Gloria Silva

Optical Detection of the Electromechanical Response of MEMS Micromirrors Designed for Scanning Picoprojectors

Single-axis rotational micromirrors actuated by comb finger structures have been designed in view of their application in reflective scanning picoprojectors for laser beam displacement along two perpendicular directions to obtain a raster scan scheme. A resonant mirror operating at a frequency around 25 kHz, suitable for horizontal scans, as well as a linear mirror, suitable for vertical scan at the typical video refresh rate (60 Hz), have been fabricated by Silicon-on-Insulator technology and are illustrated in this paper. We have in particular exploited the potentialities of semiconductor laser self-mixing interferometry, a powerful technique for characterizing the dynamic response of MEMS, for detecting the electromechanical response of both kinds of micromirrors. We report the results of the spot optical measurements performed on resonant and linear mirrors aimed at detecting the frequency of the fundamental rotational mode as well as of the in-plane and out-of-plane modes, close in frequency to the fundamental mode. We have experimentally demonstrated that the fabricated devices are suitable for high-resolution miniaturized projectors, in terms of frequency response and scanning angle.

Label-free optical detection of cells grown in 3D silicon microstructures

We demonstrate high aspect-ratio photonic crystals that could serve as three-dimensional (3D) microincubators for cell culture and also provide label-free optical detection of the cells. The investigated microstructures, fabricated by electrochemical micromachining of standard silicon wafers, consist of periodic arrays of silicon walls separated by narrow deeply etched air-gaps (50 μm high and 5 μm wide) and feature the typical spectral properties of photonic crystals in the wavelength range 1.0-1.7 μm: their spectral reflectivity is characterized by wavelength regions where reflectivity is high (photonic bandgaps), separated by narrow wavelength regions where reflectivity is very low. In this work, we show that the presence of cells, grown inside the gaps, strongly affects light propagation across the photonic crystal and, therefore, its spectral reflectivity. Exploiting a label-free optical detection method, based on a fiberoptic setup, we are able to probe the extension of cells adherent to the vertical silicon walls with a non-invasive direct testing. In particular, the intensity ratio at two wavelengths is the experimental parameter that can be well correlated to the cell spreading on the silicon wall inside the gaps.

A New Cell-Selective Three-Dimensional Microincubator Based on Silicon Photonic Crystals

In this work, we show that vertical, high aspect-ratio (HAR) photonic crystals (PhCs), consisting of periodic arrays of 5 µm wide gaps with depth of 50 µm separated by 3 µm thick silicon walls, fabricated by electrochemical micromachining, can be used as three-dimensional microincubators, allowing cell lines to be selectively grown into the gaps. Silicon micromachined dice incorporating regions with different surface profiles, namely flat silicon and deeply etched PhC, were used as microincubators for culturing adherent cell lines with different morphology and adhesion properties. We extensively investigated and compared the proliferative behavior on HAR PhCs of eight human cell models, with different origins, such as the epithelial (SW613-B3; HeLa; SW480; HCT116; HT29) and the mesenchymal (MRC-5V1; CF; HT1080). We also verified the contribution of cell sedimentation into the silicon gaps. Fluorescence microscopy analysis highlights that only cell lines that exhibit, in the tested culture condition, the behavior typical of the mesenchymal phenotype are able to penetrate into the gaps of the PhC, extending their body deeply in the narrow gaps between adjacent silicon walls, and to grow adherent to the vertical surfaces of silicon. Results reported in this work, confirmed in various experiments, strongly support our statement that such three-dimensional microstructures have selection capabilities with regard to the cell lines that can actively populate the narrow gaps. Cells with a mesenchymal phenotype could be exploited in the next future as bioreceptors, in combination with HAR PhC optical transducers, e.g., for label-free optical detection of cellular activities involving changes in cell adhesion and/or morphology (e.g., apoptosis) in a three-dimensional microenvironment.

Fibrillogenesis of human β2 -microglobulin in three-dimensional silicon microstructures.

The authors describe the interaction of biological nanostructures formed by β(2) -microglobulin amyloid fibrils with three-dimensional silicon microstructures consisting in periodic arrays of vertical silicon walls (≈3 μm-thick) separated by 50 μm-deep air gaps (≈5 μm-wide). These structures are of great interest from a biological point of view since they well mimic the interstitial environment typical of amyloid deposition in vivo. Moreover, they behave as hybrid photonic crystals, potentially applicable as optical transducers for label-free detection of the kinetics of amyloid fibrils formation. Fluorescence and atomic force microscopy (AFM) show that a uniform distribution of amyloid fibrils is achieved when fibrillogenesis occurs directly on silicon. The high resolution AFM images also demonstrate that amyloid fibrils grown on silicon are characterized by the same fine structure typically ensured by fibrillogenesis in solution.

An all-silicon optical platform based on linear array of vertical high-aspect-ratio silicon/air photonic crystals

An all-silicon optical platform (SiOP) that integrates a linear array of vertical (100-μm–deep) one-dimensional photonic crystals (1D-PhCs), with a different number of elementary silicon/air cells (from 2.5 to 11.5) and featuring a transmission peak around 1.55 μm, together with U-grooves (125-μm-wide) and end-stop-spacers for coupling/positioning/alignment of readout optical fibers in front of 1D-PhCs is reported. The SiOP is fabricated by electrochemical micromachining and characterized by measuring both reflection and transmission spectra of 1D-PhCs. An experimental/theoretical analysis of 1D-PhC features (transmissivity, quality factor, full-width-half-maximum) in transmission, around 1.55 μm, as a function of the number of elementary cells is reported.

A nanoporous gold membrane for sensing applications

Design and fabrication of three-dimensionally structured, gold membranes containing hexagonally close-packed microcavities with nanopores in the base, are described. Our aim is to create a nanoporous structure with localized enhancement of the fluorescence or Raman scattering at, and in the nanopore when excited with light of approximately 600 nm, with a view to provide sensitive detection of biomolecules. A range of geometries of the nanopore integrated into hexagonally close-packed assemblies of gold micro-cavities was first evaluated theoretically. The optimal size and shape of the nanopore in a single microcavity were then considered to provide the highest localized plasmon enhancement (of fluorescence or Raman scattering) at the very center of the nanopore for a bioanalyte traversing through. The optimized design was established to be a 1200 nm diameter cavity of 600 nm depth with a 50 nm square nanopore with rounded corners in the base. A gold 3D-structured membrane containing these sized microcavities with the integrated nanopore was successfully fabricated and ‘proof of concept’ Raman scattering experiments are described.

Fluorescence detection of fibrillar proteins on silicon microstructures

The biology and the structure of amyloid fibrils are under extensive investigation in many laboratories: they are co-causative agents of diseases such as Parkinsons and Alzheimers. We are investigating the use of a silicon micromachined structure, fabricated by electrochemical etching, as a three-dimensional supporting matrix also suitable for optically monitoring the amyloid fibrils growth. The silicon device consists in a periodic array of silicon walls with high aspect-ratio. This periodic arrangement of silicon and air gives rise to one-dimensional hybrid photonic crystals, suitable for out-of-plane (top view) imaging but, potentially, also for in-plane label-free testing. Here, we present some preliminary results relative to fluorescence microscopy analysis performed to investigate the interaction among silicon microstructures and fibrillar proteins. Samples of the highly amyloidogenic variant of human β2-microglobulin (P32G β2-m) are deposited on flat silicon dice as well as inserted into the gaps of the micromachined silicon devices. After Thioflavin T labeling, a bright emission originating only from silicon devices where polymerized amyloid fibrils are present is observed.