Showing papers by "Philippe M. Fauchet published in 2007"

Charge- and size-based separation of macromolecules using ultrathin silicon membranes

Abstract: Commercial ultrafiltration and dialysis membranes have broad pore size distributions and are over 1,000 times thicker than the molecules they are designed to separate, leading to poor size cut-off properties, filtrate loss within the membranes, and low transport rates. Nanofabricated membranes have great potential in molecular separation applications by offering more precise structural control, yet transport is also limited by micrometre-scale thicknesses. This limitation can be addressed by a new class of ultrathin nanostructured membranes where the membrane is roughly as thick (approximately 10 nm) as the molecules being separated, but membrane fragility and complex fabrication have prevented the use of ultrathin membranes for molecular separations. Here we report the development of an ultrathin porous nanocrystalline silicon (pnc-Si) membrane using straightforward silicon fabrication techniques that provide control over average pore sizes from approximately 5 nm to 25 nm. Our pnc-Si membranes can retain proteins while permitting the transport of small molecules at rates an order of magnitude faster than existing materials, separate differently sized proteins under physiological conditions, and separate similarly sized molecules carrying different charges. Despite being only 15 nm thick, pnc-Si membranes that are free-standing over 40,000 microm2 can support a full atmosphere of differential pressure without plastic deformation or fracture. By providing efficient, low-loss macromolecule separations, pnc-Si membranes are expected to enable a variety of new devices, including membrane-based chromatography systems and both analytical and preparative microfluidic systems that require highly efficient separations.

Two-dimensional silicon photonic crystal based biosensing platform for protein detection.

Abstract: We theoretically and experimentally demonstrate an ultrasensitive two-dimensional photonic crystal microcavity biosensor. The device is fabricated on a silicon-on-insulator wafer and operates near its resonance at 1.58 μm. Coating the sensor internal surface with proteins of different sizes produces a different amount of resonance redshift. The present device can detect a molecule monolayer with a total mass as small as 2.5 fg. The device performance is verified by measuring the redshift corresponding to the binding of glutaraldehyde and bovine serum albumin (BSA). The experimental results are in good agreement with theory and with ellipsometric measurements performed on a flat oxidized silicon wafer surface.

Dispersion of silicon nonlinearities in the near infrared region

Abstract: The authors present the detailed characterization of the wavelength dependence of two-photon absorption and the Kerr nonlinearity in silicon over a spectral range extending from 1.2to2.4μm. They show that silicon exhibits a significant increase in its nonlinear figure of merit with increasing wavelengths beyond the two telecommunication bands. They expect their results to provide guidance for extending nonlinear silicon photonics into new spectral regimes.

Nanoscale microcavity sensor for single particle detection

Abstract: Recently we demonstrated a biosensor based on a two-dimensional photonic crystal microcavity for detection of proteins. We present a theoretical and experimental study of a modified structure for single particle detection. With an active sensing volume of ~0.15 μm3, the device is capable of detecting ~1 fg of matter. Its performance is tested with latex spheres with sizes that fall in the size range of a variety of viruses.

Nano- to Microscale Porous Silicon as a Cell Interface for Bone-Tissue Engineering**

Abstract: An ideal material for orthopedic tissue engineering should be biocompatible, biodegradable, osteoconductive, osteoinductive, mechanically stable, and widely available. Porous silicon (PSi), a silicon-based material, fulfills these criteria. It is biocompatible and biodegradable, and supports hydroxyapatite (HA) nucleation. The micro-/nanoarchitecture of PSi may regulate cell behavior. The surface chemistry of PSi is flexible so that the interfacial properties between this material and living cells can be tailored easily by chemical modifications. Here, we report that PSi can support and promote primary osteoblast growth, protein-matrix synthesis, and mineralization. We also show that the osteoconductivity of PSi and other cellular responses can be controlled by altering the micro-/nanoarchitecture of the porous interface. With this material, we are closer to a functional biomaterial with both osteoconductivity and drug-delivery functions. Recently, tissue-engineering strategies using engineered biomaterials that support and promote bone-tissue growth have been proposed for reconstructive surgeries. The goal of a tissue-engineering approach is to repair and regenerate damaged human tissue with biomaterial-based devices. The approach requires functional cells derived from the target tissue, a matrix supporting those cells, bioactive molecules regulating cellular behavior, and the integration of this composite in the damaged tissue. Si, a semiconductor material, has the potential to achieve all the properties required for a tissue-engineering strategy. The physical and chemical properties of Si are widely known because of its wide use in the microelectronic industry. Moreover, sophisticated microfabrication techniques allow precise structures to be formed on Si substrates, some of which have been proposed for medical care. 16] The recent discovery of the biocompatibility, biodegradability, and bioactivity of PSi has opened the door for implantable applications of this Si-based material. After implantation of Si-based bioactive glass into rabbit bone, the elevation in Si concentration was only found at the implant site and not in other organs, and the implanted Si was efficiently excreted by urine. Furthermore, the large surface-to-volume ratio and the chemical flexibility of PSi makes it attractive for immobilizing bioactive molecules for drug-delivery purposes. These findings suggest that PSi can be a candidate for orthopaedic tissue engineering. Our investigations on the osteoconductivity of PSi were carried out using nanoscale (< 15 nm, NanPSi), mesoscale (ca. 50 nm, MesPSi), and macroscale (ca. 1 lm, MacPSi) pores in vitro. The PSi samples were produced by electrochemical etching of p-type Si wafers in HF-based electrolytes. The various pore configurations were achieved by changing the Si substrate, the electrolyte content, or the current density (see Supporting Information for detailed experimental information). As shown in Figure 1, MacPSi had pores with openings close to 1 lm; MesPSi had pores with pore openings around 50 nm; and NanPSi had a spongy porous structure with pore sizes under 15 nm. Unlike polished Si wafers, which do not degrade in cell media, PSi can be degraded in such a solution. Preliminary experiments showed that freshly-etched MesPSi degraded faster than MacPSi in cell media (see Supporting Information). The observation indicates that PSi, rather than Si, has potential in vivo degradation, and that MacPSi may be the most favorable candidate for bone-tissue engineering in terms of both biodegradability and stability. To protect PSi from gradual oxidation and degradation, a chemical oxidation in hydrogen peroxide was carried out after etching to form a thin oxide layer on the surface. Primary rat calvaria cells (osteoblasts) or rat osteosarcoma cells (ROS 17/2.8) were seeded onto PSi substrates for from 1 h to 5 weeks and the substrates and cells were assayed both qualitatively and quantitatively. Standard cell culture in 24-well polystyrene culture plates was used as a control. The adhesion of osteoblasts to PSi surfaces was quantified by direct counting of the attached cells. The viability of the attached cells was determined by an adenosine triphosphate (ATP)-based cell-viability assay. In adhesion studies (0.5– 4 h), PSi chips bound slightly fewer osteoblasts than the tissue-culture plate, but the difference was not statistically sigC O M M U N IC A IO N

Optical solitons in a silicon waveguide

Abstract: We observe, for the first time to our knowledge, the formation of optical solitons inside a short silicon waveguide (only 5 mm long) at sub-picojoule pulse energy levels. We measure a significant spectral narrowing in the anomalous-dispersion regime of such a waveguide, in contrast to all previous reported experiments. The extent of spectral narrowing depends on the carrier wavelength of input pulses, and the observed spectrum broadens in the normal-dispersion region. Numerical simulations confirm our experimental observations.

Label-Free Quantitative Detection of Protein Using Macroporous Silicon Photonic Bandgap Biosensors

Abstract: A label-free biosensor was demonstrated using macroporous silicon (pore size >100 nm) one-dimensional photonic band gap structures that are very sensitive to refractive index changes. In this study, we employed Tir−IBD (translocated Intimin receptor−Intimin binding domain) and Intimin−ECD (extracellular domain of Intimin) as the probe and target, respectively. These two recombinant proteins comprise the extracellular domains of two key proteins responsible for the pathogenicity of enteropathogenic Escherichia coli (EPEC). The optical response of the sensor was characterized so that the capture of Intimin−ECD could be quantitatively determined. Our result shows that the concentration sensitivity limit of the sensor is currently 4 μM of Intimin−ECD. This corresponds to a detection limit of approximately 130 fmol of Intimin−ECD. We have also investigated the dependence of the sensor performance on the Tir−IBD probe molecule concentration and the effect of immobilization on the Tir−IBD/Intimin−ECD equilibrium...

Anisotropic nonlinear response of silicon in the near-infrared region

Abstract: The authors characterize experimentally the anisotropy of two-photon absorption and the Kerr nonlinearity in silicon over a broad spectral region in the near infrared using the z-scan technique. The results show that both of these parameters decrease by about 12% along the [0 1 0] direction compared with the [011¯] direction, and this change occurs for wavelengths in the range of 1.2–2.4μm.

Betavoltaic and photovoltaic energy conversion in three-dimensional macroporous silicon diodes

Abstract: A three-dimensional p-n diode structure is presented for the generation of energy via photovoltaic and betavoltaic modes of operation. Macroporous Silicon (MPS) has a large degree of internal surface area and its vertically oriented pores, which extend deep into the bulk of the Si substrate, allow for the creation of three-dimensional structures. In this device the MPS will not only serve as a means for creating 3D diode structures, it will also serve as a host matrix for a tritium isotope which emits energetic beta particles. By varying electrochemical etching conditions and using a prepatterning technique, 1.1 μm diameter pores with a spacing of 2.5 μm were achieved. The p-n junction was created using a rapid thermal process (RTP) which relies on the diffusion from an n-type solid source into the MPS. To ensure the quality of the diode structure, devices were tested using a light source which resulted in a photovoltaic response. Finally, betavoltaic operation was demonstrated by exposing devices to a tritium gas source. The average energy conversion efficiency of the first generation 3D diode was one order of magnitude higher than that of a similar planar device.

Porous silicon as a cell interface for bone tissue engineering

Abstract: A novel cell interface has been constructed on porous silicon. We have demonstrated that nano- to macro-scale porous architectures have promising osteoconductive potentials. Macroporous silicon (pore opening 1-2 μm) is especially favorable for osteoblast adhesion, growth, protein synthesis and mineralization. An electronic/optoelectronic controllable medical implant with both scaffolding and drug delivery functions may be created for orthopaedic tissue engineering with this material.

Anisotropic nonlinear response of silicon in the near-infrared region

Abstract: We characterize experimentally the anisotropy of two-photon absorption and the Kerr nonlinearity in silicon over a broad spectral region.

Cell culture devices having ultrathin porous membrane and uses thereof

Abstract: Disclosed is a device for co-culturing two or more populations of cells using ultrathin, porous membranes positioned between cell culture chambers. Multiple chamber devices and uses thereof are described, including the formation of in vitro tissue models for studying drug delivery, cell-cell interactions, and the activity of low abundance molecular species.

Ultrafast photoluminescence dynamics of nitride-passivated silicon nanocrystals using the variable stripe length technique

Abstract: The ultrafast photoluminescence dynamics of nitride-passivated silicon nanocrystals is investigated using the variable stripe length geometry with 200 femtosecond pump pulses. We find that the luminescence lifetimes are in the nanosecond range throughout the entire spectral range. However, no evidence for optical gain is observed even when the pump fluence is in excess of 40mJ∕cm2. A comparison with similarly prepared, oxide-passivated silicon nanocrystals suggests that oxide passivation plays an important role in providing optical gain from silicon nanocrystals.

Tunable silicon-based light sources using erbium doped liquid crystals

Abstract: Tunable emission in the near infrared is demonstrated on a silicon platform. The building blocks for the tunable light sources consist of porous silicon microcavities infiltrated with erbium doped nematic liquid crystals. Erbium ions are the luminescence source, porous silicon microcavities narrow the emission band, and liquid crystals enable tuning of the peak wavelength. Greater than 10dB attenuation is achievable by thermal actuation with microcavities having a Q factor of 200. The bandwidth of the tunable emission is limited by the liquid crystal birefringence.

Alleviating Thermal Constraints While Maintaining Performance Via Silicon- Based On-chip Optical Interconnects

Abstract: The relentless pursuit of Moore’s Law by the semiconductor industry has yielded significant increases in performance, but at the cost of greater power dissipation. As CMOS technology continues to scale, increasing power densities, or “hot spots,” particularly in dense logic structures, may limit frequencies below projected targets in order to avoid circuit malfunction. A solution to this problem is to separate the hot spots by interleaving these units with cooler cache banks. This approach, however, increases the distance among processing functions, which can significantly degrade performance. While effort is made to localize communication as much as possible, global communication cannot be completely avoided, particularly in parallel applications. In this paper, the use of silicon-based on-chip optical interconnects is investigated for minimizing the performance gap created by separating processing functions due to thermal constraints. Models of optical components are presented, and used to connect the common front-end with the distributed back-ends of a large-scale Clustered MultiThreaded (CMT) processor. A significant reduction in thermal constraints (translated into an increase in clock frequency), combined with improved instructions per cycle (IPC), is demonstrated over a conventional all-electrical system. ∗This research was supported by National Science Foundation grant CCR-0304574.

Solvent Detection and Water Monitoring With a Macroporous Silicon Field-Effect Sensor

Abstract: Integration of electrical and fluidic systems for the design and fabrication of a system-on-chip (SOC) capable of sensing various liquid phase solvents is reported. A monolithic integration strategy makes use of macroporous silicon (MPS) as a gateway to interface the electrical and fluidic domains. In this application, the MPS material, acting as a sensing membrane, is used in a flow-through structure to transport an analyte from fluidic channels on one side of the chip to sensing electrodes on the other. A fluid-oxide-semiconductor interface results in the modulation of a space charge region in the semiconductor where real-time measurements are used to detect and distinguish between the presences of various solvents. The fluidic system has delivered sample volumes as small as 2 mul. Selected test solvents (i.e. acetone, ethanol, isopropyl alcohol, methanol, and toluene) have generated a measured change in capacitance up to 11%. A practical application of this sensor was demonstrated by monitoring various concentrations of isopropyl alcohol in a water supply. Undiluted samples provide characteristic responses that can be used for signature identification. The sensing device has a high degree of reusability and does not require heating or other solvent removal methods often necessitated in other sensing devices

Hybrid photonic crystal microcavity switches on SOI

Abstract: We report the development and characterization of 2-D photonic crystal (PC) microcavity devices on silicon on insulator. The transmission of light through a 2-D PC microcavity near resonance can be switched on and off by modulating the refractive index of the PC. Because silicon has poor electro-optical properties, it is advantageous to insert electro-optic materials inside the air holes. In this work, we report the design, fabrication, and characterization of such hybrid PC microcavity switches using liquid crystals as the electro-optic material. In addition, we demonstrate an electrode geometry that eliminates electric field screening by the more conducting silicon host, and thus enables switching. fabrication.

Porous silicon materials and devices

Abstract: Provided are materials and devices comprising physiologically acceptable silicon. The materials and devices can comprise a vector, including a viral vector.

Photochemical etching of silicon by two photon absorption

Abstract: Photochemical etching of silicon assisted by two-photon absorption (TPA) in the presence of hydrofluoric acid (HF) is demonstrated using a below-bandgap femtosecond laser source. We investigate the morphology of the etched silicon as a function of the laser power, exposure time, as well as the substrate doping level. Self-organized periodic silicon trenches with ∼150 nm spacing were observed at the etch front as the initial stage of the TPA photochemical etching process. Since the etching front can be precisely controlled by the focal point, this technique can be used for writing silicon microfluidic systems and for 3D micromachining.

Porous Silicon Electrical and Optical Biosensors

Abstract: Porous silicon (PSi) is a form of silicon with unique properties, distinct from those of crystalline, microcrystalline, or amorphous silicon. It was first prepared in 1956 [1] and much later it was identified as etched silicon [2]. The most common fabrication technique to produce PSi is electrochemical etching of a crystalline silicon wafer in a hydrofluoric (HF) acid-based solution [3]. The electrochemical process allows for precise control of the structural properties of PSi such as thickness of the porous layer, porosity, and average pore diameter. The morphology of PSi is important for sensing applications because the pore diameter limits the size of the species that can be captured. PSi can be prepared with a wide range of optical and electrical properties which makes it a very flexible material. The internal surface of PSi is very large, ranging from a few to hundreds of square meters per gram [4]. Therefore, the properties of PSi are affected not only by its crystalline core and nanomorphology but also by its surface. In addition, the properties of the crystalline core differ from those of bulk c-Si, when the size of the silicon structures drops below 10 nm, a size regime in which quantum mechanical effects modify its electronic states [5, 6].

Two-dimensional Si photonic crystal microcavity for single particle detection

Abstract: We theoretically and experimentally present a Si-based photonic crystal sensor for single particle detection. With a sensing area of ~40 mum2, the device is capable of detecting a sphere of ~50 nm in diameter or less.

Charge- and size-based separation of macromolecules using novel ultrathin silicon membranes

Abstract: Summary form only given.Commercial ultrafiltration and dialysis membranes have broad pore size distributions and are several orders of magnitude thicker than the molecules they are designed to separate, leading to poor size cutoff properties, filtrate loss within the membranes, and low transport rates. Nanofabricated membranes have great potential in molecular separation applications by offering more precise structural control, but either they are fragile and their preparation is cumbersome and expensive, or transport through them is still limited by mum-scale thicknesses. In this presentation, we describe a novel ultrathin porous nanocrystalline silicon membrane manufactured using straightforward silicon fabrication techniques and providing control over average pore sizes from 25 nm. These novel membranes can retain proteins while permitting the transport of small molecules at rates one order of magnitude or more faster than existing materials, separate differently sized proteins under physiological conditions, and separate similarly sized molecules carrying different charges. Despite being only several nm thick, such large-area, free-standing membranes can support a full atmosphere of differential pressure without plastic deformation or fracture. By providing efficient, low-loss macromolecule separations, these membranes are expected to enable a variety of new devices, including membrane-based chromatography systems and both analytical and preparative microfluidic systems that require highly efficient separations, including optical biosensors. In this presentation, the manufacture and physical properties of the membranes will be presented, several examples of their use for molecular separation will be discussed, and future applications in research and development as well as in the commercial sector will be outlined.

Label-free Optical Biosensor Built with Two-Dimensional Silicon Photonic Crystal Microcavity

Abstract: We experimentally demonstrate a silicon-based photonic crystal microcavity biosensor. This device is capable of detecting ~1 femtogram of analyte. Its performance is tested with glutaraldehyde-BSA model for quantitative measurements and biotin-streptavidin recognition for selectivity demonstration.

Spectral narrowing and optical soliton formation in SOI waveguides

Abstract: We observe, for the first time to our knowledge, spectral narrowing due to the formation of optical solitons inside a short 5 mm long silicon waveguide with sub-picojoule pulse energies.

Dispersion and anisotropy of Si's third-order nonlinearity from 1.2 to 2.4 μm

Abstract: We present the first detailed characterization, to the best of our knowledge, of wavelength and polarization dependence of two-photon absorption and the Kerr nonlinearity in silicon over a spectral range extending from 1.2 to 2.4 mum.

Spectral measurements of the third-order nonlinearity of bulk silicon in the near infrared region

Abstract: We report the first detailed characterization, to the best of our knowledge, of wavelength dependence of two-photon absorption and the Kerr nonlinearity in silicon over a spectral range extending from 1.2 to 2.4 mum.

Optical jitter and pulse distortion in high bit-rate, slow-light Mach-Zehnder interferometers

Abstract: Slow-light waveguides formed by photonic crystal coupled-cavities can be used to reduce the size of integrated MZI switches. We demonstrate that in these devices optical jitter causes significant pulse distortion at bit-rates above 100 Gbits/s.

Interlaced Coupled-Cavity Waveguide Platform for Silicon Photonics

Abstract: Slow-light devices are receiving significant interest in integrated photonics. We present a multi-channel platform with bandwidth above 400 Gbits/s at group velocities below 0.004c. Channel tunability makes it attractive for active devices and bio-sensing applications.

On-chip optical interconnect for reduced delay uncertainty

Abstract: Interconnect has become a primary bottleneck in the integrated circuit design process. As CMOS technology is scaled, the design requirements of delay, power, bandwidth, and noise due to the on-chip interconnects have become increasingly stringent. New design challenges are continuously emerging, such as delay uncertainty induced by process and environmental variations. It has become increasingly difficult for conventional copper interconnect to satisfy a variety of design requirements. On-chip optical interconnect has been considered as a potential partial substitute for electrical interconnect. In this paper, predictions of the performance of CMOS compatible optical devices are made based on current state-of-the-art optical technologies. Based on these predictions, the delay uncertainty in electrical and optical interconnects is analyzed, and shown to affect both the latency and bandwidth of the interconnect. The two interconnects are also compared for latency, power, and bandwidth density.

Optical jitter due to refractive index variations in slow-light photonic crystal MZI switches

Abstract: High effective index waveguides can reduce the size of integrated active MZIs significantly. We demonstrate numerically that they have high sensitivity to variations in material refractive index leading to significant pulse distortion due to jitter.