Manufacturing Bits: Feb. 8

Metalens for AR/VR
The Harvard John A. Paulson School of Engineering and Applied Sciences has developed a new lens technology for use in next-generation virtual and augmented reality systems.

Researchers have developed a so-called metalens technology. The two-millimeter achromatic metalens is capable of focusing the RGB (red, green, blue) colors at once without any aberrations.

Today, several companies are developing next-generation AR/VR headset and glasses. Virtual reality (VR) enables users to experience 3D virtual environments. Augmented reality (AR) takes computer-generated images and overlays them on the system.

Today’s AR/VR systems rely on bulky and curved lenses. Cameras and optical instruments also make use of multiple curved lenses of different thicknesses and materials.

These types of lenses are capable of correcting aberrations, but they are bulky and expensive. So, the goal is to develop a new lens technology, which presents some challenges. For example, focusing all the colors of the spectrum in today’s single lens technologies is problematic. The wavelengths of the color red move faster through the lens than blue. These two colors will reach the same location at different times, which in turn creates image distortions known as aberrations, according to Harvard.

In 2018, Harvard devised a metalens, which a lens with a flat surface. Based on various nanostructures, the metalens can focus the entire visible spectrum of light in the same spot with high resolutions.

The metalens make use of titanium dioxide nanofins, which equally focuses wavelengths of light. This in turn can eliminate chromatic aberrations.

But these lenses were too small for practical use. Now, the researchers have developed a larger achromatic metalens. They have developed a miniaturized display for virtual/augmented reality applications.

“This state-of-the-art lens opens a path to a new type of virtual reality platform and overcomes the bottleneck that has slowed the progress of new optical device,” said Federico Capasso, a professor at Harvard.

“Using new physics and a new design principle, we have developed a flat lens to replace the bulky lenses of today’s optical devices,” said Zhaoyi Li, a postdoctoral fellow at Harvard. “This is the largest RGB-achromatic metalens to date and is a proof of concept that these lenses can be scaled up to centimeter size, mass produced, and integrated in commercial platforms.”

Optical coatings using FROCs
The University of Rochester and Case Western Reserve University have developed a new class of optical coatings that can be used on filters to reflect and transmit colors at high purities.

The coatings, called Fano Resonance Optical Coatings (FROCs), are ideal for solar and other applications.

Optical coatings are key materials, which are used for every optical instrument. Optical coatings are used to improve the reflectivity of certain wavelengths of light. They are also used to improve the transmission rates in a system.

In one simple example, the coatings on tinted eyeglasses reflect, or “block out,” harmful blue light and ultraviolet rays, according to researchers from the University of Rochester and Case Western.

Until now, optical coatings were unable to simultaneously reflect and transmit the same wavelength or color. FROCs, however, can transmit and reflect the same color simultaneously. The technology is based on a scattering phenomenon called Fano resonance. The phenomenon was named after the physicist Ugo Fano.

Researchers have found a way to leverage the Fano resonance effect in optical coatings. They applied a 15nm film of germanium on a metal surface. This in turn creates a surface capable of absorbing a broad band of wavelengths.

“They combined that with a cavity that supports a narrowband resonance. The coupled cavities exhibit Fano resonance that is capable of reflecting a very narrow band of light,” according to the University of Rochester and Case Western.

“We observed that semi-transparent FROCs can transmit and reflect the same color as a beam splitter filter, a property that cannot be realized through conventional optical coatings,” said Mohamed El Kabbash of the University of Rochester in Nature Nanotechnology, a technology journal. Others contributed to this work. “Finally, FROCs can spectrally and spatially separate the thermal and photovoltaic bands of the solar spectrum, presenting a possible solution to the dispatchability problem in photovoltaics, that is, the inability to dispatch solar energy on demand. Our solar thermal device exhibited power generation of up to 50% and low photovoltaic cell temperatures (~30 °C), which could lead to a six-fold increase in the photovoltaic cell lifetime.”

Chunlei Guo, professor at Rochester’s Institute of Optics, added: “The narrowness of the reflected light is important because we want to have a very precise control of the wavelength. Before our technology, the only coating that could do this was a multilayered dielectric mirror, that is much thicker, suffers from a strong angular dependence, and far more expensive to make. Thus, our coating can be a low-cost and high-performance alternative.”