Title page for ETD etd-05082012-113216


Type of Document Dissertation
Author Daengngam, Chalongrat
Author's Email Address kengkung@vt.edu
URN etd-05082012-113216
Title Second-Order Nonlinear Optical Responses in Tapered Optical Fibers with Self-Assembled Organic Multilayers
Degree PhD
Department Physics
Advisory Committee
Advisor Name Title
Heflin, James R. Committee Chair
Davis, Richey M. Committee Member
Khodaparast, Giti A. Committee Member
Robinson, Hans D. Committee Member
Keywords
  • Patterning
  • Self-Assembly
  • Fiber taper
  • Phase matching
  • Second harmonic generation
  • Second-order nonlinear optics
  • Nanoparticles
  • Plasmonics
Date of Defense 2012-04-25
Availability unrestricted
Abstract
Owing to its centrosymmetric structure, the critical optical component of a silica fiber cannot to possess a second-order nonlinear optical susceptibility, χ(2) , preventing a sil- ica fiber from many potential applications. Here, we theoretically and experimentally demonstrate a new technique to generate large and thermodynamically stable second- order nonlinearity into silica optical tapered fibers without breaking the centrosymmetry of the silica glass. The nonlinearity is introduced by surface layers with high polar- ordering fabricated by a novel hybrid covalent/ionic self-assembly multilayer technique. Despite the overall rotational symmetry of the nonlinear fiber, we observe significant sec- ond harmonic generation with ∼ 400−500 fold enhancement of the SHG power compared to the traditional tapers. Phase matching for a SHG process in second-order nonlinear tapered fibers is also realized by the compensation of waveguide modal dispersion with material chromatic dispersion, which occurs only for submicron tapers where the modal dispersion is large. In addition, quasi-phase-matching for a nonlinear taper can be ac- complished by introducing a periodic pattern into the nonlinear film coating. We use UV laser ablation for the controlled removal of particular nonlinear film segments on a taper surface in order to produce a χ(2) grating structure. A resulting SHG enhancement from quasi-phase-matching is observed over a broadband spectrum of the pump light mainly due to the non-uniform shape of a taper waveguide. The laser ablation is a clean and fast technique able to produce well-define patterns of polymer films on either flat or curved substrate geometry. With surface layers contain- ing reactive functional groups e.g. primary amines, we demonstrate that the resulting patterned film obtained from the laser ablation can be used as a template for further self-assembly of nanoparticles with high selectivity. A pattern feature size down to ∼ 2 μm or smaller can be fabricated using this approach. We also discuss preliminary results on a novel technique to further improve spatial accuracy for selective self-assembly of nanoparticles at an unprecedented level. Different types of nanoparticles are joined in order to form well-defined, molecular-like superstruc- tures with nanoscale accuracy and precision. The technique is based on a selective surface functionalization of photosensitive molecules coated on metallic nanoparticles utilizing enhanced two-photon photocleavage at the plasmonically-active sites (hot spots) of the nanoparticles in resonance with an applied electromagnetic wave. As a result, the surface functional groups at the nanoparticle hot spots are different from the the other areas, allowing other kinds of nanoparticles to self-assemble at the hot spots with high degree of selectivity.

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