Tuning localized plasmons in nanostructured substrates for surface-enhanced Raman scattering

Comprehensive reflectivity mapping of the angular dispersion of nanostructured arrays comprising of inverted pyramidal pits is demonstrated. By comparing equivalently structured dielectric and metallic arrays, diffraction and plasmonic features are readily distinguished. While the diffraction features match expected theory, localised plasmons are also observed with severely flattened energy dispersions. Using pit arrays with identical pitch, but graded pit dimensions, energy scaling of the localised plasmon is observed. These localised plasmons are found to match a simple model which confines surface plasmons onto the pit sidewalls thus allowing an intuitive picture of the plasmons to be developed. This model agrees well with a 2D finite-difference time-domain simulation which shows the same dependence on pit dimensions. We believe these tuneable plasmons are responsible for the surface-enhancement of the Raman scattering (SERS) of an attached layer of benzenethiol molecules. Such SERS substrates have a wide range of applications both in security, chemical identification, environmental monitoring and healthcare

Nd:Ta₂O₅ rib waveguide lasers

Ta2O5 waveguides offer great potential for high-density active photonic crystal circuits and their combination with rare-earth dopants for active devices is of interest for increasing their potential functionality. To this end, neodymium-doped Ta2O5 rib waveguide lasers have been fabricated on an oxidized silicon wafer by rf sputtering and argon ion-beam milling and laser action in this material has been demonstrated. Lasing was observed at wavelengths between 1060 and 1080 nm and an absorbed pump power threshold of 87 mW was obtained

Generalised ultrafast dispersion scans of continuum generation induced by sub-50fs chirped pulses in highly nonlinear tapered planar waveguides

Ultra-high bandwidth continua generated by ultrashort fs pulses have been attracting enormous interest for applications such as general spectroscopy, Optical Coherence Tomography and metrology. Dispersion engineering is one of the key aspects of optimised continuum generation in optical waveguides. However in addition, the dispersion of the pump pulse can be continuously adapted to control bandwidth and spectral characteristics of the generated continua. In this work we report on a systematic investigation of how 2nd,and 3rd order dispersion affects the continuum generated in strongly non linear planar waveguides. A ~30 fs Ti:Sapphire tuned to 800 nm was used as a pump source delivering ~3 nJ pulses. The chirp of the pulses was controlled completely-arbitrarily by an acousto-optic programmable dispersive filter (Dazzler). The power launched into the structures was kept constant to compare the generated continua as the pulse dispersion is varied. High refractive index tantalum pentoxide waveguides grown by standard silicon processing techniques were used. The devices investigated were specially designed tapered ridges with ~5 mm2 input modal volume and zero group velocity dispersion at ~1- 3.7 mm. Self-phase modulation, which is responsible for the spectral broadening of the continua, is tracked by finely tuning the both 2nd and 3rd order dispersions. The nonlinear propagation is dramatically influenced by the simultaneous presence of these dispersive effects resulting in a change of bandwidth and spectral shape. Pulse widths of up to D1 > 100 nm for launched powers as low as 300 pJ. Spectral peak intensity can also be systematically modulated by simply scanning the 2nd and 3rd order dispersion around their relative zeros. Specific combinations of high order dispersion contribution are currently targeted as a route to control and optimise the continua bandwidths and to control dispersion lengths in specifically engineered waveguides