We have developed a low-loss, ultrasmall radio frequency ͑rf͒ metamaterial operating at ϳ76 MHz. This miniaturized medium is made up of planar spiral elements with diameter as small as ϳ / 658 ͑ is the free space wavelength͒, fashioned from Nb thin films on quartz substrates. The transmission data are examined below and above the superconducting transition temperature of Nb for both a single spiral and a one dimensional array. The validity of the design is tested through numerical simulations and good agreement is found. We discuss how superconductors enable such a compact design in the rf with high loaded-quality factor ͑in excess of 5000͒, which is in fact difficult to realize with ordinary metals. © 2010 American Institute of Physics. ͓doi:10.1063/1.3456524͔The wavelength of visible light is orders of magnitude larger than the size of the atoms in most dielectric media. Therefore, light is not sensitive to the details of the atomicscale electromagnetic fields but to coarse-grained properties of the structure. Since metamaterials mimic natural materials, artificial electromagnetic structures must have elements whose dimensions are much smaller than the free space wavelength ͑͒ at which the metamaterial operates. Up to now, research has focused on metamaterials functioning at gigahertz or higher frequencies, since the electrical size of the inclusion of such a medium can be kept reasonably small. [1][2][3][4][5] Many metamaterials are based on split ring resonators ͑SRRs͒ and their variations to create subwavelength magnetically active features. The SRR geometry is typically composed of two concentric rings, each with a capacitive split, and can be understood in terms of the inductorcapacitor ͑LC͒ analogy. However this design calls for comparatively large dimensions for radio frequency ͑rf͒ metamaterials, since the wavelength is on the order of meters.Although there are few studies available on rf metamaterials, they have great potential in applications such as magnetic resonance imaging ͑MRI͒ devices for noninvasive and high resolution medical imaging.6,7 They may also improve rf antenna efficiency and directivity as well as reduce their size.8 Microwave delay lines, 9 magnetoinductive lenses for near field imaging, 10 rf filters, and compact resonators 11 may also be enabled by rf metamaterials.The first practical demonstration of those metamaterials was implemented by Wiltshire et al.6,12,13 using a Swiss roll geometry as suggested earlier by Pendry.14 An array of the rolls made of Cu/Kapton layers wounded around a dielectric mandrel was used in an MRI machine for guiding rf flux caused by magnetic resonance to the receiver coil in the system. 6,15 Though the design had some disadvantages, such as being lossy, three-dimensional, and nonuniform, it was well-suited for such applications. 16 A planar version of the Swiss roll geometry fabricated with a thick copper film on a dielectric substrate and excited by a microstrip transmission line was found to resonate at frequencies as low as 125 MHz. However signifi...
Abstract-Superconducting metamaterials combine the advantages of low-loss, large inductance (with the addition of kinetic inductance), and extreme tunability compared to their normal metal counterparts. Therefore, they allow realization of compact designs operating at low frequencies. We have recently developed radio frequency (RF) metamaterials with a high loaded quality factor and an electrical size as small as ∼λ/658, (λ is the free space wavelength) by using Nb thin films. The RF metamaterial is composed of truly planar spirals patterned with lithographic techniques. Linear transmission characteristics of these metamaterials show robust Lorentzian resonant peaks in the sub-100 MHz frequency range below the Tc of Nb. Though Nb is a non-magnetic material, the circulating currents in the spirals generated by RF signals produce a strong magnetic response, which can be tuned sensitively either by temperature or magnetic field thanks to the superconducting nature of the design. We have also observed strong nonlinearity and meta-stable jumps in the transmission data with increasing RF input power until the Nb is driven into the normal state. We discuss the factors modifying the induced magnetic response from single and 1-D arrays of spirals in the light of numerical simulations.
We have directly imaged the anisotropic nonlinear Meissner effect in an unconventional superconductor through the nonlinear electrodynamic response of both (bulk) gap nodes and (surface) Andreev bound states. A superconducting thin film is patterned into a compact self-resonant spiral structure, excited near resonance in the radio-frequency range, and scanned with a focused laser beam perturbation. At low temperatures, direction-dependent nonlinearities in the reactive and resistive properties of the resonator create photoresponse that maps out the directions of nodes, or of bound states associated with these nodes, on the Fermi surface of the superconductor. The method is demonstrated on the nodal superconductor YBa2Cu3O 7−δ and the results are consistent with theoretical predictions for the bulk and surface contributions.Introduction -The Meissner effect is the spontaneous exclusion of magnetic flux from the bulk of a superconductor and is one of the hallmarks of superconductivity. In the presence of a magnetic field, a superconductor must invest kinetic energy in a supercurrent flow to screen out the applied field. This reduces the free energy difference between the superconducting and normal states, resulting in a reduction in magnitude of the superconducting order parameter. This in turn leads to a field-and current-dependent magnetic penetration depth, diamagnetic moment, etc. and is referred to as the nonlinear Meissner effect (NLME). Microscopically the NLME arises when Cooper pairs at the leading edge of the current-carrying Fermi surface can de-pair into available quasi-particle states at the back-end and create a quasi-particle backflow current [1]. Conventional (fully gapped) superconductors show the strongest nonlinearities near T c , and have exponentially suppressed nonlinear response at low temperatures, T ≪ T c . Unconventional superconductors with nodes in the superconducting energy gap are expected to have a strong nonlinear Meissner effect at low temperatures, due to the nodal excitations out of the superconducting ground state [2]. In addition this nonlinear response should be anisotropic, reflecting the locations of nodes of the gap on the Fermi surface.
Local and controlled delivery of therapeutic agents directly into focally afflicted tissues is the ideal for the treatment of diseases that require direct interventions. However, current options are obtrusive, difficult to implement, and limited in their scope of utilization; the optimal solution requires a method that may be optimized for available therapies and is designed for exact delivery. To address these needs, we propose the Biocage, a customizable implantable local drug delivery platform. The device is a needle-sized porous container capable of encasing therapeutic molecules and matrices of interest to be eluted into the region of interest over time. The Biocage was fabricated using the Nanoscribe Photonic Professional GT 3D laser lithography system, a two-photon polymerization (2PP) 3D printer capable of micron-level precision on a millimeter scale. We demonstrate the build consistency and features of the fabricated device; its ability to release molecules; and a method for its accurate, stable delivery in mouse brain tissue. The Biocage provides a powerful tool for customizable and precise delivery of therapeutic agents into target tissues.
Superconducting metamaterials are utilized to study the approach to the plasmonic limit simply by tuning temperature to modify the superfluid density, and thus the superfluid plasma frequency. We examine the persistence of artificial magnetism in a metamaterial made with superconductors in the plasmonic limit, and compare to the electromagnetic behavior of normal metals as a function of frequency as the plasma frequency is approached from below. Spiral-shaped Nb thin film meta-atoms of scaled dimensions are employed to explore the plasmonic behavior in these superconducting metamaterials, and the scaling condition allows extraction of the temperature dependent superfluid density, which is found to be in good agreement with expectations.
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