Localized Plasmon Modes Research Articles

Ultrafast plasmon dynamics in asymmetric gold nanodimers

We theoretically investigate the effect of symmetry breaking on the ultrafast plasmon responses of Au nanodisk (ND) dimers by varying the diameter of one of the constituent nanodisks. In the case of a single ultrafast laser pulse, we demonstrate that the ultrafast responses of Au ND homodimer can be significantly modified due to the effect of symmetry breaking. The symmetric dimer shows a single broad spectral peak, whereas the size-asymmetric dimer shows three spectral peaks. The first system displays at most one temporal maximum and no beats in ultrafast temporal, whereas the second system may have three temporal maxima and two beats due to a combination of broken symmetry and the coherent superposition between various plasmon modes induced by the ultra-short laser pulse. Moreover, the shape of temporal dynamics of the size-asymmetric dimer is significantly deformed due to the excitation of local plasmon modes with different wavelength components. Furthermore, the decay time of the amplitude of the local field is longer and oscillates with a high frequency due to the narrower linewidth and red-shifted spectral peaks. We show that the ultrafast plasmon responses of both dimers can be controlled by varying the relative phase and time delays between a pair of two pulses. Our results will open new paths to understanding ultrafast plasmon responses in asymmetric heterodimers with suitable properties for different applications.

Second harmonic generation by strongly coupled exciton-plasmons: The role of polaritonic states in nonlinear dynamics.

We investigate second harmonic generation (SHG) from hexagonal periodic arrays of triangular nano-holes of aluminum using a self-consistent methodology based on the hydrodynamics-Maxwell-Bloch approach. It is shown that angular polarization patterns of the far-field second harmonic response abide to threefold symmetry constraints on tensors. When a molecular layer is added to the system and its parameters are adjusted to achieve the strong coupling regime between a localized plasmon mode and molecular excitons, Rabi splitting is observed from the occurrence of both single- and two-photon transition peaks within the SHG power spectrum. It is argued that the splitting observed for both transitions results from direct two-photon transitions between lower and upper polaritonic states of the strongly coupled system. This interpretation can be accounted by a tailored three-level quantum model, with results in agreement with the unbiased numerical approach. Our results suggest that the hybrid states formed in strongly coupled systems directly contribute to the nonlinear dynamics. This opens new directions in designing THz sources and nonlinear frequency converters.

Second-Order Dispersion Sensor Based on Multi-Plasmonic Surface Resonances in D-Shaped Photonic Crystal Fibers

This paper proposes a scheme to determine the optical dispersion properties of a medium using multiple localized surface plasmon resonances (SPR) in a D-shaped photonic crystal fiber (PCF) whose flat surface is covered by three adjacent gold layers of different thicknesses. Using computational simulations, we show how to customize plasmon resonances at different wavelengths, thus allowing for obtaining the second-order dispersion. The central aspect of this sensing configuration is to balance miniaturization with low coupling between the different localized plasmon modes in adjacent metallic nanostructures. The determination of the optical dispersion over a large spectral range provides information on the concentration of different constituents of a medium, which is of paramount importance when monitoring media with time-varying concentrations, such as fluidic media.

Detection of electron spin resonance down to 10 K using localized spoof surface plasmon

In this study, novel use of the electromagnetic field profile of a localized spoof surface plasmonic mode to detect electron spin resonance (ESR) is being reported. The mode is supported on a resonator with a complementary metallic spiral structure, etched on the ground plane of a microstrip line having a characteristic impedance of 50 Ω. The change in characteristics of the mode of interest with lowering of temperature has been observed and analyzed. ESR spectra of three paramagnetic samples—2,2-diphenyl-1-picrylhydrazyl (DPPH), galvinoxyl, and 2,2,6,6-tetramethylpiperidine-1-oxyl are recorded using this resonator at room temperature. ESR spectra of DPPH are also captured down to 10 K. Potential application of the mode in the detection of microwave induced Rashba field-driven ESR has been discussed.

Microellipsometry study of plasmonic properties of metal–insulator–metal structures with ordered lattices of nanoparticles

Microellipsometric investigations of metal–insulator–metal (MIM) structures with regular square lattices of nanocylinders as one metallic layer are presented together with a theoretical description of all dipolar interactions in such a system. A comparison between the theoretical and experimental results is made, which clearly demonstrates the influence of surface plasmon on the optical properties of such MIM systems. The hybridization of localized and surface plasmon modes is revealed.

Second-harmonic generation in nonlinear plasmonic lattices enhanced by quantum emitter gain medium.

We report on a theoretical study of second-harmonic generation (SHG) in plasmonic nanostructures interacting with two-level quantum emitters (QEs) under incoherent energy pump. We generalize the driven-dissipative Tavis-Cummings model by introducing the anharmonic surface plasmon-polariton (SPP) mode coupled to QEs and examine physical properties of corresponding SPP-QE polariton states. Our calculations of the SHG efficiency for strong QE-SPP coupling demonstrate orders of magnitude enhancement facilitated by the polariton gain. We further discuss time-domain numerical simulations of SHG in a square lattice comprising Ag nanopillars coupled to QEs utilizing a fully vectorial nonperturbative nonlinear hydrodynamic model for conduction electrons coupled to Maxwell-Bloch equations for QEs. The simulations support the idea of gain enhanced SHG and show orders of magnitude increase in the SHG efficiency as the QEs are tuned in resonance with the lattice plasmon mode and brought above the population inversion threshold by incoherent pumping. By varying pump frequency and tuning QEs to a localized plasmon mode, we demonstrate further enhancement of the SHG efficiency facilitated by strong local electric fields. The incident light polarization dependence of the SHG is examined and related to the symmetries of participating plasmon modes.

Controllable hybridization between localized and delocalized anisotropic borophene plasmons in the near-infrared region.

In this Letter, we theoretically propose a coupled borophene plasmonic system, where an anisotropic localized plasmonic (LP) mode and a delocalized guided plasmonic (DGP) mode can be simultaneously excited. This allows us to manipulate the optical response of the strong LP-DGP coupling with exceptional flexibility in the near-infrared region, which is not possible with the conventional metallic plasmonic structures, and overcomes some shortcomings of coupled structures based on the other 2D materials. Specifically, the spatially LP-DGP coupling can arise when the system is driven into the strong coupling regime; this gives rise to a transparency window which can be well described by a coupled oscillation model. The bandwidth of the window is governed by the coupling strength which can be passively adjusted by the spacer thickness, while the center wavelength and the number of windows can be actively modulated by tuning the borophene electron density and the incident angle.

Dual-Band Plasmonic Perfect Absorber Based on the Hybrid Halide Perovskite in the Communication Regime

Due to the weak absorption of (CH3NH3)PbI3 in the communication regime, which restricts its optoelectronic applications, we design a adjustable dual-band perfect absorber based on the (CH3NH3)PbI3 to significantly enhance its absorption capability. Since the localized plasmon (LP) mode and surface plasmon (SP) mode are excited in the structure, which can both greatly enhance light absorption of the (CH3NH3)PbI3 layer, dual-band perfect absorption peaks are formed in the communication regime, and the absorption of (CH3NH3)PbI3 layer is increased to 43.1% and 64.2% at the dual-band absorption peaks by using finite-difference time-domain (FDTD) methods, respectively. By varying some key structural parameters, the dual-band absorption peaks of (CH3NH3)PbI3 can be separately shifted in a wide wavelength region. Moreover, the designed absorber can keep good performance under wide angles of incidence and manifested polarization correlation. Furthermore, not just for (CH3NH3)PbI3, the physical mechanism in this absorber can also be utilized to strengthen the absorption of other halide perovskites.

Spectral Engineering via Complex Patterns of Circular Nano-object Miniarrays: I. Convex Patterns Tunable by Integrated Lithography Realized by Circularly Polarized Light

Illumination of colloid sphere monolayers by circularly polarized beams enables the fabrication of concave patterns composed of circular nanohole miniarrays that can be transferred into convex metal nano-object patterns via a lift-off procedure. Unique spectral and near-field properties are achievable by controlling the geometry of the central nanoring and quadrumer of slightly rotated satellite nanocrescents and by selecting those azimuthal orientations that promote localized plasmon resonances. The spectral and near-field effects of hexagonal patterns composed of uniform gold nanorings and nanocrescents, which can be prepared by transferring masks fabricated by a perpendicularly and obliquely incident single homogeneous circularly polarized beam, were studied to uncover the supported localized plasmonic modes. Artificial rectangular patterns composed of a singlet nanoring and singlet nanocrescent as well as quadrumer of four nanocrescents were investigated to analyze the effect of nano-object interactions and lattice type. It was proven that all nanophotonical phenomena are governed by the azimuthal orientation independent localized resonance on the nanorings and by the C2, C1, and U resonances on the nanocrescents in case of bar {E}-field direction perpendicular and parallel to their symmetry axes. The interaction between localized surface plasmon resonances on individual nano-objects is weak, whereas scattered photonic modes have a perturbative role at the Rayleigh anomaly only on the larger periodic rectangular pattern of miniarrays. Considerable fluorescence enhancement of dipolar emitters is achievable at spectral locations promoting the C and U resonances on the constituent nano-object.

Fluorescence Emission Triggered by Radioactive β decay in Optimized Hyperbolic Cavities.

Luminescence arising from -decay of radiotracers has garnered much interest recently as a viable in-vivo imaging technique. The emitted Cerenkov radiation can be directly detected by high sensitivity cameras or used to excite highly efficient fluorescent dyes. Here, we investigate the enhancement of visible and infrared emission driven by -decay of radioisotopes in the presence of a hyperbolic nanocavity. By means of a transfer matrix approach, we obtain quasi-analytic expressions for the fluorescence enhancement factor at the dielectric core of the metalodielectric cavity, reporting a hundred-fold amplification in periodic structures. A particle swarm optimization of the layered shell geometry reveals that up to a ten-thousand-fold enhancement is possible thanks to the hybridization and spectral overlapping of whispering-gallery and localized-plasmon modes. Our findings may find application in nuclear-optical medical imaging, as they provide a strategy for the exploitation of highly energetic gamma rays, Cerenkov luminescence, and visible and near-infrared fluorescence through the same nanotracer.

Tunable plasmon-induced transparency with a dielectric grating-coupled graphene structure for slowing terahertz waves.

We present a tunable plasmon-induced transparency (PIT) structure that is composed of dielectric grating and a graphene system to manipulate terahertz (THz) waves. The graphene system consists of a graphene sheet and a graphene ribbon layer, with a spacer between them. By exploiting the diffraction coupling of THz wave with dielectric grating, graphene plasmonic resonance is efficiently excited on both graphene sheet and graphene ribbons. This leads to the surface plasmon mode of the graphene sheet and the localized plasmon mode of the graphene ribbons. The coupling between the two-plasmon modes via near-field destructive interference generates a strong PIT effect with slowing the group velocity of THz waves. A group delay over 0.2 ps and group index beyond 170 can be achievable. The group slowing effect is dynamically tunable with varying the Fermi level of graphene. The work suggests a promising scheme for on-chip graphene slow-wave devices at the THz regime.

Entanglement of quantum emitters interacting through an ultra-thin noble metal nanodisk.

Ultra-thin metallic nanodisks, supporting localized plasmon (LP) modes, are used as a platform to facilitate high entanglement between distant quantum emitters (QEs). High Purcell factors, with values above 103, are probed for a QE placed near to an ultra-thin metallic nanodisk, composed of the noble metals Au, Ag, Al, and Cu. The disk supports two sets of localized plasmon modes, which can be excited by QEs with different transition dipole moment orientations. The two QEs are placed on opposite sides of the nanodisk, and their concurrence is used as a measure of the entanglement. We observe that the pair of QEs remains entangled for a duration that surpasses the relaxation time of the individual QE interacting with the metallic disk. Simultaneously, the QEs reach the entangled steady state faster than in the case where the QEs are in free space. Our results reveal a high concurrence value for a QES separation distance of 60 nm, and a transition energy of 0.8 eV (λ = 1550 nm). The robustness exhibited by this system under study paves the way for future quantum applications.

Design and measurement of a single-dual-band tunable metamaterial absorber in the terahertz band

Abstract In this paper, a single-band tunable metamaterial absorber with metal disk array is verified in Terahertz (THz) range. An absorption peak Is achieved based on local surface plasmon (LSP) mode resonance at 14.3 THz with 90% amplitude under room temperature. In the first stage of measurement, this absorption peak is enhanced and shifted to lower frequencies with diameter increasing, while reduced and shifted to higher frequencies with lattice constant increasing. In the second stage of measurement, this peak is in sensitive to temperature. Moreover, a new absorption peak is achieved at 26.6 THz with temperature increasing. This peak is enhanced from 45% to 81% when temperature is increased from 300 k to 400 k, and shifted from 26.6 THz to 18.7 THz. Therefore, a metamaterial absorber that can both modulate new absorption peak and retain original absorption performance is verified.

Sub-10 nm Nanolaminated Al2O3/HfO2 Coatings for Long-Term Stability of Cu Plasmonic Nanodisks in Physiological Environments.

By supporting localized plasmon modes, metal-based plasmonic nanostructures can confine optical fields at deep-subwavelength scale in various applications, such as biological and chemical sensing, nanoscale light emission, and solar energy harvesting. While Cu is a low-cost complementary metal oxide semiconductor (CMOS) compatible material, its poor chemical stability limits the use of Cu plasmonic nanodevices in corrosive biochemical aqueous environments. In this paper, we demonstrate that sub-10 nm Al2O3/HfO2 nanolaminated coatings can significantly extend the lifetime of Cu nanodisk arrays from ∼5 h to ∼180 days in the physiological environment of 1× phosphate-buffered saline (PBS) at 37 °C. Cu nanodisk arrays are fabricated using freestanding Au nanohole array films as the physical vapor deposition masks and sub-10 nm nanolaminated coatings composed of alternating Al2O3 and HfO2 nanolayers are grown on Cu nanodisk arrays by atomic layer deposition (ALD). Time-dependent optical extinction measurements of Cu nanodisk arrays are conducted in 1× solutions at 37 °C to investigate the anticorrosion performance for different pure and nanolaminated ALD coatings. We observe a linear relationship between the lifetime of Cu nanodisk arrays in 1× PBS at 37 °C and the nanolaminated coating thickness, and ∼1.3 nm nanolaminated coatings of ∼10 ALD cycles can extend the lifetime of Cu plasmonics up to ∼20 days. Furthermore, we find that the anticorrosion performance of Al2O3/HfO2 nanolaminated ALD coatings strongly depends on the processing and the geometric parameters, such as the annealing temperature and the nanolaminated backbone unit size.

Self-assembled plasmonics for angle-independent structural color displays with actively addressed black states

Nanostructured plasmonic materials can lead to the extremely compact pixels and color filters needed for next-generation displays by interacting with light at fundamentally small length scales. However, previous demonstrations suffer from severe angle sensitivity, lack of saturated color, and absence of black/gray states and/or are impractical to integrate with actively addressed electronics. Here, we report a vivid self-assembled nanostructured system which overcomes these challenges via the multidimensional hybridization of plasmonic resonances. By exploiting the thin-film growth mechanisms of aluminum during ultrahigh vacuum physical vapor deposition, dense arrays of particles are created in near-field proximity to a mirror. The sub-10-nm gaps between adjacent particles and mirror lead to strong multidimensional coupling of localized plasmonic modes, resulting in a singular resonance with negligible angular dispersion and ∼98% absorption of incident light at a desired wavelength. The process is compatible with arbitrarily structured substrates and can produce wafer-scale, diffusive, angle-independent, and flexible plasmonic materials. We then demonstrate the unique capabilities of the strongly coupled plasmonic system via integration with an actively addressed reflective liquid crystal display with control over black states. The hybrid display is readily programmed to display images and video.

Nonlinear plasmon-exciton coupling enhances sum-frequency generation from a hybrid metal/semiconductor nanostructure

The integration of metallic plasmonic nanoantennas with quantum emitters can dramatically enhance coherent harmonic generation, often resulting from the coupling of fundamental plasmonic fields to higher-energy, electronic or excitonic transitions of quantum emitters. The ultrafast optical dynamics of such hybrid plasmon–emitter systems have rarely been explored. Here, we study those dynamics by interferometrically probing nonlinear optical emission from individual porous gold nanosponges infiltrated with zinc oxide (ZnO) emitters. Few-femtosecond time-resolved photoelectron emission microscopy reveals multiple long-lived localized plasmonic hot spot modes, at the surface of the randomly disordered nanosponges, that are resonant in a broad spectral range. The locally enhanced plasmonic near-field couples to the ZnO excitons, enhancing sum-frequency generation from individual hot spots and boosting resonant excitonic emission. The quantum pathways of the coupling are uncovered from a two-dimensional spectrum correlating fundamental plasmonic excitations to nonlinearly driven excitonic emissions. Our results offer new opportunities for enhancing and coherently controlling optical nonlinearities by exploiting nonlinear plasmon-quantum emitter coupling.

On the existence of a local dipolar plasmon mode in doped small gold atomic arrays

Impurity doping in atomic chains provides a potentially effective way to adjust its electronic excitation. Recently, it was reported that doping transition-metal atoms into small gold atomic chains induces a local dipolar plasmon mode which corresponds to charge oscillations around the impurity atoms, whereas our studies indicate that such a local mode does not exist. In this paper, using time-dependent density functional theory, we extensively investigate the excitation properties of plasmons in doped gold nanostructures. Our results show that all the plasmon peaks are affected by impurity, and the corresponding plasmon modes mutate into different forms compared to pure gold atomic chains. The propagation of charge oscillations in some plasmon modes is blocked in the doped chain, so that the charges oscillate mainly in half of the chain. Furthermore, the $5d$ electrons of gold atoms reduce the energies and intensities of plasmon resonances for both the transverse and longitudinal modes, and cause the end and center features of the induced charge distributions to nearly vanish in the transverse plasmon modes.

Optical field tuning of localized plasmon modes in Ag microcrystals at the nanofemto scale.

Nanoscale plasmonic field enhancement at sub-wavelength metallic particles is crucial for surface sensitive spectroscopy, ultrafast microscopy, and nanoscale energy transduction. Here, we demonstrate control of the spatial distribution of localized surface plasmon modes at sub-optical-wavelength crystalline silver (Ag) micropyramids grown on a Si(001) surface. We employ multiphoton photoemission electron microscopy (mP-PEEM) to image how the plasmonic field distributions vary with the photon energy, light polarization, and phase in coherent two-pulse excitation. For photon energy hυ > 2.0 eV, the mP-PEEM images show single photoemission locus, which splits into a dipolar pattern that straddles the Ag crystal at a lower energy. We attribute the variation to the migration of plasmon resonances from the Ag/vacuum to the Ag/Si interfaces by choice of the photon energy. Furthermore, the dipolar response of the Ag/Si interface follows the polarization state of light: for linearly polarized excitations, the plasmon dipole follows the in-plane electric field vector, while for circularly polarized excitations, it tilts in the direction of the handedness due to the conversion of spin angular momentum of light into orbital angular momentum of the plasmons excited in the sample. Finally, we show the coherent control of the spatial plasmon distribution by exciting the sample with two identical circularly polarized light pulses with delay defined with attosecond precision. The near field distribution wobbles at the pyramid base as the pump-probe delay is advanced due to interferences among the contributing fields. We illustrate how the frequency, polarization, and pulse structure can be used to design and control plasmon fields on the nanofemto scale for applications in chemistry and physics.

Resonance Modes of Tall Plasmonic Nanostructures and Their Applications for Biosensing

The collective oscillation of plasmons in metallic nanostructures generates localized surface plasmons (LSP), which are responsive to their surrounding dielectric environment and can be used for low-cost, label-free sensing platforms. However, the inherently short evanescent decay field saturates the optical response within 10–30 nm of the metal surface, hindering multi-layered functionalization strategies typically used for specific binding due to its limited surface proximity. In this work, we propose the use of tall nanostructures to engineer the plasmonic response for biosensing applications. The resonance mode characteristics are investigated, where the emergence of hybrid modes is found to arise from the decoupling of localized plasmon modes at increasing antenna height. Using high aspect ratio plasmonic nanostructures, we demonstrate its viability with up to 4.3 $\times$ higher sensitivity and 18.4 $\times$ higher figure of merit within the visible range. Coupled with a cost-effective fabrication method, the height provides an additional degree of freedom for tailoring the optical spectrum.

Strong coupling with directional absorption features of Ag@Au hollow nanoshell/J-aggregate heterostructures

Abstract Tunable plasmon-exciton coupling is demonstrated at room temperature in hybrid systems consisting of Ag@Au hollow nanoshells (HNSs) and J-aggregates. The strong coupling depends on the exciton binding energy and the localized surface plasmon resonance strength, which can be tuned by changing the thickness of the Ag@Au HNS. An evident anticrossing dispersion curve in the coupled energy diagram of the hybrid system was observed based on the absorption spectra obtained at room temperature. In this paper, strong coupling was observed twice (first at lower wavelength and then also at a higher wavelength) via a single preparation process of the Ag@Au HNS system. The first Rabi splitting energy (ħΩ) is 225 meV. Then, the extinction spectra of the bare Ag@Au HNS and the Ag@Au HNS-J-aggregate hybrid system were reproduced by numerical simulations using the finite-difference time domain method, which were in good agreement with the experimental observations. We attributed the strong coupling of the new shell hybrid system to the reduced local surface plasmon (LSP) mode volume of the Ag@Au HNS. This volume is about 1021.6 nm3. The features of the Ag@Au HNS nanostructure with a small LSP mode volume enabled strong light-matter interactions to be achieved in single open plasmonic nanocavities. These findings may pave the way toward nanophotonic devices operating at room temperature.