Theoretical Advancement Enables Unprecedented Wave Control

JOEL WILLIAMS, ASSOCIATE EDITOR
joel.williams@photonics.com

Cornell researchers have proposed a new way to modulate the absorptive and the refractive qualities of metamaterials in real time, and their findings open new opportunities to control, in time and space, the propagation and scattering of waves for applications in various areas of wave physics and engineering.

The research was conducted by doctoral students Zeki Hayran and Aobo Chen, along with their adviser Francesco Monticone, assistant professor in the school of electrical and computer engineering in the college of engineering.

“In electromagnetics and photonics, materials and structures modulated in space (gratings, photonic crystals, metamaterials) have been studied for many decades. Instead, metamaterials modulated in time have started attracting increasing interest only recently,” Hayran and his colleagues told Photonics Media. “In the literature on this emerging topic, two main classes of temporal modulations are commonly studied: temporal switching and periodic modulation.

“In this study, we have proposed and theoretically demonstrated a new, broad class of temporal modulations with some distinct properties (one-way frequency response and inherent lack of reflections). These results not only deepen our understanding of wave interaction in complex dynamic media but can also enable better performance and new functionalities in some application scenarios.”

The challenge of modulating wave propagation simultaneously in both time and space is the need to understand the intrinsic dispersion of the material, or the fact that the material properties depend on frequency.

“One needs to make sure that the modulation exhibits similar properties at all frequencies within the operational bandwidth of the device, as discussed in the paper. Another challenge is that one needs to trigger the temporal modulation upon the arrival time of the incident pulse,” Hayran and colleagues said.

The team intends to demonstrate a composite metamaterial where the refractive and absorptive properties are modulated simultaneously, but separately in distinct materials of the composite structure. Radio frequency and microwave frequency metamaterials might be particularly suitable, as standard circuit components like varactor diodes or resistors can be used to realize the desired temporal modulation with the necessary speed, the researchers said. The work could enable development of new metamaterials containing wave absorption and scattering properties far beyond what’s currently available. For example, a broadband absorber currently has to be thicker than a certain value to be effective, though the thickness is limiting to the design’s applications.

“To decease the thickness and increase the bandwidth of such an absorber, you have to overcome the limitations of conventional materials,” Hayran said. “One of the ways to bypass these limitations is through temporally modulating the structure.”

In their paper, the researchers demonstrated, theoretically, reflectionless absorption over broad bandwidths.

“This might have important implications,” Hayran said, “for example, in radar stealth technology. Moreover, we also showed that light can propagate through a temporal disturbance as if the disturbance never existed, which might be relevant for applications where robust signal guiding is required even in the presence of certain forms of temporal noise and perturbations.

The research was published in Optica (www.doi.org/10.1364/OPTICA.423089).