Most researchers actively try to suppress disordered states such as background static, as they lead to unpredictable experimental results. However, recent findings from researchers at KAUST reveal that the built-up energy inside chaotic systems can be tapped using optical waves and nanofabrication. This approach has led to producing the darkest material ever seen on Earth—a T-shaped nanoparticle with record-setting potential to store and release light energy.
“Sometimes our research is described as complex, but it is actually quite simple,” explained Andrea Fratalocchi, assistant professor of electrical engineering at KAUST. “We just follow the evolution of nature and what we see around us.” In this manner, white beetles of the genus Cyphochilus and natural thermodynamic phenomena became the inspiration behind the discovery of an advanced light trapping material.
Fratalocchi leads a team that seeks to understand and design three-dimensional systems that automatically optimize their energy trapping. As an example, he discusses the problem of delivering a precise microgram quantity of a drug powder. “Too much and the patient dies, a little less and it has no effect. If there is no scale capable of weighing such a small amount precisely, what would you do?”
The answer, he explains, is to dissolve the powder in water and use the natural tendency of molecules to move via random Brownian motion uniformly throughout the liquid. A volume containing the exact dosage can then be extracted. This diffusion process makes this system’s entropy—a parameter that quantifies thermodynamic disorder—increase irreversibly towards its maximum value.
“This is an extremely powerful effect that works every time regardless of the size and shape of a container,” Fratalocchi noted. “It’s so ubiquitous we think of it as simple, but it is actually based on very complex chaotic dynamics.”
Chaotic energy harvesting, he continued, can function the same way. “Think of the powder as being light and the container is an optical resonator, or a cavity that stores light energy,” he said. The team’s resonators make photons move wildly and irreparably so that the system’s entropy increases and condenses the maximum amount of scattered light within a single second.
Fratalocchi and the team combined theoretical simulations and polystyrene microspheres to put their strategy into practice. They discovered that by deforming the microspheres with mechanical pressure, light could scatter chaotically for significantly enhanced energy harvesting—the device held over 600 percent more energy than a similarly sized classical system.
The researchers’ next target was to fabricate a perfect black body, a material that absorbs and emits large quantities of radiation. This technology could lead to new light and thermal energy sources, but the team’s early designs relied on complex structures that were extremely difficult to fabricate.
One day, however, a tennis match between Changxu Liu from Fratalocchi’s lab and Jianfeng Huang, a chemist working with KAUST professor Yu Han, turned out to be more than just a sociable workout. Discussing their research as they played, the students realized that new particles synthesized by Huang consisting of nanospheres attached to nanorods might be used for a simpler bio-inspired approach to creating a black body device.
“We had an idea based on a beetle that uses extremely white scales to reflect light as a form of camouflage,” Fratalocchi said. “By reversing this effect with chaotic structures, we could harvest a lot of energy and create an ultra-dark material.”
Liu’s simulations revealed that the nanosphere–nanorod structures had near ideal scattering behavior, and when the team dispersed them in water the liquid turned completely black—so dark, in fact, that 99 percent of incoming light was captured. The black body turned into a new source of monochromatic light following laser excitation.
Another successful collaboration, although this one a bit more intentionally developed, involves Frederico Capaso from Harvard University. Together, Fratalocchi and Capaso are working on new types of plasmonic materials. The ultimate goal of improved energy harvesting has several applications from solar energy to commercial paint.
Finally, dielectric metamaterials that manipulate light are another target for Fratalocchi’s group. He and his team developed the concept of rogue wave-based devices that arrange photonic crystals into stadium-shaped arrangements on a microchip. Chaotic waves in these devices can build up until they release a localized waveform with exceptional amplitude, an energy localization akin to natural events such as hurricanes.
"This can permanently change how we look at catastrophic events," Fratalocchi said. "Imagine if we develop a system where we can transport energy in extremely large quantities like a tsunami wave, or where we create anomalous giant amplitude waves localized at the nanoscale for extremely precise micro-surgery or new imaging techniques. These ideas are not fiction but fully possible science."