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In this study, we investigated plasmonic field localization with trapezoidal nanopatterns under normal incident light excitation to find optimum structures for sensing and imaging. A finite element method was used to calculate the fundamental characteristics of the localized surface plasmon with varied trapezoidal nanopatterns. First, we describe how to localize the plasmonic fields on the trapezoidal patterns and then report our results from the investigation of the optimum properties of the nanopatterns for maximized field intensity. Initially, we expected that maximized field localization would lead to enhancement of the sensing sensitivity or imaging resolution in plasmon-based sensing and imaging systems. However, more interestingly, we found a field cancellation effect under specific modality conditions through the simulation. Thus, we thoroughly investigated the principle of the effect and extracted the modality conditions that induced field cancellation. In addition, specific modality conditions of nanopatterns that could be fabricated with conventional lithographic methods were numerically determined. Then, the field cancellation effect was experimentally verified using scanning nearfield optical microscopy. The results indicate that trapezoidal nanopatterns bring about enhanced field localization at the shaper edge of nanopatterns than do conventional rectangular nanopatterns and that plasmonic field cancellation can be observed under specific modality conditions of nanopatterns, even for conventional rectangular nanopatterns. Thus, it is suggested that careful fabrication and maintenance are needed to obtain strong plasmonic localization. Finally, the feasibility of providing a novel sensing platform using the field cancellation effect is suggested.

We theoretically present a tunable dual-band perfect absorber in the visible and near-infrared regions for sensing applications. The absorber consists of a periodic metal-dielectric-metal shell-core nanorod array and a thick metal film separated by a periodic dielectric nanorod array. Two narrow perfect absorption bands (98.2% and 99.7%) are achieved due to the synergy of localized surface plasmons (LSPs), propagating surface plasmons (PSPs) and lattice resonances. The two narrow bands both are sensitive to the refractive index change and the maximum sensitivity is 783.705 nm/RIU (RIU is the refractive index unit). These enable the structure to have great potential in the fields of perfect absorbers, plasmonic sensors, and filters.

Here we show that surface plasmon resonance sensors that typically use 760 nm wavelength Kretschmann–Raether coupling to a 50 nm thick gold film can have 3 times higher surface sensitivity by using local resonances from periodically arranged short-range modes in the same configuration. Considering shot noise, the resolution was found to improve fourfold. This was calculated by matching the design wavelength and minimum angle as calculated by rigorous coupled wave analysis, giving a period of 250 nm in a 10 nm thick gold film and a gap length of 40 nm. Finite-difference time-domain simulations were used to confirm that the short-range modes correspond to localized surface plasmon resonances. The present short-range plasmon approach can be used to improve the sensitivity in monitoring biomolecule interactions.

A coupling system is proposed to active control of strong exciton–plasmon–exciton coupling, which consists of a silver nanoprism separated from a monolayer WS2 by J-aggregates. The scattering spectrum of the hybrid system calculated by the finite-difference time-domain (FDTD) method is well reproduced by the coupled oscillator model theory. The calculation results show that strong couplings among WS2 excitons, J-aggregate excitons, and localized surface plasmon resonances (LSPRs) are achieved in the hybrid nanostructure, and result in three plexciton branches. We further analyze the exciton–plasmon–exciton coupling behaviors and obtain the weighting efficiencies of the original modes in three plexciton branches. The strong couplings between two different excitons and LSPRs can be active manipulated by tuning the temperature or the concentration of J-aggregates. The proposed systems make up a simple platform for the dynamic control of exciton–plasmon–exciton couplings and have potential applications in optical modulators at the nanoscale.

Chameleon complex, a coordination polymer containing Eu3+ and Tb3+ as emitting centers, is a temperature sensor whose photoluminescence (PL) intensity ratio between Eu3+ and Tb3+ varies with the temperature. Although the performance of this complex has been verified in a form of powders, it has not been used as a film, which is a commonly used form for various applications. In this study, we fabricate a temperature-sensing thin film by dissolving a chameleon complex powder in poly(methyl methacrylate) (PMMA). The chameleon complex retains the responsivity to temperature after the dispersion in PMMA. The PL intensity ratio changes almost linearly with the temperature in the experimental range of 20°C–40°C. In order to further demonstrate the temperature sensing ability of the film, we deposit the film on a plasmonic periodic array of Al nanocylinders. We simultaneously use two laser lines to excite both localized surface plasmon resonances (LSPRs) hybridized with in-plane diffraction in the array and the chameleon complex in the film. The LSPRs decay into heat increasing the temperature, which is then detected through the PL intensity ratio between Eu3+ and Tb3+. The temperature increase is in agreement with numerical calculations. The results show that the film is applicable for temperature sensing in a local area defined by the excitation spot.

Magnetic localized surface plasmon modes are supported on metallic spiral structures. Coupling mechanisms for these metamaterial resonators, which are the joint action of magnetic and electric coupling, are studied. Based on the strong coupling, spoof magnetic plasmon modes propagating in the backward direction are proposed along a chain of subwavelength resonators. The theoretical analysis, numerical simulations, and experiments are in good agreement. The proposed novel route for achieving negative-index waveguiding has potential applications in integrated devices and circuits.

The light absorption of a hybrid novel MoS2-based nanostructure is theoretically investigated by using the finite-difference time-domain (FDTD) simulations, and high-efficiency broadband absorption is achieved in the visible wavelength region. The enhancement of localized electromagnetic field owing to that localized surface plasmon resonances (LSPRs) supported by Au nanoparticles (NPs) can be used to enhance the absorption of MoS2, and the localized absorption of monolayer MoS2 are remarkably enhanced up from about 18.3% and 4.6% to about 55.2% and 84.8% at the resonant wavelengths of 467.7 nm and 557.8 nm, respectively. Furthermore, the effects of radii of Au NPs, period of Au NPs array, Au@Si NPs core-shell ratios, period numbers of the distributed Bragg mirror (DBR), and incident angle on the absorption of the proposed nanostructure have been systematically investigated. The similar design idea to enhance the light-MoS2 interaction can also be applied to other transition-metal dichalcogenides (TMDCs). This work will contribute to the design of TMDCs-based nanophotonic and optoelectronic devices.

In this work, a periodic bowtie structure based on black phosphorus (BP) is theoretically proposed and characterized. It is demonstrated that localized surface plasmons can be excited in the BP nanoantennas at terahertz (THz) frequencies. Numerical investigations, using the numerical method finite-difference time-domain (FDTD), have been utilized to analyze the the dimensions' impact on absorption spectra. Furthermore, the electric field distribution is plotted and discussed to explain the resonance wavelength tuning by different geometrical sizes of the structure. Results reveal that the optimized BP bow-tie structure can be allowed for the realization of two-dimensional nanophotonics at terahertz frequencies.

Plasmon resonances and electric field enhancements of several near-field optical antennae with plasmonic nanostructures engineered at their apices were quantitatively compared using finite difference time domain simulations. Although many probe designs have been tested experimentally, a systematic comparison of field enhancements has not been possible, due to differences in instrument configuration, reporter mechanism, excitation energy, and plasmonic materials used. For plasmonic nanostructures attached to a non-plasmonic support (e.g., a nanoparticle functionalized AFM tip), we find that the complex refractive index of the support material is critical in controlling the overall plasmonic behavior of the antenna. Supports with strong absorption at optical energies (Pt, W) dampen plasmon resonances and lead to lower enhancements, while those with low absorption (SiO2, Si3N4, Si) boost enhancement by increasing the extinction cross-section of the apex nanostructure. Using a set of physically realistic constraints, probes were optimized for peak plasmonic enhancement at common near-field optical wavelengths (633-647 nm) and those with focused ion-beam milled grooves near the apex were found to give the largest local field enhancements (~30x). Compared to unstructured metal cones, grooved probes gave a 300% improvement in field strength, which can boost tip-enhanced Raman spectroscopy (TERS) signals by 1-2 orders of magnitude. Moreover, grooved probe resonances can be easily tuned over visible and near-infrared energies by varying the plasmonic metal (Ag or Au) and groove location. Overall, this work shows that probes with strong localized surface plasmon resonances at their apices can be engineered to provide large field enhancements and boost signals in near-field optical experiments.

In this paper, we propose a microfluidic chemical sensor based on quarter-mode spoof plasmonic resonators with more compact overall size and higher sensitivity. First, a microfluidic channel engraved on polydimethylsiloxane is aligned to the upper part of the spoof plasmonic metal-insulator-metal (MIM) ring resonator where the strongest electric fields are observed at resonance. Although a resonant frequency shift of 270 MHz has been observed when the microfluidic channel is filled with pure ethanol, there is no resonant frequency shift when the ethanol concentration is changed from 40% to 60%. Then the spoof localized surface plasmons modes on the quarter corrugated MIM ring are analyzed, and a microfluidic sensor based on the quarter-mode spoof plasmonic resonator has been proposed. The proposed microfluidic sensor requires a very small amount (3.9 μL) of liquid for testing. After infilling the microfluidic channel with pure ethanol, the resonant frequency shift of 940 MHz has been observed on account of the dielectric changes. It is observed that the resonant frequency of the proposed sensor shifts from 5.07 to 6.62 GHz when the ethanol concentration is varied from 10% to 90%. It has been demonstrated that such quarter-mode spoof plasmonic resonator is well suited to a highly sensitive and compact microfluidic chemical sensor.

Metallic nanoparticles (NPs) support localized surface plasmon resonances (LSPRs), which enable to concentrate sunlight at the active layer of solar cells. However, full-wave modeling of the plasmonic solar cells faces great challenges in terms of huge computational workload and bad matrix condition. It is tremendously difficult to accurately and efficiently simulate near-field multiple scattering effects from plasmonic NPs embedded into solar cells. In this work, a preconditioned volume integral equation (VIE) is proposed to model plasmonic organic solar cells (OSCs). The diagonal block preconditioner is applied to different material domains of the device structure. As a result, better convergence and higher computing efficiency are achieved. Moreover, the calculation is further accelerated by two-dimensional periodic Green's functions. Using the proposed method, the dependences of optical absorption on the wavelengths and incident angles are investigated. Angular responses of the plasmonic OSCs show the super-Lambertian absorption on the plasmon resonance but near-Lambertian absorption off the plasmon resonance. The volumetric method of moments and explored physical understanding are of great help to investigate the optical responses of OSCs., 11 pages, 6 figures

This work numerically investigates a localized terahertz (THz) slow light phenomenon by tuning the spoof localized surface plasmon-induced transparency (PIT). A binary meta-molecule supports the interaction of the spoof localized surface plasmon (spoof-LSP), which is composed of a metallic arc and a textured circular cavity of periodic grooves. By tuning the central angle θ of the arc from 90 degrees to 170 degrees, a slow light plateau is found in the transparency window at certain frequency range. A maximum of 46 ps group delay is achieved at the θ of 135. The numerical mapping of the electromagnetic field indicates a new-born dipolar spoof-LSP that appears at the transparency windows on the circular cavity with opposite polarity to the spoof-LSP on the metallic arc. These two spoof-LSPs of opposite direction lead to a fake quadrupole, which will repel each other in magnetic dipole momentum. The slow light achieves maximum with the induced spoof-LSP and is the same as the origin spoof-LSP on the metallic arc in oscillation strength. This work paves a new way for the maximization of THz slow light.

A graphene-assisted vertical multilayer structure is proposed for high performance surface-enhanced Raman scattering (SERS) and surface-enhanced infrared absorption (SEIRA) spectroscopies on a single substrate, employing simultaneous localized surface plasmon in the visible region and magnetic plasmon resonance in the mid-infrared region. Such multilayer structure consists of a monolayer graphene sandwiched between Ag nanoparticles (NPs) and a metal-insulator-metal (MIM) microstructure, which can be easily fabricated by a standard surface micromachining process. Benefiting from the large near field enhancement by the hybrid plasmons in both visible and mid-infrared regions, a high enhancement factor of up to 107 for SERS and 105 for SEIRA can be achieved. Additionally, the strong magnetic resonance of the MIM microstructure can be tuned in broadband to selectively enhance the desired vibration modes of molecules. The strong SERS and SEIRA enhancement together with easy fabrication provides new opportunities for developing integrated plasmonic devices for multispectral detection of molecules on the same substrate.

In this paper, we propose and demonstrate an all-fiber passively Q-switched erbium doped fiber laser (EDFL) by using gold nanostars (GNSs) as a saturable absorber (SA) for the first time, to the best of our knowledge. In comparison with other gold nanomorphologies, GNSs have multiple localized surface plasmon resonances, which means that they can be used to construct wideband ultrafast pulse lasers. By inserting the GNS SA into an EDFL cavity pumped by a 980 nm laser diode, a stable passively Q-switched laser at 1564.5 nm was achieved for a threshold pump power of 40 mW. By gradually increasing the pump power from 40 to 120 mW, the pulse duration decreases from 12.8 to 5.3 μs and the repetition rate increases from 10 to 17 kHz. Our results indicate that the GNSs are a promising SA for constructing pulse lasers.

We compare different spoof localized surface plasmon (LSP) schemes for increasing the field enhancement in the subwavelength regime. Based on the mechanisms of three differently widely used corrugated metallic disk structures, we compare three widely used corrugated disks and propose a nonconcentric spiral-shaped metallic disk structure, which not only maximizes the electromagnetic field confinement but also effectively reduces the radiation loss. The performance can be further improved by adopting a hybrid system consisting of two closely spaced nonconcentric spiral structures. We show that such a dimer of nonconcentric spiral disk produces significant field enhancement compared with the previously discussed structures. Our study provides a perceptive guideline for potential applications, such as plasmonic sensors and antennas, associated with LSPs in the microwave and terahertz frequencies.

We theoretically show a high-performance refractive index (RI) sensor that consists of a two-dimensional (2D) gold nanoparticle array lying on a dielectric slab waveguide. The sensor has high RI sensitivity and figure of merit (FOM), reaching 250 nm/RIU and 28, respectively. Such a high RI sensitivity and FOM result from a sharp fanolike resonance, which is caused by the interference between the localized surface plasmon resonances (LSPRs) excited on individual gold nanoparticles and the waveguide mode propagating in the adjacent dielectric slab. The interference condition is that the electric field of the waveguide mode must be parallel to the polarization direction of the LSPRs. Our work may have potential applications in ultra-compact biomedical sensing.

Asymmetric transmission (AT) holds significant applications in controlling polarization and propagation directions of electromagnetic waves. In this paper, tilted rectangular nanohole (TRNH) arrays in a square lattice are proposed to realize an AT effect. Numerical results show two AT modes in the transmission spectrum, and they are ascribed to the localized surface plasmon resonances around the two ends of TRNH and surface plasmon polaritons on the golden film. AT properties of the TRNH strongly depend on structural parameters, such as width, length, thickness, and tilted angle of TRNH. Results provide a novel mechanism for generating AT effect and offer potential plasmonic device applications, such as asymmetric wave splitters and optical isolators.

We investigate the localized surface plasmon (LSP) effect by Al nanorings on the AlGaN/GaN multiple quantum well (MQW) structure emitting at 365 nm. For this experiment, first, the size of Al nanorings is optimized to maximize the energy transfer (or coupling) between the LSP and MQW using the silica nanospheres. Then, the Al nanorings with an outer diameter of 385 nm, which exhibit a strong absorption peak in the near-ultraviolet region, are applied to the top surface of the AlGaN/GaN MQW. The photoluminescence (PL) intensity of the MQW structure with Al nanorings increased by 227% at 365 nm compared to that without Al nanorings. This improvement is mainly attributed to an enhanced radiative recombination rate in the MQWs through the energy-matched LSPs by the temperature-dependent PL and time-resolved PL analyses. The radiative lifetime was about two times shorter than that of the structure without Al nanorings at room temperature. In addition, the measured PL efficiency at room temperature of the structure with Al nanorings was 33%, while that of the structure without Al nanorings was 19%, implying that LSP-QW coupling together with the nanoring array pattern itself played important roles in the enhancement.

We measured the transmission spectra of an array of split-ring resonators up to 10 terahertz for parallel and perpendicular polarizations. Calculations of the lattice and plasmon mode dispersion relations in combination with electromagnetic simulations confirm the presence of multiple higher order lattice and plasmon modes. We modify the quality factor of higher-order plasmon resonances by modulating the lattice-plasmon mode coupling via changes in the period of the array. We also propose single frequency switches and a broadband dual-state amplitude modulator based on structured illumination that actively modifies the period of the splitring resonator array.

The interaction of a surface plasmon polariton wave of the far-infrared regime propagating in a single-walled carbon nanotube with a drift current is theoretically investigated. It is shown that under the synchronism condition a surface plasmon polariton amplification mechanism is implemented due to the transfer of electromagnetic energy from a drift current wave into a terahertz surface wave propagating along the surface of a single-walled carbon nanotube. Numerical calculations show that for a typical carbon nanotube surface plasmon polariton amplification coefficient reaches huge values of the order of 106 сm−1, which makes it possible to create a carbon-nanotube-based spaser.

Coupling efficiency between the localized surface plasmons (LSPs) of metal nanoparticles (NPs) and incident light dominates the sensitivities of plasmon-based sensing spectroscopies and imaging techniques, e.g., surface-enhanced Raman scattering (SERS) spectroscopy. Many endogenous features of metal NPs (e.g., size, shape, aggregation form, etc.) that have strong impacts on their LSPs have been discussed in detail in previous studies. Here, the polarization-tuned electromagnetic (EM) field that facilitates the LSP coupling is fully discussed. Numerical analyses on waveguide-based evanescent fields (WEFs) coupled with the LSPs of dispersed silver nanospheres and silver nano-hemispheres are presented and the applicability of the WEF-LSPs to plasmon-enhanced spectroscopy is discussed. Compared with LSPs under direct light excitation that only provide 3–4 times enhancement of the incidence field, the WEF-LSPs can amplify the electric field intensity about 30–90 times (equaling the enhancement factor of 106–108 in SERS intensity), which is comparable to the EM amplification of the SERS “hot spot” effect. Importantly, the strongest region of EM enhancement around silver nanospheres can be modulated from the gap region to the side surface simply by switching the incident polarization from TM to TE, which widely extends its sensing applications in surface analysis of monolayer of molecule and macromolecule detections. This technique provides us a unique way to achieve remarkable signal gains in many plasmon-enhanced spectroscopic systems in which LSPs are involved.

The development of surface enhanced infrared absorption has been constrained by the limited field enhancement and narrow-band resonance of commonly used metal resonators. In this theoretical work, the design of a crescent resonator (CR) combined with graphene-enabled plasmon tuning is proposed to settle the drawbacks. The CR is similar to a split ring resonator (SRR), but exhibits a much improved field enhancement. The influence of graphene on the field enhancement of the CR has been systematically investigated. Coupling from localized plasmon of CR to propagating plasmon of graphene has been observed, and the constructive interference of the plasmon wave has led to not only better enhancement inside the gap but also usable enhancements all over the graphene film, which go beyond the localized nature of metal plasmons.

Cavity optomechanics is applied to study the coupling behavior of interacting molecules in surface plasmon systems driven by two-color laser beams. Different from the traditional force–distance measurement, due to a resonant frequency shift or a peak splitting on the probe spectrum, we have proposed a convenient method to measure the van der Waals force strength and interaction energy via nonlinear spectroscopy. The minimum force value can reach approximately 10−15 N, which is 3 to 4 orders of magnitude smaller than the widely applied atomic force microscope (AFM). It is also shown that two adjacent molecules with similar chemical structures and nearly equal vibrational frequencies can be easily distinguished by the splitting of the transparency peak. Based on this coupled optomechanical system, we also conceptually design a tunable optical switch by van der Waals interaction. Our results will provide new approaches for understanding the complex and dynamic interactions in molecule–plasmon systems.

The propagation characteristics of excited surface plasmon polaritons in a plasmonic filter are studied by controlling the geometric parameters of the waveguide. The proposed filter is composed of two nanoscale metal-insulator-metal type surface plasmon polariton based bus waveguides connected through two face-to-face concentric semi-rings with different radii placed at the top and bottom. Numerical results show that the wavelengths of the transmission spectra dips have changes of red shift when either the radius of the two semi-rings or the refractive index of the insulator is increased. This sensitivity is superior to the filter properties, and thus we believe that the proposed waveguide structure has potential application in nanoscale photonic devices integration.

We develop a compact objective-coupling surface plasmon holographic microscopy with a common-path configuration by introducing a Wollaston prism. Through off-axis hologram recording and numerical reconstruction, amplitude- and phase-contrast surface plasmon resonance (SPR) images can be obtained simultaneously. Based on the four-layer SPR model, the thin film thickness distribution in near field can be mapped unambiguously using a novel demodulation method without a priori knowledge. The technique demonstrates nondestructive and full-field measurement capabilities with sub-nanometer resolution theoretically. Furthermore, owing to the high temporal stability, the recommended system shows great potential for dynamic measurement of near-field tiny refractive index or thickness variation in fields such as chemistry and biomedicine, etc.

Using the vectorial form of the Huygens–Fresnel principle, we have studied the diffraction of a surface plasmon mode propagating on a metal–dielectric interface as it passes through a dielectric thick lens. This method, unlike the scalar diffraction theory, provides the interference and focusing patterns of the surface mode in the near field. Also, we have calculated the phase delay of the surface plasmon mode behind the lens without using the thin lens assumption. Based on the developed equations, we have analyzed the effects of lens thickness and aperture size on in-plane focusing of the surface plasmon.

Two-dimensional gold-nanorod arrays (2D-GNA) exhibit distinct resonance peaks in the visible wavelength range that are clearly associated with long and short axis plasmon oscillations. In this paper, we demonstrate a flexible and reproducible way for controlling the plasmon resonance of such 2D-GNAs in-situ, even post-fabrication process, simply by embedding free-standing nanorod arrays into an elastomer thin film. Stretching the polymer film shows the plasmon long-axis resonance to red-shift proportionally to the applied force by as much as 20~nm by increasing the center-to-center distance between individual nanorods. Releasing the load elastically relaxes the stretched polymer film, hence allowing the recording of cyclic load curves while varying the spectral response in-situ. Notably, film stretching along the substrate plane (x-axis) results in a uniaxial distortion of the nanorod lattice. We show how to account for this anisotropic strain in both the experiment and our complementary finite element modelling simulations, which then both match very well. This novel work illustrates both the feasibility and reliability when integrating 2D-GNAs for potential flexible, plasmonic applications.

When light emitters are being placed in close proximity to a plasmonic system, not only the emission but also the excitation can be strongly enhanced and both yield the surface plasmon polariton (SPP) mediated fluorescence enhancement. Here, we combine the rate equation model and coupled mode theory to formulate the excitation rate of light emitters located on a periodic metallic array. The rate is expressed in terms of quantities that can be measured by angle- and polarization-resolved reflectivity and photoluminescence spectroscopy. As a demonstration, we have studied the excitation rate of CdSeTe quantum dots deposited on a 2D Au nanohole array as a function of the propagation direction of the (−1,0) Bloch-like SPPs. At the excitation wavelength of 633 nm, we find the rate remains almost constant at ~44 ps−1 regardless of the propagation direction of SPPs, which move from the Γ-X towards the Γ-M direction in the first Brillouin zone, and the crossing of the (−1,0) and (0,-1) SPPs along the Γ-M direction where two bright and dark modes are formed. The results are supported by the finite-difference time-domain simulations. We conclude the excitation rate is an intrinsic parameter and the enhanced excitation of the quantum dots arises entirely from field enhancement.

A high sensitivity D-shaped hole double-cladding fiber temperature sensor based on surface plasmon resonance (SPR) is designed and investigated by a full-vector finite element method. Within the D-shaped hole double-cladding fiber, the hollow D-section is coated with gold film and then injected in a high thermo-optic coefficient liquid to realize the high temperature sensitivity for the fiber SPR temperature sensor. The numerical simulation results show that the peaking loss of the D-shaped hole double-cladding fiber SPR is hugely influenced by the distance between the D-shaped hole and fiber core and by the thickness of the gold film, but the temperature sensitivity is almost insensitive to the above parameters. When the thermo-optic coefficient is −2.8×10−4/°C, the thickness of the gold film is 47 nm, and the distance between the D-shaped hole and fiber core is 5 μm, the temperature sensitivity of the D-shaped hole fiber SPR sensor can reach to −3.635 nm/°C.

Transition metal nitrides are promising alternative plasmonic materials to noble metals for data storage applications as they exhibit localized surface plasmon resonances and have high melting temperatures. Here, angle dependent spectral measurements of the plasmonic resonances of nanodisk arrays made from titanium nitride are examined. Polarized light is used to excite the quadrupole and higher order resonance plasmonic modes which are required in the state-of-the-art designs of near-field transducers used in plasmonic enhanced magnetic recording. Numerical simulations compare the energy distribution and absorption efficiencies for different sized Au and Ti nanodisks. A high electric field enhancement is calculated at the termination of a lollipop plasmonic transducer made of titanium nitride which is shifted to longer wavelengths when compared with an Au transducer of the same dimensions. This, together with its outstanding material properties makes TiN a favourable material for data storage applications.

The plasmon resonance properties of gold nanoparticles (AuNPs) in liquid ionic medium with different concentration was investigated using a UV-visible/NIR spectrophotometer. The optical absorption spectra of AuNPs suspended in NaCl solutions in the range of 400 – 800 nm were measured; specifically, 10 nm bare AuNPs separately suspended in NaCl solution with molar concentrations of 0.01M and 1M resulted in different shapes and widths in the plasmon resonance peak. The optical spectral resonances were analyzed using the Mie theory, Lorenz-Drude size-dependent dielectric functions and effective medium theory. Analysis of the spectral features by means of theoretical models showed that the damping of the plasmon resonance of AuNPs in ionic medium is size dependent. The experimental results indicated that the effect of chemical interface damping is more prominent compared with the inherent size effects of AuNPs.

Sensing characteristics of D-shaped photonic crystal fiber surface plasmon resonance sensors are investigated in this paper. The finite element method is used to study the influences of side-polished depths and sensing layer thicknesses on our sensors. The results show that the sensitivity increases with the increase of the sensing layer thickness but decreases a little with the increase of the side-polished depth in a certain range. The influences of the side-polished lengths on our sensors are studied experimentally. It is revealed that a short side-polished length is conducive to improving the performance of our sensors. The highest sensitivity of our sensors is obtained to be 7381.0 nm/RIU in the refractive index environment of 1.40–1.42, when the side-polished length is controlled at 10 mm. The novel D-shaped photonic crystal fiber surface plasmon resonance sensors will bring new vitality to the application of biochemistry.

We design and numerically investigate a graphene-based asymmetric nanoantenna microstructure that can be used to realize electrically controllable, unidirectionally propagating broadband surface plasmon polaritons. The device geometry facilitates the simultaneous excitation of two localized surface plasmons resonances in the whole structure, and consequently, the asymmetric nanoantenna can be considered as being composed of two oscillating magnetic dipoles, wherein the interference of the radiated electromagnetic waves leads to a unidirectional propagation effect. Our results indicate that our proposed active device is promising for realizing compactable, tunable, terahertz plasmonic light sources.

A nanoscale germanium-gold Schottky photodetector (PD) is proposed based on a nanoring supporting localized surface plasmon excitation. A design methodology is presented based on maximizing resonant transmission power throughput of light from the active region of the PD. Numerical calculations show significant field enhancement of about 200 times in the near-field zone inside of the nanoring, which is utilized to compensate the responsivity decrease due to PD active region volume shrinkage, providing the responsivity of 338 mA/W at a 1310 nm detection wavelength. In addition, the incident wavelength is detected in about one-seventh of the absorption length of bulk germanium, yielding about 33% of the external quantum efficiency. The ratio of responsivity to the active region volume of the PD is calculated to be approximately 100 times that of similar studies.

In this paper, a numerical investigation of a new nanosandwich structure suitable for use in refractive index sensing is presented. The split ring nanosandwich consists of a stack of two gold split rings separated by a SiOsub2/subdisk spacer where the gaps in the upper and lower split rings are in perpendicular axes. The optical properties of the structure are described using the plasmon hybridization picture. This structure shows a high tunability of the optical response due to its strong hybridized modes. Its sensitivity is explored as a function of its dimensions showing strong field enhancement and high refractive index sensitivity reaching 3024 nm/RIU and FOM of 1.4.

The adhesion layer used in nanofabrication process of metallic nanostructures affects the surface plasmon modes. We characterize the localized surface plasmon resonances (SPRs) of gold nanodisks of various diameters and heights while varying the thickness of the Ti adhesion layers. Scattering, absorption, and extinction coefficient calculations show a significant dependence of the SPR on the size of nanostructures and the adhesion layer thickness. Comparisons of peak resonance wavelengths of different Ti adhesion layer thicknesses indicate a significant red shift and a reduction in amplitude as the Ti thickness increases. A comparison of spectral broadening of the plasmon mode indicates a linear increase with Ti thickness and percentage. In addition, the decay time of the plasmon mode decreased significantly as the adhesion layer size increases. These observations aid in understanding size dependent adhesion layer effects and optimized fabrication of single nanoplasmonic structures.

The interplay between localized surface plasmon (LSP) resonances and their collective responses, known as surface lattice resonances (SLRs), in metal nanoparticle arrays can lead to resonances with high Q-factors (∼100). These responses have in the past usually been studied for LSP resonances in the plane of the array of the nanoparticles (assumed to be nonmagnetic), thus restricting efficient coupling to particles separated along a specific direction. In the present study, we demonstrate that LSPs oscillating perpendicular to the plane of the surface can lead to stronger inter-particle coupling, which enhances the SLRs. This stronger coupling occurs because the out-of-plane oscillations can couple in all directions within the plane of the array. We study the resulting SLRs for square and hexagonal lattices using the discrete-dipole approximation, and we predict much larger Q-factors in the wavelength range near 650 nm. This prediction suggests that SLRs could be very useful in enhancing various optical processes, and in many applications such as sensing and nonlinear optical wave mixing.

We numerically demonstrate that spatially and temporally phase-shaped ultrafast laser pulses can spatiotemporally control ultrafast plasmon pulses generated at metal nanostructures. Excitation laser pulses designed by the superposition of high-order Hermite–Gaussian modes can localize the plasmon excitation at desired areas in nanostructures, and at the same time the temporal plasmon evolution is determined by the excitation pulse shape and the local plasmon response function. We numerically demonstrate this control scheme for local plasmons at the Au nanostructures and a surface plasmon polariton waveguide.

The sensitivity and dynamical range of an optical-fiber transducer consisting of a long-period grating coated with gold nanoparticles is investigated. For a grating with an 80 μm spatial periodicity, the resonances close to the turning point lie within the 450-900 nm spectral range. Employing a bottom-up production route, the localized surface plasmon resonance of gold nanoparticles is matched to the grating resonances; it is shown that this results in an increase in the refractive index sensitivity of the device. The device also shows increased dynamic range and enhanced refractive index sensitivity in water.

Controlling the launching efficiency and the directionality of surface plasmon polaritons is a major goal for the development of plasmonic devices. This study presents the use of the damped least-squares method to optimize the geometry parameters of a surface plasmon polariton unidirectional coupler. A damped least-squares algorithm has been constructed and the performance of a surface plasmon polariton unidirectional coupler has been obtained with COMSOL LiveLink for MATLAB. We have optimized a quasi-periodic surface plasmon polariton unidirectional grating coupler, which consists of five groups of sub-wavelength groove-doublets with an extinction ratio of nearly 71 dB. Combination of the COMSOL and the damped least-squares method in a MATLAB environment is a practicable method for the design of a surface plasmon polariton unidirectional coupler. Moreover, the general design principles behind this study can readily be extended to other numeric algorithms and other plasmonic devices.

In this paper, we discuss four different configurations of metal/dielectric thin films systems: surface plasmon resonance, coupled plasmon-waveguide resonance, metallic waveguide-coupled surface plasmon resonance, and dielectric waveguide-coupled surface plasmon resonance. For the waveguide-coupled surface plasmon resonance thin film systems, we explore several waveguide thicknesses that produce different resonant line shapes, including plasmon-induced transparency and waveguide-induced transparency. This paper presents a theoretical analysis and comparison of the intensity and phase sensitivities to changes in the index of refraction in a sensing layer external to the thin film system. We discuss the material parameters and the variations that led to the sensitivity variations in each thin film system. We show that when the surface plasmon polariton mode is coupled with dielectric waveguides, there is an enhancement of the sensitivity. The plasmon-induced transparency is used to increase the dynamic range of the system and shows a monotonically increasing intensity in the range of 0.4 refractive index units.

In this paper, we propose to achieve high-efficiency polarization conversion based on spatial dispersion modulation of spoof surface plasmon polaritons (SSPP). Different k is firstly designed in the two transverse directions by aligning an SSPP-supporting fishbone structure in y direction while maintaining free space in x direction. The orthogonal phase difference is introduced by larger k of SSPP waves for y-polarized component of incident waves. Meanwhile, to achieve high efficiency, gradient k in z-direction is designed so that the y-polarized component of incident waves can be coupled perfectly as SSPP waves. By rotating the fishbone structure with respect to the polarization direction of incident waves, different polarization states for transmitted waves can be realized. As an example, a polarization converter prototype with the central working frequency f = 8GHz was designed, fabricated, and measured. Both the simulation and experiment demonstrate the high-efficiency linear-to-circular (LTC) polarization conversion in 6.9-9.6GHz.

One of the key challenges that graphene plasmonics face is achieving efficient coupling to external light. In this paper, this difficulty is overcome by a concept that is capable of exciting localized surface plasmon polaritons (SPPs) in flat gratings formed by sinusoidally shaping graphene nanoribbons. These gratings enable the parallel-polarized light to couple into SPPs, creating a sharp notch with an ultrahigh Q-factor on the transmission spectrum. Besides, the excited SPPs can be tuned not only by adjusting the geometrical parameters (arc length of the sinusoidal grating and the ribbon width) but also by changing the Fermi level of graphene. After reasonably considering the amplitude and period of the 2D grating, both the theoretical analyses and the numerical results demonstrate the applicable properties of this structure. This work provides a framework for understanding the mechanism of plasmon excitations and designing tunable 2D plasmonic devices, such as filters, switches, and sensors.

The influences of deposition rate and alloying with aluminum (0 to 65 at.%) on the structure and optical properties of magnesium films on quartz are examined. Results show that increasing the deposition rate improves the plasmonic figure of merit (FOM) values, and decreases the surface roughness and grain size. Addition of Al to Mg blue shifts the localized surface plasmon (LSP) figure of merit peak position and improves LSP FOM values over portions of the ultraviolet (UV) region. Nanotriangles (NTs) of Mg and Mg-Al on quartz were fabricated using colloidal lithography. Mg-Al NTs show improved LSP resonance (LSPR) response compared to Mg particles due to improved structural integrity.

In this paper, a theoretical investigation on plasmon modes in a circular cylindrical double-layer graphene structure is presented. Due to the interlayer electromagnetic interaction, there exist two branches of plasmon modes, the optical plasmon mode and the acoustic plasmon mode. The characteristics of these two modes, such as mode pattern, effective mode index and propagation loss, are analyzed. The modal behaviors can be effectively tuned by changing the distance between two graphene layers, the chemical potential of graphene and the permittivity of interlayer dielectric. Importantly, the breakup of tradeoff between mode confinement and propagation loss is discovered in the distance-dependent modal behavior, which originates from the unique dispersion properties of a double-layer graphene system. As a consequence, both strong mode confinement and longer propagation length can be achieved. Our results may provide good opportunities for developing applications based on graphene plasmonics in circular cylindrical structure.

Plasmon assisted photoelectric hot electron collection in a metal-semiconductor junction can allow for sub-bandgap optical to electrical energy conversion. Here we report hot electron collection by wafer-scale Au/TiO2 metallic-semiconductor photonic crystals (MSPhC), with a broadband photoresponse below the bandgap of TiO2. Multiple absorption modes supported by the 2D nano-cavity structure of the MSPhC extend the photon-metal interaction time and fulfill a broadband light absorption. The surface plasmon absorption mode provides access to enhanced electric field oscillation and hot electron generation at the interface between Au and TiO2. A broadband sub-bandgap photoresponse centered at 590 nm was achieved due to surface plasmon absorption. Gold nanorods were deposited on the surface of MSPhC to study localized surface plasmon (LSP) mode absorption and subsequent injection to the TiO2 catalyst at different wavelengths. Applications of these results could lead to low-cost and robust photo-electrochemical applications such as more efficient solar water splitting.

Infrared light has received attention for sensor applications, including fingerprint spectroscopy, in the bioengineering and security fields. Surface plasmon physics enables the operation of a light harvesting optical antenna. Gold nanochains exhibit localized surface plasmon resonance (LSPR) in the infrared region with high frequency selectivity. However, a feasible design for optical antennae with a higher resonant efficiency and frequency selectivity as a function of structural design and periodicity is still unknown. In the present study, we investigated the relationship between the resonant efficiency and frequency selectivity as a function of the structural design of gold nanochains and explored structural periodicity for obtaining highly frequency-selective optical antennae. An optical antenna design with higher resonant efficiency is proposed on the basis of its efficient interaction with non-polarized light.

The strong plasmonic response and wide electrostatic tunability of graphene make it a promising material for developing infrared optoelectronic components. In this paper, we present a mid-infrared wideband tunable cross polarization converter using periodically perforated graphene. The polarization converter consists of a metal ground plane, an insulator layer, and a rectangle-shape periodically perforated graphene sheet. By superimposing two localized surface plasmon modes, the polarization converter transforms a linear polarization to its cross polarization over a bandwidth as wide as ~5% of the central frequency (46.8THz) with a peak conversion ratio exceeding 90%. The polarization conversion performance is maintained over a wide range of incident angles up to 50°, and is highly tunable by electrostatic tuning of the graphene Fermi energy. Our proposed device enables the manipulation of light polarization for potential mid-infrared applications.

Graphene nanoribbon (GNR), as a fundamental component to support the surface plasmon waves, are envisioned to play an important role in graphene plasmonics. However, to achieve extremely confinement of the graphene surface plasmons (GSPs) is still a challenging. Here, we propose a scheme to realize the excitation of localized surface plasmons with very strong field enhancement at the resonant frequency. By sinusoidally patterning the boundaries of GNRs, a new type of plasmon mode with field energy concentrated on the shaped grating crest (crest mode) can be efficiently excited, creating a sharp notch on the transmission spectra. Specifically, the enhanced field energies are featured by 3 times of magnitude stronger than that of the unpatterned classical GNRs. Through theoretical analyses and numerical calculations, we confirm that the enhanced fields of the crest modes can be tuned not only by changing the width, period and Fermi energy as traditional ribbons, but also by varying the grating amplitude and period. This new technique of manipulating the light-graphene interaction gives an insight of modulating plasmon resonances on graphene nanostrutures, making the proposed pattern method an attractive candidate for designing optical filters, spatial light modulators, and other active plasmonic devices.