Showing papers by "David R. Smith published in 2001"

Experimental Verification of a Negative Index of Refraction

Abstract: We present experimental scattering data at microwave frequencies on a structured metamaterial that exhibits a frequency band where the effective index of refraction (n) is negative. The material consists of a two-dimensional array of repeated unit cells of copper strips and split ring resonators on interlocking strips of standard circuit board material. By measuring the scattering angle of the transmitted beam through a prism fabricated from this material, we determine the effective n, appropriate to Snell's law. These experiments directly confirm the predictions of Maxwell's equations that n is given by the negative square root of epsilon.mu for the frequencies where both the permittivity (epsilon) and the permeability (mu) are negative. Configurations of geometrical optical designs are now possible that could not be realized by positive index materials.

Microwave transmission through a two-dimensional, isotropic, left-handed metamaterial

Abstract: We present experimental data, numerical simulations, and analytical transfer-matrix calculations for a two-dimensionally isotropic, left-handed metamaterial (LHM) at X-band microwave frequencies. A LHM is one that has a frequency band with simultaneously negative eeff(ω) and μeff(ω), thereby having real values of index of refraction and wave vectors, and exhibiting extended wave propagation over that band. Our physical demonstration of a two-dimensional isotropic LHM will now permit experiments to verify some of the explicit predictions of reversed electromagnetic-wave properties including negative index of refraction as analyzed by Veselago [Usp. Fiz. Nauk 92, 517 (1964), Sov. Phys. Usp. 10, 509 (1968)].

Plasmon resonances of silver nanowires with a nonregular cross section

Abstract: We investigate numerically the spectrum of plasmon resonances for metallic nanowires with a nonregular cross section, in the 20‐50 nm range. We first consider the resonance spectra corresponding to nanowires whose cross sections form different simplexes. The number of resonances strongly increases when the section symmetry decreases: A cylindrical wire exhibits one resonance, whereas we observe more than five distinct resonances for a triangular particle. The spectral range covered by these different resonances becomes very large, giving to the particle-specific distinct colors. At the resonance, dramatic field enhancement is observed at the vicinity of nonregular particles, where the field amplitude can reach several hundred times that of the illumination field. This near-field enhancement corresponds to surface-enhanced Raman scattering~SERS! enhancement locally in excess of 10 12 . The distance dependence of this enhancement is investigated and we show that it depends on the plasmon resonance excited in the particle, i.e., on the illumination wavelength. The average Raman enhancement for molecules distributed on the entire particle surface is also computed and discussed in the context of experiments in which large numbers of molecules are used.

Ab initio numerical simulation of left-handed metamaterials: Comparison of calculations and experiments

Abstract: Using numerical simulation techniques, the transmission and reflection coefficients, or S parameters, for left-handed metamaterials are calculated. Metamaterials consist of a lattice of conducting, nonmagnetic elements that can be described by an effective magnetic permeability μeff and an effective electrical permittivity eeff, both of which can exhibit values not found in naturally occurring materials. Because the electromagnetic fields in conducting metamaterials can be localized to regions much smaller than the incident wavelength, it can be difficult to perform accurate numerical simulations. The metamaterials simulated here, for example, are based on arrays of split ring resonators (SRRs), which produce enhanced and highly localized electric fields within the gaps of the elements in response to applied time dependent fields. To obtain greater numerical accuracy we utilize the newly developed commercially available code MICROWAVE STUDIO, which is based on the finite integration technique with the per...

A study of molybdenum influxes and transport in Alcator C-Mod

Abstract: A characterization is presented of Mo sources ΓMo, core Mo content NMo and their dependences on the Alcator C-Mod operational regimes. The primary impurity source locations are the divertor, the inner wall and the ICRF antenna limiters. Boronization is used to coat the first wall surfaces with a thin layer of B, which greatly reduces the Mo sources for a period of time, eroding away with an e folding period of 15-50 discharges. The penetration of Mo into the core plasma under different conditions is analysed using the concept of penetration factor, PF = NMo/ΓMo (s). In general, the inner wall Mo source is large (~1018s-1), but is found to be relatively uncorrelated with the core Mo content in diverted plasmas. The outer divertor source is of a similar order to that of the inner wall and has a penetration factor in the range 10-5 - 2 × 10-3 s depending on density and confinement mode. The antenna limiter Mo sources are typically smaller, but with higher penetration factors of 10-3 - 2 × 10-2 s. The behaviour of the antenna limiter sources is consistent with physical sputtering due to the influence of RF sheath rectification. The measurements of the plasma potential on field lines connected to the antenna are very high when the antennas are energized, often reaching hundreds of volts.

Non‐regularly shaped plasmon resonant nanoparticle as localized light source for near‐field microscopy

Abstract: We study numerically two-dimensional nanoparticles with a non-regular shape and demonstrate that these particles can support many more plasmon resonances than a particle with a regular shape (e.g. an ellipse). The electric field distributions associated with these different resonances are investigated in detail in the context of near-field microscopy. Depending on the particle shape, extremely strong and localized near-fields, with intensity larger than 105 that of the illumination wave, can be generated. We also discuss the spectral dependence of these near-fields and show that different spatial distributions are observed, depending which plasmon resonance is excited in the particle.

Left-Handed Metamaterials

Abstract: The response of a material to electromagnetic radiation can be entirely characterized by the material parameters: the electrical permittivity, or e, and the magnetic permeability, or μ. The range of possible values for the material parameters, as dictated by fundamental considerations such as causality or thermodynamics, extends beyond that found in naturally occurring materials. We thus seek to extend the material parameter space by creating electromagnetic metamaterials—ordered composite materials that display electromagnetic properties beyond those found in naturally occurring materials. Recently, we have demonstrated a metamaterial made of a repeated lattice of conducting, nonmagnetic elements that exhibits an effective μ and an effective e, both of which are simultaneously negative over a band of frequencies [1]. Such a medium has been termed Left-Handed [2], as the electric field (E), magnetic intensity (H) and propagation vector (k) are related by a left-hand rule. We introduce the reader to the expected properties predicted by Maxwell’s equations for Left-Handed media, and describe our recent numerical and experimental work in developing and analyzing this new metamaterial.

Microwave holographic measuring method and apparatus

Abstract: A microwave holographic measuring method is disclosed. A first electrical signal S of microwave frequency is provided. A first part S1 of the first signal S is directed to a first antenna (101). Predetermined changes of phase and amplitude are applied to a second part S2 of the first signal S to produce a second electrical signal S4 which is coherent with the first part S1 of the first signal S. Microwave radiation is detected at a plurality of locations by means of a second antenna (107) to generate a third electrical signal S5 at each location. The second S4 and third S5 electrical signals are combined to produce a fourth electrical signal.

Status of the new Los Alamos UCN source

Abstract: Ultra-cold neutrons (UCN) have been produced at reactors over the last 30 years. Although very successful, experiments often suffer from low UCN statistics—as limited by the traditional production scheme. A new type of UCN source has been developed at Los Alamos. The source combines a spallation target, a cold neutron flux trap, and a solid deuterium converter for the down-scattering of cold neutrons into the ultra-cold regime. The breakthroughs of the last year include the theoretical understanding of the production mechanism and its experimental verification. The new technique is capable of delivering orders of magnitude higher densities of UCN. The highest UCN density ever stored in an experiment is reported.

Comparison of pressure measurements and operating data for wind excitation of telescope structures

Abstract: The performance of large, ground-based optical telescopes is limited by the quality of the Image that can be delivered to the final detector (or camera). The telescope and its protective dome create atmospheric variationsdue to thermal mismatch with the local environment. This "dome seeing" effect reduces performance because it results in a blurring of the incoming image. This effect can be minimized by allowing bulk airflow across the mirror, but this comes at the price of wind buffeting of the structure, which also reduces performance. Operation of a world-class telescope requires an understanding of this performance trade-off in order to assure the users of the facility the best possible data. The current stage of construction of the Gemini South 8m (26 ft) telescope in Chile provided a unique opportunity to make simultaneous measurements of wind pressure and structural response. Further, the Gemini dome design allows for a wide range of flow configurations, from nearly enclosed to almost fully exposed to the wind. We measured the operating response of surface pressures on the primary mirror cell as well as more than 70 channels of accelerometers on the telescope structure. The data were taken in a variety of dome configurations and relative wind directions. Since the wind pressure measurements will not be available during telescope operation, an understanding of the relationship between these pressures and other measurable quantities is essential to real time monitoring of the wind buffeting of the mirror. In this paper, we present the comparison of the wind pressure measurements with the structural response. Further, we present comparisons between the wind pressure measurements and the wind speed at various locations in the dome.

Modal and operating characterization of an optical telescope

Abstract: As part of the overall assessment of the dynamic characteristics of the Gemini Optical Telescope, an experimental modal test was performed on this structure along with collection of operating data. For the modal testing, multiple reference impact testing was performed to characterize the structure. Time data was acquired and processed to compute multiple referenced frequency response functions. Modal parameters were extracted as part of the overall assessment of this optical telescope. For the operating assessment, many operating tests were conducted to determine the effects of wind loading on the structure. A variety of different structural configurations were evaluated during a series of tests at the Gemini Optical Telescope. Several days and nights were used to measure the telescope's behavior under a variety of different wind loading conditions. Operating data was collected and reduced for all of the tests conducted to identify wind loading conditions that hinder normal operations of the telescope. This paper presents some of the significant considerations regarding the data obtained and the determination of the modal and operating mode shapes. Some thoughts on the acquisition of the data and its reduction are presented along with the extraction of modal parameters and operating shapes.

Operating data and pointing measurements of a large radio telescope

Abstract: The most challenging problem facing the next generation of large, high-frequency radio telescopes is to maintain the pointing accuracy of the final structure. The required pointing tolerances are very precise (typically <0.0003 degrees, or 1 arc-second), because the size of the radio beam on the sky decreases at higher observation frequencies and also decreases with increased telescope diameter. A major difficulty is that such large structures are affected significantly by the wind. A critical unknown in any future plans for achieving required telescope performance is identifying the source and frequency content of the pointing errors. The importantstructural contributors to such errors are not obvious. The large dish-shaped primary reflector presents the largest physical area to the wind. However, the pointing accuracy is generally more sensitive to the lateral position of the secondary mirror, which is suspended far above the dish on a tripod or tetrapod structure. As a result, it is not clear which structure is more important in wind-induced errors. To address this, we examined the operating data taken at the Nobeyama 45m radio telescope, located in Japan. More than 40 channels of accelerometer data were taken simultaneously with an on-sky pointing measurement. In this paper, we identify the most significant structural contributors to wind-induced quasi-static and dynamic pointing errors, and determine the relevant frequency range.