Ruiqi Y. Chen

3D analysis of surface plasmon dispersion for SERS sensor based on inverted pyramid nanostructures

Surface enhanced Raman scattering (SERS) can be used to amplify the Raman cross-section of signals by several orders of magnitude, when a mixed photon-Plasmon mode (surface Plasmon polaritons) couples to molecules on a nano textured metallo-dielectric substrate. In this paper we demonstrate a comprehensive 3D computational model based on Rigorous coupled wave analysis (RCWA) for the purpose of analysing propagating and localised surface Plasmon polaritons supported by planar SERS substrates based on periodic array of metal coated inverted pyramidal nanostructures. Although studies [1, 2] have explored the optical properties of inverted square pyramidal pits using simulation and experimentation, there has yet been no investigation performed on rectangular inverted pyramidal pits. Here we perform 3D modelling and simulation on rectangular pit arrays with aspect ratio 1:1.2 over 400nm thick gold. We investigate the effect of incident polarisation and electric-field density within the pits and show that inverted rectangular pyramidal pit array can be used as highly effective SERS and Plasmonic substrates.

Polymer waveguide grating couplers for low cost nanoimprinted integrated optics

Waveguide grating couplers permit efficient coupling to planar waveguides, complete with relaxed alignment tolerances and the possibility of wafer scale device testing without cleaving. To date, most solutions have been implemented as 1D gratings in high index contrast waveguides (typically SOI) with high coupling strengths and lateral mode converters. Here, we report the design and optimization of 1D grating couplers in polymer waveguides with much lower index cores (n = 1.8). Basic parameters from grating theory are used as the basis for FDTD simulations scanning over etch depth and grating period. Several optimizations are tested, including top claddings, buried dielectric mirrors, and buried metal mirrors. More than 80% coupling efficiency to air is predicted for a uniform symmetric grating, 20 periods long, with a carefully positioned buried metal reflector. The designs are intended for monolithic integration in polymeric planar lightwave circuits mass-produced by a roll-to-roll nanoimprint lithography process, where metallic mirrors can be safely and successfully incorporated.

Integration of nanostructures and waveguide core for surface enhanced Raman spectroscopy: a novel excitation method

Surface Enhanced Raman Spectroscopy (SERS) allows the intensity of Raman scattering to be enhanced by a factor of 10 6 by placing molecules within a few nm of a rough metal surface. In this paper we investigate a completely different configuration for the excitation mechanism, incorporating an optical waveguide beneath a nano-structured precious metal surface. The pyramidal geometry projects the Plasmon field into free space, thus increasing the cross section of interaction between the analyte molecules and optical fields, thereby increasing device sensitivity. In this arrangement the excitation field comes from underneath and enters the nanostructures at the base. This allows the emission to reach the discrete sensing areas effectively and provides ideal parameters for maximum Raman interactions. Using FDTD modeling methods the waveguide coupled SERS nanostructures were analyzed and its performance at different gold thicknesses was determined. The model investigates efficiency of coupling between the waveguide and surface plasmons, but also investigates spatial localization around sharp features of the geometry. Thin films of aluminum oxide and silicon oxynitride were reactively sputtered and characterized to determine their suitability as the waveguide core material. It was found that silicon oxynitride slab waveguide losses were too high to be considered as the core. The 2D and 3D simulations were based on an aluminum oxide core.

Integrated waveguide and nanostructured sensor platform for surface-enhanced Raman spectroscopy

Abstract. Limitations of current sensors include large dimensions, sometimes limited sensitivity and inherent single-parameter measurement capability. Surface-enhanced Raman spectroscopy can be utilized for environment and pharmaceutical applications with the intensity of the Raman scattering enhanced by a factor of 106. By fabricating and characterizing an integrated optical waveguide beneath a nanostructured precious metal coated surface a new surface-enhanced Raman spectroscopy sensing arrangement can be achieved. Nanostructured sensors can provide both multiparameter and high-resolution sensing. Using the slab waveguide core to interrogate the nanostructures at the base allows for the emission to reach discrete sensing areas effectively and should provide ideal parameters for maximum Raman interactions. Thin slab waveguide films of silicon oxynitride were etched and gold coated to create localized nanostructured sensing areas of various pitch, diameter, and shape. These were interrogated using a Ti:Sapphire laser tuned to 785-nm end coupled into the slab waveguide. The nanostructured sensors vertically projected a Raman signal, which was used to actively detect a thin layer of benzyl mercaptan attached to the sensors.

Experimental Demonstration of On-Chip Optical Parametric Oscillation in Planar Tantalum Pentoxide Waveguides

Tantalum pentoxide ( Ta2O5 ) planar waveguides have recently been shown to possess unusually large nonlinearities, and nonlinear Kerr coefficient (n2), leading to potential applications in nonlinear integrated optics, such as supercontinuum generation. In this paper, we report the experimental demonstration of a third-order susceptibility (χ(3)) governed nonlinear optical parametric process within a 7 mm long planar tantalum pentoxide waveguide using a pump-probe configuration. When pumped at 800 nm, and seeded in the near infra-red (IR) the waveguides allow parametric conversion giving rise to signal photons in the visible spectrum. By seeding the parametric conversion process in the 1200 to 1600 nm IR telecoms range, we obtain continuously tunable output over the visible range (533 to 600 nm) from a single guide.

Anisotropic Ta2O5 waveguide etching using inductively coupled plasma etching

Smooth and vertical sidewall profiles are required to create low loss rib and ridge waveguides for integrated optical device and solid state laser applications. In this work, inductively coupled plasma (ICP) etching processes are developed to produce high quality low loss tantalum pentoxide (Ta2O5) waveguides. A mixture of C4F8 and O2 gas are used in combination with chromium (Cr) hard mask for this purpose. In this paper, the authors make a detailed investigation of the etch process parameter window. Effects of process parameters such as ICP power, platen power, gas flow, and chamber pressure on etch rate and sidewall slope angle are investigated. Chamber pressure is found to be a particularly important factor, which can be used to tune the sidewall slope angle and so prevent undercut.

Waveguide core integrated nanostructured SERS sensor platform

The intensity of Raman scattering can be enhanced by a factor of 106 using Surface Enhanced Raman Spectroscopy (SERS). In this method, molecules are placed within a few nm of a rough/nanostructured metal surface. In this paper we show fabrication and characterisation of an integrated optical waveguide beneath a nano-structured precious metal coated surface. By using a waveguide core, the excitation field comes from underneath and enters the nanostructures at the base. This allows the emission to reach the discrete sensing areas effectively and should provide ideal parameters for maximum Raman interactions. The nanostructured geometry projects the Plasmon field into free space, thus increasing the cross section of interaction between the analyte molecules and optical fields, thereby increasing device sensitivity. Thin films of silicon oxynitride were deposited using PECVD on to thermal oxide coated 4 inch wafers and annealed at various temperatures to obtain low loss layers suitable for the waveguide core material. Based on the results from our simulations, nanostructured features of various diameters/feature lengths and pitch were etched into the low loss silicon oxynitride layer. The sensor area was coated with a thin layer of gold (25nm) and a variety of optical measurements were completed for many of the processed test chips including broadband reflectrometry, normal incident Raman spectroscopy and waveguide Raman spectroscopy using a Raman probe above the sensor area. The results showed that detection of a Raman active molecule (Benzyl Mercaptan) was possible when excited from the underlying waveguide core with 104 sensitivity.

Tapered nanowire waveguide layout for rapid optical loss measurement by 'cut-back' technique

Tantalum pentoxide (Ta2O5) is a promising material for both linear and nonlinear integrated optical device fabrication due to its high refractive index, low absorption over a wide wavelength range, high nonlinear refractive index, large value of chi 3 and high optical damage threshold. In particular Ta2O5 rib and ridge waveguides provide an interesting platform for solid state Laser applications. Waveguide surface roughness and sidewall slope profile can induce significant scattering loss reducing the efficiency of the device. Optimization of these parameters is key to obtain ultimate performance of the final device. In this paper, we present a method and photolithographic mask layout suitable to allow easy measurement of optical propagation loss for planar rib or ridge waveguides. The procedure is equivalent to the standard ‘cut- back’ method, but one that does not requiring devices to be cleaved and polished multiple times. The mask incorporates a set of narrow nano-wire waveguides coupled by tapered waveguide sections to wide input /output guides. The lengths of the central nano-wire section are determined precisely by the lithographic mask. The layout is designed to allow losses of each sub-component such as taper sections and input waveguides to be removed from the measurement, giving accurate measurement of loss in the central nanowire section of the guide. Optical loss measurements are presented for Ta2O5 nanowire rib waveguides. Loss was found to be dependent on lengths and widths of nanowire waveguide sections. Measured propagation losses for the rib waveguides are found to be just slightly higher than loss of a Ta2O5 slab waveguide as measured by a commercial Metricon system, validating the low loss nanowire waveguide fabrication processes.