Engineering Tunable Plasmonic Nanostructures To Enhance Upconversion Luminescence

Plasmonic nanostructures, which can confine and manipulate light below the diffraction limit, are becoming increasingly important in many areas of optical physics and devices. One of the areas that can greatly benefit from surface-plasmon mediated confinement of optical fields is the enhancement of emission in low quantum yield materials. The resonant wavelength for plasmonic structures used for emission enhancement is either the excitation or emission wavelengths of the luminescent material. Therefore, a key component in designing plasmonic structures used in luminescent enhancement applications is the ability to engineer and tune plasmonic building blocks to create structures resonant at the desired wavelength. In this thesis, we have used two approaches to build tunable structures for luminescent enhancement: 1) using already synthesized metallic nanocrystals resonant at the desired wavelengths as building blocks, we designed structures that would result in maximum emission enhancement. 2) Designing arrays of plasmonic nanostructures with the help of simulation software to be resonant at the desired wavelength and then fabricating them with top-down nanoscale fabrication techniques. In either approach, the resulting large area structures were macroscopically studied by steady state and time-resolved photoluminescence measurements to quantify the plasmonic effects on enhancement. We were able to achieve high enhancement factors in almost all of the structures and designs. Furthermore, we were able to identify and study various effects that play a role in plasmonic enhancement processes

Plasmonic Enhancement of Nanophosphor Upconversion Luminescence in Au Nanohole Arrays

Arrays of subwavelength holes (nanoholes) in Au films were computationally designed, fabricated, and used as templates to localize and enhance the luminescence of upconversion nanophosphors (UCNPs)hexagonal phase NaYF₄ doped with Yb³⁺ and Er³⁺. The dimensions of nanohole Au arrays were designed to accept only a single UCNP upon particle filling and with a periodicity to be resonant with the excitation wavelength of the upconversion. Frequency-dependent luminescence enhancements of up to 35-fold and a concomitant shortening of the UCNP luminescence rise time were observed, consistent with simulations of plasmonic enhancement of the UCNP absorption

Plasmon-Enhanced Upconversion Luminescence in Single Nanophosphor–Nanorod Heterodimers Formed through Template-Assisted Self-Assembly

We demonstrate plasmonic enhancement of upconversion luminescence in individual nanocrystal heterodimers formed by template-assisted self-assembly. Lithographically defined, shape-selective templates were used to deterministically coassemble single Au nanorods in proximity to single hexagonal (β-phase) NaYF₄:Yb³⁺,Er³⁺ upconversion nanophosphors. By tailoring the dimensions of the rods to spectrally tune their longitudinal surface plasmon resonance to match the 977 nm excitation wavelength of the phosphors and by spatially localizing the phosphors in the intense near-fields surrounding the rod tips, several-fold luminescence enhancements were achieved. The enhancement effects exhibited a strong dependence on the excitation light’s polarization relative to the rod axis. In addition, greater enhancement was observed at lower excitation power densities due to the nonlinear behavior of the upconversion process. The template-based coassembly scheme utilized here for plasmonic coupling offers a versatile platform for improving our understanding of optical interactions among individual chemically prepared nanocrystal components

Metal-Enhanced Upconversion Luminescence Tunable through Metal Nanoparticle–Nanophosphor Separation

We have demonstrated amplification of luminescence in upconversion nanophosphors (UCNPs) of hexagonal phase NaYF₄ (β-NaYF₄) doped with the lanthanide dopants Yb³⁺, Er³⁺ or Yb³⁺, Tm³⁺ by close proximity to metal nanoparticles (NPs). We present a configuration in which close-packed monolayers of UCNPs are separated from a dense multilayer of metal NPs (Au or Ag) by a nanometer-scale oxide grown by atomic layer deposition. Luminescence enhancements were found to be dependent on the thickness of the oxide spacer layer and the type of metal NP with enhancements of up to 5.2-fold proximal to Au NPs and of up to 45-fold proximal to Ag NPs. Concomitant shortening of the UCNP luminescence decay time and rise time is indicative of the enhancement of the UCNP luminescence induced by resonant plasmonic coupling and nonresonant near-field enhancement from the metal NP layer, respectively