Researchers have demonstrated how reconfigurable metasurfaces — artificial materials with extraordinary optical properties — are critical to the future of nanotechnology by harnessing the power of ‘phase-change’ materials.
Optical lens technological advancement has long been a significant indicator of human scientific achievement. Eyeglasses, telescopes, cameras, and microscopes have all allowed us to see the world in new ways, both literally and metaphorically. Lenses are also an important component in the semiconductor industry’s production of nanoelectronics.
The development of photonic metasurfaces – artificially engineered nano-scale materials with remarkable optical properties – has been one of the most significant advances in lens technology in recent history. In a recent study published in Nature Communications, Georgia Tech researchers at the forefront of this technology demonstrated the first-ever electrically tunable photonic metasurface platform.
“Metasurfaces can make optical systems very thin, and as they become easier to control and tune, you’ll soon find them in cell phone cameras and similar electronic imaging systems,” said Ali Adibi, professor in the Georgia Institute of Technology’s School of Electrical and Computer Engineering.
The pronounced tuning measures achieved through the new platform represent a critical advancement towards the development of miniaturized reconfigurable metasurfaces. The results of the study have shown a record eleven-fold change in the reflective properties, a large range of spectral tuning for operation, and much faster tuning speed.
Metasurfaces can make optical systems very thin, and as they become easier to control and tune, you’ll soon find them in cell phone cameras and similar electronic imaging systems.
Professor Ali Adibi
Heating Up Metasurfaces
Metasurfaces are a type of nanophotonic material in which a large number of miniaturized elements are engineered to control the transmission and reflection of light at various frequencies.
“When viewed under very strong microscopes, metasurfaces resemble a periodic array of posts,” Adibi explained. “The best analogy would be a LEGO pattern formed by connecting many similar LEGO bricks next to each other.”
Metasurfaces have been used to demonstrate that very thin optical devices can affect light propagation since their inception, with metalenses (the formation of thin lenses) being the most developed application.
Despite significant progress, the majority of demonstrated metasurfaces are passive, which means that their performance cannot be changed (or tuned) after fabrication. Adibi and his team’s work, led by Ph.D. candidate Sajjad Abdollahramezani, applies electrical heat to a special class of nanophotonic materials to create a platform that can easily manufacture reconfigurable metasurfaces with high levels of optical modulation.
PCMs Provide the Answer
Metals, oxides, and semiconductors can all be used to form metasurfaces, but Abdollahramezani and Adibi’s research focuses on phase-change materials (PCMs) because they can form the most effective structures with the smallest feature sizes. PCMs are substances that absorb and release heat during the heating and cooling processes. They are referred to as “phase-change” materials because they transition from one crystallization state to another during thermal cycling. The most common example is water changing from a liquid to a solid or gas.
The Georgia Tech team’s experiments are far more complex than simply heating and freezing water. Knowing that local heating can change the optical properties of PCMs, they have maximized the potential of the PCM alloy Ge2Sb2Te5 (GST), a compound of germanium, antimony, and tellurium.
The team can change the crystalline phase of the GST to enable active tuning of the metasurface device by combining the optical design with a miniaturized electrical microheater underneath. The reconfigurable metasurfaces were created at Georgia Tech’s Institute for Electronics and Nanotechnology (IEN) and tested in characterization labs by illuminating them with laser light at various frequencies and measuring the properties of the reflected light in real time.
What Tunable Metasurfaces Mean for the Future
Driven by device miniaturization and system integration, as well as their ability to selectively reflect different colors of light, metasurfaces are rapidly replacing bulky optical assemblies of the past. Immediate impact on technologies like LiDAR systems for autonomous cars, imaging, spectroscopy, and sensing is expected.
According to Abdollahramezani and Adibi, more aggressive applications such as computing, augmented reality, photonic chips for artificial intelligence, and biohazard detection can be envisioned with further development.
“As the platform evolves, reconfigurable metasurfaces will be found everywhere,” Adibi predicted. “They will even enable smaller endoscopes to go deep inside the body for better imaging and assist medical sensors in detecting various biomarkers in blood.”
