Scientists do stuff graphene with light

Credit: Daria Sokol / MIPT

Physicists from MIPT and Vladimir State University, Russia have converted light energy into surface waves on graphene with about 90% efficiency. They relied on a laser-like energy conversion scheme and collective resonance. The paper was published Laser and Photonics Review.

Manipulation of light at the nanoscale is an important task to be able to create ultracompact devices for optical energy conversion and storage. To localize light on such a small scale, researchers transform optical radiation into so-called surface plasmon-polaritons. These SPPs are widely diffused with different refractive indices — specifically, a metal and a dielectric or air — at the interface between the two materials. Depending on the materials chosen, the degree of surface wave localization varies. It is strongest for localized light on only one atomic layer thick material, as such 2-D materials have high refractive indicators.

The efficiency of existing plans to convert light into SPP on 2-D surfaces is not more than 10%. It is possible to improve that figure by using intermediary-signal converters-nano-objects of different chemical compositions and geometries.

Mediator Intermediaries Used in Recent Studies Laser and Photonics Review Semiconductors with sizes ranging from 5 to 100 nanometers are quantum dots and similar to the solid semiconductors they are manufactured from. That said, the optical properties of a quantum dot vary greatly with its size. Therefore by changing its dimensions, researchers can tune it to the optical wavelength of interest. If an assembly of quantum dots of different sizes is illuminated with natural light, each point will respond to a particular wavelength.

Quantum dots come in different shapes – cylinders, pyramids, spheres, etc. and various chemical compositions. In their study, the team of Russian researchers used ellipsoid-shaped quantum dots 40 nanometers in diameter. The dots acted as a scatter above the surface of the graphene, illuminated with infrared light at a wavelength of 1.55 micrometers. A dielectric buffer several nanometers thick separated the graphene sheet from quantum dots.

The idea of ​​using a quantum dot as a scatter is not new. Some of the previous Graphene studies used a similar arrangement, with dots located above the 2-D sheet and interacting with light and surface electromagnetic waves at a common wavelength shared with both. This was made possible by selecting a quantum dot shape that was exactly right. While such a system is fairly easy to tune an resonance, it is susceptible to luminescence quenching – the conversion of incident light energy into heat as well as reverse light scattering. As a result, the efficiency of SPP generation did not exceed 10%.

“We investigated a scheme where a quantum dot located above graphene interacts with both incident light and a surface electromagnetic wave, but the frequency of these two interactions varies. The dot is at a wavelength of 1.55 micrometers and the surface. K interacts with light along the plane. Pilariton at 3.5 micrometers. This is enabled by a hybrid interaction scheme, “study co-author Alexey Prokhorov, senior researcher at MIPT Center for Photography and 2-D Materials, and Vladimir State University Says an associate professor at.

No harm: Scientists stuff Graphene with light

Structure credit of converting laser-light to surface-plasmon polaritons used in the study: Mikhail Gubin et al. Laser and Photonics Review

The essence of the hybrid interaction scheme is that instead of using only two energy levels – the upper and lower ones – the setup also includes an intermediate level. That is, the team used an energetic structure for the laser. The intermediate energy level serves to enable a strong relationship between the quantum dot and the surface electromagnetic wave. The quantum dot evokes excitation at the wavelength of the laser, while the surface waves are generated at the wavelength determined by the SPP-quantum dot resonance.

“We have worked with a range of materials as well as different types of graphene to manufacture quantum dots,” Prokhorov explained. “In addition to pure graphene, also called doped graphene, which incorporates elements from neighboring groups in the periodic table. Depending on the kind of doping, the chemical potential of graphene varies. We have adapted the parameters of quantum dot . ” Chemistry, geometry – as well as types of graphene, so as to maximize the efficiency of light energy conversion to surface plasmon-polaritons. Eventually we settled on doped graphene and indium antimonide as quantum dot materials. “

Despite the highly efficient energy input to Graphene via the quantum dot intermediary, the resulting waveform intensity is extremely low. Therefore, a large number of dots are to be used in a special arrangement above the Graphene layer. Researchers had to accurately detect the correct distance between dots, the correct geometry, to ensure signal amplification due to the phasing of fields near each dot. In their study, the team reported discovering such geometry and measuring a signal in Graphene, orders of magnitude more powerful than randomly arranged quantum dots. For their subsequent calculations, physicists employed self-developed software modules.

The newly proposed scheme calculates conversion efficiency as high as 90% –95%. Even accounting for all possible negative factors, which may affect this figure of merit, it will remain more than 50% of the time compared to any other competitive system.

“A large part of such research focuses on creating ultracompact devices that will be able to convert light energy into surface plasmon-polaritons with high efficiency and on a very small scale space, allowing light energy to be converted into some structure Can be recorded, ”the director said. MIPT Center for Photonics and 2-D Materials, Valentin Volkov, who co-authored the study. “In addition, you can accumulate polyritones, potentially designing an ultrathin battery composed of multiple atomic layers. It is possible to use the effect in light energy converters similar to solar cells, but with many times higher efficiency. With. Another promising application has to do with. Nano- and bio-object detection. ”

Resonant energy transfer from quantum dots to graphene

more information:
Mikhail Yu. Gubin et al. Hybrid schemes for excitation of mass resonances with surface plasmon polarities in arrays of quantum dots in proximity to graphene: Laser and Photonics Review (2020). DOI: 10.1002 / lpor.202000237

Provided by Moscow Institute of Physics and Technology

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