Optical Materials of Tomorrow

MARIE FREEBODY, CONTRIBUTING EDITOR, marie.freebody@photonics.com

From silicon photonics and quantum dots to metamaterials and carbon nanotubes, innovative materials promise a faster, brighter and more integrated future. While some materials have already enjoyed a measure of commercial success, for others, the future promises anything from niche applications right up to market disruption and displacement of existing technologies.

With the rise of high-performance digital systems and the expansion of internet traffic, the exigency for advances in optical materials is growing. At Intel, serious work is underway to introduce silicon photonics to remove networking bottlenecks, eventually replacing copper wires and interfaces with optical fibers.

Highly efficient, highly saturated red, green and blue printed QLEDs. The emitting area of each QLED is 20 × 20 mm. Courtesy of QD Vision Inc.

“The predicted growing need for optical interconnections in applications like optical waveguides, optical data storage, holographic parameters and recording materials, and laser crystals is leading to extensive research being undertaken in the field,” said Rakesh Singh, assistant manager of semiconductors at Allied Market Research, a market report specialist based in Portland, Ore. “The pioneering research in the parallel optical interconnections for cloud computing such as data transfer between backplanes, boards and chips, as well as within the chip, is the latest advancement in optical material.”

Earlier this year, unprecedented storage of 360 TB/disc was demonstrated by scientists at the University of Southampton in England. Using nanostructured glass, the team from the university’s Optoelectronics Research Center developed five-dimensional (5D) digital data with a virtually unlimited lifetime at room temperature and thermal stability up to 1000 °C.

Data is recorded using a femtosecond laser, which writes in three layers of nanostructured dots separated by 5 mm. A very stable and safe form of portable memory, the technology could be useful for organizations with large archives, such as national archives, museums and libraries, to preserve their information and records.

Metamaterials

A material that has captured the imagination and the headlines since it was first fabricated in 2006 is metamaterials. David Smith at Duke University in Durham, N.C., and fellow researchers found that when precisely arranging photonic crystals into patterns of two or more distinct materials, light would suddenly behave very strangely.

Rather than following the usual rules of nature, electromagnetic radiation could be diffracted in such a way as to be confined within the material — and the promise of invisibility cloaks was born.

Fast-forward 10 years to today and optical cloaking has yet to “materialize,” leaving some scientists dismissing optical metamaterials as no more than a headline-grabbing fantasy. But there are others who are hard at work, dogged in their attempts to bring optical metamaterials to reality.

According to a report compiled by analyst Andrew McWilliams and published by technology market research report publishers BCC Research, based in Wellesley, Mass., the total global market for metamaterials was estimated at $307.7 million in 2015, but is expected to grow to more than $1 billion by 2020, and nearly $2.5 billion by 2025.

Today’s plasmonic metamaterials incarcerate and enhance light for use as sensor elements in micrometer-sized arrays for the next generation of biosensors.

“The most remarkable development of plasmonics is its application in surface-enhanced spectroscopy in which it acts as the surface for molecule absorption in order to enhance Raman scattering,” Singh said. “Researchers are making efforts for fabricating photonic chips for fast and reliable biological detection. It will take up almost three to four years for these applications to become reality.”

A scientist with a keen focus on bringing about more commercial applications is professor Ortwin Hess, Leverhulme Chair in Metamaterials and Deputy Head of the Condensed Matter Theory group at Imperial College London.

“After all the exciting general discoveries related to imaging, cloaking and stopping of light, research and attention in the optical metamaterials field has on the one hand continued to push boundaries and explore new functionalities, but at the same time also become more application-oriented,” Hess said.

Examples of advances in optical metamaterials include the search for ways to avoid or compensate for losses, to look for new familiies of materials — particularly compound semiconductors, to facilitate integration with lasers and LEDs, and to provide an exciting platform for new types of lasers and emitters.

“The fact that we now even discuss applications and functionalities that work in scales much smaller than the wavelength of light is truly revolutionary and was not around in optics 15 years ago — and still not present in most textbooks,” Hess added.

This graphic helps to capture the differences in BT color standards. The multicolored horseshoe-shaped area represents all the colors that the average human eye can detect. The area within the dashed gray line, labeled ‘Pointer’s Gamut,’ contains all the colors found in nature. The solid dark gray line labeled BT.709 represents the colors that a standard HD TV can reproduce. Finally, BT.2020 and Nanosys Quantum Dot (QD) is also plotted here. The U′, V′ axes are arbitrary values on the color space chart introduced by the CIE (International Commission on Illumination). Courtesy of Nanosys Inc.

Hess has recently begun working with a company in Cambridge, England, to design and realize CMOS-compatible “metasurface” structures for thermal emitters used in gas sensors. Such “metasurfaces” enable radiation to be controlled by spatially selective patterning.

This innovation can also be used for circular polarization control and beamsplitters for circularly polarized light. This technology is expected to gain significant importance in the coming years and set a new paradigm in the world of optical materials and its future applications.

Incorporating metamaterial concepts into compound semiconductor platforms could open up some promising avenues to explore. However, Singh cautions that an application such as nano-sensing is unlikely to leave the lab — or even the theoretical modeling stage — anytime soon.

Carbon nanotubes

Thanks to their unique structure, carbon nanotubes (CNTs) offer an almost unlimited number of configurations, each of which offer intriguing optical properties.

When a thin sheet of carbon is rolled into a nanotube, it forms a tube with a particular diameter and twist, defined as its chirality. Each chirality absorbs a narrow range of optical wavelengths, and because several hundred different chiralities are possible, scientists can access more parts of the electromagnetic spectrum than ever before.

The spectrum created by Nanosys Hyperion Quantum Dots enables displays to create greater than 90 percent of the BT.2020 UltraHD TV broadcast standard for color without requiring an exemption to the European RoHS Directive. Courtesy of Nanosys Inc.

“Owing to their excellent conductive properties, CNT-enhanced materials are developed for flexible electronics and transparent coatings,” said Alexandre Clerbaux, marketing and business development manager at Nanocyl SA, Belgium. “Such materials are used for touchscreens to replace scarce resources like indium tin oxide and for novel organic photovoltaics systems.”

Vantablack, the so-called “super-black” material, is so dark that human eyes struggle to discern its dimension and shape. Courtesy of Surrey NanoSystems.

When it comes to making solar cells, optical materials must be combined in such a way as to maximize the number of charge carriers generated and to then efficiently transfer them to their respective electrodes. CNTs offer the opportunity to create lightweight, flexible solar cells that can be processed at low temperatures — making them cheaper to make than their silicon counterparts.

There have been tremendous advances in recent years, but market viability has yet to be achieved. With today’s CNT photovoltaics increasing their power conversion efficiencies, combined with the additional benefits of low-temperature processing and their lightweight flexible nature, scientists are convinced that they will one day become commercially attractive.

Super-black coating

As well as applications in solar energy generation, CNTs are being added to other materials to absorb light so effectively that the human eye strains to process it. Vantablack, the so-called “super-black” material, was created by U.K.-based Surrey NanoSystems Ltd. by growing a CNT forest directly on a sheet of aluminum.

Vantablack was created by U.K.-based Surrey NanoSystems by growing a CNT forest directly on a sheet of aluminum. Courtesy of Surrey NanoSystems.

“This material is so dark that human eyes struggle to discern its dimension and shape — a phenomenon that gives an impression as though one was looking into a black hole,” said Nanocyl’s Clerbaux.

Vantablack holds the world record as the darkest man-made substance and although originally developed for satellite-borne blackbody calibration systems, its unique optical properties make it ideal for a host of light-suppression and light-management problems. Applications include optical power dumps in laser and visual projection systems; aesthetic purposes in luxury goods and artworks; solar energy absorbers and concentrators; and cold shields in infrared imaging systems.

Quantum dots

Quantum dots (QDs) have gone from hype to reality. Great strides have been made in quantum efficiencies, reliability and color control, so much so that QD-based TVs or QD monitors are now available to buy.

White QLED comprising a mixture of solution-processed QDs with high color rendering index. Courtesy of QD Vision Inc.

“There has been a lot of advancement in commercializing QD technology over the past five years, solving the last mile problem including reliability, scalability and cost,” said Peter Kazlas, director of Advanced Product Development at QD Vision Inc., in Lexington, Mass. “Advanced material development takes a lot of time and commitment, but it’s worth it. Samsung invested early, and the most, in QD material development and that’s why they are leading the LCD industry today with their premium QD-based SUHD TVs even outpacing OLED TV sales.”

Most of today’s efforts are focused on packaging and integrating quantum dots into displays in new ways that enable lower cost and better performance. This includes integrating QDs into display color filters, which can more than double the power efficiency of a display, and developing flexible, emissive QD displays than can be printed at low cost.

“QLEDs that operate in electroluminescent mode present a leap-frog opportunity over OLED technology,” Kazlas said. “Better color, lower voltage and printability is a winning combination for next-generation displays and lighting.”

Two vials of photoluminescent quantum dots (right) next to a prototype blue electroluminescent QLED device (left). Courtesy of Nanosys Inc.

Nanosys Inc., QD specialists based in Milpitas, Calif., are currently working on QLEDs, which it believes is the future of QDs for displays.

“We are developing QLEDs, using the quantum dots as an emissive material — think OLED display but with quantum dots instead of organic emitters,” said Jeff Yurek, corporate communications manager at Nanosys Inc. “Today, QD displays are built just like LED displays. The quantum dots are added to the backlight of the display in the form of a translucent plastic film that’s loaded with dots. In this mode, the QDs are improving existing LED displays by enabling them to be more power efficient and deliver better color.”

Although the quantum efficiency of red and green QLEDs basically matches those of commercial OLEDs with much better color, the efficiency of blue devices remains an issue — as does large-scale printing and reliability. These challenges must first be overcome before the market can be reached.

While the timeline for Nanosys’ QLED displays remains confidential, its Hyperion Quantum Dots was launched in May this year. The technology enables displays to deliver over 90 percent of the BT.2020 (latest color standard that replaces BT.709) Ultra HD broadcast standard for color gamut.

This brings a significantly wider, more accurate range of colors to TV screens that Yurek says matches the range of colors found in the natural world, making watching TV more like looking out of a window. Crucially, this development comes with cadmium levels below the 100-ppm limit established by the European RoHS Directive.

Display makers currently have a particular interest in cadmium levels since rumors emerged that European policy makers may put to an end the current RoHS exemption for displays containing more than 100 ppm of cadmium by 2018.

“This left display makers wondering how they would support the BT.2020 color gamut standard in the next few years without an exemption,” Yurek said. “This is accomplished by combining an entirely cadmium-free red quantum dot with a green quantum dot engineered to have an exceptionally narrow emission spectrum and ultra-low cadmium.”