Traditional lenses are bulky, and getting more and more incompatible with the growing need of lightweight and low profile display that is at the center of virtual reality (VR) and augmented reality (AR) technology.
Although some of us such as teenagers can focus on things as close as 7 cm (2.8 inches) away from their eyes, most of the adults cannot see things at that kind of close distance. That’s why AR or VR reality head-mounted displays, which usually sit 3 to 7 cm in front of our eyes, require lenses that can refocus the light and allow our eyes to see images projected from the devices.
Manufacturers have put a lot of efforts to make the traditional lenses as thin as possible, but the resulted products still cannot keep up with the advancement of the technology. Here comes the next generation - metalenses. These seemingly flat and slim lenses have nano-scale structures (nicknamed nanofins) on the surface that give them better light focusing ability, and also allow engineers to shrink the size of lenses drastically.
An engineering research team from Harvard’s School of Engineering and Applied Sciences (SEAS) announced that they had developed the first lens capable of focusing the entire visible spectrum. Their study was recently published in Nature Nanotechnology.
Before this new breakthrough, most metalenses have trouble focusing well on all points across the spectrum of light. So far people have been mitigated the problem by stacking lenses to cover different colors. (Different colors move through materials at different speeds. For example, red travels through glass quickest and violet the slowest because their difference in wavelengths).
But the stacking causes a secondary problem known as chromatic aberration, meaning that lenses fail to focus on colors. Not only the aberration not only causes pictures to appear blurry but also makes the colors bleed into one another (known as “color fringing”). This is what happens when wavelengths of color get focused at different points on the focal plane.
The Harvard SEAS team has tackled this issue by developing a first-ever single lens that can focus the entire visible light spectrum including white light in one spot. Commenting on their research, Federico Capasso, SEAS professor and senior author of the study said: "Metalenses have advantages over traditional lenses. Metalenses are thin, easy to fabricate and cost-effective. This breakthrough extends those advantages across the whole visible range of light. This is the next big step."
The new metalenses use titanium dioxide nanofins. Those nanofins can equally focus various wavelengths of light. Earlier research showed that the wavelengths could be focused but at different distances. Thus, the researchers developed pairs of nanofins to control the speed of the wavelengths simultaneously. The paired nanofins control the refractive index on the meta-surface and are tuned to result in different time delays for the light passing through different fins, ensuring that all wavelengths reach the focal spot at the same time.
Another challenge was that the SEAS team need to ensure the outgoing wavelengths from all the different points of the metalens arriving at the focal point simultaneously. By combining two nanofins into one element, they tuned the speed of light in the nanostructured material, to ensure that all wavelengths in the visible are focused in the same spot, using a single metalens. This dramatically reduces thickness and design complexity compared to composite standard achromatic lenses, according to Wei Ting Chen, a postdoctoral fellow at SEAS and first author of the paper.
The researchers hope to increase the diameter of their lens to about 1 cm. This would open a whole host of new possibilities, such as applications in virtual and augmented reality.
Shrinking microscope lenses. Credit: Science