Politecnico di Milano: Controlling How Fast Graphene Cools Down

Graphene is the thinnest material ever produced, with the thickness of a single atomic layer, thinner than a billionth of a meter.

A property of its is to efficiently absorb light from the visible to the infrared through the photoexcitation of its charge carriers. After light absorption, its photoexcited charge carriers cool down to the initial equilibrium state in a few picoseconds, corresponding to a millionth of a millionth of a second. The remarkable speed of this relaxation process makes graphene particularly promising for a number of technological applications, including light detectors, sources and modulators.

A recent study published in ACS Nano has shown that the relaxation time of graphene charge carriers can be significantly modified by applying an external electrical field. The research was conceived within an international collaboration between the CNR-IFN, Politecnico di Milano, the University of Pisa, the Graphene Center of Cambridge (UK) and ICN2 of Barcelona (Spain), and it is supported by the European project Graphene Flagship.

This work paves the way to the development of devices that exploit the control of the relaxation time of charge carriers to support novel functionalities. For example, if graphene is used as saturable absorber in a laser cavity to generate ultrashort light pulses, by changing the relaxation time of the charge carriers, we can control the duration of the output pulses.

The theoretical modeling of the relaxation of the charge carriers of graphene as a function of the external electric field has allowed the identification of the physical mechanism underlying the observed phenomenon. The graphene-based device has been studied by ultrafast spectroscopy, which allowed to monitor the variation of the relaxation time of the charge carriers.

This discovery is of large interest for a number of technological applications, ranging from photonics, for pulsed laser sources or optical limiters that prevent optical components damaging, to telecommunication, for ultrafast detectors and modulators

concludes Giulio Cerullo, professor of the Department of Physics of Politecnico di Milano.