Plasmon Transmission Research Articles

Transmission of Plasmons through a Nanowire

Exact quantitative understanding of plasmon propagation along nanowires is mandatory for designing and creating functional devices. Here we investigate plasmon transmission through top-down fabricated monocrystalline gold nanowires on a glass substrate. We show that the transmission through finite-length nanowires can be described by Fabry-P\'{e}rot oscillations that beat with free-space propagating light launched at the incoupling end. Using an extended Fabry-P\'{e}rot model, experimental and simulated length dependent transmission signals agree quantitatively with a fully analytical model.

Optical control of plasmonic grating transmission by photoinduced anisotropy

Light transmission through subwavelength silver slit gratings coated with Langmuir–Blodgett films of an azo-dye compound was studied. While the coating was found to dramatically enhance the TM-polarized grating transmittance, it also enabled its low-intensity optical control: the photoinduced alignment of the azo-dye molecules yielded a significant (∼ 40 nm) shift of the extraordinary transmission peaks to shorter or longer wavelengths depending on the orientation of the induced optical axis with respect to the gratings.

2D circularly polarized antenna array fed by spoof surface plasmonic waveguide

A two-dimensional high-gain circularly polarized (CP) patch array antenna operating in Ku band is proposed in this article. To excite a novel spoof surface plasmonic transmission line (SSP-TL), which is mounted vertically on the ground, a waveguide with a transition are used for the momentum and impedance matching. A SSP-TL 4-way power divider is then used to feed a 4 × 7 circular patch array by travelling waves. Therefore, the CP radiation is generated on the patches located between the parallel SSP-TLs. Avoiding the deterioration in axial ratio, 90° phase difference between adjacent SSP-TLs is designed by tuning the height of the SSP-TLs, as the dispersion property is influenced by the height of the corrugations obviously. The simulated maximum gain of 20.6 dBi is achieved at 15.2 GHz, together with the axial ratio of 0.8 dB. A prototype is fabricated and measured to validate the proposed antenna array.

Surface plasmon bandwidth increase using chirped-pitch linear diffraction gratings.

Large-scale linear diffraction gratings with gradually varying pitch were photo-inscribed onto the surface of azobenzene thin films using a 532 nm laser and a modified Lloyd mirror set-up. By placing a cylindrical lens in front of the direct half of the inscribing beam, gratings with a chirping rate as high as 12.9 nm/mm were produced. Subsequently, when these chirped-pitch gratings were coated with silver, over three-fold bandwidth increase was observed in the surface plasmon transmission peaks at FWHM, when compared to constant-pitch gratings. This was made possible due to the simultaneous excitation of surface plasmon resonance in a band of light wavelengths.

Broadband THz radiation through tapered semiconductor gratings on high-index substrate

We numerically demonstrate an extraordinary optical transmission (EOT) through semiconductor gratings with plasmonic properties on a high-index substrate over a broad terahertz (THz) bandwidth at the plasmonic Brewster channel. The THz EOT is due to impedance matching, an inherently non-resonant mechanism, at the grating entrance and exit faces, which are periodically carved with tapered slits. The optimal grating geometry provides a transmission of over 85% over a broad 0.2–1.0 THz bandwidth at a Brewster angle of incidence of 75°. In addition, we introduce a perfect THz absorber with low-loss and metal-free properties over a broad operating bandwidth based on the plasmonic Brewster transmission concept.

Label-free cell-substrate adhesion imaging on plasmonic nanocup arrays.

Cell adhesion is a crucial biological and biomedical parameter defining cell differentiation, cell migration, cell survival, and state of disease. Because of its importance in cellular function, several tools have been developed in order to monitor cell adhesion in response to various biochemical and mechanical cues. However, there remains a need to monitor cell adhesion and cell-substrate separation with a method that allows real-time measurements on accessible equipment. In this article, we present a method to monitor cell-substrate separation at the single cell level using a plasmonic extraordinary optical transmission substrate, which has a high sensitivity to refractive index changes at the metal-dielectric interface. We show how refractive index changes can be detected using intensity peaks in color channel histograms from RGB images taken of the device surface with a brightfield microscope. This allows mapping of the nonuniform refractive index pattern of a single cell cultured on the plasmonic substrate and therefore high-throughput detection of cell-substrate adhesion with observations in real time.

Creation of multiple on-axis foci and ultra-long focal depth for SPPs.

We present the design of a plasmonic lens (PL) which is composed of pixelated nano-grooves on a gold film for the coupling and focusing of surface plasmon polaritons (SPPs) into multiple focal spots on the optical axis. The pixelated grooves are arranged along the y-axis and the x-position of each groove is optimized by the simulated annealing algorithm. PLs that implement two and three on-axis foci are presented and the designed structures have been validated with FDTD simulations. We also successfully constructed a long-focal-depth PL with a longitudinal FWHM of the focus that reached 25 plasmonic wavelengths, while its transverse field profile is maintained over 15 µm distance. The presented design method constitutes a new basis for plasmonic beam engineering, and the proposed particular SPP focal fields have potential applications in multiple imaging, particle manipulating, and plasmonic on-chip signal transmission.

Loss Analysis and Engineering of Spoof Surface Plasmons Based on Circuit Topology

We propose to systematically investigate the loss mechanism in spoof surface plasmon transmission lines (SSP-TLs) associated with surface-mounted resistors, by numerical simulation and circuit topology modeling, with the experimental validations. Distorted dispersion and loss curves introduced by the resistors are simultaneously predicted by the numerical results and interpreted by the circuit topology model. Attenuators constructed by periodically loaded resistors and T-type networks are designed according to the circuit topology model, and validated by both simulation and experiment results; hence, significant and controllable losses are achieved in a deep-subwavelength scale. The proposed structures and methods enable insightful understanding of the SSP-TLs and hold promise in other lossy SSP device study and design.

Resonances in reflection, transmission and absorption of 1-D triangular-relief metallic gratings

We present numerical simulations in order to investigate the resonances in reflection, transmission, and absorption of surface plasmons for triangular gratings. The results reveal a number of resonances depending of grating geometrical parameters, surface materials, and characteristics of incident light. For metals Cu, Ag, Au, Ni, Pd, and Pt, the intensive weakly flat grating-resonance at TM-polarized incident light under normal direction is at wavelength \(\lambda = 0.83 \pm 0.01\,\upmu{\text{m}}\), and a weaker grating resonance is at wavelength around \(0.46\,\upmu{\text{m}}\). For strongly flat grating weak resonances appear for some metals, and disappear for others.

Peptide Sequence Effects Control the Single Pot Reduction, Nucleation, and Growth of Au Nanoparticles

Peptides have demonstrated unique capabilities to fabricate inorganic nanomaterials of numerous compositions through noncovalent binding of the growing surface in solution. In this contribution, we demonstrate that these biomolecules can control all facets of Au nanoparticle fabrication, including Au3+ reduction, without the use of secondary reagents. In this regard using the AuBP1 peptide, the N-terminal tryptophan residue is responsible for driving Au3+ reduction to generate Au nanoparticles passivated by the oxidized peptide in solution, where localized residue context effects control the reducing strength of the biomolecule. The process was fully monitored by both time-resolved monitoring of the growth of the localized surface plasmon resonance and transmission electron microscopy. Nanoparticle growth occurs by a unique disaggregation of nanoparticle aggregates in solution. Computational modeling demonstrated that the oxidized residue of the peptide sequence does not impact the biomolecule’s ability t...

Plasmon transmission through excitonic subwavelength gaps.

We study the transfer of electromagnetic energy across a subwavelength gap separating two co-axial metal nanorods. In the absence of spacer in the gap separating the rods, the system exhibits strong coupling behavior between longitudinal plasmons in the two rods. The nature and magnitude of this coupling are studied by varying various geometrical parameters. As a function of frequency, the transmission is dominated by a split longitudinal plasmon peak. The two hybrid modes are the dipole-like "bonding" mode characterized by a peak intensity in the gap and a quadrupole-like "antibonding" mode whose amplitude vanishes at the gap center. When the length of one rod is varied, this mode spectrum exhibits the familiar anti-crossing behavior that depends on the coupling strength determined by the gap width. When off-resonant 2-level emitters are placed in the gap, almost no effect on the frequency dependent transmission is observed. In contrast, when the molecular system is resonant with the plasmonic line shape, the transmission is strongly modified, showing characteristics of strong exciton-plasmon coupling. Most strongly modified is the transmission near the lower frequency "bonding" plasmon mode. The presence of resonant molecules in the gap affects not only the molecule-field interaction but also the spatial distribution of the field intensity and the electromagnetic energy flux across the junction.

Surface plasmon transmission through discontinuous conducting surfaces: Plasmon amplitude modulation by grazing scattered fields

We have studied numerically the diffraction of a surface plasmon polariton (SPP) when it encounters a wide multi-wavelength slit in conducting films. As a jump process a SPP is excited beyond the slit by wave scattering at the second slit edge. The exciting radiation is produced when the incident SPP collapses at the first slit edge. We have found that the transmitted SPP supports inherent and unavoidable interference with grazing scattered radiation; the spatial modulation extends to the fields in the diffraction region where a series of low intensity spots arises. We demonstrate that the SPP generated on the second slab depends on the frequency but not on the wave vector of the collapsed SPP; a SPP is transmitted even when the two metals forming the slit are different. The numerical results were obtained using the Finite Difference Time Domain (FDTD) method with a grid size λ/100.

Artificial neural network modeling of plasmonic transmission lines.

In this paper, new models based on an artificial neural network (ANN) are developed to predict the propagation characteristics of plasmonic nanostrip and coupled nanostrips transmission lines. The trained ANNs are capable of providing the required propagation characteristics with good accuracy and almost instantaneously. The nonlinear mapping performed by the trained ANNs is written as closed-form expressions, which facilitate the direct use of the results obtained in this research. The propagation characteristics of the investigated transmission lines include the effective refractive index and the characteristic impedance. The time needed to simulate 1000 different versions of the transmission line structure is about 48 h, using a full-wave electromagnetic solver compared to 3 s using the developed ANN model.

Photometric determination of tetracycline based on surface plasmon resonance of silver nanoparticles

We report on a simple and sensitive method for the determination of tetracycline based on its reducing action on AgNO3 in alkaline medium containing ammonia and sodium hydroxide at 65°C. As a result of this reaction, silver nanoparticles (AgNPs) are formed. The AgNPs are stabilized in solution by adding poly(vinyl pyrrolidone) as a capping agent. The formed AgNPs were identified by surface plasmon resonance absorption spectrum and transmission electron microscopy image. The plasmon absorption peak at 411 nm is proportional to the concentration of tetracycline. The calibration graph is linear in the concentration range of 0.05–5.0 mg/L with a detection limit of 0.013 mg/L. This method was applied to the determination of tetracycline in pharmaceutical products.

Tunable Plasmonic Nanohole Arrays Actuated by a Thermoresponsive Hydrogel Cushion.

New plasmonic structure with actively tunable optical characteristics based on thermoresponsive hydrogel is reported. It consists of a thin, template-stripped Au film with arrays of nanoholes that is tethered to a transparent support by a cross-linked poly(N-isopropylacrylamide) (pNIPAAm)-based polymer network. Upon a contact of the porous Au surface with an aqueous environment, a rapid flow of water through the pores enables swelling and collapsing of the underlying pNIPAAm network. The swelling and collapsing could be triggered by small temperature changes around the lower critical solution temperature (LCST) of the hydrogel. The process is reversible, and it is associated with strong refractive index changes of Δn ∼ 0.1, which characteristically alters the spectrum of surface plasmon modes supported by the porous Au film. This approach can offer new attractive means for optical biosensors with flow-through architecture and actively tunable plasmonic transmission optical filters.

Propagation and localization of quantum dot emission along a gap-plasmonic transmission line.

Plasmonic transmission lines have great potential to serve as direct interconnects between nanoscale light spots. The guiding of gap plasmons in the slot between adjacent nanowire pairs provides improved propagation of surface plasmon polaritons while keeping strong light confinement. Yet propagation is fundamentally limited by losses in the metal. Here we show a workaround operation of the gap-plasmon transmission line, exploiting both gap and external modes present in the structure. Interference between these modes allows us to take advantage of the larger propagation distance of the external mode while preserving the high confinement of the gap mode, resulting in nanoscale confinement of the optical field over a longer distance. The performance of the gap-plasmon transmission line is probed experimentally by recording the propagation of quantum dots luminescence over distances of more than 4 μm. We observe a 35% increase in the effective propagation length of this multimode system compared to the theoretical limit for a pure gap mode. The applicability of this simple method to nanofabricated structures is theoretically confirmed and offers a realistic way to combine longer propagation distances with lateral plasmon confinement for far field nanoscale interconnects.

Two-Dimensional Analogies to Frequency-Selective Surfaces (FSS) on the Graphene Sheet

In this paper, we propose that two-dimensional analogies to frequency-selective surfaces (FSS) can be achieved on graphene surfaces based on transformation optics. The analogies to representative FSS structures, including the anti-reflecting coating (ARC) and the high-reflecting coating (Bragg reflector), have been investigated through both analytical effective-index method (EIM)/transfer-matrix method (TMM) and numerical simulations. Both analytical and numerical solutions have shown that the propagation of plasmons on graphene surface with periodic chemical potentials can be an analogy to the interaction of incident light with traditional FSS multilayer dielectric media in which the transmission or reflection can be obtained by EIM/TMM. Combined with the tunability of graphene, the transmission or reflection of plasmons can be tuned by adjusting the bias voltage. The proposed structures and theoretical methods may provide new visions for achieving two-dimensional analogies to traditional structures on graphene.

Ultrathin 90-degree sharp bends for spoof surface plasmon polaritons.

At low frequencies outside the plasmonic range, strongly confined surface waves can be achieved on periodically structured metal surfaces, thereby allowing for the design of compact electromagnetic guiding devices. Here, we propose an approach to realize highly efficient transmission of spoof surface plasmons around 90-degree sharp bends on ultrathin metallic films in the microwave regime. We demonstrate that by judiciously engineering the structure, the dispersion relation can be designed to reduce the scattering. Furthermore, the reflection can be suppressed by proper structural decoration at the bending corner. A one-dimensional scattering theory is employed to understand and verify the transmission properties of our waveguide bend structure. Our design scheme is not restricted to the specific structure we propose here but can be applied to other guiding components built up on two dimensional metal surfaces.

A green approach to prepare silver nanoparticles loaded gum acacia/poly(acrylate) hydrogels

In this work, gum acacia (GA)/poly(sodium acrylate) semi-interpenetrating polymer networks (Semi-IPN) have been fabricated via free radical initiated aqueous polymerization of monomer sodium acrylate (SA) in the presence of dissolved Gum acacia (GA), using N,N′-methylenebisacrylamide (MB) as cross-linker and potassium persulphate (KPS) as initiator. The semi-IPNs, synthesized, were characterized by various techniques such as X-ray diffraction (XRD), thermo gravimetric analysis (TGA) and Fourier transform infrared (FTIR) spectroscopy. The dynamic water uptake behavior of semi-IPNs was investigated and the data were interpreted by various kinetic models. The equilibrium swelling data were used to evaluate various network parameters. The semi-IPNs were used as template for the in situ preparation of silver nanoparticles using extract of Syzygium aromaticum (clove). The formation of silver nanoparticles was confirmed by surface plasmon resonance (SPR), XRD and transmission electron microscopy (TEM). Finally, the antibacterial activity of GA/poly(SA)/silver nanocomposites was tested against E. coli.

Transmission and Reflection of Terahertz Plasmons in Two-Dimensional Plasmonic Devices

Plasmons in two-dimensional semiconductor devices will be reflected by discontinuities, notably, junctions between gated and non-gated electron channels. The transmitted and reflected plasmons can form spatially- and frequency-varying signals, and their understanding is important for the design of terahertz detectors, oscillators, and plasmonic crystals. Using mode decomposition, we studied terahertz plasmons incident on a junction between a gated and a nongated channel. The plasmon reflection and transmission coefficients were found numerically and analytically and studied between 0.3 and 1 THz for a range of electron densities. At higher frequencies, we could describe the plasmons by a simplified model of channels in homogeneous dielectrics, for which the analytical approximations were accurate. At low frequencies, however, the full geometry and mode spectrum had to be taken into account. The results agreed with simulations by the finite-element method. Mode decomposition thus proved to be a powerful method for plasmonic devices, combining the rigor of complete solutions of Maxwell's equations with the convenience of analytical expressions.