Field Of Nanoplasmonics Research Articles

Infrared PINEM developed by diffraction in 4D UEM

The development of four-dimensional ultrafast electron microscopy (4D UEM) has enabled not only observations of the ultrafast dynamics of photon-matter interactions at the atomic scale with ultrafast resolution in image, diffraction, and energy space, but photon-electron interactions in the field of nanoplasmonics and nanophotonics also have been captured by the related technique of photon-induced near-field electron microscopy (PINEM) in image and energy space. Here we report a further extension in the ongoing development of PINEM using a focused, nanometer-scale, electron beam in diffraction space for measurements of infrared-light-induced PINEM. The energy resolution in diffraction mode is unprecedented, reaching 0.63 eV under the 200-keV electron beam illumination, and separated peaks of the PINEM electron-energy spectrum induced by infrared light of wavelength 1,038 nm (photon energy 1.2 eV) have been well resolved for the first time, to our knowledge. In a comparison with excitation by green (519-nm) pulses, similar first-order PINEM peak amplitudes were obtained for optical fluence differing by a factor of more than 60 at the interface of copper metal and vacuum. Under high fluence, the nonlinear regime of IR PINEM was observed, and its spatial dependence was studied. In combination with PINEM temporal gating and low-fluence infrared excitation, the PINEM diffraction method paves the way for studies of structural dynamics in reciprocal space and energy space with high temporal resolution.

Magnesium as Novel Material for Active Plasmonics in the Visible Wavelength Range.

Investigating new materials plays an important role for advancing the field of nanoplasmonics. In this work, we fabricate nanodisks from magnesium and demonstrate tuning of their plasmon resonance throughout the whole visible wavelength range by changing the disk diameter. Furthermore, we employ a catalytic palladium cap layer to transform the metallic Mg particles into dielectric MgH2 particles when exposed to hydrogen gas. We prove that this transition can be reversed in the presence of oxygen. This yields plasmonic nanostructures with an extinction spectrum that can be repeatedly switched on or off or kept at any intermediate state, offering new perspectives for active plasmonic metamaterials.

Attosecond plasmonic streaking from gold nanospheres

SynopsisTo study time-resolved photoemission from gold nanospheres, we introduce a semi-analytical quantum- mechanical model, including the plasmonic near-field-enhancement of the streaking field at the surface of the nanosphere. For an 800 nm incident near infrared (NIR) pulse and 10 nm diameter gold nanosphere, we find that the presence of the nanoplasmonic field enhances the oscillation amplitude by a factor of 3.

Ponderomotive electron acceleration in a silicon-based nanoplasmonic waveguide.

Ponderomotive electron acceleration is demonstrated in a semiconductor-loaded nanoplasmonic waveguide. Photogenerated free carriers are accelerated by the tightly confined nanoplasmonic fields and reach energies exceeding the threshold for impact ionization. Broadband (375 nm ≤ λ ≤ 650 nm) white light emission is observed from the nanoplasmonic waveguides. Exponential growth of visible light emission confirms the exponential growth of the electron population, demonstrating the presence of an optical-field-driven electron avalanche. Electron sweeping dynamics are visualized using pump-probe measurements, and a sweeping time of 1.98 ± 0.40 ps is measured. These findings offer a means to harness the potential of the emerging field of ultrafast nonlinear nanoplasmonics.

Velocity map imaging of femtosecond laser induced photoelectron emission from metal nanotips

A novel application of velocity-map imaging (VMI) is demonstrated, whereby the momentum distribution of photoelectrons ejected from a tungsten nanoscale metal tip (< 50 nm radius) is recorded following illumination with an ultrafast laser pulse. The electrostatic conditions in the VMI instrument are optimized through finite element modelling, taking into account a physically realistic geometry including all conductive elements in the vicinity of the electron trajectories. The instrument is calibrated by observing above threshold ionization in krypton gas, and simultaneous electron emission from this gas and a tungsten nanotip is presented, illustrating that the velocity mapping condition is maintained. Realizing photoelectron VMI for femtosecond laser illuminated nanoscale objects will have a significant impact on the emerging field of ultrafast nanoplasmonics and will influence the development of such devices as a source of coherent pulses of electrons with applications in time-resolved microscopy, holography and diffractive imaging.

Toward ultimate nanoplasmonics modeling.

Advances in the field of nanoplasmonics are hindered by the limited capabilities of simulation tools in dealing with realistic systems comprising regions that extend over many light wavelengths. We show that the optical response of unprecedentedly large systems can be accurately calculated by using a combination of surface integral equation (SIE) method of moments (MoM) formulation and an expansion of the electromagnetic fields in a suitable set of spatial wave functions via fast multipole methods. We start with a critical review of volume versus surface integral methods, followed by a short tutorial on the key features that render plasmons useful for sensing (field enhancement and confinement). We then use the SIE-MoM to examine the plasmonic and sensing capabilities of various systems with increasing degrees of complexity, including both individual and interacting gold nanorods and nanostars, as well as large random and periodic arrangements of ∼1000 gold nanorods. We believe that the present results and methodology raise the standard of numerical electromagnetic simulations in the field of nanoplasmonics to a new level, which can be beneficial for the design of advanced nanophotonic devices and optical sensing structures.

Breakthroughs in Photonics 2013: Research Highlights on Biosensors Based on Plasmonic Nanostructures

Label-free biomolecular sensing is by far the most common and successful application area in the emerging field of nanoplasmonics. This review paper highlights the latest progress and achievements made in this area. Key aspects of the nanoplasmonic sensor development, including performance enhancement, efforts to increase multiplexing capacity, and the progress in sensor integration and miniaturization, are discussed.

A Versatile Self-Assembly Strategy for the Synthesis of Shape-Selected Colloidal Noble Metal Nanoparticle Heterodimers

The self-assembly of individual nanoparticles into dimers—so-called heterodimers—is relevant for a broad range of applications, in particular in the vibrant field of nanoplasmonics and nanooptics. In this paper we report the synthesis and characterization of material- and shape-selected nanoparticle heterodimers assembled from individual particles via electrostatic interaction. The versatility of the synthetic strategy is shown by assembling combinations of metal particles of different shapes, sizes, and metal compositions like a gold sphere (90 nm) with either a gold cube (35 nm), gold rhombic dodecahedron (50 nm), palladium truncated cube (120 nm), palladium rhombic dodecahedron (110 nm), palladium octahedron (130 nm), or palladium cubes (25 and 70 nm) as well as a silver sphere (90 nm) with palladium cubes (25 and 70 nm). The obtained heterodimer combinations are characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), scanning transmission electron microscopy–energy dispersive X-ray spectroscopy (STEM-EDX), dynamic light scattering (DLS), and zeta-potential measurements. We describe the optimal experimental conditions to achieve the highest yield of heterodimers compared to other aggregates. The experimental results have been rationalized using theoretical modeling. A proof-of-principle experiment where individual Au–Pd heterodimers are exploited for indirect plasmonic sensing of hydrogen finally illustrates the potential of these structures to probe catalytic processes at the single particle level.

Carrier-envelope phase effects on the strong-field photoemission of electrons from metallic nanostructures

Sharp metallic nanotapers irradiated with few-cycle laser pulses are emerging as a source of highly confined coherent electron wavepackets with attosecond duration and strong directivity. The possibility to steer, control or switch such electron wavepackets by light is expected to pave the way towards direct visualization of nanoplasmonic field dynamics and real-time probing of electron motion in solid state nanostructures. Such pulses can be generated by strong-field induced tunneling and acceleration of electrons in the near-field of sharp gold tapers within one half-cycle of the driving laser field. Here, we show the effect of the carrier-envelope phase of the laser field on the generation and motion of strong-field emitted electrons from such tips. This is a step forward towards controlling the coherent electron motion in and around metallic nanostructures on ultrashort length and time scales.

Nanoprotective Layer-by-Layer Coatings with Epoxy Components for Enhancing Abrasion Resistance: Toward Robust Multimaterial Nanoscale Films

Layer-by-Layer (LbL) assembled films offer many interesting applications (e.g., in the field of nanoplasmonics), but are often mechanically feeble. The preparation of nanoprotective films of an oligomeric novolac epoxy resin with poly(ethyleneimine) using covalent LbL-assembly is described. The film growth is linear, and the thickness increment per layer pair is easily controlled by varying the polymer concentration and/or the adsorption times. The abrasion resistance of such cross-linked films was tested using a conventional rubbing machine and found to be greatly enhanced in comparison to that of classic LbL-films that are mostly assembled through electrostatic interactions. These robust LbL-films are then used to mechanically protect LbL-films that would completely be removed by a few rubbing cycles in the absence of a protective coating. A 45 nm thick LbL-film composed of gold nanoparticles and poly(allylamine hydrochloride) was chosen as an especially weak example for a functional multilayer system. The critical thickness for the protective LbL-coatings on top of the weak multilayer was determined to be about 6 layer pairs corresponding to about only 10 nm. At this thickness, the whole film withstands at least 25 abrasion cycles with a reduction of the total thickness of only about 2%.

Dipole-induced localized plasmon modes and resonant surface plasmon scattering

A metal film supports the continuum of propagating surface plasmon waves. The interaction of these waves with a dipole (nanoparticle) positioned some distance from the surface of the film can produce well defined localized plasmon modes whose frequency nonetheless resides inside the continuum. This leads to the resonant enhancement of scattering of surface plasmons off the dipole. The maximum of scattering is found to occur when the distance from the dipole to the surface of the film is equal to one half of the film thickness. The possibility of controllable plasmon scattering could be advantageous for the field of nanoplasmonics.

Efficient isolated attosecond emission in a pre-excited medium using noncarrier-envelope phase-stabilized nanoplasmonic fields

We theoretically investigate the isolated attosecond pulse generation in a pre-excited medium by using local nanoplasmonic fields with few-cycle duration. It is shown that the combined effect of the temporal ionization of the pre-excited medium and the spatial inhomogeneity of the local nanoplasmonic field can dramatically control the quantum path, leading to broadband supercontinuous harmonics with high yield. Moreover, this spatiotemporal combination is robust against the carrier-envelope phase (CEP) shift, and the broadband supercontinuum can be supported by nearly all the CEPs of the local field. It would open a route to obtain efficient isolated attosecond emission with no need for the CEP-stabilized laser technique.

Performance of Nonlocal Optics When Applied to Plasmonic Nanostructures

Semiclassical nonlocal optics based on the hydrodynamic description of conduction electrons might be an adequate tool to study complex phenomena in the emerging field of nanoplasmonics. With the aim of confirming this idea, we obtain the local and nonlocal optical absorption spectra in a model nanoplasmonic device in which there are spatial gaps between the components at nanometric and subnanometric scales. After a comparison against time-dependent density functional calculations, we conclude that hydrodynamic nonlocal optics provides absorption spectra exhibiting qualitative agreement but not quantitative accuracy. This lack of accuracy, which is manifest even in the limit where induced electric currents are not established between the constituents of the device, is mainly due to the poor description of induced electron densities.

The generation of MHz isolated XUV attosecond pulses by plasmonic enhancement in a tailored symmetric Ag cross nanoantenna with a few-cycle laser

Within a few-cycle laser, the generation of MHz isolated extreme ultraviolet (XUV) attosecond pulses via nanoplasmonic field enhancement in silver nanostructures is theoretically investigated. Numerical techniques are employed to optimize nanoantennas and attain plasmonic field enhancement factors up to 270. In a volume of 15 × 15 × 30 nm−3 in the nanoantenna, the intensity could be enhanced to 1014 W cm−2 for high harmonic generation (HHG). Optimal conditions for the production of MHz isolated attosecond pulses of 140 attosecond duration via HHG have been identified. These findings open up the possibility for the development of a compact source of ultrashort XUV pulses with MHz repetition rates. Moreover, asymmetric cross HHG is proposed to control the polarizations, select the wavelengths by varying the ratio of silver nanoantennas and generate XUV pulses in both polarized directions.

Attosecond streaking in a nano-plasmonic field

A theoretical study of the application of attosecond streaking spectroscopy to time-resolved studies of the plasmonic fields surrounding isolated, resonantly excited spherical nanoparticles is presented. A classification of the different regimes in attosecond streaking is proposed and identified in our results that are derived from Mie calculations of plasmon fields, coupled to classical electron trajectory simulations. It is shown that in an attosecond streaking experiment, the electrons are almost exclusively sensitive to the component of the field parallel to the direction in which they are detected. This allows one to probe the different components of the field individually by resolving the angle of emission of the electrons. Finally, simulations based on fields calculated by finite-difference time-domain (FDTD) are compared with the results obtained using Mie fields. The two are found to be in good agreement with each other, supporting the notion that FDTD methods can be used to reliably investigate non-spherical structures.

Silica-coated Au/Ag nanorods with tunable surface plasmon bands for nanoplasmonics with single particles

We present the synthesis and analysis of silica-coated Au/Ag bimetallic nanorods with controlled surface plasmon bands. Depending on the thickness of Ag shell deposited on the Au nanorod surface, there is a blue-shift on the longitudinal surface plasmon band of Au nanorods, which can be expressed by an approximate formula derived from the absorption profile of light in Ag films using finite difference time domain simulations. The subsequent coating of silica shell not only enhances the stability of the Au/Ag bimetallic nanorods but also provides a mesoporous host for optically active species. Minute red-shifts of the longitudinal resonance mode, induced by stepwise increased silica shell volumes, are shown. Application as carrier for fluorescent rhodamine B molecules is demonstrated by photoluminescence analysis. On the single-particle level, dark field microscopy of Au/Ag-silica nanorods was finally employed. This introduces a route towards revealing the relation between structure, shape, and optical (plasmonic) properties of complex composite metal particles as well as fabrication strategies for nanoassemblies of tailored structures in the field of nanoplasmonics.

Nanoplasmonic electron acceleration in silver clusters studied by angular-resolved electron spectroscopy

The nanoplasmonic field enhancement effects in the energetic electron emission from few-nm-sized silver clusters exposed to intense femtosecond dual pulses are investigated by high-resolution double differential electron spectroscopy. For moderate laser intensities of 1014 W cm−2, the delay-dependent and angular-resolved electron spectra show laser-aligned emission of electrons up to keV kinetic energies, exceeding the ponderomotive potential by two orders of magnitude. The importance of the nanoplasmonic field enhancement due to resonant Mie-plasmon excitation observed for optimal pulse delays is investigated by a direct comparison with molecular dynamics results. The excellent agreement of the key signatures in the delay-dependent and angular-resolved spectra with simulation results allows for a quantitative analysis of the laser and plasmonic contributions to the acceleration process. The extracted field enhancement at resonance verifies the dominance of surface-plasmon-assisted re-scattering.

Attosecond Plasma Wave Dynamics in Laser-Driven Cluster Nanoplasmas

We introduce a microscopic particle-in-cell approach that allows bridging the microscopic and macroscopic realms of laser-driven plasma physics. As a first application, resonantly driven cluster nanoplasmas are investigated. Our analysis reveals an attosecond plasma-wave dynamics in clusters with radii R is approximately equal to 30 nm. The plasma waves are excited by electrons recolliding with the cluster surface and travel toward the center, where they collide and break. In this process, energetic electron hot spots are generated along with highly localized attosecond electric field fluctuations, whose intensity exceeds the driving laser by more than 2 orders of magnitude. The ionization enhancement resulting from both effects generates a strongly nonuniform ion charge distribution. The observed nonlinear plasma-wave phenomena have a profound effect on the ionization dynamics of nanoparticles and offer a route to extreme nanoplasmonic field enhancements.

Quantum Finite-Size Effects in Graphene Plasmons

Graphene plasmons are emerging as an alternative solution to noble metal plasmons, adding the advantages of tunability via electrostatic doping and long lifetimes. These excitations have been so far described using classical electrodynamics, with the carbon layer represented by a local conductivity. However, the question remains, how accurately is such a classical description representing graphene? What is the minimum size for which nonlocal and quantum finite-size effects can be ignored in the plasmons of small graphene structures? Here, we provide a clear answer to these questions by performing first-principles calculations of the optical response of doped nanostructured graphene obtained from a tight-binding model for the electronic structure and the random-phase approximation for the dielectric response. The resulting plasmon energies are in good agreement with classical local electromagnetic theory down to ∼10 nm sizes, below which plasmons split into several resonances that emphasize the molecular character of the carbon structures and the quantum nature of their optical excitations. Additionally, finite-size effects produce substantial plasmon broadening compared to homogeneous graphene up to sizes well above 20 nm in nanodisks and 10 nm in nanoribbons. The atomic structure of edge terminations is shown to be critical, with zigzag edges contributing to plasmon broadening significantly more than armchair edges. This study demonstrates the ability of graphene nanostructures to host well-defined plasmons down to sizes below 10 nm, and it delineates a roadmap for understanding their main characteristics, including the role of finite size and nonlocality, thus providing a solid background for the emerging field of graphene nanoplasmonics.

Broad-bandwidth and ultrafast electromagnetic response of coupled bimetal nanoantennas in few-cycle laser applications

We explore the field enhancement and temporal response of coupled bi-metal Ag/Au core-shell nanoparticle antennas. The bimetal antennas exhibit ultra-broadband resonances and allow exploiting the local field enhancement for few-cycle laser applications such as elements with an ultrafast response in nanoplasmonic device. We study dimer, trimer and heptamer arrangements and find that the Ag/Au core-shell trimer shows that a very high enhancement factor with an amplitude exceeds 120, but still facilitates an ultrafast response. Such systems may be ideal for the generation of attosecond light pulses based on high harmonic generation by employing nanoplasmonic field enhancement.