Tuesday, May 17, 2022

A tunable graphene-based platform to study rare physics

Finely controlled approach enables ‘exceptional’ devices; has implications for future optoelectronics.

Light and matter can couple and interact at different levels: Weak or strong. However, controlling coupling shifts from weak to strong and back again remains challenging for advancing optoelectronic devices.

An international team- co-led by researchers at The University of Manchester‘s National Graphene Institute (NGI) in the UK and the Penn State College of Engineering in the US, has solved this challenge by developing a tunable graphene-based platform. Consisting of a graphene-based tunable THz resonator, the platform could control the interaction between light and matter in the terahertz (THz) spectrum to reveal rare phenomena known as exceptional points.

Exceptional points are points at which any two spectral values in an open system merge. They are sensitive enough to respond to even the most minor changes to the system, revealing curious yet desirable characteristics.

The platform has a bottom reflective mirror made of a gold-foil gate electrode. Above that, scientists positioned a graphene layer with electrodes. This formed a tunable top mirror. There is a non-volatile ionic liquid electrolyte layer sandwiched between the mirrors. This enabled control of the top mirror’s reflectivity by changing the applied voltage. There exist molecules of alpha lactose in the middle of the device between the mirrors.

Two adjusters control the overall device. One adjuster changes the length of the cavity by raising the lower mirror, tuning the frequency of resonation to couple the light with the collective vibrational modes of the organic sugar molecules, which serve as a fixed number of oscillators for the system. The other adjuster changes the voltage applied to the top mirror. This changes the graphene’s reflective properties to transition the energy loss imbalances to adjust coupling strength.

The delicate, fine-tuning shifts weakly coupled terahertz light and organic molecules to become strongly coupled and vice versa.

The exceptional points coexist with the crossover point between the weak and strong coupling regimes of terahertz light with collective molecular vibrations.

Coskun Kocabas, professor of 2D device materials at The University of Manchester, said, “We have demonstrated a new class of optoelectronic devices using 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.”

“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. Due to the topology of the exceptional points, we observed a significant modulation in the magnitude and phase of the terahertz light, which could find applications in next-generation THz communications.”

When scientists applied the voltage and adjusted the resonance, the system went to an exceptional point and beyond. Before, at, and beyond the exceptional point, the topology of the system change.

Scientists noted, “One such change is the 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, but their platform demonstrates unprecedented levels of phase modulation.”

Scientists moved the system through exceptional points and along loops around exceptional points in different directions. They quantified how it responds to changes. They found that the phase modulation could range from zero to four magnitudes larger. It depends on the system’s topology at the point of measurement.

First author M. Said Ergoktas said“We can electrically steer the device through an exceptional point, which enables electrical control on reflection topology. Only by controlling the system’s topology electronically could we achieve these huge modulations.”

Scientists think their tunable graphene-based platform has potential applications ranging from topological optoelectronic and quantum devices to topological control of physical and chemical processes.

Journal Reference:

  1. M. Said Ergoktas, Sina Soleymani et al. Topological engineering of terahertz light using electrically tunable exceptional point singularities. DOI: 10.1126/science.abn6528

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