Plasmonic Particles Research Articles

Electrothermoplasmonic flow in gold nanoparticles suspensions: Nonlinear dependence of flow velocity on aggregate concentration

Efficient mixing and pumping of liquids at the microscale is a technology that is still to be optimized. The combination of an AC electric field with a small temperature gradient leads to a strong electrothermal flow that can be used for multiple purposes. Combining simulations and experiments, an analysis of the performance of electrothermal flow is provided when the temperature gradient is generated by illuminating plasmonic nanoparticles in suspension with a near-resonance laser. Fluid flow is measured by tracking the velocity of fluorescent tracer microparticles in suspension as a function of the electric field, laser power, and concentration of plasmonic particles. Among other results, a non-linear relationship is found between the velocity of the fluid and particle concentration, which is justified in terms of multiple scattering-absorption events, involving aggregates of nanoparticles, that lead to enhanced absorption when the concentration is raised. Simulations provide a description of the phenomenon that is compatible with experiments and constitute a way to understand and estimate the absorption and scattering cross-sections of both dispersed particles and/or aggregates. A comparison of experiments and simulations suggests that there is some aggregation of the gold nanoparticles by forming clusters of about 2–7 particles, but no information about their structure can be obtained without further theoretical and experimental developments. This nonlinear behavior could be useful to get very high ETP velocities by inducing some controlled aggregation of the particles.

Plasmonic Bowl-Shaped Nanopore for Raman Detection of Single DNA Molecules in Flow-Through.

Plasmonic nanopores combined with Raman spectroscopy are emerging as platforms for single-molecule detection and sequencing in label-free mode. Recently, the ability of identifying single DNA bases or amino acids has been demonstrated for molecules adsorbed on plasmonic particles and then delivered into the plasmonic pores. Here, we report on bowl-shaped plasmonic gold nanopores capable of direct Raman detection of single λ-DNA molecules in a flow-through scheme. The bowl shape enables the incident laser to be focused into the nanopore to generate a single intense hot spot with no cut off in pore size. Therefore, we achieved ultrasmall focusing of NIR light in a spot of 3 nm. This enabled us to detect 7 consecutive bases along the DNA chain in flow-through conditions. Furthermore, we found a novel electrofluidic mechanism to manipulate the molecular trajectory within the pore volume so that the molecule is pushed toward the hot spot, thus improving the detection efficiency.

Defect Damping-Enhanced Plasmonic Photothermal Conversion.

Significantly increasing the photothermal conversion of plasmonic nanostructured particles (PNPs) is a common goal for all applications of thermoplasmonics, but it is still in challenge, especially for PNPs with the morphology and composition required for a specific photothermal application. Here, we present a concept of defect-induced damping-enhanced photothermal conversion, which favors PNP intrinsic properties. A model of a defect-damped harmonic oscillator is established to depict photothermal conversion correlation with the structure of PNPs and is capable of accurately reproducing the optical performance of the PNPs with the local surface plasmon resonance far from the interband transition. The theoretical model analyses demonstrate that the defect-induced damping can significantly suppress the light scattering of the PNPs and effectively improve their photothermal conversion efficiency. Especially for the PNPs with a sufficiently large size (larger than ∼100 nm for Au and Ag), we show that defect-induced damping can significantly enhance their light absorption and photothermal performances. These are experimentally confirmed. Typically, defect-enriched Au nanostars with ∼100-150 nm profile size were fabricated and showed much higher photothermal performance and a big increment by 23% in photothermal conversion efficiency, compared with the normal (or defect-impoverished) counterpart. Furthermore, the in vitro and in vivo biological experiments demonstrate that this defect-enriched PNP can indeed exhibit significantly higher photothermal performance than the normal counterpart in cells and mouse tumors, which confirms the validity of the presented strategy in typical practical applications. This work provides a strategy to intrinsically and significantly enhance plasmonic photothermal conversion of PNPs with a sufficiently large size, which is not only suitable for PNPs with the morphology and composition required for specific applications but also can be combined with existing strategies to further increase their photothermal performance.

Low-frequency noise in Au-decorated graphene–Si Schottky barrier diode at selected ambient gases

We report results of the current–voltage characteristics and low-frequency noise in Au nanoparticle (AuNP)-decorated graphene–Si Schottky barrier diodes. Measurements were conducted in ambient air with addition of either of two organic vapors, tetrahydrofuran [(CH2)4O; THF] and chloroform (CHCl3), as also during yellow light illumination (592 nm), close to the measured particle plasmon polariton frequency of the Au nanoparticle layer. We observed a shift of the DC characteristics at forward voltages (forward resistance region) when tetrahydrofuran vapor was admitted (in a Au-decorated graphene–Si Schottky diode), and a tiny shift under yellow irradiation when chloroform was added (in not decorated graphene–Si Schottky diode). Significantly larger difference in the low-frequency noise was observed for the two gases during yellow light irradiation, compared with no illumination. The noise intensity was suppressed by AuNPs when compared with noise in graphene–Si Schottky diode without an AuNP layer. We conclude that flicker noise generated in the investigated Au-decorated Schottky diodes can be utilized for gas detection.

Yb3+ NIR emission enhancement in dichroic Cu-containing glass for solar spectral conversion

Ytterbium and copper co-doped phosphate glass containing plasmonic Cu particles showing dichroism was synthesized and the impact on Yb3+ near-infrared (NIR) emission evaluated. While the co-doped precursor glass exhibited ultraviolet to NIR luminescence down-shifting owing to copper(I), the dichroic glass displayed an enhanced NIR emission which is reported herein for the first time. It was found most favorable for Cu+ excitation around 360 nm. Scattering by Cu particles is suggested to produce the emission boost attractive for solar cell applications.

Enhanced Two-Dimensional Exciton Propagation via Strong Light–Matter Coupling with Surface Lattice Plasmons

Exciton–polaritons─hybrid exciton–photon quasi-particles─have enabled exciting long-range coherent transport by taking advantage of the properties of their photonic component. However, most experimental demonstrations of strong coupling have been based on semiconductor Fabry–Pérot microcavities. Here, we report an open and versatile exciton–polariton platform by integrating two-dimensional lead halide perovskites with plasmonic nanoparticle arrays, which support hybridization between excitons and surface lattice resonances in the strong coupling regime. Benefitting from the open architecture, we directly visualized polariton-enhanced exciton transport via angle-dependent transient absorption microscopy measurements. The observed transport enhancement extends into the picosecond timescale, leading to more than 1 order of magnitude improvement of exciton migration, from ∼10 nm to 100s of nanometers. We proposed a polariton-mediated exciton transport model and obtained an average polariton velocity of ∼1 × 106 m/s for states with an ∼40% photonic fraction. These results demonstrate plasmonic particle arrays as a versatile and adaptable cavity to enhance exciton transport.

Colloidal Assemblies of Chiral Plasmonic Nanoparticles Induce Tunable Circular Dichroism Response

AbstractPlasmonic nanoparticles with strong chiral optical activity are promising systems for a wide range of applications. Self‐assembly of achiral metallic nanoparticles thanks to chiral molecules or templates is a well‐known strategy to prepare chiral plasmonic nanomaterials. In this work, the assembly of chiral plasmonic particles without using a chiral templating agent is investigated. For that purpose, previously developed chiral core‐shell Au@Ag and Au@Au particles are assembled in a controlled way by lowering the surfactant concentration in solution. The assembly leads to pseudo‐linear structures that are revealed by electron microscopy, and the plasmonic responses are characterized by circular dichroism (CD) and UV‐Vis‐NIR spectroscopies. The chiral optical response strongly depends on the system and assembly conditions. For Au@Ag, the CD signal is tuned with contributions appearing in the red region of the spectrum, while for Au@Au, the CD response is unexpectedly lost. The origin of this difference remains undetermined, it is hypothesized that the coupling of circular dichroism modes or the generation of chiral superstructures may influence the chiroptical properties. These experimental results highlight the non‐obvious and complex nature of the chirality generation mechanisms in such plasmonic systems.

Exciting-frequency-adaptive amplitude/phase hybrid holographic inscription in plasmonic polymers.

Plasmonic holography is generally regarded as an effective technology for 3D display that meets the requirements of the human visual system. However, low readout stability and large cross talk in the frequency field during a plasmonic photo-dissolution reaction set a huge obstacle for application of color holography. Herein, we propose a new, to the best of our knowledge, route toward producing exciting frequency sensitive holographic-inscription based on plasmonic nano-Ag adaptive growth. Donor-molecule-doped plasmonic polymers on polyethylene terephthalate substrates exhibit wide spectral response range, accurate optical frequency sensing, and bending durability. The resonant plasmonic particles act as optical antennas and transfer energy to surrounding organic matrices for nanocluster production and non-resonant particle growth. The surface relief hologram is also highly dependent on the excitation frequency, so we successfully obtain a controllable cross-periodic structure with amplitude/phase mixed information, as well as color holographic display. This work provides a bright way to high-density storage, information steganography, and virtual/augmented reality.

One-Shot Laser-Pulse Modification of Bare and Silica-Coated Gold Nanoparticles of Various Morphologies.

Gold nanoparticles are widely used in laser biomedical applications due to their favorable properties, mainly localized plasmon resonance. However, laser radiation can cause a change in the shape and size of plasmonic nanoparticles, thus resulting in an unwanted reduction of their photothermal and photodynamic efficiency due to a drastic alteration of optical properties. Most previously reported experiments were carried out with bulk colloids where different particles were irradiated by different numbers of laser pulses, thus making it difficult to accurately evaluate the laser power photomodification (PM) threshold. Here, we examine the one-shot nanosecond laser-pulse PM of bare and silica-coated gold nanoparticles moving in a capillary flow. Four types of gold nanoparticles, including nanostars, nanoantennas, nanorods, and SiO2@Au nanoshells, were fabricated for PM experiments. To evaluate the changes in the particle morphology under laser irradiation, we combine measurements of extinction spectra with electron microscopy. A quantitative spectral approach is developed to characterize the laser power PM threshold in terms of normalized extinction parameters. The experimentally determined PM threshold increases in series were as follows: nanorods, nanoantennas, nanoshells, and nanostars. An important observation is that even a thin silica shell significantly increases the photostability of gold nanorods. The developed methods and reported findings can be useful for the optimal design of plasmonic particles and laser irradiation parameters in various biomedical applications of functionalized hybrid nanostructures.

Using click reaction to covalently link heterogeneous gold nanorods into head‐to‐head one‐dimensional chains with an alternating sequence as plasmonic Fano resonators

AbstractWe present the deterministic linkage of short and long gold nanorods into one‐dimensional (1D) alternating nanorod chains in a head‐to‐head manner using click reactions. By precisely controlling the surface chemistry, we achieve orientation‐specific and robust covalent linkage between nanorods with high‐product yield. The optical response of the alternating nanorod chains exhibits Fano‐like resonances. This method enables the high‐throughput production of covalently linked plasmonic particle networks consisting of heterogeneous nanoparticles with specifically chosen spectral responses.

Dispersion braiding and band knots in plasmonic arrays with broken symmetries.

Periodic arrays can support highly nontrivial modal dispersion, stemming from the interplay between localized resonances of the array elements and distributed resonances supported by the lattice. Recently, intentional defects in the periodicity, i.e., broken in situ symmetries, have been attracting significant attention as a powerful degree of freedom for dispersion control. Here we explore highly nontrivial dispersion features in the resonant response of linear arrays of plasmonic particles, including the emergence of braiding and band knots caused by band folding. We show that these phenomena can be achieved within simple dipolar arrays for which we can derive closed-form expressions for the dispersion relation. These phenomena showcase powerful opportunities stemming from broken symmetries for extreme dispersion engineering, with a wide range of applications, from plasma physics to topological wave phenomena. Our theoretical model can also be generalized to higher dimensions to explore higher-order symmetries, e.g., glide symmetry and quasi-periodicity.

Photothermal Effect and Phase Transition in VO2 Enhanced by Plasmonic Particles

Phase change metasurfaces based on VO2, which are pre-heated with electric current and optically addressed by projected structured light hologram, are considered to become a new paradigm in programmed THz/middle IR flat optics. Macroscopic quasi-homogeneous arrays of Au nanoparticles show large near IR absorption and a significant photothermal effect capable of boosting a light-triggered switching of VO2 and are to be carefully examined. We propose a new approach to simultaneously probe the altered temperature and electric conductivity of a hybrid Au particle-VO2 film composite by monitoring a phase shift and attenuating a surface acoustic wave in a YX128° cut LiNbO3 substrate. The method shows a temperature resolution of 0.1 °C comparable with the best existing techniques for studying nanoobjects and surfaces. The laser-induced photothermal effects were characterized in a macroscopic array of Au nanostars (AuNSts) with different surface coverage. In a monolayer of 10 nm Au, coupled plasmonic nanoparticles were deposited on the LiNbO3 substrate. An optically triggered insulator-metal transition assisted by photothermal effect in AuNSts/VO2/TiO2/LiNbO3 composites was studied at varied light power. We believe that the proposed SAW-based method is of significant importance for the characterization and optimization of radiation absorbing or/and electrically heated elements of metasurfaces and other devices for lab-on-chip and optical communication/processor technology.

Full dynamic corrections to the Maxwell Garnett mixing formula and corresponding extensions beyond the dipolar approximation

Here we derive a full dynamic extension to the Maxwell Garnett mixing formula for the effective refractive index of a random system of particles that takes proper account of scattering by the particles even for moderate densities. The new formula exhibits the so-called dependent-scattering effects, which are strongest, in relative terms, in colloidal systems with small non-absorbing particles. Then we extend our result to systems of large particles having a multipolar electromagnetic response and offer a new approximation to the effective refractive index for these systems. The resulting formula can also be seen as an extension of the well-known Foldy-Lax approximation through dynamic local-field corrections. This formula reduces to all previously established approximations in the appropriate limits. We compare predictions of this formula for the imaginary part of the effective refractive index with experimental data of colloidal suspensions of non-absorbing particles with sizes comparable to and larger than the wavelength of the incident radiation and find excellent agreement. We provide some new insights into the origin of dependent scattering for systems of very small particles, such as nanofluids in the optical regime, and finally show predictions of the here-derived formulas for the effective refractive index of a system of plasmonic particles, showing redshifts of the resonance which differ from the ones predicted by other approximations.

Learning and Predicting Photonic Responses of Plasmonic Nanoparticle Assemblies via Dual Variational Autoencoders.

The application of machine learning is demonstrated for rapid and accurate extraction of plasmonic particles cluster geometries from hyperspectral image data via a dual variational autoencoder (dual-VAE). In this approach, the information is shared between the latent spaces of two VAEs acting on the particle shape data and spectral data, respectively, but enforcing a common encoding on the shape-spectra pairs. It is shown that this approach can establish the relationship between the geometric characteristics of nanoparticles and their far-field photonic responses, demonstrating that hyperspectral darkfield microscopy can be used to accurately predict the geometry (number of particles, arrangement) of a multiparticle assemblies below the diffraction limit in an automated fashion with high fidelity (for monomers (0.96), dimers (0.86), and trimers (0.58). This approach of building structure-property relationships via shared encoding is universal and should have applications to a broader range of materials science and physics problems in imaging of both molecular and nanomaterial systems.

Dynamic Multi-Mode Mie Model for Gain-Assisted Metal Nano-Spheres

Coupling externally pumped gain materials with plasmonic spherical particles, even in the simplest case of a single spherical nanoparticle in a uniform gain medium, generates an incredibly rich variety of electrodynamic phenomena. The appropriate theoretical description of these systems is dictated by the quantity of the included gain and the size of the nano-particle. On the one hand, when the gain level is below the threshold separating the absorption and the emission regime, a steady-state approach is a rather adequate depiction, yet a time dynamic approach becomes fundamental when this threshold is exceeded. On the other hand, while a quasi-static approximation can be used to model nanoparticles when they are much smaller than the exciting wavelength, a more complete scattering theory is necessary to discuss larger nanoparticles. In this paper, we describe a novel method including a time-dynamical approach to the Mie scattering theory, which is able to account for all the most enticing aspects of the problem without any limitation in the particle's size. Ultimately, although the presented approach does not fully describe the emission regime yet, it does allow us to predict the transient states preceding emission and represents an essential step forward in the direction of a model able to adequately describe the full electromagnetic phenomenology of these systems.

Aspects of the interaction of pectin-coated Ag nanoparticles with methylene blue with regard to photodynamic applications

The promising future of photodynamic therapy has led to progress in the design of highly effective new-generation photosensitizers based on the combination of plasmonic metal nanoparticles and fluorescent molecules to kill cancer cells and bacteria. In this work, the details of the interaction of pectin-coated Ag nanocomposites (NCs) of different sizes (8, 10, 13, and 28 nm) and various types of pectin shells with the classical photosensitizer methylene blue (MB) were investigated. Pectin-Ag NCs were characterized by UV–vis spectroscopy, DLS, TEM, and XPS. The enhanced singlet oxygen generation effect of pectin-Ag@MB complexes was size-dependent (the smaller the size of the plasmonic particles, the higher the 1O2 generation) and associated with the local concentration of the dye in different molecular forms in the pectin shell of nanoparticles. In cell experiments, the pectin-Ag@MB complexes did not show enhanced phototoxicity or cellular uptake towards cancer cells, compared to free MB. The prolonged release of MB from the complex in a phosphate-buffered saline solution (pH 7.4) was observed. The amount of irreversibly bound MB depended on the type of pectin shell and equaled 1.13, 1.45, and 2.57 µg per mg pectin for Citrus25-Ag, Classic25-Ag, and Classic10-Ag NCs, respectively. The obtained physicochemical insights supplement the present understanding of the fundamental aspects of the interaction between plasmonic polymer-capped nanoparticles and MB and could be important for the rational design of new theranostic agents.

Dynamic Nanophotonics in Epsilon‐Near‐Zero Conductive Oxide Films and Metasurfaces: A Quantitative, Nonlinear, Computational Model

The promise of dynamic nanophotonic technologies relies on the confinement and spatiotemporal control of light at the nanoscale. Confinement via plasmonics, dielectric resonators, and waveguides can be complemented with materials whose optical properties can be controlled using nonlinear effects. Transparent conducting oxides (TCOs) exhibit strong optical nonlinearities in their near‐zero permittivity spectral region, on the femtosecond timescale. Harnessing full spatiotemporal control over the nonlinear response requires a deeper understanding of the process. To achieve this, a self‐consistent multiphysics time‐domain model for the nonlinear optical response of TCOs is developed and implemented into a 3D finite‐difference time‐domain code. Simulations are compared and tuned against recently published experimental results for intense laser irradiation of thin indium tin oxide (ITO) films, achieving good quantitative agreement; the time‐domain dynamics of the nonlinear response and the phenomenon of time‐refraction are also explored. Finally, by simulating intense laser irradiation of a plasmonic particle on an ITO film, the applicability of the approach to time‐varying metasurfaces is demonstrated. As expected, significant enhancement of the nonlinear response of an ITO‐based metasurface over bare ITO thin films is found. This work thus enables quantitative nanophotonics design with conductive oxides in their epsilon‐near‐zero region.

Plasmonic properties of composition graded spherical nanoparticles in quasi-static approximation

During the operation of a localized surface plasmon resonance (LSPR) sensor made in the form of a core–shell nanoparticle with the shell acting as a sensing layer, the target molecules penetrate into the shell due to intrinsic diffusion or reaction mechanisms. As a result, these molecules or various reactants are nonuniformly distributed in the shell layer. Such sensing particles are termed composition graded plasmonic particles, and their LSPR characteristics may be quite different from those of the uniform core–shell particles. Here, under the quasi-static assumption, a perturbation theory is developed to predict the LSPR properties of composition graded plasmonic particles. The effects of the composition gradient on the LSPR properties due to a metal hydride, a dielectric, and an effective medium are either numerically calculated or analytically derived. Our results show that various configurations of the composition gradient can tune the location and the amplitude of the LSPR peak. The results are important for understanding the sensing performance of composition graded plasmonic particles, and the perturbative treatment presented here can also be used for other composition graded structures.

Self-assembled photonic structure: a Ga optical antenna on GaP nanowires.

Semiconductor nanowires are the perfect platform for nanophotonic applications owing to their resonant, waveguiding optical properties and technological capabilities providing control over their crystalline and chemical compositions. The vapor-liquid-solid growth mechanism allows the formation of hybrid metal-dielectric nanostructures promoting sub-wavelength light manipulation. In this work, we explore both experimentally and numerically the plasmonic effects promoted by a gallium (Ga) nanoparticle optical antenna decorating the facet of gallium phosphide (GaP) nanowires. Raman, photoluminescence and near-field mapping techniques are used to study the effects. We demonstrate several phenomena including field enhancement, antenna effect and increase in internal reflection. We show that the observed effects have to be considered when nanowires with a plasmonic particle are used in nanophotonic circuits and discuss the ways for utilization of these effects for efficient coupling of light into nanowire waveguide and field tailoring. The results open up promising pathways for the development of both passive and active nanophotonic elements, light harvesting and sensorics.

Optically trapped SiO2@Au particle-dye hybrid-based SERS detection of Hg2+ ions.

The selective ultra-sensitive detection of a very low concentration of analyte in a liquid environment using surface-enhanced Raman spectroscopy (SERS) is a challenging task owing to the poor reproducibility of the Raman signals arising from the nonstationary nature of the substrate. However, plasmonic metal particle-incorporated microparticles can be effectively 3-D arrested in a liquid environment that can serve as a stable SERS substrate by employing an optical trapping force. Herein, we demonstrate a 3-D optically trapped Au-attached SiO2 microparticle as an efficient SERS substrate that can detect 512 pM for Rhodamine6G and 6.8 pM for crystal violet. Further, the substrate allows the simultaneous detection of multiple analytes. By utilizing the Raman signal from Rhodamine 6G as the probe beam, the selective detection of Hg2+ ions as low as 100 pM is demonstrated.