Effect of the Interface in Plasmon-enhanced Second Harmonic Generation from Nonlinear Optical Thin Films
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1248-D11-20
Effect of the Interface in Plasmon-enhanced Second Harmonic Generation from Nonlinear Optical Thin Films Hans D. Robinson1, Kai Chen1, Cemil Durak1, Akhilesh Garg2, Richey M. Davis2 and James R. Heflin1 1 Department of Physics, Virginia Tech, Blacksburg, VA, 24060 2 Department of Chemical Engineering, Virginia Tech, Blacksburg, VA, 24060 ABSTRACT The second order nonlinear optical (NLO) properties of two different ionic selfassembled multilayer (ISAM) films combined with Ag nanoparticles have been investigated. The plasmon resonances in the Ag particles concentrate the incident light, markedly increasing in the NLO efficiencies of the films. We find that the efficiency enhancement is significantly larger in conventional ISAM films compared to films made using a hybrid covalent ISAM technique (HCISAM), even though the intrinsic bulk second order non-linear susceptibility (χ(2)) is much larger for HCISAM films. We attribute this to the interfaces in HCISAM films being much easier to disrupt by external perturbations such as the metal deposition by which the nanoparticles are fabricated. We conclude that because the plasmon decay length is very short, the plasmonic enhancement of NLO effects primarily occurs at and near the film-particle interface. To discern the importance of the interfaces, we surrounded thin ISAM and HCISAM films with NLOinactive buffer layers, which confirmed this hypothesis, particularly in the case of HCISAM films. INTRODUCTION Ionic self-assembled multilayer (ISAM) films can exhibit high χ(2) values when properly designed and fabricated1-4. These films are fabricated through layer-by-layer (LbL) deposition5, in which a charged substrate is alternately immersed in solutions of oppositely charged polyelectrolytes. When NLO active chromophores are incorporated in the films, the majority of the chromophores will orient towards the surface, so that the film acquires the polar order necessary for a nonzero χ(2). However, the order is far from perfect, with a significant number of chromophores oriented in the opposite direction. To overcome this, our group has previously developed a method for depositing a hybrid covalent ISAM (or HCISAM) film, which results in greater polar order and therefore significantly larger χ(2) values. In HCISAM film, the polyanion is replaced by an NLO active monomer, which is covalently rather than electrostatically bound to the underlying polycation layer. The following polycation layer then binds electrostatically to the ionic groups of the monomer exposed at the surface4. We has also demonstrated a novel way of increasing the second-order NLO response of ISAM films by several orders of magnitude6. This was accomplished by plasmonic enhancement achieved by depositing triangular Ag nanoparticles (NPs) on ISAM films using nanosphere lithography (NSL)7, 8. We denote these films as NP-ISAM films. The second harmonic generation (SHG) conversion efficiency of a thin ISAM film could be enhanced by as much as 1600 times due to the localized surface plasmon resonances
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