A Tunable Split Resonator Method for Non-destructive Permittivity Characterization

The split cylinder resonator method is improved for nondestructive and accurate measurement for low permittivity materials at multiple frequency points. The dielectric constants of flat substrate materials are calculated based on a rigorous mode match analysis of the TE/sub 011/ mode. The loss tangent is also approximately calculated. The dielectric properties of two commercial substrates have been measured at multiple frequencies. The results demonstrate that this technology is capable of accurately characterizing the dielectric properties of flat substrate materials versus frequency in a nondestructive way.

Molecular Plasmonics with Tunable Exciton-Plasmon Coupling Strength in J-Aggregate Hybridized Au Nanorod Assemblies

Controlling coherent electromagnetic interactions in molecular systems is a problem of both fundamental interest and important applicative potential in the development of photonic and opto-electronic devices. The strength of these interactions determines both the absorption and emission properties of molecules coupled to nanostructures, effectively governing the optical properties of such a composite metamaterial. Here we report on the observation of strong coupling between a plasmon supported by an assembly of oriented gold nanorods (ANR) and a molecular exciton. We show that the coupling is easily engineered and is deterministic as both spatial and spectral overlap between the plasmonic structure and molecular aggregates are controlled. We think that these results in conjunction with the flexible geometry of the ANR are of potential significance to the development of plasmonic molecular devices.

Mechanically tunable multiband compact quadrifilar helix antenna with dual mode operation

We show that by introducing a gap at the center of the helical sections (where the current is minimum) of a lambda/2 quadrifilar helix antenna (QHA) and varying the axial length and radial gap between the overlapping volutes, the antenna gives a 28% impedance bandwidth which is nine times the bandwidth of a conventional QHA. A 16% bandwidth with a front to back ratio of >= 14 dB is achievable with 5-14% reduction in the size of the QHA. The structure can yield a monopole radiation pattern suitable for terrestrial applications or a hemispherical pattern suitable for satellite use. The simulation results are validated by measurements at L-band.

Ultra-thin tunable microwave absorber using liquid crystals

A dynamically adaptive radar absorber is described which is based on a periodic array of microstrip patches that are printed on a 500 mu m-thick liquid crystal substrate. The measured reflectivity of the structure is less than -38 dB with a 200 MHz -10 dB bandwidth at 10.19 GHz when a +4 DC bias is applied. It is shown that a 34 dB reduction in signal loss occurs when the bias voltage is increased to 20 V.

Liquid crystal tunable mm wave frequency selective surface

A frequency selective surface (FSS) which exploits the dielectric anisotropy of liquid crystals to generate an electronically tunable bandpass filter response at D Band (110-170 GHz) is presented. The device consists of two printed arrays of slot elements which are separated by a 130-mu m thick layer of liquid crystals. A 3% shift in the filter passband occurs when the substrate permittivity is increased by applying a control signal of 10 V. Measured results show that the insertion loss increases from -3.7 dB to -10.4 dB at resonance (134 GHz), thus demonstrating the potential to create a FSS which can be switched between a transmitting and a reflecting structure.

Tunable liquid crystal reflectarray patch element

Numerical and measured results are employed at X-band to demonstrate that the electrical properties ofnematic state liquid crystal can be exploited to produce phase shifters for beam scanning printed reflectarray antennas with a tunable range greater than 180'.

THEORY AND DESIGN OF A TUNABLE QUASI-LUMPED QUADRATURE COUPLER

In this article, we present the theory and a design methodology for a unable Quasi-Lumped Quadrature Coupler (QLQC). Because of its topology, the coupler is simply reconfigured by switching the bias of two varactor diodes via a very simple DC bias circuitry. No additional capacitors or inductors are required. A prototype at 3.5 GHz is etched on a 0.130-mm-thick layer substrate with a dielectric material of relative permittivity of 2.22. The simulated and measured scattering parameters are, presented. (c) 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 2219-2222 2009: Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24526

Porphyrin and tetrabenzoporphyrin dendrimers: Tunable membrane-impermeable fluorescent pH nanosensors

The pH dependencies of the UV-vis and fluorescent spectra of new water-soluble dendritic porphyrins and tetrabenzoporphyrins were studied. Because of extended pi-conjugation and nonplanar distortion, the absorption and the emission bands of tetraaryltetrabenzoporphyrins (Ar4TBP) are red-shifted and do not overlap with those of regular tetraarylporphyrins (Ar4P). When encapsulated inside dendrimers with hydrophilic outer layers, Ar(4)Ps and Ar(4)TBPs become water soluble and can serve as pH indicators, with pKs adjustable by the peripheral charges on the dendrimers. Two new dendritic porphyrins, Gen 4 polyglutamic porphyrin dendrimer H2P-Glu(4)OH (1) with 64 peripheral carboxylates and Gen 1 poly(ester amide) Newkome-type tetrabenzoporphyrin dendrimer H2TBP-Nw(1)OH (2) with 36 peripheral carboxylates, were synthesized and characterized. The pKs of the encapsulated porphyrins (pK(H2P-Glu)(OH)(4) = 6.2 and pK(H2TBP)-Nw(1)OH = 6.3) were found to be strongly influenced by the dendrimers, revealing significant electrostatic shielding of the cores by the peripheral charges. The titration curves obtained by differential excitation using the mixtures of the dendrimers were shown to be identical to those determined for the dendrimers individually. Due to their peripheral carboxylates and nanometric molecular size, porphyrin dendrimers cannot penetrate through phospholipid membranes. Dendrimer 1 was captured inside phospholipid liposomes, which were suspended in a solution containing dendrimer 2. No response from 1 was detected upon pH changes in the bulk solution, while the response from 2 was predictably strong. When proton channels were created in the liposome walls, both compounds responded equally to the bulk pH changes. These results suggest that porphyrin dendrimers can be used as fluorescent pH indicators for proton gradient measurements.