Toward broadband plasmonics: tuning dispersion in rhombic plasmonic crystals

Ligand-Mediated Control of the Surface Oxidation States of Copper Nanoparticles Produced by Laser Ablation

We report on studies that demonstrate how the chemical composition of the surface of copper nanoparticles (CuNPs) - in terms of percentage copper(I/II) oxides - can be varied by the presence of N-donor ligands during their formation via laser ablation. Changing the chemical composition thus allows systematic tuning of the surface plasmon resonance (SPR) transition. The trialed ligands include pyridines, tetrazoles, and alkylated tetrazoles. CuNPs formed in the presence of pyridines, and alkylated tetrazoles exhibit a SPR transition only slightly blue shifted with respect to CuNPs formed in the absence of any ligand. On the other hand, the presence of tetrazoles results in CuNPs characterized by a significant blue shift of the order of 50-70 nm. By comparing these data also with the SPR of CuNPs formed in the presence of carboxylic acids and hydrazine, this work demonstrates that the blue shift in the SPR is due to tetrazolate anions providing a reducing environment to the nascent CuNPs, thus preventing the formation of copper(II) oxides. This conclusion is further supported by the fact that both AFM and TEM data indicate only small variations in the size of the nanoparticles, which is not enough to justify a 50-70 nm blue-shift of the SPR transition. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) studies further confirm the absence of Cu(II)-containing CuNPs when prepared in the presence of tetrazolate anions.

Bloch-Surface Plasmon Polariton Enhanced Amplified and Directional Spontaneous Emission from Plasmonic Hexagonal Nanohole Array

The light-matter interactions at nanoscale can be enhanced by Bloch-surface plasmon polariton (Bloch-SPP) on the plasmonic lattice. An Ag nanohole array in hexagonal arrangement served as an optical cavity to realize the directional and polarized amplified spontaneous emission (ASE) of R6G. A 100-fold enhanced ASE was observed at 15° emission angle under TM polarization when the pump power density exceeded the threshold of 198 W/cm² based on the degenerated high state density modes. Moreover, a specific polarization dependence of ASE was modulated by the Bloch-SPP modes, and the degree of polarization was enhanced from 1.3 to 2.1 when the pump power density exceeded the threshold of ASE. This work clarifies the interaction between the gain media and plasmonic systems, which lays a foundation for the plasmonic device designing.

Near Perfect Absorber for Long-Wave Infrared Based on Localized Surface Plasmon Resonance

In recent years, broadband absorbers in the long-wave infrared (LWIR) spectrum have shown great scientific value and advantages in some areas, such as thermal imaging and radiation modulation. However, designing a broadband absorber with an ultra-high absorption rate has always been a challenge. In this paper, we design a near perfect absorber that is highly tunable, angle insensitive, and has polarization independence for LWIR. By using multi-mode localized surface plasmon resonance (LSPR) of a surface metal structure, the absorber achieves a very high absorption average of 99.7% in wavelengths from 9.7 μm to 12.0 μm. For incident light, the meta-structure absorber exhibits excellent polarization independence. When the incident angle increases from 0° up to 60°, the absorption rate maintains over 85%. By modulating the size of the structure, the meta-structure absorber can also achieve a high absorption rate of 95.6%, covering the entire LWIR band (8-14 μm in wavelength). This meta-structure absorber has application prospects in infrared detecting, infrared camouflage, radiation cooling, and other fields.

Scalable two-tier protruding micro-/nano-optoelectrode arrays with hybrid optical-electrical modalities by hierarchical modular design

In situ spatiotemporal characterization of correlated bioelectrical and biochemical processes in living multicellular systems remains a formidable challenge but can offer crucial opportunities in biology and medicine. A promising approach is to develop bio-interfaced multifunctional micro-/nano-sensor arrays with complementary biophotonic-bioelectronic modalities and biomimetic topology to achieve combined bioelectrical and biochemical detection and tight device-cell coupling. However, a system-level engineering strategy is still missing to create multifunctional micro-/nano-sensor arrays that meet the multifaceted design requirements for in situ spatiotemporal characterizations of living systems. Here, we demonstrate a hierarchical modular design and fabrication approach to develop scalable two-tier protruding micro-/nano-optoelectrode arrays that extend the design space of biomimetic micro-/nano-pillar topology, plasmonic nanoantenna-based biophotonic function in surface-enhanced Raman spectroscopy (SERS), and micro-/nano-electrode-based bioelectronics function in electrochemical impedance spectroscopy (EIS). Notably, two-tier protruding micro-/nano-optoelectrode arrays composed of nanolaminate nanoantenna arrays on top of micropillar electrode arrays can support plasmonic nanocavity modes with high SERS enhancement factors (≈10⁶) and large surface-to-volume ratio with significantly reduced interfacial impedance in EIS measurements. We envision that scalable two-tier protruding micro-/nano-optoelectrode arrays can potentially serve as bio-interfaced multifunctional micro-/nano-sensor arrays for in situ correlated spatiotemporal bioelectrical-biochemical measurements of living multicellular systems such as neuronal network cultures, cancerous organoids, and microbial biofilms.

Boosting Long-Range Surface-Enhanced Raman Scattering on Plasmonic Nanohole Arrays for Ultrasensitive Detection of MiRNA

A fundamental challenge, particularly, in surface-enhanced Raman scattering (SERS) analysis is the detection of analytes that are distant from the sensing surface. To tackle this challenge, we herein report a long-range SERS (LR-SERS) substrate supporting an extension of electric field afforded by long-range surface plasmon resonance (LRSPR) excited in symmetrical dielectric environments. The LR-SERS substrate has a sandwich configuration with a triangle-shaped gold nanohole array embedded between two dielectrics with similar refractive indices (i.e., MgF₂ and water). The finite-difference time-domain simulation was applied to guide the design of the LR-SERS substrate, which was engineered to have a wavelength-matched LRSPR with 785 nm excitation. The simulations predict that the LR-SERS substrate exhibits great SERS enhancement at distances of more than 10 nm beyond its top surface, and the enhancement factor (E_F) has been improved by three orders of magnitude on LR-SERS substrates compared to that on conventional substrates. The experimental results show good agreement with the simulations, an E_F of 4.1 × 10⁵ remains available at 22 nm above the LR-SERS substrate surface. The LR-SERS substrate was further applied as a sensing platform to detect microRNA (miRNA) let-7a coupled with a hybridization chain reaction (HCR) strategy. The developed sensor displays a wide linear range from 10 aM to 1 nM and an ultralow detection limit of 8.5 aM, making it the most sensitive among the current detection strategies for miRNAs based on the SERS-HCR combination to the best of our knowledge.

Nanostructured Color Filters: A Review of Recent Developments

Color plays an important role in human life: without it life would be dull and monochromatic. Printing color with distinct characteristics, like hue, brightness and saturation, and high resolution, are the main characteristic of image sensing devices. A flexible design of color filter is also desired for angle insensitivity and independence of direction of polarization of incident light. Furthermore, it is important that the designed filter be compatible with the image sensing devices in terms of technology and size. Therefore, color filter requires special care in its design, operation and integration. In this paper, we present a comprehensive review of nanostructured color filter designs described to date and evaluate them in terms of their performance.

Scratch on Polymer Materials Using AFM Tip-Based Approach: A Review

As a brand new nanomachining method, the tip-based nanomachining/nanoscratching (TBN) method has exhibited a powerful ability at machining on polymer materials and various structures have been achieved using this approach, ranging from the nanodot, nanogroove/channel, bundle to 2D/3D (three-dimensional) nanostructures. The TBN method is widely used due to its high precision, ease of use and low environmental requirements. First, the theoretical models of machining on polymer materials with a given tip using the TBN method are presented. Second, advances of nanostructures achieved by this method are given, including nanodots/nanodot arrays, a nanogroove/channel, 2D/3D nanostructures and bundles. In particular, a useful approach called the ultrasonic vibration-assisted method introduced to integrate with TBN method to reduce the wear of the tip is also reviewed, respectively. Third, the typical applications of the TBN method and the nanostructures achieved by it are summarized in detail. Finally, the existing shortcomings and future prospects of the TBN method are given. It is confirmed that this review will be helpful in learning about this method and push the technology toward industrialization.

Polarization-Dependent Lasing Behavior from Low-Symmetry Nanocavity Arrays

This paper reports how geometric effects in low-symmetry plasmonic nanoparticle arrays can produce polarization-dependent lasing responses. We developed a scalable fabrication procedure to pattern rhombohedral arrays of aluminum anisotropic nanoparticles that support lattice plasmon modes from both first-order and second-order diffraction coupling. We found that nanoparticle shape can be used to engineer the spatial overlap between electromagnetic hot spots of different lattice modes and dye gain to support plasmonic lasing. The lasing behavior revealed that plasmon-exciton energy transfer depends on polarization, with stronger coupling and faster dynamics when the transition dipole moments of the excited gain are aligned with the electric field of the plasmon modes.

Spin-locking metasurface for surface plasmon routing

Nanophotonic circuitry requires an ability to externally control and analyze optical signals tightly confined in subwavelength volumes. Various schemes of surface plasmon (SP) routing have been presented using active and passive metasurfaces. One of the most appealing approaches is the use of plasmonic spin-orbit interaction where the incident light spin state is efficiently coupled to an orbital degree of freedom of the surface wave. Recently, a major attention has been drawn to an additional plasmonic degree of freedom - the transverse spin and some application for near-field plasmonic manipulations have been presented. Here we propose a spin-locking metasurface incorporating a transverse spin of the SP wave to selectively route the near-field beams. Owing to the combination of the oblique incidence of circularly polarized light with the accurately designed momentum matching of the grating we achieve a precise directional control over the plasmonic distributions. The experimental verification of the directional launching is performed by a time-resolved leakage radiation measurements allowing one to visualize the shape and the dynamics of the excited beam.

Fabrication of arrayed triangular micro-cavities for SERS substrates using the force modulated indention process

Based on the tip-based continuous indentation process, a novel method for the fabrication of periodic arrayed triangular micro-cavities on copper (Cu) surface is presented as SERS substrates.

Size dependence of band structures in a two-dimensional plasmonic crystal with a square lattice

A scanning transmission electron microscope (STEM) -cathodoluminescence (CL) technique is used to investigate the size dependence of the band structures in two-dimensional plasmonic crystals with a square lattice (SQ-PlCs) composed of cylindrical pillars and holes. The experimentally determined and calculated dependences of the band edge energies of the three SPP modes at the Γ point on the diameter of the cylindrical structure agree well. The photon maps reveal the field strength distributions of the standing SPP waves of the three eigenmodes. Additionally, a mechanism is proposed to explain the dependence of the contrast on the detected light polarization.

Quasiperiodic moiré plasmonic crystals

This paper describes the properties of silver plasmonic crystals with quasiperiodic rotational symmetries. Compared to periodic plasmonic crystals, quasiperiodic moiré structures exhibited an increased number of surface plasmon polariton modes, especially at high angles of excitation. In addition, plasmonic band gaps were often formed at the intersections of these new modes. To identify the origin and predict the location of the band gaps, we developed a Bragg-based indexing system using the reciprocal lattice vectors of the moiré plasmonic crystals. We showed that even more complicated quasiperiodic geometries could also be described by this indexing model. We anticipate that these quasiperiodic lattices will be useful for applications that require the concentration and manipulation of light over a broadband spectrum.

Highly efficient broadband ultrafast plasmonics

To date, considerable experimental and theoretical focus has been placed on the spatial control of Surface Plasmon Polaritons (SPPs) using nanostructured surfaces; however, research aimed toward accessing the ultrafast dynamics of SPPs remains vastly unexplored. Despite this, SPPs have the potential to exhibit some of the fastest possible optical processes, while maintaining the advantage of nanoscale spatial manipulation. Here, we present an experimental and computational investigation of a system that provides access to the efficient excitation of broadband, propagating SPP modes. To achieve this, a surface array of tailor designed, reduced symmetry nanostructures has been fabricated to enable the required control of the plasmon dispersion map to match sub 20 fs pulses in the near infra-red. Using a combination of optical spectroscopy and frequency resolved optical gating techniques, complimented by finite element computational analysis, the efficient excitation of propagating broadband plasmonic modes is demonstrated.

Large-Scale Fabrication of Three-Dimensional Surface Patterns Using Template-Defined Electrochemical Deposition

A new strategy to achieve large-scale, three-dimensional (3D) micro- and nanostructured surface patterns through selective electrochemical growth on monolayer colloidal crystal (MCC) templates is reported. This method can effectively create large-area (>1 cm2), 3D surface patterns with well-defined structures in a cost-effective and time-saving manner (<30 min). A variety of 3D surface patterns, including semishells, Janus particles, microcups, and mushroom-like clusters, is generated. Most importantly, our method can be used to prepare surface patterns with prescribed compositions, such as metals, metal oxides, organic materials, or composites (e.g., metal/metal oxide, metal/polymer). The 3D surface patterns produced by our method can be valuable in a wide range of applications, such as biosensing, data storage, and plasmonics. In a proof-of-concept study, we investigated, both experimentally and theoretically, the surface-enhanced Raman scattering (SERS) performance of the fabricated silver 3D semishell arrays.

Angle-insensitive structural colours based on metallic nanocavities and coloured pixels beyond the diffraction limit

To move beyond colorant-based pigmentation display technologies, a variety of photonic and plasmonic crystal based structures have been designed and applied as colour filters. Nanostructure based colour filtering offers increased efficiencies, low power consumption, slim dimensions, and enhanced resolution. However, incident angle tolerance still needs to be improved. In this work, we propose a new scheme through localized resonance in metallic nanoslits by light funneling. Angle insensitive colour filters up to ±80 degrees have been achieved, capable of wide colour tunability across the entire visible band with pixel size beyond the diffraction limit (~λ/2). This work opens the door to angle insensitive manipulation of light with structural filtering.

X-shaped quasi-3D plasmonic nanostructure arrays for enhancing electric field and Raman scattering

We propose and demonstrate strongly enhancing electric field and Raman scattering with a large tolerance to the light incident angle and polarization by using x-shaped quasi-3D plasmonic nanostructure arrays (X-Q3D-PNAs). The finite-difference time-domain simulations were used to study the reflectance spectra and electric field profiles of X-Q3D-PNAs. Results show that both surface plasmon polaritons and localized surface plasmon polaritons (LSPPs) can be generated at the metal/dielectric interfaces of the top gold thin film with square grating x-shaped nanoholes. The resonance of the LSPPs generated at the gold islands formed between x-shaped nanoholes at the top gold thin film greatly enhance the electric fields at the tips of the cross-sectors of the x-shaped nanoholes. Both plasmon resonances and electric field enhancements are affected by the structural dimensions. The strong electric field enhancement and the large tolerance to the laser polarization were demonstrated by surface-enhanced Raman scattering experiments. This unique plasmonic property of X-Q3D-PNAs could be attractive for photovoltaics and biosensing applications.

Eutectic liquid alloys for plasmonics: theory and experiment

We report a method based on density functional theory molecular dynamics that allows us to calculate the plasmonic properties of liquid metals and metal alloys from first principles with no a priori knowledge of the system. We show exceptional agreement between the simulated and measured optical constants of liquid Ga and the room temperature liquid In-Ga eutectic alloy (T(m) = 289 K). We then use this method to analyze the plasmonic properties of various alloy concentrations in the In-Ga system. The plasmonic performance of the In-Ga system decreases with increasing In concentration. However, the benefits of a room-temperature plasmonic liquid are likely to outweigh the minor reduction in plasmonic performance when moving from pure Ga to the eutectic composition. Our results show that density functional theory molecular dynamics can be used as a predictive tool for studying the optical properties of liquid metal systems amenable to plasmonics.

Polarization-dependent multipolar plasmon resonances in anisotropic multiscale au particles

This paper reports the fabrication and characterization of three-dimensional (3D) multiscale Au particles with different aspect ratios. Increasing the length of the particles resulted in excitation of a longitudinal mode and two different transverse modes having different multipolar orders. The multipolar orders increased for both longitudinal and transverse modes as the aspect ratio increased. Finite-difference time-domain calculations revealed that the structural asymmetry of the 3D anisotropic particles were the reason for the two distinct transverse plasmon resonances. When the 3D structural change occurred at the ends of the multiscale particle, however, the optical response showed two resonances in the longitudinal direction and only a single resonance in the transverse direction.

Extraordinary nonlinear absorption in 3D bowtie nanoantennas

This paper reports that arrays of three-dimensional (3D), bowtie-shaped Au nanoparticle dimers can exhibit extremely high nonlinear absorption. Near-field interactions across the gap of the 3D bowties at the localized surface plasmon resonance wavelengths resulted in an increase of more than 4 orders of magnitude in local field intensity. The imaginary part of the third-order nonlinear susceptibility (Im χ((3))) for the 3D bowtie arrays embedded in a dielectric material was measured to be 10(-4) esu, more than 2 orders of magnitude higher than reported for other metal nanoparticle-dielectric composites. Moreover, 3D dimers with increased nanoscale structure (such as folding) exhibited increased optical nonlinearity. These 3D nanoantennas can be used as critical elements for nanoscale nonlinear optical devices.

Isotropic broadband absorption by a macroscopic self-organized plasmonic crystal

We describe the plasmonic properties of a two-dimensional periodic metallic grating of macroscopic size obtained by gold deposition on a self-assembled silica opal. Structural characterization shows a transition from microscopic order to isotropy at macroscopic scale. Optical reflection spectra exhibit a dip of almost complete absorption due to coupling to surface-plasmon-polaritons (SPP). This is explained by theoretical calculations introducing a density of coupled SPP modes. We demonstrate, at a given incidence angle, a broad continuum of coupled wavelengths over the visible spectrum. This opens new possibilities in fields where light-plasmon coupling is required over a broad range of wavelengths and incidence orientations.