- Harvard researchers have developed metalens that can focus all visible light spectrums at one particular point in high resolution.
- It uses titanium dioxide nanofins that focus all wavelengths of light equally, eliminating chromatic aberration.
- Metalens opens a new range of possibilities, including applications in lithography, microscopy, endoscopy, virtual and mixed reality.
A metalens is a flat-surface lens that uses nanostructures to focus light. It holds all potential to replace the existing thick, curved lens. However, it remained limited in the light spectrum it can focus accurately.
Researchers at Harvard John A.Paulson School of Engineering and Applied Sciences have created a new metalens which is capable enough to focus all visible light spectrums at one particular place in high resolution. So far, this has been only possible by stacking two or more traditional lenses.
This has brought researchers one step closer to incorporate thin lenses into usual as well as advanced optical devices including camera, Augmented and Virtual Reality devices. Let’s find out in details how Harvard researchers managed to achieve this milestone.
Hurdles
It’s very difficult to focus the whole visible spectrum of light (including white color) at one point, mainly because different wavelengths travel at different speed through materials. For instance, blue light will go slower than the red one, so these two colors will arrive at a given point at different times, making the foci different and distorting the image. This distortion is called chromatic aberrations.
In order to adjust these aberrations, all optical devices use two or more curved lenses with distinct thicknesses, adding bulk to the instrument.
Metalenses and Design
Metalenses have several advantages over conventional lenses – they are easy to fabricate, thin and cost effective. The research team has leveraged these advantages across the entire visible spectrum of light.
The new metalens uses titanium dioxide nanofins that focus all wavelengths of light equally, eliminating chromatic aberration. To do this, researchers took some ideas from previous study that demonstrates different wavelength could be focused at a given point by tweaking width, height, distance and shape of the nanofins.
Electron microscope showing side-view of metalens, scale bar – 200 nm | Capasso Lab/Havard SEAS
In the new design, paired nanofins simultaneously control the speed of distinct wavelengths, and the refractive index of the surface of the metalens. This gives variable time delays to wavelengths passing through different fins, in a way that all lights arrive the focal point at the same moment.
The light speed in the nanostructured material can be tuned by merging two nanofins into one element. It significantly decreases thickness as well as design complexity compared to achromatic lenses.
Specifically, the team has demonstrated diffraction-limited achromatic focusing and imaging from 470 to 760 nanometers. The new metalens contains only a single layer of nanostructures with a thickness of order of the wavelength, and doesn’t involve spatial multiplexing or cascading.
The same design principle could be applied to other regions of the electromagnetic spectrum. Realizing achromatic metalenses with bigger diameters and larger numerical apertures requires a wider range of group delay supported by multiple combinations of nanofins with different dimensions. This could be realized by different dispersion techniques or by simply increasing the nanofins’ height.
Flat Metalens | Image credit: Jared Sisler/Harvard SEAS
In this research, titanium dioxide nanostructures have been demonstrated with approximately 4.5 micrometers height corresponding to a group delay of around 37 femtoseconds (10−15 seconds).
Reference: Nature Nanotechnology | doi:10.1038/s41565-017-0034-6 | Harvard SEAS
Cascading layers of metalenses could further increase group delay, which introduces an additional degree of freedom to correct monochromatic aberrations within a large field of view. Once can also merge a metalens that acts as an aberration corrector with a refractive spherical lens.
It looks promising as one would be able to concurrently correct chromatic and monochromatic aberrations of the spherical lens, while leveraging the benefits of bigger lens aperture and a small chromatic focal length shift.
What’s Next?
Harvard has already licensed the technology to a startup for developing it on a commercial level, and has protected the intellectual property of the project.
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Researching are now aiming to increase the diameter of the lens to 1 centimeter, which could open a new range of possibilities, including applications in lithography, microscopy, endoscopy, virtual and mixed reality.
