetre, making it suitable for consumer electronic devices.

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