Nonlinear nano-plasmonic waveguides
Creators: name: Davoyan, Artur R.
Publisher: name: The Australian National University
Year: 2011
Subjects: subject: QC176.8.P55 D38 2011
subject: Nanostructured materials Optical properties
subject: Plasmons (Physics)
subject: Nonlinear optics
subject: Nanoparticles

description: Physical processes at nanoscale (one millionth of a millimeter) started to attract strong interest from researchers in the past decade, since manufacturing and observation of such small objects became possible. In particular, plasmonics -the physics of light interaction with metallic structures at dimensions much smaller than that of the visible light wavelength -is one of the hot topics, offering numerous applications and intriguing effects. Plasmonic devices are suggested to substitute current electronic and photonic components due to their potential for miniaturizing signal processing devices including sensors, lasers, and other components for integrated optics. Moreover, high electromagnetic energy concentration in plasmonic structures gives possibility for achieving a nonlinear optical response at reasonably low signal power levels, thus providing opportunities for light manipulation and control. The purpose of this thesis is to study the light propagation in plasmonic waveguiding structures, and to reveal the fundamental effects in both linear and nonlinear plasmonic waveguides. We focus our analysis on the possibility of tuning and control over light propagation in such structures at the nanoscale. Chapter I gives a general introduction to the subject, and discusses the present state of the art in plasmonics and possible future developments in the field. The chapter also provides a detailed discussion of aims and purposes of this thesis. In Chapter II we discuss the basic electrodynamic properties of media, thereby we give an introduction to the theoretical methods and concepts employed in the analysis of original results carried out in the scope of this work (Chapters III-VI). We provide a classification of media, and give brief description of the main properties of each material type, including dispersion, second and third order nonlinear effects. The chapter is concluded with an introduction to the physics of surface plasmon-polaritons. We start our analysis of plasmonic waveguiding structures with the study of a linear planar metal-dielectric-metal waveguide, Chapter III. We show that the mode structure and corresponding dispersions strongly depend on the waveguide width and losses. We provide a classification of modes in the structure, and reveal that in lossy systems propagating and evanescent modes become mixed and cannot be distinguished from each other. We also observe mode transformation with the increase of loss strength, occurring due to several bifurcation scenarios. In Chapter IV we study multilayer metal-dielectric structures with linear chirp of the structure period. We analyze light propagation in both short and long structures. For short structures we find the spectrum of eigen-modes and reveal that practically all structures with linear gradient of period have equidistant states, corresponding to the Wannier-Stark ladder and manifesting the existence of plasmonic Bloch oscillations. For long structures we apply an asymptotic analytical method and find a novel regime of beam dynamics. In particular, we show that for long linearly chirped metal-dielectric structures it is possible to achieve different directions of energy flow at different edges of the structure, so that a paraxial beam propagating in such structure will curl. Chapters V and VI discuss nonlinear effects of plasmon propagation in waveguiding structures. First, in Chapter V we study both third and second order nonlinear processes in plasmonic systems. We show that in planar metal-dielectric{u00AC}metal waveguides the third order nonlinear optical response leads to the nonlinear dispersion of guided modes, depending on the input power level. Moreover, we reveal the symmetry breaking bifurcation, which occurs at certain power levels. We also address a more general question of plasmonic beam propagation in non-linear plasmonic wavegudies and derive an asymptotic analytical model describing beam propagation in such structures. We employ our model to the study of the plasmonic beam propagation at the metal -nonlinear dielectric interface and prove our theory with numerical simulations. We show that at certain power levels the plasmon-soliton is formed, however due to very strong losses in plasmonic structures the soliton propagates for very short distances. As for the second order nonlinear processes, we study the possibility of plasmon to plasmon frequency conversion in metal-dielectric-metal waveguide supporting plasmonic modes of different symmetry. Our analysis shows that the phase-matching condition between modes with different symmetry can be satisfied. This indicates the possibility of efficient nonlinear processes, including parametric amplification of plasmons. We discuss the conditions for plasmon-to-plasmon frequency conversion. In Chapter VI we study the plasmon propagation in nonlinear tapered waveguides. We show that for certain taper shapes it is possible to achieve the effective compensation of plasmon amplitude decay. Depending on this condition we reveal three regimes of plasmonic beam 'propagation in nonlinear tapers, and show that the tapering allows solitons to propagate for reasonably long distances, and at particular conditions three-dimensional nanofocusing in two-dimensional waveguides is possible. Finally we conclude the thesis with a brief summary of the main results and the outlook for the future of plasmonics in Chapter VII.