Point Emitters Research Articles

Increasing microscopy resolution with photobleaching and intensity cumulant analysis.

Super-resolution fluorescence microscopy and its applications for analysis of biological structures are evolving rapidly field. A number of approaches aimed at overcoming the fundamental limit imposed by diffraction have been proposed in recent years. Here we present a modification of super-resolution optical fluctuation imaging (SOFI), a technique based on spatio-temporal evaluation of the optical signal from independently fluctuating emitters. Instead of rapid, reversible photoswitching, photobleaching is used to produce irreversible transitions between emitting and nonemitting states of the fluorochrome molecules. Simulated images are used to demonstrate that, in the absence of noise, the proposed SOFI modification increases the efficiency of transfer of high spatial frequencies in a fluorescence microscope. Correspondingly, a decrease of the point spread function (PSF) width is obtained. Moreover, the modified SOFI algorithm is capable of resolving point emitters in the presence of simulated noise. Using real biological images we demonstrate that an increase of resolution is obtained in 2D optical sections through densely packed chromatin in cell nuclei and lamin layer at the nuclear envelope. Finally, the approach is extended to 3D wide-field microscopy, allowing reduction of out-of-focus image blurring.

Effect of ion bombardment on the topography and chemical composition of the surface of Pd-Ba powder composites

The effect of 7-keV Ar+-ion bombardment on the surface morphology of porous Pd-Ba composite manufactured by the powder metallurgical method is studied. It is shown that irradiation leads to the formation of cones on the surface of the target which can be used as point emitters.

High performance CNT point emitter with graphene interfacial layer

Carbon nanotubes (CNTs) have great potential in the development of high-power electron beam sources. However, for such a high-performance electronic device, the electric and thermal contact problem between the metal and CNTs must be improved. Here, we report graphene as an interfacial layer between the metal and CNTs to improve the interfacial contact. The interfacial graphene layer results in a dramatic decrease of the electrical contact resistance by an order of 2 and an increase of the interfacial thermal conductivity by 16%. Such a high improvement in the electrical and thermal interface leads to superior field emission performance with a very low turn-on field of 1.49 V μm−1 at 10 μA cm−2 and a threshold field of 2.00 V μm−1 at 10 mA cm−2, as well as the maximum current of 16 mA (current density of 2300 A cm−2).

Effect of long-range spatial correlations on the lifetime statistics of an emitter in a two-dimensional disordered lattice

The effect of spatial correlations on the Purcell effect in a bidimensional dispersion of resonant nanoparticles is analyzed. We perform extensive calculations on the fluorescence decay rate of a point emitter embedded in a system of nanoparticles statistically distributed according to a simple 2D lattice-gas model near the critical point. For short-range correlations (high temperature thermalization) the Purcell factors present a long-tailed statistic which evolves towards a bimodal distribution when approaching the critical point where the spatial correlation length diverges. Our results suggest long-range correlations as a possible origin of the large fluctuations of experimental decay rates in disordered metal films.

Uniaxial epsilon-near-zero metamaterials: from superlensing to double refraction

We investigated optical properties of nanostructured metal-dielectric multilayered lattices under the conditions of epsilon-near-zero (ENZ), a concept derived from the effective-medium approach (EMA). We theoretically found that the periodic array of metallic nanolayers may exhibit either superlensing driven by broadband canalization from point emitters or single-polarization double refraction, and conventional positive as well as negative, even at subwavelength regimes. For the latter case, we formulated a modified EMA, and subsequently a generalized refraction law, that describes both refractive behaviors concurrently. The modal coupling of plasmonic lattice resonances, and nonlocality induced by partial screening across the nanolayer length, are responsible for these distinct effects. Numerical simulations show that deep-subwavelength lensing along the optical axis of Ag-GaAs metamaterial is clearly enhanced at optical wavelengths. On the other hand, transverse-magnetic-polarized radiation that is obliquely incident on the ENZ periodic nanostructures with the same materials in the infrared (around 1.55 μm ) undergoes double refraction neighboring 50/50 beamsplitting.

Plasmons in graphene on uniaxial substrates

Placing graphene on uniaxial substrates may have interesting application potential for graphene-based photonic and optoelectronic devices. Here, we analytically derive the dispersion relation for graphene plasmons on uniaxial substrates and discuss their momentum, propagation length, and polarization as a function of frequency, propagation direction, and both ordinary and extraordinary dielectric permittivities of the substrate. We find that the plasmons exhibit an anisotropic propagation, yielding radially asymmetric field patterns when a point emitter launches plasmons in the graphene layer.

Field-emission microscopy of the surface of an Ir-C-Cs point emitter

The emissive properties of an iridium-based point emitter with various forms of carbon (chemisorbed species, two-dimensional graphite structures) and cesium atoms adsorbed on the surface has been studied by the field-electron emission microscopy (FEM) and field-desorption microscopy (FDM) techniques. The FEE and FDM images of the emitter surface corresponding to various phase states of carbon have been obtained. It is established that two-dimensional graphite structures grow predominantly in the regions of (100) and (111) faces of iridium.

Single molecule localization microscopy for superresolution

In recent years there has been a rash of developments in light microscopies circumventing traditional resolution limits associated with the diffraction of light occurring between the sample and the detector. Collectively, these techniques are referred to as ‘superresolution’ microscopies. One major family of superresolution techniques, variably referred to as PALM, FPALM and STORM, uses temporal control of the excited state of fluorophores to sequentially identify single non-overlapping emitters in time and space. Conventional images of single point emitters are fitted to sub-diffraction-limited areas, and a composite image is reconstructed from position data collected over many thousands of individual imaging frames. This paper provides a brief overview of superresolution microscopy, followed by a detailed discussion of STORM, including practical guidelines for sample preparation designed to help to make the technique more accessible to the non-specialist.

Thermo-Fluid Dynamics of an Array of Impinging Ionic Jets in a Crossflow

Laminar to weakly turbulent mixed convection in a square duct heated from the bottom side is highly strengthened by ionic jets generated by an array of high voltage points, opposite to the heated strip. Negative ion injection is activated within the dielectric liquid HFE-7100. Local temperatures on the heated wall are measured by liquid crystal thermography. Distributions of the Nusselt number are obtained at different forced flow rates, applied heat flows, and transiting electrical currents. In correspondence of the point emitters, higher Nusselt numbers in the impingement areas are measured and an analogy with the thermo-fluid dynamic behavior of an array of submerged impinging jets in a crossflow is drawn. The diameter of the ionic jets is evaluated and an electrohydrodynamic Reynolds number is employed for correlation and similarity purposes. Potential applications of the technique are high-efficiency compact heat exchangers and heat sinks.

Dipole Decay Rates Engineering via Silver Nanocones

A full control of the interaction between confined plasmons and point sources of radiation is a central issue in molecular plasmonics. In this paper, a theoretical contribution towards a physical understanding on the localized surface plasmons excited into metallic nanocones by a point dipole is given. A numerical approach based on the discrete dipole approximation is applied to determine the modifications of the dipole decay rates for varying geometrical parameters of the dipole-metal nanoparticle system. Results declare the centrality of the cone aperture to control the plasmon resonances and to handle the effects it induces on the lifetime of a point emitter. A full spectral tuning of the resonances in the decay rates can be achieved by operating on a unique spatial degree of freedom: by tailoring the aperture alone, total decay rates 105 times higher than the free-space value can be obtained at short distances from the metal in a large region of the spectral range. Quite unexpectedly, size dependence of the antenna is found to have a marginal role if only a lifetime manipulation is desired. It becomes, instead, a crucial aspect of the problem when large quantum yields are required. Results presented in this work shed light on spontaneous emission modification due to interaction with plasmonic nanocones of different shapes and are relevant for a number of applications in the fields of nanoplasmonics and fluorescence microscopy.

Ligand Receptor Binding Rate Kinetics via K-Space Image Correlation Spectroscopy: An in Silico Study

To achieve a fundamental understanding of intra-cellular signalling pathways, it will be necessary to measure the rates of reaction between chemically reacting and interacting macromolecules. The magnitude of binding rates plays a very important molecular regulatory role in a range of systems including the binding of antigens to T-cell receptors, which can lead to differing immune-responses, as well as rates of hydrolysis of GTPs with Rac or Rho proteins, which lead to temporal changes in actin filament lengths and cell motility. Here, we develop an image correlation technique which may be used to study such systems in conjunction with fluorescence microscopy. We simulate receptors diffusing in 2D with diffusion coefficients varying from 0 to 10 pixels2/frame. Receptors are only visible as point emitters when a fluorescent ligand is bound to the receptor; ligand binding kinetics are created by turning receptors ‘on' or ‘off' with average life-times varying from 3 to 20 frames drawn from a negative exponential distribution. Photobleaching is similarly incorporated by permanently turning diffusing receptors ‘off' after average times varying from 20 to 400 frames. The image time-series is generated by convolving a Gaussian function of fixed radius with the point emitter distribution to create 128x128 pixel images with background noise added to give a signal to noise ratio of 3. We use k-space image correlation spectroscopy (kICS) and develop a two-state kinetic binding model with freely diffusing receptors on a 2D membrane to capture the membrane binding kinetics of the image time-series. We show that kICS can accurately recover on/off binding rates at the 95% confidence level for over 75% of simulations. Typically, percentage errors are less than 30% for weak photobleaching and within 60% for strong photobleaching effects.

Accurate 3D Fluorophore Localization using Phase Retrieved Pupil Functions

When near the focus, the image of a point emitter has little information about the emitter's position along the optical axis. To improve the 3D localization precision, several methods such as astigmatic imaging, double helix point spread functions, and multiple focal plane strategies have been developed to encode the axial position in one or more 2D images of the same emitter. In each of these strategies, a proper model for the 3D microscope point spread function (PSF) is essential to avoid inaccuracies in localization. Generating PSF models from phase retrieved (PR) pupil functions has the advantage that the model images are noise free, can be calculated for any focal plane, and include aberrations specific to the optical system. This representation can also be extremely compact by representing the pupil function as a set of Zernike polynomials. Here we demonstrate the use of PSFs generated from PR pupil functions for 3D single molecule localization based super-resolution imaging using a dual focal plane setup. We show the phase retrieval process, as well as the details of fast PSF calculation by implementation on graphics processing unit hardware. We show a comparison between fitting with ideal, aberration free PSF models and that generated from PR pupil functions using a variety of cellular structures including the ER, microtubules and the membrane proteins STIM1 and ORAI1.

High-field formation of multipoint field-electron emitters

Using the field-ion and field-electron microscopy techniques, it is demonstrated that the efficiency of formation of field-induced-emission sources with developed (multipoint) surfaces can be increased due to the phenomenon of evaporation in a strong electric field in the presence of an active gas. The proposed emitter formation process is self-consistent and terminates upon attaining equality of local field factors over the vertices of point emitters irrespective of their initial distribution with respect to the radius of curvature and the local factor of mutual screening.

Model of corona discharge from a thin point in a space-charge-limited current regime

Exact self-similar solutions for the problem of electric field and charge density distribution in the external region of a unipolar corona discharge from a thin point emitter have been found. The obtained solutions are valid outside the framework of the traditional Deutsch approximation based on the small influence of a volume charge on the shape of equipotential surfaces. The value of a saturated discharge current is estimated assuming that the corona discharge is generated by a certain part of the point.

Electron field emission properties of vertically aligned carbon nanotube point emitters

We fabricated vertically aligned carbon nanotube (CNT) point emitters directly on Cu wires with flat or sharp tips. The nanostructures and field emission properties of the two kinds of emitters were investigated. Both emitters showed vertically aligned, multi-walled bamboo-liked CNT structures and demonstrated good field emission properties. The emitter on the Cu wire with a flat tip (F-tip) attained an emission current of 70μA at 4200V whereas the emitter on the Cu wire with a sharp tip (S-tip) reached an emission current of 65μA at 3700V. Both of them demonstrated excellent field emission stability for 40h at emission currents of about 40μA and 10μA for the F-tip and S-tip emitters, respectively.

Field emission microscopy of graphene on iridium point emitter

The field electron emission from the surface of an iridium point emitter covered by a monolayer graphene film has been studied. An analysis of the field emission images showed that electron emission takes place at the boundaries between graphene islands with dimensions up to several dozen nanometers. Intercalation of alkali metal (cesium) atoms under the graphene film decreases the work function of the emitter but does not change the image. Field ion desorption images obtained in the fields where the surface diffusion of Cs atoms is impossible reveal the presence of a submonolayer concentration of cesium at the defects representing graphene island contacts.

Point field emitters based on rod-like ZnO nanocrystals

New semiconducting anisotropic materials for field emitters, such as ZnO, are of interest from practical and scientific standpoints. Usually, rod-like nanocrystals have a length of few hundred microns and a radius of curvature less than hundred nm. In the present work, rod-like nanocrystals ZnO with high aspect ratio are investigated. They are obtained by high-temperature pyrolysis of Zn compounds. FIB Quanta 200 3D is used for preparation, structure characterization and field emission experiments. With the help of a Kleindiek micromanipulator, the most suitable crystal rods are transferred to the tungsten needle tip and fixed using tungsten gaseous injection system (GIS). As a result, ZnO point emitters are formed with a tip radius of curvature less than 50 nm. Emission characteristics revealed a field amplification coefficient (β) of about 1500 for an anode-cathode distance of 50 μm.

Variation of ion current composition during field evaporation of some binary and ternary compounds

The process of field evaporation of ion emitters made of binary (Mo-Re) and ternary (Hf-Mo-Re) alloys has been experimentally studied. Depending on the field-ion emitter pretreatment by heating to high temperatures in the presence of strong electric fields, it is possible to ensure the emission of ion fluxes with variable ion type and charge compositions with respect to all components of the initial alloy or compound as well as ions the fluxes of separate components. Using alloys and complex compounds as field-emitter materials, it is possible to create sources of ion fluxes with variable compositions and masses from the same emitter and sources of elements from which it is technically difficult or even impossible to make point emitters directly.

Radiative flux from a multiple-point bioluminescent or chemiluminescent source within a cylindrical reactor incident on a planar-circular coaxial detector. II. Rotationally symmetric radiation.

In the previous paper [J. Opt. Soc. Am. A 28, 126 (2011)], an analytical formula was presented for calculating radiative fluxes from arbitrarily distributed and arbitrarily radiating multiple-point emitters of bioluminescent or chemiluminescent sources within cylindrical reactors, when the radiation from these point emitters propagates through two homogeneous isotropic media and reaches a planar-circular coaxial detector. This formula was based on two assumptions. The first is that radiation passes across a planar boundary interface between the two media. The second is that the surface reflections on the lateral surface and on the reactor base opposite the detector may be neglected. In this paper, the formula obtained previously was simplified for the case of uniformly distributed point emitters of bioluminescent or chemiluminescent sources emitting an identical rotationally symmetric radiation. The simplified formula is suitable for optimizing and calibrating the analyzed reactor-detector system, which is most commonly used to study the bioluminescence emitted by small biological objects and the chemiluminescence from chemical reactions. Representative data were calculated, illustrated graphically, and tabulated.

Radiative flux from a multiple-point bioluminescent or chemiluminescent source within a cylindrical reactor incident on a planar-circular coaxial detector I Arbitrary radiation field

In a quantitative description of bioluminescence (BL) and chemiluminescence (CL) phenomena, it is indispensable to know the total fluxes of radiation emitted by the sources studied. Such knowledge is necessary for determining the quantum yields of the examined BL and CL reactions, for comparing the data measured by different optical radiometers (luminometers), and for expressing the power of the emitted radiation in absolute units, i.e., in watts or as a number of photons per specified time period. This paper presents an integral formula for calculating radiative fluxes incident on a planar-circular detector from a volume multiple-point BL or CL source partially or completely filling an open coaxial cylindrical reactor. The formula represents the solution to the basic radiometric equation applied to the point emitters embedded within a homogeneous substrate separated from the detector by a different optical homogeneous isotropic medium. All calculations were performed for a nontransparent cylindrical reactor when the surface reflections were neglected and when optical radiation was incident on the detector through an open end of the cylindrical reactor. No additional restrictions were made with respect to the spatial distribution of the point emitters and angular distribution of the emitted radiations, so that the formula is applicable for arbitrarily distributed and arbitrarily radiating point emitters composing a BL or CL source.