Vicinity Of Metallic Nanostructures Research Articles

(Invited) Changes in Electric Double-Layer Structures Under Localized Surface Plasmon Excitation

For the visible light energy conversion, the localized surface plasmon resonance (LSPR), which is the collective oscillations of free electrons, should be promising. With the excitation of LSPR, the highly localized electric field generates in the vicinity of metal nanostructures. It is interesting that various unique photo-response properties, such as chemical reactions or molecular excitations, can be observed due to the enhancement of light-matter interaction.[1] To understand and control such mediated chemical reactions or electrochemical reactions, the precise understanding about the electrified interfaces of plasmonic nanostructures should be required.Especially in the electrochemical system, the electric double layer is formed due to the difference in electrochemical potential between the metal interface and solution. The study of the surface potential is important for the design of chemical reactions because the surface potential at the liquid-solid interface is dependent on the physical properties of the systems. The zeta potential which is the potential of the slipping surface near the Stern layer of the diffusion layer could provide such information. From this point of view, in this study, we investigated the electric double-layer structures through the examination of the zeta potential under LSPR excitation condition. To obtain the zeta potential, we have performed the streaming potential method using Au nanoisland structures under visible light illuminations to excite the LSPR. It was interesting that the zeta potential was drastically changes under the visible light illumination conditions. This zeta potential change indicates the changes in the electric double layer structures through the excitation of LSPR. Through the investigations of incident wavelength dependence or the substrate dependences, the effects for the LSPR excitations on the electric double layer structure have been revealed. The present work would provide the fundamental knowledge about the effect on the LSPR excitation at solid-liquid interfaces.[1] H. Minamimoto et al., Acc. Chem. Res., 2022, 55(6), 809.

Silica Nanoparticles Coated with Smaller Au Nanoparticles for the Enhancement of Optical Oxygen Sensing

In the vicinity of metal nanostructures, phosphorescent intensity can be greatly enhanced through the localized surface plasmon resonance (LSPR). The enhancement of phosphorescence intensity in noble-metal nanoparticle (NP) systems is highly dependent on the overlap of the absorbance peak of a noble metal with that of phosphorescent dyes. When the peak of the NPs’ absorption spectra has a maximum overlap with that of these dyes, the phosphorescence enhancement is the largest. Here, we use a seeded growth method to synthesize SiO2/Au NPs with tunable plasmonic resonance and compare their phosphorescence enhancement for an oxygen range of 0–21%. The measurement sensitivity of oxygen is strongly dependent on the enhancement of phosphorescence. When used with PtTFPP, when the absorbance peak of the SiO2/Au NPs has a maximum overlap with that of PtTFPP, the phosphorescence enhancement factor is as high as 7; the sensitivity of oxygen concentration is as high as 75. Moreover, we also optimize the performance by varying the number of NPs to eliminate the nonradiative energy-transfer (NRET) process and self-quenching.

Electromagnetic modeling of excited-state dynamics in the vicinity of metallic nanostructures

Numerical modeling of spectroscopic response, and near-field effects including local field enhancement and local optical chirality induced by metallic non-chiral nanostructures under continuous-wave laser excitations are presented in this paper. Particularly, excited-state dynamics including plasmon resonances and local optical activity is studied. The light-matter interaction problem is described through Maxwell's equations which are numerically solved by the boundary element method. The calculations are verified by the corresponding experiments. Knowledge of excited-state dynamics of metallic nanostructures provides valuable guidelines for designing alternative functional nanomaterials that is benefited from the enhanced light-matter interaction.

Parallel mapping of optical near-field interactions by molecular motor-driven quantum dots.

In the vicinity of metallic nanostructures, absorption and emission rates of optical emitters can be modulated by several orders of magnitude1,2. Control of such near-field light-matter interaction is essential for applications in biosensing3, light harvesting4 and quantum communication5,6 and requires precise mapping of optical near-field interactions, for which single-emitter probes are promising candidates7-11. However, currently available techniques are limited in terms of throughput, resolution and/or non-invasiveness. Here, we present an approach for the parallel mapping of optical near-field interactions with a resolution of <5 nm using surface-bound motor proteins to transport microtubules carrying single emitters (quantum dots). The deterministic motion of the quantum dots allows for the interpolation of their tracked positions, resulting in an increased spatial resolution and a suppression of localization artefacts. We apply this method to map the near-field distribution of nanoslits engraved into gold layers and find an excellent agreement with finite-difference time-domain simulations. Our technique can be readily applied to a variety of surfaces for scalable, nanometre-resolved and artefact-free near-field mapping using conventional wide-field microscopes.

Inhomogeneous Double Optical Gating of High-Intensity Isolated Attosecond Pulse Generation in Crossed Metal Nanostructures

We propose and investigate an effective method for obtaining high-energy and high-intensity isolated attosecond pulses (IAPs) using the inhomogeneous double optical gating (DOG) technology in specifically designed metal nanostructures. First, using the homogeneous mid-infrared DOG technology modulated by a linearly near-infrared field, we obtain a harmonic yield of 2.5 orders of magnitudes higher than that from the single polarization gating (PG) technology. Further, introducing the crossed metal nanostructures along the driven and gating components, we can extend not only the harmonic cutoff but also enhance the harmonic yield attributed to the plasmonic field enhancement near the vicinity of metal nanostructures. As a result, we find a single harmonic plateau with smaller modulations. The supercontinuum is not very sensitive to the pulse duration of the near-infrared field, and the harmonic yields can be further enhanced with increase in the pulse intensity of the near-infrared field, showing a 108 eV supercontinuum with an intensity enhancement of 4 orders of magnitudes. Finally, by superposing the selected harmonics from the inhomogeneous DOG scheme, we obtain a 33 as SAP with an intensity increase of 4 orders of magnitudes.

Plasmonic Fields Focused to Molecular Size

AbstractEfficient use of light energy is regarded as a key factor in solving energy challenges to create a sustainable society. The highly concentrated photon energy generated by localized surface plasmon resonance excitation in the vicinity of metal nanostructures can enhance light‐matter interaction. Optimization of the interactions between plasmons and electrons in materials can lead to novel light energy applications. To overcome the current limitations for these interactions, the plasmon field must be focused to an extremely small size close to the molecular scale. Formation of the plasmonic field at the quantum limit may cause interesting phenomena with unique photoresponses. Recently, following the development of nanofabrication techniques, detailed investigations have been undertaken to understand these processes. In this focus review, we describe recent advances in the strong interactions between highly localized photons and electrons in nanomaterials, including molecules, nanocarbons, and quantized nanoparticles. First, we outline the plasmonic properties that depend on the metal nanostructures. In addition, we describe surface‐enhanced Raman scattering (SERS), which is used to detect interactions between plasmons and materials. The importance of the resonant electronic excitation process, which is a chemical effect based on the charge transfer contribution, is discussed while considering the unique molecular selectivity in SERS. We then highlight the unique photoresponse properties that are used for ultra‐sensitive detection of single molecules by the localized plasmon field. These properties are major advantages of the plasmon field. Next, we introduce strong coupling between plasmons and excitons. This coupling state is promising because of its ability to modify the intrinsic optical properties of materials via creation of a novel absorption wavelength region to accumulate the light energy. Finally, we discuss the use of plasmon excitation for effective chemical reactions accompanied by electron transfer. We conclude that reduction of light to the molecular scale would open novel routes for energy manipulation required by the next generation.

Modified optical absorption of molecules on metallic nanoparticles at sub-monolayer coverage

Enhanced optical absorption of molecules in the vicinity of metallic nanostructures is key to a number of surface-enhanced spectroscopies and of great general interest to the fields of plasmonics and nano-optics. Yet, experimental access to this absorbance has long proven elusive. We here present direct measurements of the intrinsic absorbance of dye-molecules adsorbed onto silver nanospheres, and crucially, at sub-monolayer concentrations where dye--dye interactions become negligible. With a large detuning from the plasmon resonance, distinct shifts and broadening of the molecular resonances reveal the intrinsic properties of the dye in contact with the metal colloid, in contrast to the often studied strong-coupling regime where the optical properties of the dye-molecules cannot be isolated. The observation of these shifts together with the ability to routinely measure them has broad implications in the interpretation of experiments involving resonant molecules on metallic surfaces, such as surface-enhanced spectroscopies and many aspects of molecular plasmonics.

Harmonic-emission from Rydberg states in the vicinity of metallic nanostructures

SynopsisWe theoretically demonstrate that harmonic emission could be achieved from Rydberg atoms without deteriorating the nanostructures.

Extreme Ultraviolet Sources Generation by Using the Two-Color Multi-Cycle Weak Inhomogeneous Field

An efficient method for attosecond extreme ultraviolet source generation under the two-color multi-cycle weak pulse has been theoretically presented by using the concept of the plasmonic field enhancement in the vicinity of metallic nanostructures. The results show that by properly choosing the inhomogeneity of the two-color multi-cycle (20 fs) weak pulse (1013 W/cm2), not only the harmonic cutoff has been extended, resulting in a broadband XUV continuum, but also the single short quantum path has been selected to contribute to the harmonic. As a result, two isolated XUV pulses with durations of 68as and 66as can be obtained.

Single circularly polarized attosecond pulse generation by spatially inhomogeneous fields from atoms with nonvanishing angular quantum number

We address an efficient scheme to generate single circularly polarized attosecond pulse using spatially inhomogeneous fields in the vicinity of metallic nanostructures from atom media with nonvanishing angular quantum number. Based on the numerical solution of a two-dimensional time-dependent Schrödinger equation, it is shown that using linearly polarized plasmonic fields, the harmonic intensity of two orthogonal polarization components, i.e., harmonics polarized parallel and perpendicular to the driving fields polarization direction, is comparable. Moreover, the relative phase between the two components is about π/2. As a result, near-circularly polarized isolated attosecond extreme ultraviolet or X-ray pulses can be directly produced.

Attosecond x-ray source generation from two-color polarized gating plasmonic field enhancement

The plasmonic field enhancement from the vicinity of metallic nanostructures as well as the polarization gating technique has been utilized to the generation of the high order harmonic and the single attosecond x-ray source. Through numerical solution of the time-dependent Schrödinger equation, for moderate the inhomogeneity and the polarized angle of the two fields, we find that not only the harmonic plateau has been extended and enhanced but also the single short quantum path has been selected to contribute to the harmonic. As a result, a series of 50 as pulses around the extreme ultraviolet and the x-ray regions have been obtained. Furthermore, by investigating the other parameters effects on the harmonic emission, we find that this two-color polarized gating plasmonic field enhancement scheme can also be achieved by the multi-cycle pulses, which is much better for experimental realization.

Surface-plasmon resonance for photoluminescence and solar-cell applications

The surface plasmon of metal nanostructures influences surrounding semiconductors in various ways. In particular, a surface plasmon modifies recombination rates of excitons in a semiconductor, and intensifies photon flux in the vicinity of metal nanostructures. These phenomena have contributed to the improvement of photoluminescent properties both by enhancement of radiative recombination and by electromagnetic-field amplification, even though the degree of nonradiative energy dissipation is sensitively dependent on the metal-semiconductor distance. Strong light absorption induced by surface plasmons is also attractive for photovoltaic applications, so metal nanostructures can be incorporated into diverse solar-cell systems with a reduced solar-cell thickness.

Giant Optical Response from Graphene–Plasmonic System

The unique properties of graphene when coupled to plasmonic surfaces render a very interesting physical system with intriguing responses to stimuli such as photons. It promises exciting application potentials such as photodetectors as well as biosensing. With its semimetallic band structure, graphene in the vicinity of metallic nanostructures is expected to lead to non-negligible perturbation of the local distribution of electromagnetic field intensity, an interesting plasmonic resonance process that has not been studied to a sufficient extent. Efforts to enhance optoelectronic responses of graphene using plasmonic structures have been demonstrated with rather modest Raman enhancement factors of less than 100. Here, we examine a novel cooperative graphene-Au nanopyramid system with a remarkable graphene Raman enhancement factor of up to 10(7). Experimental evidence including polarization-dependent Raman spectroscopy and scanning electron microscopy points to a new origin of a drastically enhanced D-band from sharp folds of graphene near the extremities of the nanostructure that is free of broken carbon bonds. These observations indicate a new approach for obtaining detailed structural and vibrational information on graphene from an extremely localized region. The new physical origin of the D-band offers a realistic possibility of defining active devices in the form of, for example, graphene nanoribbons by engineered graphene folds (also known as wrinkles) to realize edge-disorder-free transport. Furthermore, the addition of graphene made it possible to tailor the biochemical properties of plasmonic surfaces from conventional metallic ones to biocompatible carbon surfaces.

Generation of a broadband xuv continuum in high-order-harmonic generation by spatially inhomogeneous fields

We address an efficient scheme to generate a broadband extreme-ultraviolet (xuv) continuum from high-order harmonic generation emerging from the concept of plasmonic field enhancement in the vicinity of metallic nanostructures [Kim et al., Nature (London) 453, 757 (2008)]. Based on the numerical solution of a time-dependent Schr\"odinger equation, for moderate field intensities and depending on the inhomogeneity of the field, we are able to increase the plateau region roughly by a factor of two and generate a broadband xuv continuum. The underlying physics of the plasmon enhancement in harmonic generation is investigated in terms of the semiclassical trajectories of strong field-electron dynamics, and perfect consistency is found between quantum mechanical simulations. It is found that the field inhomogeneity plays a critical role in quantum path selection. After a critical value, we observe a systematic suppression in the long trajectories, suggesting the generation of a single isolated attosecond pulse. Finally, we investigate the dependence of cutoff position on the order of field inhomogeneity and find a ${\ensuremath{\beta}}^{2.3\ensuremath{\mp}0.2}$ scaling.

Subwavelength control of electromagnetic field confinement in self-similar chains of magnetoplasmonic core-shell nanostructures

We apply first-principles methodology to study the spatial localization of electric field enhancement at plasmonic resonance and magnetic field enhancement at gyroresonance in a self-similar chain of magnetoplasmonic core-shell nanostructures (MCSNs). Localized regions of high electric and magnetic fields in the vicinity of metal nanostructures can be created in a controlled manner by adjusting the physical parameters characterizing this system and the polarization of the external harmonic excitations. We demonstrate the high degree of control achieved on electric field confinement, of the order of 10(3), down to a feature size of λ/1000 in self-similar chains of MCSNs, where λ denotes the free space wavelength of the resonant excitation. We also compare our findings with recent investigations in related plasmonic nanostructures.

Theory of plasmon-enhanced high-order harmonic generation in the vicinity of metal nanostructures in noble gases

We present a semiclassical model for plasmon-enhanced high-harmonic generation (HHG) in the vicinity of metal nanostructures. We show that both the inhomogeneity of the enhanced local fields and electron absorption by the metal surface play an important role in the HHG process and lead to the generation of even harmonics and to a significantly increased cutoff. For the examples of silver-coated nanocones and bowtie antennas we predict that the required intensity reduces by up to three orders of magnitudes and the HHG cutoff increases by more than a factor of two. The study of the enhanced high-harmonic generation is connected with a finite-element simulation of the electric field enhancement due to the excitation of the plasmonic modes.

Hot Spots in Ag Core−Au Shell Nanoparticles Potent for Surface-Enhanced Raman Scattering Studies of Biomolecules

The vicinity of metallic nanostructures that provides intense optical fields is termed as “hot spot”. Any molecule in close proximity to these hot spots will give rise to an increased surface-enhanced Raman spectroscopic (SERS) signal. We have synthesized Ag core−Au shell (core−shell) nanoparticles (NP) with nanopores, which act as hot spots in the SERS measurements. We have demonstrated a large enhancement in SERS studies of various molecules using core−shell NP with hot spots, which is better than using silver nanoparticles. The core−shell NP with hot spots can be used for ultratrace analysis of important biomolecules such as histone acetyltranferase p300, a human transcriptional coactivator protein. The core−shell NP does not change the biomolecule's physical and chemical property upon adsorption, which makes it biocompatible. The core−shell NP carrying hot spots with high SERS enhancement would be ideally suited for in vivo studies of biological systems.

Accuracy of local field enhancement models: toward predictive models?

An accurate computation of field enhancement in the vicinity of metallic nanostructures is fundamental for the prediction of different physical phenomena such as SERS or fluorescence, and also for the design of nanostructures for specific applications. Several numerical models have been developed and are used to compute the field enhancement. Nevertheless, its evaluation can be very tedious and boring due to the plasmon resonance increasing the intensity level, and to the discontinuity of the field near the material edges. The behavior of commonly used computational codes is investigated in order to identify the convergence problems, and to propose some solutions to control the accuracy in the computation of the field enhancement.

Multipolar emission in the vicinity of metallic nanostructures

Electromagnetic emission from finite-size multipoles very close to metal surfaces exhibitingplasmon resonances can have profound consequences on the characteristics of the emittedradiation in the far field in both its integrated intensity and its spatial distribution. Theproblem is relevant for many types of spectroscopies that use metals to enhance opticalsignals, but it also represents a complementary aspect to the recent interest innear-field imaging via surface plasmon resonances. The breakdown of selection rulesfor multipolar emission and the phenomenon of radiation funnelling in the farfield are explicitly discussed. The relevance of these concepts for certain types ofspectroscopy, like surface enhanced Raman scattering (SERS), is also highlighted.