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One of the problems of energy harvesting with nanoantennas is a limited coherence length of solar or any other greybody radiation. In our previous paper on the subject, we proposed using Talbot effect to enhance, potentially indefinitely, the coherence length of blackbody radiation. Current paper extends this result by providing a theory, which takes into account a finite aperture of incipient beam. In this context we must mention that for monochromatic laser beam, the transmission through a correctly placed second grating completely restores coherence, smeared by the first grating. Because of an extended spectral range of blackbody radiation, complete reconstruction of an image obtained by metamaterial propagation is not possible and additional structures appear in a beam. We try to demonstrate extra features of blackbody radiation, which appear on propagation through the periodic structures located at average Talbot distance for the emission spectrum of the source.

1. INTRODUCTION We have previously presented a method for optical rectification that has been demonstrated both theoretically and experimentally and can be used for the development of a practical rectification and energy conversion device for the elec tromagnetic spectrum including the visible portion. This technique for optical frequency rectification is based, not on conventional material or temperature asymmetry as used in MIM or Schottky diodes, but on a purely geometric property of the antenna tip or other sharp edges that may be incorporated on patch antennas. This “tip” or e dge in conjunction with a collect or anode providing connection to the external circuit constitutes a tunnel junction. Because such devices act as both the absorber of the incident radiation and the rectifier, they are referred to as “rectennas.” Using current nanofabrication techniques and the selective Atomic Layer Deposition (ALD) process, junctions of 1 nm can be fabricated, which allow for rectif ication of frequencies up to the blue portion of the spectrum (see Section 2). In this paper we treat in detail the role of geometry in nanoscale rectennas for rectification and energy conversion. We also discuss different model analyses and mathematical treatments for electron emission from a sharp tip. which all exhibit the same focusing and tunneling features due to asymmetrical geometry. Due to the incident radiation AC currents are induced along the length of the antenna, which produce oscillating charges at the top or edge of the geometrically asymmetric tunneling junction and corresponding image currents in the anode. The presence of the constricted geometry of the tip or edge gives rise to an e nhanced field at the tip. The oscillating charges in the tunnel junction induce an AC voltage across the gap. If the induced field is sufficient for field emission, a tunneling current is produced. Due to geometric asymmetry (and possible material asymmetry or plasmonic coatings), there is a difference between the potential barriers for forward and reverse bias, which results in a rectified DC current (see section 3). An SEM image of our fabricated geometrically-asymmetric device that we use in our own research is shown in Fig. 1. Note that we have chosen a device structure that has been fabricated on a large scale using standard nanofabri cation techniques.

The exact solutions for fully three‐dimensional electrostatic quantities due to a free charge in a nanoscale tip/base junction, such as the scanning‐tunneling microscope, are given for the problem as modeled in the prolate‐spheroidal coordinate system. These exact solutions consist of summation series of integrals of conical functions, and methods for calculating these exact solutions with high accuracy are described in detail in this paper. Calculated results for the surface‐charge distribution on the tip and the base due to a free electron in a nanoscale tip/base junction are also presented. This analysis has important implications for the ultimate objective of quantifying electron tunneling, from first principles, in nonplanar geometries such as sharp vacuum field emitters.

The spatial and temporal temperature distribution for a conical shape metal emitter under laser irradiance is obtained analytically using the Green function method. In this study the tip is modeled as an infinite cone of half angle θ0 which can vary between 0 and 12π. The full three-dimensional heat diffusion equation is solved simultaneously with the Fourier equation for the heat flux assuming no radiation losses. The general solution is obtained for an arbitrary temporal and spatial distribution of the irradiance. With uniform irradiance the temperature rise in the conical tip varies almost linearly with the laser intensity and heating time and depends strongly on cone angle and thermal properties. For a tungsten tip, the temperature increase is about 2–3 orders of magnitude above ambient for a typical laser intensity of about 1 MW/cm2 in a time of a few hundred nanoseconds. To study Joule heating and the Thomson thermoelectric effect in the STM junction we first obtained the current distribution inside the emitter. Using this as a volume heat source, we used the Green function method to solve the heat conduction equation. The resulting temperature gradient was then used to obtain the thermoelectric potential. For a tungsten tip at ∼ 1000 K the thermoelectric effect generates a bias voltage on the order of 10 mV.

Spindt et al. have demonstrated that high density field emitter arrays fabricated using thin-film techniques and electron beam microlithography operate at very low applied voltages, compared with that for conventional etched wire field emission tips. These arrays are capable of operating at effective current densities far higher than can be obtained with etched emitters. The current-voltage curves obtained from Spindt devices generally follow Fowler-Nordheim behavior, but differ dramatically in slope of the F-N curves. It has been suggested that the apparent emitting area per tip is a few atomic sites. By contrast, the data for the etched emitters indicate a much greater emitting area (about 100 atomic sites), from which it is concluded that there are anomalously high fields on Spindt cathodes. This difference was explained by assuming that the emitting atoms form a protuberance on the tip of the cone with a field magnification factor of about 5. Closed form analytical solutions for the image interaction for tips modeled as a cone, paraboloid, hyperboloid, and sphere on cone have been derived. Numerical results show that the classical image interaction for these non-planar emitter geometries is diminished in magnitude relative to the planar image interaction for the same external bias potential. Simple analytic expressions for the image interaction for these non-planar geometries are obtained by fitting of the numerical results. The tunneling barriers have been constructed by adding the image potential for each model of the tip and the corresponding geometrically dependent bias potential. It is found that the bias potentials (fields) for the model emitters significantly modify the shape of the tunneling barriers, and the resulting forms predict a dramatically enhanced current relative to the “classical” Fowler-Nordheim result.

The precise definition and physical interpretation of a tunneling time is a fundamental problem in quantum mechanics. The lack of a well‐defined time operator in quantum theory precludes the calculation of time in terms of an expectation value. Consequently, model calculations and simulation studies have been proposed and used to determine the time for a particle to traverse a quantum‐mechanical barrier. The results offer both qualitatively and quantitatively, disparate predictions. One of the more commonly used approaches is the phase method, which we have applied to several one‐dimensional model potentials to show that the tunneling time is characterized by the ratio of the typical decay length in the barrier to the incident velocity, τ∼(1/κ)/v0. We also show that the phase and the spin precession methods are equivalent when the magnetic field is applied throughout the space containing the particle wave function and the tunneling barrier. In principle, the spin precession method can be regarded as an operational definition of the tunneling time. It also has been shown that a tunneling time can be defined operationally for a time‐dependent barrier, for which the model calculations predict a tunneling time given by the mean barrier width divided by the magnitude of the imaginary velocity of the particle in the barrier, τ∼d/‖v‖. This pseudokinematic result has been inferred by magnetic field dependent tunneling through heterostructures and computer simulation of wave packet tunneling through one‐dimensional square barriers. Other tunneling time theories predict one or the other of the above mentioned characteristic times; several theories predict both. We interpret these different results for τ to mean that a tunneling time is not uniquely defined as a function of particle energy and the barrier shape unless the process in which the tunneling is involved is also specified. That is, the tunneling time is a meaningful and unique concept only for particular tunneling processes or definite operational procedures.

Charge transport in diamond is studied using the Monte Carlo method, in which the scattering of electrons by phonons is considered stochastically. It is assumed that electrons are injected into the diamond conduction band with an initial equilibrium energy distribution and they are then accelerated by the internal field subject to phonon scattering. It is found that the electron energy distribution is independent of the field up to ≂0.1 V/μm. For larger fields, ‘‘hot’’ electron transport is predicted, i.e., the distribution shows a tail which depends on the internal field and the thickness of the diamond film. It implies that if electron field emission is from the conduction band in a diamond film, the transport and the energy spectrum of the emitted electrons should exhibit hot electron features.

The electron field emission from diamond surfaces is investigated theoretically using a model consisting of the projection of the energy band surfaces in the 〈111〉, 〈110〉, and 〈100〉 emission directions. The effect of the negative electron affinity, the band bending, the image interaction, and surface states is examined in detail. It is found that the conventional theory of electron field emission applied to crystalline diamond cannot explain the measured high‐current emission at low fields. We postulate two subbands in the intrinsic band gap, which may be generated by defects or impurities. With reasonable band parameters, the calculated I‐V characteristics agree with experimental data.

The geometry of an atomically sharp or nearly atomically sharp tip in proximity to a planar anode may be closely approximated in the prolate‐spheroidal coordinate system. An exact three‐dimensional electrostatic‐potential solution for a free charge in such a tip/base junction is given in this letter, including calculations for both the symmetrical on‐axis case and the asymmetric off‐axis case. An exact solution for the potential‐energy barrier is also given; this solution has immediate applications in three‐dimensional tunneling studies and in calculations of electron trajectories in micron‐ and submicron‐sized field‐emitter arrays.

Researchers disagree about why patients with schizophrenia perform poorly on memory tests. Some argue the presence of a fundamental memory deficit stemming from dysfunction in medial temporal lobe structures, principally the hippocampus. Others, stressing the contributions of impaired attention or executive failings such as a disorganized approach to learning, implicate larger neural networks. We compared data from psychometrically similar procedures, the Wechsler Memory Scale-Revised (WMS-R) and Wechsler Adult Intelligence Scale-Revised (WAIS-R), generated by 17 schizophrenia-spectrum patients and 33 psychiatric controls. We then compared our findings in detail with all published WMS-R/WAIS-R schizophrenia data. Our findings and the literature indicate that the acquisition of new information is disrupted in schizophrenia, but they provide little support for claims that memory deficits are especially pronounced relative to other weaknesses. Since schizophrenia patients exhibit reasonable retention following intervening activity, theories that place primary emphasis upon hippocampal dysfunction are not well supported.

The Wide Range Achievement Test-Revised (Level 2) provides valuable data in neuropsychological assessments, but its inclusion is costly in time. Although the Reading and Arithmetic subtests are inappropriate for abbreviation, the Spelling subtest is unnecessarily lengthy for most clinical needs. In a retrospective study of the reliability of data obtained from split-half and split-third tabulations with mixed psychiatric and forensic subjects, strong correlations were obtained between the split-halves and between split-half and full administration scores. A Spearman-Brown correction of the relationship between the split-halves yielded a correlation of .97, higher than that reported for any WAIS-R subtest (Wechsler, 1981). Standard scores generated by split-half tabulations were within 10 points of those generated from the complete raw data in 94.6% of cases. These findings suggest that the Spelling subtest could be abbreviated in clinical settings with minimal loss of reliability.

We present a new and viable method for optical rectification. This approach has been demonstrated both theoretically and experimentally and is the basis fot the development of devices to rectify radiation through the visible. This technique for rectification is based not on conventional material or temperature asymmetry as used in MIM (metal/insulator/metal) or Schottky diodes, but on a purely sharp geometric property of the antenna. This sharp “tip” or edge with a collector anode constitutes a tunnel junction. In these devices the rectenna (consisting of the antenna and the tunnel junction) acts as the absorber of the incident radiation and the rectifier. Using current nanofabrication techniques and the selective atomic layer deposition (ALD) process, junctions of 1 nm can be fabricated, which allow for rectification of frequencies up to the blue portion of the spectrum. To assess the viability of our approach, we review the development of nanoantenna structures and tunnel junctions capable of operating in the visible region. In addition, we review the detailed process of rectification and present methodologies for analysis of diode data. Finally, we present operational designs for an optical rectenna and its fabrication and discuss outstanding problems and future work.

Scanning tunneling microscope (STM) has been used to characterize the rectifying properties of metal nanojunctions. Advances in nanofabrication have made it possible to fabricate solid state nanotunnel junctions similar to point contact and STM junctions. (AIP)

The local density of states (LDOS) at an atomically sharp Si tip is calculated using a tight‐binding model. A pyramidal shaped cluster of ten layers is constructed to represent the tip. The top six layers are allowed to relax while the bottom four are fixed to simulate the bulk. The forces are calculated and used to optimize the atomic geometry. It is found that the LDOS of the topmost atom is significantly different than that in the bulk, and that it changes little with an applied bias. However, the topmost atom does not have any strongly localized occupied state, though there exists more localized p‐type states above the highest occupied states. This suggests that the discrete character of the structure plays a significant role in determining the LDOS for an atomic size tip.

The electron field emission from diamond surfaces is investigated theoretically using a model consisting of the projection of the energy‐band surfaces in the 〈111〉, 〈110〉, and 〈100〉 emission directions. The effect of the negative electron affinity, the band bending, the image interaction, and surface states are examined in detail. It is found that the tunneling from the bulk conduction and valence bands is negligible in p‐type diamond. While emission from surface states located about 1 eV below the conduction band has sufficient transmission probability to produce currents observed in experiments, there is no obvious transport mechanism in intrinsic, doped, or polycrystalline diamond to sustain a direct field emission current. We postulate two subbands in the intrinsic band gap, which may be generated by defects or impurities. With reasonable band parameters, the calculated j–F characteristics agree with experimental data.

Experiments on frequency mixing and tunnelingtime measurements have stirred renewed interest in the problem of tunneling through time dependent barriers in a vacuum diode configuration such as the scanning tunneling microscope(STM) junction. In the present work we have formulated a theory of photoassisted emission for a planar bimetallic junction with both a static dc voltage and a time dependent bias of angular frequency ω. An expression for the current is derived using kinetic theory and the transfer Hamiltonian method. The wave functions are determined from time dependent perturbation theory and are calculated recursively to infinite order. Although the current is initially calculated within the free‐electron model, this kinetic formalism allows for the inclusion of band‐structure effects through the density of states factors. The photoassisted inelastic tunneling current was calculated as a function of frequency, fields, barrier parameters, and material asymmetry. The results are applied to an analysis of the tunneling time experiments of Nguyen et al. [H. Q. Nguyen et al., IEEE Trans. Electron Devices 36, 2671 (1989)]. The conclusions impose some constraints but do not limit the experimental procedure used for extracting tunneling times in an irradiated STM junction.

In a series of recent experiments, research groups have made absolute frequency measurements with laser beams in the infrared region of the spectrum (λ ≲ 10 μm) using a metal point contact diode for generation, frequency mixing and detection. It has been postulated that the mechanism for the nonlinear current-voltage characteristic of the diode is tunnelling of electrons through an intermediate oxide film from the whisker into the metal base, i.e., the configuration is considered to be a metal-oxide-metal (MOM) tunnelling junction. Several features of the diode's operation create considerable doubt concerning the applicability of the MOM tunnelling mechanism. Analysis of the available experimental data led us to postulate an alternate solid state mechanism, namely a thermally enhanced field emission process. Such emission would be a consequence of the immersion of the whisker tip in the laser radiation resulting in (1) conduction heating which induces thermionic emission and (2) generation of an electric field at the tip necessary for electron tunnelling by field emission. In this paper we calculate rigorously the power absorbed in the metal whisker from the incident radiation. From the power absorbed, the heat conduction equation is solved for model geometries to obtain the laser induced temperature distribution at the whisker surface. Estimates of the electric field are obtained and combined with temperature calculations to obtain the nonlinear I – V characteristics of the thermally enhanced field emission model. Finally some simple experiments are proposed to test the thermal field emission hypothesis as a possible mechanism to explain the nonlinear characteristics of the metal whisker point contact diode.

This paper elaborates on the idea, recently advanced by the authors, of using the scanning tunneling microscope (STM) in new experiments on laser frequency synthesis and laser rectification at optical frequencies. We first introduce the subject of harmonic generation and mixing of laser beams by the related tunneling device called ‘‘point contact diode.’’ Then, we present new quasistatic current–voltage characteristics of several STM junctions, emphasizing their nonlinear and rectifying aspects. The physical origin of the observed asymmetrical I–V curves is discussed. Finally, we describe the proposed rectification experiment which should result in an operationally meaningful definition of a tunneling time for electrons crossing an STM junction and we present some very preliminary results.

Schottky barrier-type GaAs baseline devices (semiconductor surface etched and then immediately metalized) and GaAs conducting metal oxide-semiconductor devices are fabricated and characterized. The baseline surfaces (no purposeful oxide) are prepared by a basic or an acidic etch, while the surface for the MIS devices are prepared by oxidizing after the etch step. The metallizations used are thin-film Au, Ag, Pd, and Al. It is shown that the introduction of purposeful oxide into these Schottky barrier-type structures examined on n-type GaAs modifies the barrier formation, and that thin interfacial layers can modify barrier formation through trapping and perhaps chemical reactions. For Au- and Pd-devices, enhanced photovoltaic performance of the MIS configuration is due to increased barrier height.

A Green’s function formalism has been used to obtain, for the first time, the exact multiple‐image interactions for a hyperboloidal‐tip–planar‐anode model of a metal‐whisker point‐contact diode. I‐V characteristics, including all image interactions, have been calculated for the point‐contact junction consisting of electrodes with identical metals. These are compared with results for the parallel‐plane diode model with exact and appropriate forms of the image barrier. In contrast to the case of the parallel‐plane diode of identical metals, rectification is predicted for the point‐contact diode. Significant differences between the calculated I‐V curves are found to be a consequence of the different geometries and approximations for the image interactions.

In recent years, research groups have used metal-metal point contact diodes for frequency mixing and detection of infrared laser radiation. It has been postulated that the mechanism for the nonlinear current-voltage characteristic of the diode is the tunneling of electrons through an intermediate oxide film from the whisker tip to the metal base, i.e., the configuration is considered to be a metal-oxide-metal (MOM) tunneling junction. Several features of the diodes' operation create considerable doubt concerning the applicability of the MOM tunneling mechanism. Analysis of the available data led us to postulate an alternate solid state mechanism, namely a thermally enhanced field emission process. Such emission would be a consequence of the immersion of the whisker in the laser radiation resulting in (1) conduction heating which induces thermionic emission and (2) generation of an electric field at the tip necessary for electron tunneling. In an earlier paper, we calculated the power absorbed by the cylindrical shank of a point contact diode in an infrared radiation field. Using the absorbed power as a source, detailed calculations were made of the laser induced temperature distributions on the diode; more approximate treatments were used to obtain the electric fields developed on the tip. Values of the computed temperature and field parameters for tungsten were found to be consistent with a thermal field emission process. In this paper we present a more rigorous calculation of the voltages and fields induced on different metal whisker tips by the incident laser radiation. Linear antenna theory is used to describe the receiving properties of the diode. The actual pointed geometry of the diode tip has been taken into account using Schelkunoff's theory of the conical antenna. The electric fields at the tip are found to be comparable with those necessary for field emission. The highest fields are established on gold tips, consistent with the experiments of Green et al. who found the best responsivity occurs with gold-gold contacts. Finally we discuss the significance of the experimental results of Young et al. on metal-vacuum-metal tunneling characteristics to the MOM tunneling hypothesis.

In a series of recent experiments, research groups have made absolute frequency measurements with laser beams in the infrared region (μm) using a metal-metal point contact diode for the generation, frequency mixing and detection. At present there are two models which attempt to explain the rectification mechanism of the diode: 1) Tunneling of electrons through an intermediate oxide film from whisker to the metal base, i.e., configuration is considered to be a metal-oxide-metal (MOM) tunneling junction. 2) Rectification and nonlinear processes are the result of a thermal enchanced field emission. Such emission is a consequence of the immersion of the whisker in the laser radiation which results in conduction induced thermionic emission and/or generation of an electric field at the tip necessary for electron tunneling by field emission. The purpose of this comment is: a) to discuss qualitatively the basic difference between MOM and TFE theories as regards the origin of the nonlinearity and rectification properties of the metal point contact junction; b) to review the analyses describing the ultimate frequency response of the device; and 3) to provide a possible explanation for polarity reversal consistent with the TFE mechanism describing the operation of the whisker diode.

A Fabry‐Perot interferometer with one spherical mirror was used to measure the absorption of pure O2 and O2–Ar and O2–N2 gas mixtures. The experimental pressure range was 3.7–51 atm and the frequency range 48–81 GHz. The measured spectrum was successfully reproduced by a calculated line shape using Gordon's pressure broadening theory and 3‐ and 4‐body classical scattering calculations.

The rotational levels of the ground vibrational state of D2O have been studied utilizing 29 observed microwave transitions and 259 rotational energy differences obtained from infrared measurements. Seventeen of the microwave lines in the region 70–310 GHz have been observed in the course of this work. Eighteen linearly independent rotational constants are required to fit the observations: three constants of the second power of the angular momentum, five constants of the fourth power, seven constants of the sixth power, and the coefficients of the operators Pz8, Pz6P2, Pz10. Values of these constants and the calculated microwave transition frequencies up to 1000 GHz are reported. The coefficients of Pz2, Px2, Py2 are, respectively, A = 462 292.37 ± 0.43, B = 217 979.95 ± 0.27, and C = 145 303.34 ± 0.14 MHz.

The chemical nature of semitransparent (∼125 A) palladium on silicon Schottky‐barrier‐type devices was determined by complementary AES and ISS techniques. Postdeposition analyses of metal‐semiconductor (MS) and metal–thin‐insulator–semiconductor (MIS) devices prepared without heat treatment showed that palladium silicide is formed in the MS structures, while the presence of an ultrathin (∼30 A) purposefully grown semiconductor oxide film inhibits the chemical reaction between Pd and Si in the metal overlayer. Chemical bonding information extracted from the AES data was correlated with barrier‐height measurements obtained from capacitance‐vs‐voltage (C‐V) and current‐vs‐voltage (I‐V) electrical characteristics of these devices.

This letter presents new results on the tunneling and rectification characteristics of metal‐whisker point‐contact diodes. The model adopted here includes two new important effects, namely, the actual geometrical asymmetry of the electrodes and a temperature differential between tip and base. Calculations, for a point‐contact junction of identical metals, confirm the feasibility of a rectification mechanism based solely on the geometrical asymmetry of the tip and base.

This study was designed to investigate the hypothesis that the trot-gallop transition in running guadrupeds occurs when active cross-sectional areas of muscles or fiber populations within muscles, reach some critical point as animals increase speed within trotting. Rats (Rattus norvegicus) were used as experimental animals, and glycogen depletion was used to estimate patterns or fiber activity. Our results indicate that 1) the contribution to power output by the front limb muscles was less than that of the hind limb muscles during trotting and galloping; 2) the active cross-sectional area of plantaris muscles peaked immediately prior to the transition in gait; 3) the ankle plantar flexor group of muscles as a whole did not attain a maximum active cross-sectional area during fast trotting; and 4) no major discontinuities in whole muscle or fiber type glycogen depletion rates occurred across the gait change. Although these findings do not prove the hypothesis, they support the concept that the trop-gallop transition follows the attainment of peak active muscle cross-sectional areas as animals increase trotting speed.

In this talk we present some recent theoretical results on the study of tunneling and current-voltage (I-V) characteristics of metallic cat whisker point-contact diodes. These point-contact devices, which currently have the fastest response times (∼< 2 orders of magnitude) of all infrared and optical detectors (e.g., Josephson tunneling devices, Schottky barrier diodes, etc.) [1], have applications to precision frequency measurements [1-3] as well as in quantum electronics and solid-state physics [4], including infrared studies of superconductivity [5]. The structure is made of an ultra-fine metal tip, comparable to those used in field emission (i.e., ∼10− 2 - 1 μm radius) in contact with a flat metal anode. A fundamental application of these devices has been the most precise and accurate determination of the velocity of light [1-3,6,7].

In this paper we wish to report some recent theoretical results on the effect of the image and multiple image interactions on the tunneling and rectification characteristics of metallic cat-whisker diodes used in precision determinations of infrared laser frequencies [1-3]. The structure is an ultra fine metal tip (< 1 μm radius) in contact with a flat metal anode separated by a distance of the order of 10-40 A. It has been suggested that the diode configuration can be considered a MOM planar tunneling junction described by Simmons theory [1,4]. The authors have questioned the applicability of the Simmons MOM model, based upon planar geometry, to the cat-whisker diode configuration and refined the model, using somewhat different solid state mechanisms [5-7]. This new model includes a thermally enhanced field induced tunneling process to explain the nonlinearity of the devices and a new rectification mechanism, independent of material asymmetry, which takes into account explictly the geometrical shape of the electrodes [8-10]. That is, the actual curvature of the tip gives rise to a nonuniform electric field across the metal-vacuum-metal (MVM) or metal-oxide-metal junction. As a consequence of the nonuniform field distribution, there is a different tunneling barrier for forward and reverse bias which leads to rectification [9].

Durch Transaminierung von (I) mit einem Uberschus an Diamin in Benzol werden in uber 80%iger Ausbeute die Diamin-bis-borane (II) erhalten.

The boron cations reported represent the first examples of mononuclear 2+ species with B–H bonds and show a surprising deactivation of the boron–hydrogen bond towards electrophilic reagents.