Metalenses are flat surfaces that use nanostructures to focus light. These surfaces promise to revolutionize optics by replacing the bulky, curved lenses currently used in optical devices with a simple and flat surface. But metalenses have remained limited in the spectrum of light that they can focus on well.
A team of researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) has developed the first single lens that can focus the entire visible spectrum of light—including white light—in the same spot and in high resolution. This has only been achieved in conventionally by stacking multiple lenses.
Focusing on the entire visible spectrum and white light— a combination of all the colors of the spectrum --- is so challenging because each wavelength moves through materials at different speeds. For example, red wavelengths will move through glass faster than blue, so the two colors reach the same location at different times, which results in different foci. This creates image distortions called chromatic aberrations.
Cameras and optical instruments use multiple curved lenses of different thicknesses and materials in order to correct this irregularity, which adds to the bulk of the device.
"Metalenses have advantages over traditional lenses," said Federico Capasso, the Robert L. Wallace professor of applied physics and Vinton Hayes senior research fellow in electrical engineering at SEAS and senior author of the research. "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 Harvard Office of Technology Development (OTD) has protected the intellectual property that relates to this project and is exploring commercialization opportunities.
The metalenses developed by Capasso and his team have arrays of titanium dioxide nanofins to equally focus wavelengths of light and eliminate chromatic aberration. Other research has demonstrated that different wavelengths of light could be focused but at different distances by optimizing shape, width, distance and height of the nanofins.
In the latest design, the researchers created units of paired nanofins that control the speed of different wavelengths of light simultaneously. The paired nanofins control the refractive index on the metasurface and are tuned to result in different time delays for the light passing through different fins, which ensures that all wavelengths reach the focal spot at the same time.
"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," said Wei Ting Chen, a postdoctoral fellow at SEAS and first author of the paper. "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."
"Using our achromatic lens, we are able to perform high quality, white light imaging. This brings us one step closer to the goal of incorporating them into common optical devices such as cameras," said Alexander Zhu, co-author of the study.
The next step for the research team is to scale up the lens to about 1 cm in diameter. This would open a host of new possibilities, like applications in virtual and augmented reality.
This research was published in Nature Nanotechnology.