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We report a substantially improved localized and bulk sensing response of a plasmonic sensor based on a plasmonic nanopillar array spacer-separated from a plasmonic thin film and operated using the conventional Kretschmann configuration. This paper describes theoretical investigations of the spectral and angular interrogation of the proposed sensors to determine the spectral or angular reflectivity minima. Shifts in the reflectivity minima are determined to obtain information on perturbations in localized (local binding analyte layer sensing) and bulk refractive index (bulk medium sensing) on the sensor surface. Optimization of the structural parameters of the proposed sensor yields a highly enhanced sensor performance, with an extremely high sensitivity (9920 nm RIU−1) and figure of merit (58.2 RIU−1) for bulk sensing response, as well as extremely high sensitivity (76 nm/nm) and figure of merit (0.5 nm−1) for localized sensing response upon spectral interrogation of the sensor. The localized surface sensitivity for the proposed sensor, which is evaluated to be 76 nm/nm, is higher than what has been reported till now. The proposed sensors do not require any sophisticated optical set-ups and can be implemented on the optical setups used for the conventional Kretschmann configuration based sensors. The proposed sensor design could easily be fabricated using the state-of-the-art top-down nanofabrication approaches and could be employed for developing highly sensitive biosensing platforms. [ABSTRACT FROM AUTHOR]

• A Nano-cavity based multispectral plasmonic refractive index sensor was investigated. • Considering the sensitivity and proximity to the telecommunication window, the structure is designed. • Higher bulk and surface sensitivity for detection of biological analytes • A higher bulk sensitivity of 958.3 nm/RIU for the mode III. • Electric field enhancement and Surface charge distribution was evaluated. In this paper, a Nano-cavity based multispectral plasmonic refractive index sensor for prostate-specific antigen (PSA) detection was investigated. The proposed structure includes two cavities in the form of a T and E shape inside a metal layer. The validity of the findings was verified through the use of the 3D finite difference time domain (FDTD) method. The identification of biomolecules is difficult for conventional biosensors in which there is a very limited amount of analyte, other metrics should be used. In most cases, there is enough analyte to completely cover the sensing surface. Here, in addition to bulk sensitivity, the surface sensitivity was analyzed. The proposed structure has three resonance modes. According to our findings, the designed structure has a bulk sensitivity of 958.3 nm/RIU for the third mode (mode III), 550 nm/RIU for the second mode (mod II), and 637.1 for the first mode (mode I). Having three resonance modes makes this structure a suitable option for use in multispectral applications. The surface sensitivity of 429.4, 457.6, and 465.5 nm/RIU was obtained for surface layer thicknesses of 3.5 nm, 5 nm, and 6.5 nm, respectively. Our results show the proposed structure has good potential in different biological applications. [ABSTRACT FROM AUTHOR]

Efficient coupling of plasmonic nanomaterials to optically transparent polymers still is a challenge in order to obtain affordable, versatile, and sensitive surface plasmonic devices. The in-situ fabrication of gold and silver nanoparticles on PDMS has been reported, but the resulting bulk sensitivities (of up to 70 nm RIU) may still be improved. The authors report that few simple modifications to the general preparation of these composites (AuNPs@PDMS) can result in substantial improvements of the optical features. A two-steps growth of AuNPs@PDMS is found to be particularly effective. It includes chemical treatment of the PDMS surface before the formation of well-exposed and densely-packed 3D conglomerates of gold spheroids with enhanced bulk sensitivity. Differently from available approaches, the structures obtained by this method display sensitivity to refractive index change of about 250 nm per RIU. This is 3.5 times higher than spherical nanoparticles prepared by similar protocols and is near the optical performance of anisotropic NPs. Due to the strong 3D character of the structures, excellent plasmon coupling is realized on PDMS surface. The authors also show that these nanocomposite substrates can be subjected to external stimuli and then exhibit red shifts or blue shifts typical of induced plasmon coupling and uncoupling. Hence, the method represents a major step forward in terms of high-performance composite plasmonic nanomaterials for use in biosensing. [ABSTRACT FROM AUTHOR]

Highlights: [•] Stable SPR substrates were fabricated with standard evaporation techniques. [•] Au substrates and SiO2-covered Au substrates were compared. [•] Minimal experimental bulk sensitivity change with additional layers was observed. [•] Experimental results were confirmed by theoretical simulations. [•] Similar bio-assay results for both substrate types. [ABSTRACT FROM AUTHOR]

Cross-stacked gratings are introduced for refractive index sensing based on guided mode resonance (GMR) effect. Reflection from and transmission through cross-stacked gratings is computed after converting the diffraction problem into an equivalent transmission line formulation. This formulation, which lends itself for the analysis of a biperiodic dielectric structure, can be used to calculate diffraction characteristics of the cross-stacked gratings with a high degree of accuracy. With the help of this analysis, it is shown that the cross-stacked gratings possess two mutually coupled GMRs. The method of analysis is then applied to various cross-stacked gratings to estimate their performance when used as a biosensor. It will be shown that, in comparison with conventional single grating GMR sensor with equal available area, a roughly two-fold surface sensitivity, equal bulk sensitivity, and extremely narrow resonance linewidth can be obtained by using cross-stacked gratings. Consequently, this structure improves detection ability of the GMR biosensor particularly for small analyte detection. [ABSTRACT FROM PUBLISHER]

Soft-X-ray angle-resolved photoelectron spectroscopy (ARPES) with photon energies around 1 keV combines the momentum space resolution with increasing probing depth. The concepts and technical realisation of the new soft-X-ray ARPES endstation at the ADRESS beamline of SLS are described. The experimental geometry of the endstation is characterized by grazing X-ray incidence on the sample to increase the photoyield and vertical orientation of the measurement plane. The vacuum chambers adopt a radial layout allowing most efficient sample transfer. High accuracy of the angular resolution is ensured by alignment strategies focused on precise matching of the X-ray beam and optical axis of the analyzer. The high photon flux of up to 1013 photons s-1 (0.01% bandwidth)-1 delivered by the beamline combined with the optimized experimental geometry break through the dramatic loss of the valence band photoexcitation cross section at soft-X-ray energies. ARPES images with energy resolution up to a few tens of meV are typically acquired on the time scale of minutes. A few application examples illustrate the power of our advanced soft-X-ray ARPES instrumentation to explore the electronic structure of bulk crystals with resolution in three-dimensional momentum, access buried heterostructures and study elemental composition of the valence states using resonant excitation. [ABSTRACT FROM AUTHOR]

Abstract: A modified waveguide model for predicting the sensitivity of surface-relief guided mode resonance (GMR) biosensors is presented. The proposed model is a convenient and fast calculation method compared to the current optical simulation tools such as the rigorous couple wave analysis (RCWA). A systematic and theoretical discussion of the GMR geometric structure and sensitivity of the modified model for biosensing is also presented. For a given GMR material, the theoretical maximum sensitivity can be achieved by choosing the proper geometric structure from the model simulations. A comparison of the GMR biosensor sensitivity respectively calculated by the model and the RCWA method shows good agreement. Finally, a suspending GMR structure with a 4.5-fold increase in sensitivity compared to the traditional surface-relief GMR is proposed. [Copyright &y& Elsevier]

Abstract: Bulk sensitivity is inevitable for photoelectron spectroscopy (PES) when one studies bulk electronic structures of strongly correlated electron systems, which are often much different from surface electronic structures. Combination of soft and hard X-ray PES (SXPES and HAXPES) is a promising approach for this purpose by quantitatively evaluating the contribution of the surface in the observed angle integrated PES spectra. Even in the angle resolved PES studies (ARPES), the bulk sensitivity of the SX-ARPES is required to get the real bulk band dispersions and Fermi surface topology, which may be noticeably modified in the surface region as seen in several materials studied in this paper. Although hard X-ray ARPES is feasible, deep attention is required for the discussion of the possible recoil effects for the valence band. Besides, extremely low energy PES (ELEPES) by use of microwave excited Xe, Kr and Ar lamps will be as useful as those by synchrotron radiation and laser to realize a very high resolution of better than 5meV with bulk sensitivity under certain conditions. [Copyright &y& Elsevier]

Gold nanoantennas have been used in a variety of biomedical applications due to their attractive electronic and optical properties, which are shape- and size-dependent. Here, a periodic paired gold nanostructure exploiting surface plasmon resonance is proposed, which shows promising results for Refractive Index (RI) detection due to its high electric field confinement and diffraction limit. Here, single and paired gold nanostructured sensors were designed for real-time RI detection. The Full-Width at Half-Maximum (FWHM) and Figure-Of-Merit (FOM) were also calculated, which relate the sensitivity to the sharpness of the peak. The effect of different possible structural shapes and dimensions were studied to optimise the sensitivity response of nanosensing structures and identify an optimised elliptical nanoantenna with the major axis a, minor axis b, gap between the pair g, and heights h being 100 nm, 10 nm, 10 nm, and 40 nm, respectively. In this work, we investigated the bulk sensitivity, which is the spectral shift per refractive index unit due to the change in the surrounding material, and this value was calculated as 526–530 nm/RIU, while the FWHM was calculated around 110 nm with a FOM of 8.1. On the other hand, the surface sensing was related to the spectral shift due to the refractive index variation of the surface layer near the paired nanoantenna surface, and this value for the same antenna pair was calculated as 250 nm/RIU for a surface layer thickness of 4.5 nm. [ABSTRACT FROM AUTHOR]

Surface plasmon resonance-based sensors have emerged as commercially fostering portable biodetectors. The scientific community is engaged in extensive research to improve their performance in terms of sensitivity, selectivity, and reproducibility for the recognition of specific biomolecules. Essentially, there is a need for miniaturizing the size of existing sensors with innovative designs without compromising their bioaffinity and sensitivity performance. In this work, we propose and demonstrate a grating-coupled surface plasmon polariton (SPP) sensor on a thin flat gold layer using a hybrid configuration. The proof of concept of the grating architecture has been realized through an innovative fabrication procedure, with experimental verification of its bulk sensitivity. The geometry is identical to the prism-coupling configuration, yet with miniaturization and compactness. Characteristics of the excited modes in the spectral regime of interest are investigated using the finite-difference time-domain simulations. The effective index calculation of the resonance conditions and the accompanying field distribution can identify the excited SPP and metal-assisted guided-mode resonance modes. Detailed probing of the electric field distribution of the desired SPP mode reveals an extended evanescent decay length of 1284 nm, close to the theoretical limit, and an extended propagation length of 270 μm. The experimental demonstration of the reflectance dip with two different analyte media perceived an increased bulk sensitivity of 1133 nm/RIU. Remarkably, this resonant mode exhibits sensing capabilities for a wide range of analyte refractive indexes. We believe that the fabricated configuration with observed high sensitivity and calculated ultradeep evanescent field penetration depth along with extended propagation length can lead to the designing of a hands-on biochip for detecting large biomolecules.