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Almost everyone who knows something about graphene has heard about its many properties. Graphene possesses a number of excellent properties, and this makes it suitable for many applications—from low-tech additive to high-tech electronic applications.
Despite having excellent properties, scientists have been continually manipulating and changing the properties of graphene to find new ways of using graphene.
Manipulating the properties of graphene is nothing new. In fact, some of the most notable (and media-covered) research in recent years has revolved around manipulating the properties of graphene. Twisting it to a ‘magic angle’ is one such example.
While scientists know that graphene already has excellent properties, the nature of graphene (and its electronic structure and quantum properties) also makes it a highly tunable material. While more effort is required to see some of these properties, it opens the door for graphene to be used in more (and wider-reaching) applications.
Graphene has excellent properties that make it suitable for optoelectronic devices and has been touted as one of the most nonlinear optoelectronic materials that we know about, i.e., a material in an electronic device that changes its properties with varying voltages and currents.
Nonlinear properties are exploited in many modern-day devices, especially for switching and processing electronic signals. This also includes broadband electronic frequency multipliers and modulators at terahertz (THz) frequency ranges.
Recent research has emerged from teams in Germany and Spain that can control and tune the nonlinearities in graphene by applying an electrical voltage across it. While the researchers knew about the efficient nonlinearity properties in graphene (from previous research), controlling it had been the missing piece of the research.
Now, the team has achieved this goal, and even though it is still early days, the hope is that, by finding a way to control the nonlinearity in graphene, it could open the doors for graphene being used in electrical signal processing and signal modulation applications.
Optoelectronic Properties of Graphene
Graphene has a number of excellent properties and is often touted for its electronic properties, such as a very high electrical conductivity and charge carrier mobility. It is also known for its optical properties and is highly transparent as only 2.3% of light is blocked per atomic layer. Moreover, it can produce optical transitions in an applied electrical field and exhibit photoluminescence when excited with a near-infrared (NIR) laser. But what about its optoelectronic properties?
One of the defining features of graphene’s electronic properties lies in its electrons. The planar and delocalized nature of the graphene sheet leads to the electrons becoming massless. For this reason, the electrons in graphene are often referred to as massless Dirac fermions. This is why we see such a high electrical conductivity across the graphene sheet. But these properties can also be exploited for optoelectronic devices.
The Dirac-type electronic band structure in graphene means that graphene exhibits a very high nonlinear response to electronic fields in the THz frequency range in room temperature conditions. Graphene is both a highly efficient THz nonlinear absorber and an efficient terahertz frequency multiplier. The combination of both these properties makes graphene a suitable material for generating multiple THz harmonics in optoelectronic devices.
Tuning the Nonlinear Optoelectronic Properties of Graphene
The nonlinear response of graphene in electronic fields was found to be highly dependent on the current being passed through graphene. Moreover, graphene oscillates at frequencies that don’t exceed a few hundred GHz and has a field strength that is lower than the channel fields in current high-speed transistors.
So, in theory, you could alter the voltage across the device, and this would lead to graphene exhibiting a different response, which could enable it to be integrated into many sub-THz CMOS devices for the upconversion of signals to the THz range. This is what has been done in recent research.
A nonlinear response in graphene is directly generated by a moderate electric field. Graphene is also a highly tunable material. Graphene does not have any electronic gap between the valence and conduction bands, which is why it is conductivity is so high.
This gapless band structure means that the Fermi level can be easily tuned, and not just in one of the bands but also between the valence and conduction bands across the Dirac point. This is where a small gate voltage comes in to manipulate the nonlinear properties of graphene.
To tune the nonlinear properties of graphene, the researchers created a device similar to a transistor. In this device, a control voltage could be applied via a set of electronic contacts that acted as a gate. After the voltage was applied, ultrahigh-frequency THz signals were transmitted using the device. The transmission and transformation of these signals were then analyzed with respect to the applied voltage.
The research produced some interesting results. At a specific voltage (a few volts) it was found that the graphene went almost completely transparent. At this point, the graphene also lost all its nonlinearity response. Once the voltage deviated from this value, the graphene sheet turned into a strongly nonlinear material. This subsequently altered the strength and frequency of the transmitted THz signals.
This approach worked because of the ability to control and alter the number of electrons in the graphene sheet that can move freely (using the voltage as a control). In an applied electric field, more electrons can move freely, so the nonlinearity from the graphene is higher.
On the other hand, the greater the number of free moving electrons there are, the more likely they are to interact strongly with each other in certain circumstances, reducing the nonlinearity. By using the voltage as a control, the team was able to switch between high levels of nonlinearity and poor levels of nonlinearity in the graphene sheet.
The ability to tune the large nonlinearity properties of graphene by using a small gating voltage could lead to graphene being used in signal upconversion devices, as well as in nonlinear terahertz modulators, mixers, shutters, and switches. The work done to date reaches an important milestone towards getting graphene integrated into THz devices.
Because graphene itself is compatible with other CMOS technologies, it opens the possibility of creating hybrid devices where the initial (lower frequency) signal is produced by the longstanding semiconductor technology and then upconverted to the THz range in a controllable and predictable manner using the graphene components.
References and Further Reading
AzoNano [Online] What are the Optical Properties of Graphene. Available at: https://www.azooptics.com/article.aspx?ArticleID=1537
EurekAlert! [Online] Graphene: Everything Under Control. Available at: https://www.eurekalert.org/pub_releases/2021-04/bu-geu040821.php
Kovalev S. et al (2021) Electrical tunability of terahertz nonlinearity in graphene, Science Advances, DOI: 10.1126/sciadv.abf9809
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