Plasma asymmetric dipole antenna excited by a surface wave

A study is made of a quarter-wave asymmetric dipole antenna in which the conducting rod is replaced by a plasma column with an electron density much higher than the critical density. The parameters of such an antenna are determined by the exited surface wave, which affects the electromagnetic field structure in the near-field zone. It is shown analytically, numerically, and experimentally that the resonant length of the plasma dipole antenna is close to one-quarter of the length of the surface wav and that the conversion efficiency of plasma antenna power into radiation can be no worse than that of a metal dipole antenna. It is also shown experimentally that the plasma in a dipole antenna can be self-consistently excited by an RF oscillator and that the excited RF oscillations can be efficiently radiated into the surrounding space.     

Modeling and simulation of novel antenna for the treatment of hepatocellular carcinoma using finite element method

In this article, new interstitial antenna operating at a frequency of 2.45 GHz for the treatment of hepatocellular carcinoma (HCC) using microwave ablation has been investigated. This antenna is basically an asymmetrical miniaturized choke dipole antenna with a pointed needle at the tip. A commercial finite element method (FEM) package, COMSOL Multiphysics 3.4a, has been used to simulate the performance of needle tip choke antenna. The performance of the antenna has been evaluated numerically, taking into account the specific absorption rate, antenna impedance matching and geometry of the obtained thermal lesion, and the temperature distribution plot obtained shows that maximum temperature was attained in this simulation. The antenna is also capable of creating a spherical-shaped ablation zone. The size and shape of the ablation zone can be slightly adjusted by adjusting the choke position in order to maintain spherical ablation zones.     

Electronic Beam Switching of Circularly Polarized Plasma Magneto-Electric Dipole Array with Multiple Beams

In this paper, different array arrangements based on magneto-electric (ME) dipole antenna with wideband circular polarization (CP) characteristics are designed and investigated. Planar, triangular prism, square prism, and hexagonal prism array arrangements are considered. Each prism face has a sub-array comprises 2 × 2 ME-dipole elements. Each sub-array has wide impedance matching of 73.7%, a maximum gain of 16.6 dBi, and CP bandwidth of 78.2%. It employs the plasma frequency of the ME-dipole antenna to control its radiation characteristics. Frequency-independent lumped element equivalent circuit is constructed for a single antenna element. It is used to represent the antenna input impedance at different plasma electron densities with fixed physical structure. The proposed equivalent circuit comprises a single series section used for matching enhancement with feeder circuit, and three parallel tuned circuits corresponding to the three resonance frequencies in the input impedance. The best values of the equivalent circuit elements are computed using the particle swarm optimization (PSO) technique. Different array arrangements, planar, triangular, square, and hexagonal prism are designed to create single or multiple beams in different directions. An electronic beam switching is achieved by tuning in the plasma inside the ME-dipole in the desired direction. The radiation characteristics are analyzed and investigated using the finite integration technique (FIT).

     

Graphene-Based Photoconductive Antenna Structures for Directional Terahertz Emission

Plasmonics - Graphene-based photoconductive antenna structures for optically excited terahertz (THz) emission is reported. Initially, a dipole antenna is designed using plasmonic graphene material...     

A plasma receiving dipole antenna

Results from experimental studies of a short-wave plasma dipole transceiver antenna are presented. The efficiency of the plasma receiving antenna is estimated, and the optimal frequency range for excitation and reception under the given experimental conditions is determined.     

Effect of Long-Wavelength Magnetic Field Disturbances on the Generation of Auroral Kilometric Radiation

The effect of long-wavelength magnetic field disturbances typical of the Earth’s auroral region on the generation of auroral kilometric radiation in a narrow three-dimensional plasma cavity in which a weakly relativistic electron flow propagates against the background of cold low-density plasma is analyzed. The dynamics of the propagation and amplification of fluctuation waves with initial group velocities directed toward the higher magnetic field is considered in the geometrical optics approximation. Analysis of wave trajectories shows that the wave amplification coefficients depend on the magnetic field gradient in the reflection region. If the wave reflection point lies in the region where the gradient of the disturbed magnetic field is less than that of the undisturbed dipole field, then the wave amplification coefficients exceed those of waves propagating in the undisturbed field, and vice versa. Thus, the shape of the spectrum of generated waves changes in the presence of long-wavelength disturbances of the dipole magnetic field in such a way that segments with different curvatures can form in the spectrum.     

Spectroscopy and structure of bacteriochlorophyll dimers. I. Structural consequences of nonconservative circular dichroism spectra.

The origin of the nonconservative nature of the circular dichroism (CD) spectrum of bacteriochlorophyll dimers is investigated. It is shown that coupling between the Qy and Qx transitions can, under rather restricting circumstances, lead to an asymmetrical CD spectrum: only for a limited set of relative orientations of the monomers within the dimer is the spectrum found to be asymmetrical. The relation between intensity and asymmetry of the CD spectrum is elucidated. The results are applied to the B820 subunit of the LH1 antenna system and subsequently to the antenna system LH1 itself. Differences in the geometry of the BChls in LH1 versus the LH2 structure are discussed.     

Radiation from a pulsed dipole source in a moving magnetized plasma

The problem of radiation from a pulsed dipole source in a moving magnetized plasma described by a diagonal permittivity tensor is considered. An exact solution describing the spatiotemporal behavior of the excited electromagnetic field is obtained. The shape of an electromagnetic pulse that is generated by the source and propagates at different angles to both the direction of the external magnetic field and the direction of plasma motion is investigated. It is found that even nonrelativistic motion of the plasma medium can substantially influence the parameters of radiation from prescribed unsteady sources.     

Lateralization in the invertebrate brain: left-right asymmetry of olfaction in bumble bee, Bombus terrestris

Brain and behavioural lateralization at the population level has been recently hypothesized to have evolved under social selective pressures as a strategy to optimize coordination among asymmetrical individuals. Evidence for this hypothesis have been collected in Hymenoptera: eusocial honey bees showed olfactory lateralization at the population level, whereas solitary mason bees only showed individual-level olfactory lateralization. Here we investigated lateralization of odour detection and learning in the bumble bee, Bombus terrestris L., an annual eusocial species of Hymenoptera. By training bumble bees on the proboscis extension reflex paradigm with only one antenna in use, we provided the very first evidence of asymmetrical performance favouring the right antenna in responding to learned odours in this species. Electroantennographic responses did not reveal significant antennal asymmetries in odour detection, whereas morphological counting of olfactory sensilla showed a predominance in the number of olfactory sensilla trichodea type A in the right antenna. The occurrence of a population level asymmetry in olfactory learning of bumble bee provides new information on the relationship between social behaviour and the evolution of population-level asymmetries in animals.     

Biologically inspired coupled antenna beampattern design

We propose to design a small-size transmission-coupled antenna array, and corresponding radiation pattern, having high performance inspired by the female Ormia ochracea's coupled ears. For reproduction purposes, the female Ormia is able to locate male crickets' call accurately despite the small distance between its ears compared with the incoming wavelength. This phenomenon has been explained by the mechanical coupling between the Ormia's ears, which has been modeled by a pair of differential equations. In this paper, we first solve these differential equations governing the Ormia ochracea's ear response, and convert the response to the pre-specified radio frequencies. We then apply the converted response of the biological coupling in the array factor of a uniform linear array composed of finite-length dipole antennas, and also include the undesired electromagnetic coupling due to the proximity of the elements. Moreover, we propose an algorithm to optimally choose the biologically inspired coupling for maximum array performance. In our numerical examples, we compute the radiation intensity of the designed system for binomial and uniform ordinary end-fire arrays, and demonstrate the improvement in the half-power beamwidth, sidelobe suppression and directivity of the radiation pattern due to the biologically inspired coupling.     

Influence of dentures on SAR in the visible Chinese human head voxel phantom exposed to a mobile phone at 900 and 1800 MHz

To investigate the influence of dentures on electromagnetic energy absorption during the daily use of a mobile phone, a high-resolution head phantom based on the Visible Chinese Human dataset was reconstructed. Simulations on phantoms with various dentures were performed by using the finite-difference time-domain method with a 0.47 wavelength dipole antenna and a mobile phone model as radiation sources at 900 and 1800 MHz. The Specific energy Absorption Rate (SAR) values including 1 and 10 g average SAR values were assessed. When the metallic dental crowns with resonance lengths of approximately one-third to one-half wavelength in the tissue nearby are parallel to the radiation source, up to 121.6% relative enhancement for 1 g average SAR and 17.1% relative enhancement for 10 g average SAR are observed due to the resonance effect in energy absorption. When the radiation sources operate in the normal configuration, the 10 g average SAR values are still in compliance with the basic restrictions established by the Institute of Electrical and Electronic Engineers (IEEE) and the International Commission on Non-Ionizing Radiation Protection (ICNIRP), indicating that the safety limits will not be challenged by the usage of dentures.     

Comprehensive Surface-Wave Description for the Nano-scale Energy Concentration with Resonant Dipole Antennas

Resonant optical dipole nano-antennas allow giant field enhancement within nano-gaps. To show how the energy of external illumination waves is delivered and concentrated in nano-gaps, we build up a model by considering the dynamical launching and multiple scattering processes of surface plasmon polaritions (SPPs) on both antenna arms. The model captures the main feature of the antenna resonance as evidenced by comparison of the model prediction with fully vectorial numerical results and provides an intuitive picture that the energy of external wave is initially transferred into SPP and is then coupled into the nano-gap. The enhanced field in the nano-gap oscillates quasi-periodically with the increase of the antenna-arm length, and the resonance peaks can be predicted with a phase-matching condition derived from the model, showing that antenna resonance is due to a constructive interference of the multiple-scattered SPPs. Analytical equation for determining the complex resonance wavelength and the quality factor of the resonant modes is obtained. The model however exhibits observable deviation from fully vectorial numerical results for the lowest resonance order (for antenna with the shortest arms), evidencing that, for this case, surface waves other than SPPs contribute to the antenna resonance. The present results are helpful for clarifying the underlying physics for the energy concentration with resonant dipole antennas and may provide recipes for intuitive design of antenna devices, such as those used for optical nonlinearity enhancement and biochemical sensing.     

The effects of unilateral antennectomy on the flight behaviour of male Heliothis virescens in a pheromone plume

Abstract .Unilaterally antennectomized Heliothis virescens (F.) males flying close to the central axis of a plume of sex pheromone display no significant differences in behaviour compared to sham-operated males in course angles, track angles, airspeed and groundspeed. This demonstrates that right/left antennal information is not necessary for normal orientation movements in response to pheromone, but rather that it is 'blended' within the moth's central nervous system before pheromone-mediated manoeuvres are made. However, some unilaterally antennectomized moths (36%) make repetitive, asymmetrical, saw-tooth-shaped tracks during pheromone-mediated upwind progress, whereas control moths never make such tracks. Unilaterally antennectomized moths made such tracks on the side of the plume contralateral to the missing antenna. We hypothesize that these occasional asymmetrical tracks in unilaterally ablated males are the result of reiterative asymmetrical pheromone stimulation of a higher probability on track legs going toward rather than away from the long axis of the plume on males with a single antenna remaining on the 'away from axis' side. Combined with a greater propensity for treated moths to lock onto the plume away from the central axis on one side rather than the other, repetitive successive asymmetrical track legs (resulting in a saw-tooth-shaped track) are commonly observed in these moths. Control moths do also make asymmetric successive track legs but they rarely are repeated and thus are not readily observed.     

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.     

Dipole Mechanism Generation of Terahertz Waves under Laser-Cluster Interaction

The feasibility of dipole radiation of terahertz waves under the action of a femtosecond laser pulse on a cluster is demonstrated theoretically. It is shown that the dipole mechanism of terahertz radiation generation plays a decisive role in the interaction of a laser pulse with small-size clusters with a sufficiently high electron collision frequency. The dependences of the spectral, angular, energetic, and spatiotemporal characteristics of the terahertz signal on the density of free electrons in the cluster plasma under the conditions in which dipole radiation is dominant are investigated. It is shown that the energy of terahertz radiation is maximal under the resonance conditions, when the laser frequency coincides with the eigenfrequency of a spherical cluster.     

The effects of metamaterial on electromagnetic fields absorption characteristics of human eye tissues

The interaction of a dipole antenna with a human eye model in the presence of a metamaterial is investigated in this paper. The finite difference time domain (FDTD) method with convolutional perfectly matched layer (CPML) formulation have been used. A three-dimensional anatomical model of the human eye with resolution of 1.25 mm × 1.25 mm × 1.25 mm was used in this study. The dipole antenna was driven by modulated Gaussian pulse and the numerical study is performed with dipole operating at 900 MHz. The analysis has been done by varying the size and value of electric permittivity of the metamaterial. By normalizing the peak SAR (1 g and 10 g) to 1 W for all examined cases, we observed how the SAR values are not affected by the different permittivity values with the size of the metamaterial kept fixed.     

Polarization-Independent Reconfigurable Graphene Gas Sensor Using Crescent Plasmonic Antenna

Plasmonics - A polarization-independent gas sensor based on crescent plasmonic dipole antenna loaded with graphene monolayer is introduced in this paper for environment...     

Radiation retinopathy

Therapeutic radiation to the eye, or in close proximity to the eye, can result in damage to the retina. Manifestations of radiation retinopathy include: intraretinal hemorrhages, hard exudates, macular edema, cotton wool spots, microaneurysms, telangiectatic vessels, sheathed retinal vessels, retinal capillary non-perfusion, or neovascularization. Factors that influence the development of radiation retinopathy are discussed and a case report which includes radiation retinopathy in the differential diagnosis is presented.