Applications In Plasmonic Sensors Research Articles

Polarization-Independent Narrowband Near-Perfect Absorption Based on One-Dimension Embedded Aluminum Grating

In the past, the polarization-insensitive absorption is realized mainly employing the two-dimensional periodic structure of fourfold rotational symmetry, which greatly increases the manufacturing complexity and cost. Here, we present the numerical design of a polarization-independent near-perfect absorber incorporating one-dimensional embedded aluminum grating. The absorption peaks near 99% at 520-nm wavelength for different polarization angles are achieved. The underlying mechanism associated with the resonance is attributed to the magnetic polariton resonance and the cavity-mode resonance for TM and TE polarization, respectively, and further explained by the inductor-capacitor circuit model and the eigen equation of cavity mode. Furthermore, the effects of geometrical parameters of the nano-cavity on the absorption performance are discussed. The proposed structure may provide a new way to achieve polarization-independent absorption with one-dimensional meta-surface, which has broad applications in plasmonic sensors, photo-detectors, and so on.

Surface Plasmon Assisted Luminescence Enhancement of Ag NP/NWs-Doped SiO2-TiO2-ZrO2:Eu3+ Ternary System

Non-hydrolytic sol-gel process was employed to prepare Eu3+-doped and silver nanoparticles/nanowires (Ag NP/NWs) co-doped SiO2-TiO2-ZrO2 ternary matrix. XRD, TEM, and SAED analyses revealed that the matrix is of amorphous nature even the crystalline nanoparticles/nanowires are embedded. The formation of ternary matrix is further evidenced by the characteristic peaks in FTIR spectrum and EDS analysis. The emission intensity is enhanced by sixfold for Ag NPs and eightfold for Ag NWs embedded matrix and is probably due to Surface Plasmon Coupled Emission (SPCE). Above the critical concentration of 1 wt.% Ag NP and 1.5 wt.% Ag NW, the emission intensity starts to diminish because of quenching effects. CIE chromaticity studies confirm that the addition of Ag NP/NWs into Eu3+-doped ternary matrix does not alter the color coordinates and purity of the present system. These Eu3+-Ag NP/NWs-doped ternary systems could be used as a promising material for efficient red light emitting sources, bio-detection, and plasmonic sensor applications.

Strong Fano Resonance Excited in an Array of Nanoparticle-in-Ring Nanostructures for Dual Plasmonic Sensor Applications

A large-area (over 1 cm2) nanoparticle-in-ring array (NPIRA) with strong Fano resonance is fabricated by a low-cost shadow sphere lithography (SSL) method. Controlled narrow nanogaps of 8–20 nm are formed between the nanoparticles (NPs) and rings. Quadrupole modes are excited in the nanogaps and interact with the dipole mode of the NPs and ring, resulting in two Fano resonances. The two Fano resonances are in the visible and near-infrared region, respectively, and show a maximum refractive index sensitivity of 425 nm/RIU and a figure of merit of 4.7–7.1 RIU–1. In addition, the NPIRAs show enhanced Raman intensity due to the strong electric fields confined in the nanogaps. The structure has great potential for the dual plasmonic sensor applications, i.e., surface-enhanced Raman scattering and refractive index sensing, and provides a robust sensing performance for each.

Tunable multiband plasmonic response of indium antimonide touching microrings in the terahertz range.

In this paper, we propose a terahertz (THz) plasmonic structure that supports three resonance modes, including the charge transfer plasmon (CTP), the bonding dipole-dipole plasmon, and the antibonding dipole-dipole plasmon, which can be strongly tuned by geometrical parameters, passively, and the temperature, actively. The structure exhibits a considerable thermal sensitivity of more than 0.01THz/K. The introduced multiband and tunable THz plasmonic structures offer important applications in thermal switches, thermo-optical modulators, broadband filters, design of multifunctional molecules originating from the multiband specification of the proposed structure, and improvement in plasmonic sensor applications stemming from a detailed study of the CTP mode.

Assembly of gold nanoparticles into aluminum nanobowl array

We mimic unique honeycomb structure as well as its functions of storing honey and pollen to assemble Au nanoparticle pattern on honeycomb-like Al nanobowl array by utilizing solid state dewetting process. Patterned Au nanoarrays of ‘one particle per bowl’ with tunable plasmonic bands ranging from the visible to the near-infrared region are fabricated by finely selecting the initial thickness of Au film, the geometry of Al nanobowl array and the thermal treatment parameters. This work presents a powerful approach to assemble Au nanoparticles into high density nanoarrays with superior spatial resolution, offering highly concentrated electromagnetic fields for plasmonic sensor applications.

Thin InSb layers with metallic gratings: a novel platform for spectrally-selective THz plasmonic sensing.

We present a computational study of terahertz optical properties of a grating-coupled plasmonic structure based on micrometer-thin InSb layers. We find two strong absorption resonances that we interpret as standing surface plasmon modes and investigate their dispersion relations, dependence on InSb thickness, and the spatial distribution of the electric field. The observed surface plasmon modes are well described by a simple theory of the air/InSb/air tri-layer. The plasmonic response of the grating/InSb structure is highly sensitive to the dielectric environment and the presence of an analyte (e.g., lactose) at the InSb interface, which is promising for terahertz plasmonic sensor applications. We determine the sensor sensitivity to be 7200 nm per refractive index unit (or 0.06 THz per refractive index unit). The lower surface plasmon mode also exhibits a splitting when tuned in resonance with the vibrational mode of lactose at 1.37 THz. We propose that such interaction between surface plasmon and vibrational modes can be used as the basis for a new sensing modality that allows the detection of terahertz vibrational fingerprints of an analyte.

Structural, morphological and optical properties of Ag–AgO thin films with the effect of increasing film thickness and annealing temperature

Ag films of thickness ranging from 5 to 60nm were deposited by thermal evaporation technique followed by air annealing process with temperature varying from 50 to 250°C. Morphological properties such as particle size, shape, surface roughness and number particles density were studied by atomic force microscope (AFM). The structural transition from quasi-amorphous to nanocrystalline to crystalline upon increasing film thickness and annealing temperature were studied. Ag films with smallest particle size and surface roughness were achieved up to film thickness of 7nm. The possibility of surface oxidation of Ag on both as deposited and annealed films was studied through Raman mapping by using confocal Raman spectroscopy. Ag film was X-ray amorphous even after annealing process up to the film thickness of 7nm and above which the crystallinity reached maximum at 250°C. The surface plasmon resonance (SPR) with a symmetric line shape due to dipole–dipole interactions was found to be very strong for film thickness of 5nm at 100°C, attributed to the formation of smaller Ag NPs size of ∼22nm with least size distribution and higher particles number density of ∼1625μm−2 in a self-organized fashion. With an increase of film thickness and annealing temperature, an asymmetric broad absorption arose due to increase in damping of collective electron oscillation on bulky NPs. Theoretical absorption spectra were simulated using extended Maxwell garnet method showing a decent agreement with experimental data. The real and imaginary parts of dielectric constants were determined and plotted for different film thicknesses of as deposited Ag films. Even though the film is oxidized at the surface level, it still can be used for plasmonic sensor applications however the film thickness should be approximately 7nm for the enhanced result.

Tarnishing of silver subwavelength slit gratings and its effect on extraordinary optical transmission

We study the impact of tarnishing of silver subwavelength gratings on their optical performance. We report that in the course of months under regular laboratory conditions, the gratings undergo tarnishing very differently from plain silver films: instead of thin layer of evenly distributed silver sulfide, a random pattern of rare larger crystals is formed across the grating area. As typical of plasmonic metamaterials, the gratings appear to be very sensitive to the arising structural disorder and show a very specific modification of the optical transmittance spectra – total degradation of the Wood diffraction anomaly and attenuation of the near-infrared transmission peak. The identified ‘optical fingerprints’ of the microscopic grating contamination pave a way for prospective plasmonic sensor applications.

Honeycomb-lattice plasmonic absorbers at NIR: anomalous high-order resonance

We design, fabricate and characterize a plasmonic honeycomb lattice absorber with almost perfect absorption at 1140 nm over a wide incident angle range. This absorber also possesses a narrow-band, angle- and polarization-dependent high-order resonance in the short-wavelength range, with a bandwidth of 19 nm and angle sensitivity of 3 nm per degree. The nature of this high-order absorption band is analyzed through finite-element simulations. We believe it is due to Bragg coupling of the incident light to the backward-propagating surface plasmon polariton through the periodic modulation of the structure.Such fine absorption bands can find applications in plasmonic sensors and spectrally selective thermal emitters.

Achieving high-purity colloidal gold nanoprisms and their application as biosensing platforms

Gold nanoprisms with average edge size of ∼140 nm and thickness of ∼8 nm were achieved in high-purity (∼97%) by exploiting the electrostatic aggregation and shape effects through a modified seed-mediated approach. The proposed strategy lies in the dramatically different stability and aggregation potential between the produced gold nanoprisms and spherical gold nanoparticles, which can be modulated by varying the anion concentration in the reaction solution. Hence, the gold nanoprisms spontaneously aggregated into precipitate whereas most of the spherical ones were still kept in the solution. Moreover, this strategy is also flexible enough that ultra-small gold nanoprisms with average width less than 50 nm can be collected in good-purity. The structure and optical properties of these nanoprisms have been studied by TEM, SAED, XRD and UV–vis–NIR spectroscopy, respectively. These high-purity colloidal gold nanoprisms exhibit remarkably enhanced surface plasmon resonance (SPR) as well as strong near-infrared absorption. Furthermore, we have also investigated their potential for biosensing based on the sensitive changes of SPR band induced by the antibody–antigen recognition events. The experimental results clearly suggest that gold nanoprisms can be a promising nanostructured system for plasmonic sensor applications.