Collective Electron Oscillations Research Articles

Ultrahigh Responsivity UV Photodetector Based on Cu Nanostructure/ZnO QD Hybrid Architectures.

Strong near-surface electromagnetic field formed by collective oscillation of electrons on Cu nanostructure a shows a strong dependence on geometry, offering a promising approach to boost the light absorption of ZnO photoactive layers with enhanced plasmon scattering. Here, a facile way to fabricate UV photodetectors with tunable configuration of the self-assembled Cu nanostructures on ZnO thin films is reported. The incident lights are effectively confined in ZnO photoactive layers with the existence of the uplayer Cu nanostructures, and the interdiffusion of Cu atoms during fabrication of the Cu nanostructures can improve the carrier transfer in ZnO thin films. The optical properties of the hybrid architectures are successfully tailored over a control of the geometric evolution of the Cu nanostructures, resulting in significantly enhanced photocurrent and responsivity of 2.26 mA and 234 A W-1 under a UV light illumination of 0.62 mW cm-2 at 10 V, respectively. The photodetectors also exhibit excellent reproducibility, stability, and UV-visible rejection ratio (R370 nm /R500 nm ) of ≈370, offering an approach of high-performance UV photodetectors for practical applications.

Surface plasmon polariton in multilayered configuration

We investigate the effect of a propagation of surface electromagnetic waves along a metallic surface covered by layered dielectric structures. The optical coupling of a wave incident to collective oscillations of electrons along an interface between a metal and a dielectric is governed by the thickness of metal and gap layers. The surface Plasmon excited by an electromagnetic wave with a central wavelength of λ=486 nm and using the optic finite-difference time-domain method (OptiFDTD). For the metal, in particular a frequency on their dielectric permittivity dependence and described by the Drude-Lorentz model and using the effective-index approach and an explicit expression for the propagation constant of long rang surface plasmon polaritons (LR-SPPs) obtained for moderate metal widths.

(Invited) Enhanced Water Splitting Under Modal Strong Coupling Conditions

Metallic nanoparticles such as gold (Au) and silver (Ag) shows light absorption and scattering at the arbitrary wavelength of visible and near-infrared regions based on localized surface plasmon resonances (LSPRs). LSPRs which are collective oscillations of conductive electrons give rise to the enhancement of near-field and are expected as a light harvesting optical antenna for light energy conversion devices due to their spectrum tunability. We have successfully developed the plasmon-induced artificial photosynthesis systems such as water splitting and ammonia synthesis systems as well as solid-state plasmonic solar cells based on the principle of plasmon-induced charge separation between gold nanoparticles (Au-NPs) and the semiconductor photoelectrode.[1]-[6] Recently, the plasmon-induced charge separation has received considerable attention as a novel strategy for the solar energy conversion.[7],[8] However, for the monolayer of Au-NPs on the semiconductor in the regular Au-NPs loaded semiconductor photoelectrode, the insufficient absorption limited its solar energy conversion efficiency. Aiming at the enhancement of light absorption, in the present study, we apply the principle of modal strong coupling to plasmonic water splitting using Au-NPs/titanium dioxide (TiO2) thin-film/Au-film photoelectrode. [9] References Nishijima, K. Ueno, H. Misawa et al. J. Phys. Chem. Lett. 1, (2010)2031.Zhong, K. Ueno, Y. Mori, X. Shi, T. Oshikiri, K. Murakoshi, H. Inoue, H. Misawa, Angew. Chem. Int. Ed., 53 (2014)10350.Oshikiri, K. Ueno, H. Misawa, Angew. Chem. Int. Ed., 53, (2014)9802.Oshikiri, K. Ueno, H. Misawa, Angew. Chem. Int. Ed., 55, (2016) 3942.Nakamura, T. Oshikiri, K. Ueno, H. Misawa et al. J. Phys. Chem. Lett., 7, (2016) 1004.V. Hoang, K. Hayashi, K. Ueno, H. Misawa et al. Nat. Commun., 8, (2017)771.Ueno, T. Oshikiri, H. Misawa, ChemPhysChem, 17, (2016)199.Ueno, T. Oshikiri, Q. Sun, X. Shi, H. Misawa, Chem. Rev., 118, (2018) 2955.Shi, K. Ueno, T. Oshikiri, Q. Sun, K. Sasaki, H. Misawa, Nature Nanotech., 13, (2018) 953-958.

Simultaneous TE and TM designer surface plasmon supported by bianisotropic metamaterials with positive permittivity and permeability

Abstract Surface plasmon polaritons (SPPs) are surface modes existing at the interface between a metal and a dielectric material. Designer SPPs with a customer-defined property can be supported on the surface of suitably engineered metallic structures. They are important for various applications, ranging from chemical sensing to super-resolution imaging. In conventional systems, SPPs are transverse magnetic (TM) polarized, because of their origin in the collective electron oscillation along the surface. In this work, we show that both transverse electric (TE) and TM designer surface plasmons can be supported at the interface between a suitably designed bianisotropic metamaterial and a normal dielectric material without involving either negative permittivity or negative permeability. We further propose a realistic bianisotropic metamaterial for implementation of the double surface modes. The bianisotropic metamaterial demonstrated here may have tremendous applications in optical information processing and integrated photonic devices.

Tackling the Scalability Challenge in Plasmonics by Wrinkle-Assisted Colloidal Self-Assembly.

Electromagnetic radiation of a certain frequency can excite the collective oscillation of the free electrons in metallic nanostructures using localized surface plasmon resonances (LSPRs), and this phenomenon can be used for a variety of optical and electronic functionalities. However, nanostructure design over a large area using controlled LSPR features is challenging and requires high accuracy. In this article, we offer an overview of the efforts made by our group to implement a wrinkle-assisted colloidal particle assembly method to approach this challenge from a different angle. First, we introduce the controlled wrinkling process and discuss the underlying theoretical framework. We then set out how the wrinkled surfaces are utilized to guide the self-assembly of colloidal nanoparticles of various surface chemistry, size, and shape. Subsequently, template-assisted colloidal self-assembly mechanisms and a general guide for particle assembly beyond plasmonics will be presented. In addition, we also discuss the collective plasmonic behavior in depth, including strong plasmonic coupling due to nanoscale gap size as well as magnetic mode excitation and demonstrate the potential applications of wrinkle-assisted colloidal particle assembly method in the field of mechanoresponsive metasurfaces and surface-enhanced spectroscopy. Lastly, a general perspective in the field of template-assisted colloidal assembly with regard to potential applications in plasmonic photocatalysis, solar cells, optoelectronics, and sensing devices is provided.

Metasurface based on cross-shaped plasmonic nanoantennas as chemical sensor for surface-enhanced infrared absorption spectroscopy

Infrared spectroscopy is an effective technique extensively used in research and industry for the label-free and unambiguous identification of molecular species. However, the sensitivity of this technique is severely limited as a result of Beer’s law and, the small infrared absorption cross-section that make prohibitively weak the absorption signals, of minute amounts of analyte as those present in monolayers. This limitation can be overcome by enhancing the infrared vibration of molecules through the enhancement of the electromagnetic (EM) field. Surface Enhanced InfraRed Absorption (SEIRA) using resonant metal Nano-scale Antennas (NAs) can provide huge electromagnetic fields on the nanometer scale featuring localized collective oscillations of electrons, an effect named Localized Surface Plasmonic Resonances (LSPRsWe here report on a series of 2D arrays of cross-shaped NAs having several mm2 area coverage (metasurface) as SEIRA optimized antennas, which can be used in practical applications such as the vibrational sensing of chemical and biological analytes. Cross-shape designed NAs are insensitive to the polarization of the electromagnetic radiation impinging the active area. Due to the random orientation of the dipole moments of molecules they are particularly suitable for the construction of bio-molecular sensors. At the same time, the 2D-array configuration ensures a good near-field signal enhancement arising from the coupling between neighbour NAs Moreover, SEIRA NAs can be easily integrated with micrometre-sized channels and be suitable for the high sensitivity, real time analysis of IR emitting samples, in matrices where IR spectroscopy is severely limited due to absorption bands of liquid water. We present the design, fabrication and experimental characterization of large-area metasurfaces based on cross-shaped plasmonic NAs for the spectroscopic characterization of various types of compounds and for sensing applications in the mid-infrared range. The cross-shaped NAs we have designed exhibit SEIRA phenomena which are very sensitive to both refractive index changes in the surrounding medium and to the specific molecular vibration band emerging from surface adsorbed molecules. To test this effect on our device, we have used as model compounds small molecules (molecular weight (MW) < 500 g/mol) containing triple bond groups resonating at about 2100 cm−1 and a large polymer (MW ˜ 950,000 g/mol) containing carbonyl groups resonating at wavenumbers of about 1700 cm−1. We show a sensitivity of 600 nm/RIU at different wavelengths at a maximum amount of immobilized small molecule of 0.7 fmoles and a SEIRA enhancement factor of 48,000. We also show the device potential to reveal chemical reactions, occurring on the sensor surface at the same scale, where the nitrile group is converted to a triazole ring.

Wavefunction of plasmon excitations with space charge effects

The one dimensional (1D) driven quantum coupled pseudoforce system governing the dynamics of collective Langmuir electron oscillations is used in order to investigate the effects of variety of space charge distributions on plasmon excitations of a nearly free electron gas with an arbitrary degree of degeneracy and electron fluid temperature. A generalized closed form analytic expression for the grand wavefunction of collective excitations in the presence of an arbitrary space charge distribution is presented based on the stationary solutions of the driven coupled pseudoforce system which has been derived from the Schrödinger-Poisson model. The wavefunction and electrostatic potential profiles for some special cases such as the Heaviside charge distribution, Dirac charge sheet, impurity charge sheet in the 1D plasmonic lattice, and the Kroning-Penney Dirac charge distributions with particular applications in plasmonics and condensed matter physics are investigated in this paper. It is remarkably found that two parallel Dirac charged sheets completely shield all interior plasmon excitations with any given energy value from outside electrostatic fields and charge densities. It is also found that the presence of even a weakly charged impurity layer within a perfect 1D plasmonic crystal profoundly alters the periodic electrostatic field of the crystal lattice, and hence, the Bloch character of the wavefunction is considered in the bandgap theory of solids. The current investigation of electron excitations in arbitrary degenerate electron gas in the presence of static charge distributions may be used to develop analytic models for a variety of real physical situations. It also helps in further developments of the rapidly growing fields of nanotechnology and plasmonics.

Enhancement of the Surface Plasmon Polaritons Excitation Efficiency

Surface Plasmon Polaritons (SPPs) are propagating excitations that arise from the coupling of light with collective oscillations of electrons on the metal surface. This paper describes a new approach for increasing the SPP excitation efficiency. Using a computational electromagnetics approach, potential techniques for reducing incident power reflectivity were investigated using the Kretschmann configuration. The effects of different parameters such as incident angle, metal thickness, and dielectric permittivity were tested and optimized to improve the efficiency of SPPs by minimizing reflectivity. An 30% increase in efficiency was obtained. In addition, the optical response of a thin metal film on a glass substrate was numerically examined in terms of SPP excitation. The dependency of the response on the incident angle, materials, and dimensions were demonstrated. This paper describes how an improvement in efficiency can improve the effectiveness of bio-sensing applications through the proper choice of layer dimensions and material permittivity of different layers. The relationship between incident angle and reflectivity for different permittivities can be used for bio-sensing applications such as blood glucose biosensors, etc.

Graphene plasmon enhanced infrared spectroscopy

Infrared spectroscopy can accurately reflect the information of molecular vibration, and it is an important technology to characterize the composition and structure of materials. However, since the interaction between nanomaterials and infrared light is very weak due to the significant size mismatch, it is challenging to obtain the spectral information of nanomaterials in the field of infrared spectroscopy. The plasmon is a collective electron oscillation on the surface of the material inducing by the incident light, and it has excellent light field confinement, which can significantly enhance the interaction between light and nanomaterials. Graphene plasmon has prominent properties, such as high light field confinement, dynamic adjustment, and low intrinsic attenuation. Therefore it is an important solution to enhance the infrared absorption of nanomaterials. This article systematically introduces the infrared plasmon materials system. Then it summarizes the characteristics of graphene plasmon and their advantages on surface enhanced infrared spectroscopy, and it emphasizes the recent important researches and applications of graphene plasmon enhanced infrared spectroscopy in the world, including single molecular layer biochemical detection, gas identification, refractive index sensing, etc. Further prospects for the development and potential applications of graphene plasmon enhanced infrared spectroscopy are also demonstrated.

Photostimulated Synthesis of Noble Metals Nanoparticles

Nowadays, one of the most relevant areas of modern materials science is the science of nanoparticles and nanomaterials, as well as nanotechnology. Composition, size and shape of nanomaterials at the nanoscale determines its electronic, optical, magnetic, catalytic, etc. properties. Due to the unique optical and catalytic properties, noble metals nanoparticles (silver and gold ones) today are one of the most intensively studied types of nanoobjects. The properties of silver and gold nanoparticles are extremely important and promising for technological use in such areas as electronics, optics, solar energy, information storage, communications, biomedicine, environmental research and others. A number of promising applications of noble metal nanoparticles are due to the effect of localized surface plasmon resonance, which consists in the collective oscillation of conduction electrons relatively to the ions in metallic crystal lattice bounded by the nanoparticle surface at the resonant excitation frequency. The dimensional dependence of the basic physical and chemical properties of nanoparticles makes specific demands on the synthesis, which should provide the necessary particles’ diameter and size distribution, the possibility of surface functionalization, particles’ stability in the manufacturing process, subsequent storage and operation for its further practical application. Existing methods for obtaining noble metals nanoparticles, including physical, thermal, chemical, photochemical, electrochemical, etc. do not provide the required reproducibility or are too expensive for mass use. In addition, most currently known methods allow to obtain metal nanoparticles only with a wide distribution of shapes and sizes. Careful control of the reaction parameters, such as time, process temperature, stirring rate, concentration of reactans and stabilizing additives, allows to narrow the size distribution of nanoparticles, but not always to the desired limits. According to recent studies, monodisperse colloidal solutions of noble metals nanoparticles can be obtained by excitation of plasmon-stimulated reactions in the reaction mixture. This review, based on a rage of experimental studies, demonstrates how light can be used to control the processes of growth, shape and size of noble metals nanoparticles, and to convert heterogeneous populations of metal nanoparticles into populations with high monodispersity. The manifestation of localized surface plasmons in the optical spectra of metal nanoparticles of different sizes and shapes was also considered. In addition, there were also discussed photophysical processes, associated with the excitation of localized surface plasmon resonance in metal nanoparticles, which allow to control chemical reactions at the nanoscale, namely the photothermal effect; concentration of light near the surface of nanoparticles, which leads to an increase in the electromagnetic field and the intensity of the photon flux for molecules near the particles and the generation of hot electron-hole pairs that can participate in charge transfer between nanoparticles and nearby molecules.

Surface plasmon mediated chemical reaction

Surface plasmons are collective oscillations of free electrons at the interface between metal and dielectric. Surface plasmons can break through the diffraction limit of light, because the electromagnetic field is confined in a very small space near the surface of the nanostructure, which provides a possibility for nanometer-scale light manipulation. By using surface plasmon resonance, the local surface electromagnetic field can be strongly enhanced, which can be used to enhance the molecular fluorescence and Raman signals. In addition, the plasmon relaxation induces thermal electrons which can drive the catalytic reaction of surface molecules to achieve a selective catalytic reaction at normal temperature, which is so-called plasmon mediated chemical reaction (or plasmonic catalysis). As a new type of catalytic system, plasmonic catalysis can mediate chemical reactions that are difficult to occur under various conventional conditions. Due to the complexity and diversity of plasmon catalyzed reactions, it is still a huge challenge to fully characterize the reaction kinetics and understand its reaction mechanism. Characterizing the intermediate and final products in the catalytic reaction accurately and obtaining more detailed information in the reaction process are essential for exploring the theoretical mechanism of plasmon catalysis. In this paper, we review the characterization techniques used in plasmon catalysis in detail in the progress of plasmon catalysis. First, the basic concepts of plasmon catalysis and several common catalytic mechanisms are introduced. Second, the Raman spectroscopy, including the application of surface and tip-enhanced Raman spectroscopy in plasmon catalytic in situ monitoring are reviewed. Then, the other techniques such as gas chromatography, gas chromatography-mass spectrometry, high performance liquid chromatography, scanning transmission electron microscopy, scanning tunneling microscopy, scanning electrochemical microscopy and UV-visible absorption spectroscopy for monitoring plasmon catalyzed reaction are introduced in detail. Finally, the characteristics and advantages of these characterization techniques in the study of kinetic catalytic process and catalytic mechanism of plasmon, and the future development and challenge are mentioned and analyzed.

Research progress of plasmonic nanofocusing

Surface plasmons (SPs) are the surface waves of collective oscillations of free electrons at metal-dielectric interface, which have the ability to overcome the diffraction limit and to enhance the giant near-field. Tapered metallic nanostructures that support surface plasmons’ propagation are highly attractive to nanophotonic applications because of their waveguiding and field-focusing properties. This distinct morphologic feature enables the functionality known as nanofocusing. As a result, the plasmons can be guided in these nanostructures and finally focused on the sharp apex to greatly enhance the local field. This attractive effect can be widely used for effective remote-excitation detection/sensing. In this paper, we review various types of plasmonic nanofocusing structures operating in the visible and infrared region. We focus on their fundamentals, fabrications, and applications. Firstly, we discuss the mechanisms of the plasmonic nanofocusing. Then, the characteristics of various tapered metallic nanostructures of SPs are reviewed, including on-chip waveguides, metal tips and bottom-up fabricated nanowires. For applications, some prototypes of plasmonic nanofocusing for bio/chemo sensing are demonstrated. Finally, a summary and outlook of plasmonic waveguides are given.

Distinct plasmon resonance enhanced microwave absorption of strawberry-like Co/C/Fe/C core-shell hierarchical flowers via engineering the diameter and interparticle spacing of Fe/C nanoparticles.

Strawberry-like Co/C/Fe/C core–shell hierarchical flowers (CSHFs) consisting of separated Fe/C nanoparticles (NPs) anchoring on a Co HF surface were prepared by decomposing Fe(CO)5 in the presence of Co HFs. Changing the decomposition temperature (Td) and Fe(CO)5 volume (δ) could also facilely modulate the phase structure, surface morphology and composition of the products. The low Td and small δ helped form Co/C/Fe/C CSHFs with a strawberry-like plasmon surface. The diameter and interparticle spacing-dependent electromagnetic properties were investigated at 2–18 GHz. The interparticle-spacing-to-diameter ratio determines the plasmon resonance and coupling. The permittivity and permeability enhanced by strong plasmon resonance were exhibited by Co/C/Fe/C CSHFs formed at δ = 3–4 mL with the interparticle-spacing-to-diameter ratio of 1.36–0.76. The collective oscillation of the conduction band electrons and near field on the Co/C and Fe/C surfaces generated a surface plasmon resonance and coupling, which were responsible for significantly enhanced permittivity and permeability with negative values. In view of the synergistic effect of the enhanced permittivity and permeability, dual dielectric relaxations, dual magnetic resonances, high attenuation and good impedance matching, Co/C/Fe/C CSHFs with particle size of 110 ± 20–380 ± 100 nm and interparticle spacing of 150 ± 50 nm were excellent absorbers that feature strong absorption, broad bandwidth and light weight. An optimal reflection loss (RL) of −45.06 was found at 17.92 GHz for an absorber thickness of 1.6 mm, and the frequency range (RL ≤ −20 dB, 99% absorption) was over 2–18 GHz. Our findings demonstrated that optimally designed plasmonic heterostructures must be fabricated to improve microwave absorption performances for future applications.

Enhancement of photoluminescence from zinc oxide by aluminum nanoparticle surface plasmon

During the past few decades, surface plasmons (SPs) have become a research hotspot. The SPs are the collective oscillations of free electrons at the interface between metal and dielectric surrounding. Localized surface plasmon resonance (LSPR) for metal nanoparticles (NPs) has a wide application in the light emission enhancement by the selective photon absorption and by increasing local electromagnetic field. Nowadays, many achievements of SP-enhanced-emissions are applied to light emitting diodes. With the advantages of the direct wide band gap (3.37 eV) and large exciton binding energy (60 meV), zinc oxide (ZnO), which is considered as a potential material, has a wide range of applications, especially in ultraviolet (UV) optoelectronic devices. However, the low photoluminescence efficiency of ZnO limits the commercial applications of ZnO-devices. The relevant research shows that the selection of different metal NPs, such as platinum (Pt), aluminum (Al), argentum (Ag), aurum (Au), is one of the approaches to improving the UV emission from ZnO. In this study, two-dimensional arrays of Al NPs are used to improve the LSPR photoluminescence efficiency from ZnO grown by the atomic layer deposition (ALD). The two-dimensional arrays of Al NPs are fabricated on the surfaces of p-type Gallium nitride (GaN) substrates by colloid lithography. With the air-liquid interface self-assembly, the monolayer masks for colloid lithography are obtained on the substrates of p-type GaN. Then, after a 50-nm Al layer is deposited by thermal evaporation, the Al NPs’ arrays are gained by being dipped into toluene and extra sonication to remove the masks. Finally, 15 nm Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; and 200 nm ZnO films are deposited in sequence by ALD at a temperature of 125 ℃. The extinction spectra of Al NPs’ arrays are acquired by an ultraviolet-visible spectrophotometer. The results of the extinction spectra suggest that the radiative recombination rate is increased by the resonance coupling between the localized surface plasmons (LSP) of the Al NPs arrays and the excitons of the ZnO. A 1.91-fold enhancement of photoluminescence integral intensity in band-edge emission is measured because of the Al NP arrays coupled with ZnO. The result means that the LSP of the Al NPs’ arrays can increase the UV-emission of the ZnO. Therefore, this cost-effective and facile approach can be used in high-performance optoelectronic devices.

Spectroscopic studies of plasmons in topological materials

Plasmonics plays an important role in the development of nanophotonics, which allows breaking diffraction limit and controlling light in deep-subwavelength scale due to the strong interaction between light and free carriers. Noble metals and 2-dimensional electron gas have been the main platforms for studying plasmonics over the past decade. The metal-based plasmonic devices have exhibited great potential in various applications, including integrated photonic systems, biological sensing, super-resolution imaging and surface-enhanced Raman scattering, etc. Because of the high carrier density, plasmons of noble metals are realized in the near-infrared to visible frequency range. With the rapid development of new materials, many other plasmonic materials are discovered to exhibit new properties. One example is the graphene plasmons working in the mid-infrared and terahertz spectral range, which exhibit strong field confinement and frequency tunability due to the massless Dirac fermions and other exotic electrical and optical properties. Recently, topological materials, the band structures of which are composed of cones with linear dispersion like in graphene, are discovered, such as the topological insulators, Dirac semimetals, Weyl semimetals and nodal line semimetals, providing another platform to study the Dirac plasmons. Such linear dispersion results in small electron mass and unique carrier density dependence of plasmons. In addition, topological materials possess a tremendous amount of exotic electron properties, such as the ultrahigh mobility, topological surface states and chiral anomaly in Weyl semimetals, etc. Many of these electronic properties can be inherited by the collective oscillation of free electrons, promising new possibility for plasmonics. Here, the experimental observations of plasmons in topological insulators and topological semimetals are reviewed, with special focus on the studies based on electron energy loss spectrum and Fourier transform infrared spectroscopy. At the end, other topological materials with potential for hosting 2D plasmons are discussed. This review provides an overview of plasmons in topological semimetals and may stimulate further quest of more exotic features for plasmons.

Acoustic Far-Field Hypersonic Surface Wave Detection with Single Plasmonic Nanoantennas.

The optical properties of small metallic particles allow us to bridge the gap between the myriad of subdiffraction local phenomena and macroscopic optical elements. The optomechanical coupling between mechanical vibrations of Au nanoparticles and their optical response due to collective electronic oscillations leads to the emission and the detection of surface acoustic waves (SAWs) by single metallic nanoantennas. We take two Au nanoparticles, one acting as a source and the other as a receptor of SAWs and, even though these antennas are separated by distances orders of magnitude larger than the characteristic subnanometric displacements of vibrations, we probe the frequency content, wave speed, and amplitude decay of SAWs originating from the damping of coherent mechanical modes of the source. Two-color pump-probe experiments and numerical methods reveal the characteristic Rayleigh wave behavior of emitted SAWs, and show that the SAW-induced optical modulation of the receptor antenna allows us to accurately probe the frequency of the source, even when the eigenmodes of source and receptor are detuned.

Au nanoclusters in the plasmonic applications

One of the new applications of gold nanoparticles is the use of the plasmon effect, which occurs due to the interaction of an electromagnetic wave with free electrons in nanostructures. Particles of gold with a size smaller than the wavelength of a visible electromagnetic wave can strongly absorb light due to the collective oscillation of conduction electrons. Bulk gold has a high-symmetry fcc structure, but controlling the collection of metal atoms in solution can also lead to the growth of many anisotropic forms. The use of these different structures has a significant effect on the plasmonic properties, since defects at twin boundaries can serve as scattering centers of conduction electrons in a metal. Thus, on the basis of controlling the size, shape, and structure of Au nanoparticles, it is possible to control light with unprecedented accuracy, and therefore the determination of the morphology of individual gold nanoparticles is of great interest for studying their possible optical interaction.

Influence of Interaction Retardation and Radiation Reaction on the Fano Resonance Efficiency in the System of Nanoparticles

The system of metallic nanospheroids in the field of monochromatic light wave under conditions of manifestation of the Fano resonance is considered. The dimensions of the nanospheroids are much smaller than the wavelength of the incident radiation, but distances between them can be arbitrary. An analysis is made of the influence of the interparticle interaction and the reaction field on the inhomogeneity of the energy distribution of the collective oscillations of electrons in the system (the Fano resonance efficiency). It is shown that both the retardation and the reaction field contribute to an increase in the resonance efficiency.

Colloidal Plasmonic Nanoparticles for Ultrafast Optical Switching and Laser Pulse Generation

Light-matter interactions are greatly enhanced and spatially well-confined in plasmonic nanostructures in the spectral resonance range. In colloidal plasmonic nanoparticle (NP), this effect is manifested by the strong absorption caused by the collective oscillation of conduction electrons in NPs, known as the localized surface plasmon resonance (LSPR). The strong spectral resonance due to LSPR also contributes to the large enhancement in optical nonlinearity, such as nonlinear absorption and optical Kerr effect, and the ultrafast transient photo-bleaching in those materials making them ideal platforms for exploring optical switching behavior that is needed in all optical devices. Here, we review the recent advances in the use of colloidal processed noble metal, metalloid, chalcogenide and oxide plasmonic NPs for ultrafast optical switches to drive pulse laser generation in both solid state and fiber lasers. The broadband and ultrafast response of these plasmonic NPs offers a significant advantage in devices performance and allows the pulse generation from near infrared (NIR) to middle infrared (MIR) spectral range with pulse duration down to 100-femtosecond level. Compared with commercial products and other optical switches based on 2D materials, these plasmonic NPs are expected to provide a competitive and cost-efficient material solution to compact ultrafast pulse lasers and relevant devices.

From plasmon-enhanced molecular spectroscopy to plasmon-mediated chemical reactions

The excitation of surface plasmons (SPs) — collective oscillation of conduction-band electrons in nanostructures — can afford photon, electron and heat energy redistribution over time and space. Making use of this ability, plasmon-enhanced molecular spectroscopy (PEMS) techniques with ultra-high sensitivity and surface selectivity have attracted much attention and have undergone considerable development over the past four decades. Recently, the development of plasmon-mediated chemical reactions (PMCRs) has shown the potential to have a large impact on the practice of chemistry. PMCRs exhibit some obvious differences from and potential advantages over traditional thermochemistry, photochemistry and photocatalysis. However, our physicochemical understanding of PMCRs is still far from complete. In this Review, we analyse the similarities and distinctive features of PEMS and PMCRs and compare PMCRs with traditional photochemical and thermochemical reactions. We then discuss how PMCRs can be improved by rationally designing and fabricating plasmonic nanostructures, selecting suitable surface and interface mediators and teaming them synergistically. Surface plasmons can redistribute photoenergy over different time, space and energy scales and have been exploited in new spectroscopic techniques. This Review reports on how surface plasmons can also drive chemical reactions by localizing photon, electronic and/or thermal energies.