Nonlinear optical effects in two dimensional and thin film materials

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.Engineering and Physical Sciences Research Council (EPSRC

Origins of All-Optical Generation of Plasmons in Graphene

This is the final version. Available on open access from Nature Research via the DOI in this recordGraphene, despite its centrosymmetric structure, is predicted to have a substantial second order nonlinearity, arising from non-local effects. However, there is disagreement between several published theories and experimental data. Here we derive an expression for the second order conductivity of graphene in the non-local regime using perturbation theory, concentrating on the difference frequency mixing process, and compare our results with those already published. We find a second-order conductivity (σ (2)≈ 10−17AmV−2 ) that is approximately three orders of magnitude less than that estimated from recent experimental results. This indicates that nonlinear optical coupling to plasmons in graphene cannot be described perturbatively through the electronic nonlinearity, as previously thought. We also show that this discrepancy cannot be attributed to the bulk optical nonlinearity of the substrate. As a possible alternative, we present a simple theoretical model of how a non-linearity can arise from phFundacio Privada Cellex, Spanish MINECO Severo Ochoa ProgrammeCERCA Programme/Generalitat de CatalunyaEngineering and Physical Sciences Research Council (EPSRC)Royal SocietyTATAEuropean Commissio

All-optical switching of an epsilon-near-zero plasmon resonance in indium tin oxide (article)

This is the final version. Available on open access from Nature Research via the DOI in this recordThe dataset associated with this article is available in ORE at https://doi.org/10.24378/exe.3004Nonlinear optical devices and their implementation into modern nanophotonic architectures are constrained by their usually moderate nonlinear response. Recently, epsilon-near-zero (ENZ) materials have been found to have a strong optical nonlinearity, which can be enhanced through the use of cavities or nano-structuring. Here, we study the pump dependent properties of the plasmon resonance in the ENZ region in a thin layer of indium tin oxide (ITO). Exciting this mode using the Kretschmann-Raether configuration, we study reflection switching properties of a 60 nm layer close to the resonant plasmon frequency. We demonstrate a thermal switching mechanism, which results in a shift in the plasmon resonance frequency of 20 THz for a TM pump intensity of 70 GW cm−2. For degenerate pump and probe frequencies, we highlight an additional two-beam coupling contribution, not previously isolated in ENZ nonlinear optics studies, which leads to an overall pump induced change in reflection from 1% to 45%.Engineering and Physical Sciences Research Council (EPSRC)Royal SocietyUS Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and EngineeringThe original version of the Supplementary Information associated with this article contained an error in Supplementary Figure 5, in which panel c of Supplementary Figure 5 had an error on the vertical axis. The HTML of the online article has been updated to include a corrected version of the Supplementary Information. See the online correction at https://doi.org/10.1038/s41467-021-22020-7 for full information. NOTE: the original Supplementary Information file in this record has been replaced with the corrected Supplementary Information file (on 2021-05-06)Note that there is an author correction to the supplementary information at https://doi.org/10.1038/s41467-021-22020-

Emancipation from the Whirlwind: Piety and Rebellion among Jewish-American Post-Holocaust and Christian Liberation Readings of Job

This article focuses upon the manner in which the Book of Job’s dissonant messages of theological radicalism and conservatism have been utilised within discussion of two specific episodes of innocent suffering in the modern world – the murder of six million Jews during the Holocaust and the suffering of the oppressed in the developing world. Overlaying the discussion, the following model is proposed: that, firstly, Christian liberation theologians emphasise the more theologically conservative messages that can be drawn from Job while asserting radical political opposition to those who possess power. Conversely, Jewish Holocaust theologians empathise with Job’s more theologically radical elements, yet do so within outlooks committed to conservatively maintaining the security and power of the state of Israel after two thousand years of Jewish powerlessness. This model is tested by focusing upon seven treatments of Job associated with liberation or Holocaust theologies. It is concluded that, although there are significant complications, in broad terms the model largely holds ” offering a comparative insight into contextual Christian and Jewish interpretations of the Bible in which political radicalism and theological radicalism are found to be at odds with one another

All-Optical Switching of an Epsilon-Near-Zero Plasmon Resonance in Indium Tin Oxide (dataset)

The data to reproduce the figures in the Nature Communication Publication "All-Optical Switching of an Epsilon-Near-Zero Plasmon Resonance in Indium Tin Oxide". The codes to plot the graphs are based on Python 3.8.5 and Mathematica 12. The final collection in an SVG file was done using Inkscape 1.0.The article associated with this dataset is available in ORE at: http://hdl.handle.net/10871/124741This is the dataset used for the Bohn et al. (2021) article "All-Optical Switching of an Epsilon-Near-Zero Plasmon Resonance in Indium Tin Oxide" published in Nature Communications.Engineering and Physical Sciences Research Council (EPSRC)Royal SocietyUS Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineerin