For the first time, they have demonstrated that graphene can convert electronic signals with frequencies in the gigahertz (GHz) range.
Today's silicon-based electronic components operate at clock rates of several hundred GHz, that is, they are switching several billion times per second. The electronics industry is currently trying to access the terahertz (THz) range, i.e. up to thousand times faster clock rates. A promising material and potential successor to silicon could be graphene, which has a high electrical conductivity and is compatible with all existing electronic technologies.
Theory has predicted that graphene could be a very efficient "nonlinear" electronic material – that is a material that can efficiently convert an applied oscillating electromagnetic field into fields with a much higher frequency. However, all experimental efforts to prove this effect in graphene over the past decade have not been successful.
"We have now been able to provide the first direct proof of frequency multiplication from GHz to THz in a graphene monolayer and to generate electronic signals in the THz range with remarkable efficiency," explains Dr. Michael Gensch of HZDR.
Moreover, HZDR’s cooperation partners led by Prof. Dmitry Turchinovich, experimental physicist at the UDE, have succeeded in describing the measurements quantitatively well using a simple model based on fundamental physical principles of thermodynamics.
The researchers used graphene that contains many free electrons, which come from the interaction of graphene with the substrate onto which it is deposited, as well as with the ambient air. According to the team, these mobile electrons are excited by an oscillating electric field, they share their energy very quickly with the other electrons in graphene, which then react much like a heated fluid: From an electronic "liquid", figuratively speaking, an electronic "vapour" forms within the graphene. The change from the "liquid" to the "vapour" phase occurs within trillionths of a second and causes particularly rapid and strong changes in the conductivity of graphene. This is the key effect leading to efficient frequency multiplication, the researchers explain.
The scientists used electromagnetic pulses from the TELBE facility with frequencies between 300 and 680 gigahertz and converted them in the graphene into electromagnetic pulses with three, five and seven times the initial frequency, i.e. up-converted them into the terahertz frequency range.
Prof Turchinovich adds: “Graphene is possibly the electronic material with the strongest nonlinearity known to date. The good agreement of the measured values with our thermodynamic model suggests that we will also be able to use it to predict the properties of ultrahigh-speed nanoelectronic devices made of graphene.”
The experiment was performed using the superconducting-accelerator-based TELBE THz radiation source at the ELBE Center for High-Power Radiation Sources at the HZDR. Its hundred times higher pulse rate compared to typical laser-based THz sources made the measurement accuracy required for the investigation of graphene possible in the first place. A data processing method developed as part of the EU project EUCALL allows the researchers to use the measurement data taken with each of the 100,000 light pulses per second.