Dielectric Tip Research Articles

Combating Pathogens Using Carbon-Fiber Ionizers (CFIs) for Air Purification: A Narrative Review

Airborne pathogens, though a minor fraction of airborne particles, can cause infections, intoxications, or allergic reactions through respiration, dermal contact, or ingestion. The SARS-CoV-2 pandemic has underscored the significance of mitigating airborne microbial threats. Traditional air ionization methods, such as dielectric barrier discharge and metal tip corona discharge, produce ozone, a reactive and potentially harmful byproduct. However, carbon-fiber ionizers (CFIs) generate high concentrations of ions with minimal ozone production, making them a safer alternative. Operating at voltages below 5 kV, CFIs are more efficient than their metallic counterparts. This review focuses on the antimicrobial efficacy of CFIs, which produce unipolar ions that can disrupt microbial membranes, leading to cell death. Compared to ultraviolet light sterilization, CFIs are cost-effective and suitable for small spaces. The literature review highlights the need for comprehensive studies to evaluate the real-world application and effectiveness of CFIs. Many existing studies are limited by small-scale testing and insufficient data reporting, complicating comparative analyses. Our work aims to provide a detailed perspective on CFIs, examining their impact on various microorganisms, ion efficacy, ionization outcomes, and ozone generation levels. By addressing these aspects, the review seeks to offer an updated understanding of CFIs’ antimicrobial capabilities and to identify limitations in current research, paving the way for more informed and effective air purification strategies.

Electromagnetic jet produced with loaded waveguide ended by an optimized multi-rectangular sections tip

Previous works have demonstrated that a parallel plate waveguide, equipped at the end with a tip, can be used to initiate an “electromagnetic jet (Ej)” (this name generalizes the photonic jet (Pj) phenomenon regardless of the wavelength). In addition to being easy to manufacture, a rectangular shape improves energy concentration, however concurrently the full-width at half maximum (FWHM) is slightly degraded. This article aims to introduce tips with multi-rectangular sections reducing the drawbacks previously mentioned. This article proposes the use of Aperiodic Polynomial Modal Method (A-PMM) as an efficient yet robust method to model and optimize an outgoing Ej, equipped with this kind of dielectric tips. Thus, a three rectangular sections tip can improve energy concentration and can reduce the FWHM beyond the diffraction limit compared to a single section tip.

Comparative Apex Electrostatics of Atom Probe Tomography Specimens.

Rigorous electrostatic modeling of the specimen-electrode environment is required to better understand the fundamental processes of atom probe tomography (APT) and guide the analysis of APT data. We have developed a simulation tool that self-consistently solves the nonlinear electrostatic Poisson equation along with the mobile charge carrier concentrations and provides a detailed picture of the electrostatic environment of APT specimen tips. We consider cases of metals, semiconductors, and dielectrics. Traditionally in APT, and regardless of specimen composition, the apex electric field has been approximated by the relation , which was originally derived for sharp, metallic conductors; we refer to this equation as the "k-factor approximation". Here, is tip-electrode bias, is the radius of curvature of the tip apex, and is a dimensionless fitting parameter with . As expected, our Poisson solver agrees well with the k-factor approximation for metal tips; it also agrees remarkably well for semiconductor tips-regardless of the semiconductor doping level. We ascribe this finding to the fact that even if a semiconductor tip is fully depleted of majority carriers under the typical conditions used in APT, an inversion layer will appear at the apex surface. The inversion forms a thin, conducting layer that screens the interior of the tip -thus mimicking metallic behavior at the apex surface. By contrast, we find that the k-factor approximation applied to a purely dielectric tip results in values far greater than the typical range for metallic tips. We put our numerical results into further context with a brief discussion of our own, separate, experimental work and the results of other publications.

Charge Calibration Standard for Atomic Force Microscope Tips in Liquids.

An electric charge standard with nanoscale resolution is created using the known charge distribution of a single tobacco mosaic virus coat protein combined with the known packing of these proteins in the virus capsid. This advances the ability to measure charge on nanometric samples. Experimental atomic force microscope (AFM) force-distance curves are collected under aqueous conditions with controlled pH and ion concentration. A mathematical model that considers a polarizable dielectric tip immersed in an electrolyte is used to obtain charge density from the AFM measurements. Interactions between the tip and the sample are modeled using theory that includes monopolar electrostatic interactions, dipolar interactions, screening from both the dielectric nature of ambient water and solvated ions as described by the linear Poisson-Boltzmann equation, and hard-core repulsion. It is found that the tip charge density changes on a timescale of hours requiring recalibration of the tip for experiments lasting more than an hour. As an example of how a charge-calibrated tip may be used, the surface charge densities on 20 individual carboxylate-modified polystyrene (PS) beads are measured. The average of these AFM-measured bead charge densities is compared with the value obtained from conventional titration combined with electron microscopy. The two values are found to agree within 20%. While the comparison demonstrates similarity of the two charge measurements, hypotheses are put forward as to why the two techniques might be expected not to provide identical mean charge densities. The considerations used to build these hypotheses thus underscore the relevance of the method performed here if charge information is required on individual nanoparticles.

Collecting quantum dot fluorescence with a hybrid plasmonic probe

Collecting quantum emitter fluorescence with high efficiency and high spatial resolution is a crucial topic in quantum and nanophotonic fields. In the general cases of far-field objective lens collection, though the collection efficiency can be very high, the spatial resolution is diffraction limited. Or one can use near-field probe, such as metal-coated dielectric tips, to break this diffraction limit, while the collection efficiency is very low. In this work, a new method is proposed to collect the fluorescence of quantum dots (QDs) with a fiber-integrated silver nanowire (AgNW) waveguide. Fluorescence lifetime measurement is performed to investigate the coupling between QDs and different plasmonic modes. Compared with previous near-field collection methods, the AgNW-fiber probe can realize much higher collection efficiency with similar spatial resolution. This fiber-integrated plasmonic probe may be useful in the area of fluorescence imaging and is also promising for quantum information devices.

Ring of charge probed with Atomic Force Microscopy dielectric tip

We consider a dielectric sphere in the presence of a charged ring and derive expressions for their interaction force. New mathematical closed form expressions for electrostatic problems involving dielectric objects are always interesting, both for their intrinsic foundational value, and also for their practical use as test beds for numerical solutions. The chosen geometry is motivated by the importance to probe charged rings with Atomic Force Microscopy dielectric tips. The interaction forces obtained are further analyzed to identify notorious features of the force versus separation curves. This analysis provides practical guidelines to measure charge and size of rings from experiments.

Ion transmission through a dielectric hollow tip for scanning probe microscopy

We present a new concept for scanning probe microscopy characterization of molecular microstructures. It is based on a thin capillary using as a sharp tip to probe and map the morphology of a surface. In our experiment a collimated ion beam is formed by tungsten ions passing through a quartz tapered capillary with a 100-nm aperture and enters a 2D position-sensitive detector. We demonstrate that such ions are capable of producing the image of a dielectric nanoaperture in the case of low-dose ion beam. Ion transmission through a nanoscale capillary opens the door to observing photodesorption of large organic molecular ions with high spatially-element resolution using the combination of a hollow-tip vacuum scanner with time-of-flight mass spectrometer.

Wavelength-Dependent Optical Force Imaging of Bimetallic Al-Au Heterodimers.

Many important applications of nanometer-scale metallic complexes arise from the light-induced, near-field interactions between their component structures. Here we examine the near-field interactions in bimetallic Al-Au plasmonic nanodisk heterodimers, where the coupling between the primitive plasmons of nanostructures composed of two different metals is studied. Understanding the correlations between nanoparticle morphology and near-field optical properties, particularly for nanostructures composed of two different metals, requires spectrally resolved near-field spatial information. An ideal tool for such investigations is the recently developed photoinduced force microscopy, where the electromagnetic forces between an optically excited plasmonic nanostructure and an adjacent scanning nanoscale tip are measured. Using this approach, we visualize the wavelength-dependent near-field interactions in these bimetallic heterodimers. This system provides a prime example of the diabatic, antenna-reactor picture of plasmon coupling where for a given wavelength the more resonant primitive "driving" plasmon induces a response, the "forced" plasmon, in the off-resonant component. We critically examine spectrally resolved tip-nanostructure forces, comparing experiment with theory, for tips and nanoscale structures of realistic dimensions relative to frequently used approximations for tip geometries. The contrasting effects of dielectric versus metallic tips on acquired spectral force profiles are also examined.

Modified field confinement and enhanced optical forces in hybrid dielectric wedge tip-loaded plasmonic waveguide

We proposed and theoretically investigated a hybrid plasmonics waveguide consisting of a tiptilted quadrate nanowire, which was embedded in a low-permittivity dielectric and placed on a metal substrate with a small gap distance. Due to the corner effect, the hybrid mode with extremely local field enhancement has the long propagation length and strong coupling strength between the dielectric nanowire and metal. By employing the simulations with different geometric parameters, the proposed waveguide can obtain better performances than the previous hybrid plasmonics waveguide, particularly in the subwavelength confinement (as small as λ2/1600), long-range propagation (millimeter range), and optical trapping forces (2.12 pN/W). The use of a naturally dielectric wedge tip of quadrate nanowire that can be chemically synthesized provides an efficient approach to circumvent the fabrication difficulty of shape wedge tips. The present structure provides an excellent platform for nanophotonic waveguides, nanolasers, nanoscale optical tweezers, and biosensing.

Electric-Field Sensing with a Scanning Fiber-Coupled Quantum Dot

We demonstrate the application of a fiber-coupled quantum-dot-in-a-tip as a probe for scanning electric field microscopy. We map the out-of-plane component of the electric field induced by a pair of electrodes by measurement of the quantum-confined Stark effect induced on a quantum dot spectral line. Our results are in agreement with finite element simulations of the experiment. Furthermore, we present results from analytic calculations and simulations which are relevant to any electric field sensor embedded in a dielectric tip. In particular, we highlight the impact of the tip geometry on both the resolution and sensitivity.

Mapping the near fields of plasmonic nanoantennas by scattering‐type scanning near‐field optical microscopy

AbstractNear‐field optical microscopy techniques provide information on the amplitude and phase of local fields in samples of interest in nanooptics. However, the information on the near field is typically obtained by converting it into propagating far fields where the signal is detected. This is the case, for instance, in polarization‐resolved scattering‐type scanning near‐field optical microscopy (s‐SNOM), where a sharp dielectric tip scatters the local near field off the antenna to the far field. Up to now, basic models have interpreted S‐ and P‐polarized maps obtained in s‐SNOM as directly proportional to the in‐plane ( or ) and out‐of‐plane () near‐field components of the antenna, respectively, at the position of the probing tip. Here, a novel model that includes the multiple‐scattering process of the probing tip and the nanoantenna is developed, with use of the reciprocity theorem of electromagnetism. This novel theoretical framework provides new insights into the interpretation of s‐SNOM near‐field maps: the model reveals that the fields detected by polarization‐resolved interferometric s‐SNOM do not correlate with a single component of the local near field, but rather with a complex combination of the different local near‐field components at each point (, and ). Furthermore, depending on the detection scheme (S‐ or P‐polarization), a different scaling of the scattered fields as a function of the local near‐field enhancement is obtained. The theoretical findings are corroborated by s‐SNOM experiments which map the near field of linear and gap plasmonic antennas. This new interpretation of nanoantenna s‐SNOM maps as a complex‐valued combination of vectorial local near fields is crucial to correctly understand scattering‐type near‐field microscopy measurements as well as to interpret the signals obtained in field‐enhanced spectroscopy. image

Plasmonic focusing of infrared SNOM tip patterned with asymmetric structures.

Several scattering type metal tips patterned with asymmetric metal/dielectric bump gratings are studied and proved to be efficient in focusing light energy into nano 'hot spot'. The dielectric bump tip shows complex mechanisms including local geometric resonance, surface plasmon polariton (SPP) standing wave resonance and Fano effect in the near-field enhancement. Additionally, considering the practical situation, we also demonstrate that, for the case of bending tip surface, the grating coupling method for plasmonic nano-focusing is still applicable if the intervals between neighboring bumps are well designed according to the surface bending curvature. With practical realizations, our results could benefit not only infrared scanning near-field optical microscopes (SNOMs) but also many other applications in nanotechnology such as sensing and lithography.

Propagation of a Positive Streamer Toward a Dielectric Tip in Pure Nitrogen and in Air Under Voltage Pulses With Subnanosecond Rise Time

We simulate a positive streamer discharge approaching a dielectric tip (e.g., of a dielectric rod) under a voltage pulse with a subnanosecond rise time. In air, the streamer reaches the tip, whereas in nitrogen, it does not.

Optical near-fields & nearfield optics

Optical near-fields & nearfield optics

Dielectric bow-tie nanocavity

We propose a novel dielectric bow-tie (DBT) nanocavity consisting of two opposing tip-to-tip triangle semiconductor nanowires, whose end faces are coated by silver nanofilms. Based on the advantages of the dielectric slot and tip structures, and the high reflectivity of the silver mirror, light can be confined in this nanocavity with low loss. We demonstrate that at 4.5 K (300 K) around the resonance wavelength of 1550 nm, the mode excited in this nanocavity has a deep subwavelength mode volume of 2.8×10(-4) μm³ and a high quality factor of 4.9×10(4) (401.3), corresponding to an ultrahigh Purcell factor of 1.6×10(7) (1.36×10(5)). This DBT nanocavity may find applications for integrated nanophotonic circuits, such as high-efficiency single photon sources, thresholdless nanolasers, and strong coupling in cavity quantum electrodynamics experiments.

Impact of the apex of an elongated dielectric tip upon its light absorption properties

This paper discusses the impact of the hemispherical ending of a conical dielectric tip on its optical absorption capabilities. We show that the bottleneck for light coupling via the apex is its ability to resonantly interact with the incident light, which is only possible for apex radii commensurate with the wavelength. Once light has been fed into the apex, however, the fundamental guided mode of the dielectric tip is excited independently from the apex size. The existence and propagation of the fundamental mode from the apex into the shank are supported experimentally by the observation of periodic ripples close to the apex of a silicon tip irradiated with high-fluence green laser pulses. The periodicity and attenuation of these ripples along the tip axis are in excellent agreement with the theoretical properties of the fundamental guided mode.

Ultrafast desorption of molecular ions by XUV-photons, passing through dielectric hollow tip

Pulsed desorption of organic conducting polymer by XUV-photons, formed by a thin capillary collimator, has been investigated. Short-wavelength radiation has been resulted from a metal target irradiated by a sharply focused Ti:Sa laser beam (0.8 μm, 40 fs, 3 mJ∕pulse) and has been filtered by Mylar-gold substrate. Single shot and 1-kHz pulses regimes of driving femtosecond laser have been compared using a time-of-flight mass spectrometer. A stepwise function of photoion signal vs laser pulse energy has been observed.

Near-Field Optical Analysis of Plasmonic Nano-Probes for Top-Illumination Tip-Enhanced Raman Scattering

Top-illumination tip-enhanced Raman scattering (TI-TERS) has recently emerged as a promising near-field vibrational spectroscopy method that can be adapted on an upright optical microscope. With a relatively simplified optics, TI-TERS can probe both opaque and transparent samples making them a promising tool in nanoscale chemical analysis. One of the critical components of TI-TERS is the plasmonic nano-tip used to enhance the Raman spectroscopic signature. Herein, we numerically studied the near-field optical properties of conventional gold tip (20 nm radius of curvature) and two varieties of optical antenna-based tips in the context of TI-TERS. Optical antenna-based tips included a 40-nm gold nanoparticle attached to a dielectric tip and a 50-nm equilateral gold nanotriangle attached to a dielectric tip. We evaluated the Raman enhancement spectra as a function of experimental variables such as underlying substrate and angle of the tip with respect to substrate normal. Our analysis revealed that conventional gold tip facilitates superior enhancement and optical antenna-based tips facilitate superior spectral bandwidth and lateral resolution in TI-TERS configuration. Tips with higher enhancement can be harnessed for ultra-sensitive measurements, and tips with broader spectral bandwidth can be utilized to enhance both Stokes and anti-Stokes component of the Raman spectra.

Simulations of a Dielectric Slot Tip for Scanning Near-Field Microwave Microscope

Simulations of a Dielectric Slot Tip for Scanning Near-Field Microwave Microscope

Contrast inversion in electrostatic force microscopy imaging of trapped charges: tip–sampledistance and dielectric constant dependence

We present a numerical and analytical study of the behavior of both electrostatic force andforce gradient created by a charge trapped below the surface of a dielectric on an atomicforce microscope tip as a function of the dielectric constant and tip–sample distance. Asexpected, the force decreases monotonously when the dielectric constant increases.However, a maximum in the dielectric constant dependence of the force gradient is found.This maximum occurs in the typical experimental parameters’ range and depends on thetip–sample distance and the sample thickness. The analytical study permits us tounderstand the physical origin of this phenomenon and is in good agreement withthe numerical simulation for small tip–sample distances. We also report a studyexemplifying a possible contrast inversion in electrostatic force microscopy (EFM)signals while scanning, at different heights, two charges trapped in a sample havingheterogeneous dielectric domains. In addition to this particular contrast inversion effect,this study can be considered as a way to gain insight into the mechanisms ofEFM image formation as a function of the dielectric constant and tip–sample.