Mode-Coupling Generation Using ITO Nanodisk Arrays with Au Substrate Enabling Narrow-Band Biosensing.

Plasmonic metal nanostructures have promising applications in biosensing due to their ability to facilitate light-matter interaction. However, the damping of noble metal leads to a wide full width at half maximum (FWHM) spectrum which restricts sensing capabilities. Herein, we present a novel non-full-metal nanostructure sensor, namely indium tin oxide (ITO)-Au nanodisk arrays consisting of periodic arrays of ITO nanodisk arrays and a continuous gold substrate. A narrow-band spectral feature under normal incidence emerges in the visible region, corresponding to the mode-coupling of surface plasmon modes, which are excited by lattice resonance at metal interfaces with magnetic resonance mode. The FWHM of our proposed nanostructure is barely 14 nm, which is one fifth of that of full-metal nanodisk arrays, and effectively improves the sensing performance. Furthermore, the thickness variation of nanodisks hardly affects the sensing performance of this ITO-based nanostructure, ensuring excellent tolerance during preparation. We fabricate the sensor ship using template transfer and vacuum deposition techniques to achieve large-area and low-cost nanostructure preparation. The sensing performance is used to detect immunoglobulin G (IgG) protein molecules, promoting the widespread application of plasmonic nanostructures in label-free biomedical studies and point-of-care diagnostics. The introduction of dielectric materials effectively reduces FWHM, but sacrifices sensitivity. Therefore, utilizing structural configurations or introducing other materials to generate mode-coupling and hybridization is an effective way to provide local field enhancement and effective regulation.

Near-infrared surface plasmon resonance sensor with a graphene-gold surface architecture for ultra-sensitive biodetection.

Binding behaviors of proteins are important for applications in the field of biochemistry. Though a standard assay has a favorable limit of detection (LOD), it is mainly limited to indirect observation via fluorescence labeling. We reported and demonstrated a novel label-free sensing approach based on a near-infrared (NIR) surface plasmon resonance (SPR) sensing chip modified with a graphene-gold surface architecture in this paper. The NIR excitation wavelength can greatly improve the sensitivity of SPR sensing derived from the wavelength modulation-based methodology. Moreover, benefiting from the excellent electro-optical properties of graphene in NIR range, the graphene-gold surface architecture was built to further improve the sensing sensitivity. Experimental results proved the superiority over most of those reported previously in terms of ultra-sensitivity (39,160 nm/RIU) and resolution (5.107 × 10-7 RIU). We detected human immunoglobulin G (IgG) to confirm the ability to enhanced-sensitive detection with a graphene overlayer. This sensor provides surface-specific detection schemes with a large linear dynamic range of ng/ml (pM) to fg/ml (aM) and a LOD of 7.2 fg/ml (48 aM) using gold nanoparticles (GNPs) as amplification labels. The proposed method provides a simple and effective strategy to improve sensitivity and LOD for biochemical detection in a rapid, ultrasensitive, and nondestructive manner.

Electromagnetically Induced Transparency-Like Effect by Dark-Dark Mode Coupling.

Electromagnetically induced transparency-like (EIT-like) effect is a promising research area for applications of slow light, sensing and metamaterials. The EIT-like effect is generally formed by the destructive interference of bright-dark mode coupling and bright-bright mode coupling. There are seldom reports about EIT-like effect realized by the coupling of two dark modes. In this paper, we numerically and theoretically demonstrated that the EIT-like effect is achieved through dark-dark mode coupling of two waveguide resonances in a compound nanosystem with metal grating and multilayer structure. If we introduce |1⟩, |2⟩ and |3⟩ to represent the surface plasmon polaritons (SPPs) resonance, waveguide resonance in layer 2, and waveguide resonance in layer 4, the destructive interference occurs between two pathways of |0⟩→|1⟩→|2⟩ and |0⟩→|1⟩→|2⟩→|3⟩→|2⟩, where |0⟩ is the ground state without excitation. Our work will stimulate more studies on EIT-like effect with dark-dark mode coupling in other systems.

Bidirectional band-switchable nano-film absorber from narrowband to broadband.

We propose a switchable perfect absorber with broadband and narrowband absorption based on alternating dielectric and metal nano-film structures in this paper. The lithography-free pattern is equipped with polarization insensitivity, good ductility and manufacturability, which has great significance in practical device development and applications. The quasi-complete selective absorption of incident light can be originated from asymmetric Fabry-Perot resonance, which combines the destructive interference in dielectric layers with inherent absorption in metal layers. When the light incidents on the surface covered with ultra-thin metal film of this structure, it acts as a narrowband absorber with over 99.90% absorption at 771 nm wavelength and a full wave at half maximum of 20 nm. When the light incidents on other surfaces covered with anti-reflective dielectric film, it achieves broadband perfect absorption with an average absorption exceeding 96.02% in a 500-1450 nm wavelength range. The absorption spectrum of oblique incidence shows that the broadband absorption behaves big angle range tolerance while the narrowband absorption exhibits angular dependence. The band-switchable performance of this absorber makes it valuable for energy harvesting/re-radiation applications in solar thermal photovoltaic systems.

Thymine-Functionalized Gold Nanoparticles (Au NPs) for a Highly Sensitive Fiber-Optic Surface Plasmon Resonance Mercury Ion Nanosensor.

Mercury ion (Hg2+) is considered to be one of the most toxic heavy metal ions. Once the content of Hg2+ exceeds the quality standard in drinking water, the living environment and health of human beings will be threatened and destroyed. Therefore, the establishment of simple and efficient methods for Hg2+ ion detection has important practical significance. In this paper, we present a highly sensitive and selective fiber-optic surface plasmon resonance (SPR) Hg2+ ion chemical nanosensor by designing thymine (T)-modified gold nanoparticles (Au NPs/T) as the signal amplification tags. Thymine-1-acetic acid (T-COOH) was covalently coupled to the surface of 2-aminoethanethiol (AET)-modified Au NPs and Au film by 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride/N-Hydroxysuccinimide (EDC/NHS) activation effect, respectively. In the presence of Hg2+ ions, the immobilized thymine combines specifically with Hg2+ ions, and forms an Au/thymine-Hg2+-thymine/Au (Au/T-Hg2+-T/Au) complex structure, leading to a shift in SPR wavelength due to the strong electromagnetic couple between Au NPs and Au film. Under optimal conditions, the proposed sensor was found to be highly sensitive to Hg2+ in the range of 80 nM-20 µM and the limit of detection (LOD) for Hg2+ was as low as 9.98 nM. This fiber-optic SPR sensor afforded excellent selectivity for Hg2+ ions against other heavy metal ions such as Fe3+, Cu2+, Ni2+, Ba2+, K+, Na+, Pb2+, Co2+, and Zn2+. In addition, the proposed sensor was successfully applied to Hg2+ assay in real environmental samples with excellent recovery. Accordingly, considering its simple advantages, this novel strategy provides a potential platform for on-site determination of Hg2+ ions by SPR sensor.

Freestanding bilayer plasmonic waveguide coupling mechanism for ultranarrow electromagnetic-induced transparency band generation.

Strong electromagnetic coupling among plasmonic nanostructures paves a new route toward efficient manipulation of photons. Particularly, plasmon-waveguide systems exhibit remarkable optical properties by simply tailoring the interaction among elementary elements. In this paper, we propose and demonstrate a freestanding bilayer plasmonic-waveguide structure exhibiting an extremely narrow transmission peak with efficiency up to 92%, the linewidth of only 0.14 nm and an excellent out of band rejection. The unexpected optical behavior considering metal loss is consistent with that of electromagnetic induced transparency, arising from the destructive interference of super-radiative nanowire dipolar mode and transversal magnetic waveguide mode. Furthermore, for slow light application, the designed plasmonic-waveguide structure has a high group index of approximately 1.2 × 105 at the maximum of the transmission band. In sensing application, its lowest sensing figure of merit is achieved up to 8500 due to the ultra-narrow linewidth of the transmission band. This work provides a valuable photonics design for developing high performance nano-photonic devices.

Plasmonic hybridization generation in self-aligned disk/hole nanocavities for multi-resonance sensing.

Plasmonic nanostructures have proven an extensive practical prospect in ultra-sensitive label-free biomolecule sensing due to their nanoscale localization and large near-field enhancement. Here, we demonstrate a photonic plasmonic hybridization in the self-aligned disk/hole nanocavity array under two specific cases of nanogap and nanooverlap achieved by adjusting pillar height embedded into hole. The proposed disk/hole arrays in above two cases exhibit three hybridized modes with extremely high absorption, mainly arising from the in-phase (bonding) and out-of-phase (antibonding) coupling of dipolar modes of their parent disk and hole. Surprisingly, when the nanogap feature of the disk/hole array is transformed to the nanooverlap, crossing the quantum effect region, the bonding mode in the disk/hole array has an enormous transition in the resonant frequency. In comparison with the counterpart in the nanogap structure, the bonding mode in the nanooverlap structure supports strongest near-field localization (i.e., the decay length down to merely 3.8 nm), although charge transfer channel provided by the geometry connect between disk and hole quenches partial field enhancement. Furthermore, we systematically investigate the sensing performances of multiple hybridized modes in above two cases by considering two crucial evaluating parameters, bulk refractive index sensitivity and surface sensitivity. It is demonstrated that, in the nanogap structure, the bonding mode possesses both high bulk refractive index sensitivity and surface sensitivity. Dissimilarly, for the nanooverlap structure, the bonding and antibonding modes show different surface sensitivities in different regions away from the surface, which can be used to monitoring different bio-molecular sizes and achieve the most optimum sensitivity. Due to its unique sensing features, this disk/hole array mechanism is very valuable and promising for developing of high sensitivity sensing platform.

Numerical Investigation on Multiple Resonant Modes of Double-Layer Plasmonic Grooves for Sensing Application.

A high-performance multi-resonance plasmonic sensor with double-layer metallic grooves is theoretically constructed by introducing a polymethyl methacrylate groove with a numerical simulation method. Multiple resonance wavelengths can be generated at the oblique incidence, and the number and feature of resonant mode for sensing detection is different for various incident angles. Specifically, at the incident angle of 30°, the reflection spectrum exhibits two resonant dips, in which the dip at the wavelength of 1066 nm has an extremely narrow line width of ~4.5 nm and high figure of merit of ~111.11. As the incident angle increases, the electric dipole mode gradually weakens, but the surface plasmon resonance and cavity resonance mode are enhanced. Therefore, for an incident angle of 65°, three resonance dips for sensing can be generated in the reflection spectrum to realize three-channel sensing measurement. These double-layer plasmonic grooves have potential in the development of advanced biochemical surface plasmon polariton measurements.

Au nanoparticles as label-free competitive reporters for sensitivity enhanced fiber-optic SPR heparin sensor.

A label-free Au NPs-enhanced surface plasmon resonance (SPR) sensor was developed for the ultrasensitive detection of heparin based on competitive adsorption behavior of heparin and Au NPs on the poly (dimethyl-diallylammonium chloride) (PDDA)-modified optical fiber surface and the corresponding change in the resonance wavelength of SPR. Due to the high affinity between heparin and PDDA, the present senor shows good analytical performance with respect to heparin detection. Two obvious advantages of the proposed heparin sensor over other reported methods are: its much wider linear concentration range (10-6-10-10 g/mL) and lower limit of detection (0.0257 ng/mL). The analysis of heparin in serum demonstrated that the present sensor exhibited high sensitivity and selectivity. It should be noted that the sensing strategy takes advantage of a portable fiber-optic SPR sensing system and avoids the need for complex processes for labeled-Au NPs, and thus the present sensor promises to be a practical tool for the point-of-care monitoring of heparin.

Electromagnetically induced transparency in an all-dielectric nano-metamaterial for slow light application.

Slow light technique has significant potential applications in many contemporary photonic device developments for integrated all-optical circuit, such as buffers, regenerators, switches and interferometers. In this paper, we present an efficient coupling mechanism of an electromagnetically induced transparency like (EIT-like) effect in an all-dielectric nano-metamaterial. This EIT-like effect is generated by destructive interference between a radiative Fabry-Perot (FP) mode and a dark waveguide (WG) mode, which is based on a combined structure of a dielectric grating and multilayer films. The dark WG mode is excited by guided mode of dielectric grating instead of radiative FP mode. In analogy to the molecular transition process, the FP mode, guided mode and WG mode are denoted by excited states of |1〉, |2〉 and |3〉. The two coupling pathways of the EIT-like effect in our metamaterial are |0〉 → |1〉 and |0〉 → |2〉 → |3〉 → |1〉, where |0〉 is the ground state. The simulated resonant wavelength of WG mode is consistent with theoretical result. We further confirm this EIT-like effect through a two-oscillator coupling analysis. We achieve a group refractive index of 913.6 by adjusting these two modes coupling of the EIT-like effect, which is useful for developing slow light device. This work provides a valuable solution to realize electromagnetically induced transparency in all-dielectric nanomaterial.

Near-Field Enhancement and Polarization Selection of a Nano-System for He-Ne Laser Application.

In this paper, we focus on transmission behavior based on the single aperture with a scatter. Both the near-field enhancement and polarization selection can be achieved numerically with a proposed nano-system under He-Ne laser wavelength. The nano-system consists of an Ag antenna, a wafer layer, an Ag film with an aperture and a dielectric substrate. Numerical results show that the near-field enhancement is related to the FP-like resonance base on surface plasmon polaritons (SPPs) in the metal-isolator-metal (MIM) waveguide for transverse magnetic (TM) polarization. The near-field optical spot is confined at the aperture export with a maximal electric intensity 20 times the value of the incident field for an antenna length of 430 nm. The transmission cutoff phenomenon for transverse electric (TE) polarization is because the transmission is forbidden for smaller aperture width. High extinction ratios of 9.6×10-8 (or 70.2 dB) and 4.4×10-8 (or 73.6 dB) with antenna lengths of 130 nm and 430 nm are achieved numerically with the nano-system. The polarization selective property has a good angular tolerance for oblique angles smaller than 15°. The spectral response is also investigated. We further demonstrate that the nano-system is applicable for another incident wavelength of 500 nm. Our investigation may be beneficial for the detection of polar molecules or local nano polarized nanosource.

Hybrid Nanodisk Film for Ultra-Narrowband Filtering, Near-Perfect Absorption and Wide Range Sensing.

The miniaturization and integration of photonic devices are new requirements in the novel optics field due to the development of photonic information technology. In this paper, we report that a multifunctional layered structure of Au, SiO2 and hexagonal nanodisk film is advantageous for ultra-narrowband filtering, near-perfect absorption and sensing in a wide refractive index (RI) region. This hexagonal nanostructure presented two remarkable polarization independent plasmon resonances with near-zero reflectivity and near-perfect absorptivity under normal incidence in the visible and near-infrared spectral ranges. The narrowest full width at half maximum (FWHM) of these resonances was predicted to be excellent at 5 nm. More notably, the double plasmon resonances showed extremely obvious differences in RI responses. For the first plasmon resonance, an evident linear redshift was observed in a wide RI range from 1.00 to 1.40, and a high RI sensitivity of 600 nm/RIU was obtained compared to other plasmonic nanostructures, such as square and honeycomb-like nanostructures. For the second plasmon resonance with excellent FWHM at 946 nm, its wavelength position almost remained unmovable in the case of changing RI surrounding nanodisks in the same regime. Most unusually, its resonant wavelength was insensitive to nearly all structural parameters except the structural period. The underlying physical mechanism was analyzed in detail for double plasmon resonances. This work was significant in developing high-performance integrated optical devices for filtering, absorbing and biomedical sensing.

Mercaptopyridine-Functionalized Gold Nanoparticles for Fiber-Optic Surface Plasmon Resonance Hg2+ Sensing.

As a highly toxic heavy metal ion, divalent mercuric ion (Hg2+) is one of the most widely diffused and hazardous environmental pollutants. In this work, a simple, portable, and inexpensive fiber-optic sensor based on surface plasmon resonance (SPR) effect was developed for Hg2+ detection, which takes advantage of 4-mercaptopyridine (4-MPY)-functionalized Au nanoparticles (Au NPs/4-MPY) as a signal amplification tag. Based on the coordination between Hg2+ and nitrogen in the pyridine moiety, we developed the sensor by self-assembling 4-MPY on Au film surfaces to capture Hg2+ and then introducing Au NPs/4-MPY to generate a plasmonic coupling structure with the configuration of nanoparticle-on-mirror. The coupling between localized SPR increased changes in SPR wavelength, which allowed highly sensitive Hg2+ sensing in aqueous solution. The sensor exhibited superior selectivity for Hg2+ detection compared with other common metal ions in water. The sensor's Hg2+ detection limit is 8 nM under optimal conditions. Furthermore, we validated the sensor's practicality for Hg2+ detection in tap water samples and demonstrated its potential application for environmental water on-site monitoring.

Refractory Ultra-Broadband Perfect Absorber from Visible to Near-Infrared.

The spectral range of solar radiation observed on the earth is approximately 295 to 2500 nm. How to widen the absorption band of the plasmonic absorber in this range has become a hot issue in recent years. In this paper, we propose a highly applicable refractory perfect absorber with an elliptical titanium nanodisk array based on a silica⁻titanium⁻silica⁻titanium four-layer structure. Through theoretical design and numerical demonstration, the interaction of surface plasmon resonance with the Fabry⁻Perot cavity resonance results in high absorption characteristics. Our investigations illustrate that it can achieve ultra-broadband absorption above 90% from a visible 550-nm wavelength to a near-infrared 2200-nm wavelength continuously. In particular, a continuous 712-nm broadband perfect absorption of up to 99% is achieved from wavelengths from 1013 to 1725 nm. The air mass 1.5 solar simulation from a finite-difference time domain demonstrates that this absorber can provide an average absorption rate of 93.26% from wavelengths of 295 to 2500 nm, which can absorb solar radiation efficiently on the earth. Because of the high melting point of Ti material and the symmetrical structure of this device, this perfect absorber has excellent thermal stability, polarization independence, and large incident-angle insensitivity. Hence, it can be used for solar cells, thermal emitters, and infrared detection with further investigation.

Fiber-optic surface plasmon resonance glucose sensor enhanced with phenylboronic acid modified Au nanoparticles.

A highly sensitive surface plasmon resonance (SPR) sensor is reported for glucose detection using self-assembled p-mercaptophenylboronic acid (PMBA) monolayer on Au coated optical fibers. The cis-diol group of saccharides, such as for glucose, interacted with the self-assembled PMBA monolayers on the optical fibers, but the low molecular mass of glucose is insufficient for measuring a significant shift in SPR wavelength. The response for glucose was thus enhanced with Au nanoparticles (Au NPs) modified with 2-aminoethanethiol (AET) and PMBA. Selectivity was assured since glucose has the ability to capture the signal amplification tags (Au NPs/AET- PMBA) through secondary binding with another set of syn-periplanar diol groups and the PMBA on the gold surface. Accordingly, a glucose concentration-dependent sandwich structure was formed and the coupling between Au NPs and Au film results in the red shift of SPR resonance wavelength. The experimental results demonstrated that this SPR sensor responded to glucose within a range of 0.01-30 mM better than to fructose and galactose. The minimum concentration for quantify glucose is as low as 80 nM, which is lower than the physiological blood glucose level. Glucose was then accurately detected in urine sample, which indicated the potential application of the sensor for the analysis of glucose in urine. We believe that our proposed PMBA-modified single amplification tag and sensing principle can also be used for biomolecules consisting of carbohydrate structures, particularly for DNA-associated bioanalysis.

Micro-capillary-based self-referencing surface plasmon resonance biosensor for determination of transferrin.

A novel self-referencing surface plasmon resonance (SPR) biosensor for detection of transferrin is demonstrated using a micro-capillary as the sensing element. The biosensor employs the SPR mode as a measuring signal and the Fabry-Perot (FP) mode as a referencing signal. The SPR mode is generated in the gold film that is coated on the outside of the capillary; instead, the FP mode is excited in the capillary, which is filled with de-ionized water. The FP mode is sensitive to temperature and insensitive to refractive index, which can be used as a referencing signal to compensate the effects caused by the temperature fluctuation. The sensor provides a high sensitivity of 1783.943 nm/RIU (refractive index unit) and a resolution of about 7.287×10-5 RIU. The self-referencing biosensor was applied to measurement of transferrin protein. It can monitor the interaction of transferrin protein with anti-transferrin in real time (0-5.228 µM). The simple and low-cost SPR sensor can be used for highly sensitive self-referencing biosensing for further investigations.

Narrow-band wavelength tunable filter based on asymmetric double layer metallic grating.

In this paper, the optical properties of asymmetric double layer metallic gratings are presented theoretically. The asymmetric structure is achieved by two main factors: one corresponding to moving alternatively metal nanowires of the top layer metallic grating, the other corresponding to possessing different thickness of the top and down layer metallic gratings. Our proposed structure shows one remarkable narrow-band transmission dip at normal incidence, which is distinct different from that of symmetric structure. The results are further confirmed by using different numerical computation methods, and explained by the analytical model of Fano-like resonance. We find that, only when the thickness of the down layer metallic grating has certain fixed value, transmission dip can be transformed from two to only one dip even if the existence of symmetry breaking. However, the wavelength position of the dip can be easily controlled by adjusting the thickness of the top layer metallic grating without the need to modify the structure period, and the width of metal nanowire. Moreover, the influence of other structure parameters on the dip is also investigated. Surprisingly, in order to keep the appearance of one dip in the transmission spectrum of designed structure, there is a good linear approximation between the refractive index of waveguide layer and the thickness of down layer metallic grating, and the relation of waveguide layer thickness and the thickness of down layer metallic grating satisfy secondary polynomial fitting. This work can be used to develop subwavelength metallic-grating-based and narrow-band tunable wavelength filters.