Abdallah Chahadih

Terahertz Corrugated and Bull's-Eye Antennas

The radiation and temporal properties of one-dimensional and two-dimensional (Bulls-eye) corrugated antennas are investigated numerically and experimentally at terahertz. The thickness of the antenna is miniaturized, within the fabrication limits, by using the transverse slot resonance rather than the longitudinal resonance. Square and triangular corrugations are discussed. The comparison between these two profiles shows that, in terms of return loss and gain, the antenna is robust to the corrugation shape, which alleviates the fabrication complexity. The temporal analysis of the Bulls eye antenna is also shown to demonstrate the contributions coming from the leakage of the surface wave at each groove. This insight allows us to engineer the temporal shape of the output pulse by varying independently the depth of each groove. The antennas presented here hold promise for manipulating with very low profiles pulse- and beam-shapes of THz radiation.

Metamaterial-Inspired Bandpass Filters for Terahertz Surface Waves on Goubau Lines

This paper is focused on the application of split ring resonators (SRRs) to the design of compact bandpass filters for terahertz surface waves on single-wire waveguides, the so-called planar Goubau lines (PGLs). Through equivalent circuit models, electromagnetic simulations, and experiments, it is shown that, while a pair of SRRs coupled to a PGL inhibits the propagation of surface waves along the line, introducing a capacitive gap to the PGL switches the bandstop behavior to a bandpass behavior. In order to highlight the potential application of the proposed structure to the design of practical higher order terahertz bandpass filters, two types of compact bandpass filters are designed and fabricated: 1) third-order periodic bandpass filters based on SRR/gap-loaded PGL and 2) coupled-resonator bandpass filters. It is shown that, while the frequency response of the both filter types can be controlled by altering the physical dimensions of the structure, a wider bandwidth can be achieved from the coupled-resonator filters. The design concept and simulation results are validated through experiments.

H2-induced copper and silver nanoparticle precipitation inside sol-gel silica optical fiber preforms

Ionic copper- or silver-doped dense silica rods have been prepared by sintering sol-gel porous silica xerogels doped with ionic precursors. The precipitation of Cu or Ag nanoparticles was achieved by heat treatment under hydrogen followed by annealing under air atmosphere. The surface plasmon resonance bands of copper and silver nanoparticles have been clearly observed in the absorption spectra. The spectral positions of these bands were found to depend slightly on the particle size, which could be tuned by varying the annealing conditions. Hence, transmission electron microscopy showed the formation of spherical copper nanoparticles with diameters in the range of 3.3 to 5.6 nm. On the other hand, in the case of silver, both spherical nanoparticles with diameters in the range of 3 to 6 nm and nano-rods were obtained.

Direct-writing of PbS nanoparticles inside transparent porous silica monoliths using pulsed femtosecond laser irradiation

Pulsed femtosecond laser irradiation at low repetition rate, without any annealing, has been used to localize the growth of PbS nanoparticles, for the first time, inside a transparent porous silica matrix prepared by a sol-gel route. Before the irradiation, the porous silica host has been soaked within a solution containing PbS precursors. The effect of the incident laser power on the particle size was studied. X-ray diffraction was used to identify the PbS crystallites inside the irradiated areas and to estimate the average particle size. The localized laser irradiation led to PbS crystallite size ranging between 4 and 8 nm, depending on the incident femtosecond laser power. The optical properties of the obtained PbS-silica nanocomposites have been investigated using absorption and photoluminescence spectroscopies. Finally, the stability of PbS nanoparticles embedded inside the host matrices has been followed as a function of time, and it has been shown that this stability depends on the nanoparticle mean size.

Room temperature direct space-selective growth of gold nanoparticles inside a silica matrix based on a femtosecond laser irradiation

Melting point phenomena of micron-sized indium particles embedded in an aluminum matrix were studied by means of acoustic emission. The acoustic energy measured during melting increased with indium content. Acoustic emission during the melting transformation suggests a dislocation generation mechanism to accommodate the 2.5% volume strain required for melting of the embedded particles. A geometrically necessary increase in dislocation density of 4.1 x 10^13 m^-2 was calculated for the 17 wt% indium composition.

Laser-induced growth of nanocrystals embedded in porous materials

Space localization of the linear and nonlinear optical properties in a transparent medium at the submicron scale is still a challenge to yield the future generation of photonic devices. Laser irradiation techniques have always been thought to structure the matter at the nanometer scale, but combining them with doping methods made it possible to generate local growth of several types of nanocrystals in different kinds of silicate matrices. This paper summarizes the most recent works developed in our group, where the investigated nanoparticles are either made of metal (gold) or chalcogenide semiconductors (CdS, PbS), grown in precursor-impregnated porous xerogels under different laser irradiations. This review is associated to new results on silver nanocrystals in the same kind of matrices. It is shown that, depending on the employed laser, the particles can be formed near the sample surface or deep inside the silica matrix. Photothermal and/or photochemical mechanisms may be invoked to explain the nanoparticle growth, depending on the laser, precursor, and matrix. One striking result is that metal salt reduction, necessary to the production of the corresponding nanoparticles, can efficiently occur due to the thermal wrenching of electrons from the matrix itself or due to multiphoton absorption of the laser light by a reducer additive in femtosecond regime. Very localized semiconductor quantum dots could also be generated using ultrashort pulses, but while PbS nanoparticles grow faster than CdS particles due to one-photon absorption, this better efficiency is counterbalanced by a sensitivity to oxidation. In most cases where the reaction efficiency is high, particles larger than the pores have been obtained, showing that a fast diffusion of the species through the interconnected porosity can modify the matrix itself. Based on our experience in these techniques, we compare several examples of laser-induced nanocrystal growth in porous silica xerogels, which allows extracting the best experimental conditions to obtain an efficient particle production and to avoid stability or oxidation problems.

Bandpass filters based on coupled split ring resonators for surface waves on planar Goubau lines

This paper demonstrates a method for enhancing the performance of recently introduced compact bandpass filters for terahertz surface waves on single wire waveguides, the so-called planar Goubau lines (PGLs). It is firstly shown numerically and validated experimentally that a gapped PGL loaded with a pair of split ring resonators (SRRs) acts as a bandpass filter. The concept and simulation result are validated through experiment. Furthermore, in order to achieve an improved frequency response, a third-order filter based on coupled SRRs is proposed. It is shown that while the size of the proposed filter is further reduced, it additionally benefits from a higher inband transmission, improved selectivity, and a controllable wide bandwidth.

Low loss transitions and microstrip lines on cyclo-olefin co-polymer substrate for terahertz applications

We present low loss microstrip transmission line with compact transitions from coplanar waveguide for sub-terahertz applications. The microstrip (MS) transmission line is fabricated on the surface of a thin cyclic olefin copolymer dielectric layer. Among different optimization parameters, we present here the influence of CPW-to-MS transition length. Low loss transmission of the MS line has been demonstrated using Vector network analyzer (VNA) and its loss value is approximately -0.9 dB/mm. We also demonstrate that this topology is well suited for terahertz filters obtained by the combination of split rings resonators along the MS line.

Low loss terahertz Planar Goubau Line on high resistivity silicon substrate

Improvement of THz waveguides with reduced losses has seen a rapid progress in the recent years. Among designs with the low loss, Planar Goubau Line have been designed and fabricated on high resistivity silicon substrate. In this contribution, we discuss two kinds of Planar Goubau lines: rectangular cross section line for low loss transmission and corrugated line in order to slow down and confine the electromagnetic waves. Moreover, in this paper we present our low-loss waveguides and focus mainly on the influence of strip width and the length of the PGL on the S-parameters. The measured loss at 210 GHz is 2.5 dB. This value of loss could be tuned depending on certain parameters such as length, width of the rectangular cross section, the dimension of the coplanar waveguide transition or the thickness of the silicon substrate, etc. At the end of this communication, we show an example of the corrugated PGL that could be used first to confine and to slow down the electromagnetic propagation, and second to be a candidate for a filter application in the THz domain.

Energy loss of terahertz electromagnetic waves by nano-sized connections in near-self-complementary metallic checkerboard patterns

The design of a self-complementary metallic checkerboard pattern achieves broadband, dispersion-less, and maximized absorption, concentrating in the deep subwavelength resistive connections between squares, without any theoretical limitation on the energy absorbing area. Here, we experimentally and numerically investigate the electromagnetic response in the limit of extremely small connections. We show that finite conductivity and randomness in a near-self-complementary checkerboard pattern plays a crucial role in producing a frequency-independent energy loss in the terahertz frequency region. Here metals behave like an almost perfect conductor. When the checkerboard pattern approaches the perfect self-complementary pattern, the perfect conductor approximation spontaneously breaks down, owing to the finite conductivity at the nano- scale connection, leading to broadband absorption. It is also shown that the random connections between metallic squares also lead to broadband and maximized energy loss through scattering loss, similar to finite conductivity.