Electrically controlled third-harmonic generation in graphene has ultrawide bandwidth

Electrical control of third harmonic generation (THG) can be obtained in single-layer graphene. In THG three low-frequency photons (shown as red) sum to generate one high-frequency photon (shown as blue). (Credit: Giancarlo Soavi, University of Cambridge)

Researchers at Graphene Flagship, a graphene-centered joint coordinated research program by the European Commission, have for the first time demonstrated gate-tunable third-harmonic generation (THG) in graphene. The research, led by Graphene Flagship partner University of Cambridge (Cambridge, UK) in collaboration with Politecnico di Milano (Milan, Italy) and IIT-Istituto Italiano di Tecnologia in Genova (Genoa, Italy), could enable on-chip broadband optical switches for data transport in optical systems.

Optical harmonic generation occurs when high-intensity light interacts with a nonlinear optical material. THG produces photons with three times the energy of the incident photons. THG exploits a nonlinear interaction between high intensity light from a laser and a material. In principle, all materials can generate new frequencies of light by THG; however, the efficiency of this process is typically small and cannot be controlled externally. Graphene has strong light/matter interaction and a strong third-order nonlinear response, thus offering great potential for THG.

The researchers showed experimentally, for the first time, gate tuneable THG in graphene -- or, in other words, electrical control of the nonlinear optical response of graphene.  This can enable applications such as gate-tunable switches and frequency converters. The researchers showed that the strong THG in graphene can be controlled by an external electric field and also increased in efficiency over an ultrabroad bandwidth.

"Our work shows that the third-harmonic-generation efficiency in graphene can be increased by over 10 times by tuning an applied electric field," says Giancarlo Soavi from the Cambridge Graphene Centre at the University of Cambridge.

There are currently commercial devices using nonlinear optics for optical switches in spectroscopy. However, using graphene for THG can enable integration into devices working over an ultrabroad bandwidth. "Our initial research demonstrates the feasibility of this approach, so now we want to move closer to producing integrated devices in optical fibers and waveguides," says Soavi.


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