Researchers from Germany’s University of Regensburg, Russia’s MIPT, and the American University of Kansas and MIT have found abnormally strong absorption of light in magnetized graphene. The effect appears when normal electromagnetic waves are converted into ultra-slow surface waves running along graphene. The phenomenon could help to develop new ultra-compact signal receivers with high absorption efficiency for future telecommunications.
Daily experience teaches us that the efficiency of light energy harvesting is proportional to the area of the absorber, as indicated by the “farms” of solar panels covering large areas. But can an object absorb radiation from an area larger than itself? It appears that way, and it is possible when the frequency of the light is in resonance with the movement of the electrons in the absorber. In this case, the radiation absorption area is on the order of the squared wavelength of light, although the absorber itself may be extremely small.
For example, a hydrogen atom has a surface of the order of one angstrom squared. But if illuminated by radiation whose frequency is synchronous with the transition between electron orbits, the absorption area can increase by a factor of about two hundred thousand.
Resonant light absorption phenomena are actively “tamed” to receive electromagnetic waves – from radio frequency to the ultraviolet range. The most practical resonant receiver is an antenna, a metal rod is the simplest example. Resonant conditions require a certain antenna size. A sensitive metal antenna should be comparable in size to the wavelength, and if smaller, it loses a lot of sensitivity.
For example, the frequency of 0.1 terahertz is proposed for 6G mobile data transmission. This would require antennas of around 3mm, taking up a large and expensive area on the smartphone chip. Researchers are therefore seeking to create ultra-compact and resonant radiation absorbers.
Two classes of resonances are of interest in this regard, both observed in semiconductors. The first is called plasmon resonance and is associated with the synchronous movement of electrons and the electromagnetic field from one sample boundary to another. The second is called cyclotron resonance. This happens when the frequency of the electromagnetic wave coincides with the frequency of rotation of the electrons in the circular orbit in the magnetic field.
Both resonances have been successfully studied experimentally. However, the absorption enhancement effect in most semiconductors studied so far has been weak for practical applications.
In recent work by the team, the absorption of electromagnetic waves has been studied under conditions where both resonances – cyclotron and plasmon – exist simultaneously. The frequency of the electromagnetic waves was chosen in the vicinity of the terahertz. First, because of the practical importance of the terahertz electromagnetic range, and second, because of the convenience of observing resonance effects at these frequencies.
The terahertz experiments were carried out at the University of Regensburg. The material of choice was graphene. The high electronic quality of graphene allows long-lasting plasma oscillations. The fact is that oscillating electrons can pass from one sample boundary to another without ever encountering impurities.
The magnetization of graphene “spins” the electrons in orbit, creating the conditions for cyclotron resonance. Already at small field values - about one Tesla – the frequency of the cyclotron resonance falls into the desired terahertz range. In the experiment, graphene was illuminated by radiation from a terahertz laser. The more light is absorbed, the more the graphene heats up and the more its resistance changes. Thus, the change in resistance of graphene under the influence of light is a measure of its absorptive capacity.
The surprising result of the experiment was the super strong absorption of radiation by graphene at twice the frequency of the cyclotron resonance. The signal at the conventional cyclotron frequency was relatively weak. A detailed comparison of the experiment with the theory showed that the strong absorption is due to the interaction (“hybridization”) of the double cyclotron and plasmon resonances. Near the frequency of the double cyclotron resonance, the plasma waves are considerably slowed down – their speed drops to almost zero. Light incident on graphene is captured and transformed into an ultra-slow surface wave; these waves are “trapped” in the graphene and remain there until they are absorbed.
“The fact that absorption is enhanced when slow surface waves are excited has been known for some time,” explains Denis Bandurin, one of the paper’s lead authors, “However, it was previously thought that surface waves surface in semiconductors could not be slower than the electrons moving in the wave.For graphene, the speed of electrons is somewhere around 300 times slower than the speed of light.Our research shows that ‘there is actually no limit to the freezing of light, and it can be slowed to a complete stop when a small magnetic field is activated.
An unusual property of graphene here is that it combines three roles – antenna, absorber and photocurrent generator. Usually, in semiconductor engineering, these roles are assigned to different materials and different devices. At the same time, strong absorption in graphene can be achieved with extremely small device size (sub-wavelength).
“We expect that graphene in a magnetic field may turn out to be a super-absorber,” comments corresponding author of the paper Dmitry Svintsov, head of the 2D Materials Laboratory for Optoelectronics at the MIPT Center for photonics and 2D materials,” that is, it will not just capture light from an area larger than its geometric size. It will be able to capture light from an area larger than the square of the wavelength. The abnormally low speed of plasmons in magnetized graphene creates all the prerequisites for this.”
“In this study, graphene proved to be a very convenient platform for observing abnormally strong terahertz absorption,” comments the paper’s lead author, Professor Sergey Ganichev (University of Regensburg), “however, the observability of the phenomenon is not limited to graphene alone – many natural materials and nanostructures based on them support ultra-slow surface waves.
One of the research team’s immediate goals is to create compact super-absorbers that don’t require low temperatures and strong magnetic fields.