Metamaterial perfect absorber simulations for intensifying the thermal gradient across a thermoelectric device.

The thermal gradient across a thermoelectric device is the key to convert heat energy into electricity. Here, we propose a metamaterial perfect absorber (MPA) that increases the thermal gradient across a thermoelectric device by local heat generation through absorbing thermal radiation emitted from an infinite-size blackbody radiator. The MPA, when attached on top of a bismuth telluride thermoelectric device, generates local heat that propagates to the device, resulting in an additional thermal gradient. The amount of local heat generated at the MPA and the output power of the thermoelectric device loaded with the MPA are examined through numerical calculations.

Coherently tunable metalens tweezers for optofluidic particle routing.

Nanophotonic particle manipulation exploits unique light shaping capabilities of nanophotonic devices to trap, guide, rotate and propel particles in microfluidic channels. Recent introduction of metalens into microfluidics research demonstrates the new capability of using nanophotonics devices for far-field optical manipulation. In this work we demonstrate, via numerical simulation, the first tunable metalens tweezers that function under dual-beam illumination. The phase profile of the metalens is modulated by controlling the relative strength and phase of the two coherent incident light beams. As a result, the metalens creates a thin sheet of focus inside a microchannel. Changes to the illumination condition allow the focus to be swept across the microchannel, thereby producing a controllable and reconfigurable path for particle transport. Particle routing in a Y-branch junction, for both nano- and microparticles, is evaluated as an example functionality for the tunable metalens tweezers. This work shows that tunable far-field particle manipulation can be achieved using near-field nano-engineering and coherent control, opening a new way for the integration of nanophotonics and microfluidics.

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 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.

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.

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.

Size-controlled simple fabrication of free-standing, ultralong metal nanobelt array.

Free-standing, ultralong (up to several millimeters) nanobelts of gold, silver, and copper were fabricated by a template approach. Firstly, a metal nanofin array was prepared on a substrate via metal nanocoating of the template surface and selective removal of the metal top layer and template. Electroless plating and sputtering were employed for the metal nanocoating. In this approach, the minimum width and thickness of the Au nanobelt were 95 and 30 nm, respectively. Systematic control of the nanobelt width (from 95 to 350 nm) was successfully achieved by adjusting the template height. Free-standing nanobelts of several millimeters in length were fabricated and maintained their unique structure and alignment, even on a mesh grid.

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.

Mechanisms of photocatalytic remote oxidation.

The mechanisms of photocatalytic remote oxidation have been elucidated. Hydrogen peroxide, generated on a photocatalyst (TiO2, Pt-TiO2, or Ag-TiO2), is transported in air and then photocleaved into hydroxyl radicals by UV light in the vicinity of a substrate, and the hydroxyl radicals attack the substrate.

Detection of H2O2 released from TiO2 photocatalyst to air.

H2O2 generated and released from TiO2 photocatalysts to the gas phase was detected. A flow-through cell packed with TiO2-coated glass beads was irradiated with UV light, and the gas flowing out of the cell was flushed through a collecting solution containing 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulforic acid) diammonium salt (ABTS) and peroxidase. Oxidative coloration of ABTS was observed only in the absence of catalase, indicating the presence of H2O2 in the gas. The quantum yield of the H2O2 generation was estimated to be > 1 x 10(-7). The detected amount of H2O2 decreased as the TiO2 thickness decreased. H2O2 was not detected when dry air or nitrogen was used.