Common-mode plasmon sensing scheme as a high-sensitivity compact SPR sensor.

A deep metal grating enables quasi-phase-matched simultaneous excitation of two counterpropagating surface plasmon modes by means of its +1st and -2nd diffraction orders. The resulting angular reflection spectra of the scattered -1st and zeroth orders exhibit three interleaved zeros and maxima in a range centered around the Littrow angle. The spectra differ thoroughly from the usual reflection dip resulting from single-order plasmon coupling that produces strong absorption. The zeroth and -1st orders exhibit two crossing angles enabling high-sensitivity common-mode detection schemes designed to reject variations in source power and environmental noise. The proof of concept and experimental assessment of this new surface plasmon resonance (SPR) sensing scheme are demonstrated by monitoring gases in a pressure-controlled chamber. A limit of detection (LOD) of 2 × 10-7 refractive index unit (RIU) was achieved.

Metal-coated concave cone in a fused-silica rod as a multi-function plasmonic element.

A collimated light beam parallel to the axis of a fused-quartz cylinder impinging on a 90° apex angle concave cone cut in a quartz rod is transformed into a cylindrical wave by total internal reflection. A thin metal film at the quartz-air interface enables excitation of the plasmon mode at the air side that can polarize the cylindrical wave and/or has the potential to monitor physical, chemical, or biological quantities or events at the inner wall of the cone. The present Letter first analyzes the plasmon coupling mechanism and conditions. It then describes the diamond-grinding technique achieving a smooth cone wall and the finest possible tip. The experimental evidence of the polarization conversion is brought on a diamond-grinded section of fused-silica rod and gold coating of the concave wall.

A versatile technology for colloidal crystal transfer using parylene coatings and hydrosoluble polymers.

We propose a novel versatile colloidal crystal transfer technique compatible with a wide range of water-insoluble substrates regardless of their size, material, and wettability. There are no inherent limitations on colloidal particles material and size. The method possibilities are demonstrated via the colloidal transfer on quartz, glass substrates with a flat and curved surface, and via the fabrication of 3D colloidal structure with 5 overlaid colloidal monolayers. The process occurs at a room temperature in water and is independent from the illumination conditions, which makes it ideal for experimental manipulations with sensitive functional substrates. We performed the nanosphere photolithography process on a photosensitive substrate with a transferred colloidal monolayer. The metallized hexagonal arrays of nanopores demonstrated a clear resonant plasmonic behavior. We believe that due to its high integration possibilities the proposed transfer technique will find applications in a large-area surface nanotexturing, plasmonics, and will speed up a device fabrication process.

Effect of roughness on surface plasmons propagation along deep and shallow metallic diffraction gratings.

The roughness of shallow or deep metallic diffraction gratings modifies the propagation of surface plasmon mode along the metallic-air interface. The scattering losses lead to a spectral or angular broadening of the surface plasmon resonance (SPR) and to a shift of the resonance wavelength and coupling angle. This mechanism is deeply analyzed both experimentally and theoretically to overcome these effects when such structures, in particular deep ones, are used as SPR-based sensors.

Compensation of disorder for extraordinary optical transmission effect in nanopore arrays fabricated by nanosphere photolithography.

The work considers the effect of extraordinary optical transmission (EOT) in polycrystalline arrays of nanopores fabricated via nanosphere photolithography (NPL). The use of samples with different qualities of polycrystalline structure allows us to reveal the role of disorder for EOT. We propose a phenomenological model which takes the disorder into account in numerical simulations and validate it using experimental data. Due to the NPL flexibility for the structure geometry control, we demonstrate the possiblity to partially compensate the disorder influence on EOT by the nanopore depth adjustments. The proposed experimental and theoretical results are promising to reveal the NPL limits for EOT-based devices and stimulate systematic studies of disorder compensation designs.

Systematic study of resonant transmission effects in visible band using variable depth gratings.

The article focuses on depth-dependent visible band transmission effects in a symmetrical "insulator-metal-insulator" diffraction system based on a variable depth grating. These effects were studied both experimentally and theoretically in TM and TE polarizations. In particular, the existence of an optimized grating depth for plasmon-mediated resonant transmission was confirmed experimentally, and differences in TE and TM transmission behavior are discussed. We utilize a simple and flexible fabrication approach for rapid synthesis of apodized structures with adiabatically varying depth based on a beat pattern of two interferential lithography exposures. The present study can be useful in the fields of transmission-based optical security elements and biosensors.

Surface Plasmon Resonance Based Temperature Sensors in Liquid Environment.

The aim of this work is to measure the temperature variations by analyzing the plasmon signature on a metallic surface that is periodically structured and immersed in a liquid. A change in the temperature of the sample surface induces a modification of the local refractive index leading to a shift of the surface plasmon resonance (SPR) frequency due to the strong interaction between the evanescent electric field and the metallic surface. The experimental set-up used in this study to detect the refractive index changes is based on a metallic grating permitting a direct excitation of a plasmon wave, leading to a high sensibility, high-temperature range and contactless sensor within a very compact and simple device. The experimental set-up demonstrated that SPR could be used as a non-invasive, high-resolution temperature measurement method for metallic surfaces.

Periodic TiO2 Nanostructures with Improved Aspect and Line/Space Ratio Realized by Colloidal Photolithography Technique.

This paper presents substantial improvements of the colloidal photolithography technique (also called microsphere lithography) with the goal of better controlling the geometry of the fabricated nano-scale structures-in this case, hexagonally arranged nanopillars-printed in a layer of directly photopatternable sol-gel TiO2. Firstly, to increase the achievable structure height the photosensitive layer underneath the microspheres is deposited on a reflective layer instead of the usual transparent substrate. Secondly, an increased width of the pillars is achieved by tilting the incident wave and using multiple exposures or substrate rotation, additionally allowing to better control the shape of the pillar's cross section. The theoretical analysis is carried out by rigorous modelling of the photonics nanojet underneath the microspheres and by optimizing the experimental conditions. Aspect ratios (structure height/lateral structure size) greater than 2 are predicted and demonstrated experimentally for structure dimensions in the sub micrometer range, as well as line/space ratios (lateral pillar size/distance between pillars) greater than 1. These nanostructures could lead for example to materials exhibiting efficient light trapping in the visible and near-infrared range, as well as improved hydrophobic or photocatalytic properties for numerous applications in environmental and photovoltaic systems.

Condensation phenomenon detection through surface plasmon resonance.

The aim of this work is to optically detect the condensation of acetone vapor on an aluminum plate cooled down in a two-phase environment (liquid/vapor). Sub-micron period aluminum based diffraction gratings with appropriate properties, exhibiting a highly sensitive plasmonic response, were successfully used for condensation experiments. A shift in the plasmonic wavelength resonance has been measured when acetone condensation on the aluminum surface takes place due to a change of the surrounding medium close to the surface, demonstrating that the surface modification occurs at the very beginning of the condensation phenomenon. This paper presents important steps in comprehending the incipience of condensate droplet and frost nucleation (since both mechanisms are similar) and thus to control the phenomenon by using an optimized engineered surface.

High resolution group refractive index measurement by broadband supercontinuum interferometry and wavelet-transform analysis.

In this paper we propose group refractive index measurement with a spectral interferometric set-up using a broadband supercontinuum generated in an air-silica Microstructured Optical Fibre (MOF) pumped with a picosecond pulsed microchip laser. This source authorizes high fringes visibility for dispersion measurements by Spectroscopic Analysis of White Light Interferograms (SAWLI). Phase calculation is assumed by a wavelet transform procedure combined with a curve fit of the recorded channelled spectrum intensity. This approach provides high resolution and absolute group refractive index measurements along one line of the sample by recording a single 2D spectral interferogram without mechanical scanning.