Optics: Metalens, “beyond” lens
The concept of metamaterials has been around for a while, but the last two years has seen a surge in scientific and fabrication breakthroughs that could soon usher in a new age of optical techniques and capabilities based on metalenses. The term “metamaterials” was coined from the Greek word meta, which means “beyond”-metamaterials go beyond normal materials in the sense that the natural properties of the materials have been altered.
Metalenses are tiny optical elements that can manipulate electromagnetic waves (light)-just like traditional lenses-but are thinner than a sheet of paper. The technology is exciting to developers across many industries due to their small size, ease and cost-efficiency to manufacture at scale, and their potential to revolutionize multiple optical applications.
A brief introduction to metamaterials and metalenses and how they work.
The history of modern technology development has been a steady progression towards miniaturization, as computer chips and other electronic components have gotten increasingly smaller. For example, all the computing power that NASA used for the 1969 moon landing was far less than what now fits in your pocket on an average cell phone.
However, until metamaterials, there had been no way to significantly shrink lenses used in cameras, microscopes, telescopes, and other optical equipment. The size lenses has been constrained by the material properties ( refractive index and dispersion) of glass-or in recent decades, plastics. In essence, we are still using a medieval tool (the glass lens) and 19th century optical technology that we’ve not been able to improve on beyond achieving greater precision in shaping and grinding. 1
The Structure of Metalenses
Traditional lenses work by bending light waves. Lenses are formed with areas of varying thickness, so that as a wavefront passes through the lens, light waves at different parts of the lens are refracted at different angles. Often multiple individual lenses are stacked together (called compound lenses) to direct and control light in specific ways.
A typical spherical convex (converging) lens bends light waves to meet at a focal point. (Image: Mini Physics)
In contrast to traditional lenses, the concept of a metalens is a single thin, flat structure with multiple waveguides-like tiny pillars-arranged in specific patterns. These pillars, which have also been likened to antennae, are nanoelements made from titanium dioxide (TiO2) that measure approximately 600 nanometers (nm) long-shorter than the length of 10 hydrogen atoms laid side by side.
Because metalenses are flat (planar) and ultra-thin, they do not produce chromatic aberrations-they are “achromatic”-because all wavelengths of light pass through virtually simultaneously. Their advantages also include tunable dispersion-the ability to manipulate how colors of light are dispersed-as opposed to glass or other tradition materials that have fixed dispersions. Additionally, metasurfaces can be mass produced in existing CMOS semiconductor foundries, enabling massively-parallel wafer-scale integration of optical systems. 2
Examples of two possible metalens patterns, captured by electron microscope. (Image source: Science Magazine video)
A significant milestone in the development of metalenses came in 2016 when researchers from Harvard University’s John A. Paulsen School of Engineering and Applied Sciences (SEAS) demonstrated the first planar lens that worked efficiently within the range of visible light, covering the entire spectrum of colors from red to blue. Calling the waveguides “nanofins”, the SEAS team, led by Federico Capasso, designed their metalens to focus light onto a single point approximately 400 nm across. Their metalens was the first to be able to focus the entire spectrum of visible light and offered optical performance better than any current commercial lenses.
This development was followed in 2018 by an announcement of the first two-dimensional metalenses with nanostructures the same size as the wavelengths of light that pass through them, created by a team from the University of Washington and Taiwan’s National Tsing Hua University.
Four ultra-thin metalenses developed by University of Washington / Taiwan researchers visualized under a microscope. (Image: Nano Letters)
Soon after, a Columbia University Engineering team created the first flat lens capable of correctly focusing a large range of colors of any polarization to the same focal spot without the need for any additional elements. Only a micron thick, their flat lens uses “meta-atoms” to focus light with wavelengths ranging from 1.2 to 1.7 microns-in the invisible near-infrared range-to the same focal spot.
“Broadband imaging in the near-infrared using a single meta-lens, showing the ability of the lens in correcting chromatic aberrations, and manufacturing of meta-lenses using planar fabrication techniques similar to those used in the integrated circuits industry.”3
Since then, additional metalens developments have come fast and furiously-we could almost call 2019 the Year of the Metalens.
- In January, the SEAS team at Harvard announced the development of a new “polarization-insensitive metalens comprising non-symmetric nanofins that can achromatically focus light across the visible spectrum without aberrations.”4Previously metalenses could only focus circular non-polarized light, effectively losing half of the incident light which did not match the polarization.
- In May, mathematicians at the Massachusetts Institute of Technology (MIT) published the results of their work to develop a computational technique that quickly determines the optimal makeup and arrangement of metalens nanoelements. This will enable makers to design metalens patterns to meet specific objectives, such as controlling colors or creating different shaped beams.
- In July, researchers at the University of Michigan announced a new technique that uses a set of metalenses to focus light into a specific pattern, rather than a single point. This approach has implications for any beam-shaping applications such as laser cutting, annealing, and selective crystallization.
- Also in July, researchers at Saudi Arabia’s King Abdullah University of Science and Technology (KAUST), develop a method of twisting a slack of metalens films to further control the properties of light. For example, they developed a bifocal metalens with controllable focal length and intensity ratio.
Related to metalenses is the rise of wafer-level optics. Wafer-level fabrication and integration of micro-optical components is enabling the application of micro-structure technologies to camera module manufacturing. Essentially, camera lenses can be manufactured using semiconductor processes to imprint individual microlenses on wafers. Wafer-level camera modules have tight tolerances for alignment shape, thickness/uniformity, but can offer more efficient fabrication at high volume.
Metalens technology is now poised to revolutionize cameras, displays, and other optical devices. Some of the exciting potential applications include:
- Chip manufacture — The University of Michigan’s method for creating patterns of light can be applied by chip manufacturers who use light to carve specific patterns onto the surface of a silicon slab. Using metalenses will potentially enable more complex patterns to be produced more efficiently and with a low rate of defects.
- AR/VR/MR — developers of XR systems continue to wrestle with the challenge of incorporating bulky hardware systems into devices to be worn on the head. Metalenses offer the prospect of tiny optical elements incorporated into small, high-performing, lightweight headsets and smart glasses.
- Medicine — the enhanced optical capabilities of metalenses can enable more precise diagnostic imaging than ever before, higher-resolution imaging tools such as endoscopes, and new microscope form factors, enabling radiologists, physicians, and lab technicians to see details that were previously invisible.
A team from Harvard Medical School has been working on developing “optical coherence tomography” (OCT) and a nano-optic endoscope to achieve ultra-high-resolution diagnostic imaging capabilities. (Image Source)
There are still challenges to overcome: “expenses are still high because it is difficult to precisely place nanoscale elements on a centimeter-scale chip…So far metalenses do not transmit light as efficiently as traditional lenses do-an important capability for such applications as full-color imaging. In addition, they are too small to capture a large quantity of light, which means that, at least for now, they are not suited to snapping high-quality photographs.” 5
Nevertheless, meta-technologies hold great promise for the continuing development of optical, imaging, and display systems. With Radiant’s 20+ years of experience in precision measurement of light and color and our work with optical and display systems at both full- and nano-scale, we’re excited to see what comes next.
- Capasso, F., “Flat optics: from high-performance metalenses to structured light”, keynote presentation from SPIE Photonics West, 2018, Mary 9, 2018.
- Devlin, R., “Optical Metasurfaces: From Fundamental Science to Application”, Drexel University College of Engineering, February 13, 2019.
- Chen, W., et al., “A broadband achromatic polarization-insensitive metalens consisting of anisotropic nanostructures”, Nature Communications 10, Article #355, January 21, 2019.
- Evarts, H., “Revolutionary Ultra-thin “Meta-lens” Enables Full-color Imaging”, Columbia Engineering, October 3, 2018.
- Moscatelli, A., “Tiny Lenses Will Enable Design of Miniature Optical Devices”, Scientific American, July 1, 2019.