Smartphone camera technology is growing in leaps and bounds in the past couple of years and continues to be a major point to differentiate their products. More and more technologies are being incorporated including larger sensors, better lenses, ultra-thin lens, optical image stabilization technologies, dual camera optical zoom technology that also improve low light performance and remove noise.
Curved lenses like those in cameras or telescopes are stacked to reduce distortions and clarify images. That’s why high-powered microscopes are so big and telephoto lenses so long. 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 the visible spectrum of light, covering the whole range of colors from red to blue. 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.
NIKON, Sony and Canon are reported to be in race to develop and market curved sensor camera that operates using lens designs with fewer elements, less weight, less light loss, less internal reflection, less distortion and less aberration, all at lower cost. At present Sony is leading the race and already has a patent for a 35mm F1.8 compact camera.
A thinner, flatter lens
The lens can resolve nanoscale features separated by distances smaller than the wavelength of light. It uses an ultrathin array of tiny waveguides, known as a metasurface, which bends light as it passes through. The research is described in the journal Science.
“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,” said Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering, the senior author of the paper. “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.”
“Correcting for chromatic spread over the visible spectrum in an efficient way, with a single flat optical element, was until now out of reach,” said Bernard Kress, partner optical architect at Microsoft, who was not part of the research. “The Capasso Group’s meta-lens developments enable the 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.”
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. The team used titanium dioxide, a material found in everything from paint to sunscreen, to create the nanoscale array of smooth and high-aspect ratio nanostructures that form the heart of the meta-lens. “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.
“Normal lenses have to be precisely polished by hand,” said co-author Wei Ting Chen, a postdoctoral fellow in the Capasso Lab. “Any kind of deviation in the curvature, any error during assembling makes the performance of the lens go way down. Our lens can be produced in a single step — one layer of lithography and you have a high-performance lens, with everything where you need it to be.
“This new breakthrough solves one of the most basic and important challenges, making a visible-range meta-lens that satisfies the demands for high numerical aperture and high efficiency simultaneously, which is normally hard to achieve,” said Vladimir M. Shalaev, professor of electrical and computer engineering at Purdue University, who was not involved in the research.
One of the most exciting potential applications, said Khorasaninejad, is in wearable optics such as virtual reality and augmented reality.
“Any good imaging system right now is heavy because the thick lenses have to be stacked on top of each other. No one wants to wear a heavy helmet for a couple of hours,” he said. “This technique reduces weight and volume and shrinks lenses thinner than a sheet of paper. Imagine the possibilities for wearable optics, flexible contact lenses, or telescopes in space.”
The paper was co-authored by Jaewon Oh and Alexander Zhu of SEAS. It was supported in part by a MURI grant from the Air Force Office of Scientific Research, Draper Laboratory, and Thorlabs Inc.
Flat optical lenses that can be easily mass-produced and integrated with image sensors
A classical lens made of plastic or glass has a curved shape that bends the path of incoming light toward a single focal point. This is because light travels faster through the thinner glass at the edges of the lens than through the thicker glass at the center.
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 flat lens optical system. Their work was published in Nature Communications on November 28.
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.
“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.
This research was funded by Samsung Electronics, the Defense Advanced Research Projects Agency, the National Science Foundation, and the Department of Energy.
Curved Sensor Camera
“A curved image sensor can potentially improve the chief ray incidence angle (CRA) on the sensor, as well as the aberration balancing, image quality, packaging, and manufacturing tolerance sensitivity, “explains Dmitry Reshidko and José Sasian in SPIE. Digital image sensors become less efficient when the incident light is at higher obliquity or larger chief ray incidence angle (CRA) on the sensor. The field of view (FOV) of the mobile camera is large, and the CRA proportional to the FOV. “However, if the image sensor is curved, the light rays get to the sensor in straighter lines and CRA is significantly reduced. Moreover, decreased incidence angles on the sensor reduce crosstalk between adjacent pixels,”
Comparatively, curved sensors can be paired with simpler, flatter lenses with larger apertures – that means more light reaches the sensors. Small f-number lenses provide better quality and low-light imaging, and can accommodate a larger number of sensor pixels, leading to better resolution.
And the process of bending CMOS sensors has a surprising advantageous effect. When you bend a silicon sensor you alter its band gap, lowering the ambient noise level caused by the dark current which continues to move through a pixel in the absence of light.
Sony’s curved CMOS sensor boosts light sensitivity
Announcing its technology in 2014, Sony engineers reported that they have created a set of curved CMOS image sensors using a “bending machine” of their own construction. The engineers claimed their sensor possesses higher sensitivity to light compared to current flat sensors, 1.4 times more sensitive at the center of the sensor, and twice as sensitive at the edges.
The change in geometry allow the sensor to work with a flatter and a larger aperture lens, allowing more light to reach the sensor. Bending the sensor induces strain on the sensor, alters its bandgap that results in lower “dark current” flowing through a pixel, thereby improving the image quality further.
Equally important is the development of mass manufacture method, Sony engineers have produced in the vicinity of 100 full-size sensors with their bending machine.
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