Optical Materials Bending the Rules, Shaping Our World

MARIE FREEBODY, CONTRIBUTING EDITOR

From the emergence of diverse 2D materials (perovskites for efficient solar cells) and twisted bilayer graphene (superconductive at a “magic angle”) to the boom in polymer science and the promise of quantum photonics, materials science is rapidly evolving.

An artistic rendering of discrete random media made of nanoparticles, with a light field scattered by the nanoparticles. Courtesy of LENS at the University of Florence.

In the process of research, intermediary results sometimes surprise and can lead researchers to explore avenues that differ from their original intent. Materials research is vast and prone to unexpected changes in direction.

More than 40 years ago, Corning scientists developed a highly pure optical glass that could transmit light over long distances. The material — optical fiber — would become instrumental in a world-changing technological achievement: the internet. The fibers that form its backbone are also a critical component of semiconductor lasers and microprocessors, which together have revolutionized communications (Figure 1).

Figure 1. Glass fiber is made using a chemical vapor deposition process that results in superb optical qualities. Courtesy of Corning.

Currently, more than 2 billion kilometers of optical fiber link people, businesses, communities, countries, and continents. This fiber enables electronic commerce, tele-education, telemedicine, social media, and even connected cars.

“I believe the next two decades will be the decades of chemistry and materials science,” said Waguih Ishak, division vice president and chief technologist at California-based Corning Research and Development Corp. Ishak also serves as an adjunct professor in Stanford University’s Department of Electrical Engineering. “We need novel materials that can sense signals, process information, store information, and interact with users in unprecedented ways.”

According to Ishak, if these materials can be produced, numerous breakthrough products will be realized. Some of the innovations, he believes, will stem from the intriguing optical behaviors that occur only in and around materials at the nanoscale.

Nanophotonics

At the nanometer level, classical rules of light no longer apply, and unexpected, even fortuitous, things begin to happen. Metamaterials, quantum photonics, and surface plasmon polaritons have all been born out of experiments with materials at the nanoscale.

Incorporating, for example, plasmonic lenses into semiconductor electronics will allow some critical components to shrink in size — which will enable complex and integrated devices, such as advanced sensors and trackers, to be created. Ishak and his team at Corning intend to take advantage of such innovations and are building R&D teams to address opportunities in communications, displays, the automotive industry, and quantum photonics.

Although potential benefits are wide-ranging, the ostensibly infinite ways of experimenting with a material pose a challenge in itself. At the genesis of a project, it is difficult to predict if, or when, an end result will find a use, be marketable, or receive sufficient recognition to warrant further development. Materials informatics and machine learning will play increasingly important roles in helping to make these determinations.

Another challenge is sourcing enough graduates in the field of materials research from top universities. Ishak said he has noticed during the past two decades that many graduates choose to specialize in software-based areas, and he emphasized the need to encourage graduate students to move into the physical science domain.

Polymers

Graduate student Florian Le Roux, from the Department of Physics at Oxford University, and professor Donal Bradley, who has joint appointments at Oxford and King Abdullah University in Saudi Arabia, are studying the intricacies of conjugated polymer-based strong-light interactions, specifically polaritons.

Polaritons are formed when a conjugated polymer is placed inside a microcavity. They are hybrid particles with distinct physical properties and emission characteristics that open up a plethora of new phenomena and possible applications.

“We hope to bring forward a generation of LEDs exhibiting narrow, highly saturated emissions, and polarization-sensitive devices with minimal footprints, thanks to precise control over the polymer structure and patterning of this structure at the submicrometer scale,” Le Roux said.

“In order to achieve this, we use a range of novel techniques, including dip-pen patterning (Figure 2) and noncontact photoalignment layers for thermotropic conjugated polymer liquid crystal orientation” (Figure 3).

Figure 2. A submicron stripe of the β-phase (red) of poly(9,9-dioctylfluorene) (PFO) patterned using dip-pen technology. The β-phase possesses a different refractive index relative to the glassy phase (purple/blue). This patterning process enables the fabrication of nanophotonic elements. See Reference 1. Courtesy of A. Perevedentsev.

Figure 3. Photoluminescence (PL) images of photonic elements made of aligned polymer chains patterned using an underlying azobenzene sulfuric dye photoalignment layer. The PL contrast reveals the orientation of polymer chains within the film. The bright yellow and black regions are respectively oriented parallel (top) and perpendicular (bottom) to the PL excitation light polarization. The weak background is the emission from unoriented polymer chains. See Reference 2. Courtesy of A. Perevedentsev.

Polymer coatings are employed in several other key technologies, including light harvesting in thin-film photovoltaics, solid-state lighting, and xerographic charge generation layers for photocopiers.

“Profoundly novel results can be uncovered in materials that were previously thought to be relatively well understood, and this motivates our goal to continue to advance fundamental knowledge in the field of conjugated polymer science, engineering, and application,” Le Roux said.

As of today, OLED display devices are made predominantly from vacuum-deposited small molecules, a process that brings with it fabrication and yield challenges, especially for intermediate-size RGB high-resolution displays.

Solution-processed materials deposited by coating and printing techniques offer an alternative approach that is gaining traction, whether using conjugated polymers, molecules, or even quantum dots.

“It is clear that semiconducting conjugated polymers have much more to offer in many current applications and should remain a key topic for ongoing study,” Le Roux said.

Creating responsive polymers that can be tuned using an electrical or optical stimulus brings a new level of flexibility to materials. Polymers that can change their shape or refractive index open up applications in microrobotics, biomedical engineering, sensing, and computing.

Soft robotics mimic natural world

Some groups are taking inspiration from the natural world, which provides examples of beautiful photonic architectures — some with outstanding optical performance — that scientists are only recently learning to mimic and adapt (Figure 4). Courtesy of H. Zhang.

Figure 4. A Cyphochilus beetle. Its elytra are covered with small scales containing a disordered nanostructure made of chitin and are famous for their outstanding capability to diffusely reflect light despite their thickness of about 7 µm. Courtesy of Andy Parnell/CC BY 4.0. Courtesy of Andy Parnell/CC BY 4.0.

The retina, for example, changes shape upon absorption of photons and triggers a cascade of processes ending with specific neural signals. Synthetic materials that make use of molecules that change their shape under selective irradiation can make it possible to dynamically control several properties (Figure 5).

Figure 5. A photograph of the diffracted beams from a microstructured beam steering device. The shape-changing diffraction grating made by a photoresponsive material can be controlled by light, tuning the diffracted beam directions. Courtesy of LENS at the University of Florence.

These materials find application in so-called soft robotics, an emerging area that scientists at the European Laboratory for Non-linear Spectroscopy (LENS) at the University of Florence in Italy are exploring. Diederik Wiersma, president of the National Institute of Metrological Research and head of photonics research at LENS, is attempting to make machines from smart polymers with “intrinsic intelligence” that are capable of simple decision-making.

“For polymeric robots, thanks to the material’s ability to sense the environment and act in response to selected stimuli, we envision a first form of artificial intelligence that resides intrinsically in the material properties,” Wiersma said. “We demonstrated a micrometric robotic hand that can autonomously recognize particles of different color.”

As companies move toward sustainable supply chains, it is perhaps the technological breakthroughs themselves that will be most beneficial. As new materials enable devices to shrink, become more efficient, and use less power or utilize renewable energy, precious resources can be better protected.

“Photonic materials can contribute in many ways, starting from things such as environmental sensors (we are currently thinking about technology that can detect airborne RNA and DNA clusters, relevant for early detection of virus infections in public places) and clean energy conversion, going all the way to light-driven microrobotics and biomedical applications,” Wiersma said (Figure 6).

Phase-change materials

Materials that respond to an external stimulus are nothing new. Neither are those that can switch between various states — for example, transforming from liquid to solid. They are known as phase-change materials (PCMs). PCMs can be made to convert from a crystalline to an amorphous structure, and subsequently result in a different physical property, such as electrical conductivity, optical reflectivity, mass density, or thermal conductivity.

What is new, however, is an optically transparent PCM, such as that developed by professor Juejun Hu’s research group at MIT. Unlike the commonly used opaque PCMs, which have been used for decades inside rewritable CDs and DVDs, Hu’s optically transparent PCM opens up the possibility of a wide spectrum of reconfigurable or reprogrammable photonic devices.

“The Achilles heel of traditional phase-change alloys used in CDs and DVDs is that they are optically opaque, which limits their applications,” Hu said. “We developed a new phase-change material uniquely exhibiting broadband transparency in the infrared while maintaining a very large index contrast between the two states.”

This type of material can be used in three ways: in flat lenses to adjust focus, as optical switches that can be controlled to route light signals on photonic chips, and in transient optical test ports used on wafers that can be turned off permanently once wafer-level inspection is completed.

Hu believes that transparent PCMs will lead to nontraditional computing, such as neuromorphic or in-memory computing, or agile reconfiguration capabilities for optical computing architectures.

“There are now lots of exciting opportunities, including data and telecommunications, lidar for self-driving vehicles, sensing and spectroscopy, as well as optical analog computing,” he said.

References

1. A. Perevedentsev et al. (2015). Dip-pen patterning of poly(9,9-dioctylfluorene) chain-conformation-based nano-photonic elements. Nat Commun, Vol. 6, Article 5977.

2. H. Zhang et al. (2020). Azobenzene sulphonic dye photoalignment as a means to fabricate liquid crystalline conjugated polymer chain-orientation-based optical structures. Adv Opt Mater, Vol. 8, Issue 8, pp. 1901-1958.