Impedance matching in negative index 2D air hole array was addressed by the retrieval of the effective parameters. By
solving the eigenvalues problem, we first stress the major difference between an electromagnetic confinement in air for
the ground right handed branch and in the host matrix for the left handed one. We then calculate the complex
transmission and reflection coefficients for a finite slab from which the effective refractive index and impedance are
deduced by using a Fresnel inversion technique. The criterion n = -1 was found incompatible with the impedance
matching condition z = 1. Also, the relevance of the dispersion characteristics was assessed by a technique based on
spatial Fourier transform.
An original all-dielectric design that performs cloaking at terahertz frequencies is demonstrated. The cloak consists of
radially positioned discretized micrometer-sized cylindrical elements. Based on Mie theory and under adequate
excitation conditions (H along the rod axis), high-κ cylinders exhibit a strong magnetic resonance dependent on the
cylinder radii and material properties. Full-wave simulations coupled with a field-summation retrieval technique were
employed to adjust the electromagnetic response of individual ferroelectrics rods (Ba0.5Sr0.5TiO3; ε = 200 - tan δ = 2.10-2).
The rods magnetic plasma frequency was engineered such that the full cloak displays a progressive variation in its
permeability radial component; hence satisfying, for this polarization, the reduced equations derived from the conformal
transformation theory. The cloaking performance was assessed by modelling the complete micro-structured device.
Results unambiguously show that cloaking of any wavelength scaled objects located inside the cloak is achieved above
the Mie resonance frequency at 0.58 THz for the present device. In particular, the phase fronts of the electric field behind
the device are well reconstructed with a high value in transmission of the incident plane wave. This also means that the
absorption losses are small within the cloak in comparison with the metallic systems originally proposed. Although
cloaking is observed in a narrow band, this all-dielectric configuration provides an attractive route for designing cloaking
devices at microwave and terahertz frequencies.
There has been an increasing interest in using photonic crystals as negative refraction index slabs for integrated
nanophotonics. According to the superlensing criteria, a refractive index equal to -1 is needed to operate at an arbitrary
wavelength [1-2]. The field distribution is the result of multiple propagation phenomena such as reflection, diffraction,
self collimation and negative refraction. We report on the optimization of focusing properties of a triangular air hole
lattice etched in a III-V semiconductor matrix and present the demonstration of negative refraction by FDTD 3D
calculations. Under isotropy and finite length conditions, light transmission in the second band was investigated for an
incident wave tilted by 0°, 2°, 7° and 15° (E Field parallel to the air holes). The advantage of our method lies in the
existence of Fabry-Pérot effect resulting of interferences between the front and the rear interface of the slab. From the
comparison of each transmission spectrum, the filling factor was adjusted to obtain simultaneously n = -1 and a
maximum of signal to operate at a wavelength of 1.55 micrometers. At least, the validity of this method to produce an
intensity maximum behind the slab was checked by mapping the field with FDTD 3D simulations.
In this paper, we review some of the key issues for designing, fabricating and assessing electromagnetic properties of left-handed propagation media, aimed at operating in the infrared region. We show how lefhandedness can be experimentally demonstrated and we address the important issue of loss. In the far infrared region, we consider in details the various stages in the study of a left handed transmission line with a direct evidence of backward propagation in the time domain. We also compared the respective advantages and drawbacks of dielectric based microstructures namely Photonics crystals and of metal nanotructures starting from shrunk split ring resonators.
In this paper we have characterized the refractive indexed and absorption coefficient of negative photoresist NANO XP SU-8 from 0.1-1.6 THz using THz time-domain spectroscopy. Over the measured frequency range it was found that the refractive index is a relatively flat function of frequency, decreasing from 1.8 to 1.7. The value of absorption coefficient is seen to increase in a line fashion over the given frequency range, being 25 cm-1 at 1 THz. From this data we have extracted functions of dielectric constant and dielectric loss tangent versus frequency, quantities which will be of use for future THz circuit design. In addition to these measurements, we have demonstrated two novel applications of SU-8. First, we have fabricated membrane-like features by using a multi-exposure photolithographic technique on a single layer of SU-8, and second we have utilized a thin layer of SU-8 as a patternable adhesion layer as part of a semiconductor epitaxial lift-off process designed to transfer III-V semiconductor epitaxial layers onto lower loss host substrates.
The recent advances in telecommunication and teledetection systems are creating a demand for system developments in the upper part of the millimeter wave spectrum and beyond that available in this frequency range. It is thought that novel device schemes, relying on quantum effect,s will play an increasing role in this development. At last, new micromachining technologies are currently being developed for these wavelengths not only for the fabrication of passive components but also for active devices. This paper gives an overview of the Terahertz components, currently fabricated at IEMN, which are aimed at operating in THz receivers. This concerns mixers and harmonic multipliers, engineered for ultra-fast electron dynamics and strongly reactive and resistive non linearities. Special attention has been paid to high performance InP-based Heterostructure Barrier Varactors for harmonic multiplication. Double Barrier Heterostructure Resonant Tunnelling Didoes for fundamental generation and T-gate Schottky's for sub- harmonic mixing. Novel ideas will be presented in order to control the particle and displacement currents and to overcome the intrinsic and extrinsic limitations.
In this communication, we report on the design and the fabrication of quantum well barrier varactor structures with state of the art results in terms of capacitance ratio over a narrow voltage range. Basically, the fact to consider is a barrier cladded by two quantum wells with respect to a single barrier heterostructure. It has several consequences for the non linear character of the device. The capacitance mechanism is governed at low voltage by the electron population rates off the quantum well rather than the conventional depletion mode process. A true band gap capacitance engineering is here demonstrated with thee kinds of structures either in the InP material system with a InGaAs/InAs/AlAs heterostructure or in the GaAs material system with GaAs/InGaAs/AlAs pseudomorphic epilayers and lattice matched AlGaAs/GaAs/AlAs heterojunctions. Self- consistent simulations, based on the solution of Poisson and Schroedinger coupled equations system, were performed in order to calculate the electron wave function and the conduction band bending. High capacitance ratios can be predicted depending on material parameters and structure geometry. Test samples were then fabricated and rf tested. The devices very high capacitance ratios is excess of 5 to 1 over a 1 Volt range.
In the context of very high frequency components, where quantum well devices have already shown promise, we report, in this communication, one the optimization, realization and characterization of GaAs and InP-based resonant tunneling diodes. A fully planar technology has been achieved for a GaAs triple-well structure, with the use of ion implantation techniques, whereas mesa-etched technology together with airbridge integration have been used for the InP-based devices. In terms of performance, both material system have shown excellent DC characteristics with a current density of 60 kA/cm2 associated with a current contrast of 6:1 for the GaAs-based RTD. An even higher current density has been achieved for the InP-based devices with 215 kA/cm2 while still preserving a peak-to-valley current ratio of 9:1. In addition, 1 micrometers 2-area InP-based RTD's have been found to be unconditionally stable without the necessity of a stabilizing network. These anticipated properties for very small area devices, which meet the stability criteria, enables us to perform small signal characteristics over the whole range of the negative differential resistance region. Analysis of the measured scattering parameters up to 50 GHz shows an increase in the capacitance-voltage characteristics.
In a fiber optic communication system, the receiver must have high gain and low noise. The latter is generated in the photodetector, the load resistor, and the preamplifier. The photodetector 'gain' can be improved by use of an Avalanche Photodiode (APD) in which an internal gain is obtained from the impact ionization multiplication process. If, however, the ionization rates of electrons ((alpha) ) and holes ((beta) ) have the same value, the noise associated to the multiplication process will be high, according to McIntyre theory. This means that most of the III-V semiconductors are unsuitable for APD with low noise because (alpha) and (beta) are about the same. One way to improve the noise performance of the III-V compound semiconductor APDs is to alter the ratio (beta) /(alpha) (or (alpha) /(beta) ) by the use of a multi-quantum well (MQW) structure in the multiplication region of the APD. For a GaAs/GaAlAs MQW superlattice there is a large difference between the conduction and the valence band discontinuities ((Delta) Ec and (Delta) Ev) at the GaAs-GaAlAs edges. (Delta) Ec higher than (Delta) Ev leads to (alpha) >(beta) : electrons have more ability than holes to impact ionize. The authors obtained from multiplication factors and noise measurements a ratio k-1 equals (alpha) /(beta) equals 8...10, which is similar to the value given by F. Capasso.
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