Graphene for Better Terahertz-Control

A small graphene-based device allows better control of terahertz radiation. (Source: U Geneva)

The terahertz waves span frequency ranges between the infrared spectrum and giga­hertz waves. Terahertz waves allow for the detection of materials that are undetec­table at other frequencies. However, the use of these waves is severely limited by the absence of suitable devices and materials allowing to control them. Researchers at the University of Geneva, working with the Federal Polytechnic School in Zurich ETHZ and two Spanish research teams, have developed a technique based on the use of graphene, which allows for the poten­tially very quick control of both the inten­sity and the polari­zation of tera­hertz light. This disc­overy paves the way for a practical use of terahertz waves, in particular for imaging and tele­communica­tions.

Graphene is a single atomic layer of carbon atoms that form a honey­comb network. In the Department of Quantum Matter Physics of University Geneva’s Faculty of Sciences, Alexey Kuzmenko’s team has been working on graphene’s physical proper­ties for several years. “The inter­action between terahertz radiation and the electrons in graphene is very strong and we have therefore come to the hypo­thesis that it should be possible to use graphene to manage tera­hertz waves,” Kuzmenko explains.

Working within the framework of the European project Graphene Flagship, scientists have made a graphene-based tran­sistor adapted to tera­hertz waves. “By combining the electrical field, which enables us to control the number of electrons in graphene and thus allows more or less light to pass through, with the magnetic field, which bends the elec­tronic orbits, we have been able to control not just the intensity of the tera­hertz waves, but also their polarization,” comments Jean-Marie Poumirol, a member of the UNIGE research team. “It is rare that purely electrical effects are used to control magnetic phenomena.” Scientists are now able to apply such control over a complete range of tera­hertz frequen­cies.

A graphene based device allows better control of terahertz radiation. (Source: UNIGE)

Today, the Geneva research team’s focus is to move on from the prototype, and develop practical appli­cations and new oppor­tunities by control­ling terah­ertz waves. Their objective is to make terahertz waves indus­trially compe­titive in the next few years. There are two main areas of appli­cation for this inno­vation, the first being communi­cations. “Using a film of graphene asso­ciated with terahertz waves, we should be poten­tially able to send fully-secured infor­mation at speeds of about 10 to 100 times faster than with Wi-Fi or radio waves, and do it securely over short distances,” explains Poumirol. This would present a signi­ficant advantage in tele­communica­tions.

The second sphere of appli­cation is that of imaging. Being non-ionising, tera­hertz waves do not alter DNA and there­fore are very useful in medicine, biology and pharmacy. Addi­tionally, the control of the circular polari­zation of the tera­hertz waves will allow distinc­tion between different symmetries of biological molecules, which is a very important pro­perty in medical appli­cations. Further­more, there is potentially a very powerful appli­cation of these waves in homeland security. Kuzmenko continues, “Tera­hertz waves are stopped by metals and are sensitive to plastics and organic matter. This could lead to more effective means of detecting firearms, drugs and explosives carried by indi­viduals, and could perhaps serve as a tool to strengthen airport safety.” (Source: UNIGE)

Reference: J.-M. Poumirol et al.: Electrically controlled terahertz magneto-optical phenomena in continuous and patterned graphene, Nat. Comm. 8, 14626 (2017); DOI: 10.1038/ncomms1462

Link: Dept. of Quantum Matter Physics, University of Geneva, Geneva, Switzerland

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