Vacuum Emission Research Articles

Comparative analysis of Trichuris muris surface using conventional, low vacuum, environmental and field emission scanning electron microscopy

The whipworm of the genus Trichuris Roederer, 1791, is a nematode of worldwide distribution and comprises species that parasitize humans and other mammals. Infections caused by Trichuris spp. in mammals can lead to various intestinal diseases of human and veterinary interest. The morphology of Trichuris spp. and other helminths has been mostly studied using conventional scanning electron microscopy of chemically fixed, dried and metal-coated specimens, although this kind of preparation has been shown to introduce a variety of artifacts such as sample shrinking, loss of secreted products and/or hiding of small structures due to sample coating. Low vacuum (LVSEM) and environmental scanning electron microscopy (ESEM) have been applied to a variety of insulator samples, also used in the visualization of hydrated and/or live specimens in their native state. In the present work, we used LVSEM and ESEM to analyze the surface of T. muris and analyze its interaction with the host tissue using freshly fixed or unfixed hydrated samples. Analysis of hydrated samples showed a set of new features on the surface of the parasite and the host tissue, including the presence of the secretory products of the bacillary glands on the surface of the parasite, and the presence of mucous material and eggs on the intestinal surface. Field emission scanning electron microscopy (FESEM) was also applied to reveal the detailed structure of the glandular chambers in fixed, dried and metal coated samples. Taken together, the results show that analysis of hydrated samples may provide new insights in the structural organization of the surface of helminth parasites and its interaction with the infected tissue, suggesting that the application of alternative SEM techniques may open new perspectives for analysis in taxonomy, morphology and host–parasite interaction fields.

Photon-enhanced thermionic emission from heterostructures with low interface recombination

Photon-enhanced thermionic emission is a method of solar-energy conversion that promises to combine photon and thermal processes into a single mechanism, overcoming fundamental limits on the efficiency of photovoltaic cells. Photon-enhanced thermionic emission relies on vacuum emission of photoexcited electrons that are in thermal equilibrium with a semiconductor lattice, avoiding challenging non-equilibrium requirements and exotic material properties. However, although previous work demonstrated the photon-enhanced thermionic emission effect, efficiency has until now remained very low. Here we describe electron-emission measurements on a GaAs/AlGaAs heterostructure that introduces an internal interface, decoupling the basic physics of photon-enhanced thermionic emission from the vacuum emission process. Quantum efficiencies are dramatically higher than in previous experiments because of low interface recombination and are projected to increase another order of magnitude with more stable, low work-function coatings. The results highlight the effectiveness of the photon-enhanced thermionic emission process and demonstrate that efficient photon-enhanced thermionic emission is achievable, a key step towards realistic photon-enhanced thermionic emission based energy conversion.

Quantum vacuum emission in a nonlinear optical medium illuminated by a strong laser pulse

A strong light pulse propagating in a nonlinear Kerr medium produces a change in the refractive index, which makes light travel at different speeds inside and outside the pulse. By tuning the pulse velocity, an analog black hole horizon can be obtained in a suitable frequency window. In this paper, we develop a quantum theory of light propagation for this system, including the frequency dispersion of the refractive index of the medium by coupling the electromagnetic field to matter polarization fields. In a configuration with a single black hole horizon, the spectrum of spontaneously emitted particles presents some similarities with Hawking radiation. Furthermore, even in horizonless systems spontaneous vacuum emission is still possible due to the dispersive nature of the medium, yet with dramatically different spectral properties.

Field Emission of ZnO Nanowires in Low Vacuum Following Various Enhancements Made by Exposure to UV

High-density vertical ZnO nanowires (NWs) were synthesized by vapor phase transport deposition without a metal catalyst or template. The work function φ of ZnO NWs was reduced to 3.7 eV by exposure to UV. At a low vacuum of 10-3 Torr, the UV light improved the field emission performance and reduced the turn-on voltage such that they were better than those of normal field emission in a high vacuum. The depletion layer of NWs disappeared because surface O2- oxygen molecules were removed by exposure to UV light. The enhancement by UV remained even after the UV light had been turned off for 20 min.

Kinematic study of the effect of dispersion in quantum vacuum emission from strong laser pulses

A strong light pulse propagating in a nonlinear medium causes an effective change in the local refractive index. With a suitable tuning of the pulse velocity, the leading and trailing edge of the pulse were predicted to behave as analogue black and white horizons in the limit of a dispersionless medium. In this paper, we study a more realistic situation where the frequency dispersion of the medium is fully taken into account. As soon as positive frequency modes with negative norm are present in the comoving frame, spontaneous emission of quantum vacuum radiation is expected to arise independently of the presence of horizons. We finally investigate the kinematic constraints put on the emission and we show that the optimal directions to observe Hawking-like emission form a narrow angle with the direction of propagation of the pulse.

Feeling the Texture of Vacuum: Laser Acceleration toward PeV

We contemplate if there is a way to reach energies that allow us to probe the tiniest structures in the physical world in a laboratory. Identified are the method of laser acceleration to meet this challenge and a set of ballpark parameters for laser, plasma, and accelerator technologies that are defined for accelerated electron energies reaching as high as PeV. These parameters are carved out from the theoretical laws that govern the physics of laser acceleration, theoretically suggested and experimentally explored over a wide range in recent years. We extrapolate this knowledge toward PeV energies (over 1000 stages of TeV). In the density regime on the order of 10 cm−3, it is possible to consider the application of the existing NIF (or LMJ) or its extended lasers to their appropriate retrofitting for this purpose. Such energies by themselves may allow us to begin to feel and study the physics of the ‘texture of vacuum’. This is an example of fundamental physics exploration without the need of luminosity paradigm and even the production of a single particle at PeV per shot amounts to useful physics at such an extreme energy that far surpasses the contemporary energy horizon in the laboratory. By converting accelerated electrons with extreme energies to like-energy gamma photons, and let them propagate through vacuum over a sufficient distance, these extremely high energy (and therefore short wavelength) photons experience smallest vacuum structures and fluctuations. We introduce an ultrashort detection method of PeV energy γ’s with sub-fs sensitivity by the novel combination of the Schwinger-like vacuum emission of e−e+ by the PeV γ and the carrier-envelope phase (CEP) laser streaking technique with the attosecond phase accuracy. This compilation of the ability to reach PeV and to measure the fs time resolution of PeV γ photons can provide valuable data if and how gamma photons still obey the premise of relativity or the vacuum texture begins to alter such fundamentals. The only method currently available to look at this problem may be to study astrophysical data of the primordial gamma ray bursts (GRBs), which are compared with the presently suggested approach.

Comparison of ion beam and electron beam induced transport of hot charge carriers in metal-insulator-metal junctions

ABSTRACTThe generation of hot charge carriers within a solid bombarded by charged particles is investigated using biased thin film metal-insulator-metal (MIM) devices. For slow, highly charged ions approaching a metal surface the main dissipation process is electronic excitation of the substrate, leading to electron emission into the vacuum and internal electron emission across the MIM junction. In order to gain a deeper understanding of the distribution and transport of the excited charge carriers leading to the measured device current, we compare ion induced and electron induced excitation processes in terms of absolute internal emission yields as well as their dependence on the applied bias voltage.

Focus on cryogenics and vacuum

Focus on cryogenics and vacuum

Positronium Cooling and Emission in Vacuum from Nanochannels at Cryogenic Temperature

High formation yield and a meaningful cooled fraction of positronium below room temperature were obtained by implanting positrons in a silicon target in which well-controlled oxidized nanochannels (5-8 nm in diameter) perpendicular to the surface were produced. We show that by implanting positrons at 7 keV in the target held at 150 K, about 27% of positrons form positronium that escapes into the vacuum. Around 9% of the escaped positronium is cooled by collision with the walls of nanochannels and is emitted with a Maxwellian beam at 150 K. Because positronium quantum confinement limits the minimum achievable positronium energy, the tuning of the nanochannel's size is crucial for obtaining positronium gases in vacuum at very low temperature.

Nanocrystalline diamond film as cathode for gas discharge sensors

Nanocrystalline diamond (NCD) film was deposited on a silicon substrate utilizing microwave plasma-enhanced chemical vapor deposition in a mixed flow of methane, hydrogen and argon. The deposited film had a cauliflower-like morphology, and was composed of NCD, carbon clusters and mixed sp 2- and sp 3-bonded carbon. Electron field emission (EFE) in vacuum and electrical discharges in Ar, N 2 and O 2 using the NCD film as the cathode were characterized. The turn-on field for EFE and the geometric enhancement factor for the NCD film were 8.5 V/μm and 668, respectively. The breakdown voltages for Ar, N 2 and O 2 increased with pressures from 1.33 × 10 4 Pa to 1.01 × 10 5 Pa, following the right side of the normal Paschen curve.

Thermionic emission energy distribution from nanocrystalline diamond films for direct thermal-electrical energy conversion applications

In the ongoing quest for energy production by nonconventional methods, energy conversion by vacuum and solid-state thermionic emission devices is one of the potentially efficient pathways for converting thermal energy directly into electrical power. The realization of practical of thermionic energy conversion devices strongly depends on achieving low work function materials, which is thus far a limiting factor. In an attempt to develop a new low work function thermionic material, this work reports thermionic emission energy distributions (TEEDs) from nanocrystalline diamond (NCD) films in the temperature range from 700 to 900 °C that reveal a consistent effective work function of 3.3 eV. The NCD films also exhibit emission peaks corresponding to higher work functions as indicated by shifts in their energy position and relative intensity as a function of temperature. These shifts thus appear to be related to instabilities in the NCD’s surface chemistry. The analysis of these data yields information on the origin of the low effective work function of NCD.

Irving Langmuir and the light bulb

Irving Langmuir's principal contribution to lighting was a major improvement in the efficiency of the incandescent lamp. He also used experiments on these lamps to provide fundamental new insight into a number of other areas of physics, including evaporation of metals, the space charge limited current in a vacuum and thermionic emission of electrons from metallic surfaces. This paper describes his experiments, and the chain of ideas which led him to new discoveries.

Synthesis and Optical Characteristics of Green-Emitting ( Y , Gd ) Al3 ( BO3 ) 4 : Tb3 + Phosphor for Plasma Display Panel Applications

The objective of this work is to find a green phosphor for plasma display panels which has both high luminescence and a short decay time. A new green phosphor, (, ), was synthesized by a flux-assisted solid reaction and its vacuum UV excitation and emission characteristics were examined. We found that under 147 and excitations, phosphors showed a strong green emission at corresponding to the electric dipole transition of . The luminance of at the excitation was higher than that of , while the spectrum and decay time of the Tb ion was about the same as that of the commercial phosphor .

Hollow cathode conditioning and discharge initiation

In the framework of developing a fast-starting hollow cathode, one must understand how a cathode is conditioned for operation. The desorption of known contaminates as a function of heater power was measured to quantify cathode conditioning characteristics. In addition to conditioning, a study of hollow cathode emission characteristics under vacuum conditions was performed. Results of these two studies are described herein along with measurements of the temperature of the cathode as a function of time and heater power. The data are used to validate a transient surface diffusion model that describes the migration and surface coverage fraction of low-work-function material from the interior of the hollow cathode to the orifice barrel and to the exterior surface of the orifice plate of the cathode. A sufficiency condition is presented for our cathode geometry of vacuum emission current level and the ability to start a hollow cathode discharge once gas flow is initiated.

Direct observation of tetrahertz electromagnetic waves emitted from intrinsic Josephson junctions in single crystalline Bi2Sr2CaCu2O8+δ

We have observed intense, coherent, continuous and monochromatic electromagnetic (EM) emission at terahertz frequencies generated from a single crystalline mesa structure of the high-Tc superconductor Bi2Sr2CaCu2O8+δ intrinsic Josephson junction system. The mesa is fabricated by the Argon-ion-milling and photolithography techniques on the cleaved surface of Bi2Sr2CaCu2O8+δ single crystal. The frequency, ν, of the EM radiation observed from the sample obeys simple relations: ν=c/nλ=c/2nw and ν=2eV/hN, where c is the light velocity in vacuum, n the refractive index of a superconductor, λ the wave length of the EM emission in vacuum, w the shorter width of the mesa, V the voltage applied to the mesa, N the number of layers of intrinsic Josephson junctions, e and h are the elementary charge and the Planck constant, respectively. These two relations strongly imply that the mechanism of the emission is, firstly, due to the geometrical resonance of EM waves to the mesa like a cavity resonance occuring in the mesa structure, and forming standing waves as cavity resonance modes, and secondly, due to the ac-Josephson effect, which works coherently in all intrinsic Josephson junctions. The peculiar temperature dependence of the power intensity emitted form samples shows a broad maximum in a temperature region between 20 and 40K, suggesting that the nonequilibrium effect plays an essential role for the emission of EM waves in this system. The estimated total power is significantly improved in comparison with the previous report [L. Ozyuzer et al., Science 318 (2007) 1291, K. Kadowaki, et al., Physica C 437–438 (2006) 111, I.E. Batov, et al., Appl. Phys. Lett. 88 (2006) 262504], and reached as high as 5μW from single mesa with w=60μm at 648GHz, which enables us to use it for some of applications. So far, we succeeded in fabricating the mesa emitting EM waves up to 960GHz in the fundamental mode in the w=40μm mesa, whereas the higher harmonics up to the 4-th order were observed, resulting in a frequency exceeding 2.5THz. In sharp contrast to the previous reports [K. Kadowaki, et al., Physica C 437–438 (2006) 111 , M.-H. Bae, et al., Phys. Rev. Lett. 98, (2007) 027002], all the present measurements were done in zero magnetic field. Lastly, a plausible theoretical model for the mechanism of emission is discussed.

Influence of dielectric environment on quantum-well luminescence spectra

A fully microscopical theory for the photoluminescence of a quantum-well in an arbitrary one-dimensional stack structure is presented. For strong-coupling configurations, the full semiconductor luminescence equations are solved. For the weak-coupling regime, a frequency-dependent filter function is directly derived from the semiconductor luminescence equations with the knowledge of the dielectric structure. Via that filter function, the detected luminescence can be related to the pure quantum-well emission in vacuum. The approach is generalized to include corrections to the emitted peak width due to the photonic-environment-dependent radiative decay, and the corrections are shown to be obtainable from the mode functions alone. The applicability of the method is thoroughly tested up to the onset of normal-mode coupling.

Materials characteristics and surface morphology of a cesium iodide coated carbon velvet cathode

Cesium iodide (CsI) coated carbon fiber cathodes have shown promise as a cold cathode for microwave and x-ray devices. In particular, the cathodes have demonstrated over 1 000 000 shots lifetime at operating voltages at or in excess of 165 kV and current densities greater than 25A/cm2. While the vacuum emission characteristics have been well studied, the materials characteristics of the cathodes themselves, particularly after operation, have received little attention. Furthermore, while researchers at University of Wisconsin have demonstrated a reduction in work function of carbon due to the CsI coating, the details of the emission mechanism remain poorly understood. This article gives results of a series of materials diagnostics investigating the cathode surface morphology as well as the changes in the carbon fiber structure with cathode shot history. We demonstrate that the cathode surface undergoes several changes in relation to the bond line along the fiber-substrate interface as well as at the fiber tips. While the exact mechanisms leading to these changes have not been determined, we offer several possible explanations for the changes, as well as the means by which these mechanisms can be ascertained.

Forms and behaviour of vacuum emission electronic devices comprising diamond or other carbon cold cathode emitters

Nanocarbon-derived electron emission devices, specifically nanodiamond lateral field emission (FE) diodes and gated carbon nanotube (CNT) triodes, are new configurations for robust nanoelectronic devices. These novel micro/nanostructures provide an alternative and efficient means of accomplishing electronics that are impervious to temperature and radiation. For example, nitrogen-incorporated nanocrystalline diamond has been lithographically micropatterned to use the material as an electron field emitter. Arrays of laterally arranged 'finger-like' nanodiamond emitters constitute the cathode in a versatile diode configuration with a small interelectrode separation. A low diode turn-on voltage of 7V and a high emission current of 90 microA at an anode voltage of 70V (electric field of approx. 7V microm(-1)) are reported for the nanodiamond lateral device. Also, a FE triode amplifier based on aligned CNTs with a low turn-on voltage and a small gate leakage current has been developed.

Field enhancement at a triple junction in arrangements consisting of three media

The field singularity occurs at an edge point where wedge-like interfaces of dielectrics meet. This phenomenon is well known as Takagi-effect or triple-junction effect in the cases of two dielectrics. Its significance is twofold; (1) the negative phenomena of possible discharge or breakdown inception, and (2) the positive application of the field enhancement to field emission or electron beam generation in vacuum. This paper describes the analytical expression which gives the exponent of the field singularity in three dielectric media, and applications of this expression in various simple conditions. The analytical expression is also compared with the value given from the numerical field calculation.

Diamond and boron carbide field emission in medium vacuum

AbstractField electron emission current–time and current–voltage measurements were made on single field emitters of synthetic diamond and boron carbide in medium vacuum (10−5–10−8 Torr), and two diamond specimens were characterized by scanning electron microscopy (SEM) and field ion microscopy (FIM). Generally, we found that the boron carbide emitters were less noisy than the diamond emitters, and that emission current stability improved with increasing current, at least up to 6 × 10−6 A. Use of the conventional field electron microscope (FEM) configuration enabled direct visualization of the surface condition during emission current and voltage measurements, and the combination of FIM and FEM permitted an estimation of current density for one diamond specimen, 1 × 106 A/cm2. Copyright © 2007 John Wiley & Sons, Ltd.