Fabrication of multipoint side-firing optical fiber by laser micro-ablation.

A multipoint, side-firing design enables an optical fiber to output light at multiple desired locations along the fiber body. This provides advantages over traditional end-to-end fibers, especially in applications requiring fiber bundles such as brain stimulation or remote sensing. This Letter demonstrates that continuous wave (CW) laser micro-ablation can controllably create conical-shaped cavities, or side windows, for outputting light. The dimensions of these cavities determine the amount of firing light and their firing angle. Experimental data show that a single side window on a 730 µm fiber can deliver more than 8% of the input light. This can be increased to more than 19% on a 65 µm fiber with side windows created using femtosecond laser ablation and chemical etching. Fine control of light distribution along an optical fiber is critical for various biomedical applications such as light-activated drug-release and optogenetics studies.

Monolithic NPG nanoparticles with large surface area, tunable plasmonics, and high-density internal hot-spots.

Plasmonic metal nanostructures have shown great potential in sensing, photovoltaics, imaging and biomedicine, principally due to the enhancement of local electric field by light-excited surface plasmons, i.e., collective oscillation of conduction band electrons. Thin films of nanoporous gold have received a great deal of interest due to the unique 3-dimensional bicontinuous nanostructures with high specific surface area. However, in the form of semi-infinite thin films, nanoporous gold exhibits weak plasmonic extinction and little tunability in the plasmon resonance, because the pore size is much smaller than the wavelength of light. Here we show that by making nanoporous gold in the form of disks of sub-wavelength diameter and sub-100 nm thickness, these limitations can be overcome. Nanoporous gold disks not only possess large specific surface area but also high-density, internal plasmonic "hot-spots" with impressive electric field enhancement, which greatly promotes plasmon-matter interactions as evidenced by spectral shifts in the surface plasmon resonance. In addition, the plasmonic resonance of nanoporous gold disks can be easily tuned from 900 to 1850 nm by changing the disk diameter from 300 to 700 nm. Furthermore, nanoporous gold disks can be fabricated as either bound on a surface or as non-aggregating colloidal suspension with high stability.

Surface-enhanced Raman spectroscopy with monolithic nanoporous gold disk substrates.

Monolithic hierarchical nanoporous gold disks, 500 nm in diameter, 75 nm in thickness and 3.5 nm in pore radius, have been fabricated by hybrid processes. A surface-enhanced Raman scattering enhancement factor of at least 10(8) has been obtained on individual disks using benzenethiol self-assembled monolayer with 785 nm laser excitation.

Effects of intermediate dielectric films on multilayer surface plasmon resonance behavior.

The effects of intermediate dielectric films on multilayer surface plasmon resonance (SPR) behavior were studied in terms of biosensing applications. Ten simple and complex oxides and fluoride, including MgF(2) and MgO, SiO(2), TiO(2) and complex PZT family dielectric materials, were evaluated. The materials cover a wide range of refractive indices, from 1.19 for the porous silica film to 2.83 for the TiO(2) film. The resonance curves of the multilayer SPR configurations were taken from an angular modulated Kretschmann set-up under a fixed incident wavelength of 543.5 nm. The intermediate dielectric layer has no strong effect on the SPR resonance angle and minimum reflectance at the resonance point. Some intermediate dielectric films, such as MgF(2), porous silica, TiO(2) and PLZT, apparently reduce the width of the resonance curves, resulting in sharper resonance dips. Better performance of the multilayer SPR biosensor incorporating these dielectric films is expected.