Von Willlebrand Adhesion to Surfaces at High Shear Rates Is Controlled by Long-Lived Bonds

Von Willebrand factor (vWF) adsorbs and immobilizes platelets at sites of injury under high-shear-rate conditions. It has been recently demonstrated that single vWF molecules only adsorb significantly to collagen above a threshold shear, and here we explain such counterintuitive behavior using a coarse-grained simulation and a phenomenological theory. We find that shear-induced adsorption only occurs if the vWF-surface bonds are slip-resistant such that force-induced unbinding is suppressed, which occurs in many biological bonds (i.e., catch bonds). Our results quantitatively match experimental observations and may be important to understand the activation and mechanical regulation of vWF activity during blood clotting

Defect filtering for thermal expansion induced dislocations in III-V lasers on silicon

Epitaxially integrated III-V semiconductor lasers for silicon photonics have the potential to dramatically transform information networks, but currently, dislocations limit performance and reliability even in defect tolerant InAs quantum dot (QD) based lasers. Despite being below critical thickness, QD layers in these devices contain previously unexplained misfit dislocations, which facilitate non-radiative recombination. We demonstrate here that these misfit dislocations form during post-growth cooldown due to the combined effects of (1) thermal-expansion mismatch between the III-V layers and silicon and (2) precipitate and alloy hardening in the active region. By incorporating an additional sub-critical thickness, indium-alloyed misfit dislocation trapping layer, we leverage these mechanical hardening effects to our advantage, successfully displacing 95% of misfit dislocations from the QD layer in model structures. Unlike conventional dislocation mitigation strategies, the trapping layer reduces neither the number of threading dislocations nor the number of misfit dislocations. It simply shifts the position of misfit dislocations away from the QD layer, reducing the defects' impact on luminescence. In full lasers, adding a misfit dislocation trapping layer both above and below the QD active region displaces misfit dislocations and substantially improves performance: we measure a twofold reduction in lasing threshold currents and a greater than threefold increase in output power. Our results suggest that devices employing both traditional threading dislocation reduction techniques and optimized misfit dislocation trapping layers may finally lead to fully integrated, commercially viable silicon-based photonic integrated circuits.Comment: 9 pages, 6 figure

Dislocation-induced structural and luminescence degradation in InAs quantum dot emitters on silicon

We probe the extent to which dislocations reduce carrier lifetimes and alter luminescence and growth morphology in InAs quantum dots (QD) grown on silicon. These heterostructures are key ingredients to achieving a highly reliable monolithically integrated light source on silicon necessary for photonic integrated circuits. We find up to 20-30% shorter carrier lifetimes at spatially resolved individual dislocations from both the QD ground and excited states at room temperature using time-resolved cathodoluminescence spectroscopy. These lifetimes are consistent with differences in the intensity measured under steady-state excitation suggesting that trap-assisted recombination limits the minority carrier lifetime, even away from dislocations. Our techniques also reveal the dramatic growth of misfit dislocations in these structures under carrier injection fueled by recombination-enhanced dislocation glide and III-V/Si residual strain. Beyond these direct effects of increased nonradiative recombination, we find the long-range strain field of misfit dislocations deeper in the defect filter layers employed during III-V/Si growth alter the QD growth environment and introduce a crosshatch-like variation in the QD emission color and intensity when the filter layer is positioned close to the QD emitter layer. Sessile threading dislocations generate even more egregious hillock defects that also reduce emission intensities by altering layer thicknesses, as measured by transmission electron microscopy and atom probe tomography. Our work presents a more complete picture of the impacts of dislocations relevant for the development of light sources for scalable silicon photonic integrated circuits.Comment: 15 pages, 6 figure

Polymer Fiber Probes Enable Optical Control of Spinal Cord and Muscle Function In Vivo

Restoration of motor and sensory functions in paralyzed patients requires the development of tools for simultaneous recording and stimulation of neural activity in the spinal cord. In addition to its complex neurophysiology, the spinal cord presents technical challenges stemming from its flexible fibrous structure and repeated elastic deformation during normal motion. To address these engineering constraints, we developed highly flexible fiber probes, consisting entirely of polymers, for combined optical stimulation and recording of neural activity. The fabricated fiber probes exhibit low-loss light transmission even under repeated extreme bending deformations. Using our fiber probes, we demonstrate simultaneous recording and optogenetic stimulation of neural activity in the spinal cord of transgenic mice expressing the light sensitive protein channelrhodopsin 2 (ChR2). Furthermore, optical stimulation of the spinal cord with the polymer fiber probes induces on-demand limb movements that correlate with electromyographical (EMG) activity.National Science Foundation (U.S.) (EEC-1028725)National Science Foundation (U.S.) (Career Award)National Science Foundation (U.S.) (DMR-0819762)McGovern Institute for Brain Research at MIT (Neurotechnology Grant)Massachusetts Institute of Technology. Simons Center for the Social Brai

Electrically pumped continuous wave quantum dot lasers epitaxially grown on patterned, on-axis (001) Si

High performance III-V lasers at datacom and telecom wavelengths on on-axis (001) Si are needed for scalable datacenter interconnect technologies. We demonstrate electrically injected quantum dot lasers grown on on-axis (001) Si patterned with {111} v-grooves lying in the [110] direction. No additional Ge buffers or substrate miscut was used. The active region consists of five InAs/InGaAs dot-in-a-well layers. We achieve continuous wave lasing with thresholds as low as 36 mA and operation up to 80°C

Reliability of InAs quantum dot lasers grown on silicon: Physics, Limits and Solutions

Direct crystal growth approaches for III-V/silicon integration are of great technological interest for photonic integrated circuit applications. However, historically, these approaches have been hindered due to the generation of large numbers of interfacial dislocations that form to mediate materials property mismatches between GaAs/InP based films and the silicon substrate. The presence of dislocations at this interface, far from critical device regions, is not inherently problematic. But their ends, termed threading dislocations, rise upward through all the subsequent device layers, facilitating non-radiative recombination losses where they cross the light-emitting ‘active’ region. Further, the energy released in these defect-assisted processes facilitates ongoing dislocation growth during laser operation. As a result, these runaway ‘recombination enhanced dislocation climb’ and ‘recombination enhanced dislocation glide’ (REDC and REDG, respectively) processes culminate in device failure. Thus, laser reliability engineers have traditionally focused on efforts to reduce threading dislocation densities and to develop dislocation tolerant infrared emitters. However, even with decades of research seeking to reduce threading dislocation densities and to develop more dislocation tolerant gain materials such as p-modulation doped InAs quantum dots (QD), device lifetimes at industrially relevant temperatures (near 80°C) remained too short by orders of magnitude.Seeking to better understand the mechanism by which even relatively low densities of threading dislocations led to failure in InAs QD lasers, my colleagues and I identified that the threading dislocations give rise to in plane misfit dislocations lying at the QD layers. This however was puzzling: these layers are designed to be too thin for misfit dislocations to form due to lattice mismatch during growth. We therefore proposed that these specific misfit dislocations formed instead during post-growth cooldown due to (1) thermal expansion mismatch between the (Al)GaAs layers and the silicon and (2) local mechanical hardening effects in the QD layers. To test this hypothesis, we then used metallurgical principles to intentionally widen the mechanically hardened region and thus displace misfit dislocation formation to less critical regions of the device. This was very successful. Placing a single, thin, alloy-hardened ‘misfit dislocation trapping layer’ approximately 100-200 nm to either side of the quantum dot active region reduces the total QD adjacent misfit dislocation length by >10x and trapped misfit dislocations show no evidence of recombination enhanced dislocation climb (REDC) after more than 1000 hours of continuous operation at 60°C. Furthermore, lasers with misfit dislocation trapping layers operating 80°C demonstrate median extrapolated lifetimes significantly greater than 100,000 hours—meeting the requirements for data center operation. Thus, these results simultaneously emphasize both the importance of moving in earnest towards the advanced photonic integration schemes and the importance of continuing fundamental materials studies on III-V semiconducting systems

Characterization and connectorization of optoelectronic neural probes

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015.Cataloged from PDF version of thesis.Includes bibliographical references (pages 45-46).Reliability of interfaces between the nervous system and the neuroprosthetics can be significantly improved through the use of flexible polymer and polymer composite neural stimulation and recording systems. Furthermore, recent advances in optical neural stimulation methods would benefit from seamless integration of optical waveguides into neural probes. In this thesis, we describe electronic and optical characterization of polymer-based probes produced through thermal drawing process. Our results indicate that polymer-based fiber-probes maintain low-loss optical transmission even in the presence of 90-270* bending deformation with radii of curvature as low as 500 pim over multiple deformation cycles. These probes were robust enough to chronically function in the brain of freely moving mice. Furthermore, these flexible devices enabled direct optical stimulation in the spinal cord, which for the first time allowed for direct spinal optical control of lower limb muscles. In addition to optical characterization, we have developed a method for high-throughput connectorization of the fiber-probes with microscale features to external electronics. This required the development of custom printed circuit boards and involved a multi-step lithographic process. Finally, in a three-months long study we have demonstrated that probes characterized in this thesis yield significantly reduced tissue response in the brain as compared to the steel microwires traditionally used by neuroscientists.by Jennifer Selvidge.S.B

Reliability of InAs quantum dot lasers grown on silicon: Physics, Limits and Solutions

Direct crystal growth approaches for III-V/silicon integration are of great technological interest for photonic integrated circuit applications. However, historically, these approaches have been hindered due to the generation of large numbers of interfacial dislocations that form to mediate materials property mismatches between GaAs/InP based films and the silicon substrate. The presence of dislocations at this interface, far from critical device regions, is not inherently problematic. But their ends, termed threading dislocations, rise upward through all the subsequent device layers, facilitating non-radiative recombination losses where they cross the light-emitting ‘active’ region. Further, the energy released in these defect-assisted processes facilitates ongoing dislocation growth during laser operation. As a result, these runaway ‘recombination enhanced dislocation climb’ and ‘recombination enhanced dislocation glide’ (REDC and REDG, respectively) processes culminate in device failure. Thus, laser reliability engineers have traditionally focused on efforts to reduce threading dislocation densities and to develop dislocation tolerant infrared emitters. However, even with decades of research seeking to reduce threading dislocation densities and to develop more dislocation tolerant gain materials such as p-modulation doped InAs quantum dots (QD), device lifetimes at industrially relevant temperatures (near 80°C) remained too short by orders of magnitude.Seeking to better understand the mechanism by which even relatively low densities of threading dislocations led to failure in InAs QD lasers, my colleagues and I identified that the threading dislocations give rise to in plane misfit dislocations lying at the QD layers. This however was puzzling: these layers are designed to be too thin for misfit dislocations to form due to lattice mismatch during growth. We therefore proposed that these specific misfit dislocations formed instead during post-growth cooldown due to (1) thermal expansion mismatch between the (Al)GaAs layers and the silicon and (2) local mechanical hardening effects in the QD layers. To test this hypothesis, we then used metallurgical principles to intentionally widen the mechanically hardened region and thus displace misfit dislocation formation to less critical regions of the device. This was very successful. Placing a single, thin, alloy-hardened ‘misfit dislocation trapping layer’ approximately 100-200 nm to either side of the quantum dot active region reduces the total QD adjacent misfit dislocation length by >10x and trapped misfit dislocations show no evidence of recombination enhanced dislocation climb (REDC) after more than 1000 hours of continuous operation at 60°C. Furthermore, lasers with misfit dislocation trapping layers operating 80°C demonstrate median extrapolated lifetimes significantly greater than 100,000 hours—meeting the requirements for data center operation. Thus, these results simultaneously emphasize both the importance of moving in earnest towards the advanced photonic integration schemes and the importance of continuing fundamental materials studies on III-V semiconducting systems