A breakthrough in electrically tunable graphene devices could pave the way for “beyond-5G” wireless technology to emerge.

To explore uncommon physics, researchers create electrically controllable graphene devices.

The discovery could pave the way for the development of “beyond-5G” wireless technology for high-speed networks.

Researchers from The University of Manchester’s National Graphene Institute (NGI) in the United Kingdom and Penn State College of Engineering in the United States have developed a tunable graphene-based platform that allows for fine control over the interaction between light and matter in the terahertz (THz) spectrum to reveal rare phenomena known as exceptional points. The team’s findings were published in Science today (April 7, 2022).

According to the researchers, the work could develop optoelectronic technologies to better generate, manipulate, and perceive light, as well as potentially improve communications. They showed how to manipulate THz waves, which have frequencies between microwaves and infrared waves. The achievement could help to advance “beyond-5G” wireless technology for high-speed communication networks.

Weak and strong interactions

Light and matter can interact at several levels: weakly, where they are correlated but do not modify one another’s constituents; or powerfully, where their interactions can fundamentally transform the system. Controlling how the coupling goes from weak to strong and back again has been a major issue in the development of optoelectronic devices, but researchers have now overcome it.

Co-corresponding author Coskun Kocabas, professor of 2D device materials at the University of Manchester, said, “We have developed a new class of optoelectronic devices employing principles of topology—a branch of mathematics examining features of geometric structures.” “We show that topological principles may be used to construct optoelectronic devices that provide novel approaches to manipulating terahertz light using exceptional point singularities.”

The Henry Royce Institute for Advanced Materials, based in Manchester, is also linked with Kocabas.

Spectral singularities—places where any two spectral values in an open system converge—are exceptional points. According to co-corresponding author Sahin K. Zdemir, associate professor of engineering science and mechanics at Penn State, they are unusually sensitive and respond to even the smallest changes in the system, exhibiting peculiar yet desirable qualities.

“We have demonstrated a new class of optoelectronic devices using concepts of topology — a branch of mathematics studying properties of geometric objects, Using exceptional point singularities, we show that topological concepts can be used to engineer optoelectronic devices that enable new ways to manipulate terahertz light.”

said co-corresponding author Coskun Kocabas, professor of 2D device materials at The University of Manchester.

Zdemir, who is also affiliated with Penn State’s Materials Research Institute, said, “The energy landscape of the system is significantly affected at an unusual point, resulting in reduced dimensionality and skewed topology.” This, in turn, improves the system’s reactivity to perturbations, changes the local density of states, increasing spontaneous emission rates and causing a slew of other effects. Controlling exceptional sites and the physical processes that take place there could lead to improved sensors, cameras, lasers, and other technologies. “

Two adjusters are in charge of the system. To adjust the length of the cavity, one raises the bottom mirror, tweaking the resonation frequency to couple the light with the collective vibrational modes of the organic sugar molecules, which act as a fixed number of oscillators for the system. The other adjuster alters the voltage delivered to the top graphene mirror, causing the graphene’s reflective characteristics to vary, allowing the energy loss imbalances to be transitioned and the coupling strength to be adjusted. Weakly linked terahertz light and organic molecules become strongly coupled as a result of the precise, fine tuning.

Zdemir stated that the crossover point between the weak and strong coupling regimes of terahertz light with collective molecular vibrations coincides with exceptional spots.

According to him, these singularity points are commonly investigated and observed in the connection of similar modes or systems, such as two optical modes, electronic modes, or acoustic modes.

“This is one of the few examples when extraordinary points are shown to develop in the connection of two modes with different physical sources,” Kocabas added. “We noticed a large modulation in the magnitude and phase of the terahertz light due to the topology of the unique spots, which could find use in next-generation THz communications.”

In the THz spectrum, there’s never been anything like it.

The researchers drive the system to an extraordinary point and beyond by applying voltage and adjusting the resonance. The geometric aspects of the system—its topology—alter before, at, and beyond the exceptional point.

The phase modulation, which defines how a wave changes as it propagates and interacts in the THz field, is one such alteration. According to the researchers, controlling the phase and amplitude of THz waves is a technological difficulty, but their platform shows unparalleled degrees of phase modulation. The researchers changed the system’s behavior by moving it through exceptional spots, as well as looping around exceptional points in different directions, and measuring how it responded. Phase modulation can range from zero to four magnitudes larger, depending on the topology of the system at the time of measurement.

  1. Remarkably, Ergoktas, the first author, said, “We can electrically steer the device through an unusual point, which permits electrical control on reflection topology.” “We could only produce these massive modulations by changing the topology of the system electronically.”

According to the researchers, the graphene-based platform’s topological control of light-matter interactions around an exceptional point has applications ranging from topological optoelectronic and quantum devices to topological control of physical and chemical processes.

“Electrically tuneable exceptional point singularities for topological engineering of terahertz light” is a reference. Science, 7 April 2022.

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