Efficiently Converting Light Energy Into Surface Waves on Graphene

Dotty graphene and doping: Whatever it considers Russia’s record plasmonics to shine.

Physicists from MIPT and Vladimir State University, Russia, have actually attained an almost 90% performance transforming light energy into surface area waves on graphene. They depend on a laser-like energy conversion plan and cumulative resonances. The paper came out in Laser & Photonics Reviews.

Manipulating light at the nanoscale is a job essential for having the ability to produce ultracompact gadgets for optical energy conversion and storage. To localize light on such a little scale, scientists transform optical radiation into so-called surface area plasmon-polaritons. These SPPs are oscillations propagating along the user interface in between 2 products with considerably various refractive indices — particularly, a metal and a dielectric or air. Depending on the products picked, the degree of surface area wave localization differs. It is the greatest for light localized on a product just one atomic layer thick, since such 2D products have high refractive indices.

The existing plans for transforming light to SPPs on 2D surface areas have an effectiveness of no greater than 10%. It is possible to enhance that figure by utilizing intermediary signal converters — nano-objects of different chemical structures and geometries.

The structure for transforming laser light to surface-plasmon polaritons utilized in the research study. Credit: Mikhail Gubin et al./Laser & Photonics Reviews

The intermediary converters utilized in the current research study in Laser & Photonics Reviews are semiconductor quantum dots with a size of 5 to 100 nanometers and a structure comparable to that of the strong semiconductor they are made from. That stated, the optical residential or commercial properties of a quantum dot differ substantially with its size. So by altering its measurements, scientists can tune it to the optical wavelength of interest. If an assembly of otherwise sized quantum dots is lit up with natural light, each dot will react to a specific wavelength.

Quantum dots can be found in different shapes — cylinders, pyramids, spheres, etc. — and chemical structures. In its research study, the group of Russian scientists utilized ellipsoid-shaped quantum dots 40 nanometers in size. The dots functioned as scatterers placed above the surface area of graphene, which was lit up with infrared light at a wavelength of 1.55 micrometers. A dielectric buffer a number of nanometers thick separated the graphene sheet from the quantum dots (fig. 1).

The concept to utilize a quantum dot as a scatterer is not brand-new. Some of the previous graphene research studies utilized a comparable plan, with the dots placed above the 2D sheet and engaging both with light and with surface area electro-magnetic waves at a typical wavelength shared by the 2 procedures. This was enabled by selecting a quantum dot size that was precisely ideal. While such a system is relatively simple to tune to a resonance, it is vulnerable to luminescence quenching — the conversion of event light energy into heat — along with reverse light scattering. As an outcome, the performance of SPP generation did not surpass 10%.

“We investigated a scheme where a quantum dot positioned above graphene interacts both with incident light and with the surface electromagnetic wave, but the frequencies of these two interactions are different. The dot interacts with light at a wavelength of 1.55 micrometers and with the surface plasmon-polariton at 3.5 micrometers. This is enabled by a hybrid interaction scheme,” remarks research study co-author Alexei Prokhorov, a senior scientist at the MIPT Center for Photonics and 2D Materials, and an associate teacher at Vladimir State University.

The essence of the hybrid interaction plan is that instead of utilizing simply 2 energy levels — the upper and lower ones — the setup likewise consists of an intermediate level. That is, the group utilized an energetic structure similar to that of the laser. The intermediate energy level serves to make it possible for the strong connection in between the quantum dot and the surface area electro-magnetic wave. The quantum dot goes through excitation at the wavelength of the laser brightening it, whereas surface area waves are created at the wavelength identified 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 described. “Apart from pure graphene, there is also what’s 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.”

Despite the extremely effective energy input into graphene through the quantum dot intermediary, the strength of the resulting waves is exceptionally low. Therefore, great deals of dots need to be utilized in a particular plan above the graphene layer. The scientists needed to discover specifically the ideal geometry, the ideal range in between the dots to make sure signal amplification due to the phasing of the near fields of each dot. In their research study, the group reports finding such a geometry and determining a signal in graphene that was orders of magnitude more effective than for arbitrarily set up quantum dots. For their subsequent estimations, the physicists utilized self-developed software application modules.

The determined conversion performance of the freshly proposed plan is as high as 90%-95%. Even accounting for all the prospective unfavorable elements that may impact this figure of benefit, it will stay above 50% — a number of times greater than any other contending system.

“A large part of such 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,” stated the director of the MIPT Center for Photonics and 2D Materials, Valentyn Volkov, who co-authored the research study. “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.”

Reference: “Hybrid Schemes for Excitation of Collective Resonances with Surface Plasmon Polaritons in Arrays of Quantum Dots in the Proximity of Graphene” by Mikhail Yu. Gubin, Alexander V. Shesterikov, Alexei V. Prokhorov and Valentyn S. Volkov, 28 September 2020, Laser & Photonics Reviews.
DOI: 10.1002/lpor.202000237

The research study reported in this story was supported by Grant No. 20-12-00343 of the Russian Science Foundation.