Nonlinear optical effects in two dimensional and thin film materials

Abstract

This thesis comprises work on three different projects in nonlinear optics. Those are the second order conductivity of graphene, plasmon enhanced four-wave mixing in graphene and intensity dependent differential reflection of ITO. \par The second order conductivity of graphene has been a contentious area for some time as, in a centro-symmetric material like graphene, one would expect no response within the dipole approximation. As experiments have shown strong second order signals for graphene, the field has sought explanations outwith the dipole approximation to explain the results. However, there has not been a consensus that agrees with experiments. Here we derive the second order conductivity of graphene through perturbation theory and, agreeing that this result cannot account for observed responses, seek alternate explanations for second order signals observed in experiment. This work concludes with the presentation of a photothermal model, based on the Seebeck effect, that predicts response two order of magnitude higher than the results of perturbation theory. This work provides a very promising first step to advancing the understanding the discrepancy between theory and experiment for second order nonlinear effects in graphene. \par Graphene is predicted to have large third order nonlinear effects due to its linear band structure in the vicinity of the Dirac points. This work presents a detailed study into the possibility of plasmon enhanced resonant cascaded four-wave mixing. An experimental setup is designed such that a four-wave mixing signal is generated from a graphene sheet and the DFG mode of the same incident beams can couple to the surface plasmon polariton in graphene through a difference frequency generation interaction. This work shows that the signal from a cascaded effect is not strong enough to contribute significantly to the wavemixing signal observed. This implies that the enhancement of electric field at the plasmon resonance is not sufficient to overcome the inherent weakness of multiple second order processes in graphene. \par Indium tin oxide (ITO) has recently been identified as a high promising nonlinear material, in particular for use optical switching devices, where the reflectance of one beam is changed through intensity of itself or another pump beam. This work presents a new geometry that predicts and measures a change in reflectivity at a glass-ITO-air interface larger than previously measured and for significantly smaller intensities. Further to this finding, models are presented that reveal constraints to the use of ITO as a nonlinear material. In particular, both the switching and pumping of the ITO must occur on femtosecond timescales, at higher picosecond timescales the strong electron-phonon coupling and high specific heat capacity of the lattice prevent such effects being observed. This work provides an excellent platform for moving towards an optical switch with ITO as the nonlinear active component.