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Scientists fill graphene with light

No losses: Scientists fill graphene with light

Credit: Daria Sokol / MIPT

Physicists at MIPT and Vladimir State University, Russia, have converted light energy into surface waves on graphene with almost 90% efficiency. They relied on a laser-like circuit to convert energy and collective resonances. The report was published in Laser and photonics reviews.

The manipulation of light on a nanoscale is a crucial task for the creation of ultra-compact devices for optical conversion and storage of energy. To localize light on such a small scale, researchers are converting optical radiation into so-called surface plasmon polaritons. These SPPs are oscillations that propagate along the boundary between two materials with drastically different refractive indices ̵

1; in particular metal and dielectric or air. Depending on the selected materials, the degree of localization of surface waves varies. It is strongest for light localized on a material with a thickness of only one atomic layer, as such 2-D materials have high refractive indices.

Existing schemes for converting light into SPP on 2-D surfaces have an efficiency of no more than 10%. It is possible to improve this figure by using intermediate signal converters – nano-objects with different chemical composition and geometry.

The intermediate converters used in the recent study in Laser and photonics reviews are semiconductor quantum dots with a size of 5 to 100 nanometers and a composition similar to that of the solid semiconductor from which they are made. However, the optical properties of a quantum dot vary considerably depending on its size. So by changing its size, researchers can adjust it to the optical wavelength of interest. If a group of quantum dots of different sizes is illuminated with natural light, each dot will respond to a certain wavelength.

Quantum dots are available in different shapes – cylinders, pyramids, spheres, etc. – and in different chemical composition. In their study, the team of Russian researchers used quantum dots in the shape of an ellipsoid with a diameter of 40 nanometers. The dots served as sprinklers located above the surface of graphene, which was illuminated with infrared light with a wavelength of 1.55 micrometers. A dielectric buffer several nanometers thick separates the graphene sheet from the quantum dots.

The idea of ​​using a quantum dot as a diffuser is not new. Some previous graphene studies have used a similar arrangement, with the dots located above 2-D sheets and interacting with both light and surface electromagnetic waves at a total wavelength shared by the two processes. This was made possible by choosing the exact size of the quantum dot. While such a system is quite easy to tune to resonance, it is susceptible to luminescence quenching – the conversion of incident light energy into heat, as well as the backscattering of light. As a result, the efficiency of SPP generation does not exceed 10%.

“We studied a scheme in which the quantum dot above graphene interacts with both incident light and the surface electromagnetic wave, but the frequencies of these two interactions are different. The dot interacts with light with a wavelength of 1.55 micrometers and the surface plasmon-polariton at 3.5 micrometers. This is possible through a hybrid interaction scheme, “said study co-author Alexei Prokhorov, a senior researcher at the MIPT Center for Photonics and 2-D Materials and an associate professor at Vladimir University.

No losses: Scientists fill graphene with light

The structure for conversion of laser light into surface-plasmon polaritons used in the study Credit: Mikhail Gubin et al. Laser and photonic reviews

The essence of the hybrid interaction scheme is that instead of using only two energy levels – the upper and lower, the setting also includes an intermediate level. That is, the team uses an energetic structure similar to that of a laser. The intermediate energy level serves to ensure the strong connection between the quantum dot and the surface electromagnetic wave. The quantum dot is excited at the wavelength of the laser that illuminates it, while the surface waves are generated at the wavelength determined by the resonance of the SPP-quantum dot.

“We have worked with a number of materials for the production of quantum dots, as well as with different types of graphene,” Prokhorov explained. “Apart from pure graphene, there is also the so-called alloyed graphene, which includes elements from neighboring groups in the periodic table. Depending on the type of alloying, the chemical potential of graphene varies. We optimized the parameters of the quantum dot – its chemistry, geometry – and graphene so as to maximize the efficiency of converting light energy into surface plasmon-polaritons. In the end, we focused on doped graphene and indium antimonide as the material of the quantum dots. “

Despite the highly efficient introduction of energy into graphene through the mediator of quantum dots, the intensity of the resulting waves is extremely low. Therefore, a large number of points must be used in a specific arrangement above the graphene layer. The researchers had to find the exact geometry, the perfect distance between the points, to provide signal amplification due to the phasing of the nearby fields at each point. In their study, the team reported finding such a geometry and measuring a signal in graphene that is orders of magnitude more powerful than a random order of quantum dots. For their subsequent calculations, physicists used self-developed software modules.

The calculated conversion efficiency of the newly proposed scheme reaches 90% -95%. Even if it takes into account all the potential negative factors that could affect this value, it will remain above 50% – several times higher than any other competing system.

“Much of this research is focused on creating ultra-compact devices that could convert light energy into surface plasmon-polaritons with high efficiency and on a very small scale in space, thus recording light energy into some structure,” he said. the director of the Center for Photonics and 2-D materials of MIPT, Valentin Volkov, who is a co-author of the study. “In addition, you can accumulate polaritons by 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 several times higher efficiency. Another promising application is related with the detection of nano- and bio-objects. ”

Resonant energy transfer from quantum dots to graphene

More info:
Mikhail Y. Gubin et al. Hybrid schemes for excitation of collective resonances with surface plasmon polaritons in arrays of quantum dots near graphene, Laser and photonics reviews (2020). DOI: 10.1002 / lpor.202000237

Provided by the Moscow Institute of Physics and Technology

Quote: No losses: Scientists fill graphene with light (2020, November 16), extracted on November 17, 2020 from https://phys.org/news/2020-11-losses-scientists-graphene.html

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