Sharp Metal Tip Research Articles

Development of wireless, chipless neural stimulator by using one-port surface acoustic wave delay line and diode–capacitor interface

For the first time, a wireless and chipless neuron stimulator was developed by utilizing a surface acoustic wave (SAW) delay line, a diode–capacitor interface, a sharp metal tip, and antennas for the stimulation of neurons in the brain. The SAW delay line supersedes presently existing complex wireless transmission systems composed of a few thousands of transistors, enabling the fabrication of wireless and chipless transceiver systems. The diode–capacitor interface was used to convert AC signals to DC signals and induce stimulus pulses at a sharp metal probe. A 400 MHz RF energy was wirelessly radiated from antennas and then stimulation pulses were observed at a sharp gold probe. A ∼5 m reading distance was obtained using a 1 mW power from a network analyzer. The cycles of electromagnetic (EM) radiation from an antenna were controlled by shielding the antenna with an EM absorber. Stimulation pulses with different amplitudes and durations were successfully observed at the probe. The obtained pulses were ∼0.08 mV in amplitude and 3–10 Hz in frequency. Coupling-of-mode (COM) and SPICE modeling simulations were also used to determine the optimal structural parameters for SAW delay line and the values of passive elements. On the basis of the extracted parameters, the entire system was experimentally implemented and characterized.

Self-aligned grating couplers on template-stripped metal pyramids via nanostencil lithography.

We combine nanostencil lithography and template stripping to create self-aligned patterns about the apex of ultrasmooth metal pyramids with high throughput. Three-dimensional patterns such as spiral and asymmetric linear gratings, which can couple incident light into a hot spot at the tip, are presented as examples of this fabrication method. Computer simulations demonstrate that spiral and linear diffraction grating patterns are both effective at coupling light to the tip. The self-aligned stencil lithography technique can be useful for integrating plasmonic couplers with sharp metallic tips for applications such as near-field optical spectroscopy, tip-based optical trapping, plasmonic sensing, and heat-assisted magnetic recording.

Development of chip-less and wireless neural probe functioning stimulation and reading in a single device

A chip-less, wireless neural probe system was developed for stimulation of neurons and for reading of neural communications in the brain. The developed neural probe system is composed of a one-port surface acoustic wave (SAW) reflective delay line, neural firing-dependent ferroelectric capacitor, two antennas, and a network analyzer for reading of neural communications, as well as Schottky diode, static capacitor, and sharp metal tip for stimulation of neurons. The one-port SAW reflective delay line replaces existing wireless transceiver system composed of ~5000 electronic components and makes chip-less, wireless interrogation system possible. The probe for reading of neural signals employs PVDF ferroelectric capacitor which changes polarization and volume depending on neural firings. A 4.3nH inductor was also connected between one-port SAW reflective delay line and reading probe to obtain a large linearity and high sensitivity through impedance matching effect. As electrical pulses imitating neural signals were applied to the reading probe at a 0.9% saline solution, amplitude changes in the reflection peaks were observed depending on the magnitude of applied electrical pulses. Good linearity and sensitivity were observed at the amplitude variations in terms of electrical pulses. The probe system for stimulation of neurons consists of Schottky diode, capacitor, and sharp metal probe. By modulating RF input power and the number of cycle from interrogator, stimulation amplitude and duration time of the resultant output pulses are manipulated. Coupling-of-mode (COM) modeling was also performed to predict device performances and to find optimal device parameters.

Local detection efficiency of a NbN superconducting single photon detector explored by a scattering scanning near-field optical microscope.

We propose an experiment to directly probe the local response of a superconducting single photon detector using a sharp metal tip in a scattering scanning near-field optical microscope. The optical absorption is obtained by simulating the tip-detector system, where the tip-detector is illuminated from the side, with the tip functioning as an optical antenna. The local detection efficiency is calculated by considering the recently introduced position-dependent threshold current in the detector. The calculated response for a 150 nm wide detector shows a peak close to the edge that can be spatially resolved with an estimated resolution of ∼ 20 nm, using a tip with parameters that are experimentally accessible.

Ultrafast Infrared Nanoscopy with Sub-Cycle Temporal Resolution

Ultrafast microscopy of surfaces with simultaneous nanometer spatial resolution and femtosecond temporal resolution can be achieved by coupling ultrafast optical pulses to sharp metal tips [1-10]. In the case of scattering-type near-field scanning optical microscopy (s-NSOM), evanescent fields scattered from the tip apex provide a window into a drastically subwavelength world. Moreover, since these scattered fields can be measured using established far-field technologies, ultrafast infrared pulses emerging from a nanoscale volume can be detected with the best possible time resolution: faster than a single optical oscillation cycle [1].

Selective excitation of plasmons superlocalized at sharp perturbations of metal nanoparticles

Sharp metal corners and tips support plasmons localized on the scale of the curvature radius —superlocalized plasmons. We analyze plasmonic properties of nanoparticles with small and sharp corner- and tip-shaped surface perturbations in terms of hybridization of the superlocalized plasmons, which frequencies are determined by the perturbations shape, and the ordinary plasmons localized on the whole particle. When the frequency of a superlocalized plasmon gets close to that of the ordinary plasmon, their strong hybridization occurs and facilitates the excitation of an optical hot spot near the corresponding perturbation apex. The particle is then employed as a nano-antenna that selectively couples the free-space light to the nanoscale vicinity of the apex providing precise local light enhancement by several orders of magnitude.

Multicomponent surface materials: subsequent surface structuring and deposition

The surface properties of materials can be modified by tailoring the topography and chemical composition. Functional surfaces with desirable properties have been produced by precisely controlled surface structures and chemical composition at the micro-nanometer scale. Until now, the challenge has been the ability to manipulate surface morphology and chemistry at the micro-millimeter scale. This work presents a new surface modification method capable of permitting subsequent structuring and material deposition. A micro tip patterning technique is based on sharp metal tips that are used for modifying the topography and chemistry of various material surfaces at the micro-millimeter scale. The method is appropriate for characterizing different topographies by probing conductive material surfaces with a needle tip, which permits a precisely controlled surface structuring with continuous 3D profiles. The desired locations of the scanned surface can be chemically modified by delivering materials via the needle tip. The method is appropriate for the multi-object placement of numerous substances such as functional reagents and biological components in a complex microenvironment. The accurate structuring of complex 3D surface topographies and subsequent chemical deposition provides a precisely controlled functionalization of material surface properties suitable for different applications.

Self-prevention of instability in a low-power microwave Ar plasma jet for biomedical applications

The behavior of a low-power microwave Ar plasma jet according to the target shape and distance is investigated. The plasma jet shows distinct behavior when it contacts a human finger or grounded metals. No plasma channel and no attraction of the jet to the human finger and metal plate are observed in contrast to low-frequency plasmas. Glow-to-arc transition does not occur even at a very small target distance (<1 mm) between a sharp metal tip and bare electrodes. It is a highly favorable property of the microwave plasma for biomedical applications. Reflection coefficient, current, electric field and electron density are investigated to find the mechanism. This unique phenomenon is caused by the characteristic of microwave frequency systems. A decrease of the target distance induces impedance mismatching leading to the reduction of net input power. It is found that the change in the geometry of the plasma jet is the dominant factor for impedance mismatching. This prevents changes in the discharge regime including glow-to-arc transition, similar to ballast. The mechanism is different from the instability prevention methods including the dielectric barrier in low-frequency systems. Insignificant electric field induced on the metal plate by the impedance mismatching can be the reason for the absence of the plasma channel. Emission intensities of reactive species of the plasma jet are almost uniform regardless of the target distance. Electrical safety and performance can be ensured by the low-power microwave plasma jet.

Ultrafast oscilloscope based on laser-triggered field emitters.

Laser-triggered electron emission from sharp metal tips has been demonstrated in recent years as a high brightness, ultrafast electron source. Its possible applications range from ultrafast electron microscopy to laser-based particle accelerators to electron interferometry. The ultrafast nature of the emission process allows for the sampling of an instantaneous radio frequency (RF) voltage that has been applied to a field emitter. For proof-of-concept, we use an RF signal derived from our laser's repetition rate, mapping a 9.28 GHz signal in 22.4 fs steps with 28 mv accuracy.

Vibrational spectroscopy in the electron microscope.

Vibrational spectroscopies using infrared radiation, Raman scattering, neutrons, low-energy electrons and inelastic electron tunnelling are powerful techniques that can analyse bonding arrangements, identify chemical compounds and probe many other important properties of materials. The spatial resolution of these spectroscopies is typically one micrometre or more, although it can reach a few tens of nanometres or even a few ångströms when enhanced by the presence of a sharp metallic tip. If vibrational spectroscopy could be combined with the spatial resolution and flexibility of the transmission electron microscope, it would open up the study of vibrational modes in many different types of nanostructures. Unfortunately, the energy resolution of electron energy loss spectroscopy performed in the electron microscope has until now been too poor to allow such a combination. Recent developments that have improved the attainable energy resolution of electron energy loss spectroscopy in a scanning transmission electron microscope to around ten millielectronvolts now allow vibrational spectroscopy to be carried out in the electron microscope. Here we describe the innovations responsible for the progress, and present examples of applications in inorganic and organic materials, including the detection of hydrogen. We also demonstrate that the vibrational signal has both high- and low-spatial-resolution components, that the first component can be used to map vibrational features at nanometre-level resolution, and that the second component can be used for analysis carried out with the beam positioned just outside the sample--that is, for 'aloof' spectroscopy that largely avoids radiation damage.

Theory of Spatial Coherence in Near-Field Raman Scattering

A theoretical study describing the coherence properties of near-field Raman scattering in two- and one-dimensional systems is presented. The model is applied to the Raman modes of pristine graphene and graphene edges. Our analysis is based on the tip-enhanced Raman scheme, in which a sharp metal tip located near the sample surface acts as a broadband optical antenna that transfers the information contained in the spatially correlated (but nonpropagating) near field to the far field. The dependence of the scattered signal on the tip-sample separation is explored, and the theory predicts that the signal enhancement depends on the particular symmetry of a vibrational mode. The model can be applied to extract the correlation length Lc of optical phonons from experimentally recorded near-field Raman measurements. The coherence properties of optical phonons have been broadly explored in the time and frequency domains, and the spatially resolved approach presented here provides a complementary methodology for the study of local material properties at the nanoscale.4 MoreReceived 28 March 2014DOI:https://doi.org/10.1103/PhysRevX.4.031054This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical Society

Using a sharp metal tip to control the polarization and direction of emission from a quantum dot.

Optical antennas can be used to manipulate the direction and polarization of radiation from an emitter. Usually, these metallic nanostructures utilize localized plasmon resonances to generate highly directional and strongly polarized emission, which is determined predominantly by the antenna geometry alone, and is thus not easily tuned. Here we show experimentally that the emission polarization can be manipulated using a simple, nonresonant scanning probe consisting of the sharp metallic tip of an atomic force microscope; finite element simulations reveal that the emission simultaneously becomes highly directional. Together, the measurements and simulations demonstrate that interference between light emitted directly into the far field with that elastically scattered from the tip apex in the near field is responsible for this control over polarization and directionality. Due to the relatively weak emitter-tip coupling, the tip must be positioned very precisely near the emitter, but this weak coupling also leads to highly tunable emission properties with a similar degree of polarization and directionality compared to resonant antennas.

Towards quantitative off-axis electron holographic mapping of the electric field around the tip of a sharp biased metallic needle

We apply off-axis electron holography and Lorentz microscopy in the transmission electron microscope to map the electric field generated by a sharp biased metallic tip. A combination of experimental data and modelling provides quantitative information about the potential and the field around the tip. Close to the tip apex, we measure a maximum field intensity of 82 MV/m, corresponding to a field k factor of 2.5, in excellent agreement with theory. In order to verify the validity of the measurements, we use the inferred charge density distribution in the tip region to generate simulated phase maps and Fresnel (out-of-focus) images for comparison with experimental measurements. While the overall agreement is excellent, the simulations also highlight the presence of an unexpected astigmatic contribution to the intensity in a highly defocused Fresnel image, which is thought to result from the geometry of the applied field.

Scaling in light scattering by sharp conical metal tips.

Using the electrostatic approximation, we analyze electromagnetic fields scattered by sharp conical metal tips, which are illuminated with light polarized along the tip axis. We establish scaling relations for the scattered field amplitude and phase, and verify the validity with numerical simulations. Analytic expressions for the wavelength at which the scattered field near the tip changes its direction and for the field decay near the tip extremity are obtained, relating these characteristics to the cone angle and metal permittivity. The results obtained have important implications for various tip-enhanced phenomena, ranging from Raman and scattering near-field imaging to photoemission spectroscopy and nano-optical trapping.

Individual Template-Stripped Conductive Gold Pyramids for Tip-Enhanced Dielectrophoresis.

Gradient fields of optical, magnetic, or electrical origin are widely used for the manipulation of micro- and nanoscale objects. Among various device geometries to generate gradient forces, sharp metallic tips are one of the most effective. Surface roughness and asperities present on traditionally produced tips reduce trapping efficiencies and limit plasmonic applications. Template-stripped, noble metal surfaces and structures have sub-nm roughness and can overcome these limits. We have developed a process using a mix of conductive and dielectric epoxies to mount template-stripped gold pyramids on tungsten wires that can be integrated with a movable stage. When coupled with a transparent indium tin oxide (ITO) electrode, the conductive pyramidal tip functions as a movable three-dimensional dielectrophoretic trap which can be used to manipulate submicrometer-scale particles. We experimentally demonstrate the electrically conductive functionality of the pyramidal tip by dielectrophoretic manipulation of fluorescent beads and concentration of single-walled carbon nanotubes, detected with fluorescent microscopy and Raman spectroscopy.

Critical behavior of optical singularities near sharp metal corners and tips

Critical behavior of optical singularities near sharp metal corners and tips

Ultrashort laser oscillator pulses meet nano-structures: from attosecond physics at metal tips to dielectric laser accelerators

We report on two experiments about the interaction of femtosecond laser pulses with nano-scale structures. In the first experiment, we have observed strong-field effects at nanometer sharp metal tips, such as rescattering, the high-energy cut-off and a strong carrier-envelope phase dependence. All these effects are hallmarks of attosecond physics. We have used the new understanding of these processes at a solid tip to build a near-field sensor with record sensitivity of 1 nm. In the second experiment, we have demonstrated the acceleration of non-relativistic electrons with the optical near-field at a photonic grating structure, with an acceleration gradient of 25MeV/m. The grating serves to generate a near-field mode co-propagating with and efficiently imparting momentum onto the electron, which is impossible in vacuum over a macroscopic distance. For relativistic electrons we expect an acceleration gradient of more than 1 GeV/m with two-sided grating structures.

Laboratory x-ray microscopy using a reflection target system and geometric magnification

This paper describes a laboratory X-ray microscopy setup, based on geometric magnification. The setup uses a sharp metal tip as a reflection target and a highly focused electron beam of a scanning electron microscope. Here we will describe the structuring process for these metal targets. To demonstrate the capabilities of our system, we show radiographs of test structures corresponding to resolutions below 100nm. There are abilities for 3D imaging in later updates of the system. Further we discuss the first imaging examples for high- and low-absorbing samples.

Microwave Near-Field Detection of the Ion Concentration in Sealed Fluidic Systems

We have developed a near-field scanning microwave microscopy (NSMM) system that contains a high-quality factor resonator, a sharp metallic probe tip, a 6-degree scanning stage and a vector network analyzer. By applying incident GHz microwaves through the probe tip over samples under test and measuring the magnitude and phase shift of the reflection parameter S11, we have precisely detected the ion concentration of electrolytes in a variety of fluidic systems which are sealed under a 50 μm thick dielectric cover. As expected, the measured magnitude of S11 monotonically increases with the ion concentration, but not linearly, and is sensitive to the tip-sample spacing. This technique offers a useful way for on-site, real-time monitoring of the changes in electrolyte fluids of limited volume in a sealed device. Further work is needed to reveal the exact correlation between the deflection magnitude and ion concentration.

Scale invariance of a diodelike tunnel junction

We measure the current vs voltage (I-V) characteristics of a diodelike tunnel junction consisting of a sharp metallic tip placed at a variable distance d from a planar collector and emitting electrons via electric-field assisted emission. All curves collapse onto one single graph when I is plotted as a function of the single scaling variable Vd^{-\lambda}, d being varied from a few mm to a few nm, i.e., by about six orders of magnitude. We provide an argument that finds the exponent {\lambda} within the singular behavior inherent to the electrostatics of a sharp tip. A simulation of the tunneling barrier for a realistic tip reproduces both the scaling behavior and the small but significant deviations from scaling observed experimentally.