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A classical lens made of plastic or glass has a curved shape that bends the path of incoming light toward a single focal point on a piece of photographic film or an electronic sensor. This is because light travels faster through the thinner glass at the edges of the lens than through the thicker glass at the center.
Conventional cameras, whether used in smartphones or for microscopy, require focusing to ensure that the details of an object are sharp. If there are multiple objects at different distances from the camera, each object must be focused separately. Also, Light captured at the very edges of a curved glass lens does not line up correctly with the rest of the light, creating a fuzzy image at the edge of the frame. To correct this, lenses use extra pieces of glass, adding bulk, complexity, and mass. Conventional cameras also use multiple lenses to keep different colors of light in focus simultaneously.
Researchers have now developed flat lens, lens whose flat shape allows it to provide distortion-free imaging, potentially with arbitrarily-large apertures. The term is also used to refer to other lenses that provide a negative index of refraction. Using a single lens that is about one-thousandth of an inch thick, researchers have created a camera that does not require focusing. Researchers have also developed flat metalens with built-in chromatic aberrations corrections so that a single lens is required
This technology is potentially revolutionary because it works in the visible spectrum, which means it has the capacity to replace lenses in all kinds of devices, from microscopes to cameras to displays and cell phones. The technology offers considerable benefits over traditional cameras such as the ones in most smartphones, which require multiple lenses to form high-quality, in-focus images. Mealens will enable integration of broadband imaging systems in a very compact form, allowing for next generations of optical sub-systems addressing effectively stringent weight, size, power, and cost issues, such as the ones required for high performance AR/VR [augmented reality/virtual reality] wearable displays.
Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering and three partners have started a company called Metalens, targeting applications that do not require achromatic lenses, such as for security cameras, biometrics, authentication, and face recognition.
While lens technology has improved, it is still difficult to make a compact and thin lens. But researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have demonstrated the first flat — or planar — lens that works highly efficiently within a continual bandwidth of colors, from blue to green. This bandwidth, close to that of an LED, paves the way for new applications in imaging, spectroscopy and sensing.
Long-wave infrared (LWIR) imaging refers to imaging in the wavelength band approximately from 8 μm to 12 μm, and is important for applications ranging from defense, medicine, and agriculture to environmental monitoring. In order to attain high transparency, conventional refractive lenses in the LWIR band require materials such as silicon, germanium, or chalcogenide glasses. The weight of these conventional lenses can be too high for many applications. The increased weight limits the range of operation of unmanned aerial vehicles. In addition, such optics render head-mounted night vision goggles heavy, and cause neck and head injuries in soldiers as well as reducing their situational awareness.
Researchers from University of Utah led by Monjurul Meem have demostrated that, by appropriately designing thin Multilevel Diffractive Lenses (MDLs), they can correct for image aberrations, including chromatic aberrations in the LWIR band, and thereby reduce the weight of such lenses by over 2 orders of magnitude when compared to conventional refractive lenses. In addition, since our MDLs are very thin, that is, thickness of ∼λ0, the design wavelength, and the resulting absorption losses are low, we can utilize polymers for the lens material, which are easier to manufacture.
Flat Lens technology
Conventional refractive optics is comprised of curved surfaces and become thicker with increasing resolution. That is, in order to bend light at larger angles, the radius of curvature must be lowered, and, consequently, the lens becomes thicker and heavier. Recently, metalenses have been proposed as a means to reduce the thickness of refractive lenses. Metalenses are comprised of constituent units that act as scattering elements (of subwavelength thickness), which render a prescribed local phase shift to light upon scattering.
Flat lenses require a refractive index close to −1 over a broad angular range. Flat lenses employ metamaterials, that is, electromagnetic structures engineered on subwavelength scales to elicit tailored polarization responses.
Left-handed responses typically are implemented using resonant metamaterials composed of periodic arrays of unit cells containing inductive–capacitive resonators and conductive wires. Negative refractive indices that are isotropic in two and three dimensions at microwave frequencies have been achieved in resonant metamaterials with centimetre-scale features. Metamaterials can image infrared, visible, and, most recently, ultraviolet wavelengths
The first flat lens used a thin wafer of silicon 60 nanometers thick coated with concentric rings of v-shaped gold nanoantennas to produce photographic images. The antennas were systematically arranged on the silicon wafer and refract the light so that it all ends up on a single focal plane, a so-called artificial refraction process. The antennas were surrounded by an opaque silver/titanium mask that reflected all light that did not strike the antennas. Varying the arm lengths and angle provided the required range of amplitudes and phases. The distribution of the rings controls focal length.
later flat lens is made of a sandwich of alternating nanometer-thick layers of silver and titanium dioxide. It consists of a stack of strongly-coupled plasmonic waveguides sustaining backward waves and exhibits a negative index of refraction regardless of the incoming light’s angle of travel. The waveguides yield an omnidirectional left-handed response for transverse magnetic polarization. Transmission through the metamaterial can be turned on and off using higher frequency light as a switch, allowing the lens to act as a shutter with no moving parts.
With the advances in micro- and nanofabrication techniques, continued miniaturization of the conventional optical lenses has always been requested for various applications such as communications, sensors, data storage, and a wide range of other technology-driven and consumer-driven industries. Specifically, ever smaller sizes as well as thinner thicknesses of micro lenses are highly needed for subwavelength optics or nano-optics with extremely small structures, particularly for visible and near-IR applications. Also, as the distance scale for optical communications shrinks, the required feature sizes of micro lenses are rapidly pushed down.
Researchers have also utilized the excellent properties of graphene oxide to provide novel solutions to overcome the challenges of current planar focusing devices. Specifically, giant refractive index modification (as large as 10^-1), which is one order of magnitude larger than the current materials, between graphene oxide (GO) and reduced graphene oxide (rGO) have been demonstrated by dynamically manipulating its oxygen content using direct laser writing (DLW) method. As a result, the overall lens thickness can be potentially reduced by more than ten times.
Also, the linear optical absorption of GO is found to increase as the reduction of GO deepens, which results in transmission contrast between GO and rGO and therefore provides amplitude modulation mechanism. Moreover, both the refractive index and the optical absorption are found to be dispersionless over a broad wavelength range from visible to near infrared. Finally, GO film offers flexible patterning capability by using the maskless DLW method, which reduces the manufacturing complexity and requirement.
In the near future, meta-lenses will be manufactured on a large scale at a small fraction of the cost of conventional lenses, using the foundries that mass-produce microprocessors and memory chips.”
Meatsurface based Flat lens to work across a continuous bandwidth allows new control of light reported in 2016
Engineers at Caltech have developed a system of flat optical lenses that can be easily mass-produced and integrated with image sensors, paving the way for cheaper and lighter cameras in everything from cell phones to medical devices.
The technology relies on stacking two metasurfaces to create a lens system that can capture and focus light from a 70-degree angular range, making the technology useful for the first time in microscope and camera imaging applications. Metasurfaces are sheets of material whose electromagnetic properties can be altered on demand. Each metasurface is dotted with tens of millions of silicon nanoposts or cylinders smaller than a micron across that alter the way light passes through them.
The cylinders just 600 nanometers tall and with varying diameters in the hundreds of nanometers. (For scale, a strand of human hair is 100,000 nanometers wide.) Light travels faster through nanoposts with smaller diameters than through nanoposts with larger diameters, so controlling the width of the nanoposts allows the engineers to finely adjust the path of light passing through the metasurface to create flat lenses.
To design an achromatic lens — a lens without chromatic dispersion — the team optimized the shape, width, distance, and height of the nanopillars that make up the heart of the metalens. As in previous research, the researchers used abundant titanium dioxide to create the nanoscale array.
In order to focus red, blue, and green light — light in the visible spectrum — the team needed a material that wouldn’t absorb or scatter light, said Rob Devlin, a graduate student in the Capasso Lab and co-author of the paper. “We needed a material that would strongly confine light with a high refractive index,” he said. “And in order for this technology to be scalable, we needed a material already used in industry.
“We wanted to design a single planar lens with a high numerical aperture, meaning it can focus light into a spot smaller than the wavelength,” said Mohammadreza Khorasaninejad, a postdoctoral fellow in the Capasso Lab and first author of the paper. “The more tightly you can focus light, the smaller your focal spot can be, which potentially enhances the resolution of the image.” The team designed the array to resolve a structure smaller than a wavelength of light, around 400 nanometers across. At these scales, the meta-lens could provide better focus than a state-of-the art commercial lens.
Metasurfaces like these can be easily mass produced, much the way computer chips are,” Caltech’s Andrei Faraon says. “That means this could be a cheap and easily scalable way to create tiny lenses just a few millimeters in diameter.”
In addition, the lenses can be seamlessly integrated with CMOS (complementary metal-oxide semiconductor) image sensors because they are made using the same materials and fabrication techniques. CMOS image sensors are the tiny chips that underpin digital photography, and were developed at JPL. Flat, lightweight, and cheap lenses are in demand for various consumer electronics equipped with cameras, or medical devices such as endoscopes, Faraon says. Next, the team plans to integrate these lenses into miniaturized cameras and microscopes, and extend their functionality and operation bandwidth.
Faraon, assistant professor of applied physics and materials science in Caltech’s Division of Engineering and Applied Science, collaborated with Caltech postdoctoral researcher Amir Arbabi and Seunghoon Han from Samsung Electronics to develop the lens system. Their work was published in Nature Communications on November 28.This research was funded by Samsung Electronics, the Defense Advanced Research Projects Agency, the National Science Foundation, and the Department of Energy.
Functional optical metalens made from 2D materials reported in Nov 2018
New types of ultrathin materials such as hexagonal boron nitride and molybdenum disulfide could replace the conventional glass lenses used in cameras and imaging systems. Typically the new sorts of metalenses are not made of glass. Instead, they consist of materials constructed at the nanoscale in arrays of columns or fin-like structures like Fresnel lenses.
A team from the University of Washington, MD, USA, (UW) and National Tsing Hua University, Taiwan, have developed functional metalenses that measure between one-tenth to one-half the thickness of the wavelengths of light that they focus. The metalenses, constructed from layered 2D materials, are as thin as 190 nm across. The work has been described in Nano Letters.
“This is the first time that researchers have shown that it is possible to create a metalens out of 2D materials,” commented article author Arka Majumdar, an assistant professor of physics and of electrical and computer engineering at UW. The design principles can be used for the creation of metalenses with more complex, tunable features, added Majumdar, who is also a faculty researcher with the UW’s Molecular Engineering & Sciences Institute and Institute for Nano-Engineered Systems.
Overcoming design limitation
The challenge in this project was to overcome an inherent design limitation in metalenses: in order for a metalens material to interact with light and achieve optimal imaging quality, the material had to be roughly the same thickness as the light’s wavelength in that material. This restriction ensures that a full zero to two-pi phase shift range is achievable, which guarantees that any optical element can be designed. For example, a metalens for a 500nm lightwave (green light) would itself need to be about 500nm thick, although this thickness can be decreased as the refractive index of the material increases.
Majumdar and his team have synthesized functional metalenses that are much thinner than this theoretical limit — one-tenth to one-half the wavelength. First, they constructed a metalens from sheets of layered 2D materials; the team employed widely-studied 2D materials such as hexagonal boron nitride and molybdenum disulfide. A single atomic layer of these materials provides a small phase shift that is unsuitable for efficient lensing. So instead the UW team used multiple layers to increase the thickness, although the thickness remained too small to reach a full two-pi phase shift.
“We had to start by deciding what type of design would yield the best performance given the incomplete phase,” commented co-author Jiajiu Zheng, a doctoral student in electrical and computer engineering. To make up for the shortfall, the team employed mathematical models that were originally formulated for liquid-crystal optics. These, in conjunction with the metalens structural elements, allowed the researchers to achieve high efficiency even if the whole phase shift is not covered.
Promising work
In addition to achieving a new approach to metalens design at record-thinness levels, the team says that this work shows the promise of making new devices for imaging and optics entirely out of 2D materials. “These results open up an entirely new platform for studying the properties of 2D materials, as well as constructing fully functional nanophotonic devices made entirely from these materials,” commented Majumdar. “Additionally, these materials can be easily transferred on any substrate, including flexible materials, paving a way towards flexible photonics.”
Graphene optical lens a billionth of a meter thick breaks the diffraction limit
Researchers from Swinburne University of Technology has developed a graphene microlens one billionth of a meter thick weighing just a microgram and that can take sharper images of objects of the size of a single bacterium. At the same time, it has a precise and adjustable three-dimensional focus that allows a detailed view of objects at wavelengths ranging from visible to near infrared.
This technology has overcome one of the key obstacles in advances in optical microscopes or its lens, the diffraction limit that is focusing less than half the wavelength of light. One of the approaches has been to explore the use of ultrathin flat lenses that are etched with concentric circles and act like tiny Fresnel lenses.
Swinburne’s breakthrough came when Xiaorui Zheng, a PhD student at the Centre for Micro-Photonics, used graphene oxide to form a lens. This material allowed the team to make ultrathin flat lenses that are 300 times thinner than a sheet of paper and weigh a microgram.
The team, led by Associate Professor Baohua Jia, have earlier developed a three-dimensional printer that could quickly and cheaply produce the lens using a sprayable graphene oxide solution. Lasers were used to precisely pattern the surface, creating three concentric rings of reduced graphene oxide, which enabled its extraordinary focus.
Once the technology is mature, the team sees a gamut of applications like thinner mobile phones with thermal imaging capabilities, creating a much smaller, safer and more sophisticated endoscope for noninvasive surgery by integrating the lens with fibre , and to increase the efficiency of photonic chips in supercomputers and superfast broadband distribution.
In the future the technology has potential to reduce the size and weight of mobile phones in which cameras are currently dependent on thick and heavy lenses. It would also allow focusing of the infrared spectrum, allowing thermo-imaging and possible remote medical diagnosis.
This would also useful to reduce the cost of manufacturing of nanosatellites as well as obtaining better pictures of Earth and space. The optical lenses used by current nanosatellites weigh a couple of hundred grams whereas the new lenses developed by the Swinburne group weigh just a microgram.
Metalens Technology Expected to Reach Commercial Market in 2022
Metalenz, the Harvard University-originating startup that last week received exclusive license to flat-optics innovations developed in the lab of Federico Capasso, said that the long-awaited metalens technology will likely see introduction into the commercial market as early as next year. Rob Devlin, CEO and co-founder of Metalenz, told Photonics Media that he expects the company to ship its first consumer device in early 2022.
The technology, which manipulates light at the nanometer scale through wafer-thin metasurfaces, was introduced in Capasso’s group in 2011. Early demonstrations from Capasso and his then-student Nanfang Yu, now a professor of applied physics at Columbia University, showed an unseen level of control over light with just a single layer of nanoscale antennas. Ten years ago, the results were promising, though inefficient and unable to form high-quality images.
In 2016, during Devlin’s time in Capasso’s lab pursuing his Ph.D., the group advanced the technology by implementing nanofabrication methods that improved efficiency to the point where they could be used in practical applications, and be developed in methods similar to integrated circuits.
The metalenses developed by Capasso and his team use arrays of titanium dioxide nanofins to equally focus wavelengths of light and eliminate chromatic aberration. Previous research demonstrated that different wavelengths of light could be focused but at different distances by optimizing the shape, width, distance, and height of the nanofins. 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, ensuring that all wavelengths reach the focal spot at the same time.
This gives us a lot of freedom to improve the performance and to overall make very complicated systems much smaller in size and in some cases actually add functionality,” said Alex Zhu, Capasso Group member and graduate student. In current practices, multiple lenses are required to focus all of the wavelengths in the same spot without distortion. Capasso told Photonics Media that in order to correct the distortion, six or seven lenses may be required, as is the case with cameras in cell phones.
“With a single lens or just a couple, you can actually do the work of traditional optics,” he said. “They are thinner and it’s easier to stack them and align them optically, and it’s easier to actually correct the distortions.” Capasso added that it is also easier to machine the lenses because they don’t require the same curvature as refractive lenses. This technology could change the way that cameras are made.
The technology, Rob Devlin, CEO and co-founder of Metalenz said, is able to provide an entirely new tool for designing compact and complex optical systems by providing functionality that can’t be achieved with conventional lenses. “However, they also come with their own set of challenges,” Devlin said.
One of those challenges involves stray light that the metalenses introduce. This light is not found in ordinary refractives, and, if not addressed through careful design and process controls, the performance of certain sensing modules can suffer. The company, Devlin said, was able to develop a set of tools tailored to meet those issues without compromising the benefits of the technology.
Namely, Devlin and Capasso see implementation potential for the meta-optics in applications ranging from those in the health care and automotive industries to consumer electronics. “The metalens platform has the potential to drive a revolution in imaging and sensing, from the ubiquitous cameras in cellphones, cars, and self-driving vehicles to AR/VR, and in the future to widespread use in drones and CubeSats,” Capasso said.
References and Resources also include:
https://www.photonics.com/a63628/Capasso_Group_Develops_Flat_Lenses_with_Same
