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A Thesis Submitted for the Acquisition of Phd in Electronics and Computer UNIVERSITY OF SOUTHAMPTON Faculty of Physical Sciences and Engineering Electronics and Computer Science Dynamic Modulation of Plasmon Excitations in Monolayer Graphene by Nikolaos Matthaiakakis A Thesis submitted for the acquisition of PhD in Electronics and Computer Science by examination and dissertation ___________________________________________________________________ P a g e | 2 Chapter: Introduction ___________________________________________________________________ UNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF PHYSICAL SCIENCES AND ENGINEERING School of Electronics and Computer Science Doctor of Philosophy DYNAMIC MODULATION OF PLASMON EXCITATIONS IN MONOLAYER GRAPHENE by Nikolaos Matthaiakakis Abstract Plasmonic devices based on noble metals have offered solutions in numerous scientific and com- mercial fields over the past decades. Nevertheless the optical properties of noble metals are hardly tuneable thus not allowing for dynamic control of device properties. Offering a solution for achiev- ing efficient dynamically tuneable plasmonic devices is a crucial since it would significantly widen the range of plasmonic applications and open the way for on-chip photonic logic systems. Graphene has demonstrated high quantum efficiency for light matter interactions, strong optical nonlinearity, high optical damage threshold, and plasmons with high confinement and long propa- gation distances. Having a linear dispersion, zero bandgap, as well as very few free electrons avail- able under zero doping conditions, has made this material a strong candidate for realising dynamic and highly tuneable photonic and plasmonic devices. The interest of graphene as a platform for photonic applications is enormous with numerous pub- lications focusing on the realisation of electrostatically controlled optical devices utilizing novel properties offered by this material. Graphene plasmonics in particular have great promise in realis- ing highly efficient on-chip modulators, optical interconnects, waveguides, sensors, and even pho- tonic logic gates. Naturally, several issues need to be overcome in order for such devices to reach commercialization. Obtaining strong coupling of light with plasmons in graphene while also providing efficient long range frequency and intensity modulation of the plasmon absorption is a crucial and highly antici- pated goal for graphene based plasmonic devices. This work overcomes these issues by utilizing a novel diffraction grating/gold-insulator-graphene combined structure to dynamically couple, enhance, and manipulate plasmons in a graphene mon- olayer. The proposed structure consists of a two-dimensional inverted pyramid grating on a Si wa- fer, which acts as a phase matching component, and utilizes a gold back reflector and a transparent spacer in order to enhance coupling of plasmons on the graphene layer that lies above. An extra ionic gel layer above the monolayer of graphene is used to achieve efficient electrostatic control of the plasmon frequency and absorption efficiency. The pyramid grating structure properties were studied experimentally. Theoretical calculations as well as Rigorous Coupling Wave Analysis simulations of the final device setup provide evidence of extremely efficient plasmon modulation both in terms of frequency and absorption efficiency, reaching even total optical absorption under certain conditions. Furthermore the device configura- tion allows for dynamic switching of plasmon excitations thus providing a possible solution for pho- tonic switching applications. Finally, alternative materials for achieving tuneable plasmonic devices are also discussed. P a g e | 3 Chapter: Introduction ___________________________________________________________________ P a g e | 4 Chapter: Introduction ___________________________________________________________________ Table of Contents Abstract ........................................................................................................................... 3 Table of Contents ......................................................................................................... 5 Acknowledgements .................................................................................................... 11 Academic Thesis: Declaration of Authorship ...................................................... 13 1. Introduction ........................................................................................................ 15 2. Thesis Outline .................................................................................................... 19 3. Literature ............................................................................................................. 21 3.1. General Background .................................................................. 21 3.1.1. Plasmonics ........................................................................................................... 21 3.1.2. Diffraction and Wood-Rayleigh Anomalies ................................................. 24 3.1.3. Band Structure of Graphene ........................................................................... 25 3.1.4. Graphene Photonics .......................................................................................... 26 3.1.5. Photonic Devices Based on Graphene ......................................................... 28 3.1.6. Graphene Plasmonics ....................................................................................... 30 3.1.7. Excitation and Tuning of Graphene Plasmons .......................................... 32 3.1.8. Optimising the Plasmon Absorption of Graphene ................................... 34 3.1.9. Plasmonic Devices Based on Graphene ....................................................... 35 3.2. Growth and Transfer Process of Graphene ................................ 42 3.2.1. Graphene Growth ............................................................................................... 42 3.2.2. Chemical Vapour Deposition of Graphene ................................................. 43 3.2.3. Transfer Process of Graphene ........................................................................ 46 3.2.4. Chemical Doping of Graphene ....................................................................... 50 3.3. Characterization of Graphene ................................................... 52 3.3.1. Raman Spectroscopy of Graphene ................................................................ 52 3.3.2. Scanning Electron Microscopy Imaging of Graphene ............................. 55 3.3.3. Atomic Force Microscopy Imaging of Graphene ...................................... 56 3.3.4. Optical, Scanning Tunnelling and Transmission Electron Microscopy of Graphene ....................................................................................................................... 57 3.3.5. Comparison of Characterization Methods ................................................. 58 4. Theoretical Modelling and Simulations ....................................................... 61 P a g e | 5 Chapter: Introduction ___________________________________________________________________ 4.1. Tuneable Graphene Plasmonics Device Based on a 2D Grating ... 61 4.1.1. Theory for Electrostatic Tuning of Optical Properties of Graphene ... 61 4.1.2. Phase Matching .................................................................................................. 67 4.1.3. Rigorous Coupled Wave Analysis of the Device ....................................... 69 4.1.4. Optimisation of Structure Geometry............................................................ 72 4.1.5. Incident Light Polarization .............................................................................. 74 4.1.6. Dissipative Losses in Graphene .................................................................... 75 4.1.7. Conclusion ........................................................................................................... 77 4.2. Tuneable Total Optical Absorption Device Based on Graphene ... 78 4.2.1. Limitations of Basic Device ............................................................................. 78 4.2.2. Improved Device Concept ............................................................................... 79 4.2.3. Strong Enhancement of Optical Absorption in Graphene ..................... 81 4.2.4. Simulation Results ............................................................................................. 81 4.2.5. Deconvolution and Explanation of Plasmon Coupling Mechanisms .. 84 4.2.6. Contribution of the Salisbury Screen to the Absorption Spectra ........ 86 4.2.7. Angle of Incidence and Polarization ............................................................ 87 4.2.8. Rapid Optical Switching ................................................................................... 90 4.2.9. Tuneable Sensors and Couplers ................................................................... 91 4.2.10. Effect of losses on the Graphene Layer .................................................... 91 4.2.11. Conclusion ......................................................................................................... 92 4.3. Epsilon-Near-Zero Tuneable Plasmonic Device ........................... 93 4.3.1. Triply Resonant MIM/Salisbury Screen Device .......................................... 93 4.3.2. Background for Epsilon-Near-Zero ITO devices ........................................ 97 4.3.3. Tuneable Metal-ENZ-Insulator-Metal device .............................................
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