Showing papers by "David R. Smith published in 2005"

Electromagnetic parameter retrieval from inhomogeneous metamaterials

Abstract: We discuss the validity of standard retrieval methods that assign bulk electromagnetic properties, such as the electric permittivity « and the magnetic permeability m, from calculations of the scattering sSd parameters for finite-thickness samples. S-parameter retrieval methods have recently become the principal means of characterizing artificially structured metamaterials, which, by nature, are inherently inhomogeneous. While the unit cell of a metamaterial can be made considerably smaller than the free space wavelength, there remains a significant variation of the phase across the unit cell at operational frequencies in nearly all metamaterial structures reported to date. In this respect, metamaterials do not rigorously satisfy an effective medium limit and are closer conceptually to photonic crystals. Nevertheless, we show here that a modification of the standard S-parameter retrieval procedure yields physically reasonable values for the retrieved electromagnetic parameters, even when there is significant inhomogeneity within the unit cell of the structure. We thus distinguish a metamaterial regime, as opposed to the effective medium or photonic crystal regimes, in which a refractive index can be rigorously established but where the wave impedance can only be approximately defined. We present numerical simulations on typical metamaterial structures to illustrate the modified retrieval algorithm and the impact on the retrieved material parameters. We find that no changes to the standard retrieval procedures are necessary when the inhomogeneous unit cell is symmetric along the propagation axis; however, when the unit cell does not possess this symmetry, a modified procedure—in which a periodic structure is assumed—is required to obtain meaningful electromagnetic material parameters. DOI: 10.1103/PhysRevE.71.036617

Gradient index metamaterials.

Abstract: Metamaterials—artificially structured materials with tailored electromagnetic response—can be designed to have properties difficult or impossible to achieve with traditional materials fabrication methods. Here we present a structured metamaterial, based on conducting split ring resonators sSRRsd, which has an effective index of refraction with a constant spatial gradient. We experimentally confirm the gradient by measuring the deflection of a microwave beam by a planar slab of the composite metamaterial over a range of microwave frequencies. The gradient index metamaterial may prove an advantageous alternative approach to the development of gradient index lenses and similar optics, especially at higher frequencies. In particular, the gradient index metamaterial we propose may be suited for terahertz applications, where the magnetic resonant response of SRRs has recently been demonstrated.

Impact of inherent periodic structure on effective medium description of left-handed and related metamaterials

Abstract: We study the frequency dependence of the effective electromagnetic parameters of left-handed and related metamaterials of the split ring resonator and wire type. We show that the reduced translational symmetry speriodic structured inherent to these metamaterials influences their effective electromagnetic response. To anticipate this periodicity, we formulate a periodic effective medium model which enables us to distinguish the resonant behavior of electromagnetic parameters from effects of the periodicity of the structure. We use this model for the analysis of numerical data for the transmission and reflection of periodic arrays of split ring resonators, thin metallic wires, cut wires, as well as the left-handed structures. The present method enables us to identify the origin of the previously observed resonance-antiresonance coupling as well as the occurrence of negative imaginary parts in the effective permittivities and permeabilities of those materials. Our analysis shows that the periodicity of the structure can be neglected only for the wavelength of the electromagnetic wave larger than 30 space periods of the investigated structure.

Simulation and testing of a graded negative index of refraction lens

Abstract: A gradient index (GRIN) lens using a negative index of refraction material (NIM) has been designed and tested. The GRIN lens was fabricated using a NIM slab with a variable index of refraction perpendicular to the propagation direction. Ray tracing calculations based on the isotropic Eikonal equation determined the index of refraction gradient required for a given focal length. An electromagnetic code was then used to design the required ring and wire unit cells. Finally, the index of refraction was approximated using ten discrete steps in an effective medium simulation for the GRIN lens that agreed with the experimental measurements.

Experimental characterization of magnetic surface plasmons on metamaterials with negative permeability

Abstract: We study the surface plasmons (SPs) that exist at the interface between air and a metamaterial constructed of split ring resonators (SRRs). The SRR metamaterial possesses a frequency band in the microwave regime $(12.5--14\phantom{\rule{0.3em}{0ex}}\mathrm{GHz})$ over which the permeability is negative. We apply an attenuated total reflection technique in the Otto configuration in which a beam of microwaves is reflected from a higher dielectric (polycarbonate) prism to excite and probe the surface plasmons. The resulting evanescent microwave fields on the transmission side of the prism couple to SPs on the metamaterial and are indicated by a dip in the reflected power. The experimental data are compared with analytic solutions in which the metamaterial slab is approximated as an infinite half space, for which the frequency-dependent permeability (and permittivity) is derived from finite-element simulations on an SRR structure with the same parameters as those measured.

Negative index lens aberrations

Abstract: We examine the Seidel aberrations of thin spherical lenses composed of media with refractive index not restricted to be positive. We find that consideration of this expanded parameter space allows for the reduction or elimination of more aberrations than is possible with only positive index media. In particular, we find that spherical lenses possessing real aplanatic focal points are possible only with a negative index. We perform ray tracing, using a custom code that relies only on Maxwell's equations and conservation of energy, that confirms the results of the aberration calculations.

Progress towards high performance plasmas in the National Spherical Torus Experiment (NSTX)

Abstract: The major objective of the National Spherical Torus Experiment (NSTX) is to understand basic toroidal confinement physics at low aspect ratio and high βT in order to advance the spherical torus (ST) concept. In order to do this, NSTX utilizes up to 7.5 MW of neutral beam injection, up to 6 MW of high harmonic fast waves (HHFWs), and it operates with plasma currents up to 1.5 MA and elongations of up to 2.6 at a toroidal field up to 0.45 T. New facility, and diagnostic and modelling capabilities developed over the past two years have enabled the NSTX research team to make significant progress towards establishing this physics basis for future ST devices. Improvements in plasma control have led to more routine operation at high elongation and high βT (up to ~40%) lasting for many energy confinement times. βT can be limited by either internal or external modes. The installation of an active error field (EF) correction coil pair has expanded the operating regime at low density and has allowed for initial resonant EF amplification experiments. The determination of the confinement and transport properties of NSTX plasmas has benefitted greatly from the implementation of higher spatial resolution kinetic diagnostics. The parametric variation of confinement is similar to that at conventional aspect ratio but with values enhanced relative to those determined from conventional aspect ratio scalings and with a BT dependence. The transport is highly dependent on details of both the flow and magnetic shear. Core turbulence was measured for the first time in an ST through correlation reflectometry. Non-inductive start-up has been explored using PF-only and transient co-axial helicity injection techniques, resulting in up to 140 kA of toroidal current generated by the latter technique. Calculated bootstrap and beam-driven currents have sustained up to 60% of the flat-top plasma current in NBI discharges. Studies of HHFW absorption have indicated parametric decay of the wave and associated edge thermal ion heating. Energetic particle modes, most notably toroidal Alfven eigenmodes and fishbone-like modes result in fast particle losses, and these instabilities may affect fast ion confinement on devices such as ITER. Finally, a variety of techniques has been developed for fuelling and power and particle control.

How to Build a Superlens

Abstract: Conventional lenses are subject to the diffraction limit, which means that they cannot resolve objects placed closer together than one-half of the wavelength of the illuminating light. As Smith explains in his Perspective, this limit occurs because conventional optical components leave behind the electromagnetic "near-fields"--those components that contain the subwavelength spatial information about an object. In 2000, John Pendry proposed a new type of imaging device, based on a material with a negative index of refraction, which could recover the near-fields. Using a thin silver film as a superlens, Fang et al. now demonstrate that negative refraction can indeed produce a high-resolution optical image, opening the door to a new breed of optical devices and applications.

Sub-diffraction imaging with compensating bilayers

Abstract: We derive a general expression for the material properties of a compensating bilayer, which is a pair of material layers which transfer the field distribution from one side of the bilayer to the other with resolution limited only by the deviation of the material properties from specified values. One of the layers can be free space, a special case of which is the perfect lens, but the layers need not have equal thickness. Compensating a thick layer of free space with a thin layer creates a focusing device with increased working distance, and employs an anisotropic material. It is also possible to achieve compensation of materials with property tensors that are neither positive nor negative definite. In this case, we refer to such media as indefinite, and we analyse, in detail, bilayers of these media which support coupling of internal propagating waves to incident waves of any transverse wave vector. In this case, we find that the enhanced spatial resolution provided by large transverse wave vectors is far less sensitive to loss than that of the perfect lens.

The role of Cr substitution on the ferromagnetic properties of Ga1−xCrxN

Abstract: Angular-dependent channeling Rutherford backscattering spectroscopy (c-RBS) has been used to quantify the fraction of Cr atoms on substitutional, interstitial, and random sites in epitaxial Ga1−xCrxN films grown by reactive molecular-beam epitaxy. The morphology of these films and correlation with their magnetic properties has been investigated. Films grown at temperatures below ∼750°C have up to 90% of Cr occupying substitutional sites. Post-growth annealing at 825°C results in a systematic drop in the fraction of substitutional Cr as well as a fall off in the ferromagnetic signal. The roles of nonsubstitutional Cr in transferring charge from the Crt2 band and segregation of substitutional Cr in the loss of magnetism are discussed. Overall, these results provide strong microscopic evidence that Cr-doped III–N systems are dilute magnetic semiconductors.

Gaia astrometric CCDs and focal plane

Abstract: ESA's Gaia astrometry mission is due for launch in 2011. The astrometric instrument focal plane will have an area of up to 0.5m2 and will contain more than 100 CCDs. These will be operated in Time Delay and Integration mode in order to track and observe sources whilst the telescopes continuously scan the sky. Gaia's target for astrometric precision of a few millionths of an arc second, places extreme demands on focal plane thermo--mechanical stability and electronics performance. The CCDs themselves are large area, back illuminated, full--frame, four phase devices. They require maximum efficiency for observing the majority of (faint) objects, yet must simultaneously be able to handle very bright objects that will regularly cross the field of view. Achieving the final astrometric precision will also require excellent noise performance and MTF. In addition to demanding excellent performance from each CCD, they will need to be produced in large numbers which raises production and yield issues. When analyzing Gaia data it will be essential to understand and calibrate CCD behaviour correctly, including the expected performance degradation due to radiation damage. This is being addressed through comprehensive testing and the development of CCD models.

Application of electron multiplying CCD technology in space instrumentation

Abstract: Electron multiplying CCD (EMCCD) technology has found important initial applications in low light surveillance and photon starved scientific instrumentation. This paper discusses the attributes of the EMCCD which make it useful for certain space instruments, particularly those which are photon starved, and explores likely risks from the radiation expected in such instruments.

Symmetrical Realization for Predistorted Microwave Filters

Abstract: This paper presents a symmetric realization for a predistorted bandpass filter using a general cross-coupled resonator structure. The new method enables much simpler conversion from an existing filter design without predistortion. Hence both design and tuning efforts are reduced. A 10-4-4 dielectric resonator filter was built to verify the validity of the new method. The filter, coupled with an input circulator and output isolator was tested over temperature to validate the suitability for practical applications such as satellite transponder input multiplexers.

Superlens breaks optical barrier

Abstract: One of the best known properties of light is that it diffracts, bending or spreading around objects that lie in its path. A familiar example is when a collimated beam of light passes through a small aperture in an opaque barrier. If the aperture is large, the light emerges as a beam with the same radius as that of the aperture. But if the size of the aperture is similar to the wavelength of the incident light, the emerging light flares out from the aperture and forms a diffraction pattern whereby the intensity of the transmitted light has a broad central peak.

Status and Plans for the National Spherical Torus Experimental Research Facility

Abstract: An overview of the research capabilities and the future plans on the MA-class National Spherical Torus Experiment (NSTX) at Princeton is presented. NSTX research is exploring the scientific benefits of modifying the field line structure from that in more conventional aspect ratio devices, such as the tokamak. The relevant scientific issues pursued on NSTX include energy confinement, MHD stability at high beta, non-inductive sustainment, solenoid-free start-up, and power and particle handling. In support of the NSTX research goal, research tools are being developed by the NSTX team. In the context of the fusion energy development path being formulated in the US, an ST-based Component Test Facility (CTF) and, ultimately a high beta Demo device based on the ST, are being considered. For these, it is essential to develop high performance (high beta and high confinement), steady-state (non-inductively driven) ST operational scenarios and an efficient solenoid-free start-up concept. We will also briefly describe the Next-Step-ST (NSST) device being designed to address these issues in fusion-relevant plasma conditions.

MOS CCDs for the wide field imager on the XEUS spacecraft

Abstract: In recent years the XEUS mission concept has evolved and has been the subject of several industrial studies. The mission concept has now matured to the point that it could be proposed for a Phase A study and subsequent flight programme. The key feature of XEUS will be its X-ray optic with collecting area ~30-100x that of XMM. The mission is envisaged at an orbit around the L2 point in space, and is formed from two spacecraft; one for the mirrors, and the other for the focal plane detectors. With a focal length of 50m, the plate scale of the optic is 6.5x that of XMM, which using existing focal plane technology will reduce the effective field of view to a few arc minutes. Cryogenic instrumentation, with detector sizes of a few mm can only be used for narrow field studies of target objects, and a wide field instrument is under consideration using a DEPFET pixel array to image out to a diameter of 5 arcminutes, requiring an array of dimension 70mm. It is envisaged to extend this field of view possibly out to 15 arcminutes through the use of an outer detection ring comprised of MOS CCDs

Proton irradiation of e2v technologies L3Vision devices

Abstract: This paper describes the proton irradiation and subsequent analysis of 8 e2v technologies CCD65 L3Vision devices with the intention of assessing the suitability of L3Vision technology to applications in space. In particular the use of L3Vision charge coupled devices (CCDs) on the Radial Velocity Spectrometer (RVS) instrument of the planned European Space Agency GAIA mission is discussed. The operational properties of the devices were characterised before irradiation with protons to a 10 MeV equivalent fluence of 2.5 × 1 0 9 protons cm - 2 . The devices were then characterised again before undergoing a second proton irradiation which increased the total 10 MeV equivalent proton fluence received by each device to 2.1 × 1 0 10 protons cm - 2 . All 8 devices functioned as expected after each irradiation, with no catastrophic failures occurring even after the second irradiation which increased the total proton fluence to approximately 10 times the expected 6 year RVS fluence.

Imaging dielectric objects from scalar intensity patterns by means of indirect holography

Abstract: An indirect microwave holographic technique for the reconstruction of complex scattered fields and the imaging of objects from a single holographic intensity pattern is described. The holographic intensity pattern is constructed by combining the signal scattered from the dielectric object with a synthesised reference signal. This dispenses with the need for expensive phase measuring equipment. Reconstructed magnitude and phase patterns of the original object have been determined using an adaptation of optical techniques. This work describes how dielectric objects of low contrast to the background when viewed using reconstructed magnitude patterns can be imaged from reconstructed phase patterns.

Infrared spectroscopy and ellipsometry of magnetic metamaterials

Abstract: We present S and P polarized measurements of artificial bianisotropic magnetic metamaterials with resonant behavior at infrared frequencies. These metamaterials consist of an array of micron sized (~40μm) copper rings fabricated upon a quartz substrate. Simulation of the reflectance is obtained through a combination of electromagnetic eigenmode simulation and Jones matrix analysis, and we find excellent agreement with the experimental data. It is shown that although the artificial magnetic materials do indeed exhibit a magnetic response, care must be taken to avoid an undesirable electric dipole resonance, due to lack of reflection symmetry in one orientation. The effects of bianisotropy on negative index are detailed and shown to be beneficial for certain configurations of the material parameters.

Fabrication and optical measurements of nanoscale meta-materials: terahertz and beyond

Abstract: This paper reports on efforts to create fully left-handed meta-materials at multi-THz frequencies, detailed optical characterizations of various structures fabricated, and understanding to date of what the materials limitations are for making these structures smaller and smaller Presently, authors are using CXRO's nano-writer to create sub-micron structures with 60 nm smallest features Present results on these and other nano-fabricated structures with resonance frequencies in the 100 THz and higher range

Negative Index Lenses

Abstract: The prelims comprise: Introduction Geometric Optics Gaussian Optics Aberrations References

Mechanical Design of the NSTX High-k Scattering Diagnostic

Abstract: The NSTX high-k scattering diagnostic measures small-scale density fluctuations by the heterodyne detection of waves scattered from a millimeter wave probe beam at 280 GHz and lambda=1.07 mm. To enable this measurement, major alterations were made to the NSTX vacuum vessel and neutral beam armor. Close collaboration between the PPPL physics and engineering staff resulted in a flexible system with steerable launch and detection optics that can position the scattering volume either near the magnetic axis (rhoap.1) or near the edge (rhoap.8). 150 feet of carefully aligned corrugated waveguide was installed for injection of the probe beam and collection of the scattered signal into the detection electronics

New optics using negative refraction

Abstract: We summarize the benefits that negative index materials (NIMs), realizable in artificially constructed metamaterials, can have on both geometrical and wave optics. NIMs may lead to novel or improved future optical devices.