Nanowire Pairs Research Articles

Enhanced magnetic response in a gold nanowire pair array through coupling with Bloch surface waves

We numerically study the coupling of magnetic plasmon polaritons (MPPs) with Bloch surface waves (BSWs) in a system composed of a one-dimensional gold nanowire pair array lying on a periodic dielectric multilayer. At an appropriate period of the dielectric multilayer, maximum coupling takes place between the MPP and the BSW. It results in two branches of hybridized MPPs with a Rabi-type splitting as large as 125 meV. The maximal magnetic field intensity achieved in the center of nanowire pairs is enhanced greatly and an enhancement factor >1.5 is observed compared with that achieved by a nanowire pair array lying directly on a substrate. This has potential applications in nonlinear optics and near-field enhanced spectroscopy.

Little–Parks oscillations at low temperatures: Gigahertz resonator method

A thin-film Fabry–Perot superconducting resonator is used to reveal the Little and Parks (LP) effect [Phys. Rev. Lett. 9, 9 (1962)], even at temperatures much lower than the critical temperature. A pair of parallel nanowires is incorporated into the resonator at the point of the supercurrent antinode. As the magnetic field is ramped, Meissner currents develop, changing the resonance frequency of the resonator. The LP oscillation is revealed as a periodic set of distorted parabolas observed in the transmission of the resonator and corresponds to the states of the wire loop having different vorticities. We also report a direct observation of single and double phase slip events.

Optical Coupling of Deep-Subwavelength Semiconductor Nanowires

Systems of coupled resonators manifest a myriad of exciting fundamental physical phenomena. Analogous to the synthesis of molecules from single atoms, the construction of photonic molecules from stand-alone optical resonators represents a powerful strategy to realize novel functionalities. The coupling of high quality factor (Q) dielectric and semiconductor microresonators is by now well-understood and chipscale applications are abound. The coupling behavior of low-Q nanometallic structures has also been exploited to realize high-performance plasmonic devices and metamaterials. Although dense arrays of semiconductor nanoparticles and nanowires (NWs) find increasing use in optoelectronic devices, their photonic coupling has remained largely unexplored. These high refractive index nano-objects can serve as low-Q optical antennas that can effectively receive and broadcast light. We demonstrate that the broad band antenna response of a pair of NWs can be tuned significantly by engineering their optical coupling and develop an intuitive coupled-mode theory to explain our observations.

Fabrication of metal nanowires by ion-beam irradiation of oxides through high aspect ratio resist masks

A method of ion-beam-induced reduction in oxides is used to produce metal-bismuth nanowires embedded into a matrix of a highly insulating dielectric—bismuth oxide. The metal film is formed in the process of reduction of the metal oxide by selective removal of oxygen atoms under irradiation by the beam of protons through a mask. The mask containing pairs of parallel nanowires with contact pads was fabricated using 50 kV electron-beam lithography in a single layer of 200-nm-thick ZEP-520 (Zeon Chemicals L.P., ZEP-520 electron-beam resist) electron-beam resist. Electrical properties of the fabricated nanowires have been studied. Broadening of the fabricated metal wires with respect to the initial mask width was found to be dependent on the energy of irradiating protons. This effect may be attributed to the scattering of protons in the oxide-film. It is shown that the method of selective atom removal combined with high aspect ratio e-beam lithography is a feasible technique for fabrication of metal nanowires embedded in a dielectric matrix of metal oxide.

Effects of cross-section shape on fast electron beams excited plasmons in the surface of nanowire pairs

In this paper,we investigate the gap plasmon excitation by swift electron beams in nanowire pairs, in particular, with different cross-section shapes (including circular, wedge and irregular shape). For all these nanowire pairs, our results show that fast electron beams can efficiently excite low-order monopole-monopole gap plasmons, which possess long propagation distance and well spatial localization. Furthermore, in contrast to high-order plasmons, the excited gap plasmons do not depend highly on cross-section shape of nanowire pairs through comparing the merit parameters in these three kinds of waveguides. Besides, the excited gap mode will be more confined in nanowire pairs with wedge cross-section. Our study will be helpful for the experiments of gap plasmon generation using electron microscope.

Optical forces in nanowire pairs and metamaterials

We study the optical force arising when isolated gold nanowire pairs and metamaterials with a gold nanowire pair in the unit cell are illuminated with laser radiation. Firstly, we show that isolated nanowire pairs are subject to much stronger optical forces than nanospheres due to their stronger electric and magnetic dipole resonances. We also investigate the properties of the optical force as a function of the length of the nanowires and of the distance between the nanowires. Secondly, we study the optical force in a metamaterial that consists of a periodic array of nanowire pairs. We show that the ratio of the size of the unit cell to the length of the nanowires determines whether the electric dipole resonance leads to an attractive or a repulsive force, and we present the underlying physical mechanism for this effect.

Cavity Plasmonics: Large Normal Mode Splitting of Electric and Magnetic Particle Plasmons Induced by a Photonic Microcavity

We couple localized plasmon modes in nanowire pairs with resonator modes of a microcavity. Depending on the position of the nanowire pair in the resonator, the electric (symmetric) or magnetic (antisymmetric) plasmon mode is coupled, manifested by a huge anticrossing in the dispersion diagram. We explain this behavior by taking the symmetry and spatial distribution of the electric fields in the resonator into account. Experimental spectra verify the predicted mode-splitting due to the resonant coupling and agree well with theory. Our work can serve as a model system for far-field plasmon-plasmon coupling and paves the way toward enhanced localized plasmon-plasmon interaction in photonically coupled three-dimensional Bragg structures.

Controllable excitation of gap plasmons by electron beams in metallic nanowire pairs

We show that electron beams can efficiently excite gap-plasmon modes in metallic waveguides defined by nanowire pairs. These modes are characterized by monopole-monopole charge symmetry. Significant excitation yields of higher-order dipole-dipole and quadrupole-quadrupole modes are also predicted. Our results are based on rigorous numerical solution of Maxwell's equations in which the electron is described as a classical external current. In our analysis, we find it convenient to simulate electron energy-loss spectra, which are resolved in the frequency and parallel momentum transferred from the electron to the wires for various perpendicular electron trajectories. The charge symmetry of these modes is inferred from the distribution of the induced near fields. The emission yield for the gap modes is as high as ${10}^{\ensuremath{-}4}$ plasmons per incoming electron over the visible range of frequencies. Higher-order modes can be selectively excited by playing with the orientation of the electron trajectory whereas the gap plasmon is preferentially launched in nonsymmetric configurations and it is totally suppressed in symmetric arrangements. The effect of curvature along the direction of the wires is also explored by considering the excitation of plasmons defined in the gap between two circularly shaped wires. Our results provide full support for the excitation of gap plasmons with designated electron-beam sources in nanocircuits.

Modelling and Adaptive Control of Nanowire-Driven Micromanipulators

The design problem of an adaptive controller for a robotic micromanipulator is considered in this article. The micromanipulator's dynamic model is appropriately modified in order to include the interaction of the attractive Van der Waals (VdW) forces that are typically encountered in the microworld. For modelling purposes every link is decomposed into a series of elementary particles (e.g. spheres), each one interacting with the robot's neighboring objects during its motion. This interaction induces nonlinear additive terms in the model, attributed primarily to the overall effect of the VdW---forces and secondarily by the nanoscale friction. Sparked by recent advancements in nanowires the actuation is achieved by a nanowire-driven system in a tendon-like configuartion. At each joint, a pair of nanowires are attached and act in a passive antagonistic manner. The kinematic and dynamic analysis of the nanowire-driven actuation mechanism is offered. Consequently, the microrobot's model is shown to be linearly parameterizable. Subject to this observation, a globally stabilizable adaptive control scheme is derived, estimating the unknown parameters (masses, generalized VdW---forces) and compensating any variations of those. Simulation studies on a 2-DOF and a 10-DOF planar micromanipulator are offered to highlight the effectiveness of the proposed scheme.

Nanocircuits: Fabrication of a 3D Nanoscale Crossbar Circuit by Nanotransfer‐Printing Lithography (Small 15/2010)

The cover picture shows the structure and a scanning electron microscopy image of a 3D nanoscale crossbar circuit fabricated by nanotransfer printing lithography. An array of parallel Au nanowires of 100 nm width and 100 nm spacing are printed onto an electrically switchable polymer layer, and the printing process can be repeated to fabricate a 3D circuit with multiple layers of the polymers and nanowires aligned along different directions. The conductance of the polymer sandwiched between each pair of nanowires can be modified by applying voltage biases on the nanowires. The 3D architectures can significantly increase the device density and complexity in electronic circuits, generate photonic crystals or plasmonic structures with novel optical properties, and mimic 3D neural networks with field configurable plasticity. For more information, please read the Full Paper “Fabrication of a 3D Nanoscale Crossbar Circuit by Nanotransfer-Printing Lithography” by Y. Chen and co-workers, beginning on page 1663.

Creating Well-Defined Hot Spots for Surface-Enhanced Raman Scattering by Single-Crystalline Noble Metal Nanowire Pairs

Well-defined surface-enhanced Raman scattering (SERS) active systems were fabricated by single-crystalline noble metal nanowires. Crossed and parallel nanowire pairs were constructed by using a nanomanipulator to create SERS hot spots in the form of nanowire junction. SERS spectra of brilliant cresyl blue (BCB), p-mercaptoaniline (pMA), and p-mercaptobenzoic acid (pMBA) were observed at the junction of two nanowires. The SERS enhancement and polarization dependence are correlated well with the enhanced electric field intensities calculated by the finite difference time domain (FDTD) method for specific nanowire geometries. These simple and effective SERS active systems have a practical advantage that the hot spots can be readily located and visualized by an optical microscope. These well-defined SERS active systems based on noble metal nanowires can be further developed to find applications in a variety of biological and chemical sensing.

Robust Plasmon Waveguides in Strongly Interacting Nanowire Arrays

Arrays of parallel metallic nanowires are shown to provide a tunable, robust, and versatile platform for plasmon interconnects, including high-curvature turns with minimum signal loss. The proposed guiding mechanism relies on gap plasmons existing in the region between adjacent nanowire pairs and multiwire arrays. We focus on square and circular silver nanowires in silica, which we demonstrate to perform much better than previous schemes in terms of a relevant figure of merit measuring the degree of allowed integration. Our work provides the tools for designing plasmon-based interconnects and achieving a high degree of integration with minimum cross talk between adjacent plasmon guides, which should be relevant ingredients for future multiplexed biosensors.

Surface plasmon propagation in a pair of metal nanowires coupled to a nanosized optical emitter

The coupling, propagations, and far-field emissions of surface plasmons in a pair of Au nanowires with a dipole emitter have been investigated using the finite-difference time domain method. The surface plasmon wavelength is tunable from 650 to 380 nm by adjusting the distance between the two wires, which leads to an enhancement of coupling constant and density of states of the surface plasmon. The converted energy from the dipole emitter to the propagating surface plasmon as well as the far-field emission intensity of a pair of Au nanowires increase to approximately four times as large as those of a single nanowire.

Domain-wall induced magneto-resistive memory in ferromagnetic (Ga, Mn)As nanostructures

Novel magneto-resistive memory effect is demonstrated, which appears in lithographically patterned Y-shaped, three-terminal nanostructures, consisting of three nanowires joined in one point, formed in the epitaxial layer of a ferromagnetic (Ga,Mn)As semiconductor. The effect, in which the zero-field resistance of a pair of nanowires depends on the direction of the previously applied magnetic field, results from a rearrangement of magnetic domain walls, contributing an extra resistance, between different pairs of nanowires in the structure. Two-state behaviour of the nanostructure resistance provides its usefulness for applications in non-volatile memory devices.

Measurement of the Mutual Coherence Function of an Incoherent Infrared Field with a Gold Nano-wire Dipole Antenna Array

The first direct measurement of the mutual coherence function of a spatially incoherent infrared beam was performed at 10.6 μm using a pair of infrared dipole nano-wire antennas that were connected to a common bolometer in the center of the pair by short lengths of coplanar strip transmission line. A spatially incoherent source was constructed by dithering a BaF2 diffuser near the focus of a CO2 laser beam. The distance from the diffuser to the nano-wire antenna pair was held constant while the distance from the focus of the laser beam to the diffuser was varied to control the effective diameter of the source. The measured bolometer signal was proportional to the magnitude of the mutual coherence function at the plane of the antennas. The experimental results were found to match the predicted performance closely. If this technology can be extended to large arrays, a form of synthetic aperture optical imaging based on the Van Cittert-Zernike theorem is possible, similar to that performed at microwave frequencies now by astronomers. This has the potential to greatly increase the angular resolution attainable with optical instruments.

A computational study of the interaction of light with silver nanowires of different eccentricity

The interaction of light with isolated and pairs of silver nanowires with ellipsoidal cross-sections is studied with the finite-difference time-domain (FDTD) method. A Drude model frequency-dependent dielectric constant, with parameters fit to empirical data, is employed. We determine the optical spectra, surface plasmon resonances and near-field profiles for various eccentricities and alignments relative to linearly polarized light. For one nanowire, the surface plasmon resonance position red (to a larger extent) and blue shifts as the ellipse is further elongated when the major (long) axis of the ellipse is oriented along or perpendicular to the direction of incident polarization. We also consider the case of two silver nanowires with circular and ellipsoidal cross-sections.

Evaluation of gold nanowire pairs as a potential negative index material

A metamaterial consisting of pairs of metallic nanowires, separated by a dielectric spacer, has been fabricated and spectrally characterized in the visible and near-infrared spectral domain. It is shown, that the structure exhibits both a plasmonic and a magnetic resonance depending on its geometry and orientation with respect to the illuminating wave field. In particular, we investigate the influence of the thickness of the spacer layer on the spectral position of the resonances and show that, for an appropriate adjustment, both resonances coincide. Measurements of the amplitude and the phase of the transmitted wave are presented. It is also shown that the material is highly anisotropic with respect to the angle of incidence, as the plasmonic resonance wavelength depends strongly on it whereas the magnetic resonance does not show this sensitivity. All experimental results are supported by numerical simulations.

Optical Interference from Pairs and Arrays of Nanowires

The angle dependence of the scattered light from pairs and one-dimensional arrays of nanowires was studied. The intensity of the scattered light varied distinctly during rotation of the structure. The results could be theoretically modeled by treating a pair of nanowires as a double slit and an array of nanowires as a grating. The correspondence between theory and experimental results conclusively proves that the variations are due to the proposed interference effects.

Optimization of optical transmittance of a layered metamaterial on active pairs of nanowires

Optical metamaterials with a negative value of the refractive index can be fabricated by means of patterning techniques developed for microelectronics. One of those is a layered metamaterial, where the electric and magnetic response comes from coupled parallel subwavelength size wires. We simulate propagation of EM waves through such a metamaterial. Its properties depend on the density of pairs of nanowires oriented in parallel in one layer. There is a tradeoff between high transmittance and large negative refractive index value n. The smaller is the density of nanowires; 1 masculine - the narrower the range of frequencies, where n is negative; 2 masculine - the less negative is n; 3 masculine - the higher is the transmission.

Electric and magnetic-field tuning of tubular image states above suspended nanowires

Recently, we have shown that suspended metallic nanowires support Rydberg-like electron image states. Here, we investigate the possibility of tuning such `tubular image states', formed around pairs of parallel nanowires, by electric and magnetic fields. In the presence of a magnetic field, directed along the nanowires, we observe the formation of `Landau-like' image states, with simple elliptic-like orbits, that are highly detached from the surfaces of both nanowires. An additional electric field, induced by opposite charging of the two nanowires, spatially shifts these `molecular' states to one of the wires, while strongly chaotic nodal patterns emerge.