Scientists Demonstrate New Opportunities For Controlling Light In Optical Graphene
This will help scientists and engineers to better understand the properties of light propagating in materials with a hexagonal structure, and use this knowledge in fundamental research and the development of optical devices. Physicists from ITMO took part in this research. The article was published in Nature Photonics.
In the last 15 years, scientists all over the world have been actively researching various properties of graphene, a two-dimensional material that’s composed of carbon atoms arranged in a hexagonal lattice. Simply put, carbon atoms in graphene compose hexagonal cells. Such a structure of a crystal lattice makes it so that electrons in this material can behave as particles with a zero effective mass despite having mass. This is what gives graphene its unique properties.
What happens to electrons in graphene is described by the laws of quantum mechanics. This area of physics does not perceive the electron as a particle that moves around the atom’s nucleus, much like a planet moves around the sun, but as a wave of matter. Therefore, we can logically assume that there are similar materials in which other waves propagate in an unusual way. This is how scientists came up with the idea of photon graphene, a material with a lattice composed of hexagons but which is intended for working with photons rather than electrons.
After such materials were created, it turned out that what happens with light in them can be described with the equations from classical physics. At the same time, these phenomena are similar to quantum effects that emerge in regular graphene. For example, the Spin Hall effect when electrons with different spin divert in different directions when moving. Spin (something like a particle’s inner momentum) is a quantum category, there’s no description for it in classical physics.
Analogy principle
Then again, we all know that physics speaks the language of maths. If we represent a physical phenomena as equations, they can look really similar, despite the difference in their nature. This is how the analogy principle works in physics.
An international team of scientists that includes representatives of Sheffield University, the University of Iceland, and ITMO University showed that in photonic systems, when their classical description is valid, it’s possible to observe effects that are similar to quantum ones and are well-known in solid-state physics. While studying the properties of photonic graphene, scientists succeeded in identifying and describing an effect analogous to the Dresselhaus effect.
The research studied a photonic graphene that was excited by a focused laser emission falling under specific angles. For one, they studied in detail the instance when the correlation between the frequency of excitation and the wave vector was close to the Dirac cone. This takes place close to the so-called K points where quasi particles have zero mass.
For comparison, the scientists also studied excitation of waves with small quasi impulses in the vicinity of the so-called Gamma points. In this case, quasi particles have a finite mass. It was shown that due to polarization effects, the electromagnetic wave frequency depends on its spatial period much like how electron energy depends on their impulse in the presence of spin-orbit interaction. It was experimentally demonstrated that the structure (we want to remind you that a wave’s structure is defined by four components) depends on the propagation direction differently if the system is excited in the vicinity of Gamma and K points.
The explanation of the observed effects was given in terms of the field theory, and it was also shown that the discovered effect can be interpreted as a movement in a non-Abelian gauge field. This way, the synthesis of solid-state physics and field theory methods showed its efficiency in researching purely optical effects in microstructured systems. The computing simulation made it possible to reproduce the experimental results with high precision and confirmed the analytical conclusions.
Prospects
The discovered effect can find various applications in optics, as it offers to make use of polarization as an instrument for controlling waves in two-dimensional wave structures.
One of the next directions that this research might take is studying the interaction of light in structures when the properties of a sample, among other things, depends on the intensity of the emission that propagates in it. Such systems are called nonlinear, and the propagation of high and low intensity waves in them can happen very differently. For one, in the context of photonic graphene, we can speak about a nonlinear (dependent on the intensity of light) spin-orbit interaction. This can be interesting not only from the fundamental standpoint but also useful in practical applications, for example for the generation of optical emission.
The fact that different physical systems can be identical at the abstract level of maths opens up new opportunities for theoretical physicists. The research methods that have been developed and successfully used for one kind of system can be adapted for the analysis of systems that have a different nature. With certain restrictions, we can expect that the use of theoretical physics methods will find application in researching biological and economic systems.
Finally, the similarities discovered between the descriptions of processes in optics and solid-state physics encourages to look for the same similarities in other systems, for example acoustic graphene.
Source: ITMO University