Wakana Kubo

Au double nanopillars with nanogap for plasmonic sensor.

We propose a simple, precise, and wafer-scale fabrication technique for Au double nanopillar (DNP) arrays with nanogaps of several tens of nanometers. An Au DNP was simply constructed by alternately laminating thin layers of Au and polymer on a template and selectively removing the thin layers. This DNP array was expected to exhibit a specific plasmonic property induced by its narrow gap. When measuring the refractive index sensitivity (RIS), Au DNP arrays with 33 nm gaps exhibited a high RIS of 1075 nm RIU(-1) and showed a higher sensor figure of merit than the alternative structures, which did not have a nanogap structure but had almost the same surface area. This indicated that the enhanced plasmon electromagnetic field induced by the nanogap structure improved sensor performance. Our fabrication technique and the optical properties of the nanogap structure will provide useful information for developing new plasmonic applications with nanogap structures.

Super-hydrophobic/super-hydrophilic patterning of gold surfaces by photocatalytic lithography

Super-hydrophobic and super-hydrophilic gold surfaces were prepared by modifying microstructured gold surfaces with thiols. The perfluorodecanethiol (PFDT)-modified rough gold surface was converted from super-hydrophobic (water contact angle = 150–160°) to super-hydrophilic (0–10°) by photocatalytic remote oxidation using a TiO2 film. During the remote oxidation, oxygen-containing groups were introduced to the thiol, and finally, even sulfur atoms were removed. Super-hydrophobic/super-hydrophilic patterns were also obtained by photocatalytic lithography, by using a TiO2-coated photomask. On the basis of this technique, enzymes and algal cells were patterned on the gold surfaces to fabricate biochips.

Photocatalytic remote oxidation with various photocatalysts and enhancement of its activity

Photocatalytic remote oxidation with various photocatalysts, anatase TiO2, oxygen deficient TiO2, brookite TiO2, ZnO, WO3, metal (Au, Pt, Ag and Pd)-loaded anatase TiO2 and metal (Au, Pt and Ag)-loaded ZnO, were investigated. All these photocatalysts exhibited activities for remote oxidation. Among them, Pt-loaded TiO2 exhibited the highest activity; it took 2 min to make the octadecyltriethoxysilane-modified glass surfaces super-hydrophilic, while bare anatase TiO2 took 24 min. Since the remote oxidation activity of a TiO2 film that was prepared without organic compounds was almost the same as that of a normal TiO2 film, which contains organic impurities, the active species essential to the remote oxidation is not generated photocatalytically from the organic impurities but from ambient species such as O2 and H2O. The photocatalytic remote oxidation by TiO2 and Pt–TiO2 could be applied to poly(vinylidenefluoride) and many other materials but not to Teflon. The same trend was observed for Fenton and H2O2-UV reactions, in which hydroxyl radical is generated, suggesting that an oxidizing species as strong as a hydroxyl radical is involved in the remote oxidation.

Projection method for improving signal to noise ratio of localized surface plasmon resonance biosensors

This paper presents a simple and accurate method (the projection method) to improve the signal to noise ratio of localized surface plasmon resonance (LSPR). The nanostructures presented in the paper can be readily fabricated by nanoimprint lithography. The finite difference time domain method is used to simulate the structures and generate a reference matrix for the method. The results are validated against experimental data and the proposed method is compared against several other recently published signal processing techniques. We also apply the projection method to biotin-streptavidin binding experimental data and determine the limit of detection (LoD). The method improves the signal to noise ratio (SNR) by one order of magnitude, and hence decreases the limit of detection when compared to the direct measurement of the transmission-dip. The projection method outperforms the established methods in terms of accuracy and achieves the best combination of signal to noise ratio and limit of detection.

Bolometric photodetection using plasmon-assisted resistivity change in vanadium dioxide

Vanadium oxide is a key sensing material for bolometric photodetection, thanks to its strong temperature dependence of resistivity close to room temperature. Here we demonstrate the photodetection of a stoichiometric vanadium dioxide thin film integrated with silver nanorods. The nanorods convert light into heat, consequently suppressing the resistivity of vanadium dioxide via localised surface plasmon resonance. Incorporation of this thermo-plasmonic effect into bolometric photodetection allows for wavelength and polarisation sensitivity. This work opens the path to a broad family of photodetection functionalities for vanadium dioxide-based microbolometers.

Improved self-referenced biosensing with emphasis on multiple-resonance nanorod sensors

We present a novel approach to improve self-referenced sensing based on multiple-resonance nanorod structures. The method employs the maximum likelihood estimation (MLE) alongside a linear response model (LM), relating the sensor response (shifts in resonance wavelengths) to the changes due to surface binding and bulk refractive index. We also provide a solution to avoid repetitive simulations, that have been previously needed to determine the adlayer thickness sensitivity when measuring biological samples of different refractive indices. The finite element method (FEM) was used to model the nanorod structure, and the nanoimprint lithography was employed to fabricate them. The standard deviation of the results based on the MLE method is lower than that associated with the LM results. The method can be applied to an extended number of resonances to achieve a higher accuracy and precision.

Resonance enhancement of difference-frequency generation through localized surface plasmon excitation

We report the experimental observation of difference-frequency generation in gold nanoparticles under localized surface plasmon excitation. A zero-delay peak is detected in the differential transmission signal for a gold nanoparticle film with a MgF2 overlayer, showing that the energy transfer from pump light to probe light through the difference-frequency generation is resonantly enhanced by the excitation. This peak of differential transmission decreases in strength with higher probe fluences. Both the enhancement and the power dependence of the difference-frequency generation are explained by modeling the localized surface plasmons as a nonlinear Lorentz resonator.

Manipulation of a one dimensional molecular assembly of helical superstructures by dielectrophoresis

We demonstrate that it is possible to manipulate helical superstructures composed of self-assembled chiral lipids by dielectrophoresis while preserving the shape of the superstructures. Alignment and migration of the helical fibers were only observed for an applied ac field of 1 kHz–1 MHz. The structural and physical properties of the helical fibers were preserved even under ac electric fields. This is the first report on the manipulation of a helical molecular assembly wherein the original properties of the assembly remained unchanged even after manipulation.

Improved method for estimating adlayer thickness and bulk RI change for gold nanocrescent sensors

This paper presents a novel method employing the maximum likelihood estimation (MLE) technique alongside a nonlinear sensor response model to improve and extract more quantitative sensing results for localized surface plasmon resonance biosensors. The nonlinear response model treats the sensor response as a nonlinear function of the biomolecular adlayer thickness. This method makes use of the multiple resonance characteristic of nanocrescent structures in order to estimate the adlayer thickness and bulk refractive index (RI) change. Nanoimprint lithography is used here to fabricate the nanostructures. The finite element method (FEM) is used to model the nanocrescents and numerically validate the nonlinear-MLE method. Comparing to the established linear model, the proposed nonlinear-MLE method achieves 75% improvement in the limit of detection based on the estimated adlayer thickness and improves the bulk RI resolution by two orders of magnitude.

Plasmonic vanadium dioxide microbolometers with wavelength and polarisation sensitivity

Uncooled microbolometric photodetection is a key technology for low cost, reliable and lightweight infrared sensing but suffers in performance compared to cooled photodetectors. Introducing new microbolometer functionality such as wavelength and polarisation sensitivity will improve current device performance and encourage new market opportunities. One method is to introduce metallic nanostructures, which are widely known to exhibit strong localised surface plasmon resonances (LSPR) that are sensitive to incident wavelength and polarisation. This work presents the integration of plasmonic silver nanorods into the material vanadium dioxide VO2. An experimental correlation between suppression of VO2 resistivity and dips in transmission spectra was observed. Subsequent optical and thermal simulations of VO2 films, both on sapphire Al2O3 and suspended in air, demonstrate how LSPR-driven electric field enhancement leads to localised heating around the nanorods and subsequent temperature distribution on the nanoscale. This work opens the path to a broad family of photodetection functionalities for vanadium dioxide-based microbolometers.