Frequency-domain calculation of Smith-Purcell radiation for metallic and dielectric gratings.

The intensity of Smith-Purcell radiation from metallic and dielectric gratings (silicon, silica) is compared in a frequency-domain simulation. The numerical model is discussed and verified with the Frank-Tamm formula for Cherenkov radiation. For 30 keV electrons, rectangular dielectric gratings are less efficient than their metallic counterparts, by an order of magnitude for silicon, and two orders of magnitude for silica. For all gratings studied, radiation intensity oscillates with grating tooth height due to electromagnetic resonances in the grating. 3D and 2D numerical models are compared.

Evaluation of blackbody radiation emitted by arbitrarily shaped bodies using the source model technique.

Planck's famous blackbody radiation law was derived under the assumption that the dimensions of the radiating body are significantly larger than the radiated wavelengths. What is unique about Planck's formula is the fact that it is independent of the exact loss mechanism and the geometry. Therefore, for a long period of time, it was regarded as a fundamental property of all materials. Deviations from its predictions were attributed to imperfections and referred to as the emissivity of the specific body, a quantity which was always assumed to be smaller than unity. Recent studies showed that the emission spectrum is affected by the geometry of the body and in fact, in a limited frequency range, the emitted spectrum may exceed Planck's prediction provided the typical size of the body is of the same order of magnitude as the emitted wavelength. For the investigation of the blackbody radiation from an arbitrarily shaped body, we developed a code which incorporates the fluctuation-dissipation theorem (FDT) and the source model technique (SMT). The former determines the correlation between the quasi-microscopic current densities in the body and the latter is used to solve the electromagnetic problem numerically. In this study we present the essence of combining the two concepts. We verify the validity of our code by comparing its results obtained for the case of a sphere against analytic results and discuss how the accuracy of the solution is assessed in the general case. Finally, we illustrate several configurations in which the emitted spectrum exceeds Planck's prediction as well as cases in which the geometrical resonances of the body are revealed.

Critical phenomenon in tapered dielectric structures.

We demonstrate the existence of a critical behavior of a single electromagnetic mode propagating in a tapered dielectric structure. This behavior is described in terms of a critical phase velocity in the case of an adiabatic tapering. In the vicinity of this critical phase velocity, the tapered structure no longer confines the radiation and a significant fraction of the power escapes transversely.

A novel eyelid motion monitor.

BACKGROUND: Eyelid motion analysis can provide important information about ophthalmic, neurologic, and systemic diseases. Routine assessment of eyelid function is currently based mainly on clinical examination estimating Levator Function and static palpebral fissure measurements. Most clinical tools developed to date are cumbersome expensive and difficult to operate. Currently there is no widely available, affordable device providing user friendly precision based evaluation of eyelid kinematics. Our goal is to develop a novel device for evaluation of eyelid kinematics providing rapid defined diagnosis of diseases involving eyelid movement. METHODS: A real-time prototype eyelid motion monitoring system was designed based on magnetic field sensors detecting movement of a tiny magnet located on the upper eyelid. Motion is recorded and analyzed using specially developed hardware and software, respectively, enabling both real-time and off-line data presentation. The Eyelid Motion Monitor correlates between blinking characteristics of eyelid movement and the output voltages produced by the system. Blink detection is defined as peak in voltage, caused by eyelid closure or opening. The device was tested on 20 healthy volunteers with normal clinical blinking patterns. RESULTS: The Eyelid Motion Monitor succeeded in detecting full blink motion. The system easily extracts different parameters of eyelid kinetics. CONCLUSIONS: An inexpensive prototype novel device was developed for monitoring and analyzing eyelid motion characteristics, including the inter-blink interval, eye closing/opening duration and entire blink duration. The device should allow early objective non- invasive diagnosis and follow-up of disease progression. It could be of great potential value in many ophthalmic, neurologic, and systemic diseases.

Enhanced Cherenkov-wake amplification by an active medium.

A Cherenkov wake confined by perfectly reflecting transverse walls is amplified if the dielectric medium is active. Because of the multiple-reflections process, the effective gain of the wake is enhanced compared to a ray propagating in a straight line. Higher enhancement occurs when the electron velocity is close to the Cherenkov velocity. This Cherenkov wake can then accelerate a second bunch of electrons trailing the first. Gradients larger than 1 GV/m are predicted before saturation becomes a major impediment.

Electrostatic tapering for efficient generation of radiation.

We demonstrate the feasibility of electrostatic (dc) tapering such that the net average force of the electromagnetic (ac) field is compensated by a dc field, which at resonance may be interpreted as "direct" energy transfer from the dc to the ac field. This combination persists in all three components of the setup-e-gun, resonant zone, and collector, in each one playing a different role. In equilibrium, the two field components field-emit at the cathode a density modulated cylindrical beam which is accelerated along the e-gun by the dc field; the latter also focuses the e-beam. Radiation confinement perpendicular to the e-beam is ensured by an array of dielectric Bragg-mirrors and an array of metallic hollow electrodes impose synchronous bunches and wave. This double periodic structure ensures the coexistence of dc and ac fields in the same volume. Numerical simulations demonstrate the feasibility of generation of order of 1[W] power at 1 THz from a volume on a scale of few mm^{3} with efficiency of the order of 25%.

Exposure to static magnetic field facilitates selective attention and neuroplasticity in rats.

Static magnetic fields (SMF) have neuroprotective and behavioral effects in rats, however, little is known about the effects of SMF on cognition, motor function and the underlying neurochemical mechanisms. In this study, we focused on the effects of short-term (5-10d) and long-term (13-38d) SMF exposure on selective attention and motor coordination of rats, as well as associated alterations in expression level of neuroplasticity-related structural proteins and cryptochrome (CRY1) protein in the cortex, striatum and ventral midbrain. The results showed that 6d SMF exposure significantly enhanced selective attention without affecting locomotor activity in open field. All SMF exposures non-significantly enhanced motor coordination (Rotarod test). Neurochemical analysis demonstrated that 5d SMF exposure increased the expression of cortical and striatal CRY1 and synapsin-1 (SYN1), striatal total synapsins (SYN), and synaptophysin (SYP), growth associated protein-43 (GAP43) and post-synaptic density protein-95 (PSD95) in the ventral midbrain. Exposure to SMF for 14d increased PSD95 level in the ventral midbrain while longer SMF exposure elevated the levels of PSD95 in the cortex, SYN and SYN1 in all the examined brain areas. The increased expression of cortical and striatal CRY1 and SYN1 correlated with the short-lasting effect of SMF on improving selective attention. Collectively, SMF's effect on selective attention attenuated following longer exposure to SMF whereas its effects on neuroplasticity-related structural biomarkers were time- and brain area-dependent, with some protein levels increasing with longer time exposure. These findings suggest a potential use of SMF for treatment of neurological diseases in which selective attention or neuroplasticity is impaired.

Enhancing X-ray generation by electron-beam-laser interaction in an optical bragg structure.

We demonstrate that x-ray radiation emitted by relativistic electrons scattered by a counter-propagating laser pulse guided by an adequate Bragg structure surpasses by about 2 orders of magnitude the energy generated by a conventional free-space Gaussian-beam configuration, given the same e beam and injected laser power in both configurations.

Static Magnetic Field Exposure In Vivo Enhances the Generation of New Doublecortin-expressing Cells in the Sub-ventricular Zone and Neocortex of Adult Rats.

Static magnetic field (SMF) is gaining interest as a potential technique for modulating CNS neuronal activity. Previous studies have shown a pro-neurogenic effect of short periods of extremely low frequency pulsatile magnetic fields (PMF) in vivo and pro-survival effect of low intensity SMF in cultured neurons in vitro, but little is known about the in vivo effects of low to moderate intensity SMF on brain functions. We investigated the effect of continuously-applied SMF on subventricular zone (SVZ) neurogenesis and immature doublecortin (DCX)-expressing cells in the neocortex of young adult rats and in primary cultures of cortical neurons in vitro. A small (3 mm diameter) magnetic disc was implanted on the skull of rats at bregma, producing an average field strength of 4.3 mT at SVZ and 12.9 mT at inner neocortex. Levels of proliferation of SVZ stem cells were determined by 5-ethynyl-2'-deoxyuridine (EdU) labelling, and early neuronal phenotype development was determined by expression of doublecortin (DCX). To determine the effect of SMF on neurogenesis in vitro, permanent magnets were placed beneath the culture dishes. We found that low intensity SMF exposure enhances cell proliferation in SVZ and new DCX-expressing cells in neocortical regions of young adult rats. In primary cortical neuronal cultures, SMF exposure increased the expression of newly generated cells co-labelled with EdU and DCX or the mature neuronal marker NeuN, while activating a set of pro neuronal bHLH genes. SMF exposure has potential for treatment of neurodegenerative disease and conditions such as CNS trauma and affective disorders in which increased neurogenesis is desirable.

Electromagnetic forces on the dielectric layers of the planar optical Bragg acceleration structure.

Optical Bragg acceleration structures are waveguides with a vacuum core and dielectric layers as a cladding, designed to guide laser light at the speed-of-light TM mode and accelerate charged particles. In this study, we analyze the electromagnetic forces exerted on the dielectric layers of a planar structure by both the guided laser light and the wake-field of moving charges. The distribution of the volume force densities, as well as the surface force densities, in the interfaces between the layers as a result of the laser propagation is given, and analytic scaling laws for the maximal values are obtained. Separation of the wake-field into the structure's eigenmodes is essential in order to determine the different contributions of the wake-field to the total impulse that acts on the structure. It is shown that the impact of the wake-field on the structure results almost entirely from the fundamental TM mode. While the total force on the dielectric layers may be significantly stronger than the gravitational force, we show that for typical structures, the pressures that develop are orders of magnitude below the damage threshold.

Particle acceleration by stimulated emission of radiation: theory and experiment.

The interaction of electromagnetic radiation with free electrons in the presence of an active medium has some appealing outcomes. Among them is particle acceleration by stimulated emission of radiation (PASER). In its framework, energy stored in an active medium (microscopic cavities) is transferred directly to an e -beam passing through. We have developed a two-dimensional analytic model for the evaluation of the energy exchange occurring as a train of electron microbunches traverses a dilute resonant medium. Efficient interaction occurs at resonance-namely, when the frequency of the train matches the resonance frequency of the medium. It is shown that the energy exchange is gamma independent for relativistic energies and it drops dramatically with an increase of the beam's radius. Based on this model, we have evaluated the relative change in the kinetic energy of a 0.1-nC 45-MeV macrobunch traversing an excited CO2 gas mixture-the former being modulated at the CO2 laser wavelength. Good agreement is found between the theoretical predictions and the results of the PASER experiment performed recently at Brookhaven National Laboratory.

Mirror manipulation by attractive and repulsive forces of guided waves.

Two mirrors guiding laser light may experience an either attractive or repulsive force, according to the type of eigenmode they guide. We propose a method for the control over the motion of a mirror by changing the operation wavelength along the dispersion curve of the mode. In addition, a novel method for trapping a mirror in a stable equilibrium, based on a superposition of two modes, is presented. The mirror is then trapped by being exposed to light only from one of its sides.

Bragg reflection waveguides with a matching layer.

It is demonstrated that Bragg reflection waveguides, either planar or cylindrical, can be designed to support a symmetric mode with a specified core field distribution, by adjusting the first layer width. Analytic expressions are given for this matching layer, which matches between the electromagnetic field in the core, and a Bragg mirror optimally designed for the mode. This adjustment may change significantly the characteristics of the waveguide. At the particular wavelength for which the waveguide is designed, the electromagnetic field is identical to that of a partially dielectric loaded metallic or perfect magnetic waveguide, rather than a pure metallic waveguide. Either a planar or coaxial Bragg waveguide is shown to support a mode that has a TEM field distribution in the hollow region.

Energy recovery in an optical linear collider.

It is demonstrated that recovery of the electromagnetic energy of the fundamental mode at the output of an acceleration structure leads to a significant efficiency enhancement. When using a single bunch, the number of electrons accelerated is rather small. In fact, this number is virtually identical to the case when no feedback loop is employed. To increase this number, in parallel with the efficiency enhancement associated with the feedback loop, it is necessary to split the bunch into a train of microbunches-this last process leads to suppression of high-order modes.

Optical Bragg accelerators.

It is demonstrated that a Bragg waveguide consisting of a series of dielectric layers may form an excellent optical acceleration structure. Confinement of the accelerating fields is achieved, for both planar and cylindrical configurations by adjusting the first dielectric layer width. A typical structure made of silica and zirconia may support gradients of the order of 1 GV/m with an interaction impedance of a few hundreds of ohms and with an energy velocity of less than 0.5c. An interaction impedance of about 1000 Omega may be obtained by replacing the Zirconia with a (fictitious) material of epsilon=25. Special attention is paid to the wake field developing in such a structure. In the case of a relatively small number of layers, it is shown that the total electromagnetic power emitted is proportional to the square of the number of electrons in the macrobunch and inversely proportional to the number of microbunches; this power is also inversely proportional to the square of the internal radius of the structure for a cylindrical structure, and to the width of the vacuum core in a planar structure. Quantitative results are given for a higher number of dielectric layers, showing that in comparison to a structure bounded by metallic walls, the emitted power is significantly smaller due to propagation bands allowing electromagnetic energy to escape.

Collisions of the second kind in a Penning trap.

Employing a simple analytic model, we demonstrate that electrons oscillating in a Penning trap may drain the energy stored in an active medium. For this process to happen, the electrons must become bunched, and the energy imparted to these electrons allows them to leave the trap resulting in a train of bunches, their angular frequency corresponding to the medium's resonance.

Non-thermal electromagnetic radiation damage to lens epithelium.

High frequency microwave electromagnetic radiation from mobile phones and other modern devices has the potential to damage eye tissues, but its effect on the lens epithelium is unknown at present. The objective of this study was to investigate the non-thermal effects of high frequency microwave electromagnetic radiation (1.1GHz, 2.22 mW) on the eye lens epithelium in situ. Bovine lenses were incubated in organ culture at 35°C for 10-15 days. A novel computer-controlled microwave source was used to investigate the effects of microwave radiation on the lenses. 58 lenses were used in this study. The lenses were divided into four groups: (1) Control lenses incubated in organ culture for 10 to15 days. (2) Electromagnetic radiation exposure group treated with 1.1 GHz, 2.22 mW microwave radiation for 90 cycles of 50 minutes irradiation followed by 10 minutes pause and cultured up to 10 days. (3) Electromagnetic radiation exposure group treated as group 2 with 192 cycles of radiation and cultured for 15 days. (4) Lenses exposed to 39.5°C for 2 hours 3 times with 24 hours interval after each treatment beginning on the second day of the culture and cultured for 11 days. During the culture period, lens optical quality was followed daily by a computer-operated scanning laser beam. At the end of the culture period, control and treated lenses were analyzed morphologically and by assessment of the lens epithelial ATPase activity. Exposure to 1.1 GHz, 2.22 mW microwaves caused a reversible decrease in lens optical quality accompanied by irreversible morphological and biochemical damage to the lens epithelial cell layer. The effect of the electromagnetic radiation on the lens epithelium was remarkably different from those of conductive heat. The results of this investigation showed that electromagnetic fields from microwave radiation have a negative impact on the eye lens. The lens damage by electromagnetic fields was distinctly different from that caused by conductive heat.

Two-slab all-optical spring.

It is demonstrated that a waveguide consisting of two dielectric slabs may become an all-optical spring when guiding a superposition of two transverse evanescent modes. Both slabs are transversally trapped in stable equilibrium due to the optical forces developed. A condition for stable equilibrium on the wavenumbers of the two modes is expressed analytically. The spring constant characterizing the system is shown to have a maximal value as a function of the equilibrium distance between the slabs and their width.

Experimental observation of direct particle acceleration by stimulated emission of radiation.

We report the first experimental evidence for direct particle acceleration by stimulated emission of radiation. In the framework of this proof-of-principle experiment, a 45 MeV electron macrobunch was modulated by a high-power CO2 laser and then injected into an excited CO2 gas mixture. The emerging microbunches experienced a 0.15% relative change in the kinetic energy, in a less than 40 cm long interaction region. According to our experimental results, a fraction of these electrons have gained more than 200 keV each, implying that such an electron has undergone an order of magnitude of 2 x 10(6) collisions of the second kind.