Researchers design an electrically tunable graphene device to study rare physics

An international team, co-led by researchers from the National Graphene Institute (NGI) at the University of Manchester in the UK and Penn State College of Engineering in the US, has developed a tunable graphene-based platform that enables precise control of the interaction between light and matter in the terahertz (THz) spectrum to reveal rare phenomena called exceptional points. The team published their findings today (April 8) in Science.

The work could advance optoelectronic technologies to better generate, control and detect light and potentially impact wireless communications, the researchers say. They demonstrated a way to control THz waves, which exist at frequencies between those of microwaves and infrared waves. This feat could contribute to the development of wireless technology beyond 5G for high-speed communication networks.

Weak and strong interactions

Light and matter can couple, interacting on different levels: weakly – where they can be correlated but do not change each other’s constituents – or strongly – where their interactions can fundamentally change the system. The ability to control how the coupling transitions from weak to strong and back again has been a major challenge in advancing optoelectronic devices, one that researchers have now solved.

“We have demonstrated a new class of optoelectronic devices using concepts of topology – a branch of mathematics studying the properties of geometric objects,” said co-corresponding author Coskun Kocabas, professor of 2D device materials at the University of Manchester. “Using exceptional point singularities, we show that topological concepts can be used to design optoelectronic devices that enable new ways to manipulate terahertz light.” Kocabas is also affiliated with the Henry Royce Institute for Advanced Materials, headquartered in Manchester.

Exceptional points are spectral singularities – points at which any two spectral values ​​in an open system merge. They are, unsurprisingly, exceptionally sensitive and responsive to even the smallest changes in the system, revealing curious but desirable characteristics, according to co-corresponding author Şahin K. Özdemir, associate professor of engineering science and mechanics at Penn State.

“At an exceptional moment, the energy landscape of the system is significantly altered, resulting in reduced dimensionality and asymmetric topology,” said Özdemir, who is also affiliated with the Materials Research Institute, Penn State. “This, in turn, enhances the system’s response to perturbations, alters the local density of states leading to improved spontaneous emission rates, and leads to a plethora of phenomena. Controlling exceptional points and the physical processes that occur there could lead to applications for better sensors, imaging, lasers and much more.

Composition of the platform

The platform developed by the researchers consists of a tunable THz resonator based on graphene, with a gold foil gate electrode forming a lower reflecting mirror. Above, a layer of graphene is terminated by electrodes, forming a tunable top mirror. A layer of non-volatile ionic liquid electrolyte sits between the mirrors, allowing the reflectivity of the upper mirror to be controlled by changing the applied voltage. In the middle of the device, between the mirrors, are molecules of alpha lactose, a sugar commonly found in milk.

The system is controlled by two regulators. One raises the lower mirror to change the length of the cavity – adjusting the resonant frequency to couple the light with the collective vibrational modes of the organic sugar molecules, which serve as the fixed number of oscillators for the system. The other adjuster changes the voltage applied to the top graphene mirror, changing the reflective properties of graphene to transition between energy loss imbalances and adjust the coupling strength. The delicate and fine tuning shifts loosely coupled terahertz light and organic molecules to become tightly coupled and vice versa.

“The outstanding points coincide with the crossover point between the weak and strong coupling regimes of terahertz light with the collective molecular vibrations,” Özdemir said.

He noted that these singularity points are usually studied and observed in the coupling of analogous modes or systems, such as two optical modes, electronic modes or acoustic modes.

“This work is one of the rare cases where exceptional points are demonstrated to emerge in the coupling of two modes with different physical origins,” Kocabas said. “Due to the exceptional point topology, we observed significant amplitude and phase modulation of terahertz light, which may find applications in next-generation THz communications.”

Unprecedented phase modulation in the THz spectrum

When researchers apply voltage and adjust resonance, they drive the system to an exceptional point and beyond. Before, at and beyond the exceptional point, the geometric properties — the topology — of the system change.

One of these changes is phase modulation, which describes how a wave changes as it propagates and interacts in the THz field. Controlling the phase and amplitude of THz waves is a technological challenge, the researchers said, but their platform demonstrates unprecedented levels of phase modulation. The researchers moved the system through exceptional points, as well as along loops around exceptional points in different directions, and measured how it reacted to the changes. Depending on the system topology at the measurement point, the phase modulation can range from zero to four larger magnitudes.

“We can electrically steer the device through an exceptional point, which allows electrical control over the reflection topology,” said first author Dr. Said Ergoktas. “Only by electronically controlling the topology of the system could we achieve these huge modulations.”

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

Contributors include Kaiyuan Wang, Gokhan Bakan, Thomas B. Smith, Alessandro Principi and Kostya S. Novoselov, from the University of Manchester; Sina Soleymani, graduate student in Penn State’s Department of Engineering and Mechanical Sciences; Sinan Balci, Izmir Institute of Technology, Turkey; Nurbek Kakenov, who conducted work for this article while at Bilkent University, Turkey.

The European Research Council, Consolidator Grant (SmartGraphene), Air Force Office of Scientific Research Multidisciplinary University Research Initiative Award on Programmable Systems with Non-Hermitian Quantum Dynamics, and Air Force Office of Scientific Research Award supported this work.

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