There is a limited variety of pore shapes that can be attained by electrochemical etching itself. We show that these
limitations can be overcome and new pore geometries can be realized by additional post-etching treatment of
macroporous silicon. Repeated oxidation and subsequent oxide-removal steps are used to correct the initially faceted
pore cross-section and to obtain cylindrical pores. We demonstrate that the anisotropy of oxidation process is just
opposite to the anisotropy exhibited by the electrochemical etching and accounts for the observed evolution of pore
shape from a rounded square towards circular one. On the other hand, alkaline post-etching treatment is used to fabricate
pores with square cross-section. Careful choice of concentration, alcohol additives and temperature of alkaline solution
allows for certain crystallographic directions to be preferentially etched. In this way, pores with square, eight-sided
(octagonal) or rotated square shapes can be attained. When applied on 2D macropore arrays with modulated pores, such
post-etching treatment enables the realization of truly 3D structures with very complex geometries.
In recent years it became clear the relevance of photonic crystals when considering a nonlinear interaction. It has been shown in many occasions that the structuring of the material results in a clear enhancement of the nonlinear interaction. However, not too many structuring technologies can be applied successfully to materials that exhibit very good physical properties for the nonlinear generation of light. One of these is KTiOPO<sub>4</sub> (KTP), an inorganic material with high nonlinearity, large electrooptical coefficients, and very good transparency in the near infrared and visible range. In this paper, we propose a novel technique for growing two-dimensional KTP-air photonic crystals by liquid phase epitaxy employing a two-dimensional ordered macroporous Si matrix as a template.
Macroporous silicon structures have been fabricated by electrochemical etching. Such fabrication process is known to result in the presence of a thin microporous Si layer at the walls of the macropores and at the surface. Photoluminescence measurements conducted in plan-view and cross-section exhibit a wide emission peak around 650nm which can be attributed to the microporous Si. The combination of a photonic crystal and a light emitter in one structure represents a potential for applications that has not been studied previously. This preliminary study shows the influence of the main fabrication parameters, namely the current density and the etchant solution, on the emission properties of the microporous Si layer.
We study the photonic band gap formation in 2D photonic crystals comprising rods covered with a thin interfacial layer. The dielectric constant of the interfacial layer is different from that of the rods and background material. The rod together with the surrounding interfacial layer can therefore be treated as a single rod having a core and cladding regions. We study how the thickness and the dielectric constant of the cladding material affect the properties of photonic gaps in 2D photonic lattices. Specifically, we consider triangular and honeycomb lattices consisting of air rods drilled in silicon matrix and silicon rods in air, respectively. Photonic band simulations of such structures are presented performed using both finite-difference time-domain and plane-wave expansion methods. We show that the physical properties of the cladding layer strongly influence the photonic gap parameters. In particular, the existence of dielectric cladding reduces the absolute PBG in case of air rods drilled in a dielectric host, but may lead to larger absolute gaps in case of dielectric rods embedded in air. We also discuss the practical technological feasibility of these structures and their experimental realization.
In order to compare their gas sensing properties two kinds of sensors based on silicon cantilevers of similar characteristics have been fabricated: On one side we fabricated gravimetric gas sensors based on silicon cantilevers acting as resonators. The active layers consisted of polymer films deposited on top of the cantilevers. Sensors were maintained oscillating at their natural resonance frequency with electronic circuitry also developed in this work. Basically they consist of mass-spring mechanical resonators in which the mass increment due to gas sorption in the polymer provokes a shift on the resonance frequency. The output signal is a sinusoidal voltage extracted directly from the oscillator, and the amount of gas absorbed is related to the frequency of this output signal. The second type of sensors consisted of capacitors in which one electrode is a silicon cantilever and the other is a fixed metallic electrode fabricated parallel to the silicon cantilever. The silicon cantilever of these devices is covered with the same polymer films as for the resonators. The sensing principle in this case relies on the bending produced by the internal mechanical stress induced by the absorption of the gas in the polymeric layer. In these devices the signal is obtained by measuring the capacitance between the two plates of the capacitor, in this case the out coming signal was the current of the capacitor: an amplitude modulated signal. The gas response of both types of sensors have been characterized and a comparison is presented in this paper.
In 1985 Gerd Binnig and Calvin F. Quate from Stanford and Christopher Gerber from IBM (Zurich) designed the Atomic Force Microscope (AFM), since then a big interest arose around one of its main components; micromachined cantilevers. During all these years, authors have employed these tiny devices to sense multiple physical and chemical parameters through diverse transduction principles. Micromachined cantilevers offer many different transduction mechanisms: force sensing, bimetallic effect, mass loading, medium viscoelasticity, thermogravimetry, stress sensing, and more. Along with all these transduction mechanisms, a great variety of detection methods can be employed with cantilever-based sensors as for example: optical detection, resonance frequency measurement, piezoelectric integrated resistors, etc. The design and fabrication process of a micromachined Silicon capacitive gas sensor are described in this paper. Design and testing of the interface circuitry are also shown with preliminary results.