Surface Plasmon Resonance Frequency Research Articles

Photonic crystal fiber based on graphene surface plasmon resonance for high-sensitivity terahertz refractive index sensing

A photonic crystal fiber (PCF) sensor based on surface plasmon resonance (SPR) with a graphene coating on the cladding is designed for refractive index (RI) detection in the range of 0.3–0.5 THz, especially for liquid bioanalytical sensing. The adjustability of the graphene chemical potential (E f ) enables dynamic tuning of the loss spectra over a wide frequency range with a tuning sensitivity of 570 GHz/eV at the SPR frequency. According to the analysis by the finite element method (FEM), the highest wavelength sensitivity and maximum amplitude sensitivity of 4254.11 µm/RIU and 25.62RIU−1 (n a =1.34) are achieved in the RI range of 1.15–1.35, respectively, together with a resolution of 5.93×10−5RIU. The graphene PCF-SPR sensor boasting high-sensitivity detection in a wide RI range has broad application prospects in multiple fields.

Investigation of the SERS application of gold films deposited on uncured polydimethylsiloxane with tunable LSPR

We present an approach to investigate the localized surface plasmon resonance and surface-enhanced Raman scattering of gold films deposited on uncured polydimethylsiloxane via thermal evaporation. Differing from solid substrates, the liquid surface of uncured polydimethylsiloxane can serve as an isotropic substrate on which gold atoms nucleate and disperse to form characteristic microstructures in a controlled manner. By adjusting experimental parameters during film deposition, the absorption of resonant plasmon modes can be tuned in the visible spectral range due to the control of particle size and distribution in Au films. Furthermore, Raman measurements are conducted to investigate the enhancement of Raman signals in these films, and the experimental results are verified by simulation analysis. This work exhibits tunability of surface plasmon resonance frequency and enhanced Raman detection capability by depositing metal films on liquid surfaces, thus providing potential applications of these films in flexible biosensors and chemical detection.

Thermally generated Au–Ag nanostructures with tunable localized surface plasmon resonance as SERS activity substrates

Various Au–Ag bimetallic alloy nanostructures were obtained as sensitive surface-enhanced Raman scattering (SERS) substrates by changing the thermal annealing sequence. The atomic force microscopy (AFM) and scanning electron microscopy (SEM) results confirm that Au/Ag bimetallic clusters and Ag–Au core-shell like structures can be designed by thermal annealing. The absorption spectra showed that the localized surface plasmon resonance (LSPR) frequency of the annealed Au/Ag bimetallic alloy structure could effectively shift from the near ultraviolet to the visible region. At the same time, the Au/Ag bimetallic alloy films modified by thermal annealing have shown satisfactory performance as SERS substrates. Raman enhancement mechanism of Au–Ag bimetallic alloy films is verified by finite-difference time-domain (FDTD) simulation results.

D-Shaped photonic crystal fiber with graphene coating for terahertz polarization filtering and sensing applications

A D-shaped photonic crystal fiber with graphene coating on the polished flat surface is designed. Graphene is used as the plasmonic material and surface plasmon resonance (SPR) effect is obtained at the sub-terahertz frequency. Tunable loss spectra, polarization filtering and sensing properties are studied by using the finite element method. Tuning sensitivity of the SPR frequency is 595 GHz/eV. Bandwidth of 23.46 GHz centered at 0.48THz with crosstalk above 20 dB can be obtained when fiber length is 0.5 cm. The average wavelength sensitivity of 305.5 μm/RIU and the maximum amplitude sensitivity of 30.85RIU−1 are obtained for analyses with refractive index in the range of 1.3 to 1.4. Refractive index resolution of the order of 10−4 is predicted. The designed fiber can be used as terahertz polarization filter or biosensor.

Synthesis of Controllable Cu Shells on Au Nanoparticles with Electrodeposition: A Systematic in Situ Single Particle Study.

Bimetallic Cu on Au nanoparticles with controllable morphology and optical properties were obtained via electrochemical synthesis. In particular, multilobed structures with good homogeneity were achieved through the optimization of experimental parameters such as deposition current, charge transfer, and metal ion concentration. A hyperspectral dark field scattering setup was used to characterize the electrodeposition on a single particle level, with changes in localized surface plasmon resonance frequency correlated with deposition charge transfer and amount of Cu deposited as determined by electron microscopy. This demonstrated the ability to tune morphology and spectra through electrochemical parameters alone. Time-resolved in situ measurements of single particle spectra were obtained, giving an insight into the kinetics of the deposition process. Nucleation of multiple cubes of Cu initially occurs preferentially on the tips of Au nanoparticles, before growing and coalescing to form a multilobed, lumpy shell. Modifying the surface of Au nanoparticles by plasma treatment resulted in thicker and more uniform Cu shells.

Enhanced Terahertz Fingerprint Sensing Mechanism Study of Tiny Molecules Based on Tunable Spoof Surface Plasmon Polaritons on Composite Periodic Groove Structures

Highly sensitive detection of enhanced terahertz (THz) fingerprint absorption spectrum of trace-amount tiny molecules is essential for biosensing. THz surface plasmon resonance (SPR) sensors based on Otto prism-coupled attenuated total reflection (OPC-ATR) configuration have been recognized as a promising technology in biomedical detection applications. However, THz-SPR sensors based on the traditional OPC-ATR configuration have long been associated with low sensitivity, poor tunability, low refractive index resolution, large sample consumption, and lack of fingerprint analysis. Here, we propose an enhanced tunable high-sensitivity and trace-amount THz-SPR biosensor based on a composite periodic groove structure (CPGS). The elaborate geometric design of the spoof surface plasmon polaritons (SSPPs) metasurface increases the number of electromagnetic hot spots on the surface of the CPGS, improves the near-field enhancement effect of SSPPs, and enhances the interaction between THz wave and the sample. The results show that the sensitivity (S), figure of merit (FOM) and Q-factor (Q) can be increased to 6.55 THz/RIU, 4234.06 1/RIU and 629.28, respectively, when the refractive index range of the sample to measure is between 1 and 1.05 with the resolution 1.54×10-5 RIU. Moreover, by making use of the high structural tunability of CPGS, the best sensitivity (SPR frequency shift) can be obtained when the resonant frequency of the metamaterial approaches the biological molecule oscillation. These advantages make CPGS a strong candidate for the high-sensitivity detection of trace-amount biochemical samples.

Interaction of obliquely incident lasers with anharmonic CNTs acting as dipole antenna to generate resonant THz radiation

The present paper explores the terahertz (THz) generation by the interaction of obliquely incident laser beams with the array of vertically aligned anharmonic carbon nanotubes (CNTs) acting as dipole antennas. In the scheme, anharmonicity arises due to the nonlinear variation of restoration force on the various electrons of CNTs and it plays a key role in the enhancement of THz generation. The anharmonic CNTs help in broadening the resonance peak, which paves the way for the enhancement of the normalized THz amplitude. The laser beams incident obliquely on the close-packed array of vertically aligned anharmonic CNTs grown over the glass substrate and set oscillations in the CNTs so that each CNT act as the oscillatory dipole to generate THz radiation. The THz electric field shows enhancement at surface plasmon resonance frequency , where is the characteristic parameter of CNTs, is the relative permittivity of lattice and is the plasma frequency. This scheme is quite suitable to generate THz radiations in the milliwatt range of optimized values of the laser and CNTs parameters. We also explore the impact of polarization, S-parameter, critical angle, and length-matching effects of CNT antennas on the THz generation.

Binary Pt/Te Nanoheterostructures with High Photothermal Conversion Efficiency and Anti-inflammatory Action for Enhanced Photothermal Therapy of 4T1 Breast Tumors Guided by Photoacoustic Imaging

Photothermal therapy is a powerful candidate for tumor treatment. However, photothermal therapy still faces some challenges, such as lacking photothermal agents with high photothermal conversion efficiency and undesirable inflammatory responses, which may result in tumor recurrence and therapeutic resistance. Here, the Pt/Te nanoheterostructures (PT) were synthesized by a simple hydrothermal reaction. The photothermal conversion efficiency was up to 51.84%. The outstanding photothermal conversion capacity of PT was attributed to the unique localized surface plasmon resonance frequency of metals and semiconductors and the increased circuit paths of electron transitions from nanoheterostructures. After coating with the murine mammary carcinoma (4T1) cell membrane, the camouflaged PT (mPT) exhibits excellent biocompatibility and effective homologous targeting capacity. Benefiting from antioxidative activity, mPT can efficiently scavenge inflammation-related reactive oxygen species and cytokines (such as tumor necrosis factor (TNF)-α, interleukin (IL)-1β, and IL-6) caused by hyperthermia to alleviate inflammation in vitro and in vivo. The in vitro and in vivo therapeutic results showed that mPT could effectively inhibit 4T1 breast tumors. In addition, the in vivo therapy could be guided by photoacoustic imaging. These results demonstrated that these multifunctional mPT provide a paradigm for biomimetic metal and semiconductor nanoheterostructures for enhanced photothermal therapy and anti-inflammatory action on tumors.

Tunable surface plasmon resonance sensor based on graphene-coated photonic crystal fiber in terahertz.

A terahertz surface plasmon resonance (SPR) sensor is designed based on photonic crystal fiber (PCF). Graphene is selectively coated in the cladding hole of the PCF and used as plasmonic material. The coupling mechanism, loss properties, tunability, and refractive index sensing performance of the designed SPR sensor are investigated using the finite element method. The peak of the loss spectrum corresponding to the SPR frequency can be dynamically tuned by adjusting graphene's chemical potential, and a tuning sensitivity of 767.5 GHz/eV is obtained. The SPR frequency red shifts linearly with an increase in the refractive index of analyte from 1.0 to 1.5. An average frequency sensitivity of 208.14 GHz/RIU is obtained. This research provides theoretical guidance for the design of terahertz in-fiber SPR sensors and filters.

Enhanced tunneling distance of near field radiative energy with high-index dielectric resonators

By placing high-index dielectric resonators on surfaces supporting surface plasmons in the near field, strong magnetic resonance can be observed in the high-index dielectric resonators with appropriate heights around the surface plasmon resonance frequencies. The strong magnetic resonance allows strong thermal photon tunneling across a 1 μm gap, which is one order longer than the previous demonstrations of near field radiation with surface plasmons. The thermal photon tunneling happens when the horizontal wavenumber is kx∼4πw with w is the width of the high-index resonators. The height of the high-index dielectric resonators should provide enough retardation of the electric field between the top and bottom of the resonator to form a displacement current loop. Therefore, similar magnetic field resonance occurs in the resonator when we triple rather than double the height of the high-index dielectric resonators. The usage of dielectric resonators to amplify the thermal electric field in the near field domain can be a potential method to increase the quasi-monochromatic radiation distance of an emission domain by one order or more at the frequencies of the surface waves.

Laser-assisted synthesis of Ag–Cu alloy nanoparticles with tunable surface plasmon resonance frequency in presence of external electric field

The present study aimed to synthesize the colloidal Ag–Cu alloy nanoparticles (NPs) in the presence of an external electric field. Synthesis of Ag–Cu alloy NPs is important because of their unique properties for antibacterial and antimicrobial activities. Colloidal Ag and Cu NPs were produced individually with nanosecond pulsed laser ablation in distilled water and were mixed in equal volume. The mixed colloid was postirradiated in the presence of an external DC electric field. The obtained colloidal NPs were characterized using various diagnostic methods. The results of this study demonstrated the long-term stability and oxidation resistance of colloidal Ag–Cu alloy NPs compared to the Ag and Cu NPs. Also, the results of this study showed that the external electric field reduces the average size of alloy NPs, and tune the surface plasmon resonance frequency.

Synthesis, characterization and nonlinear optical response of polyelectrolyte-stabilized copper hydroxide and copper oxide colloidal nanohybrids

In the present work, the synthesis and characterization of polyelectrolyte-stabilized copper-based nanohybrids with tunable sizes and chemical composition are reported. More precisely, highly stable poly(sodium-4-styrenesulfonate) PSS-Cu(OH)2 and PSS-CuO/Cu(OH)2 nanohybrids are synthesized and characterized in terms of their chemical composition and morphology by X-ray Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM) respectively. Moreover, their nonlinear optical (NLO) response is systematically investigated by means of the Z-scan technique under different excitation conditions, i.e., 50 fs, 800 nm and 4 ns, 532 nm laser pulses. The findings denote that the size and chemical composition of nanoparticles, defining the surface plasmon resonance frequency, along with the excitation laser parameters (pulse duration and wavelength) affect greatly the NLO response of these nanohybrids. To the best of our knowledge it is the first time that such copper-based nanohybrids are synthesized and investigated towards their third-order NLO response. The results suggest an efficient strategy for preparing copper hydroxide and copper oxide colloidal nanohybrids exhibiting at-will NLO properties for specific optoelectronic and photonic applications.

Electromagnetic Scattering from a Graphene Disk: Helmholtz-Galerkin Technique and Surface Plasmon Resonances

The surface plasmon resonances of a monolayer graphene disk, excited by an impinging plane wave, are studied by means of an analytical-numerical technique based on the Helmholtz decomposition and the Galerkin method. An integral equation is obtained by imposing the impedance boundary condition on the disk surface, assuming the graphene surface conductivity provided by the Kubo formalism. The problem is equivalently formulated as a set of one-dimensional integral equations for the harmonics of the surface current density. The Helmholtz decomposition of each harmonic allows for scalar unknowns in the vector Hankel transform domain. A fast-converging Fredholm second-kind matrix operator equation is achieved by selecting the eigenfunctions of the most singular part of the integral operator, reconstructing the physical behavior of the unknowns, as expansion functions in a Galerkin scheme. The surface plasmon resonance frequencies are simply individuated by the peaks of the total scattering cross-section and the absorption cross-section, which are expressed in closed form. It is shown that the surface plasmon resonance frequencies can be tuned by operating on the chemical potential of the graphene and that, for orthogonal incidence, the corresponding near field behavior resembles a cylindrical standing wave with one variation along the disk azimuth.

Roles of the Debye Length and Skin Depth in the Characterization of Space Charge Interactions in Semiconductor Nanoparticles

Non-degenerate semiconductor nanoparticles with bulk plasma frequency in the terahertz frequency range are of substantial current interest for device and sensing applications in that part of the electromagnetic spectrum. The penetration of electromagnetic field in a material containing mobile charges is governed by two length scales, namely the Debye length and the skin depth. Their relative influence on the response of a semiconductor nanoparticle to an external terahertz electric field is examined in this Letter. A transport-based formulation to describe charge-field interactions by coupling the Boltzmann equation with the field equations is employed to characterize space-charge effects. Numerical results computed with the charge transport formulation reveal that plasmonic interactions in a semiconductor nanoparticle remains a surface phenomenon up to the surface plasmon resonance frequency, beyond which it evolves into a bulk phenomenon as the inertia effect of the charge carriers subdues their response to the field, which penetrates deeper into the particle as the frequency is increased. Examination of the spectra of charge, field, and current distributions allows for the identification of the influence of particle size, Debye length and skin depth on space charge interactions in a semiconductor nanoparticle.

Optimized 3D Finite-Difference-Time-Domain Algorithm to Model the Plasmonic Properties of Metal Nanoparticles with Near-Unity Accuracy

The finite difference time domain (FDTD) method is a grid-based, robust, and straightforward method to model the optical properties of metal nanoparticles (MNPs). Modelling accuracy and optical properties can be enhanced by increasing FDTD grid resolution; however, the resolution of the grid size is limited by the memory and computational requirements. In this paper, a 3D optimized FDTD (OFDTD) was designed and developed, which introduced new FDTD approximation terms based on the physical events occurring during the plasmonic oscillations in MNP. The proposed method not only required ~52% less memory than conventional FDTD, but also reduced the calculation requirements by ~9%. The 3D OFDTD method was used to model and obtain the extinction spectrum, localized surface plasmon resonance (LSPR) frequency, and the electric field enhancement factor (EF) for spherical silver nanoparticles (Ag NPs). The model’s predicted results were compared with traditional FDTD as well as experimental results to validate the model. The OFDTD results were found to be in excellent agreement with the experimental results. The EF accuracy was improved by 74% with respect to FDTD simulation, which helped reaching a near-unity OFDTD accuracy of ~99%. The λLSPR discrepancy reduced from 20 nm to 3 nm. The EF peak position discrepancy improved from ±5.5 nm to only ±0.5 nm.

Design and Numerical Analysis of a Graphene-Coated SPR Biosensor for Rapid Detection of the Novel Coronavirus.

In this paper, a highly sensitive graphene-based multiple-layer (BK7/Au/PtSe2/Graphene) coated surface plasmon resonance (SPR) biosensor is proposed for the rapid detection of the novel Coronavirus (COVID-19). The proposed sensor was modeled on the basis of the total internal reflection (TIR) technique for real-time detection of ligand-analyte immobilization in the sensing region. The refractive index (RI) of the sensing region is changed due to the interaction of different concentrations of the ligand-analyte, thus impacting surface plasmon polaritons (SPPs) excitation of the multi-layer sensor interface. The performance of the proposed sensor was numerically investigated by using the transfer matrix method (TMM) and the finite-difference time-domain (FDTD) method. The proposed SPR biosensor provides fast and accurate early-stage diagnosis of the COVID-19 virus, which is crucial in limiting the spread of the pandemic. In addition, the performance of the proposed sensor was investigated numerically with different ligand-analytes: (i) the monoclonal antibodies (mAbs) as ligand and the COVID-19 virus spike receptor-binding domain (RBD) as analyte, (ii) the virus spike RBD as ligand and the virus anti-spike protein (IgM, IgG) as analyte and (iii) the specific probe as ligand and the COVID-19 virus single-standard ribonucleic acid (RNA) as analyte. After the investigation, the sensitivity of the proposed sensor was found to provide 183.33°/refractive index unit (RIU) in SPR angle (θSPR) and 833.33THz/RIU in SPR frequency (SPRF) for detection of the COVID-19 virus spike RBD; the sensitivity obtained 153.85°/RIU in SPR angle and 726.50THz/RIU in SPRF for detection of the anti-spike protein, and finally, the sensitivity obtained 140.35°/RIU in SPR angle and 500THz/RIU in SPRF for detection of viral RNA. It was observed that whole virus spike RBD detection sensitivity is higher than that of the other two detection processes. Highly sensitive two-dimensional (2D) materials were used to achieve significant enhancement in the Goos-Hänchen (GH) shift detection sensitivity and plasmonic properties of the conventional SPR sensor. The proposed sensor successfully senses the COVID-19 virus and offers additional (1 + 0.55) × L times sensitivity owing to the added graphene layers. Besides, the performance of the proposed sensor was analyzed based on detection accuracy (DA), the figure of merit (FOM), signal-noise ratio (SNR), and quality factor (QF). Based on its performance analysis, it is expected that the proposed sensor may reduce lengthy procedures, false positive results, and clinical costs, compared to traditional sensors. The performance of the proposed sensor model was checked using the TMM algorithm and validated by the FDTD technique.

Platinum-Doped Prussian Blue Nanozymes for Multiwavelength Bioimaging Guided Photothermal Therapy of Tumor and Anti-Inflammation.

Developing appropriate photothermal agents to meet complex clinical demands is an urgent challenge for photothermal therapy of tumors. Here, platinum-doped Prussian blue (PtPB) nanozymes with tunable spectral absorption, high photothermal conversion efficiency, and good antioxidative catalytic activity are developed by one-step reduction. By controlling the doping ratio, PtPB nanozymes exhibit tunable localized surface plasmon resonance (LSPR) frequency with significantly enhanced photothermal conversion efficiency and allow multiwavelength photoacoustic/infrared thermal imaging guided photothermal therapy. Experimental band gap and density functional theory calculations further reveal that the decrement of free carrier concentrations and increase in circuit paths of electron transitions co-contribute to the enhanced photothermal conversion efficiency of PtPB with tunable LSPR frequency. Benefiting from antioxidative catalytic activity, PtPB can simultaneously relieve inflammation caused by hyperthermia. Moreover, PtPB nanozymes exhibited good biosafety after intravenous injection. Our findings provide a paradigm for designing safe and efficient photothermal agents to treat complex tumor diseases.

Computational investigation of the plasmonic properties of TiN, ZrN, and HfN nanoparticles: the role of particle size, medium, and surface oxidation

Group 4 transition metal nitride (TMN) nanoparticles (NPs) display strong plasmonic responses in the visible and near-infrared regimes, exhibit high melting points and significant chemical stability, and thus are potential earth-abundant alternatives to Au and Ag based plasmonic applications. However, a detailed understanding of the relationship between TMN NP physical properties and plasmonic response is required to maximize their utility. In this study, the localized surface plasmon resonance (LSPR) frequency, bandwidth, and extinction of titanium nitride (TiN), zirconium nitride (ZrN), and hafnium nitride (HfN) NPs were examined as a function of the particle size, surface oxidation, and refractive index of the surrounding medium using finite element method (FEM). A linear redshift in the LSPR frequency and a linear increase in the associated full width at half maximum (FWHM) was observed with increasing the particle size, oxidation layer thickness, and medium refractive index. We show that the effect of surface oxidation on plasmonic properties of TMN NPs is strongly size-dependent with a significant LSPR redshift, intensity reduction, and broadening in small NPs compared with larger NPs. Furthermore, the performance and efficiency of HfN, ZrN, and TiN, as well as Au NPs for narrowband (photothermal therapy, PTT) and broadband (solar energy conversion) applications, was investigated in detail. The results indicate that narrowband and broadband photothermal performance of NPs strongly depend on the particle size, surface properties, and in case of narrowband absorption, excitation wavelength.

Kinetic theory of electroconductivity of metal nanoparticles in the condition of surface plasmon resonance

In recent years, there has been increasing interest in the study of the optical properties of metallic nanostructures. This interest is primarily related to the practical application of such nanostructures in quantum optical computers, micro- and nanosensors. These applications are based on the fundamental optical effect of excitation of surface plasmons. Surface plasmons are electromagnetic excitations of electron plasma of metals at the metal-dielectric interface, which are accompanied by fluctuations in the surface charge density. The consequence of this phenomenon is surface plasmon resonance (SPR) – an increase in the energy absorption cross-section of a metal nanoparticle as the light frequency (laser irradiation) approaches to the frequency of the nanoparticle SPR. The SPR frequency for metallic nanoparticles in a dielectric matrix is found. Using the kinetic subdivision, the components of the optical power tensor for ellipsoidal metal nanoparticles were developed.

Measuring temperature heterogeneities during solar-photothermal heating using quantum dot nanothermometry.

Small metallic nanoparticles with appropriate surface plasmon resonance frequencies can be extremely efficient absorbers of solar radiation. This efficient absorption can lead to localized heating and highly heterogeneous temperatures. These unique optical properties have inspired research into the development of environmentally relevant solar-to-heat conversion technologies that are based on the light absorption of nanomaterials. The development of robust, reliable, and straight-forward techniques for measuring spatially resolved temperatures in photothermally heated systems can be an indispensable tool to aid future work in this area. Herein, we consider the application of a fluorescent technique that can measure spatially resolved temperatures in solar photothermal systems using CdSe quantum dots (<10 nm diameter). The local temperature of the quantum dot can be determined by monitoring the shift in its fluorescence wavelength resulting from the dilatation of the lattice with increasing temperature. To exploit this property, we fabricated Au nanorod-quantum dot architectures using linkers of varying lengths, and measured the light induced temperature change increasing more rapidly closer to the surface of an Au nanorod. We also compared the effect of Au nanorod coatings and found that silica coating leads to higher overall temperatures compared to organic stabilized Au nanorods.