Oct 17 2016
Band diagram of the graphene – tungsten disulphide – graphene structure explaining the principle of plasmon generation. The application of interlayer voltage V results in the enrichment of one layer by electrons (blue), and the emergence of free states (called holes) in the opposite layer (red). An electron can tunnel from an occupied state to an empty state (dashed line), and its excess energy can be spent to excite a plasmon (red wavy line). Credit: MIPT
The possibility to develop compact sources of coherent plasmons has been theoretically demonstrated by researchers from MIPT’s Laboratory of 2D Materials’ Optoelectronics, Institute of Radioengineering and Electronics, and Tohoku University (Japan).
These plasmons are considered the fundamental building blocks for potential optoelectronic circuits. The performance of the device is based on the unique characteristics of van der Waals heterostructures, which are composites of graphene and relevant layered materials. The journal Physical Review B features a paper explaining the study.
Referred as a quasi-particle, the plasmon is a mixture of oscillating electrons and the electromagnetic field combined with them. Plasmons are ideal for generating, transmitting, and receiving signals in integrated circuits. Plasmons are capable of behaving as mediators between light waves and electrons in majorly efficient photodetectors and sources, especially in the terahertz range that has been actively explored.
It is possible to store plasmon energy at a length scale extremely smaller than the wavelength of light. This highlights the potential of plasmonic devices to be more compact than their photonic counterparts.
The plasmons that are greatly compressed are those that are coupled to the conducting planes, and the most compact optoelectronic devices can be developed using these plasmons.
The question that arises at this point is from where to find a conducting plane supporting ultra-confined plasmons. For over forty years, these objects have been developed by sequential growth on nanometer-thin semiconductors with affine crystal structures.
In this process, particular layers are enriched with electrons and excellent electrical conductivity is obtained. These “layer-cakes” are known as heterostructures, and Russian physicist Zhores Alferov was awarded the 2000 Nobel Prize in Physics for the development of these heterostructures.
Growing nanoscale layers is not the only possible way to obtain flat semiconductors. Over the past decade researchers focused on graphene, which is a different, intrinsically two-dimensional material. Graphene, a one-atom-thick layer of carbon, can be obtained by effortlessly slicing a graphite crystal.
The MIPT alumni Andre Geim and Konstantin Novoselov received a Nobel prize awarded in 2010 for the study on the unique electronic properties of graphene, which radically differ from those of standard heterostructures. An increasing number of graphene-based devices have already been developed, including the first prototypes of lasers, ultrafast photodetectors and transistors receiving high-frequency signals.
Placing graphene on another material with a similar crystal structure further enhances its properties. The “layer-cake” heterostructures can be developed with the help of materials that are very much like graphene. However, in this case the building blocks of the structures are connected by van der Waals forces, which is the reason why they are known as van der Waals heterostructures.
In this work, the researchers demonstrate that a heterostructure comprising of two graphene layers divided by a thin layer of tungsten disulphide supports the compact two-dimensional plasmons and also generates them upon the application of interlayer voltage.
The structure we are modeling is essentially the gain medium for plasmons. More common examples of gain media are the neon-helium mixture in a gas laser, or a semiconductor diode in a laser pointer. When passing through such a medium, the light is amplified, and if the medium is placed between two mirrors, the medium will generate the light by itself. The combination ‘gain medium plus mirrors’ is at the heart of any laser, while the gain medium for plasmons is a necessary element of a plasmonic laser, or spaser. If the gain medium is switched on and off, the plasmonic pulses can be obtained on demand, which could be used for signal transmission in integrated circuits. The plasmons generated in the gain medium can also be uncoupled from the graphene layers and propagate as photons in free space. This allows one to create tunable sources of terahertz and far infrared radiation.
Dmitry Svintsov, MIPT
The gain medium is actually not a perpetuum mobile, and the particles developed by it, either plasmons or photons, must obtain their energy from a specific source. In neon-helium lasers, this energy is acquired from an electron thrown on a high atomic orbital by the electric discharge.
In semiconductor lasers, the photon obtains its energy by collapsing negative and positive charge carriers, holes, and electrons, which are provided by the current source. In the double graphene layer structure that has been proposed, the plasmon acquires its energy from an electron jumping from a layer with increased potential energy to a layer comprising low potential energy. The development of a plasmon due to this jump is very much like the way in which waves develop as a diver enters the water.
The transition of an electron from one layer to another is similar to soaking through the barrier instead of jumping over it. This phenomenon is known as tunneling, and generally the probability of tunneling is extremely low for nanometer-thin barriers. The case of resonant tunneling is one exception, where each element from a single layer has a well-prepared place existing in the opposite layer.
The principle of plasmon generation studied by our group is similar to the principle of the quantum cascade laser proposed by the Russian scientists Kazarinov and Suris and realized in the USA (Faist and Capasso) more than twenty years afterwards. In this laser, the photons take energy from electrons tunneling between gallium arsenide layers through the AlGaAs barriers. Our calculations show that in this principal scheme, one can profitably replace gallium arsenide with graphene, while tungsten disulphide can act as a barrier material. This structure is able to generate not only photons, but also their compressed counterparts—plasmons. The generation and amplification of plasmons was previously thought to be a very challenging problem, but the structure we have proposed brings us one step closer to the solution.
Dmitry Svintsov, MIPT
In July 2016, the paper written by Dmitry Svintsov, Zhanna Devizorova, Victor Ryzhii, and Taiichi Otsuji received the Alferov’s Foundation Young Scientist Award at the 24th International Symposium “Nanostructures: Physics and Technology,” held in Saint Petersburg.