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Nonlinear optics in graphene: Detailed characterization for application in photonic circuits

par Dremetsika, Evdokia
Président du jury Gorza, Simon-Pierre
Promoteur Kockaert, Pascal
Co-Promoteur Emplit, Philippe
Publication Non publié, 2018-01-18

Thèse de doctorat

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Résumé :

In the quest for ultrathin materials compatible with CMOS technology for all-optical signal processing applications in integrated photonics, graphene appears to be a promising candidate, with broadband1 optical properties and a high and broadband optical nonlinearity. However, researchers do not agree on the value of its nonlinear refractive index, and commonly used characterization methods do not provide a clear picture of the optical nonlinearity, in terms of its tensor nature or relaxation time. In the first part of this thesis, apart from the previously used Z-scan method, we have also used the ultrafast Optical Kerr Effect method coupled to Optical Heterodyne Detection (OHD-OKE) for the characterization of the third order optical nonlinearity of monolayer CVD graphene at telecom wavelengths. This method allows to separately measure the real and the imaginary part of the third-order nonlinearity, as well as their dynamics. With respect to the Z-scan method, OHD-OKE presents the major advantage of being robust against inhomogeneities of the sample. As such, we have demonstrated that graphene has a negative nonlinear refractive index, contrary to previously reported results. In addition, we have studied the real and imaginary part of graphene’s nonlinearity, when electrostatic gating is applied to change the chemical potential of graphene. Furthermore, we have proposed an enhanced version of the OHD-OKE method, together with the appropriate theoretical framework, in order to extract the tensor elements of the nonlinearity including the out-of-plane tensor elements. In particular, we have measured separately the time response of the two main tensor elements of the nonlinear susceptibility and we have experimentally verified that the out-of-plane tensor components are negligible. In the second part of this thesis, we have investigated, from an experimental point of view, the use of the nonlinear optical response of graphene for all-optical switching applications in integrated photonics. Namely, we have designed simple silicon nitride waveguide structures that constitute basic building blocks of switching devices, which were then fabricated and covered by graphene patches. Finally, we have experimentally tested the graphene-covered structures at low and high power levels and discussed the results.