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 > 106 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.
In this paper we demonstrate ultra-low loss transmission across a photonic crystal super-prism device consisting of 600 lattice periods etched into a slab waveguide at wavelengths both above and below the primary band-gap. By modifying the refractive index of the holes we have reduced overall insertion loss to 4.5 dB across the entire visible region of the spectrum, greatly enhancing transmission and extinction in higher order stop-bands. In addition we show that the remaining loss is predominantly due to impedance mismatch at the boundary between patterned/unpatterned slab waveguide regions and so is no longer proportional to the length of the photonic crystal or the number of lattice periods. This is an important step forward for the realization of functional photonic crystal time delay elements, dispersion compensators and super-prism spectrometer devices. Experimental loss measurements compare extremely well with Finite difference time domain simulations which were used to investigate the effect of etch depth on scattering loss. We find that partial penetration into the underlying buffer region causes massive scattering loss to substrate modes due to loss of waveguiding in the holes.
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.
We demonstrate the fabrication, characterization and simulation of visible wavelength superprism devices in photonic crystal waveguides. We studied the super refraction dependence on lattice symmetry orientation and for propagation angles close to the main symmetry orientation. A variety of rectangular lattices devices with various pitches and hole diameters as well as number of rows have been fabricated. We used our previously developed automated broadband spectral and angular measurement to map the chromatic refractivity. We found the refraction angles and sign to be dependent on the lattice orientation and bandgap. As the lattice was rotated away from the main symmetry direction the magnitude of the angular dispersion increased indicating enhanced super-refractive properties away from symmetry direction. We found the chromatic refraction to be up to 1°/nm close to the band edge of the principal bandgaps, 10x more than equivalent gratings, and 100x more than equivalent prisms [[xiv]]. Dispersion curve obtained from plane wave simulation allowed us to model the Bloch mode propagation directions in the periodic structure. We found these simple models to be in excellent agreement with the experimental results, allowing us to design a range of effective superprism devices.
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.
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.
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