Surface Plasmon Modes Research Articles

Strong Coupling in Two‐Phase Metamaterials Fabricated by Sequential Self‐Assembly

AbstractSelf‐assembly processes provide the means to achieve scalable and versatile metamaterials by “bottom‐up” fabrication. Despite their enormous potential, especially as a platform for energy materials, self‐assembled metamaterials are often limited to single phase systems, and complex multi‐phase metamaterials have scarcely been explored. A new approach based on sequential self‐assembly (SSA) that enables the formation of a two‐phase metamaterial (TPM) composed of a disordered network metamaterial with embedded nanoparticles (NPs) is proposed. Taking advantage of both the high‐spatial and high‐energy resolution of electron energy loss spectroscopy (EELS), inhomogeneous localization of light in the network is observed, concurrent with dipolar and higher‐order localized surface plasmon modes in the nanoparticles. Moreover, it is demonstrated that the coupling strength deviates from the interaction of two classical dipoles when entering the strong coupling regime. The observed energy exchange between two phases in this complex metamaterial, realized solely through self‐assembly, implies the possibility to exploit these disordered systems for plasmon‐enhanced catalysis.

Characteristics of eco-friendly perovskite solar cell with moth-eye nanostructure array

A novel design of tin perovskite (CH3NH3SnI3) solar cell (PSC) is proposed and analyzed for energy harvesting application. The suggested PSC is lead free where moth-eye nanostructures are implemented in the active material to improve the light trapping and hence the light absorption. The suggested SC is numerically studied using finite difference time domain (FDTD) via Lumerical software package. The geometrical parameters and position of the nanostructures are studied to maximize the absorption and hence the optical efficiency. The reported PSC covered by the moth-eye nanostructures exhibits marked light trapping compared to the conventional planar structure with photocurrent density of 46.0082 (mA/cm2), an optical generation rate of 3.38 e28 (m−3. s−1) and an ultimate efficiency of 31.76%. Therefore, an enhancement of 14.496% is obtained compared to the traditional PSC due to the localized surface plasmons (LSP) modes around the moth eye nanostructures. The suggested design is an efficient replacement to lead—perovskite owing to excellent photovoltaic properties, cheap fabrication cost, suitable band gap of 1.02 eV, eco-friendly and great performance in converting sunlight to electrical energy.

Dynamically Adjusting Borophene-Based Plasmon-Induced Transparency in a Polymer-Separated Hybrid System for Broadband-Tunable Sensing.

Borophene, an emerging two-dimensional (2D) material platform, is capable of supporting highly confined plasmonic modes in the visible and near-infrared wavebands. This provides a novel building block for light manipulation at the deep subwavelength scale, thus making it well-suited for designing ultracompact optical devices. Here, we theoretically explore a borophene-based plasmonic hybrid system comprising a continuous borophene monolayer (CBM) and sodium nanostrip gratings (SNGs), separated by a polymer spacer layer. In such a structure, a dynamically tunable plasmon-induced transparency (PIT) effect can be achieved by strongly coupling dark and bright plasmonic modes, while actively controlling borophene. Here, the bright mode is generated through the localized plasmon resonance of SNGs when directly excited by TM-polarized incident light. Meanwhile, the dark mode corresponds to a propagating borophene surface plasmon (BSP) mode in the CBM waveguide, which cannot be directly excited, but requires phase matching with the assistance of SNGs. The thickness of the polymer layer has a significant impact on the coupling strength of the two modes. Owing to the BSP mode, highly sensitive to variations in the ambient refractive index (RI), this borophene-based hybrid system exhibits a good RI-sensing performance (643.8 nm/RIU) associated with a wide range of dynamically adjustable wavebands (1420-2150 nm) by tuning the electron density of borophene. This work offers a novel concept for designing active plasmonic sensors dependent on electrically gating borophene, which has promising applications in next-generation point-of-care (PoC) biomedical diagnostic techniques.

Common-mode plasmon sensing scheme as a high-sensitivity compact SPR sensor.

A deep metal grating enables quasi-phase-matched simultaneous excitation of two counterpropagating surface plasmon modes by means of its +1st and -2nd diffraction orders. The resulting angular reflection spectra of the scattered -1st and zeroth orders exhibit three interleaved zeros and maxima in a range centered around the Littrow angle. The spectra differ thoroughly from the usual reflection dip resulting from single-order plasmon coupling that produces strong absorption. The zeroth and -1st orders exhibit two crossing angles enabling high-sensitivity common-mode detection schemes designed to reject variations in source power and environmental noise. The proof of concept and experimental assessment of this new surface plasmon resonance (SPR) sensing scheme are demonstrated by monitoring gases in a pressure-controlled chamber. A limit of detection (LOD) of 2 × 10-7 refractive index unit (RIU) was achieved.

Epsilon-near-zero surface waves enhance infrared vibrational spectroscopy

Thin film epsilon-near-zero (ENZ) supports guided modes analogous to surface plasmon modes in the infrared, where surface plasmons are difficult to excite. Leveraging these modes can enhance the signal of surface-enhanced infrared absorption (SIERA) sensors by a factor of three, enabling the detection of minute amounts of materials. However, material losses can significantly impact sensor performance. To address this issue, we establish a condition that limits the impact of losses while still preserving the advantage of ENZ-surface waves. Our findings may pave the way for the identification and exploration of particularly suitable materials for highly efficient photonic SIERA sensors.

Electron cooling in graphene enhanced by plasmon-hydron resonance.

Evidence is accumulating for the crucial role of a solid's free electrons in the dynamics of solid-liquid interfaces. Liquids induce electronic polarization and drive electric currents as they flow; electronic excitations, in turn, participate in hydrodynamic friction. Yet, the underlying solid-liquid interactions have been lacking a direct experimental probe. Here we study the energy transfer across liquid-graphene interfaces using ultrafast spectroscopy. The graphene electrons are heated up quasi-instantaneously by a visible excitation pulse, and the time evolution of the electronic temperature is then monitored with a terahertz pulse. We observe that water accelerates the cooling of the graphene electrons, whereas other polar liquids leave the cooling dynamics largely unaffected. A quantum theory of solid-liquid heat transfer accounts for the water-specific cooling enhancement through a resonance between the graphene surface plasmon mode and the so-called hydrons-water charge fluctuations-particularly the water libration modes, which allows for efficient energy transfer. Our results provide direct experimental evidence of a solid-liquid interaction mediated by collective modes and support the theoretically proposed mechanism for quantum friction. They further reveal a particularly large thermal boundary conductance for the water-graphene interface and suggest strategies for enhancing the thermal conductivity in graphene-based nanostructures.

Tamm-surface plasmon resonances from nanograting-coupled plasmonic-photonic multilayer structure for an integrated fiber-optic sensing application

Simultaneous observation of Tamm plasmon (TP) and surface plasmon (SP) resonances has attracted much attention to improve the sensing performance and broaden the application field of plasmonic biosensors. Currently, enormous efforts are being devoted to simultaneously excite both SP and TP modes by exploiting total internal reflection. However, this method requires a bulky and complicated prism or microscopy setup to fulfill the phase-matching condition, limiting the miniaturization and integration of sensors. Herein, we propose and theoretically demonstrate a nanograting-coupled plasmonic-photonic multilayer structure potentially integrated on the optical fiber tip, comprising a multilayer of photonic crystal (PC) inserted between a flat gold film and a gold nanograting. The proposed plasmonic-photonic multilayer structure enables the simultaneous excitation of TP and SP modes at the normal incidence by using the zeroth and first-order diffraction light of the nanograting, respectively. The resonant positions of both SP and TP modes can be independently adjusted by changing the nanograting period and the center wavelength of the PC bandgap. Due to the insensitivity of the TP mode to ambient surroundings, the proposed structure can be served as a self-reference SP resonance sensor for robust measurement where the sensitivity of the SP resonance sensor is determined by the wavelength shift of SP mode relative to TP mode. Moreover, the spectral overlapping of both TP and SP modes results in the abrupt change of reflection intensity, which enables a rapid judgment to the refractive index of ambient surroundings by observing the red or blue shifts of SP resonant position relative to the case of TP. The simultaneous excitation of both TP and SP modes in the proposed structure opens a new path for improving detection accuracy and broadening application scenarios of fiber-optic SP resonance sensors.

Spatio-spectral metrics in electron energy loss spectroscopy as a tool to resolve nearly degenerate plasmon modes in dimer plasmonic antennas

Abstract Electron energy loss spectroscopy (EELS) is often utilized to characterize localized surface plasmon modes supported by plasmonic antennas. However, the spectral resolution of this technique is only mediocre, and it can be rather difficult to resolve modes close in the energy, such as coupled modes of dimer antennas. Here, we address this issue for a case study of the dimer plasmonic antenna composed of two gold discs. We analyze four nearly degenerate coupled plasmon modes of the dimer: longitudinal and transverse bonding and antibonding dipole modes. With a traditional approach, which takes into account the spectral response of the antennas recorded at specific points, the modes cannot be experimentally identified with EELS. Therefore, we employ the spectral and spatial sensitivity of EELS simultaneously. We propose several metrics that can be utilized to resolve the modes. First, we utilize electrodynamic simulations to verify that the metrics indeed represent the spectral positions of the plasmon modes. Next, we apply the metrics to experimental data, demonstrating their ability to resolve three of the above-mentioned modes (with transverse bonding and antibonding modes still unresolved), identify them unequivocally, and determine their energies. In this respect, the spatio-spectral metrics increase the information extracted from electron energy loss spectroscopy applied to plasmonic antennas.

Mode-Coupling Generation Using ITO Nanodisk Arrays with Au Substrate Enabling Narrow-Band Biosensing.

Plasmonic metal nanostructures have promising applications in biosensing due to their ability to facilitate light-matter interaction. However, the damping of noble metal leads to a wide full width at half maximum (FWHM) spectrum which restricts sensing capabilities. Herein, we present a novel non-full-metal nanostructure sensor, namely indium tin oxide (ITO)-Au nanodisk arrays consisting of periodic arrays of ITO nanodisk arrays and a continuous gold substrate. A narrow-band spectral feature under normal incidence emerges in the visible region, corresponding to the mode-coupling of surface plasmon modes, which are excited by lattice resonance at metal interfaces with magnetic resonance mode. The FWHM of our proposed nanostructure is barely 14 nm, which is one fifth of that of full-metal nanodisk arrays, and effectively improves the sensing performance. Furthermore, the thickness variation of nanodisks hardly affects the sensing performance of this ITO-based nanostructure, ensuring excellent tolerance during preparation. We fabricate the sensor ship using template transfer and vacuum deposition techniques to achieve large-area and low-cost nanostructure preparation. The sensing performance is used to detect immunoglobulin G (IgG) protein molecules, promoting the widespread application of plasmonic nanostructures in label-free biomedical studies and point-of-care diagnostics. The introduction of dielectric materials effectively reduces FWHM, but sacrifices sensitivity. Therefore, utilizing structural configurations or introducing other materials to generate mode-coupling and hybridization is an effective way to provide local field enhancement and effective regulation.

All-in-one photoelectric logic gates by Dember photodetectors

Photoelectric logic gates (PELGs) are the key component in integrated electronics due to their abilities of signal conversion and logic operations. However, traditional PELGs with fixed architectures can realize only very limited logic functions with relatively low on–off ratios. We present a self-driving polarized photodetector driven by the Dember effect, which yields ambipolar photocurrents through photonic modulation by a nested grating. The ambipolar response is realized by exciting the whispering-gallery mode and localized surface plasmon resonances, which leads to reverse spatial carrier generation and therefore the contrary photocurrent assisted by the Dember effect. We further design a full-functional PELG, which enables all five basic logic functions (“AND”, “OR”, “NOT”, “NAND”, and “NOR”) simultaneously in a single device by using one source and one photodetector only. Such an all-in-one PELG exhibits a strong robustness against structure size, incident wavelength, light power, and half-wave plate modulation, paving a way to the realization of ultracompact high-performance PELGs.

Photoconductive control of high-order localized surface plasmon modes in Au-Si-Au nanodisk stacking

Photoconductive control of high-order localized surface plasmon modes in Au-Si-Au nanodisk stacking

Enhanced detection accuracy and signal-to-noise ratio of surface plasmon resonance based refractive index sensor with the addition of thicker layer of Silicon

The improvement in the sensitivity performance of conventional surface plasmon resonance (SPR) based sensors with the addition of thin layers of various high index dielectrics (e.g. 5 nm–10 nm of Silicon) is well established. This improvement in sensitivity comes at a price of reduced detection accuracy, deteriorated signal-to-noise ratio (SNR) and small value of Figure of Merit (FOM). Sensors with high detection accuracy and SNR find applications in the direct detection of small molecular (few hundreds of Daltons) interactions or low molecular concentrations (physiological concentration) on the surface of the sensor. The current paper presents design methodology and theoretical analysis of the performance of SPR based refractive index sensor with enhanced SNR, detection accuracy, and FOM attained when a thicker layer of Silicon (60 nm–400 nm thickness) is introduced in the conventional Kretschmann configuration employing angular interrogation technique. The analysis is valid for any high index dielectric layer that is used to enhance the electromagnetic field associated with the surface plasmon (SP) mode excited at the metal-dielectric interface. In the present paper, we show that the SP mode which participates in the sensing applications can be excited at the metal-dielectric interface for multiple thicknesses of Silicon. Analytical expressions are derived to compute the multiple thicknesses of Silicon using a modal analysis as well as the transfer matrix method (TMM) by considering the SPR sensor as a multi-layered optical waveguide structure. The refractive index SPR sensor with thicker silicon layer is shown to exhibit a long propagation length and large penetration depth in the analyte region, resulting in a higher depth-to-width ratio (defined as the SNR) and detection accuracy in its SPR reflectivity spectrum, hence enhancing the measurement precision of the sensor.

Structure-independent flat bands induced by discontinuity plasma-air interface in photonic crystals

The band structures of transverse electric (TE) polarized dispersive photonic crystals are generally more complicated than those of transverse magnetic (TM) polarized photonic crystals. Here, by simplifying the governing equations of a TE polarized dispersion system and conducting the band structure analysis, we deduce the Hermitian eigenstates of the vector electric field. The results exhibit the double degeneracy eigenstates in the triangular lattice for lower eigenfrequencies and numerous plasmon-induced flat band modes in the interface of the plasma and air for higher eigenfrequencies. Moreover, we illustrate that the surface plasmon modes tend to present the quantized eigenfrequencies, which is due to the boundary condition imposed by the cylindrical geometry. However, when the eigenfrequency approaches the surface plasma frequency, the localization of the field on the boundary becomes stronger, which weakens the coupling between sites and reduces the dependence of the flat bands on the lattice structure. Unlike the generation mechanism of the lattice-induced flat bands, such plasma frequency-dependent localization at the interface enables a robust flat band characteristic immune to the lattice disorder and provides a novel degree of freedom to control the energy band. Our findings are expected to be useful for electromagnetic wave manipulation and field enhancement.

Excitation of surface plasmon mode in bulk semiconductor lasers.

We propose a realistic process for the excitation of surface plasmon polariton (SPP) modes in a silicon photonic waveguide (WG). The process involves the placement of buried oxide (BOX) composed of silica between a WG and silicon substrate. When the BOX thickness is manipulated, different amounts of modal power leak toward the BOX into the substrate and simultaneously acquire compensation from a semiconductor located on the WG. The compensation related to the leakage can be used to infer transparency gain. Similar to the case for a semiconductor laser cavity, the lowest transparency gain among WG modes can be favored; thus, only one mode can survive in the WG, and it is in the region with the specified BOX thickness. Finally, we propose a credible mechanism suitable for demonstrating the region requirements of the existence of SPP modes.

Numerical evaluation of an external environment effectiveness to the surface plasmon mode in Au slot waveguide for design the sensor simulation

The effective permittivity of gold thin films with an external thin dielectric layer on the top and core of the slot waveguide was evaluated as an effective compound semi-metal layer of the surface plasmonic polariton slot waveguide that will vary due to some environmental factors. By uses simple parallel and series permittivity of compound material, this value need for the computation of the effective propagating vector βsp. This takes for a sensor model designing which the external environment effectiveness was changed by the surface plasmonic mode in its slot waveguide. So the resonance length and grating period are evaluated for the sensor model in the first. Further, the propagation of the SPP surface wave mode along the interface between the compound cladding and Si dielectric waveguide is computed, while the effective refractive index of the compound cladding of the slot waveguide is increased from 1 − 1.50. The computation results showed that the TM0 mode power transfer and propagation length seem to vary with this effective refractive index of the compound cladding on the surface plasmonic polariton waveguide. The 140 nm gap width and 3.70 mm long waveguide is also simulated by the finite different time domain method in the optiwave program, where the TM0 mode transition output is corrected, while the refractive index of the environment layer is changed. The results of the simulation showed good agreement with the computation in that the TM0 mode depends on the external refractive of index and input wavelength. This result shows that loss peak ships vary with the external refractive index thin film layer and input wavelength. Thus, they seem to be useful for sensor applications such as the solution concentration analytic.

Voltage Modulation of Nanoplasmonic Metal Luminescence from Nano-Optoelectrodes in Electrolytes.

Metallic nanostructures supporting surface plasmon modes can concentrate optical fields and enhance luminescence processes from the metal surface at plasmonic hotspots. Such nanoplasmonic metal luminescence contributes to the spectral background in surface-enhanced Raman spectroscopy (SERS) measurements and is helpful in bioimaging, nanothermometry and chemical reaction monitoring applications. Although there is growing interest in nanoplasmonic metal luminescence, its dependence on voltage modulation has received limited attention in research investigations. Also, the hyphenated electrochemical surface-enhanced Raman spectroscopy (EC-SERS) technique typically ignores voltage-dependent spectral background information associated with nanoplasmonic metal luminescence due to limited mechanistic understanding and poor measurement reproducibility. Here, we report a combined experiment and theory study on dynamic voltage-modulated nanoplasmonic metal luminescence from hotspots at the electrode-electrolyte interface using multiresonant nanolaminate nano-optoelectrode arrays. Our EC-SERS measurements under 785 nm continuous wavelength laser excitation demonstrate that short-wavenumber nanoplasmonic metal luminescence associated with plasmon-enhanced electronic Raman scattering (PE-ERS) exhibits a negative voltage modulation slope (up to ≈30% V-1) in physiological ionic solutions. Furthermore, we have developed a phenomenological model to intuitively capture the plasmonic, electronic, and ionic characteristics at the metal-electrolyte interface to understand the observed dependence of the PE-ERS voltage modulation slope on voltage polarization and ionic strength. The current work represents a critical step toward the general application of nanoplasmonic metal luminescence signals in optical voltage biosensing, hybrid optical-electrical signal transduction, and interfacial electrochemical monitoring.

Plasmon mediated extra-ordinary optical transmission through an apertureless plasmonic metagrating

Extra-ordinary optical transmission (EOT) through subwavelength plasmonic nanoapertures is possible due to the funneling of light via surface plasmons (SPs) at the resonant wavelengths through the apertures. In this Letter, we experimentally demonstrate EOT through a plasmonic metagrating which does not have any open apertures. The plasmonic metagrating was fabricated by deposition of silver (Ag) on a one-dimensionally patterned flexible and transparent polydimethylsiloxane grating obtained via pattern imprinting and subsequent peeling off a commercially available blue ray disk. For normal incidence of transverse magnetic-polarized light on the top surface of plasmonic metagrating, transmission of light through it was obtained in the visible wavelength range of electromagnetic spectrum. Control experiments on variation of Ag film thickness were performed to attain optimal parameters for maximum transmission, followed by polarization and refractive index (RI) dependent performance of the plasmonic metagrating. Electric fields and Poynting vector profiles were simulated using a finite element method to explain the interaction of light with the plasmonic metagrating and the mechanism of plasmon mediated optical transmission. Such a large optical transmission is possible because the SP modes generated at metal–air interface penetrate through metagrating and couple with those supported by the metal–substrate interface. As a model application, RI sensing using the plasmonic metagrating was demonstrated. The present study shows that optical transmission is possible from apertureless structures and enriches literature with better understanding of EOT. Moreover, it opens avenues for development of flexible, cost-effective plasmonic metagratings for sensors, spectral filters, polarizers, etc.

Surface plasmon enhanced fluorescence: self-consistent classical treatment in the quasi-static limit

The problem of enhanced molecular emission in close proximity to dielectric and metallic interfaces is of great importance for many physical and biological applications. Here we present an exact treatment of the problem from the view point of classical electromagnetism. Self-consistent analytical theory of the surface enhanced fluorescence (SEF) is developed for configurations consisting of an emitter in proximity to core–shell metal-dielectric nanoparticles. The dependence of the fluorescence enhancement on the excitation laser and fluorescence frequencies and distance of the emitter to the nanoparticle interface are studied. The developed theory predicts enhanced fluorescence at intermediate distances as well as emission quenching into non-radiative surface plasmon (SP) modes dominating the response for short distances. The conditions for optimal emission enhancement for two core–shell configurations are determined and a comparison to published experimental data is performed showing a good correspondence between theory and experiment. The developed model can be applied toward analyzes and optimizations of various applications related to SP enhance fluorescence spectroscopy.

Tunable Layered Gold Nanochips for High Sensitivity and Uniformity in SERS Detection

Reasonable design and controllable fabrication of plasmonic substrates are the keys to achieving surface-enhanced Raman spectroscopy (SERS) detection with high sensitivity and uniformity. The coupling of the two main modes of surface plasmon, the localized surface plasmon resonance (LSPR) mode and the surface plasmon polariton (SPP) mode, has been proven to be an appealing way to obtain excellent SERS responses. Nevertheless, it remains an obstacle to further improve the LSPR-SPP coupling efficiency due to the dramatic nonuniformity of the nanostructure obtained by traditional wet chemical methods. Here, we report a controllable electron-beam lithography (EBL) process for the fabrication of layered Au nanochips. For the optimized Au CFN (cap and film nanostructure) structure, benefiting from the remarkable electric field enhancement induced by the effective LSPR-SPP coupling at the junction of the Au cap and Au film structure, an improved SERS performance with an enhancement factor (EF) of 2.8 × 106 and a low detection limit of 10–13 M are achieved using 4-nitrothiophenol (4-NTP) as the probe molecule. In addition, the Au CFN substrate exhibits a high uniformity over a large area. Such a new strategy for the design of plasmonic substrates with superior LSPR-SPP coupling can be applied in various SERS detection scenarios.

Simulation of a Self-Referenced Meta-Grating Sensor With High Figure of Merit in NIR Communication Window

A meta-grating-based surface plasmon resonance (SPR) sensor was theoretically investigated to act as a self-referenced sensor in the near-infrared region using a spectral interrogation technique. Introduction of a high refractive index (RI) dielectric material, silicon (Si) oxynitride (SiON) film, in between the dielectric metasurface and metal layer gives rise to a self-referencing operation in the sensor. The Si meta-grating excites the SPR at the metal–analyte interface which is sensitive to the changes in the RI of the analyte and the surface plasmon (SP) mode at the metal–dielectric interface remains fixed. The geometrical parameters of the proposed sensor were optimized to obtain the self-referencing operation with superior sensing performance. The self-referencing behavior considerably reduces the error contributions due to an unsteady ambience. The maximum sensitivity of this sensor has been achieved upto 1150 nm/RIU with the figure of merit (FOM) as high as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$442.31\,\,\text {RIU}^{-{1}}$ </tex-math></inline-formula> .