Plasmon Modes Research Articles

Origin of plasmonic Fano resonance in metal-hole/split-ring-resonator metamaterials disclosed by temporal coupled-mode theory.

In plasmonic Fano resonance, the interaction between a discrete plasmonic mode and a continuum of plasmonic mode gives rise to an asymmetric line shape in the scattering or absorption spectrum, enabling a wide range of applications such as sensing, switching, and slow light devices. Here, we establish a theoretical solution in the framework of temporal coupled-mode theory (TCMT) to study the three-dimensional (3D) and two-dimensional (2D) Fano resonances induced by strong coupling between metal hole (MH) and split ring resonator (SRR) array. We first separately analyze the transmission spectra of the MH array and SRR array under different polarized light excitation. We further investigate the electromagnetic field and charge density distribution corresponding to the resonant modes at the peak or valley wavelength of the transmission spectrum and figure out the electric/magnetic dipole feature of these resonance modes. We then establish a theoretical solution by TCMT for Fano resonances arising from the coupling of these modes. The calculated transmission spectrum is closely matching with the numerically simulated transmission spectrum for these Fano resonances in the MH-SRR array, which effectively elucidates that the asymmetry of the Fano resonances is caused by the coupling between bright and dark plasmonic modes involved in the two structures. Our results can help to understand the profound physics in such coupled plasmonic systems.

Identification of dark axisymmetric plasma modes in partially gated two-dimensional electron gas disk

We investigate the dark axisymmetric plasmon spectrum for partially gated two-dimensional gas of the disk shape. The extension of the central gate spot provides a change in plasmon dispersion from the root to a linear dependence on the wave vector. Intriguingly, the interaction between neighboring modes occurs because of stepwise change in carrier screening across the circumference of the gate. This behavior is unexpected when changing from a fully screened to unscreened two-dimensional gas by increasing the dielectric width under the gate [A.L.Fetter, Phys.Rev.B 33, 5221 (1986)]. Our results provide the accurate identification of axisymmetric plasmon modes recently observed in experiment and, in addition, pave the way for feedback plasmon resonator study.

Air-silica core microstructured optical fiber-based SPR sensor for temperature and refractive index measurement

We propose an air-silica core microstructured optical fiber-based surface plasmon resonance (SPR) sensor to simultaneously measure temperature and refractive index (RI). The sensing channel is formed by coating the outside of the fiber with a gold film and a polydimethylsiloxane (PDMS) layer as a temperature sensing medium, and then being immersed into the liquid analyte. The plasmon mode can penetrate through the PDMS layer and then into the analyte, therefore both the temperature and the analyte RI changings can lead to the variations of SPR spectra that will be measured. Our numerical results demonstrate that the proposed sensor can support two resonance peaks in the x-polarized core mode and one resonance peak in the y-polarized core mode, therefore providing two detection approaches, peak-based and polarization-based approaches. By measuring the two peaks in the x-polarized core mode, for the peak-based approach, the temperature coefficients are −2.077 nm/°C and −2.723 nm/°C, and the RI coefficients are 1252 nm/RIU and 1931 nm/RIU, respectively. While by measuring the second peak in x-polarized core mode and the peak in y-polarized core mode, for the polarization-based approach, the temperature coefficients are −2.723 nm/°C and −3.401 nm/°C, and the RI coefficients are 1931 nm/RIU and 2973 nm/RIU, respectively.

Si3N4-plasmonic ferroelectric MZIR modulator for 112-Gbaud PAM-4 transmission in the O-band.

This paper presents a simulation-based analysis on the performance of plasmonic ferroelectric Mach-Zehnder in a ring (MZIR) versus symmetric Mach-Zehnder modulators (MZMs) on Si3N4 targeting O-band operation. The detailed investigation reveals the tradeoff between Au and Ag legacy noble metals providing lower modulator losses and CMOS compatible Cu featuring low cost. The numerical models also show that by opting for the MZIR layout there is a reduction in the Vπ x L product of 46% for Ag, 39% for Au and 30% for Cu versus MZMs. Time-domain simulations verify the successful generation of 112 Gbaud PAM-4 Signals from both MZIRs and MZMs for as low as 2 × 1.3 Vpp and 5µm long plasmonic phase shifters (PSs) with MZIRs providing a ΔQ signal improvement over MZMs of 2.9, 2.4, and 1.3 for Ag, Au, and Cu metals respectively. To the best of our knowledge, this is the first theoretical demonstration of such a low-loss, low-voltage, high-speed, and CMOS compatible plasmonic modulator on Si3N4, in the O-band.

Super-Radiant SERS Enhancement by Plasmonic Particle Gratings.

Despite recent developments, surface-enhanced Raman spectroscopy (SERS) applications face challenges in achieving both high sensitivity and uniform Raman signals over a large area. Using the directional self-assembly of plasmonic nanoparticles in lattice structures, we show how one can increase the SERS signal 43-fold over randomly aligned gold nanoparticles without relying on the photoluminescence of Rhodamine 6G. For this study, we have chosen the lattice constant for an off-resonant case that matches the lattice resonance and super-radiant plasmon mode along the particle chain. Supported by electromagnetic simulations, we systematically analyze the radiative components of the plasmon modes by varying the particle size while keeping the lattice periodicity constant. We perform polarization-dependent SERS measurements and compare them with other standard SERS excitation wavelengths. Using the self-assembled plasmonic particle lattice, we have developed an effective SERS substrate that provides a significantly higher signal with 73% less surface coverage. This colloidal approach enables the cost-effective and scalable fabrication of highly sensitive, uniform, and polarization-dependent SERS substrates.

Enhanced Asymmetric Light‐Plasmon Coupling in Graphene Nanoribbons for High‐Efficiency Transmissive Infrared Modulation

AbstractGraphene plasmonic modulators can manipulate the mid‐infrared light in transmission mode, which is currently challenging for traditional liquid crystal and digital micromirror devices, opening up a new avenue for infrared scene projection, infrared optical communication, and hyper spectra imaging. Nevertheless, their efficiencies are not high enough due to the single‐layer atomic thickness and low free carrier density of graphene. Here, it is demonstrated that the modulation efficiency can be significantly improved by enhancing the asymmetric light‐plasmon coupling. A general theoretical model is established to describe the modulation behaviors of the modulator, revealing the critical role of the asymmetric coupling rate. By using dielectric environment engineering and graphene structure design experimentally to enhance the asymmetric coupling rate from 0.45×1012 to 7.05×1012 s−1, the modulation efficiency has been improved from 4% to 41% at 1530 cm−1, while maintaining the bandwidth as large as 230 cm−1. The modulator outperforms previous transmission‐type graphene plasmonic modulators in both efficiency and bandwidth, presenting great potential in next‐generation infrared integrated photonics platforms.

Designing a perfect surface plasmon resonance absorber based on graphene and hexagonal boron nitride photonic crystal nanorods

In this work, absorbers in the terahertz region based on one-dimensional photonic crystal nanorods coated with phosphorene, phosphorene-graphene, and phosphorene–hexagonal boron nitride layers were designed. On addition of a graphene layer to the structure, strong localized surface plasmon resonance was observed in two different plasmonic modes via two significant peaks at wavelengths of 10.09 μm and 12.47 μm. The absorption of the structure was affected by the presence of an antireflection layer, and the two modes showed an absorption rate of 99% at wavelengths of 9.72 μm and 9.86 μm, increasing the absorption band to 0.54 μm full width at half maximum. By our changing the dielectric environment with a higher refractive index, better localization and greater absorption occurred. Furthermore, a perfect absorber with two absorption peaks at wavelengths of 11.39 μm and 14.02 μm with 99% and 97% absorption and full width at half maximum of 0.35 μm and 1.1 μm, respectively, was created. Finally, a perfect absorber with an absorption bandwidth of 2.1 μm was obtained in the structure in which hexagonal boron nitride had been placed as a spacer between the monolayers of phosphorene.

Plasmonic Lattice Modes with High Quality Factor in Inhomogeneous Environment

Plasmonic lattice modes (PLM) with high quality factor is formed by periodic Au hemisphere arrays on the quartz substrates. The formation of PLM is attributed to the coupling between the localized surface plasmon resonance of Au hemispheres and the diffracted waves inside the substrates rather than the diffracted waves in the air. By changing the hemispheres’ size and the arrays’ period, the PLM wavelength is adjustable. It is also found that the refractive index of the substrates has no effect on the quality factor of PLM. As the refractive index increases, the position of the PLM wavelength has a redshift. While the surroundings where periodic Au hemisphere arrays on the quartz substrates are located affect the quality factor of PLM.

Plasmons in AlGaN/GaN grating-gate structure probing with 300 K background illumination

We show that terahertz plasmons in AlGaN/GaN grating-gate structures efficiently modulate the reflection of room temperature thermal radiation, leading to spectra that are in agreement with the measurements of plasmon absorption using high-power external sources. For typical samples of a few square millimeters in size, the reflected radiation intensity is relatively weak, and measurements need the use of gate voltage plasmon modulation and lock-in detection techniques. We show that unintentional use of lock-in techniques may lead to artifacts and demonstrate what kind of special precautions need to be taken into account. We show that drain voltage modulation also leads to modulation of the reflected thermal radiation by plasmons. Our results are of key importance for the research on new resonant plasmon-based terahertz sources because of the always present superposition of electrically excited terahertz emission and background radiation reflected from the structures.

Controlling Electron Mobility of Strongly Coupled Organic Semiconductors in Mirrorless Cavities

AbstractRecent experiments on the strong light‐matter coupling between an organic semiconductor and a plasmonic mode propose an unconventional way to enhance conductivity. Herein, it is shown that mirrorless cavities can boost conductivity by simply structuring the refractive indices of the multilayers in a commercially available metal oxide semiconductor field effect transistor (MOSFET). Perylene diimide (an organic semiconductor dye) molecules are deposited on a MOSFET device. The refractive index mismatch between the silicon/silicon dioxide/dye/air results in light confinement. The frequency of this confined light is tuned by changing the thickness of the organic semiconductor layer. Interestingly, an increase in electron mobility was observed once the electronic transition of the dye molecules and the second‐order cavity mode enter into the strong coupling regime. Whereas resonance tuning to the first‐order mode does not affect the electron transport. Here, the system is still in a weak coupling regime. These results are further correlated by experimental dispersion measurements and supported with transfer matrix simulations. The increase in electron mobility is not large due to high dissipation or low‐quality factors of the cavity modes. However, the mirrorless configuration presented here may offer a simpler way of boosting the properties of functional materials.

Silicene-2DEG heterostructures: Collective excitations investigations

This paper investigates collective excitations in a heterostructure consisting of a silicene sheet and a parallel GaAs quantum well by employing the random-phase approximation within exchange-correlation effects. We find that two plasmonic modes exist in the systems, corresponding to in-phase (optical) and out-of-phase (acoustical) oscillations of charged particles in two layers. Calculations show that interactions between electrons in two layers decrease the energy of both two plasmon modes, completely differing from other double-layer structures. The increase in inter-layer separation significantly decreases both optical and acoustical modes’ frequencies. We observe that taking into account local field correction and increasing quantum well width pronouncedly decrease acoustical mode frequency. However, the increase in the bandgap, caused by an external electric field, decreases only the frequency of the optical collective mode. Finally, as carrier density in quantum well decreases, the acoustical mode energy noticeably declines while that of the optical one seems extremely stable.

Enhanced circular dichroism in twisted double layer nanoparticle arrays with the excitation of hybridized surface lattice resonances

The realization of enhanced circular dichroism using chiral nanostructures is crucial for the development of functional nanophotonic devices. This study proposes to construct chiral structures by adjusting the relative orientation between two vertically shifted rectangular nanodisk arrays. In comparison to localized plasmon modes, the twisted array allows selective excitation of hybridized surface lattice resonances by left- or right-handed circularly polarized incidence, and the strong interactions among individual nanoparticles result in enhanced circular dichroism. The calculation results demonstrate a significant differential transmittance of up to 0.61, which is more than one order of magnitude stronger than that of localized modes. Moreover, the chiral responses can be effectively controlled by manipulating geometry parameters such as inter-layer separation, twisted angle, and lattice spacing, as these factors govern the coupling efficiency with the external field and the radiative damping of the hybridized modes. The proposed twisted array structure offers a versatile platform for manipulating chiral light fields.

Emission Enhancement of Ge/Si Quantum Dots in Hybrid Structures with Subwavelength Lattice of Al Nanodisks.

The effects of resonance interaction of plasmonic and photonic modes in hybrid metal-dielectric structures with square Al nanodisk lattices coupled with a Si waveguide layer were investigated using micro-photoluminescence (micro-PL) spectroscopy. As radiation sources, GeSi quantum dots were embedded in the waveguide. A set of narrow PL peaks superimposed on the broad bands were observed in the range of quantum dot emissions. At optimal parameters of Al nanodisks lattices, almost one order increasing of PL intensity was obtained. The experimental PL spectra are in good agreement with results of theoretical calculations. The realization of high-quality bound states in the continuum was confirmed by a comparative analysis of the experimental spectra and theoretical dispersion dependences. The results demonstrated the perspectives of these type structures for a flat band realization and supporting the slow light.

Manipulating multiple plasmon modes by coupling fields for broadband filtering in terahertz metamaterials

Multiple modes resonance provides the possibility of broadband filtering and has great potential applications in terahertz devices, including miniaturized spectrometers, hyperspectral imagers and optical switches in communication systems. However, it is difficult to further expand the bandwidth range due to the inability to stack an infinite number of resonant structures in a finite space. Here, an innovative method is reported for manipulating two localized surface plasmon polaritons (LSPP) modes by multiscale-coupled plasmonic fields to realize broadband filtering at terahertz (THz) frequencies. The interaction between two LSPP modes was verified theoretically and experimentally, exhibiting a prominent Rabi splitting and two peaks with high transmittance. Particularly, broadband filtering is accomplished by utilizing a double-layer structure that can be regarded as a cavity for inducing far-field interference. Experimental result demonstrates that the maximum transmission range of the bilayer metamaterial reaches 0.63 THz. Besides, the two wide bands can be integrated into one broadband through near-field coupling effects. The integration of strong coupling and far-field interference expands the transmission range of the double-layer metamaterial, further enhancing its capability for broadband filtering. This unique approach for manipulating multi-mode plasmonic coupling forges a convenient route for realizing broadband filtering.

Strong Acousto-Plasmonic Coupling in Film-Coupled Nanoparticles Mediated by Surface Acoustic Waves

The interaction between phonons and localized plasmons in film-coupled nanoparticles designs can be exploited both for modulating the scattered electromagnetic field and the understanding of the mechanical vibrations at nanoscale. In this paper, we show by finite element numerical analysis an enhanced optomechanical interaction in a film-coupled gold nanoridges or pillars mediated by surface acoustic waves. The metallic nanoparticles are placed atop a multilayer structure consisting of a thin dielectric spacer covering a gold film layer on a silicon dioxide/or silicon substrate. Optical simulations reveal the existence of surface localized plasmons in the infrared range confined under the nanoparticles in the dielectric spacer and/or in between such particles. Optomechanical coupling between the plasmonic modes and localized phonons is evaluated from the shift in the plasmon eigenfrequency. It is found that the compressional, the in-phase compressional and the out-of-phase flexural modes, yield the highest coupling rates. Such phonons are excited by means of SAW launched from the system inlet in front of the particles. The findings in this paper could help design new generation of acousto-optic modulators monitored by fast coherent surface acoustics.

Surface plasmon enhanced THz spontaneous emission near metal-cored wires coated with graphene

This work deals with the theoretical study of the spontaneous emission of a point dipole source placed near a cylindrical structure composed of a central metallic core and a dielectric shell covered with graphene. We show that the spontaneous emission rate is dominated by coupling into surface plasmon (SP) modes on graphene, which present high values of the propagation constant along the cylinder symmetry axis. When the dielectric shell thickness is reduced, the plasmon dispersion is modified into a linear dependence and the modal SP propagation constant is further incremented, involving an improved field confinement. This effect is manifested as a redshift of the emission frequency spectrum for each individual cylindrical harmonic mode. We propose a simple model that provides us a link between the dispersion characteristics and maximum emission rate for each SP cylindrical mode.

Close-packed small nanocubes assemblies as efficient SERS substrates

Silver-based nanostructures, especially the anisotropic ones such as triangular nanoplates, nanorods, nanocubes or silver nanowires, are used as Surface enhanced Raman scattering substrates for different analytes sensing, with outstanding results, in terms of the limit of detection (LOD), as well as the enhancement factor (EF), being applicable for early clinical diagnosis of cancer, trace level detection of different organic pollutants, or for the pesticides’ detection, and even explosives. Among all these shapes, cubes like nanoparticles (AgNCs) can generate intense hot-spots due to their corners and edges, due to some additional plasmonic modes that occur at different resonance wavelengths in the spectrum, thus, improving SERS sensitivity.In this work we have successfully synthesized silver AgNCs of 60 nm, that were then assembled on a planar silicon substrate, and use it as a simple SERS substrate towards Rh6G dye. We were able to obtain an averaged calculated EF of 1.78 × 108, with a limit of detection of 4. 16 × 10−12 M. Furthermore, the effect of nanocubes’ configurations was investigated through the numerical calculations, being revealed that the edge-to-edge arrangement exhibits the highest EF of about 109, followed by the edge-to-face (8.6 × 108) and face-to-face (9 × 107) arrangements, in good agreement with experimental calculated value, ∼108, considering the frequency of appearance of each configuration.Our studies related to the use of assemblies of smaller AgNCs on planar silicon surface, with no further nanostructuration process, as SERS based substrates towards one of the most known analyte molecules, as well as an organic dye pollutant, are very encouraging, giving the chance to explore the emerging properties of small nanocubes towards low concentration detection of different organic molecules, thus showing great applicability in both scientific research and practical application, like water pollutants or even foodborne pathogen detection.

Quantifying Efficiency of Remote Excitation for Surface-Enhanced Raman Spectroscopy in Molecular Junctions.

Surface-enhanced Raman spectroscopy (SERS) is enabled by local surface plasmon resonances (LSPRs) in metallic nanogaps. When SERS is excited by direct illumination of the nanogap, the background heating of the lattice and electrons can prevent further manipulation of the molecules. To overcome this issue, we report SERS in electromigrated gold molecular junctions excited remotely: surface plasmon polaritons (SPPs) are excited at nearby gratings, propagate to the junction, and couple to the local nanogap plasmon modes. Like direct excitation, remote excitation of the nanogap can generate both SERS emission and an open-circuit photovoltage (OCPV). We compare the SERS intensity and the OCPV in both direct and remote illumination configurations. SERS spectra obtained by remote excitation are much more stable than those obtained through direct excitation when the photon count rates are comparable. By statistical analysis of 33 devices, the coupling efficiency of remote excitation is calculated to be around 10%, consistent with the simulated energy flow.

Impact of coherent coupling of plasmonic dipole modes in far-field scattering of templated metallic nanoislands

Impact of coherent coupling of plasmonic dipole modes in far-field scattering of templated metallic nanoislands

Dielectric properties and plasmon modes of gapped momentum systems of different dimensionality

Dielectric properties and plasmon modes of gapped momentum systems of different dimensionality