A new class of Surface-Enhanced Raman Scattering (SERS) substrates have been engineered by exploiting both Photonic Crystal (PC) and semiconductor technologies. Gold coated inverted pyramids nanotextured substrates allow reproducibility <10% and enhancement factors > 10<sup>6</sup> over large areas. Modelling and optical characterization of the engineered structures is demonstrated. Examples of applications to amino acids and illicit drug detection are given. Concentrations as low as ppm-ppb (mg/mL to ng/mL) have been measured depending on the adsorbed analytes. Information on structure and conformation of the molecule is inferred due to the richer nature of SERS spectra.
Continuum Generation (CG) in optical waveguides has been recently attracting widespread interest in fields requiring large spectral bandwidth such as metrology and Optical Coherence Tomography (OCT). Real time and in-vivo tissue imaging with cell resolution (Δz<1μm) is rapidly becoming the ultimate frontier of several OCT medical applications. CG wavelength and bandwidth are the pertinent criteria to obtain ultra high imaging resolution. The axial resolution in tissues is inversely proportional to the bandwidth whereas the central wavelength is chosen according to the minimum absorption of water and hemoglobin. Therefore optimal candidates for OCT low coherence sources1 are continua around 1μm as this is the zero group velocity dispersion wavelength of water.
In this work we demonstrate for the first time a low-noise continuum at very low powers in high index planar waveguides pumped at 1.04 μm. Bandwidths in excess of 150 nm at -3dB are generated with launching energies <1nJ/pulse in a ~2μm<sup>2</sup> single mode ridge waveguides pumped in the normal dispersion regime. Self-Phase Modulation (SPM) had proven to be the only nonlinear process responsible for the CG. The polarization of the generated continua is highly preserved. Great flexibility in engineering waveguide dispersion, mode matching and optical functionality on chip is demonstrated by the planar approach.
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 2<i><sup>nd</sup></i>, and 3<i><sup>rd</sup></i> order dispersion affects the continuum generated in strongly nonlinear 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 (<i>Ta</i><sub>2</sub><i>O</i><sub>5</sub>) waveguides grown by standard silicon processing techniques were used. The devices investigated were specially designed tapered ridges with ~5 mm<sup>2</sup> input modal volume and zero group velocity dispersion
at ~l - 3.7 mm. Self-phase modulation, which is responsible for
the spectral broadening of the continua, is tracked by finely tuning the both 2<i><sup>nd</sup></i> and 3<i><sup>rd</sup></i> 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 Dl > 100 nm for launched powers as low as 300 pJ. Spectral peak intensity can also be systematically modulated by simply scanning the 2<i><sup>nd</sup></i> and 3<sup><i>rd</i></sup> 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.
Ultra-high bandwidth continuum generation has been attracting enormous interest for applications in optical frequency metrology, low-coherence tomography, laser spectroscopy, dispersion measurements, sensor techniques and others. The acceptance of this new technology would greatly benefit from the availability of compact and user-friendly sources. High index planar devices provide a versatile and unique approach to continuum generation. The dispersion can be carefully engineered by choosing the material and the geometry of the waveguides. Optical integration can also be provided on the same platform. Hundreds of different waveguides having different and calibrated dispersions can be integrated in few tens of millimeters. Input and output of the 2D guides can be tailored to provide mode matching to fibers and pump lasers by means of single element bulk optics. In this paper for the first time we demonstrate a low-noise, ultra-high bandwidth continuum at 1.55 μm. A bandwidth in excess of 390 nm is obtained by launching energy as low as 50 pJ in a 12 mm short tapered planar waveguides. The pump wavelength was in the normal dispersion regime and was provided by a compact, fiber-based sub-100 femtosecond source.
The trivalent thulium ion is an interesting activator for silica fiber lasers because of the near infrared transition which is broadband tunable ( diode pumpable can be operated with photon conversion efficiencies greater than 100 and has yielded in excess of 1W output power when pumped by a cw Nd:YAG laser. The paper will review progress on this system and indicate some potential future developments.