http://ilp-www.mit.edu/ List of 40 most recent project additions, as compiled by the Industrial Liaison Office. en-us Copyright 2009 MIT ILP Sat, 4 Jul 2009 06:31:19 GMT http://ilp-www.mit.edu/favicon.ico http://ilp-www.mit.edu/display_page.a4d?key=P5d http://ilp-www.mit.edu/display_project.a4d?projectId=19971 How credible would the Nash equilibrium be as a framework for behavior prediction, if there were no dynamics by which the game play could converge to such an equilibrium within a non-prohibitively large number of iterations? Motivated by this question we study the computational complexity of the Nash equilibrium. We show first that finding a Nash equilibrium is an intractable problem, which makes it unlikely that there are efficient dynamics for it. Since by Nash’s theorem an equilibrium is guaranteed to exist, the Nash equilibrium belongs to the class of problems in NP which always have a solution, and previous work establishes that such problems are unlikely to be NP-complete. We show instead that the problem is as hard as any fixed-point computation problem in a precise technical sense, which is motivated by simplicial algorithms for the computation of fixed points. In view of this hardness result, we discuss algorithms for computing approximate Nash equilibria and provide efficient algorithms for a large class of multi-player games, called anonymous, in which the players’ utilities, although potentially different, do not differentiate among the identities of the other players. Examples of such games arise in congestion, social interactions, and certain auction settings. 06/29/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19970 The Community Outreach and Education Core (COEC) of the MIT Center for Environmental Health Sciences (CEHS) focuses on educational activities for health care professionals, for the K-12 school community and for families. COEC is directed by Kathy Vandiver and Bevin Engelward, with Amy Fitzgerald as Consultant. The major objectives of the CEHS COEC are to inform children and adults about the impact of the environment on human health and to empower people to make wise choices about lifestyle and their relationship to the environment. COEC achieves these objectives by: (*) translating and "demystifying" the latest research findings in environmental health science (*) lowering the barriers between academia and the K-12 community (*) fostering strong partnerships with community health care and educational groups through activities. 06/25/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19968 This Research Core, directed by David Schauer, is to understand, holistically, the relationships that link ecological processes and human health. This includes the traditional 'fate and transport' model (in which chemical releases are transported and modulated by processes in the ecosystem, thus governing the extent of human exposure to the chemicals). However, advances over the past decade mandate a broader view of environment-health linkages, in which genomics and ecology play an increasingly prominent and important role. Future advances require better understanding of evolution, gene flow, and ecosystem processes along with progress in chemical and physical modeling and measurement. Gene flow, for example, can affect the distribution of pathogenicity, or the acquisition of antibiotic resistance or biodegradative capability in microbial communities. Ecosystem processes govern the nature of coexisting populations at scales from that of the gut to that of continents, with direct effects on humans at all scales. Examples of projects ongoing in this Core include: the environmental geochemistry of toxic metals, population dynamics of pathogenic and non-pathogenic Vibrio species in natural waters, the ecology of the lower gut and how that influences cancer susceptibility, the ecology and evolution of microorganisms in nature, and studies on arsenic in drinking water in Bangladesh (a result of a tradeoff between chemical toxins and environmentally transported pathogens). We envision that this Research Core will ultimately represent a bridge from the Systems Biology approach to the Earth System approach in addressing questions related to the effects of environment on human health. 06/24/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19967 Within the major oceanic subduction systems, the interaction of dense subducting lithosphere with the surrounding mantle controls the geometry of the descending slab, the motion of the trench with respect to the adjacent plates, and the directions and rates of global plate motions. Because complex feedback systems between plate motions and subduction zones exist on a global scale, such plate boundaries must be understood by studies of mantle convection at a global (or nearly global) scale. Within the Mediterranean region, where subduction boundaries are typically less than a thousand kilometers in length, subduction probably exerts little to no effect on the motions of the major tectonic plates, implying that feedback system between subduction and global plate motions is negligible. Thus subduction processes within the Mediterranean can be studied at a regional scale. This proposal is for study of the active Hellenic subduction system, located in the east-central Mediterranean region and sandwiched between the slowly converging African and Eurasian plates. Because the Hellenic subduction system exhibits both spatial and temporal variability in subduction rate, it offers an excellent opportunity to quantify the relationships among subduction rate, trench retreat, density of subducted lithosphere and deformation of the over-riding lithosphere. It also offers an excellent opportunity to study the dynamic interaction between subducting lithosphere and the surrounding mantle. 06/23/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19966 The Cambridge/Boston Biotechnology Cluster has a large number of newly founded biotechnology firms located in a small geographic area in Boston and Cambridge, MA. An extensive literature has developed in recent years arguing for the benefits of firms sharing a common technology to cluster geographically. Some of these benefits include the attraction of specialized staff and the promotion of venture capital, suppliers, support services, etc. Claims have also been made for the synergistic benefits of firms sharing scientific knowledge, especially if there are university laboratories near the cluster. Prior studies have inferred inter-firm communication from the evidence of co-publishing and co-patenting across firms, however, a good amount of scientific exchange may occur that does not appear in such publicly accessible records. An investigation of whether less formal scientific exchange across firms really occurs, and if so, to what degree. Characterizaton of the actual dynamics and assessment of the results of communicaiton within a biotech cluster. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19962 Interest in light sources emitting in the mid-infrared (MIR) wavelength region (2-5 micron) is growing due to their use in telecommunication and molecular spectroscopy applications. The former takes advantage of an atmospheric transparent window for MIR wavelengths and the latter due to the strong absorption lines of certain carbon-based and polluting gases. The goal of this project is to fabricate a room temperature, continuous wave, antimony-based laser and use this light source in photo-acoustic spectroscopy system for the detection of trace amounts of volatile impurities in petrochemicals. For quantum well semiconductor lasers, the wavelength is a function of well width, valence and conduction band offsets, carrier effective mass, and the band gap of the quantum well material. Hence, simulations were performed to determine the quaternary alloy compositions and the relative thicknesses of the quantum wells and barriers in order to achieve the desired wavelength. Apart from the active region, adjusting the cladding layer thickness also allows the optical mode, the confinement factor and loss to be varied. The laser structure that is shown in Figure 1 provides emission near 2.3 micron based on MATLAB simulations. Currently, the molecular beam epitaxial growth conditions that are necessary to grow the aforementioned laser structure are being determined. The MBE growth conditions include the substrate temperature, V/III flux ratio, and growth rate. Each quaternary film will be characterized after growth using the high resolution x-ray diffractometer in the Center for Material Science and Engineering (CMSE) to determine the layer composition, the layer thickness, the surface roughness, and lattice relaxation. The surface roughness can also be determined by using an atomic force microscope (AFM) and/or a scanning electron microscope (SEM); the composition will also be confirmed by Auger Electron Spectroscopy (AES). The MBE growth conditions can be optimized by measuring the full-width at half-maximum of the photoluminescence and/or x-ray diffraction peaks. Optical and electrical properties will be measured by photoluminescence, ellipsometry and Hall measurements. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19961 The demand for speed, access, and data delivery over the Internet initially fueled the development of photonic integrated chip (PIC) technology. PIC technology held the promise of being able to fully leverage underutilized fiber bandwidth while simultaneously increasing network flexibility and efficiency. Incidentally, the inherent qualities of PIC technology envisaged new applications of interest outside of its intended purpose. Specifically, PIC technology found use in so-called lab-on-a-chip applications where evanescent fields are used to interrogate chemical and biological environments for drug discovery, chemical detection and protein characterization. Moreover PIC technology has been identified as the technology to supplant bandwidth-limited copper wires in order to better serve high-speed central processing units (CPU) communication in state-of-the-art computer systems. A five-generation family of planar, electromechanically-reconfigurable, high-index-contrast optical switches was developed in order to further extend the functionality of PIC technology. A family of mechanical switches were designed to operate at a center wavelength of 1550nm with desired per device loss of less than 0.3dB, with an isolation and cross-talk of less than -30dB, and bandwidths greater than 100nm. Although these figures of merit for the mechanical optical switches may preclude their use in very large scale integration applications, these optical switches are more than capable of meeting the needs of small scale integration as well as being useful in applications that do not require premium signal-fidelity. On the other hand, these reconfigurable optical switches can be tailored for use in switching fabrics with the monolithic integration of waveguide-based optical amplifiers. Furthermore, switching fabrics that are based on planar electromechanical optical switches, can either be used for broadband switching as-is or for wavelength-specific switching with the monolithic integration of arrayed waveguide gratings. The family of planar, electromechanically-reconfigurable, high-index-contrast optical switches rely on butt-coupling, directional-coupling, and adiabatic-coupling in order to facilitate the transfer of light between waveguides; electromechanical parallel plate actuators facilitate the spatial reconfiguration of the waveguides. In particular, the in-plane adiabatic directional-coupler switch (generation 5 of the family of switches as shown in Figures 1 and 2) features electromechanical switching for a variety of waveguide-based devices (e.g., lasers, biologically-functionalized waveguides) without imposing design restrictions (e.g., doping levels, material alloy compositions) on the waveguide structure. In general, all of the generations of switches can be implemented in a variety of material systems with the only requirement being that the chosen material system has a high-index of refraction (n~3). 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19960 The goal of this project is to create an optical arbitrary waveform generator. To achieve this goal, light from a laser that generates ultra short pulses with a high repetition rate is first broken down into its various frequency components. By modifying the phase and amplitude of each frequency component, the resulting pulse of the recombined components results in an optical pulse with an arbitrary amplitude and phase. The aim of this research is to design, develop and fabricate a modulator. By creating an array of these modulators, the phase and amplitude of the various frequency components from an ultra short pulse, high repetition rate laser can be modified in order to create an optical arbitrary waveform generator. To create an arbitrary optical waveform at wavelengths that are centered at 800nm, ultra broad band modulator arrays are required. Since these modulators are to operate around 800nm, the material choices are limited to relatively high Al content AlGaAs and to In0.5 (GaxAl1-x)0.5P layers that are lattice-matched to GaAs. In addition, since GaAs absorbs light with a wavelength less than 870nm, the lower cladding layer of the modulator must be relatively thick in order to isolate the modulator from the GaAs substrate. To create the largest mode possible and to minimize the coupling loss, the index contrast between the waveguiding layers and the cladding layers should be minimized. To minimize the index contrast, a dilute waveguide structure in which thin layers of high index material are embedded in a low index material is employed. The resulting layered structure has an effective index slightly higher than the low index material and is determined by the layer thicknesses as well as the refractive index of the two materials that comprise the dilute waveguide. The modulator structure that was grown by molecular beam epitaxy is an Al0.8Ga0.2As-based structure in which the dilute waveguide consists of alternating layers of Al0.8Ga0.2As and InGaP. The structure is challenging in terms of the epitaxial growth. Although the use of Al0.8Ga0.2As for the cladding layer minimized the lattice-mismatch problem, achieving high quality, high Al content AlGaAs cladding layers is difficult due to the low Al adatom mobility on the surface during growth. To minimize free carrier loss, P-I-N structures are employed in which the Si and Be dopants are graded from the contact layers to the dilute waveguide region. Photoluminescence (PL) measurements from the arsenide-based structure show a weak PL peak at ~650nm from the InGaP layers in the dilute waveguide. The Al0.8Ga0.2As and Al0.5Ga0.5As layers as well as the InAlP layers have indirect band gaps and hence do not exhibit photoluminescence. Due to the high etch selectivity between the arsenide and phosphide layers, the uppermost high index layer of the dilute waveguide also acts as an etch stop. In addition to this original structure, a second Metal-Oxide-Semiconductor-type structure has also been grown which differs from the previous design by the addition of two oxidized AlAs layers, enabling a strongly confined optical mode in the middle of the structure. The AlxOy layers will allow the device to be capable of withstanding higher operating voltages. Furthermore, the device can be unipolar. The structure also contains an InAlP etch stop to facilitate fabrication. The optical properties of the dilute waveguide in both structures have been simulated using OptiBPM (Optiwave Corporation). The Al0.8Ga0.2As-based structure is designed to support a single optical mode within a 2 micron wide ridge waveguide; the fundamental mode for the arsenide-based structure is roughly 2 micron x 1 micron (W x H). The MOS-type structure is also designed to support a single optical mode, which is roughly 1.5 micron x 1 micron (W x H) as simulated by OptiBPM. If the dilute waveguide of the Al0.8Ga0.2As-based structure is not completely etched, due to the low index contrast of the dilute waveguide, the bending radius is quite large, on the order of a millimeter. Ultimately, the modulator will the incorporated into an array waveguide grating, therefore the loss needs to be considered. A new self-aligned fabrication process, which defines both the passive devices and the powered modulators in the same step, has been developed that is compatible with both the MOS-type structure and the Al0.8Ga0.2As-based design. The only difference in the fabrication process is the addition of the AlAs oxidation step that is inserted after the reactive ion etching that is used to define the waveguides. The mask set associated with this process has been designed and fabricated. The mask set contains Mach Zehnder interferometer modulators of various lengths with multimode interference couplers or Y-splitters. The Mach Zehnder interferometer modulators as well as conventional modulators are oriented both parallel and perpendicular to the major flat of the 2" GaAs (100) wafers. The mask set also contains a variety of passive components such as Y-splitters, multimode interference couplers as well as straight and curved waveguides. Arbitrary waveform generation is obtained by the phase and amplitude modulation of the individual frequency components within a frequency comb. Hence, optical wavelength demultiplexers and multiplexers are necessary for the spatial separation and recombination of wavelength components prior to and following modulation. Therefore, the structure and performance of arrayed waveguide gratings (AWG) have been modeled and a mask containing the AWG is currently being designed. The AWG has eight input and output waveguides that are each 2 microns wide. As the input aperture of the free propagation region (FPR) is approached, the waveguide width gradually increases to 3 microns over a length of 50 microns. The output waveguides taper in width at the output aperture, scaling back from 3 microns to 2 microns over a similar length. Adjusting the waveguide width, allows the optical mode to smoothly transition from the confined waveguides to the dispersive free propagation region. The thirty waveguides in the phased array section similarly taper from a width of 4 microns to 3 microns. At the first FPR output, where the waveguides are 4 microns wide, there is no space between the waveguides, encouraging full transmission of the diffracted power from the first FPR to the phased array waveguides and on to the second FPR. The AWG is designed and simulated to have 10 GHz channel spacing with -30dB to -40dB of optical cross-talk between output waveguides. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19959 Broadband Saturable Bragg Reflectors (SBR) consisting of monolithically integrated absorbers onto GaAs-based Bragg-mirrors have been used in a variety of ultra-short pulse lasers. The absorber, high-index layers and layer thicknesses are selected based on the laser's wavelength. The low-index layer can either be aluminum oxide that was created by the wet oxidation of AlAs layers or high Al content AlGaAs layers. Infrared SBRs are composed of AlGaAs/AlxOy mirrors with InGaAs-based absorbers which strain the structure and, depending on the absorber thickness, may lead to delamination during the AlAs oxidation process. For oxidation temperatures between 410°C and 435°C, delamination occurs between the absorber and mirror layers. More severe delamination occurs at higher oxidation temperatures. In an alternate SBR design, the additional strain introduced by the InP cladding layers generally increases the observed amount of delamination. A controlled temperature ramp before and after oxidation has greatly reduced the delamination of the SBR structures despite the presence of strain. The same AlAs oxidation technique also enables the fabrication of visible SBRs. Using In0.5Ga0.15Al0.35P as the high-index layer and AlxOy, Bragg mirrors are created for operation below 800 nm. Along with a GaAs absorber layer, these visible SBRs are nominally unstrained and may mode-lock a variety of lasers including Ti:Sapphire, Cr:LiSAF, Cr:LiCAF, and Cr:LiSGaF. A variety of SBRs with GaAs/AlGaAs distrubuted Bragg reflectors were grown for use in an Er-Yb laser. The saturable absorber sections contain either one or two InGaAs quantum well(s); the InGaAs quantum wells are clad with either GaAs or InP. The reflectivity characteristics of the different saturable Bragg reflectors were assessed. As the number of InGaAs quantum wells increased from one to two, the overall reflectivity decreased due to the absorption of light within the additional InGaAs quantum well. Although the SBRs were designed to mode-lock the Er-Yb laser, only one of the SBRs actually mode-locked the laser. The two SBRs with InP cladding layers and the SBR with a single quantum well with GaAs cladding layers exhibited both Q-switching as well as mode-locking. The two SBRs with two InGaAs quantum wells with GaAs cladding layers should have yielded similar results, however, only one of the samples successfully mode-locked the laser. Further optical measurements of the SBRs are underway. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19958 Future optical networks require the number of optical-to-electrical-to-optical conversions to be minimized. To allow the optical data to remain in the optical domain, the optical core requires some logic functionality. Therefore, the aim of this project is to model and to produce a modular monolithically-integrated all-optical unit cell capable of performing a complete set of Boolean operations at speeds of 100s of gigabits per second. Optical logic operations, wavelength conversion, and other advanced optical switching schemes can be implemented using the design. The basic structure consists of a balanced Mach-Zehnder interferometer with an InGaAsP-based semiconductor optical amplifier in each arm. By investigating the device design and fabrication tolerances using the beam propagation method and finite-difference time-domain techniques, the critical device dimensions were modeled prior to fabrication. A set of design and simulation tools is used to develop the design rules, to identify tradeoffs, to determine fabrication tolerances, and to estimate the effects of imperfections in semiconductor processing on the device’s performance. The beam propagation method (BPM) simulations are used to model passive waveguides, multimode interference couplers, and asymmetric twin-waveguide structures with adiabatic tapers. Three-dimensional, finite-difference, time-domain (FDTD) calculations were used to estimate the reflections between the various components by considering a small computational domain around each abrupt interface. The FDTD computations confirm that the reflectivity of the adiabatic tapers with blunt tips is well below 0.0001. Custom MATLAB scripts are used to assess the tradeoffs in the SOA performance for both linear and non-linear applications in photonic integrated circuits. One of the goals of this work is to produce design rules that specifically address the design of the SOAs for switching applications. The phase in the SOAs is modeled in order to study cross-phase modulation in a balanced MZI with an SOA in each arm. The creation of the optical unit cell requires multiple components such as SOAs and multi-mode interference couplers to be integrated together via low-loss passive waveguides. The passive waveguide consists of three layers of In0.8Ga0.2As 0.45P0.55 separated by InP. The SOA consists of a 200 nm thick layer of In0.56Ga0.44 As0.94P0.06 with 100 nm thick In0.8Ga0.2As 0.45P0.55 cladding layers. The asymmetric twin waveguide approach is employed for the monolithic integration of active devices with passive components in which the active devices are stacked vertically on a lower passive waveguide. The use of an adiabatic taper coupler allows the optical signal to move from the passive waveguide to the active waveguide. Prototypes of the all-optical logic unit cell have been fabricated using the facilities within the Microsystems Technology Laboratory (MTL), the Nanostructures Laboratory (NSL) and MIT Lincoln Laboratory. The current generation design combines both the active and passive devices into single die suitable for a step-and-repeat mask set, allowing for sharper tapers and smoother waveguide bends. Processing improvements include depositing the base metal for the top-side contact prior to any III-V etching, minimizing the amount of InP-based etching through the use of trenches, (as seen in the figures) and using dedicated III-V ICP RIE etcher at Lincoln Laboratory. Complete optical logic structures have been fabricated. The fabricated devices are currently being characterized both at M.I.T. and at M.I.T. Lincoln Laboratory. The passive waveguides have a measured loss of 0.89dB/cm. In addition, diode characteristics have been measured for the SOAs. An effort to improve upon the fabricated devices in order to lower the contact resistance and to improve the planarization uniformity across the chip is underway. A new contact metal mask has been designed to improve the current injection and to allow for easier probing of the SOAs. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19957 Super-collimation (SC) is the propagation of light without diffraction using the intrinsic properties of photonic crystals (PhCs). Successful fabrication and measurement of SC have been achieved for planar PhCs composed of silicon rods (Figure 1) as well as air holes etched into silicon (Figure 2). The super-collimating PhC is fabricated on a silicon-on-insulator (SOI) wafer. The low-index silicon dioxide layer is used to minimize radiation loss into the high-index silicon substrate. The rods are defined using interference lithography and pattern transfer is achieved using reactive ion etching (RIE). Infrared images of the light that is scattered normal to the plane of the super-collimator that is composed of Si-rods on a SOI wafer are shown in Figure 3. Super-collimation is observed at a wavelength of 1530nm. In principle, creating a beam which does not diverge for long distances is possible by making the distribution of the beam's constituent eigenmodes sufficiently narrow in k-space, i.e. as the beam approaches a single Bloch mode or plane wave. On the other hand, a super-collimator allows for nearly divergent-less propagation for beam widths only a few times the lattice constant of the PhC. A method of exploring the design space for super-collimating devices has been developed; the bandwidth for super-collimation for the photonic crystal of holes is wider than the bandwidth for that of rods. Hence, depending on the application, a photonic crystal that is composed of air holes may be more suitable than a photonic that is composed of dielectric rods. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19955 The creation of a new class of materials, functionally graded nanocrystalline metals (FGNMs), offers the unique ability to optimize different material properties in separate locations. In mechanical applications involving cyclic loading, for example, a gradient from small grains (which slow crack initiation at the surface) to larger grains (which slow crack propagation in the interior) could lead to the optimization of fatigue properties throughout a coating. Current work in the group explores the fundamental processing science involved in creating FGNMs through periodic reverse pulse electrodeposition of Ni-W alloys. By modifying the current waveform, this technique allows precise in-situ control of composition and grain size. The goal of this project is to understand the mismatch of composition, structure, and properties between adjacent layers in a graded nanocrystalline structure and to, in turn, define a window of processing variables in which stable, artifact-free, and stress-free FGNMs can be synthesized. Profilometry methods are presently being used to characterize the residual stress inherent in these deposits while the interfacial structure is being investigated using scanning electron and transmission electron microscopy methods. In addition, parallel studies of mechanical properties of these materials is underway. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19954 The unusual properties exhibited by nanocrystalline metals have motivated extensive fundamental studies over recent years, with great emphasis on the scaling of mechanical properties with grain size reduction and the breakdown of the characteristic Hall-Petch relationship. In this group, we use experiments as well as theory and simulations to study the unique properties that emerge at this scale. Some of our recent work has probed hardness, rate sensitivity, and pressure sensitivity of deformation, all of which exhibit inflections at a finite nanocrystalline grain size. We compile indentation curves from various nanocrystalline specimens and scanning electron micrographs of the associated residual impressions. As evidenced by the discrete discontinuities in the load-displacement response and shear offsets in the pile-up of the d = 3 nm sample, inhomogeneous shear banding characteristic of metallic glasses appears as the grain size approaches the amorphous limit, thus signaling a shift to glass-like deformation behavior. Experimental results such as this are also connected directly to simulations. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19953 Nanostructured metals exhibit interesting and useful properties owing to their extremely fine structural length scale. Unfortunately, controlling grain size in the nanocrystalline regime has proven difficult as these materials represent a classical far-from-equilibrium state, containing a large volume fraction of high-energy interfaces. Alloying presents an opportunity to reduce the energy penalty associated with nanostructure formation. In our group, we employ experiments and atomistic computer simulations to study the role of alloying elements in nanostructure formation and stabilization. In the experimental approach, we synthesize nanostructured Ni-W and Al-Mn using electrodeposition in aqueous and non-aqueous medium respectively. The figure below shows that for both systems, as the solute content increases, the grain size decreases monotonically. For the Ni-W system, atom probe tomography experiments and computer simulations show that the solute atoms (W) preferentially segregate to the grain boundaries, resulting in a thermodynamic reduction in grain boundary energy. On the other hand, scanning transmission electron microscopy experiments indicate that for the Al-Mn system, Mn does not preferentially partition to the grain boundaries. Instead, grain refinement can be attributed to electrode kinetics, where increasing Mn content in the electrolytic solution causes the grain nucleation rate to increase during electrodeposition, thus resulting in smaller grains. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19952 Heterogeneity is present in materials at various length scales and in different forms. For example, polycrystalline materials are heterogeneous in that they are composed of crystals of different orientations and that they contain defects of various dimensions, e.g., dislocations and grain boundaries. Another type of heterogeneity is the variation in compositions, such as in composites and porous media. These structural heterogeneities result in a distribution of local materials properties. From this known distribution we can evaluate the apparent or effective properties of heterogeneous materials. The goal is to quantitatively correlate the effective properties with the connectivity among various microstructural elements, which is the object of percolation theory. Combining homogenization schemes with percolation concepts offers a robust approach to understanding microstructure-property relationships. We study discrete network problems and also more common continuum problems using both theoretical modeling and computer simulations. For example, we show the effective diffusivity of a ternary grain boundary network calculated from the model (color surface plot) and from a simulation (gray dots). We also experimentally seek to improve the mechanical properties of metallic materials by manipulating the connectivity of the heterogeneous interfacial networks. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19951 The study of microstructure is only useful as it relates to observable properties though, and a variety of the effective tensor properties of materials are governed primarily by the crystallographic texture, or the distribution of crystal orientations in a material. As a result, the analysis of the microstructure in (a) may often be simplified by neglecting the spatial information and considering the microstructure from the standpoint of texture alone. The remaining information is nevertheless considerable, and requires the development of updated and more powerful means to represent and analyze orientation information. Historically, Euler angles are used for the analysis and presentation of texture information. Euler angles describe an orientation by three sequential rotations about fixed axes, as is depicted in part (b) of the figure below. One conspicuous feature of Euler angles are their asymmetry, since they more naturally describe rotations about the x and z axes than about the y axis. The asymmetry has a number of repercussions, among which is a singularity in the definition of certain rotations and in functions of Euler angles as well. Other descriptions of orientations used by the materials science community rely on the description of a rotation by an angle of rotation about a variable axis, and do not exhibit the asymmetry that is inherent to Euler angles. Although there are other benefits to using these representations, they are generally not used as frequently as Euler angles. One reason for the continued preference for Euler angles is that the only method to analytically represent a texture has historically been as a linear combination of functions of Euler angles. The esearch group has recently developed an alterative method that represents a texture as a linear combination of functions of the axis and angle of rotation. This is enabling the refinement of our ability to interpret and analyze texture information, and continues to be an area of active research. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19950 Shape memory materials exhibit pseudo-elastic behaviors and shape memory properties. They can be deformed 'elastically' up to 7-8% or even higher strain which will completely recover upon unloading. If deformed at a low temperature, they will also recover their original shapes when heated to above a certain critical temperature. A lot of energy is dissipated in one pseudo-elastic loading/unloading cycle, making shape memory materials excellent candidates for mechanical damping applications. What is more, the coupling between thermal and mechanical fields enables these materials to be used as actuators or components in multifunctional composites and devices. The group is investigating the pseudo-elastic and shape memory properties of Cu-Al-Ni shape memory alloys. Cu-based alloys are relatively inexpensive compared to other alloys, and have good thermal and electrical conductance. We are making fine structures of these alloys and studying the size effects of the damping capabilities and shape memory properties. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19949 Bulk metallic glass matrix composites (BMGMCs) are a new class of composite material, designed to take advantage of the impressive strength and high elastic limit of metallic glasses, while improving toughness and suppressing localized failure through the introduction of microstructure. Depending upon the system thermodynamics and processing kinetics, the reinforcement phase in BMGMCs may itself be inherently ductile or brittle, and the second phase volume fraction is also often quite tailorable. For example, the figure below shows an example of Zr-based glass composites with four different levels of reinforcing crystalline phase. Our group studies the microstructure-property connections in BMGMCs, with special emphasis on mechanical properties. At high temperatures, we investigate the flow rheology of composites for its implications in shape forming. At low temperatures we study strength, plasticity, toughness, and interaction of shear bands with inclusion phases. We also employ computer simulations to study the mechanics of this new class of composites. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19948 The mechanisms associated with deformation in an amorphous metal, or a metallic glass, present an interesting topic for research, as it has proven difficult to identify a single process as the underlying mechanism for the observed deformation behavior. In elucidating the nature of the mechanisms associated with deformation in an amorphous metal, our group employs different modeling techniques to access different length scales. Molecular statics simulation studies have focused on the pressure dependence of the deformation mechanism through molecular statics simulations. Recent saddle-point search methods have been employed to investigate the influence of thermal activation on the mechanisms that account for plastic flow of a glass. In an attempt to investigate larger time and length scales we have also developed mesoscale simulations, which approximate atomistic behavior by utilizing the ‘shear transformation zone’ as the primary carrier of plasticity. These mesoscale studies are aimed at understanding the link between local deformation events and the shear banding and localization that is observed at a macroscopic scale. Using these various techniques, we aim to provide information about deformation in a metallic glass across a wide range of time and length scales. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19947 Membrane fouling, which can simplistically be described as the clogging of a membrane due to the adsorption of feed components, is one of the major obstacles faced by the membrane industry. It drives up energy consumption as well as cleaning and membrane replacement, and is especially severe in processes where the feed has high concentrations of biomolecules, such as in wastewater treatment, and in food and pharmaceutical industries. The most common way to prevent fouling is to graft a hydrophilic polymer from the membrane surface, but this is often an expensive and poorly controlled process. My project aims to develop improved membranes that resist fouling making use of the self-organization of copolymers, specifically comb copolymers with a hydrophobic backbone and hydrophilic side-chains. Polyacrylonitrile-graft-poly(ethylene oxide) (PAN-g-PEO) is such a polymer: It has a backbone of PAN, a glassy polymer used in membrane industry, and side-chains of PEO, a hydrophilic polymer well known for its resistance to adsorption. When this copolymer is added to the casting solution during the manufacture of porous ultrafiltration (UF) membranes, the side-chains are driven to the polymer/water interface due to their hydrophilic nature. The polymer is pinned down by the PAN backbone, creating a brush of PEO chains on the membrane surface as well as lining all the pores. Membranes prepared using this method were found to resist irreversible fouling completely to a range of foulants, including protein, humic acid and alginate solutions, and oil-well produced water: The membrane can be cleaned simply by water, potentially decreasing cleaning costs and increasing membrane life. These membranes are very promising for waste-water treatment and membrane bioreactors. Another application, this time used to prepare membranes with selectivity in the small molecule scale, relies on the microphase separation of PAN-g-PEO. In this process, a porous support membrane is coated with a thin (0.2-2 micron) film of PAN-g-PEO. The copolymer microphase separates into a bicontinuous network of each phase. The PEO phase, which is hydrophilic, allows the passage of water and molecules smaller than its diameter, acting as PEO-lined "nanochannels". The membranes produced have size-based selectivity in the nanofiltration scale, and can be used to fractionate small-molecule dyes by size. They also have a high pure water permeability, and complete resistance to irreversible fouling. Furthermore, the size cut-off of these membranes is responsive to a range of factors that affect the conformation of PEO chains, such as temperature, pressure, and ionic strength. Amphiphilic comb copolymers impart fouling resistance to polymer membranes, an increasingly important part of the world's water supply sustainability. Research is underway to expand the application of the morphology and properties of these combs to create membranes with regenerative fouling-resistant surfaces, nanosieving membranes, and improved desalination membranes. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19946 Some colors in nature do not come from material's inherent properties, but are as a result of light interference. This kind of color, called 'structural color,' can be seen in some species such as butterflies and beetles. We are investigating the methods and conditions for creating structural color using layer-by-layer assembly of various nanoparticles. By building alternating layers of nanoparticles with low and high refractive indices, it is possible to obtain high reflectance with any color. With optical simulation programs, we are able to design nanoparticle assembly and obtain any color with more than 90% reflectance. The ultimate goal is to control the angle-dependence of the reflectance and to create omni-directional structural color by a layer-by-layer assembly process. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19945 We are investigating the use of polymer coatings formed using layer by layer assembly to create coatings capable of preventing the growth of a biofilm. We are researching the effects of key surface properties, such as mechanical stiffness and surface charge, on bacteria attachment and cytotoxicity. Using the layer by layer approach and tuning synthesis and post-assembly conditions, chemically identical films can be created with different elastic moduli and chemical functional group densities. These films are then used to isolate the effects of various properties. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19944 The ultimate goal of my thesis is to design and synthesize mechanomutable nanotubes that exhibit reversible and tunable mechanical responses to different types of external stimuli via layer-by-layer assembly. The design of such highly refined heteronanomaterials, by the incorporation of constituents from a wide range of materials as the fundamental units, provides versatility and variability in mechanical properties. Mechanomutable heteronanomaterials can be useful for the development of multi-responsive tunable sensor arrays, synthetic extracellular matrix, and dynamic armor coatings. The layer-by layer assembly technique provides a versatile and inexpensive approach to the design and synthesis of mechanomutable heteronanomaterials. The sequential adsorption of oppositely-charged species enables the precise design and control over the molecular architectures of the film, which can be manipulated for different functionalities. The synthesis of hollow, cylindrical nanotubes using a porous-templated layer-by-layer approach is of particular interest arising from their interesting dimensions. In contrast to previously reported systems, the synthesis of mechanomutable nanotubes via layer-by-layer assembly can be designed in many different ways that result in materials that exhibit reversible and tunable mechanical responses to different types of external stimuli. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19943 Diffraction based pulse shaping, developed by the Nelson group, can produce multiple and arbitrarily shaped coherent femtosecond pulses, with excellent phase stability and high temporal resolution. The spatial and temporal control afforded by this method provides a unique platform for staging phase-matched degenerate four-wave mixing experiments, coherent control experiments, and two-dimensional Fourier Transform spectroscopy experiments at optical frequencies. We are currently utilizing this novel pulse shaping scheme for two-dimensional FT optical spectroscopy experiments of semiconductor quantum wells and quantum dots. The excellent phase stability offered allows us to probe the coupling and dynamics of multiple electronic states, including those of exciton and biexciton states in quantum well structures. Moreover, the nonlinear signal of interest is detected in the rotating frame, analogous to 2D FT NMR methods developed for radiofrequency pulses. Other topics we are interested in investigating in the future include interactions among coupled electronic states in J-aggregates and photosynthetic systems, and higher order, i.e. chi(5) and chi(7), experiments, both resonant and non-resonant. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19942 Many structural and chemical dynamics in the solid state occur irreversibly and with a build-up of reaction products. Irreversible or long-lived solid-state dynamics cannot be studied using conventional ultrafast spectroscopy, in which repeated measurements are made on a sample that returns to its initial state after each laser shot. We have developed a novel method for real-time measurement of ultrafast dynamical events in a single laser shot. An excitation laser pulse is followed by 400 probe pulses, which arrive at the sample at different times and at slightly different angles. Figure 1 below illustrates the generation of a time-structured probe pulse. The crossed echelons transform the input pulse into a 2D array of spatially and temporally resolved pulses. Each probe pulse, after transmission through or reflection off the sample, arrives at a different region of a CCD camera. Thus a single laser shot yields a complete time-dependent record of the sample response. We employed single-shot femtosecond spectroscopy to study the effect of the surrounding lattice structure on the photodissociation and recombination of the triiodide ion (I3-). The panels on the left of Figure 3 illustrate the structures of three different organic crystals (tetrabutylammonium triiodide: TBAT, tetraethylammonium triiodide: TEAT, and tetraphenylphosphonium triiodide: TPPT). The different crystal structures provide different circumstances to the reactant. The panels on the right of Figure 3 show the transient absorption spectra of photofragment I2- after photolysis of triiodide in the corresponding crystals. As shown, the dynamics of dissociation and recombination differ dramatically depending on the environment. This example demonstrates the significance of lattice structure on solid state photochemistry as well as the potential of single-shot femtosecond spectroscopy as a tool for the study of irreversible solid state dynamics. For more details, see reference. Recently, we investigated the semiconductors Bi, Sb, Te, and GeTe under highly non-equilibrium conditions. Past studies have shown that large laser-induced changes in carrier occupation in metals and semiconductors can lead to substantial band structure renormalization and even to structural phase transitions. Our experiments are aimed at researching the fundamental limits of the interaction of light with matter and the behavior of matter under extreme carrier densities. By monitoring the coherent phonon response of these materials upon laser excitation, we observe different types of behavior. Bi, Sb, and Te support coherent excitation of fully symmetrical phonon modes at low excitation intensities. At high excitation intensities, all three materials undergo non-thermal melting, as indicated by the absence of the phonon modes which are present in the low-intensity regime. Non-thermal melting is a recently understood effect, which only occurs upon the interaction of semiconductors with femtosecond laser pulses. GeTe has a similar response under low-intensity excitation. However, as the laser excitation increases, the material undergoes a structural phase transition from the rhombohedral to the cubic form. Depending on the laser intensity, the phase transition is reversible or non-reversible. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19941 What is a Phonon Polariton? A phonon polariton is a wave (in many respects similar to an ocean wave) that propagates through a solid. It is a mixture of an oscillating electric field and an optic phonon, that is, oscillating dipoles within the crystal. We generally create phonon-polaritons in ferroelectric crystals, crystals that have a permanent electric dipole caused by an off-center atom in the crystal lattice. It is the oscillation of this charged, off-center atom that is responsible for phonon polariton. We generally use LiNbO3 and LiTaO3 crystals where wave frequencies are in the 0.1-15 THz range. Phonon Polariton Generation and Observation Phonon polaritons are driven and detected using femtosecond laser pump-probe techniques. Two pump beams are crossed at the surface of the crystal, and the wavelength of their interference pattern determines the wavelength of the phonon polariton. Because of the ferroelectric crystal’s large dielectric constant, the wave propagates primarily parallel to the surface of the crystal, perpendicular to the excitation pulse. This facilitates easy separation of phonon-polaritons from the pump light and enables imaging. As the wave propagates through the crystal, it changes the index of refraction, which can be observed using a broad probe beam and imaging onto a CCD camera. The jargon term for this is “Talbot phase-to-amplitude conversion.” The red beam is the pump beam, which is split into two and crossed to make an interference pattern. The expanded blue beam is the probe, which is sensitive to the index change and can be imaged onto the camera. This by itself was an extraordinary breakthrough. It was the first time that electromagnetic waves were imaged directly, appearing much like ripples in a pond. Each probe pulse generates one image on the camera. Varying the delay between pump and probe allows sub-picosecond time resolution, and the creation of “movies”, where each frame in the movie has a different delay between pump and probe. A collection of these can be viewed in the movie section of the website. (See them in our movietheater .) What do we do with them? (*) Waveguides (*) Thin films (*) Phase Transitions (*) Quantum dot spectroscopy (*) And more… One important interest in the group is polaritonics, which encompasses the generation, detection, and control of phonon polaritons. The frequency regime (1011 – 1013 Hz) places these waves between the frequencies of electronics (generation, control and observation of electrons) and photonics (generation, control, and observation of optical photons). Like these other two, polaritonics has potential applications in next-generation computers. Polaritons can be controlled directly in the LiNbO3 and LiTaO3 crystals by using thin-films and waveguides. Custom waveguides are made in-house using femtosecond laser machining. Essentially, a high energy, femtosecond laser pulse is focused on the crystal surface where it vaporizes some of the material. We have precise control over the beam and crystal, and so can cut any desired pattern into the surface. Figure 4 shows one such laser-machined pattern, and its effect on the terahertz wave as it propagates through the crystal. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19940 The objectives of this project are: (1) To study the dynamic material response of polymers and polymer-based nanocomposites to laser-driven shock waves; (2) To understand the performance of protective materials under shock loading conditions; and (3) To measure acoustic/mechanical properties of ISN samples for shock mitigation. The approach for these ojbectives are: (*) Impulsive Stimulated Thermal Scattering (ISTS) measurements provided acoustic & elastic modulus for PMMA-TiO2 multilayer (*) Group velocity optimization for shock mitigation (*) ISTS measurements synchronized with laser shock generation & first ISTS measurements of laser-shocked samples conducted Phononic structure yields frequency ranges with near-zero acoustic group velocity -- could break up shock front in impacted structure. Shock Generation -- The shock is generated by the rapid thermal expansion of an aluminum ablation layer, which is optically excited with a high intensity shaped 300 picosecond laser pulse. The lateral expansion of the aluminum predominately affects the adjacent polymer layer as it is much softer than the rigid quartz substrate. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19939 The Deathstar Project studies the interaction of high-frequency acoustic modes as they propagate through thin glass and liquid films. Optical generation and detection techniques enable us to access frequencies as high as 400 GHz. These acoustic frequencies correspond to wavelengths down to 10 nm and open up unique possibilities for characterization of complex structural correlation lengths, in addition to the correlation times associated with structural relaxation. The current research under this project is taking place on two fronts. The first and foremost goal is to extend the timescale of the experiment into the ps regime, where beta relaxation is measured. This is accomplished by penetrating thin metal films with ultra-short pulses of light, to generate very high frequency acoustic waves. This technique is known as Picosecond Ultrasonics. The laser system used is a regenerative amplifier (Coherent RegA) and oscillator (Coherent Mira) pumped by an 18 Watt Coherent Verdi laser, yielding 200 fs 4 uJ pulses at 250 kHz repetition rate. A single laser pulse will generate a broadband acoustic wave packet with frequency components up to 400 GHz. The frequency dependence of wave packet dispersion and absorption (or the acoustic modulus) give a direct measurement of the distribution of timescales of liquid motion. To improve signal-to-noise and to simplify data analysis, a narrow-band tunable pulse-shaper (known as the Deathstar) has been built, allowing measurements of liquid dynamics on timescales between 1 ns and 1 ps. With this, it should be possible to make the quantitative link between fast and slow dynamics as predicted by mode coupling theory. The second goal of current research in this lab is to understand the details of acoustic propagation at such high frequencies. Not only are there fast dynamics in a glass or liquid, but there are also structural inhomogeneities which can Rayleigh-scatter phonons and complicate analysis of the motion. The separation of the static and dynamic parts of signal contribution is crucial to fully understanding glasses. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19938 In recent years there has been great progress in the exploration of the electromagnetic spectrum between 0.5 and 3 THz. This region gives access to many interesting physical properties of semiconductors, molecular crystals, ferroelectrics and biological objects. The spectroscopic methods are based on the generation and detection of single-cycle THz pulses generated using femtosecond lasers. Nonlinear THz-Spectroscopy requires high bandwidth and high luminosity. May be used to study: (*) Lattice vibrational anharmonicity (*) Coherent control of crystal lattice dynamics (*) Study low frequency motions in liquids and disordered systems 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19965 The selection of where to place patient recruitment sites of clinical trials is a critical strategic decision for developers of drugs and biologics. Such complex decision making is becoming increasingly important as companies move towards the global concomitant registration model, in which the information generated in a global clinical development program allows simultaneous access to the new medication in multiple markets across the globe. The allocation of global clinical trials depends on numerous factors, including: characteristics of the patient population; clinical research infrastructure; costs; market potential of individual countries; patient recruitment speed and compliance; local regulatory requirements; business environment; local standards of care; and engagement of key opinion leaders. The relative importance of each of these factors is expected to vary according to the condition being studied. For some diseases, access to patients may be the most important factor, while for other conditions that require more complex trial design and/or long term follow-up, clinical research infrastructure may be the critical factor. Approach to this research: (*) Characterize the allocation decisions made by planners of global trials on serious and/or life-threatening conditions (*) Identify the critical drivers of allocation decisions for each condition through focused interviews with planners of pivotal global trials and analysis of business intelligence related to clinical research (*) Characterize the competitiveness of individual countries in attracting trials on each condition, with comparative analysis of major traditional and emerging nations. 06/22/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19933 The goal of this program is to leverage the low jitter properties of mode-locked lasers and the high resolution available at optical frequencies to achieve the long-term sub-femtosecond/attosecond precision photonic or optoelectronic systems. These developments are most important in the areas of large-scale timing distribution and synchronization of next generation accelerators and light sources and analog-to-digital conversion. Purely electronic systems are currently limited to about 100 fs timing accuracy because of the jitter performance of integrated microwave oscillators used in such systems and intrinsic limitations in time resolution by the speed and stability of electronic components. Timing Jitter in Mode-locked Lasers -- The timing jitter of optical pulse trains from passively mode-locked solid-state lasers has been theoretically predicted to be below a femtosecond in the high frequency range. In each ultrashort optical pulse, a large number of photons are concentrated in an extremely short pulse (~100 fs or below), which makes it robust against perturbations in timing from noise photons. In the last few years, there has been a remarkable progress in high repetition rate and ultralow noise mode-locked lasers, largely driven by the motivation for high-precision time/frequency control and measurement. Recently, our group has demonstrated low-jitter 200-MHz fiber laser and the scaling of its repetition rate to 2 GHz by external cavity as well as a 400-MHz integrated Er-waveguide laser which has a potential for CMOS-compatible integrated on-chip mode-locked lasers. Many high-precision applications are possible with the realization of low-jitter mode-locked laser such as large-scale timing distribution and synchronization systems and photonic analog-to-digital converters. For study and optimization of noise properties of such lasers, it is important to have an ultra-sensitive characterization method for measuring the jitter with attosecond resolution. We recently developed sub-fs resolution timing jitter characterization method based on balanced cross-correlation techniques. The timing jitter measurement of a commercial passively mode-locked Er-fiber laser has shown that the integrated timing jitter of the optical pulse train is indeed less than 1 fs and 5 fs in the 100 kHz – 10 MHz and the 10 kHz – 10 MHz ranges, respectively. Note that, due to the universal physical mechanism of pulse formation and stabilization, this nearly sub-fs noise performance is not restricted to specific lasers but is generally obtainable from any ultrashort pulse (~100 fs or below) passively mode-locked lasers carefully designed to minimize technical noise sources. We are currently working on the development of higher repetition rate, lower jitter and more compact mode-locked lasers and the optimization of noise performances of the lasers by theoretical and experimental study. Long-Term Stable Timing Distribution and Synchronization -- One of the most fascinating aspects of mode-locked lasers is that they can simultaneously provide ultralow-noise optical and microwave signals in the form of optical pulse trains. Owing to this dual property of optical pulse trains, one can build a high-precision timing network that tightly synchronizes multiple microwave and optical sources. As modern large-scale scientific facilities, such as particle accelerators, X-ray free-electron lasers (FELs) and phased-array antennas for radio astronomy, require extremely high timing accuracy between multiple remotely-located microwave and optical sources, mode-locked laser-based techniques have been expected to clock such facilities with an unprecedented timing accuracy of femtosecond (and even sub-femtosecond) level. However, lack of long-term stable synchronization techniques has hindered the realization of this pervasive clocking idea. Since the first proposal of the pervasive synchronization for FELs [5], we have developed and refined the key technologies for large-scale, long-term stable timing distribution and synchronization, i.e., timing distribution with timing-stabilized fiber links [6], optical-to-optical synchronization [6,7], and optical-to-RF synchronization [8,9,10]. We have recently achieved, for the first time, sub-10-femtosecond timing accuracy maintained over more than 10 hours [11]. The demonstrated relative timing stability in timing distribution, optical-optical synchronization, and optical-microwave synchronization is 2.5?10-20, 9?10-21, and 1.9?10-19, respectively, which represents multiple orders of magnitude improvements compared to previous work. The equivalent accuracy of synchronization is keeping the timing with less than a second accumulated error since the birth of the universe. The demonstrated techniques are already transferred and actively employed in real-world scientific facilities such as the next generation FELs in DESY (Hamburg, Germany) and Elettra (Trieste, Italy). We are currently working on improvements of these techniques to attosecond precision for opening up further potentials in high-precision measurement and control capabilities. Photonic Analog-to-Digital Conversion -- Several photonic ADC techniques have been investigated in recent years. The photonic ADC architecture pursued here in the form of an electronic-photonic integrated circuit is known as time-interleaved optical sampling using wavelength-division multiplexing (WDM) techniques. A chirped optical clock signal from a mode-locked laser is channelized in time using precisely-tuned WDM filters to create time-interleaved optical sampling signals, each operating at the rate of the mode-locked laser (here, 2 GHz). The total sampling rate is then the optical clock rate times the number N of WDM channels (here, N=20). However, in order to realize the high resolution, the sampling times of the interleaved channels must be uniform, the converter gains from each channel must be closely matched, and the sample memory effects must be minimal. These characteristics require monitoring and tight feedback control of the WDM filters. A signal recovery algorithm has been developed that enables reconstruction of the actual RF-signal in the presence of small but deterministic errors in filter spacing and unequal converter gain. The ADC chip requires the development of a number of devices: Thermally tunable WDM filter banks with large FSR, wideband optical modulators, Ge- photodetectors, and low jitter femtosecond lasers, potentially also integrated. These devices are pursued in a close collaboration between research groups at MIT Campus and MIT Lincoln Laboratory in various technologies. All these devices and techniques must be integrated on a CMOS compatible technology platform. As an example the proposed ADC chip requires filters with large free spectral range (FSR) and low loss. These two key requirements call for microring filters fabricated in a high-index contrast (HIC) material system. Another type of photonic ADC has been also investigated, i.e., a downconverting ADC for digitizing a narrowband microwave signal with a very high carrier frequency (40 GHz and higher). This ADC leverages an ultralow timing jitter of optical pulse trains for effective optical downconversion of microwave signals. For experimental demonstration, sub-10 fs jitter optical pulse train has been used to downconvert and digitize a microwave signal at 40 GHz carrier frequency. Higher than 7 effective-number-of-bit (ENOB) resolution ADC is achieved at 40 GHz over an effective bandwidth of 2 MHz, which is already limited by the quality of the sampled microwave signal itself. The resolution limited by pulse train timing jitter and shot noise is projected to be higher than 9 ENOB. 06/19/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19932 In optical parametric chirped-pulse amplification (OPCPA) [1] a femtosecond pulse is stretched to picosecond or nanosecond durations for high-energy amplification and then recompressed to the original pulse duration, which enables the generation of high-intensity laser pulses for high-field physics experiments. Parametric amplification has several distinct advantages over conventional laser amplification: ultra broadband phase matching can be achieved with reduced gain narrowing, it is wavelength agile, and it leads to low accumulated nonlinear phase and high contrast ratio. For these reasons, OPCPA has been intensively investigated over last 10 years as an alternative method to chirped-pulse amplification (CPA) based on a conventional laser amplification technique. However, the requirements on pump sources for OPCPA are much more delicate than that of CPA because parametric amplification with high efficiency while maintaining good beam quality is only possible with good spatio-temporal characteristics of the pump beam. The development of a large-average-power picosecond pump source that can be synchronized with seed beams at either high or low repetition rate is therefore one of the most important challenges for future OPCPA technologies and their applications. Besides the OPCPA applications, the large-average-power picosecond lasers can be widely used for driving other nonlinear optical processes such as frequency conversion. Over the last years, high-power high-repetition-rate picosecond laser technologies have been developed both on the basis of fiber and bulk amplifiers. Recently, a cryogenically-cooled Yb:YAG laser has proven to be a good candidate for average power scaling because of its good thermo-optic properties, small quantum defect, and low saturation fluence. At cryogenic temperatures (70 K), Yb:YAG has an emission bandwidth of 1.5 nm, suitable for picosecond pulse amplification. A high-power CW Yb:YAG laser with output powers up to >450 W and a picosecond amplifier at tens of kHz with 24 W of average power have been demonstrated. Using a cryogenically-cooled cw Yb:YAG amplifier developed at MIT Lincoln Laboratory, we demonstrated the amplification of 5.5-ps pulses at a repetition rate of 78 MHz to 287 W of average output power, which is the highest average power picoseconds pulse source at MHz repetition rates ever demonstrated. Figure 1(a) illustrates the optical layout of the fiber laser/amplifier chain to seed the 287-W cryogenically-cooled Yb:YAG amplifier. The combination of a femtosecond fiber laser oscillator and a narrowband amplification system as an OPCPA pump source provides a self-synchronization capability to a given OPCPA system. The chirped volume Bragg gratings (CVBGs) enable narrowband chirped-pulse amplification in a high-power fiber amplifier without any spectral broadening via pulse stretching and re-compression. The compressed 6-W, 4.5-ps pulses out of the fiber CPA chain is boosted up to 287 W at the cryogenically-cooled Yb:YAG amplifier. 06/19/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19930 As recognized by the 2005 Nobel Prize in Physics, frequency combs based on femtosecond lasers are immensely powerful tools for a wide range of applications. The precise spectral and temporal qualities of stabilized frequency combs have applications in optical clocks based on atomic references, femtosecond stability timing distribution systems, tests of quantum mechanics and fundamental physical constants astrophysical spectrograph calibration, high harmonic generation or soft x-ray generation, and atomic and molecular spectroscopy to name only a few. Frequency combs are fundamentally based on mode locked laser technology, which has been available since the 1960’s. The fundamental shift was the development first of microstructure fibers that allowed tremendous coherent broadening of the output spectrum from mode locked lasers. However the broader range of applications has come with the maturing of mode locked Ti:Sapphire lasers and chirped mirror technology for dispersion control, as well as impressive advances in Er:fiber laser technology. Currently, frequency combs directly obtained from femtosecond lasers cover a wavelength range from 0.6 µm to 2 µm. Other wavelength ranges may be achieved via frequency conversion of an available comb, a nonlinear optical process that guarantees the transfer of accuracy and stability of the initial comb. One of the more recently proposed applications for frequency comb technology is the calibration of astronomical spectrographs. Highly precise and highly accurate calibration of astronomical spectrographs is necessary to enable astronomers to use the radial velocity method to search for planets outside our solar system and to unravel other cosmological mysteries. In this method, astronomers monitor the light emitted from stars to observe a slight periodic shift in the emitted spectrum caused by the motion of the star induced by an orbiting planet. In this application, both the accuracy and precision of the frequency comb will be utilized to enable searches for earth like planets and solar systems. 06/19/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19929 Ultrashort pulse lasers are an enabling technology for a whole range of fundamental and applied research areas as well as industrial applications. The pulse width typically ranges from picoseconds down to few femtoseconds in duration. To generate such short pulses typically multiple longitudinal modes in a laser resonator are excited, such that a pulse much shorter than the cavity roundtrip time is created inside the laser resonator. The shortest pulse that can build up is then limited by the gain bandwidth of the amplifying medium. Figure 1 shows the progress in generating short optical pulses from various laser materials. Over the last decades, our group has made major contributions to both the understanding of the pulse generation mechanisms involved in the various types of lasers and the emerging technologies leading to shorter and shorter pulse durations. The progress of pulse shortening culminated in the generation of the shortest pulses directly from a laser oscillator approaching a single optical cycle at 800 nm from Kerr-lens mode-locked Ti:sapphire lasers, developed in our group. The laser is made up of an optically-pumped laser crystal (Ti:sapphire), an additional material (BaF2) for precise dispersion-compensation in each arm of the laser and broadband mirrors, which compensate for the dispersion in the laser crystal. Intra cavity optical components and the air in the laser resonator. The broadband mirrors are designed in pairs (green and blue), called double-chirped mirror pairs. They have a custom designed group delay over one octave of bandwidth as well as a high reflectivity over the same bandwidth and one of them has a pump transmission window to transmit the pump light into the cavity. We show a up-converted spectral interferogram obtained using the two-dimensional spectral shearing (2DSI) pulse characterization technique developed in the group. We can verifiy the effectiveness of the 2DSI technique by comparing the computed interferometric autocorrelation (IAC) from the reconstructed pulse with the directly measured IAC. Laser systems similar to the one discussed here are widely used in many of research projects in our group, such as femtosecond laser frequency combs, optical clocks and attosecond science. 06/19/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19928 Nanometer-scale fluidic structures have gained considerable attention in the last few years, because they provide unique capability in biomolecular manipulation and control. Nanofluidic structures with a critical size smaller than 100 nm can put a physical constraint on the biomolecules in solution, therefore controlling the molecules in a unique and useful way. For nanofluidic applications, one critical issue is the availability of reliable, reproducible fabrication strategies for nanometer-sized structures. In this research, we conduct the full experimental characterization of planar nanochannel fabrication using standard photolithography and wafer bonding techniques (anodic bonding and glass-glass fusion bonding), without resorting to the complex, expensive nanolithography and/or thin film deposition techniques. We have demonstrated that nanofluidic channels, as thin as 20 nm with low aspect ratio of 0.004 (depth to width) on silicon substrate and 25 nm with aspect ratio of 0.0005 on glass substrate, can be achieved with anodic bonding and developed glass-glass bonding technique, respectively, as shown in Figure 1. The thickness uniformity of sealed nanofluidic channels are confirmed by the cross-sectional SEM analysis after bonding. Such a uniform, flat nanofluidic channel can be used for more careful, controlled study of molecular and fluidic transport in nanopores or confined space. This result will be useful in designing next-generation nanofluidic devices that can be used for protein separation and biomolecule preconcentration. 06/19/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19927 With increasing demands from researchers on discovering novel drug targets and early disease markers, people found current proteomics technologies fall short on dealing with protein complexities and abundance variation. Since human proteome has more than 10,000 different proteins, with high abundant proteins having 109 higher concentrations than low abundant ones, identifying low abundant proteins (biomarkers) in complex mixtures is one of the major challenges in proteomics. As a result, before one can identify any target proteins, at least one separation step must be performed. This project focuses on studying and applying the physiochemical nanofluidic channels (~40 nm). We have developed a nanofluidic preconcentrator that can concentrate biomolecular samples up to 10 million fold. Due to the electrical double layer overlapping, sub 100 nm nanochannels have preferential transfer over counterions (or counterion current). As a result, a well known phenomenon called concentration polarization can be observed. However, once a higher bias is applied, the system will be driven into the over-limiting current regime, where the charge neutrality in the bulk no longer exists and the extended space charge layer (SCL) is formed. The detail mechanism is not well understood so far. However, by coupling a tangential field across the SCL, we can have a fast accumulation of charged molecules in front of it. In short, this device collects charged biomolecules based on two features: (i) the energy barrier for charged biomolecules generated by the induced space charge layer near the nanofluidic filter; (ii) a faster nonlinear electroosmotic flow for sample deliveries. Currently, we are able to achieve more than a million fold enhancement factor in 30 mins. The preconcentration factors and collection speed are close to those of the PCR for nucleic acids, which is an essential step for many genomics researches. Applications included biomolecular preconcentration and fluid pumping using electroosmotic flow (EOF) of the 2nd kind. 06/19/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19926 Ion concentration polarization is the fundamental transport phenomenon that occurs near ion-selective membranes, but this important membrane phenomenon has been poorly understood due to theoretical and experimental challenges. Here, we report the first direct measurements of detailed flow and electric potential profiles within and near the depletion region. This work is an important step towards a full characterization of this coupled transport problem. Using microfabricated electrodes integrated with the microfluidic device, we measured and confirmed that the electric field inside an ion depletion region is amplified more than 30 fold compared to outside of the depletion zone due to the highly non-uniform ion concentration distribution along the microchannel. As a result, the electrokinetic motion of both fluid (electroosmosis) and particle (electrophoresis) was significantly amplified. The detailed flow profile within the depletion zone was also measured for the first time by optically tracking photobleached neutral dye molecules. We further showed that the amplified electrokinetic flows generated in this device may be used as a field-controlled, microfluidic fluid pump and switch. We have measured both the electric field and flow profile within the strongly depleted ICP regions near perm-selective nanojunctions and determined the exact flow mechanism in this coupled electrokinetic flow system. This study has several important implications in understanding the amplified electrokinetic response due to ICP. Once ion depletion is triggered, the electric field distribution in the system becomes highly non-uniform, generating extremely high electric fields within the ion depletion zone. As a result, electrokinetic responses are significantly amplified within that region and significantly affect the overall motion of both fluids and particles. Based on current results, efficient concentration of peptides and proteins previously reported may be explained by AEK flow motions. Fast convection within the microchannel seems to prevent any significant development of space charge layer, as demonstrated by our concentration estimation within the depletion zone (via field measurement). However, the role of nonlinear electroosmotic slip, which is expected at the nanojunction interface, demands further investigation, possibly with higher spatial resolution in flow measurements. The AEK mechanism presented here is also an attractive candidate for microfluidic flow pumping and switching, as it can generate much higher flow rate at lower driving potentials than needed for equilibrium EOF and can be independent of the fluid’s ion concentration. These systems can potentially replace pneumatic pumping actuation with field-driven, high throughput microfludic pumping and switching, with wide applicability to the field of microfluidics. 06/19/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19925 Natural cellular materials such as (a) cork (b) balsa wood (c) sponge (d) trabecular bone (e) coral (f) cuttlefishh bone (g) iris leaf and (h) stalk of a plant are observed. 06/19/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19924 Cell Contraction -- A fibroblast on a strut in a collagen-GAG scaffold. Left: Cell at time 2 minutes. Right: Cell at 42 minutes, showing buckling of the strut that the cell is attached to. Knowing the Young’s modulus of the collagen-GAG strut, the contractile force of a single cell can be calculated. Cell Migration -- Confocal microscopy image of fibroblasts (stained green on a collagen-GAG scaffold (stained red). Right: Spheres indicate the centroids of the cells while the coloured lines track the position of the centroid as a function of time. This data is used to obtain the cell migration speed. This work was done in collaboration with Professor Lauffenburger’s group at MIT. 06/19/09 http://ilp-www.mit.edu/display_project.a4d?projectId=19923 Collagen-GAG scaffolds -- These collagen-GAG scaffolds were made using a freeze-drying technique developed in Prof. Yannas’ lab and modified in collaboration with our group. Scaffolds like this are used for regenerating skin in burn patients. Our group has worked in collaboration with Prof. Yannas’ group, as well as Prof. Bonfield’s group at Cambridge University, on modifying the scaffolds for use in regenerating cartilage. Mineralized collagen-GAG scaffolds -- The mineralized collagen-GAG scaffolds shown above were made using a freeze-drying technique developed in collaboration with Prof. Yannas’ group, as well as Prof. Bonfield’s group at Cambridge University. These scaffolds are designed for bone regeneration. Osteochondral Scaffold -- A micro-computed tomography image of an osteochondral scaffold, showing the upper collagen-GAG compartment as well as the lower collagen-GAG-calcium phosphate compartment. The scaffold is designed to regenerate cartilage as well as the underlying bone. This work was done in collaboration with Prof. Yannas’ group, as well as Prof. Bonfield’s group at Cambridge University. 06/19/09