Graphene shows its full spectrum
Graphene—a single-atom thick layer of carbon atoms arranged in a honeycomb lattice—has been a fascinating subject for scientists and engineers for several years already. In recent years, it has been shown that in addition to the extraordinary mechanical, thermal and electrical properties, graphene shows great potential in manipulating electromagnetic fields. In particular at terahertz frequencies, graphene is an interesting platform to manipulate electromagnetic waves, allowing, for example, for extremely localized surface plasmons and deep subwavelength frequency selectivity. One the main advantages of graphene with respect to other materials is the extraordinary tunability of its conductivity, which can be achieved electrically by means of a back gate, or optically through the excitation of photocarriers.
Writing in Physical Review B, Vincent Ginis, Philippe Tassin, Thomas Koschny and Costas Soukoulis demonstrate that this tunability of graphene can be used in a novel setup for the generation of frequency combs. A frequency comb is a series of ultra-sharp electromagnetic pulses that recently gained a lot of attention because of its applicability in a plethora of disciplines, including spectroscopy, astronomical observations, chemical sensing, and attosecond pulse generation. Traditionally, these frequency combs are generated using nonlinear materials, e.g. Ti:Sapphire lasers, fibre lasers, or nonlinear microresonators. Currently, there is a lot of ongoing work to construct frequency combs for terahertz frequencies.
The researchers studied the interaction between light and time-dependent graphene sheets, including both dispersion and explicit time-dependence of the conductivity. Based on this model, they demonstrated that frequency combs can be generated without material nonlinearities. Indeed, at terahertz frequencies—a frequency range where there is a large need for novel components—it is possible to modulate the linear, time-dependent conductivity of graphene to obtain a large variety of frequency combs. These results open the way for highly tunable frequency comb generation in a miniaturized setup.
Vincent Ginis, Philippe Tassin, Thomas Koschny, and Costas Soukoulis, Physical Review B 91, 161403(R) (2015). Editor's Suggestion.