Dye-assisted gain of strongly confined surface plasmon polaritons in silver nanowires

The influence of single layer MoS2flake on the propagated surface plasmons of silver nanowire

We demonstrate enhancing the excitation and transmission efficiency of the propagated surface plasmon (SP) of an Ag nanowire (Ag NW) in hybrid Ag-MoS₂structures by contrasting the SP propagation of the Ag NW on different substrates, including SiO₂and monolayer MoS₂, or partially overlapping the Ag NW on MoS₂flakes. The simulation results show that the leaky radiation of the hybrid plasmonic modes H₁and H₂can be prominently suppressed due to the high refractive index dielectric layer of MoS₂, which provides an optical barrier for blocking the leaky radiation, resulting in reduced propagation loss. This paper provides a feasible and effective method to improve the SP propagation length.

A top-down fabricated gold nanostrip on a silicon-on-insulator wafer: a promising building block towards ultra-compact optical devices

A plasmonic waveguide is a fundamental building block for high speed, large data transmission capacity, low energy consumption optical communication and sensing. Controllable fabrication and simultaneous optimization of propagation loss and coupling efficiency with free space light are essential for the realization of ultra-compact passive and active plasmonic components. Here, we proposed gold nanostrips on a silicon-on-insulator wafer as plasmonic waveguides and first demonstrated the direct free-space light coupling and end-scattering detection of the top-down fabricated plasmonic waveguide. The scattering intensity from the terminal of a 6 μm long nanostrip can be improved experimentally over 34 times larger than that on the silica substrate. The controllable fabrication process renders the gold nanostrip on a silicon-on-insulator substrate a promising building block for ultracompact, monolithic integration and CMOS-compatible plasmonic devices in optical communication and sensing.

Direct Visualization of Gap-Plasmon Propagation on a Near-Touching Nanowire Dimer

Dimers of metallic nanowires (NWs) with nanometric gaps could be an alternative to overcome the limitations of existing plasmonic waveguides. The gap-surface plasmon polaritons (gap-SPPs) of the dimers may propagate along the NW without crosstalk and greatly enhance the coupling efficiency with an emitter, enabling ultracompact optical circuits. Such a possibility has not been realized, and we experimentally show its possibility. The gap-SPPs of the AgNW-molecule-AgNW structure, with a gap of 3-5 nm defined by the molecules, are visualized using the surface-enhanced Raman scattering (SERS) of the molecules. The SERS images, representing the gap-field intensity distribution, reveal the decay and beating of the monopole-monopole and dipole-dipole gap modes. The propagation lengths of the two (l₁ = 0.5-2 μm and l₂ = 5-8 μm) closely follow the model prediction with a uniform gap, confirming that the scattering loss induced by the gap irregularities is surprisingly low.

Superficial-Layer-Enhanced Raman Scattering (SLERS) for Depth Detection of Noncontact Molecules

Although the strength of Raman signals can be increased by many orders of magnitude on noble metal nanoparticles, this enhancement is confined to an extremely short distance from the Raman-active surface. The key to the development of Raman spectroscopy for applications in diagnosis and detection of cancer and inflammatory diseases, and in pharmacology, relies on the capability of detecting analytes that are noninteractive with Raman-active surfaces. Here, a new Raman enhancement system is constructed, superficial-layer-enhanced Raman scattering (SLERS), by covering elongated tetrahexahedral gold nanoparticle arrays with a superficial perovskite (CH₃ NH₃ PbBr₃ ) film. Plasmonic decay is depressed along the vertical direction away from the noble metal surface and the penetration depth is increased in the perovskite media. The vertical penetration of SLERS is verified by the spatial distribution of the analytes via Raman imaging in layer-scanning mode.

Strongly Asymmetric Spectroscopy in Plasmon-Exciton Hybrid Systems due to Interference-Induced Energy Repartitioning

Recent intense effort has been devoted to exploring different manifestations of resonant excitations of strongly coupled plasmons and excitons, but so far such studies have been limited to situations where the Fano- or Rabi-type spectra are largely symmetric at zero detuning. Using a newly developed full quantum mechanical model, here we reveal the existence of a highly asymmetric spectroscopic regime for both the Rabi splitting and transparency dip. The asymmetric nature is inherently tied to the non-negligible exciton absorbance and is caused by substantial interference-induced energy repartitioning of the resonance peaks. This theoretical framework can be exploited to reveal the quantum behaviors of the two excitation entities with varying mutual coupling strengths in both linear and nonlinear regimes. We also use prototypical systems of rhodamine molecules strongly coupled with AuAg alloyed nanoparticles and well-devised control experiments to demonstrate the validity and tunability of the energy repartitioning and correlated electronic state occupations, as captured by the variations in the asymmetric spectroscopy and corresponding nonlinear absorption coefficient as a function of the Au:Ag ratio. The present study helps to substantially enrich our microscopic understanding of strongly coupled plasmon-exciton systems.

Tailored Surfaces/Assemblies for Molecular Plasmonics and Plasmonic Molecular Electronics

Molecular plasmonics uses and explores molecule-plasmon interactions on metal nanostructures for spectroscopic, nanophotonic, and nanoelectronic devices. This review focuses on tailored surfaces/assemblies for molecular plasmonics and describes active molecular plasmonic devices in which functional molecules and polymers change their structural, electrical, and/or optical properties in response to external stimuli and that can dynamically tune the plasmonic properties. We also explore an emerging research field combining molecular plasmonics and molecular electronics.

Far-Field Super-resolution Detection of Plasmonic Near-Fields

We demonstrate a far-field single molecule super-resolution method that maps plasmonic near-fields. The method is largely invariant to fluorescence quenching (arising from probe proximity to a metal), has reduced point-spread-function distortion compared to fluorescent dyes (arising from strong coupling to nanoscopic metallic features), and has a large dynamic range (of 2 orders of magnitude) allowing mapping of plasmonic field-enhancements regions. The method takes advantage of the sensitivity of quantum dot (QD) stochastic blinking to plasmonic near-fields. The modulation of the blinking characteristics thus provides an indirect measure of the local field strength. Since QD blinking can be monitored in the far-field, the method can measure localized plasmonic near-fields at high throughput using a simple far-field optical setup. Using this method, propagation lengths and penetration depths were mapped-out for silver nanowires of different diameters and for different dielectric environments, with a spatial accuracy of ∼15 nm. We initially use sparse sampling to ensure single molecule localization for accurate characterization of the plasmonic near-field with plans to increase density of emitters in further studies. The measured propagation lengths and penetration depths values agree well with Maxwell finite-difference time-domain calculations and with published literature values. This method offers advantages such as low cost, high throughput, and superresolved mapping of localized plasmonic fields at high sensitivity and fidelity.

Role of Resonances in the Transmission of Surface Plasmon Polaritons between Nanostructures

Understanding how surface plasmon polaritons (SPPs) propagate in metal nanostructures is important for the development of plasmonic devices. In this paper, we study the transmission of SPPs between single-crystal gold nanobars on a glass substrate using transient absorption microscopy. The coupled structures were produced by creating gaps in single nanobars by focused ion beam milling. SPPs were launched by focusing the pump laser at the end of the nanobar, and the transmission across the gaps was imaged by scanning the probe laser over the nanostructure. The results show larger losses at small gap sizes. Finite element method calculations were used to investigate this effect. The calculations show two main modes for nanobars on a glass surface: a leaky mode localized at the air-gold interface, and a bound mode localized at the glass-gold interface. At specific gap sizes (approximately 50 nm for our system), these SPP modes can excite localized surface plasmon modes associated with the gap, which dissipate energy. This increases the energy losses at small gap sizes. Experiments and simulations were also performed for the nanobars in microscope immersion oil, which creates a more homogeneous optical environment, and consistent results were observed.

Selective nanomanipulation of fluorescent polystyrene nano-beads and single quantum dots at gold nanostructures based on the AC-dielectrophoretic force

We introduced the selective manipulation of polystyrene (PS) nano-beads and single quantum dots (QDs) at a gold nanostructure by using the AC-dielectrophoretic (DEP) force. Manipulation in three degrees of freedom (end-facet, side, and position-selective manipulation) was accomplished in gold nanostructures between microelectrodes. A 10 μm gap between the microelectrodes, which has a 100 nm-wide nanowire and 200 nm-wide vortex nanostructures at the inside of the gap, was fabricated, and nanostructures were not connected with the electrodes. We also performed theoretical calculations to verify the selective manipulation through the floating AC-DEP force. A sufficiently high gradient of the square of the electric field (∇|E|(2), ∼10(19) V(2) m(-3)) was accomplished and controlled for achieving a strong attaching force of nanoparticles using the gap between microelectrodes and nanostructures as well as the rotation of structures. Fluorescent PS nano-beads and QDs were attached at the designed end facet, side, and position of nanostructures with high selectivity. A single QD attachment was also realized at gold nanostructures, and the attached QDs were verified as single using optical "blinking" measurements.

One-Dimensional Dielectric/Metallic Hybrid Materials for Photonic Applications

Explorations of 1D nanostructures have led to great progress in the area of nanophotonics in the past decades. Based on either dielectric or metallic materials, a variety of 1D photonic devices have been developed, such as nanolasers, waveguides, optical switches, and routers. What's interesting is that these dielectric systems enjoy low propagation losses and usually possess active optical performance, but they have a diffraction-limited field confinement. Alternatively, metallic systems can guide light on deep subwavelength scales, but they suffer from high metallic absorption and can work as passive devices only. Thus, the idea to construct a hybrid system that combines the merits of both dielectric and metallic materials was proposed. To date, unprecedented optical properties have been achieved in various 1D hybrid systems, which manifest great potential for functional nanophotonic devices. Here, the focus is on recent advances in 1D dielectric/metallic hybrid systems, with a special emphasis on novel structure design, rational fabrication techniques, unique performance, as well as their wide application in photonic components. Gaining a better understanding of hybrid systems would benefit the design of nanophotonic components aimed at optical information processing.

Stretchable Heater Using Ligand-Exchanged Silver Nanowire Nanocomposite for Wearable Articular Thermotherapy

Thermal therapy is one of the most popular physiotherapies and it is particularly useful for treating joint injuries. Conventional devices adapted for thermal therapy including heat packs and wraps have often caused discomfort to their wearers because of their rigidity and heavy weight. In our study, we developed a soft, thin, and stretchable heater by using a nanocomposite of silver nanowires and a thermoplastic elastomer. A ligand exchange reaction enabled the formation of a highly conductive and homogeneous nanocomposite. By patterning the nanocomposite with serpentine-mesh structures, conformal lamination of devices on curvilinear joints and effective heat transfer even during motion were achieved. The combination of homogeneous conductive elastomer, stretchable design, and a custom-designed electronic band created a novel wearable system for long-term, continuous articular thermotherapy.