D. Lippens

Mie resonance-based dielectric metamaterials

Increasing attention on metamaterials has been paid due to their exciting physical behaviors and potential applications. While most of such artificial material structures developed so far are based on metallic resonant structures, Mie resonances of dielectric particles open a simpler and more versatile route for construction of isotropic metamaterials with higher operating frequencies. Here, we review the recent progresses of Mie resonance-based metamaterials by providing a description of the underlying mechanisms to realize negative permeability, negative permittivity and double negative media. We address some potential novel applications.

Magnetic control of negative permeability metamaterials based on liquid crystals

We report on the tunability, by a magnetic field, of a negative permeability metamaterial consisting of stacked arrays of broadside-coupled split ring resonators infiltrated with liquid crystals (LCs). The resonant frequency shift was numerically assessed by a rigorous anisotropic analysis of the reorientation of LC molecules. Experiments were carried out with a prototype designed and fabricated for X-band operation and infiltrated with a nematic compound with optical birefringence Δn=0.18. Scattering parameters vectorial analysis shows a good agreement between the resonant frequency shifts predicted under anisotropic conditions and those measured under static magnetic control.

Magnetically tunable left handed metamaterials by liquid crystal orientation

The tunability of an omega-type left handed metamaterial was demonstrated at microwave frequencies via the magnetic control of liquid crystal (LC) orientation. From the experimental and simulation results, it is shown that the left handed pass-band can be tuned by 220 MHz by changing the orientation of LC molecules by 90 degrees. A maximum index variation of 0.25 was obtained in the negative index regime with a measured LC birefringence of 0.05 in the 10 - 12 GHz frequency band.

Terahertz frequency difference from vertically integrated low-temperature-grown GaAs photodetector

We report on the development of a photoconductive detector based on low-temperature-grown GaAs which is vertically integrated with terahertz spiral antennas. A non steady-state velocity overshoot effect was expected in the photoresponse with a responsivity of 0.04 A/W at a bias voltage of 8 V. Photomixing experiments using two optical 0.8 μm beating lasers show a 3 dB bandwith of 700 GHz with a radiation power at terahertz frequency of 0.5 μW under 2×30 mW optical pumping.

Fabrication and performance of InP-based heterostructure barrier varactors in a 250-GHz waveguide tripler

High-performance InGaAs-InAlAs-AlAs heterostructure barrier varactors (HBVs) have been designed, fabricated, and RF tested in a 250-GHz tripler block. The devices with two barriers stacked on the same epitaxy are planar integrated with coaxial-, coplanar-, and strip-type configurations. They exhibit state-of-the-art capacitance voltage characteristics with a zero-bias capacitance C/sub 3//sup 0/ of 1 fF//spl mu/m/sup 2/ and a capacitance ratio of 6:1. Experiments in a waveguide tripler mount show a 9.8-dBm (9.55-mW) output power for 10.7% conversion efficiency at 247.5 GHz. This is the highest output power and efficiency reported from an HBV device at J-band (220-325 GHz).

Voltage tunable short wire-pair type of metamaterial infiltrated by nematic liquid crystal

In this letter, we report an experimental demonstration on voltage tunable short wire-pair metamaterial with negative permeability at microwave frequencies. After infiltrated by nematic liquid crystal, magnetic resonance of short wire-pair can be effectively tuned via relatively low static electric field. The results show, by increasing bias voltage from 0 to 100 V, magnetic resonance is continuously and reversibly shifted from 9.91 down to 9.55 GHz. Moreover, the effective permeability of metamaterial, for operation frequency around magnetic resonance, varies from negative to positive values. Numerical analysis has a good agreement with experimental results.

Effect of cathode spacer layer on the current‐voltage characteristics of resonant tunneling diodes

The effect of a lightly doped cathode spacer layer on resonant tunneling of Al0.3Ga0.7As‐GaAs double‐barrier heterostructures is examined. A self‐consistent band‐bending calculation combined with a quantum calculation of current‐voltage characteristics is used to model the experimentally observed I‐V curves. It is found that the I‐V characteristics show additional structures, sensitively dependent upon temperature. These effects result from the formation of a wide barrier by space‐charge reaction in the spacer layer leading to two possible resonant states. The validity of the theoretical approach is supported by the good agreement with experimental results.

A 360

Paraelectric BaSrTiO3 films deposited by sol-gel have been investigated from 1 kHz to 40 GHz using coplanar waveguides, resonators, and inter-digitated capacitances. At room temperature and without bias, the dielectric constant epsivr is around 300 and the loss tangent is 5times10-2. Tunability is 40% for a 40-V bias voltage. Analog phase-shifter circuits were subsequently fabricated in order to obtain a 360deg phase shift in the Ka-band with a moderate bias voltage. The lowest bias value was 17 V at 40 GHz, whereas 40 V were necessary at 30 GHz. For the latter condition, the insertion loss was -6dB. The best figure-of-merit of the phase shifters with a loading factor of 1.4 was 27deg/dB. These results show an improvement in terms of voltage-controlled tunability

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We report on a negative-zero-positive metamaterial based on an omega-type microstructure with special attention on the nonvanishing group velocity for a zero refractive index. We first investigate the dispersion characteristics by full wave analysis, by stressing the necessary conditions of equality between the electric and magnetic plasma frequencies which are characteristic of the dispersion of the effective permittivity and permeability. Also, tuning of the gapless transition frequency between the left and right-handed dispersion branches was analyzed when the permittivity of the host substrate is changed. Last, we demonstrate experimentally the balanced composite character of the dispersion by frequency and angle-resolved transmission measurements, carried out at centimeter wavelengths on slabs and wedge-type prototypes, respectively.

BST Phase Shifter With Moderate Bias Voltage at 30 GHz

The coupling effects of subwavelength high-permittivity (?r?>?100) arrayed ceramics which exhibit magnetic and electric Mie resonances are investigated by electromagnetic full-wave analysis. Special attention was paid to the symmetry properties of both magnetic- and electric-induced dipoles by varying independently the array periodicity. In agreement with the interactions between electric and magnetic dipoles, it is shown that resonance frequency shifts toward lower (higher) frequencies can be obtained, which depends on the longitudinal (transverse) dipole coupling strengths. Moreover, the emergence of quasi-bound states between tightly coupled basic cells is pointed out for the electric Mie resonances, which shows an unexpected frequency shift with a reverse variation.