Gap-plasmon-enhanced gold nanoparticle photoluminescence is studied experimentally at the single-particle level. The photoluminescence spectra of gold nanoparticles on an Al 2 O 3 -coated gold film under both 532 and 633 nm excitation show a clear peak near the measured gap-plasmon resonance wavelength. Comparing the collected emission spectrum with that from a gold reference film under 633 nm excitation, a peak photoluminescence enhancement factor of 28 000 is observed. The spectral shape and absolute magnitude of the enhancement factors for both excitation wavelengths are reproduced using numerical calculations without the use of any free parameters. The photoluminescence enhancement is explained in terms of a gap-mode-enhanced e−h pair generation rate and a wavelength-dependent enhancement of the emission efficiency.

Voltage controlled wavelength tuning of the localized surface plasmon resonance of gold nanoparticles on an aluminum film is demonstrated in single particle microscopy and spectroscopy measurements. Anodization of the Al film after nanoparticle deposition forms an aluminum oxide spacer layer between the gold particles and the Al film, modifying the particle-substrate interaction. Darkfield microscopy reveals ring-shaped scattering images from individual Au nanoparticles, indicative of plasmon resonances with a dipole moment normal to the substrate. Single particle scattering spectra show narrow plasmon resonances that can be tuned from ~580 to ~550 nm as the anodization voltage increases to 12 V. All observed experimental trends could be reproduced in numerical simulations. The presented approach could be used as a general postfabrication resonance optimization step of plasmonic nanoantennas and devices.

ARTICLE This journal isThe effect of nanoscale surface roughness on the surface plasmon resonance of gold nanoparticles on thermally evaporated gold films is investigated experimentally and numerically. Single-particle scattering spectra obtained from 80 nm diameter gold particles on a gold film show significant particle-to-particle variation of the peak scattering wavelength of ±28 nm. The experimental results are compared with numerical simulations of gold nanoparticles positioned on representative rough gold surfaces, modeled based on atomic force microscopy measurements. The predicted spectral variation and average resonance wavelength show good agreement with the measured data. The study shows that surface roughness can have a significant impact on the spectral performance of particlebased plasmonic devices.

Gold nanoparticles on Al 2 O 3 -coated gold films are presented as a chemically and thermally robust platform for molecular sensing. Single particle spectroscopy as a function of Al 2 O 3 coating thickness shows reproducible gold nanoparticle scattering spectra in the range from 690 to 610 nm as the Al 2 O 3 thickness increases from 0 to 3.4 nm. Numerical simulation of these structures indicates that surface-enhanced Raman spectroscopy enhancement factors in excess of 10 6 can be achieved. The stability of the Al 2 O 3 -coated structures under high-power laser irradiation was tested, revealing stable scattering spectra upon irradiation with 100 W/mm 2 at the particle resonance wavelength. The presented structure solves challenges with thermal stability, wavelength tuning range, and Raman background signal associated with previously attempted approaches.

A spatially varying effective refractive index gradient using chalcogenide glass layers is printed on a silicon wafer using an optimized electrospray (ES) deposition process. Using solution-derived glass precursors, IR-transparent GeSbS and AsS glass films of programmed thickness are fabricated to yield a bilayer structure, resulting in an effective gradient refractive index (GRIN) film. Optical and compositional analysis tools confirm the optical and physical nature of the gradient in the resulting high-optical-quality films, demonstrating the power of direct printing of multimaterial structures compatible with planar photonic fabrication protocols. The potential application of such tailorable materials and structures as they relate to the enhancement of sensitivity in chalcogenide glass based planar chemical sensor device design is presented. This method, applicable to a broad cross section of glass compositions, shows promise in directly depositing GRIN films with tunable refractive index profiles for bulk and planar optical components and devices.