Surface Plasmons In Nanowires Research Articles

Modulation of Cathodoluminescence by Surface Plasmons in Silver Nanowires (Small 33/2023)

Modulation of Cathodoluminescence by Surface Plasmons in Silver Nanowires (Small 33/2023)

Modulation of Cathodoluminescence by Surface Plasmons in Silver Nanowires.

The waveguide modes in chemically-grown silver nanowires on silicon nitride substrates are observed using spectrally- and spatially-resolved cathodoluminescence (CL) excited by high-energy electrons in a scanning electron microscope. The presence of a long-range, travelling surface plasmon mode modulates the coupling efficiency of the incident electron energy into the nanowires, which is observed as oscillations in the measured CL with the point of excitation by the focused electron beam. The experimental data are modeled using the theory of surface plasmon polariton modes in cylindrical metal waveguides, enabling the complex mode wavenumbers and excitation strength of the long-range surface plasmon mode to be extracted. The experiments yield insight into the energy transfer mechanisms between fast electrons and coherent oscillations in surface charge density in metal nanowires and the relative amplitudes of the radiative processes excited in the wire by theelectron.

Promising Photoluminescence Enhancement of Tris(8-hydroxyquinoline)aluminum by Simultaneous Localized and Propagating Surface Plasmons of Ag Nanostructures

The continuous performance optimization of tris(8-hydroxyquinoline)aluminum (Alq3) materials is of great significance during the commercialization process of organic light-emitting diodes (OLEDs). In incorporating Ag nanostructures into Alq3, the photophysical properties are greatly improved by the plasmon–exciton coupling effect. Localized surface plasmons (LSPs) in Ag nanoparticles (NPs) efficiently increased the absorption ability. The coexistence of LSPs and propagating surface plasmons (PSPs) in Ag nanowires (NWs) leads to a PL enhancement of 5.3-fold and a full-width at half maximum (FWHM) narrowed by 10 nm. Temperature-dependent PL measurements exhibit that the plasmonic density of states (DOS) increases with decreasing temperature below 40 °C, and the thermal exchange can be accelerated by the introduction of Ag nanostructures. Effective suppression of the thermal accumulation effect is further proved by excitation intensity (EI)-dependent PL measurements. We also found that Ag nanostructures could mainly change the y coordinates in International Commission on Illumination (CIE), leading to a higher brightness. The 5372 K color temperature of an Ag NWs-embedded composite is suitable for daylight-type fluorescent OLEDs. The results would pave an effective way for further optimizing the optical performance of light-emitting materials in OLEDs.

Tailored Nanoscale Plasmon-Enhanced Vibrational Electron Spectroscopy.

Atomic vibrations and phonons are an excellent source of information on nanomaterials that we can access through a variety of methods including Raman scattering, infrared spectroscopy, and electron energy-loss spectroscopy (EELS). In the presence of a plasmon local field, vibrations are strongly modified and, in particular, their dipolar strengths are highly enhanced, thus rendering Raman scattering and infrared spectroscopy extremely sensitive techniques. Here, we experimentally demonstrate that the interaction between a relativistic electron and vibrational modes in nanostructures is fundamentally modified in the presence of plasmons. We finely tune the energy of surface plasmons in metallic nanowires in the vicinity of hexagonal boron nitride, making it possible to monitor and disentangle both strong phonon–plasmon coupling and plasmon-driven phonon enhancement at the nanometer scale. Because of the near-field character of the electron beam–phonon interaction, optically inactive phonon modes are also observed. Besides increasing our understanding of phonon physics, our results hold great potential for investigating sensing mechanisms and chemistry in complex nanomaterials down to the molecular level.

Utilizing niobium plasmonic perfect absorbers for tunable near- and mid-IR photodetection.

With the fast development of single photon-based technologies such as quantum computing and quantum cryptography, conventional avalanche photodiodes as single photon detectors are not the optimum tools anymore. They are currently replaced by Superconducting Nanowire Single Photon Detectors (SNSPDs) based on the superconducting to normal conducting phase transition. The current challenge with SNSPDs lies in overcoming the trade-off between detection efficiency and recovery time. While a large active area will lead to high detection efficiency, the associated high kinetic inductance causes a long recovery time. Plasmonic effects can play an important role in the absorption enhancement of SNSPDs. Nanostructuring with a suitable geometry can provide a high-absorption cross-section at the intrinsic nanowire surface plasmon resonance, which can be significantly larger than their geometric cross-section. We present a photodetector based on the intrinsic localized surface plasmon resonance of a niobium nanowire, which is one of the common superconductors with low kinetic inductance. Additionally, we are increasing the absorption of our nanostructures even further using a plasmonic perfect absorber scheme. We fabricated a plasmonic perfect absorber superconducting photodetector, investigated its response to external light at resonance, and proved its plasmonic behavior as evidenced by its polarization dependence.

Color-Tunable ZnO/GaN Heterojunction LEDs Achieved by Coupling with Ag Nanowire Surface Plasmons.

Color-tunable light-emitting devices (LEDs) have a great impact on our daily life. Herein, LEDs with tunable electroluminescence (EL) color were achieved via introducing Ag nanowires surface plasmons into p-GaN/n-ZnO film heterostructures. By optimizing the surface coverage density of coated Ag nanowires, the EL color was changed continuously from yellow-green to blue-violet. Transient-state and temperature-variable fluorescence emission characterizations uncovered that the spontaneous emission rate and the internal quantum efficiency of the near-UV emission were increased as a consequence of the resonance coupling interaction between Ag nanowires surface plasmons and ZnO excitons. This effect induces the selective enhancement of the blue-violet EL component but suppresses the defect-related yellow-green emission, leading to the observed tunable EL color. The proposed strategy of introducing surface plasmons can be further applied to many other kinds of LEDs for their selective enhancement of EL intensity and effective adjustment of the emission color.

Excitation of nanowire surface plasmons by silicon vacancy centers in nanodiamonds

Silicon vacancy (SiV) centers in diamonds have emerged as a very promising candidate for quantum emitters due to their narrow emission line resulting in their indistinguishability. While many different quantum emitters have already been used for the excitation of various propagating plasmonic modes, the corresponding exploitation of SiV centers has emained so far uncharted territory. Here, we report on the excitation of surface plasmon modes supported by silver nanowires using SiV centers in nanodiamonds. The coupling of SiV center fluorescence to surface plasmons is observed, when a nanodiamond situated close to a nanowire is illuminated by the pump, as radiated emission from the distal nanowire end. The effect of coupling is also seen as a change in the SiV center lifetime. Finally, we discuss possible avenues for strengthening the SiV center coupling to surface plasmon modes.

Plasmonic bio-sensing for the Fenna-Matthews-Olson complex

We study theoretically the bio-sensing capabilities of metal nanowire surface plasmons. As a specific example, we couple the nanowire to specific sites (bacteriochlorophyll) of the Fenna-Matthews-Olson (FMO) photosynthetic pigment protein complex. In this hybrid system, we find that when certain sites of the FMO complex are subject to either the suppression of inter-site transitions or are entirely disconnected from the complex, the resulting variations in the excitation transfer rates through the complex can be monitored through the corresponding changes in the scattering spectra of the incident nanowire surface plasmons. We also find that these changes can be further enhanced by changing the ratio of plasmon-site couplings. The change of the Fano lineshape in the scattering spectra further reveals that “site 5” in the FMO complex plays a distinct role from other sites. Our results provide a feasible way, using single photons, to detect mutation-induced, or bleaching-induced, local defects or modifications of the FMO complex, and allows access to both the local and global properties of the excitation transfer in such systems.

Enhanced near-UV electroluminescence from p-GaN/i-Al2O3/n-ZnO heterojunction LEDs by optimizing the insulator thickness and introducing surface plasmons of Ag nanowires

Enhanced near-UV electroluminescence is achieved from p-GaN/i-Al2O3/n-ZnO LEDs by optimizing the Al2O3 insulator thickness and introducing Ag nanowire surface plasmons.

Scattering of nanowire surface plasmons coupled to quantum dots with azimuthal angle difference.

Coherent scatterings of surface plasmons coupled to quantun dots have attracted great attention in plasmonics. Recently, an experiment has shown that the quantum dots located nearby a nanowire can be separated not only in distance, but also an angle ϕ along the cylindrical direction. Here, by using the real-space Hamiltonian and the transfer matrix method, we analytically obtain the transmission/reflection spectra of nanowire surface plasmons coupled to quantum dots with an azimuthal angle difference. We find that the scattering spectra can show completely different features due to different positions and azimuthal angles of the quantum dots. When additionally coupling a cavity to the dots, we obtain the Fano-like line shape in the transmission and reflection spectra due to the interference between the localized and delocalized modes.

Coupling Emitters and Silver Nanowires to Achieve Long-Range Plasmon-Mediated Fluorescence Energy Transfer.

The development of quantum plasmonic circuitry requires efficient coupling between quantum emitters and plasmonic waveguides. A major experimental challenge is to simultaneously maximize the surface plasmon propagation length, the coupling efficiency into the plasmonic mode, and the Purcell factor. Addressing this challenge is also the key to enabling long-range energy transfer between quantum nanoemitters. Here, we use a dual-beam scanning confocal microscope to carefully investigate the interactions between fluorescent nanoparticles and surface plasmons on single-crystalline silver nanowires. By exciting the fluorescent nanoparticles via nanowire surface plasmons, we maximize the light-matter interactions and reach coupling efficiencies up to 44% together with 24× lifetime reduction and 4.1 μm propagation lengths. This improved optical performance enables the demonstration of long-range plasmon-mediated fluorescence energy transfer between two nanoparticles separated by micrometer distance. Our results provide guidelines toward practical realizations of efficient long-range fluorescence energy transfer for integrated plasmonics and quantum nano-optics.

Surface modes of big clusters and resonances in generalized LC–model of metal-dielectric nanocomposites

Random impedance networks with inductive L and capacitive C bonds have been widely addressed in a literature to describe properties of disordered metal-dielectric nanocomposites. In this paper we show that networks with single bonds in the form of parallel oscillatory LC-circuits and capacitances C are more appropriate model for metal-dielectric composite in optical frequency range. Resonant spectrum of such networks demonstrates absence of resonances at frequencies higher than the plasma frequency of metal ωp. Eigenmodes of big ordered clusters are studied. Their resonances are shown to be surface modes having the form of multipoles of various order. It is in agreement with theory of surface plasmons in metallic nanowires. The idea for metal-metal and metal-semiconductor nanocomposites with different plasma frequencies is put forward.

Excitation of surface plasmons from silver nanowires embedded in polymer nanofibers.

We report an excitation of surface plasmons in silver nanowires (AgNWs) which were embedded in flexible polymer nanofibers. Using waveguiding excitation, surface plasmons in AgNWs were excited and propagated. By directing light of 650, 532, and 473 nm wavelengths into the nanofiber, surface plasmons in an embedded single AgNW (average diameter 400 nm, length 4.3 μm) were excited and the corresponding propagation lengths for the three wavelengths are 10.6, 7.7, and 5.1 μm. It was also found that, when a spatially incoherent white light of a halogen lamp with an excitation optical power of 80 μW was coupled into the polymer nanofiber, a surface plasmon mediated interference fringe was observed. In addition, on the basis of surface plasmon excitation, two adjacent AgNWs embedded in the polymer nanofibers were demonstrated to serve as coupled plasmonic waveguides.

Single-mode graphene-coated nanowire plasmonic waveguide.

We propose in this Letter a single-mode graphene-coated nanowire surface plasmon waveguide. The single-mode condition and modal cutoff wavelength of high order modes are derived from an analytic model and confirmed by numerical simulation. The mode number diagram of the proposed waveguide in the wavelength-radius space is also demonstrated. By changing the Fermi level of graphene, the performance of the proposed waveguide could be tuned flexibly, offering potential application in tunable nanophotonic devices.

Looking into meta-atoms of plasmonic nanowire metamaterial.

Nanowire-based plasmonic metamaterials exhibit many intriguing properties related to the hyperbolic dispersion, negative refraction, epsilon-near-zero behavior, strong Purcell effect, and nonlinearities. We have experimentally and numerically studied the electromagnetic modes of individual nanowires (meta-atoms) forming the metamaterial. High-resolution, scattering-type near-field optical microscopy has been used to visualize the intensity and phase of the modes. Numerical and analytical modeling of the mode structure is in agreement with the experimental observations and indicates the presence of the nonlocal response associated with cylindrical surface plasmons of nanowires.

Surface plasmons in nanowires with toroidal magnetic structure.

We consider the problem of the influence of a toroidal magnetization on a cylindrical surface plasmon polariton (SPP) propagating along a nanowire of a circular cross section. It follows from the dispersion equations that the SPP wavenumber linearly depends on the toroidal moment and the effect of magneto-optical nonreciprocity appears. The numerical solution of the dispersion equations demonstrates that the corresponding splitting of the SPP dispersion curves for two opposite directions of the toroidal moment is increased by an order of magnitude with respect to the planar case. The largest values of this splitting are observed for systems with relatively low optical losses, as is demonstrated by calculations for SPPs in a gold cylinder surrounded by rare-earth bismuth iron garnet.

Remotely excited Raman optical activity using chiral plasmon propagation in Ag nanowires

We experimentally investigated remotely excited Raman optical activity (ROA) using propagating surface plasmons in chiral Ag nanowires. Using chiral fmoc-glycyl-glycine-OH (FGGO) molecules, we first studied the local surface plasmon-enhanced ROA. We found that the Raman intensity can be excited by left- and right-circularly polarized lights and that the circular intensity difference (CID) can be significantly enhanced. Second, by selecting vibrational modes with large Raman and ROA intensities that are not influenced by chemical enhancements, we studied remotely excited ROA imaging and the CID of FGGO molecules by propagating a plasmonic waveguide using Ag chiral nanostructures. When laser light was radiated on one of the Ag terminals, the measured CID of the FGG at the other terminal showed little change compared to the local excited CID. Meanwhile, when the laser light was radiated on the Ag nanowires (not on the terminals) and was coupled to the nearby nanoantenna, the CID of the ROA could be manipulated by altering the coupling angle between the Ag nanowires. To directly demonstrate the propagation of ROA along the nanowire and its remote detection, we also measured the remotely excited ROA spectra. Our experimental method has the potential to remotely determine the chirality of molecular structures and the absolute configuration or conformation of a chiral live cell.

Examining non-locality and quantum coherent dynamics induced by a common reservoir

If two identical emitters are coupled to a common reservoir, entanglement can be generated during the decay process. When using Bell's inequality to examine the non-locality, however, it is possible that the bound cannot be violated in some cases. Here, we propose to use the steering inequality to examine the non-locality induced by a common reservoir. Compared with the Bell inequality, we find that the steering inequality has a better tolerance for examining non-locality. In view of the dynamic nature of the entangling process, we also propose to observe the quantum coherent dynamics by using the Leggett-Garg inequalities. We also suggest a feasible scheme, which consists of two quantum dots coupled to nanowire surface plasmons, for possible experimental realization.

Quantum entangling gates using the strong coupling between two optical emitters and nanowire surface plasmons

We propose a scheme to generate quantum entangling gate using one-dimensional surface plasmon waveguide. The protocol is based on the detection of the transmission spectrum of the single optical plasmons passing through two separate three-level emitters on metallic nanowire waveguide. It is shown that the low efficiency in direct detection of the single photon can be avoided by repeating the measurement of the transmission spectrum.

Probing polarization modes of Ag nanowires with hot electron detection on Au/TiO2 nanodiodes

We report the effect of surface plasmons of silver nanowires (AgNWs) on the generation of hot electrons. The AgNWs were deposited on ultra-thin Au/TiO2 Schottky nanodiodes using a spraying method. As surface coverage of the AgNWs on the thin gold film increased, the short-circuit current under illumination increased, showing the effect of the AgNWs on the energy conversion efficiency. Detection of the hot electron flow enhanced by the AgNWs on the Au/TiO2 nanodiodes provides evidence of transverse (3.28 eV) and bulk silver (3.52 eV) plasmon modes that were confirmed by the measurement of the absorbance spectra of the AgNWs.