Eric Lheurette

Capacitance engineering for InP-based heterostructure barrier varactor

We investigate new schemes of InP-based heterostructure barrier varactors with the aim of enhancing the capacitance nonlinearity of the devices. Starting from a generic step-like InGaAs/InAlAs/AlAs single barrier heterostructure, planar-doped and buried InAs quantum-well barrier heterostructures were successfully fabricated. It is shown that both solutions lead to more efficient screening of electric field near equilibrium and hence to improvement in the capacitance-voltage ratios with values as high as /spl sim/7:1. Under bias, the capacitance modulation is governed by an escaping mechanism in contrast to the conventional depletion operation mode observed for conventional varactors.

Negative-Zero-Positive Refractive Index in a Prism-Like Omega-Type Metamaterial

Negative-zero-positive index refraction was demonstrated numerically on the basis of full-wave analysis of a microstructured omega-type array and experimentally via angle resolved transmission measurement of a prism-type prototype. The experimental results are interpreted in terms of characteristic impedance and refractive index retrieved by a Fresnel inversion technique. The possibility to balance the dispersion characteristics with a negative-zero-positive index is demonstrated over X - and Ku -bands.

Experimental verification of negative refraction for a wedge-type negative index metamaterial operating at terahertz

We report on angle-resolved time domain spectroscopy (TDS) carried out on a prismlike negative index metamaterial operating around 0.5 THz. The wedge-type devices are constituted of hole arrays etched in gold thin films, which are stacked according to a sequential mask shift. By means of a goniometric TDS setup and subsequent analysis of the temporal waveforms, negative refraction is demonstrated with values close to n=−1 around 0.5 THz. The dispersion of refractive index retrieved from the Snell–Descartes law shows comparable trends in comparison with the dispersion deduced from complex transmission and reflection measurements on slab-type samples.

Left-handed electromagnetic properties of split-ring resonator and wire loaded transmission line in a fin-line technology

We report on experiments carried out on a backward transmission line, which consists of a fin-line periodically loaded by split-ring resonators and shunt wires. We first demonstrate the left-handed character of these frequency-selective transmission lines via: 1) dispersion diagram calculations which have been compared to the frequency dependence of measured scattering parameters; 2) the tracking of the phase fronts; and 3) the phase offset between two lines of various lengths. The effective refraction index found experimentally was approximately -4. Taking benefit of the use of a closed electromagnetic system and successful matching, the losses are especially analyzed. The ratio between incident and absorbed energy is 65% for a three-cell prototype and reaches 91% for ten cells.

Bandwidth enhancement in disordered metamaterial absorbers

In this letter, we show numerically and experimentally that a positional disorder of a collection of absorbing electromagnetic wave resonators operating in the long wavelength regime dramatically enhances the absorption bandwidth. The demonstration is performed by using single-size ring-shaped thin metal pieces periodically or randomly positioned onto a back-grounded dielectric layer. For periodic array, an optimum in the periodicity is pointed out with a narrow bandwidth, while the increasing influence of coupling effects for resonators in close vicinity explains a three-fold bandwidth enhancement in the case of positional disorder.

Left handed dispersion of a stack of subwavelength hole metal arrays at terahertz frequencies

We report on the electromagnetic response of a stack of subwavelength hole metal arrays. The samples were designed for exhibiting left handed dispersion branches under normal incidence, and their transmissivities were optimized via the fabrication of elliptical-shaped holes. They are constituted of benzocyclobutene layers with tens of micron thicknesses and submicron-thick gold films patterned by photolithography. Experimental evidence, achieved by time-domain terahertz spectroscopy and supported by full wave simulations, of a ground left handed dispersion branch is found around 0.45 THz. The insertion losses are −3 dB for a five-layer structure, this good level being explained by the matching of the impedance.

FROM REJECTION TO TRANSMISSION WITH STACKED ARRAYS OF SPLIT RING RESONATORS

We report on free space transmission experiments carried out on stacked split ring resonator (SRRs) arrays operating at microwave frequencies. We start from the case of a single frequency selective surface which exhibits a rejection at the SRR resonance frequency. By stacking SRR arrays in the propagation direction, we then show experimentally the possibility to induce a transmission band just below this resonance frequency. Full wave analysis shows the role played by the longitudinal and transverse coupling efiects in the electromagnetic properties of such bulk metamaterials, with the appearance of a transmission band resulting from an artiflcial magnetic activity.

Lumped elements circuit of terahertz fishnet-like arrays with composite dispersion

We have analyzed and characterized a terahertz stacked holes array over a broad transmission bandwidth. In this context, we identify the key contributions of the surface plasmon modes related to the periodicity of array, and of the subcut off transmission through the holes responsible of a ground left-handed dispersion branch. The experimental verification of a multimode electromagnetic behavior is performed by time domain spectroscopy, with a complementary assessment by full-wave analysis. A circuitlike approach appears suitable for the understanding of a zero-gap transition between the left- and right-handed characteristics, opening the way of nonvanishing group velocity in a near-zero index propagation medium. Such a balanced composite operation, pointed out over the past in the dual element transmission line approaches, is thus generalized for a stacked subwavelength holes array, whose operation can be compared to the one of the so-called fishnet structures.

Ferroelectric tunable balanced right- and left-handed transmission lines

The tuning of the dispersion characteristics of a balanced composite right- and left-handed transmission lines is demonstrated at centimeter and millimeter wavelengths. To this aim, coplanar waveguides, periodically loaded by interdigitated capacitances and stublike inductances, were fabricated onto a Ba0.5Sr0.5TiO3 ferroelectric thin film. The scattering parameter measurements show that the lines are left handed from 8 to 18 GHz and right handed above. The results are interpreted by the retrieval of the effective permittivity and permeability, which exhibit Drude-like dispersion characteristics with a magnetic plasma frequency, which can be tuned via the voltage-controlled permittivity variation in the BST thin film.

Non Linear Transmission Line Quintupler Loaded by Heterostructure Barrier Varactors

The potential of Non Linear Transmission Lines for harmonic multiplication was investigated on the basis of experimental capacitance-and current-voltage characteristics of high performance InP-based Heterostructure Barrier Varactors. The fabricated devices, which are vertically and laterally stacked, exhibit a voltage handling capability up to 50 V and a steep nonlinearity with a capacitance ratio of 6:1. These state-of-the art electrical characteristics enable the design of high efficiency quintuplers as a result of HBVs C-V curve symmetry and soliton-like propagation.