A. de Lustrac

Ultradirective antenna via transformation optics

Spatial coordinate transformation is used as a reliable tool to control electromagnetic fields. In this paper, we derive the permeability and permittivity tensors of a metamaterial able to transform an isotropically radiating source into a compact ultradirective antenna in the microwave domain. We show that the directivity of this antenna is competitive with regard to conventional directive antennas (horn and reflector antennas), besides its dimensions are smaller. Numerical simulations using finite element method are performed to illustrate these properties. A reduction in the electromagnetic material parameters is also proposed for an easy fabrication of this antenna from existing materials. Following that, the design of the proposed antenna using a layered metamaterial is presented. The different layers are all composed of homogeneous and uniaxial anisotropic metamaterials, which can be obtained from simple metal-dielectric structures. When the radiating source is embedded in the layered metamaterial, ...

Tunable bilayered metasurface for frequency reconfigurable directive emissions

The directive emission from a bilayered metamaterial surface is numerically and experimentally reported. The LC-resonant metasurface is composed of both a capacitive and an inductive grid constituted by copper strips printed on both sides of a dielectric board. By the incorporation of varactor diodes in the capacitive grid, resonance frequency and phase characteristics of the metamaterial can be tuned. The tunable phase metasurface is used as a partially reflecting surface in a Fabry–Perot resonance cavity. Far field radiation patterns obtained by direct measurements show the reconfigurability of emission frequency while maintaining an enhanced directivity.

Toward controllable photonic crystals for centimeter- and millimeter-wave devices

In this paper, we present several experimental and theoretical studies showing the feasibility of active photonic crystal controlled either by electrical elements or by light. The controllability of photonic crystals at centimeter wavelengths is proposed with the periodic insertion of diodes along the wires of a two-dimensional (2-D) metallic structure. For only three crystal periods with commercially available devices, more than 30 dB variations of the crystal transmission are predicted over a multigigahertz range by switching the diodes. From calculation models, a tight analogy is shown between these crystals and those consisting of discontinuous metallic rods with dielectric inserts. The numerical models as well as the proposed technology are validated by experimental measurements on 2-D crystals with either continuous or discontinuous metallic rods. The partial control of a 3-D layer-by-layer dielectric structure at millimeter wavelengths is also demonstrated in the second part of the work. A laser light is used to modulate the transmission level of defect modes by photo-induced free carrier absorption. The overall results are expected to contribute to further developments of switchable electromagnetic windows as well as to tunable waveguide structures in the microwave and millimeter wave domains.

HIGH-TRANSMISSION DEFECT MODES IN TWO-DIMENSIONAL METALLIC PHOTONIC CRYSTALS

An experimental and numerical study of point defect modes in a two-dimensional photonic crystal of metallic rods is presented. A method is developed to optimize the characteristics of transmission resonances in the second (and true) photonic gap of these structures. Transmission maxima close to −2 dB have been obtained by using a combination of a small number of point defects. Such a value is among the highest ones reported to date for two-dimensional photonic crystals with point defects. Besides, the width of the transmission window is shown to be adjustable in a wide range by changing the rod diameter. These results could be applicable to the fabrication of low-cost and compact filters in both the microwave and near-terahertz domains.

Experimental demonstration of electrically controllable photonic crystals at centimeter wavelengths

Electrically controllable photonic crystals have been fabricated by inserting p-i-n diodes in two-dimensional metallic lattices. A first structure uses a square lattice of thin and discontinuous metallic wires. A second structure is fabricated using stacks of printed circuits with metallic strips. The p-i-n diodes are soldered along the different metallic wires or strips. The crystals have been characterized between 1 and 20 GHz. We show that they can be operated as wideband switchable electromagnetic windows with high transmission or reflection contrast between on and off states. A ∼25 dB transmission modulation is reported within the first transmission band of a two-period crystal. We also show that the switching domain and modulation rate can be varied with a separate bias control for each crystal plane. Finally, the distance between crystal planes is used to tune the operating frequency range.

Experimental demonstration of complete photonic band gap in graphite structure

We experimentally demonstrate the existence of complete photonic band gap in graphite-type photonic crystals, thereby confirming theoretical predictions reported in previous studies. Experiments are performed at microwave frequencies from 27 to 75 GHz using hexagonal lattices of alumina rods. Transmission spectra measured for E (TM) and H (TE) polarizations and for different orientations of the two-dimensional lattice are found to be in excellent agreement with numerical calculations. The complete photonic band gap results from the overlap of E7 and H5 forbidden bands. Attenuations larger than 30 dB are measured for structures comprised of only four rows of alumina rods.

High-directivity planar antenna using controllable photonic bandgap material at microwave frequencies

In this letter, we experimentally demonstrate the capability of a controllable photonic bandgap (CPBG) material to conform the emitted radiation of a planar antenna at 12 GHz. The CPBG material is a variable conductance lattice fabricated with high-frequency PIN diodes soldered along metallic stripes on dielectric printed boards. Depending on the diode bias, the emitted radiation of the antenna can be either transmitted or totally reflected by the material. In the transmission state, the antenna radiation is spatially filtered by the CPBG material in a sharp beam perpendicular to the surface of the material.

Low temperature electroluminescence spectroscopy of high electron mobility transistors on InP

Electroluminescence spectroscopy of short gate high‐electron‐mobility transistors (HEMTs) on InP substrates is performed at cryogenic temperatures. Electroluminescence is a reliable tool to investigate impact ionization as compared to studies based on gate current which depend on the weakness of the intrinsic gate current intensity. In on‐state biased devices, a low energy (0.7–0.9 eV) recombination band is observed which is related to radiative recombination of carriers created by impact ionization in the low band gap InGaAs channel. The evolution of the luminescence intensity versus bias applied to the device shows that the electroluminescence intensity and impact ionization depend on two competing parameters: the electric field in the gate–drain access area and the drain current intensity. We show that the so‐called ‘‘kink’’ effect, which is a noticeable increase of the output conductance and which is observed at relatively moderate drain bias (600–750 mV) in our devices, is not correlated with impact ...

Photonic band gap materials for devices in the microwave domain

Materials with a periodically structured dielectric constant may exhibit forbidden photonic band gaps (PBG), that is, frequency domains where electromagnetic fields cannot propagate. The position and width of forbidden gaps can be controlled via the geometrical parameters of the structures and the contrast between the different permittivities. PBG materials have potential applications to a variety of devices in the microwave domain such as waveguides, couplers, reflectors and antenna substrate. This work reports on first experimental and theoretical studies of microwave guides and ring couplers based on PBG materials. Experiments are performed in the 27-75 GHz frequency range. Different coupling situations are given in illustration.

Solving the Poisson's and Schrodinger's equations to calculate the electron states in quantum nanostructures using the finite element method

In recent years, the sizes of semiconductor nanostructures have become so small that we have to take into account quantum effects. Simultaneously the real geometry of the device is important. In this paper, the two dimensional electron wave functions and the quantized states energies are calculated from the Schrodingers equation coupled with Poissons equation using a finite element method. The system of equations is solved iteratively to a sell consistent solution. We have simulated two devices with different carriers confinement. We obtain the carriers density and energy, conduction band and potential in these structures.