Extra Level Helps Convert Light Energy into Surface Waves on Graphene with Increased Efficiency

Collaborating physicists from two Russian universities have successfully converted light energy into surface waves on graphene at nearly 90% efficiency. A so-called laser-like energy conversion process with collective resonances, and semiconductor quantum dots acting as intermediary converters are central to the plasmonics demonstration.

Intermediary signal converters — nano-objects of varying chemical compositions and physical geometries — are used to heighten the efficiency (otherwise in the range of approximately 10%) of processes that convert light to surface plasmon-polaritons (SPPs) on 2D surfaces. The quantum dots in the research, which was led by scientists from Moscow Institute of Physics and Technology (MIPT) and Vladimir State University, are 40 nm in diameter; their composition is similar to that of the solid semiconductor from which they are manufactured.

The differing sizes of the quantum dots influence their optical properties. By altering the dimensions of the dots, the researchers were able to tune the optical wavelength(s) of interest. When natural light illuminated an assemblage of variously sized dots, each dot responded to a particular wavelength.

The researchers used quantum dots in the shape of ellipsoids, and the dots functioned as “scatterers” above the graphene surface. Infrared light at a wavelength of 1.55 μm illuminated the surface, which was physically separated from the dots by a dielectric buffer.

The structure for converting laser light to surface plasmon- polaritons used in the study. Courtesy of Mikhail Gubin et al.

Similar systems are not new, though previous versions of the work were susceptible to luminescence quenching, which occurs as incident light energy is converted into heat, as well as to reverse light scattering. The deficiencies prevented the efficiency of SPP generation from exceeding 10%.

The new work builds on earlier attempts at increasing efficiency by featuring both graphene-incident light and graphene-surface electromagnetic wave interactions, and at different frequencies, said Alexei Prokhorov, a senior researcher at the MIPT Center for Photonics and 2D Materials and a co-author of the paper describing the research. Where a dot interacted with light at a wavelength of 1.55 μm, it interacted with the SPP at 3.5 μm. A hybrid interaction scheme enabled the differentiation. As opposed to reliance upon the two present energy levels alone, namely, the upper and lower energy levels in the system, the new setup incorporated a third, intermediate level: an energetic structure that functioned as a laser.

The addition enabled a strong interaction between quantum dot and surface electromagnetic wave. At the wavelength maintained by the laser that illuminated it, the quantum dot was excited. Surface waves, meanwhile, are generated at the wavelength determined exclusively by the SPP-quantum dot resonance.

“We have worked with a range of materials for manufacturing quantum dots, as well as with various types of graphene,” Prokhorov said. “Apart from pure graphene, there is also what is called ‘doped graphene,’ which incorporates elements from the neighboring groups in the periodic table.

“Depending on the kind of doping, the chemical potential of graphene varies. We optimized the parameters of the quantum dot — its chemistry, geometry — as well as the type of graphene, so as to maximize the efficiency of light energy conversion into surface plasmon-polaritons. Eventually we settled on doped graphene and indium antimonide as the quantum dot material.”

The intensity of the resulting waves is low, despite the highly efficient, quantum dot intermediary-enabled energy input into graphene; a high number of dots is necessary, and in a precisely specific arrangement above the graphene layer to drive the process. The researchers also developed the exact geometry and optimal distance between the dots to ensure signal amplification, due to the phasing of the near fields of each dot.

The team first reported measuring a signal in graphene that was orders of magnitude more powerful than exhibited/created by quantum dots in a random arrangement. The scientists used self-developed software modules to make subsequent calculations, which showed a conversion efficiency as high as 95%.

Even considering negative factors, the researchers said, the conversion efficiency employing the newly developed scheme will remain above 50% — several times higher than that delivered by any competing system.

“A large part of [this type of] research focuses on creating ultracompact devices that would be capable of converting light energy into surface plasmon-polaritons with a high efficiency and on a very small scale in space, thereby recording light energy into some structure,” said Valentyn Volkov, co-author of the study and director of the MIPT Center for Photonics and 2D Materials. “Moreover, you can accumulate polaritons, potentially designing an ultrathin battery composed of several atomic layers. It is possible to use the effect in light energy converters similar to solar cells, but with a several times higher efficiency. Another promising application has to do with nano- and bio-object detection.”

The research was supported by a grant from the Russian Science Foundation and was published in Laser & Photonics Reviews (www.doi.org/10.1002/lpor.202000237).