Caterina M. Netti

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

Reflectivity measurements of gold nanostructures graded in pitch and aperture size allow investigation of localized plasmons. A simple model confirmed by simulations explains the plasmon resonances. Such arrays are highly suitable Raman scattering substrates.

Observation of the developing optical continuum along a nonlinear waveguide

We describe what is to our knowledge the first nondestructive measurement of the evolution of an optical continuum as a function of distance along a nonlinear waveguide. Spectral mapping is achieved on a subwavelength scale by utilizing near-field microscopy to probe the waveguides evanescent field. The measured continuum broadening along the waveguide agrees in general form with predictions of broadening from theoretical calculations, but differs in some important details. Subwavelength resolution measurements are made both along and across the waveguide to reveal spectral variations not seen before by other techniques.

Group velocity measurement using spectral interference in near-field scanning optical microscopy

Near-field scanning optical microscopy provides a tool for studying the behavior of optical fields inside waveguides. In this experiment the authors measure directly the variation of group velocity between different modes of a planar slab waveguide as the modes propagate along the guide. The measurement is made using the spectral interference between pulses propagating inside the waveguide with different group velocities, collected using a near-field scanning optical microscope at different points down the guide and spectrally resolved. The results are compared to models of group velocities in simple guides.

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

Reflectivity measurements of gold nanostructures graded in pitch and aperture size allow investigation of localized plasmons. A simple model confirmed by simulations explains the plasmon resonances. Such arrays are highly suitable Raman scattering substrates.

Octave-wide continuum generation in high-index planar waveguide by 1.5-μm femtosecond pump

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.

Low-noise self-phase modulation continuum generation in high index tapered planar waveguide at 1040 nm

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 ~2μm2 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.

Tuning surface-enhanced raman scattering with resonant localised plasmons

We demonstrate how the enhancement of SERS is correlated to the matching of localized plasmons at the pump laser wavelength, by designing and measuring a variety of nanostructured gold surfaces, showing both tuning and reproducibility.

Anisotropic localised plasmons in rectangular pits

We demonstrate how localized plasmons on nanostructured gold surfaces can be made to have two independently tuneable resonant wavelengths using anisotropic voids. This allows double resonance conditions for scattering (such as SERS) to be obtained.

Generalized 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 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 (Ta2O5) 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 ~l - 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 Dl > 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.