23 April 2018 Graphene hot electrons and nonlinear plasmonics

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ICFO researchers find the mechanism that explains how incoherent optical nonlinearities arise in graphene nanostructures. In the recent study “Transient nonlinear plasmonics in nanostructured graphene” published in Optica, ICFO researchers Joel Cox and ICREA Prof. Javier García de Abajo, from the Nanophotonics theory research group, elucidate the mechanism for strong optical nonlinearity arising due to plasmon-assisted electronic heating in graphene nanostructures.

Being able to confine light at the nano-scale has proven to be of major importance for the future development of technologies concerning optical sensing, photovoltaics, and optical communications, among others. Control of light-by-light at the nano-scale can be achieved through nonlinear plasmonics, which seeks to overcome the inherently weak nonlinear optical response of conventional materials through the amplification of local electric fields provided by plasmon resonances.

In this context, graphene has proven to be an ideal material for nonlinear plasmonics, as its linear electronic dispersion already results in a large intrinsic nonlinear response to optical fields, which can be further enhanced by the extreme concentration of light by its 2D plasmons.

Plasmons in highly-doped graphene can boost light absorption in the atomically-thin material well beyond its intrinsic 2.3% level in pristine samples. When resonantly illuminated by intense laser pulses, the absorbed light energy dramatically increases the temperature of electrons and causes a shift in the chemical potential. Through rigorous time-domain quantum-mechanical simulations of graphene nanoribbons, the researchers have shown that the combined optically-induced changes in electronic temperature and chemical potential effectively detune the graphene plasmon resonances, giving rise to a large incoherent nonlinear optical response that significantly reduces the light-intensity threshold for saturable absorption.

The results of their study predict that the incoherent process dominates the response under illumination by light pulses of longer duration, limiting the coherent manipulation of light down to very short pulse durations, but opening new avenues for transient plasmon-assisted light modulation in nanophotonic devices.