X. Melique

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).

Experimental evidence of backward waves on terahertz left-handed transmission lines

Left-handed transmission lines have been characterized by electro-optic sampling. The transmission lines used a coplanar strip technology periodically loaded by series capacitances and shunt inductances printed by electron-beam lithography onto a low-κ substrate. The experiments by optoelectronic sampling were conducted using low-temperature-grown GaAs and AlGaAs patches for probing the time-domain transmission properties. The devices exhibit a high-frequency response above 100 and up to 400 GHz which shows direct evidence of a backward propagation by tracking the time dependence of transmitted signals and phase analysis.

Comparison of fully distributed and periodically loaded nonlinear transmission lines

Two different approaches to realizing nonlinear transmission lines (NLTLs) are investigated in detail. In the first approach, the nonlinearity is continuously distributed along the line; in the second, the line is periodically loaded (PL) with discrete nonlinear elements. Measured heterostructure-barrier varactor (HBV) characteristics are used as the nonlinearities in both pulse-compression and harmonic-generation (20-60-GHz tripler) simulations. We point out that the choice of simulation step size is critical in the case of fully distributed (FD) NLTLs, and should be made sufficiently small that no numerical Bragg cutoff frequency appears. For the frequency tripler considered in this paper, simulations show that with PL (PL) NLTLs, 21% efficiency at 210-mW output power and 30% bandwidth can be obtained, whereas only 4.8% efficiency is possible using FD NLTLs. For pulse compression, we find that when properly matched, the FD NLTLs can deliver pulses that are five times sharper than can be obtained with the PL NLTLs. Measured results for an HBV-based PL NLTL frequency multiplier are reported that agree with our simulations, in particular, the 30% bandwidth. The confirmation of the role of the Bragg cutoff frequency in preventing the generation and propagation of undesired harmonics (this improving the conversion efficiency) is obtained from experimental results carried out from hybrid Schottky diodes NLTL measurements.

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.

Deposition of Ferroelectric BST Thin Films by Sol Gel Route in View of Electronic Applications

We report on the deposition of Ba 0.7 Sr 0.3 TiO 3 thin films, BST (70/30), by a sol-gel process. After deposition by spin coating, the film is annealed at 750°C, one hour in air. BST films have been deposited on (111) silicon substrates (bare and platinum coated) and on (0001) sapphire. X-Ray diffration patterns show that the films are polycristalline with perovskite structure and free of secondary phase. From SEM images, we infer that the film surface is smooth, the grain size is about 50 nm and the thickness of the BST layer is 300 nm. Electrical characterizations are mainly presented on BST deposited on platinized silicon. The dielectric constant l r is about 400 at 10 kHz and slightly decreases with frequency. The leakage current density is low: typically 10 nA/cm 2 at +5 Volts. At ambient temperature, the existence of an hysterisis cycle attest that a BST (70/30) film is in the ferroelectric state. As concerns the dissipation factor, the lower value has been obtained with a BST film deposited on sapphire: tg i = 0.014 at 100 kHz.

Determination of the mechanical properties of thin polyimide films deposited on a GaAs substrate by bulging and nanoindentation tests

In order to characterize the mechanical behavior of thin polyimide films deposited on a GaAs substrate in a membrane configuration, bulging and nanoindentation tests are performed. By these means, Young’s modulus of polyimide films, with thicknesses on the micron scale is found to be equal to 2.8 GPa. These two different techniques, which do not rely on the same assumptions, give results in very good agreement. The tensile residual stress in the membranes due to the fabrication process is a decreasing function of the thickness: 12<σ0<25 MPa. These membranes are already employed as mechanical support for coplanar wave guides up to the millimeter wave range.

Micromachining and mechanical properties of GaInAs/InP microcantilevers

Abstract Micro cantilever beams have been fabricated by selective wet etching of GaInAs/InP heterostructures with InP layer used as a sacrificial layer and GaInAs regions acting as semiconductor masks and/or etch stop layers. With deep front side micromachining, microstructures of one micron thick were fabricated and subsequently characterized by means of a nanoindentation system. For GaInAs microstructures, a Young’s modulus between 70 and 100 GPa range depending on analysis assumptions was calculated. For tri-layered structures with one micron thick sacrificial layer, we faced the problem of stiction. For addressing this issue, we fabricated several series of beam sets which differ mainly in length and width. A critical length corresponding to the transition between pinned and self-sustaining structures was found and subsequently analysed in terms of Young’s modulus with values consistent with mechanical measurements.

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.

Interface engineering for improved light transmittance through photonic crystal flat lenses

We present photonic crystal flat lenses with interfaces engineered to improve the light transmittance thanks to a broad angles impedance matching. The interface engineering consists in the realization of antireflection gratings on the edges of the lenses which are designed to reduce the propagative waves reflectivity over a wide range of incident angles. The fabricated structures were measured in optical near-field and a four times enhancement of the light transmission efficiency is reported.