arXiv subjects

K. Marinov

Publications and source records attributed to K. Marinov.

11 recordsLinked to original sources

arXiv2017

Medical therapy and imaging fixed-field alternating-gradient accelerator with realistic magnets

NORMA is a design for a normal-conducting race track fixed-field alternating-gradient accelerator (FFAG) for protons from 50 to 350 MeV. In this article we show the development from an idealised lattice to a design implemented with field maps from rigorous two-dimensional (2D) and three-dimensional (3D) FEM magnet modelling. We show that whilst the fields from a 2D model may reproduce the idealised field to a close approximation, adjustments must be made to the lattice to account for differences brought about by the 3D model and fringe fields and full 3D models. Implementing these lattice corrections we recover the required properties of small tune shift with energy and a sufficiently-large dynamic aperture. The main result is an iterative design method to produce the first realistic design for a proton therapy accelerator that can rapidly deliver protons for both treatment and for imaging at up to 350 MeV. The first iteration is performed explicitly and described in detail in the text.

physics.acc-ph↗

arXiv2015

High Responsivity, Large-Area Graphene/MoS2 Flexible Photodetectors

We present flexible photodetectors (PDs) for visible wavelengths fabricated by stacking centimetre-scale chemical vapour deposited (CVD) single layer graphene (SLG) and single layer CVD MoS2, both wet transferred onto a flexible polyethylene terephthalate substrate. The operation mechanism relies on injection of photoexcited electrons from MoS2 to the SLG channel. The external responsivity is 45.5A/W and the internal 570A/W at 642nm. This is at least two orders of magnitude higher than bulk-semiconductor flexible membranes and other flexible PDs based on graphene and layered materials. The photoconductive gain is up to 4x10^5. The photocurrent is in the 0.1-100 uA range. The devices are semi-transparent, with just 8% absorption at 642nm and work stably upon bending to a curvature of 6cm. These capabilities and the low voltage operation (<1V) make them attractive for wearable applications.

cond-mat.mtrl-sci↗

arXiv2013

Radiation Hardness of Graphene and MoS2 Field Effect Devices Against Swift Heavy Ion Irradiation

We have investigated the deterioration of field effect transistors based on twodimensional materials due to irradiation with swift heavy ions. Devices were prepared with exfoliated single layers of MoS2 and graphene, respectively. They were characterized before and after irradiation with 1.14 GeV U228+2 ions using three different fluences. By electrical characterization, atomic force microscopy and Raman spectroscopy we show that the irradiation leads to significant changes of structural and electrical properties. At the highest fluence of 4 x 102^11 ions/cm^2, the MoS2 transistor is destroyed, while the graphene based device remains operational, albeit with an inferior performance.

cond-mat.mtrl-sci↗

arXiv2008

Strong gyrotopy in a chiral toroidal medium

In this letter, we present the first experimental study of a new chiral metamaterial consisting of toroidal wire windings. We show that the metamaterial exhibits three bands of circular dichroism in the GHz range. We discuss the response of the structure in terms of multipole moments, including the (magnetic) toroidal dipole moment.

physics.optics↗

arXiv2007

Toroidal metamaterial

It is shown that a new type of metamaterial, a 3D-array of toroidal solenoids, displays a significant toroidal response that can be readily measured. This is in sharp contrast to materials that exist in nature, where the toroidal component is weak and hardly measurable. The existence of an optimal configuration, maximizing the interaction with an external electromagnetic field, is demonstrated. In addition, it is found that a characteristic feature of the magnetic toroidal response is its strong dependence on the background dielectric permittivity of the host material, which suggests possible applications. Negative refraction and backward waves exist in a composite toroidal metamaterial, consisting of an array of wires and an array of toroidal solenoids.

physics.class-ph↗

arXiv2006

Radiation enhancement and radiation suppression by a left-handed metamaterial

The perfect lens property of a dispersive and lossy left-handed metamaterial (LHM) disk is exploited to superimpose a source of electromagnetic radiation onto its mirror image, formed as a result of reflection from a perfect electric conductor (PEC) or a perfect magnetic conductor (PMC). The superposition of a vertical wire-dipole antenna with its PEC-image results in an increase of the radiation resistance of the antenna compared to that of an antenna emitting in free space. On the other hand, if the same antenna is coupled to a PMC-image it is shown that the result is the formation of a non-radiating configuration. The finite-difference time-domain (FDTD) analysis is performed and this allows a detailed characterization of the systems. It is shown that the non-radiating system allows relatively large amounts of electromagnetic energy to be stored in the LHM-disk and that is indicative of strong electromagnetic fields inside the material. This property is employed in a second-harmonic generation (SHG) process and the potential of a non-radiating configuration as an efficient nonlinear device is demonstrated.

physics.class-ph↗

arXiv2006

Electromagnetic energy in a dispersive metamaterial

An expression for the electromagnetic field energy density in a dispersive, lossy, left-handed metamaterial, consisting of an array of split-ring resonators and an array of wires is derived. An electromagnetic field with general time-dependence is considered. The outcome is compared with previously published results. In the absence of losses, agreement with the general result for the energy density in a dispersive material is obtained. The formulae are verified using the finite-difference time-domain (FDTD) numerical method. The applicability of two commonly used permeability models to the problem of calculating the energy stored in an array of split-ring resonators is discussed.

physics.class-ph↗

arXiv2005

Non-radiating and radiating configurations driven by left-handed metamaterials

It is shown that a pair of identical emitters (e.g. wire dipole antennas) in the focal points of a disc, made of left-handed metamaterial (a "perfect" lens), form a non-radiating electromagnetic configuration. The emitters are fed with voltages of equal magnitude and pi-out-of-phase. Detailed finite-difference time-domain (FDTD) modeling shows that there are non-propagating electromagnetic fields generated - fields that remain confined within the region between the emitters and the lens. The energy balance of the system shows that the radiation resistance of the system is very low. This means that the input power is converted to heat in the volume of the lens and only a small fraction of it is radiated. The system performance shows that disturbing the configuration of the non-propagating electromagnetic fields with the presence of an externally introduced object stimulates radiation. This suggests possible detector applications. In-phase feeding voltages are also studied with the consequence that the radiation resistance of the antennae is increased.

physics.class-ph↗

arXiv2005

Dispersion properties of non-radiating configurations: Finite-Difference Time-Domain modeling

A finite-difference time-domain (FDTD) numerical analysis is used to demonstrate that a toroidal solenoid, coaxial with an electric dipole, is a remarkable non-radiating configuration. It can be used to measure the dielectric permittivity of any ambient matter. It becomes a directional radiator at an interface between two dielectric media, depositing energy in the material with the highest polarizability.

physics.class-ph↗

arXiv2005

Nonradiating toroidal structures

Some basic properties of nonradiating systems are considered. A simple connection is established between the existence of residual electromagnetic potentials and the current density spectrum of the system. The properties of specific configurations based on toroidal and supertoroidal currents are modeled with the finite-difference time-domain method. Possible applications are discussed. A design of a new type of nonradiating system, based on a left-handed metamaterial is proposed and the system performance is modeled numerically.

physics.class-ph↗

arXiv2001

Stability of narrow beams in bulk Kerr-type nonlinear media

We consider (2+1)-dimensional beams, whose transverse size may be comparable to or smaller than the carrier wavelength, on the basis of an extended version of the nonlinear Schrödinger equation derived from the Maxwell`s equations. As this equation is very cumbersome, we also study, in parallel to it, its simplified version which keeps the most essential term: the term which accounts for the {\it nonlinear diffraction}. The full equation additionally includes terms generated by a deviation from the paraxial approximation and by a longitudinal electric-field component in the beam. Solitary-wave stationary solutions to both the full and simplified equations are found, treating the terms which modify the nonlinear Schrödinger equation as perturbations. Within the framework of the perturbative approach, a conserved power of the beam is obtained in an explicit form. It is found that the nonlinear diffraction affects stationary beams much stronger than nonparaxiality and longitudinal field. Stability of the beams is directly tested by simulating the simplified equation, with initial configurations taken as predicted by the perturbation theory. The numerically generated solitary beams are always stable and never start to collapse, although they display periodic internal vibrations, whose amplitude decreases with the increase of the beam power.

nlin.PS↗