Search of Extremely Sensitive Near-Infrared Plasmonic Interfaces: A Theoretical Study

The sensitivity of the wavelength position of localized surface plasmon resonance (LSPR) in metal nanostructures to local changes in the refractive index has been widely used for label-free detection strategies. Tuning the optical properties of the nanostructures from the visible to the infrared region is expected to have a drastic effect on the refractive index sensitivity. Here, we theoretically investigate the optical response of a newly designed plasmonic interface to changes in the bulk refractive index by the finite difference time domain method. It consists of a structured interface, where the planar interface is superposed with dielectric pillars 30 nm in height and 125 nm in length with a separation distance of 15 nm. The pillars are covered with U-shaped gold nanostructures of 50 nm in height, 125 nm in length, and 5 nm of gold base thickness. The whole structure is finally covered with a 5-nm thick dielectric layer of n ₂?=?2.63. This plasmonic structure shows bulk refractive index sensitivities up to 1750 nm/RIU (RIU : refractive index unit) in the near infrared (λ?=?2621 nm). The enhanced sensitivity is a consequence of the extremely enhanced electrical field between the gold nanopillars of the plasmonic interface.     

Utilizing the Metallic Nano-Rods in Hexagonal Configuration to Enhance Sensitivity of the Plasmonic Racetrack Resonator in Sensing Application

A high sensitive plasmonic refractive index sensor based on metal-insulator-metal (MIM) waveguides with embedding metallic nano-rods in racetrack resonator has been proposed. The refractive index changes of the dielectric material inside the resonator together with temperature changes can be acquired from the detection of the resonance wavelength, based on their linear relationship. With optimum design and considering a tradeoff among detected power, structure size, and sensitivity, the finite difference time domain simulations show that the refractive index and temperature sensitivity values can be obtained as high as 2610 nm per refractive index unit (RIU) and 1.03 nm/°C, respectively. In addition, resonance wavelengths of resonator are obtained experimentally by using the resonant conditions. The effects of nano-rods radius and refractive index of racetrack resonator are studied on the sensing spectra, as well. The proposed structure with such high sensitivity will be useful in optical communications that can provide a new possibility for designing compact and high-performance plasmonic devices.     

Plasmonic Biosensor Based on Triangular Au/Ag and Au/Ag/Au Core/Shell Nanoprisms onto Indium Tin Oxide Glass

Gold–silver core–shell triangular nanoprisms (Au/AgTNPs) were grown onto transparent indium tin oxide (ITO) thin film-coated glass substrate through a seed-mediated growth method without using peculiar binder molecules. The resulting Au/AgTNPs were characterized by scanning electron microscopy, atomic force microscopy, X-ray diffraction, UV–vis spectroscopy, and cyclic voltammograms. The peak of dipolar plasmonic resonance was located at near infrared region of ～700 nm, which showed the refractive index (RI) sensitivity of 248 nm/RIU. Moreover, thin gold shells were electrodeposited onto the surface of Au/AgTNPs in order to stabilize nanoparticles. Compared with the Au/AgTNPs, this peak of localized surface plasmon resonance (LSPR) was a little red-shift and decreased slightly in intensity. The refractive index sensitivity was estimated to be 287 nm/RIU, which showed high sensitivity as a LSPR sensing platform. Those triangular nanoprisms deposited on the ITO substrate could be further functionalized to fabricate LSPR biosensors. Results of this research show a possibility of improving LSPR sensor by using core–shell nanostructures.     

Tunable In-Plane Optical Anisotropy of Ag Nanoparticles Deposited by DC Sputtering onto SiO<Subscript>2</Subscript> Nanocolumnar Films

This work reports an easy-to-handle method for growing two-dimensional assemblies of Ag nanostructures presenting a tunable in-plane optical anisotropy. Ag is deposited by DC sputtering in an Ar plasma at room temperature onto bundled nanocolumnar SiO₂ thin films grown by glancing angle physical vapor deposition. In contrast with previously reported processes involving the grazing angle deposition of the metal, DC sputtering is performed at normal incidence. By varying the deposition angle of SiO₂ and the Ar pressure, it was possible to tune the deposited amount of Ag and thus the topology of the Ag deposit from isolated spherical Ag nanoparticles with isotropic optical properties to strongly dichroic Ag nanostripes oriented along the bundling direction of the SiO₂ nanocolumns. Based on simple calculations taking into account the shadowing effects during metal deposition, it is proposed that the width and shape of the tip of the bundled SiO₂ nanocolumns influence significantly the metal local atom flux arriving to them and thus the final structure of the deposit.     

Gamma–Radiation-Assisted Synthesis of Luminescent ZnO/Ag Heterostructure Core–Shell Nanocomposites

There is increasing interest in tuning the physical properties of semiconductor nanostructures using metal nanoparticles. In this work, ZnO nanosphere covered with Ag nanoparticles were synthesized using gamma–radiation-assisted method. The amount of deposited Ag nanoparticles is controlled by changing irradiation dose in the range of 30–100 kGy in order to tune the semiconductor–metal interaction. The successful deposition of Ag on the ZnO nanoparticles is examined by analyzing the morphology, microstructure, optical, and magnetic properties of ZnO/Ag nanoparticles through field emission scanning electron (FESEM), microscopy X-ray diffraction spectra, UV-visible absorption, photoluminescence measurement, and vibrating sample magnetometer. FESEM and elemental mapping results confirmed that Ag nanoparticles have been concentrated at the surface of spherical ZnO particles. Moreover, formation of pure metallic Ag nanoparticles has been confirmed by XRD analysis. UV-visible absorption spectra of obtained ZnO/Ag showed two combined peaks, a weak peak at the shoulder around 360 nm corresponds to ZnO and a sharp absorption at 420 nm refers to spherical Ag nanoparticles. Obtained results from photoluminescence revealed that the near-band-edge emission and defect-related visible emission bands of ZnO could be enhanced dramatically at the same time by deposition of Ag nanoparticles, which was ascribed to localized surface plasmon–exciton coupling and surface plasmon scattering. Controlling the semiconductor and metal coupling effect is interesting because of its application in highly efficient optoelectronic devices and biosensor.     

Localized Surface Plasmon Resonance and Refractive Index Sensitivity of Metal–Dielectric–Metal Multilayered Nanostructures

The localized surface plasmon resonances of multilayered nanostructures are studied using finite difference time domain simulations and plasmon hybridization method. Concentric metal–dielectric–metal (MDM) structure with metal core and nanoshell separated by a thin dielectric layer exhibits a strong coupling between the core and nanoshell plasmon resonance modes. The coupled resonance mode wavelengths show dependence on the dielectric layer thickness and composition of core and outer layer metal. The aluminum-based MDM structures show lower plasmon wavelength compared with Ag- and Au-based MDM nanostructures. The calculated refractive index sensitivity (RIS) factor is in the order Ag–Air–Ag>Au–Air–Au>Al–Air–Al for monometallic multilayered nanostructures. Bimetallic multilayered nanostructures support strong and tunable plasmon resonance wavelengths as well as high RIS factor of 510 nm/refractive index unit (RIU) and 470 nm/RIU for Al–Air–Au and Ag-Air-Au, respectively. The MDM structures not only exhibit higher index sensitivity but also cover a wide ultraviolet–near-infrared wavelengths, making these structures very promising for index sensing, biomolecule sensing, and surface-enhanced Raman spectroscopy.     

Analytical Methods of Equivalent Circuit Model and Transmission Matrix for a Plasmonic Switch with Sensing Capability in Near-Infrared Region

In this paper, the simultaneous switching and sensing capabilities of a compact plasmonic structure based on a conventional rectangular hole in a silver film are proposed and investigated. The proposed structure has ultrahigh sensitivity up to 3000 nm/RIU and high figure of merit of 170 RIU⁻¹. Also, the simulation results show the potential of the presented refractive index sensor to detect malaria infection, cancer cells, bacillus bacteria, and solution of glucose in water. Simultaneously, by changing the incident lightwave polarization, the structure behaves like a plasmonic switch, which has high extinction ratios of 15.81, 31.20, and 25.03 dB at three telecommunication wavelengths of 850, 1310, and 1550 nm, respectively. The ultrafast response time of 20 fs is achieved for the wideband application of the switching capability at the wavelength range of 1056 to 1765 nm. Moreover, the equivalent circuit model and transmission (ABCD) matrix methods are derived to validate the simulated results. Simple design, good agreement between the numerical and analytical results, biomedical applications, ultrahigh sensitivity, and ultrafast performance of the proposed structure help this idea to open up paths for design and implementation of other multi-application plasmonic devices in near-infrared region. To the best of our knowledge, the mentioned analytical methods have not been studied former at near-infrared wavelengths. Therefore, the achievements could pave the way for verifying the simulation results of plasmonic nanostructures in future investigations.

     

Ultrasmall Plasmonic Cavity for Chemical Sensing

We propose an ultrasmall plasmonic cavity based on the channel waveguides for chemical sensing. The plasmonic mode gap due to cutoff angular frequency enables strong optical confinement in a subwavelength volume and suppression of radiation loss. Due to strong field overlap of the surface plasmon polariton mode with environmental material, large sensitivity (1,100 nm/refractive index unit) and a high figure of merit (330) are achieved in the plasmonic cavity with a small physical size of 600?×?800?×?2,500 nm having a telecommunication resonant wavelength. This plasmonic cavity can introduce a broad range of applications including biochemical sensing and strong light–matter interactions.     

Optimization of Surface Plasmon Resonance Biosensor with Ag/Au Multilayer Structure and Fiber-Optic Miniaturization

In this paper, we report a novel wavelength interrogation-based surface plasmon resonance (SPR) system, in which a film of three Ag layers and three Au layers are alternately deposited on a Kretschmann configuration as sensing element. This multilayer film shows higher sensitivity for refractive index (RI) measurement by comparing with single Au layer structure, which is consistent with its theoretical calculation. A sensitivity range of 2056–5893 nm/RIU can be achieved, which is comparable to RI sensitivities of other wavelength-modulated SPR sensors. Compared with Ag film, this Ag/Au multilayer arrangement offers anti-oxidant protection. This SPR biosensor based on a cost-effective Ag/Au multilayer structure is applicable to the real-time detection of specific interactions and dissociation of low protein concentrations. To extend the application of this highly-sensitive metal film device, we integrated this concept on an optical fiber. The range of RI sensitivities with Ag/Au multilayer was 1847–3309 nm/RIU. This miniaturized Ag/Au multilayer-based fiber optic sensor has a broad application in chemical and biological sensing.     

Improved Absorbance and Near-Infrared Dispersion of AuGe Nanoparticles over Au Nanoparticles Prepared with Similar Thermal Annealing Environment

The difficulty in patterning the structures at sub-wavelength range leads to employ the bottom-up approach to form nanostructures of metals as well as dielectric components that disperse them in host media. The optical properties of nanoparticles are studied with UV-Vis 750 (lambda) NIR spectroscopy and fit with empirical relations. The refractive index is about the volume fraction of particles. The AuGe nanoparticles demonstrate improved absorbance, lower refractive index, and higher extinction than Au nanoparticles formed with similar thermal process. Surface plasmon resonance (SPR) phenomena are highly sensitive to the bonding between atoms, atomic structure, and the electronic configuration in atoms of the given material. If one takes into account the structure of materials, then the literature on eutectic alloys predicts that alloying gold with germanium (AuGe) with varying compositions will also change the x-ray diffraction peak positions of gold itself. The peak shift can be interpreted as the change in grain size or shift in grain boundaries implying a corresponding change in material’s atomic arrangement within lattice structure. As a result, there will be a change in the charge distribution of free electron cloud in original gold ultimately affecting a change in the plasmon resonance frequency and thereby modulating the various optical phenomena such as absorbance, reflectance, and refractive index. This alloying also brings a change in the dielectric constant of the material such that the plasmonic behavior may shift among different regions (UV, visible, NIR, MWIR, and LWIR). Metal semiconductor eutectic alloy which is widely popular as a soldering material would have scope in futuristic photonic applications due to its tuneable optical properties. In this work, we study the effects of Au and AuGe nanoparticle deposition on GaAs films grown by molecular beam epitaxy (MBE). Au and AuGe thin films (12-nm thick) were annealed in the temperature ranges of 400–800 and 300–700 °C, respectively, to form Au and AuGe nanoparticles. The formation of these nanoparticles was confirmed by scanning electron microscopy (SEM) measurements. Optical absorption spectroscopy measurements showed plasmon resonance peaks at around 670 and 535 nm for the AuGe-deposited 300 °C-annealed sample and Au-deposited 600 °C-annealed sample on sapphire, respectively, thereby confirming the plasmonic effect. Correlation of Raman spectroscopy measurement results with X-ray diffraction measurement results reveal that the transverse optical mode intensity and full width at half maximum of the GaAs (400) peak increased with an increase in annealing temperature, indicating degradation of the crystalline properties of GaAs film at higher annealing temperatures. The highest increments of the photoluminescence (PL) intensities in comparison to that of the bare GaAs film were observed to be 37 and 77% for the Au-deposited 600 °C-annealed and AuGe-deposited 300 °C-annealed samples, respectively. These enhancements of PL spectra are an indication of the significant scattering of photons by Au and AuGe nanoparticles, and they are attributed mainly to the contribution of the local surface plasmon resonance of these nanoparticles. A comparative analysis of PL enhancements revealed that AuGe nanoparticles induced a greater enhancement than Au nanoparticles. The calculated activation energies of the Au-deposited 600 °C-annealed sample, AuGe-deposited 300 °C-annealed sample, and bare GaAs sample were around 18, 24, and 33 meV, respectively. We found one-order increment in peak responsivity of AuGe plasmonic-based trilayer InAs quantum dot detector in comparison to as-grown detector at 80 K. Therefore, this study is expected to be very useful in the realization of high-performance plasmonic-based optoelectronic and sensing devices.     

Two-dimensional PtSe2 Theoretically Enhanced Goos-Hänchen Shift Sensitive Plasmonic Biosensors

Platinum diselenide (PtSe₂), an emerging two-dimensional transition metal dichalcogenide, exhibits thickness-dependent refractive index, and hence, intriguing optical properties. Here, we employ it as a plasmonic sensing substrate to achieve significant enhancement in Goos-Hänchen shift sensitivity. Through systematic optimization of all parameters, four optimum sensing configurations have been achieved at different wavelengths ranging from visible to near-infrared region, where the Goos-Hänchen shift sensitivity receives four times enhancement in comparison with the conventional bare gold sensing substrate. There is a linear range of Goos-Hänchen shift with the tiny change of refractive index for each optimal configuration. The detection limit of the refractive index change can be as low as 5 × 10⁻⁷ RIU which is estimated to be lower by 2 orders of magnitude, and the corresponding sensitivity of biomolecules has a 1000-fold increment compared with that of bare gold-based sensors.

     

Plasmonically Tunable Blue-Shifted Emission from Coumarin 153 in Ag Nanostructure Random Media: A Demonstration of Fast Dynamic Surface-Enhanced Fluorescence

Enhancement of intensity and wavelength tunability of emission are desirable features for light-emitting device applications. We report on the large and tunable blue shift (60 nm) in emission from an environment-sensitive fluorophore (Coumarin153) embedded in Ag plasmonic random media. Coumarin 153 having emission at 555 nm, show a systematic blue shift (to 542, 503 and 495 nm) upon infiltration into random media fabricated by Ag nanowires of different aspect ratio (hence, surface plasmon resonances at 426, 445 and 464 nm). The blue shift is due to the fast dynamic surface-enhanced fluorescence mechanism and can be tuned by controlling the surface plasmon resonance and hotspot density in random media. Enhanced emission at desired wavelength is achieved by using nanostructures having higher extinction coefficient but same-surface plasmon resonance. Ag nanostructures of different aspect ratio used for fabricating the random media are synthesized by chemical route.     

Characterisation of a Gold Nanorod Sol–Gel Utilising Inter-particle Coupling to Yield High Refractive Index Sensitivity

The development of gold nanorod plasmonic sol?Cgel polymer is presented and characterised with respect to its sensitivity to refractive index change. Structural characterisation of the polymer was conducted using Raman microscopy and energy dispersion spectroscopy while plasmonic function was investigated using UV/VIS spectroscopy. Refractive index sensitivities utilising the peak wavelength shift of the localised surface plasmon resonance were shown to be of the order of 2,338?nm per refractive index unit; in addition, peak absorbance was considered as an alternative measure. Furthermore, demonstration of sensitivity to biomolecule interaction has been shown as a model study. Strong photoluminescence was observed during Raman studies that restricts the potential use of the polymer as a surface-enhanced Raman substrate.     

Platinum Layers Sandwiched Between Black Phosphorous and Graphene for Enhanced SPR Sensor Performance

Highly sensitive surface plasmon resonance (SPR) sensor consisting of Ag-Pt bimetallic films sandwiched with 2D materials black phosphorus (BP) and graphene over Pt layer in Kretschmann configuration is analyzed theoretically using the transfer matrix method. Numerical results show that upon suitable optimization of thickness of Ag-Pt layers and the number of layers of BP and graphene, sensitivity as high as 412°/RIU (degree/refractive index unit) can be achieved for p-polarized light of wavelength 633 nm. This performance can be tuned and controlled by changing the number of layers of BP and graphene. Furthermore, the addition of graphene and heterostructures of black phosphorus not only improved the sensitivity of the sensor but also kept the FWHM of the resonance curve much smaller than the conventional sensor utilizing Au as plasmonic metal and hence improved the resolution to a significant extent. We expect that this new proposed design will be useful for medical diagnosis, biomolecular detection, and chemical examination.

     

Analysis of a Surface Plasmon Resonance Probe Based on Photonic Crystal Fibers for Low Refractive Index Detection

A photonic crystal fiber (PCF)-based surface plasmon resonance (SPR) probe with gold nanowires as the plasmonic material is proposed in this work. The coupling characteristics and sensing properties of the probe are numerically investigated by the finite element method. The probe is designed to detect low refractive indices between 1.27 and 1.36. The maximum spectral sensitivity and amplitude sensitivity are 6 × 10³ nm/RIU and 600 RIU^?1, respectively, corresponding to a resolution of 2.8 × 10^?5 RIU for the overall refractive index range. Our analysis shows that the PCF-SPR probe can be used for lower refractive index detection.     

Scaling of Surface Plasmon Resonances in Triangular Silver Nanoplate Sols for Enhanced Refractive Index Sensing

The plasmonic spectra of solution phase ensembles of triangular silver nanoplates have been analysed in order to examine the fundamental properties underlying their size-dependent enhanced refractive index sensitivities. Linewidth studies highlight variations in the response of these solution phase nanostructures to those previously reported for single immobilized triangular nanostructures. The observation of insignificant broadening of the resonance linewidth for larger edge length nanoplates highlights minimal contribution of radiative damping processes at these dimensions. Comparative single nanoplate studies using discrete dipole approximations were performed to analyse the dephasing processes contributing to these reduced linewidths and to determine the key parameters defining the underlying plasmonic response. These single nanoplate approximations highlight the dominance of absorption processes over radiative processes and demonstrate that this dominance can be attributed to the platelet nature/geometry of the nanoplates. These calculations indicate that the higher aspect ratio allows for the maintenance of coherent plasmon oscillations as the edge length of the triangular platelet increases within the sols. Thickness studies verify that this reduction in radiation damping is due to high aspect ratio and can act to confine electromagnetic fields at the nanoplate surface, thereby increasing near-field enhancement and hence the resultant plasmonic refractive index sensitivity.     

Tuning Plasmonic Properties of Truncated Gold Nanospheres by Coating

The influence of TiO₂ coating on resonant properties of gold nanoisland films deposited on silica substrates was studied numerically and in experiments. The model describing plasmonic properties of a metal truncated nanosphere placed on a substrate and covered by a thin dielectric layer has been developed. The model allows calculating a particle polarizability spectrum and, respectively, its surface plasmon resonance (SPR) wavelength for any given cover thickness, particle radius and truncation parameter, and dielectric functions of the particle, the substrate, the coating layer, and the surrounding medium. Dependence of the SPR position calculated for truncated gold nanospheres has coincided with the measured one for the gold nanoisland films covered with titania of different thicknesses. In the experiments, gold films with thickness of 5 nm were deposited on a silica glass substrate, annealed at 500 °C to form nanoislands of 20 nm in diameter, and covered with amorphous titania layers using atomic layer deposition technique. The resulting structures were characterized with scanning electron microscopy and optical absorption spectroscopy. The measured dependence of the SPR position on titania film thickness corresponded to the one calculated for truncated sphere-shaped nanoparticles with the truncation angle of ~50°. We demonstrated the possibility of tuning the SPR position within ~100 nm range by depositing to 30 nm thick titania layer.

     

Rational Selection of Nanorod Plasmons: Material, Size, and Shape Dependence Mechanism for Optical Sensors

The localized surface plasmon resonance dependence on surrounding medium refractive index of Ag, Al, Au, and Cu nanoparticles is examined by electrodynamic approach. The refractive index sensitivity and sensing figure of merit (FOM) dependence of selected metal nanoparticles with similar geometry shows that although, sensing relevant parameters are shape (i.e., aspect ratio), and material dependent below the width 20 nm, but above this size these parameters are material independent under similar geometrical conditions. We have concluded that at optimum size, however, Al shows much higher refractive index sensitivity (RIS) in comparison to Au, Cu, and Ag, but FOM is higher for Ag in comparison to other metals. The observed sensing behavior is expected due to parameters like surface scattering, dynamic depolarization, radiation damping, and interband transitions, which may influence the nanorod plasmons.     

Highly Sensitive Plasmonic Sensor Based on Fano Resonance from Silver Nanoparticle Heterodimer Array on a Thin Silver Film

We investigate the optical spectrum of a multilayer metallic slab using multiple-scattering formalism. A thin silver film is attached to a periodic array of heterodimers consisting of two vertically spaced silver nanoparticles of different radii. Depending on the radius of nanoparticles, heterodimer array presents a simple nanoscale geometry which gives rise to remarkable plasmonic properties of multipolar resonances. Due to the coherent interference of the localized nanoparticle plasmons (discrete mode) and surface plasmon polaritons of metallic film (continuous mode), the reflection spectrum represents a sharp asymmetric Fano resonance dip, which is strongly sensitive to the refractive index of the surrounding embedded dielectric host. The physical features contribute to a highly efficient plasmonic sensor for refractive index sensing with sensitivity of ～1.5?×?10^?3 RIU/nm.     

Triangular Au�CAg Nanoframes with Tunable Surface Plasmon Resonance Signal from Visible to Near-Infrared Region

In recent years, metal hollow nanostructures are intriguing to be synthesized and studied because they exhibit unique surface plasmonic properties. Although many methods for tuning the surface plasmonic absorption peaks of silver nanostructures have been reported, it still remains a great challenge to produce hollow Ag nanostructure with controllable surface plasmon resonance (SPR) via a facile method. In this paper, triangular Au–Ag nanoframes were successfully fabricated using triangular silver nanoplates as templates, through galvanic replacement reaction between the silver nanoplates and HAuCl₄, exhibiting tuneable SPR response from visible (605 nm) to near-infrared region (1,235 nm).