Plasmonic interactions between a perforated gold film and a thin gold film

Based on the finite difference time domain method, we investigated theoretically the optical properties and the plasmonic interactions between a gold film perforated with periodic sub-wavelength holes and a thin gold film. We showed that the plasmon resonant energies and intensities depend strongly on the thicknesses of the two films and the lattice constant. Based on the distributions of normal electric field component Ez, tangential electric field component Ey and total energy, we showed that the optical transmission is due to the collaboration of the localized waveguide resonance, the surface plasmon resonance and the coupling of the fiat-surface plasmon of the two layers.

Energy Levels of Coupled Plasmonic Cavities

We demonstrate the hybridization of the plasmonic modes in directly coupled whispering gallery cavities fabricated on silver films and present the mode patterns and energy levels using cathodoluminescence spectroscopy. Although the energy of the most antisymmetrically coupled modes is higher than that of the corresponding symmetrically coupled ones, the contrary cases happen for small quantum number modes. We attribute the phenomenon to the different surface plasmon polariton paths between the symmetrically and antisymmetrically coupled modes; These results provide an understanding of the resonant properties in coupled plasmonic cavities, which have potential applications in nanophotonic devices.

A High-Sensitivity Refractive-Index Sensor Based on Plasmonic Waveguides Asymmetrically Coupled with a Nanodisk Resonator

A high-sensitivity plasmonic refractive-index sensor based on the asymmetrical coupling of two metal-insulator- metal waveguides with a nanodisk resonator is proposed and simulated in the finite-difference time domain. Both analytic and simulated results show that the resonance wavelengths of the sensor have an approximate linear relationship with the refractive index of the materials which are filled into the slit waveguides and the disk- shaped resonator. The working mechanism of this sensor is exactly due to the linear relationship, based on which tile refractive index of the materials unknown can be obtained from the detection of the resonance wavelength. The measurement sensitivity can reach as high as 6.45 × 10-7, which is nearly five times higher than the results reported in the recent literature [Opt. Commun. 300 （2013） 265]. With an optimum design, the sensing value can be further improved, and it can be widely applied into the biological sensing. Tile sensor working for temperature sensing is also analyzed.

Coupling model for an extended-range plasmonic optical transformer scanning probe

The expansion of nanoscale optics has generated a variety of scanning probe geometries that yield spatial resolution below 10 nm.In this work,we present a physical model for coupling far-field radiation to plasmonic modes on the surface of a scanning probe,and propose a scheme for extending the working distance of such a probe.In a subsurface application,an optical transformer at the tip of a probe can be coupled to a remote near-field antenna placed inside the sample at a distance away from the surface,expanding the effective working distance up to 100 nm.

A BIOSENSOR USING COUPLED PLASMON WAVEGUIDE RESONANCE COMBINED WITH HYPERSPECTRAL FLUORESCENCE ANALYSIS

We developed a biosensor that is capable for simultaneous surface plasmon resonance(SPR)sensing and hyperspectral fuores cence analysis in this paper.A symmetrical metal-dielectric slabscheme is employed for the excitation of coupled plasnon waveguide resonance(CPWR)in thepresent work.Resonance bet ween surface plasmon mode and the guided waveguide mode gen-erates narrower full width half-maximum of the refective curves which leads to increased pre.cision for the determination of refractive index over conventional SPR sensors.In addition,CPWR also fers longer surface propagation depths and higher surface electric field strengthsthat enable the excitation of fluorescence with hyperspectral technique to maintain an appreci-able signal-to-noise ratio.The refractive index information obtained from SPR sensing and thechemical properties obt ained through hyperspectral fluorescence analysis confirm each other toexclude false-positive or false-negative cases.The sensor provides a comprehensive understandingof the biological events on the sensor chips.

DNA Nanostructure-Guided Plasmon Coupling Architectures

Plasmon coupling architectures with specific spatial and orientational arrangement configurations possess unique and tailored plasmonic properties and hold promise for advancements in nano-optics,nanoantennas,and biosensors.Numerous research has focused on the construction of plasmonic assemblies with predetermined configurations.DNA nanostructures with arbitrary geometry,high compatibility with metal nanoparticles,and spatial addressability meet the requirement for precise spatial and orientation arrangement.Currently,DNA nanostructures are widely exploited as structural materials to generate plasmonic structures with well-defined topologies.We review the evolution of DNA nanostructureguided plasmon coupling architectures,including the introduction of DNA nanostructures,DNA modification on the surface of plasmonic nanoparticles,and three strategies for constructing complex plasmonic nanostructures.Then we focus on the emerging applications of DNA nanostructure-guided architectures with engineered local electromagnetic enhancement for modulating plasmon coupling,amplifying emitter signals,and serving as biosensors.Finally,we will critically discuss the challenges and opportunities in this field.

Selective deposition of a MOF at the spikes of Au nanostars for SERS detection

In the pursuit of advancing molecular sensing through surface-enhanced Raman spectroscopy(SERS),the combination of plasmonic nanoparticles and metal-organic frameworks(MOFs)has emerged as a highly effective approach to enhance the sensitivity and selectivity of SERS substrates.However,most prior investigations have predominantly focused on MOF-coated plasmonic nanoparticles in core@shell or layer-by-layer configurations,leaving a notable knowledge gap in exploring alternative configurations.Herein we present a facile method to construct a particle-on-mirror architecture by selectively coating a MOF,zeolitic imidazolate framework-8(ZIF-8),onto the tips of Au nanostars and subsequently depositing the resultant nanoparticles onto a Au film.This design integrates the electric field enhancement at the sharp tips and nanogaps,along with the molecular enrichment function within the porous MOF immobilized at the tips and nanogaps,leading to a substantial boost in the SERS signal intensity.Such a unique SERS platform enables consistent and outstanding SERS performance for analytes of different sizes.This work opens up a promising strategy for constructing multifunctional nanostructures for sensitive SERS detection in real-life scenarios.

Fabrication of chiral plasmonic oligomers using cysteine-modified gold nanorods as monomers

Generation of circular dichroism （CD） beyond the UV region is of great interest in developing chiral sensors and chiroptical devices. Herein, we demonstrate a simple and versatile method for fabrication of plasmonic oligomers with strong CD response in the visible and near IR spectral range. The oligomers were fabricated by triggering the side-by-side assembly of cysteine-modified gold nanorods. The modified nanorods themselves did not exhibit obvious plasmonic CD signals; however, the oligomers show strong CD bands around the plasmon resonance wavelength. The sign of the CD band was dictated by the chirality of the absorbed cysteine molecules. By adjusting the size of the oligomers, the concentration of chiral molecules, and/or the aspect ratio of the nanorods, the CD intensity and spectral range were readily tunable. Theoretical calculations suggested that CD of the oligomers originated from a slight twist of adjacent nanorods within the oligomer. Therefore, we propose that the adsorbed chiral molecules are able to manipulate the twist angles between the nanorods and thus modulate the CD response of the oligomers.

Monitoring transient nanoparticle interactions with liposome-confined plasmonic transducers

The encapsulation of individual pairs of plasmonic nanoparticles(NPs)in liposomes is introduced as a new strategy for utilizing plasmon coupling to monitor interactions between co-confined NPs in a nanoconfinement that ensures high local NP concentrations.We apply the approach to monitor transient binding contacts between noncovalently tethered 55 nm diameter gold NPs,which were functionalized with cytosine(C)-rich DNAs,in acidic and mildly basic buffer conditions.At pH=8,a rich spectral dynamics indicates DNA-mediated transient binding and unbinding of co-confined NPs due to weak attractive interparticle interactions.A decrease in pH from 8 to 4 is observed to favor the associated state for some co-confined NPs,presumably due to a stabilization of the bound dimer configuration through noncanonical C-C^(+)bonds between the DNA-functionalized NPs.Plasmonic nanoemitters whose spectral response switches in response to chemical cues(in this work pH)represent optical transducers with a rich application space in chemical sensing,cell analysis and nanophotonics.

Tunable plasmon resonance coupling in coaxial gold nanotube arrays

The optical properties and plasmon resonance coupling of double coaxial gold nanotube arrays are inves- tigated. The results show that the optical transmission is highly tunable by varying the thicknesses of the inner and outer nanotubes, the separation between the inner and outer nanotubes, and the dielectric parameters inside, between, and outside the two nanotubes. The shorter-wavelength transmission bands are very sensitive to the modification of the wall thickness of the outer nanotube, the separation, and the dielectric parameters between the double nanotubes. The dipole and multipolar plasmon modes are excited in our model. However, for small separation and refractive index, the dipole normal mode has a leading function in the transmission properties. Compared with the dipolar modes, the contribution of higher-order modes becomes larger as the parameters increase.

Numerical analysis of end-fire coupling of surface plasmon polaritons in a metal-insulator-metal waveguide using a simple photoplastic connector

We propose a design for efficient end-fire coupling of surface plasmon polaritons in a metal-insulator-metal(MIM) waveguide with an optical fiber as part of a simple photoplastic connector. The design was analyzed and optimized using the three-dimensional finite-difference time-domain method. The calculated excitation efficiency coefficient of the waveguide is 83.7%(-0.77 dB) at a wavelength of 405 nm. This design enables simple connection of an optical fiber to a MIM waveguide and highly efficient local excitation of the waveguide.Moreover, the length of the metallic elements of the waveguide, and thus the dissipative losses, can be reduced.The proposed design may be useful in plasmonic-type waveguide applications such as near-field investigation of live cells and other objects with super-resolution.

Multiple plasmon couplings in 3D hybrid Au-nanoparticles-decorated Ag nanocone arrays boosting highly sensitive surface enhanced Raman scattering

Plasmon coupling is an essential strategy to realize strong local electromagnetic(EM)field which is crucial for high-performance plasmonic devices.In this work,multiple plasmon couplings are demonstrated in three-dimensional(3D)hybrid plasmonic systems composed of polydimethylsiloxane-supported ordered silver nanocone(AgNC)arrays decorated with high-density gold nanoparticles(AuNPs)which are fabricated by a template-assisted physical vapor deposition process.Strong interparticle coupling,particle-film coupling,inter-cone coupling,and particle-cone coupling are revealed by numerical simulations in such composite nanostructures,which produce intense and high-density EM hot spots,boosting highly sensitive and reproducible surface enhanced Raman scattering(SERS)detection with an enhancement factor of-1.74×10^(8).Furthermore,a linear correlation between logarithmic Raman intensity and logarithmic concentration of probe molecules is observed in a large concentration range.These results offer new ideas to develop novel plasmonic devices,and provide alternative strategy to realize flexible and high-performance SERS sensors for trace molecule detection and quantitative analysis.

Adhesive surface-enhanced Raman scattering Cu-Au nanoassembly for the sensitive analysis of particulate matter

Surface-enhanced Raman scattering(SERS)has been used in atmospheric aerosol detection as it enables the high-resolution analysis of particulate matter.However,its use in the detection of historical samples without damaging the sampling membrane while achieving effective transfer and the high-sensitivity analysis of particulate matter from sample films remains challenging.In this study,a new type of SERS tape was developed,consisting of Au nanoparticles(NPs)on an adhesive double-sided Cu film(DCu).The enhanced electromagnetic field generated by the coupled resonance of the local surface plasmon resonances of AuNPs and DCu led to an enhanced SERS signal with an experimental enhancement factor of 10^(7).The AuNPs were semi-embedded and distributed on the substrate,and the viscous DCu layer was exposed,enabling particle transfer.The substrates exhibited good uniformity and favorable reproducibility with relative standard deviations of 13.53%and 9.74%respectively,and the substrates could be stored for 180 days with no signs of signal weakening.The application of the substrates was demonstrated by the extraction and detection of malachite green and ammonium salt particulate matter.The results demonstrated that SERS substrates based on AuNPs and DCu are highly promising in real–world environmental particle monitoring and detection.

Simultaneous monitoring of the fluorescence and refractive index by surface plasmon coupled emission: A proof-of-concept study

Simultaneous acquisition of fluorescence property and refractive index using a single surface plasmon coupled emission(SPCE)measurement has been achieved,thus achieving synchronicity in real time.The SPCE sensor was employed for monitoring the adsorption of volatile organic compounds(VOCs)by dyeencapsulated metal-organic frameworks(Dye@MOFs).Refractive index can reveal surface molecular adsorption and the fluorescence with information on refractive index can provide a comprehensive analysis of the adsorption events of VOCs on the interface.Meantime,the signal intensity can be amplified by combining the responses caused by changes in refractive index and the fluorescence property in parallel.This all-in-one method opens up a route to monitoring multiple processes simultaneously occurring on the interface.

Synthesis and Raman Performance Enhancement of Multilayer AuAg Heterostructures with Magnetic Resonance

Significant amplification of surface enhanced Raman scattering(SERS)signals can be achieved mainly by the electric field enhancement in metal core-shell nanostructures,and the enhanced magnetic field is rarely studied.In this study,we prepared multi-gap Au/AgAu core-shell hybrid nanostructures by using gold nanocup as the core.The overgrowth processes to grow one,two,and three layers of AgAu hybrid nanoshells can produce Au/AgAu^(1),Au/AgAu^(2),and Au/AgAu^(3) heteronanostructures.The strong plasmon coupling between the core and shell leads to significant electromagnetic field enhancement.Under the synergistic effect of electromagnetic plasmon resonance and plasmon coupling,Au/AgAu core-shell hybrid nanostructures exhibit excellent SERS signals.We also investigate the effect of the interstitial position of the rhodamine B(RhB)molecule on Raman enhancement in Au/AgAu~3 heteronanostructures.This study can provide new ideas for the synthesis of multi-gap Raman signal amplifiers based on magnetic plasmon coupling.

Plasmon enhancement for Vernier coupled single-mode lasing from ZnO/Pt hybrid microcavities

It is essential to develop a single mode operation and improve the performance of lasing in order to ensure practical applicability of microlasers and nanolasers. In this paper, two hexagonal microteeth with varied nanoscaled air-gaps of a ZnO microcomb are used to construct coupled whispering-gallery cavities. This is done to achieve a stable single mode lasing based on Vernier effect without requiring any complicated or sophisticated manipulation to achieve positioning with nanoscale precision. Optical gain and the corresponding ultraviolet lasing performance were improved greatly through coupling with localized surface plasmons of Pt nanoparticles. The ZnO/Pt hybrid microcavities achieved a seven-fold enhancement of intensity of single mode lasing with higher side- mode suppression ratio and lower threshold. The mechanism that led to this enhancement has been described in detail.

Significantly enhanced transmission achieved with double-layered metallic aperture arrays with sub-skin-depth Ag film

We present both theoretical and experimental investigation on significantly enhanced transmission through （Ag/Au） double-layered metallic aperture arrays with sub-skin-depth Ag film due to the coupling role of a surface plasmon polariton at the Ag/Au interface by evanescent waves. The results indicate that the enhanced transmittance is highly dependent on the Ag film thickness. When the Ag film thickness increases, the peak transmit- tance firstly increases and then decreases. Moreover, other metal material properties are also discussed. The highest peak transmittance is obtained when the Ag film thickness is 4 nm. The finite-difference time-domain simulations agree well with the experimental results. This finding provides an effective way to control the enhanced transmis- sion for double-layered metallic aperture arrays, which has potential applications in designing a high-performance plasmonic thermal emitter.

Coupled quantum molecular cavity optomechanics with surface plasmon enhancement

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 inmolecule–plasmon systems.

Enhanced modulation of magnetic field on surface plasmon coupled emission(SPCE) by magnetic nanoparticles

The obvious enhancement effect of magnetic nanoparticles(MNPs) introduced in Cr/Co/Cr/Au substrate on the pulsed magnetic field-modulated surface plasmon coupled emission(SPCE) was investigated,and the observed enhancement factor was 4 comparing with the magnetic field modulated SPCE without MNPs.This is the new observation for the magnetic field modulated SPCE,and this method was designed as a biosensor,which to our knowledge,is the first application of magnetic field-modulated SPCE in biosensing and detection field.This strategy is a universal approach to increase the fluorescence signal and helps to build the new SPCE based stimulus-response system.