Olivier Vanbésien

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).

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.

Modal analysis of guiding structures patterned in a metallic photonic crystal

We present a modal analysis of guiding structures patterned in a two-dimensional metallic photonic crystal. Measurements are carried out on a T-stub structure with various dimensions in the 22–40 GHz frequency band. The pronounced resonances extracted from measured transmissivity spectra are interpreted in terms of multimode propagation phenomena in the T-stub region. Experimental data are assessed by the solution of a suitable form of the 2D Helmholtz equation established for metallic propagation media.

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.

Theoretical analysis of a branch line quantum directional coupler

We propose a new quantum directional coupler which consists of two parallel electron waveguides coupled by branch lines. The component is based on a high mobility modulation‐doped heterostructure, where the channel is constricted by a double H gate pattern at nanometer scale. The theoretical analysis is conducted by the solving of the two‐dimensional Schrodinger equation in the channel using mode matching techniques. Transmission and reflexion spectra calculated at the four ports of the structure show that 3 dB coupling coefficient along with a directivity of 17 dB can be achieved.

Left-handed electromagnetism obtained via nanostructured metamaterials: Comparison with that from microstructured photonic crystals

In this work, we review some of the key issues for designing dielectric and metallic arrays in the diffraction or refraction regimes with main emphasis on left-handed electromagnetism. We first discuss dispersion characteristics of periodic dielectric arrays which are structured on the wavelength scale (photonic crystals for optics and electromagnetic band gaps for microwaves) with special attention paid to propagation and refraction effects. Special attention was also paid to the isotropy properties in the Brillouin zone with the prospects of defining a negative refractive index. Then, we considered metallic structures which permit one to synthesize double-negative media with the goal of pushing their operation frequency into the infrared region. For both classes of microstructures and nanostructures, the technological challenges will be addressed by considering air hole arrays in a high refractive index semiconductor substrate and embedded C-shaped and wire metal arrays patterned on low index substrates.

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.

Photonic-crystal-based cloaking device at optical wavelengths

We present a photonic crystal cloaking device at optical wavelengths based on the association of two lattices working in different regimes, namely, stop band and negative refraction. The idea is to reconstruct in phase an incident cut Gaussian modulated plane wave by using the photonic crystal dispersion properties to ensure that no light penetrates in the core of the device. It is believed that such a cloaking device could become a building block for future generations of 3D integrated optical circuits.

Towards focusing using photonic crystal flat lens

We report on the numerical simulation and fabrication of a two-dimensional flat lens based on negative refraction in photonic crystals. The slab acting as a lens is made of an hole array (operating at the wavelength of 1.5 μm) etched in a InP/InGaAsP/InP semiconductor layer. We first study the key issues for the achievement of a negative refractive index taking advantage of folding of dispersion branches with main emphasis in dispersion properties rather than the opening of forbidden gaps. The diffraction and refraction regimes are analysed according to the comparison of the wave-vector with respect to the relevant dimensions of the hole array. In the second stage, we illustrate technological challenges in terms of e-beam lithography on a sub-micron scale and deep reactive ion etching for an indium phosphide based technology.

Photonic band gap material for integrated photonic application: technological challenges

We report on the fabrication of two-dimensional photonic crystals (PCs), aimed at operating in compact photonic systems, for optical communications. The targeted device is an optical add drop multiplexer (OADM) whose operation relies on the wavelength-selective and directional coupling of two optical waveguides. The microstructures are fabricated in an InP-based technology by means of a high resolution electron beam patterning generator (HR-EPBG), silicon nitride mask transfer and deep reactive ion etching (DRIE). Successful fabrication of arrays of holes with nano-scale dimensions is demonstrated with aspect ratios of the order of seven for devices operating in the long wavelength windows.