SiGe electro-absorption modulators for applications at 1550nm

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008.Includes bibliographical references (p. [76]-80).A novel SixGe₁-x, electro-absorption modulator design is experimentally demonstrated. The device is waveguide integrated, butt-coupled into high index contrast Si/SiO2 waveguides. 0.75% Silicon concentration in the alloy is optimized for 1550nm applications. With its 400nm height, 600nm width, and 50,pm length, the device has a footprint smaller than 30[mu]m². Low effective driving voltage, <2.5V, is needed to achieve an extinction ratio of 5.2dB in the broad 1510-1555nm wavelength operation range. At 1550nm, an extinction ratio of 6.5dB is achieved with an applied effective bias of -2.5V. High frequency measurements determine the device can reach a 3dB frequency of 1.2GHz. Electrical characterization of the device shows high series resistance (~15k[omega]) which is caused by fabrication over-etching during metal contact deposition. Series resistance reduction to ~100[omega] would allow the device to reach the predicted 3dB frequency of 100GHz with 10dB extinction ratio. A pseudo-linear relation is found between the achieved extinction ratio and the applied effective bias. The ratio between these two quantities, the modulation efficiency, can be considered as a new figure of merit of the device. The slope of this pseudo-linear relation measures 2.2dB/V for extinction ratio values ranging between 0 and 5.5dB. In terms of modulation depth it is equivalent to a slope of 40%/V in the range 0.5V-2V. Finally, an ultra-low power consumption per bit of 34fJ/bit is measured for a capacitance of 11fF and an effective applied reverse bias of 2.5V.by Sarah Bernardis.S.M

Modeling the Flow and Diffusion of Lidocaine Through Tooth and Gum

The movement of anesthesia around and through the tooth and gum was modeled in order to design a novel procedure to effectively anesthetize a single tooth with minimal side effects. This model includes the injection of lidocaine into the gum near the tooth, and the ensuing diffusion of anesthesia through the tooth and gum. The geometry of the tooth and gum has been drawn in two dimensions in COMSOL. The Brinkman’s equations were used to model the fluid flow resulting from the initial injection through the porous media, and the mass transfer equation was used to model how the anesthesia flows through the tooth and gum after the completion of the injection through the coupling of the velocities calculated by the Brinkman’s equations. The concentration of anesthesia in this region was calculated, in order to determine the amount of time the tooth is numb and the distance the anesthesia has travelled. The model showed that the optimum procedure for numbing the tooth has a long injection time of five minutes dispensing lidocaine at a slower velocity, specifically 0.018 mol/m3. This is an important process to model because with the longer injection time the patient’s discomfort can be decreased, numbness can be quickly achieved and anesthesia can last for the entirety of an average dental procedure protecting the patient from unnecessary pain

In-polar InN grown by plasma-assisted molecular beam epitaxy

We study the effect of different deposition conditions on the properties of In-polar InN grown by plasma-assisted molecular beam epitaxy. GaN buffer layers grown in the Ga-droplet regime prior to the InN deposition significantly improved the surface morphology of InN films grown with excess In flux. Using this approach, In-polar InN films have been realized with room temperature electron mobilities as high as 2250 cm(2)/V s. We correlate electron concentrations in our InN films with the unintentionally incorporated impurities, oxygen and hydrogen. A surface electron accumulation layer of 5.11x10(13) cm(-2) is measured for In-polar InN. Analysis of optical absorption data provides a band gap energy of similar to 0.65 eV for the thickest InN films. (c) 2006 American Institute of Physics

CMBPol Mission Concept Study: Probing Inflation with CMB Polarization

We summarize the utility of precise cosmic microwave background (CMB) polarization measurements as probes of the physics of inflation. We focus on the prospects for using CMB measurements to differentiate various inflationary mechanisms. In particular, a detection of primordial B-mode polarization would demonstrate that inflation occurred at a very high energy scale, and that the inflaton traversed a super-Planckian distance in field space. We explain how such a detection or constraint would illuminate aspects of physics at the Planck scale. Moreover, CMB measurements can constrain the scale-dependence and non-Gaussianity of the primordial fluctuations and limit the possibility of a significant isocurvature contribution. Each such limit provides crucial information on the underlying inflationary dynamics. Finally, we quantify these considerations by presenting forecasts for the sensitivities of a future satellite experiment to the inflationary parameters.Comment: 107 pages, 14 figures, 17 tables; Inflation Working Group contribution to the CMBPol Mission Concept Study; v2: typos fixed and references adde

Observing the Evolution of the Universe

How did the universe evolve? The fine angular scale (l>1000) temperature and polarization anisotropies in the CMB are a Rosetta stone for understanding the evolution of the universe. Through detailed measurements one may address everything from the physics of the birth of the universe to the history of star formation and the process by which galaxies formed. One may in addition track the evolution of the dark energy and discover the net neutrino mass. We are at the dawn of a new era in which hundreds of square degrees of sky can be mapped with arcminute resolution and sensitivities measured in microKelvin. Acquiring these data requires the use of special purpose telescopes such as the Atacama Cosmology Telescope (ACT), located in Chile, and the South Pole Telescope (SPT). These new telescopes are outfitted with a new generation of custom mm-wave kilo-pixel arrays. Additional instruments are in the planning stages.Comment: Science White Paper submitted to the US Astro2010 Decadal Survey. Full list of 177 author available at http://cmbpol.uchicago.ed

CMB-S4 Science Book, First Edition

This book lays out the scientific goals to be addressed by the next-generation ground-based cosmic microwave background experiment, CMB-S4, envisioned to consist of dedicated telescopes at the South Pole, the high Chilean Atacama plateau and possibly a northern hemisphere site, all equipped with new superconducting cameras. CMB-S4 will dramatically advance cosmological studies by crossing critical thresholds in the search for the B-mode polarization signature of primordial gravitational waves, in the determination of the number and masses of the neutrinos, in the search for evidence of new light relics, in constraining the nature of dark energy, and in testing general relativity on large scales

Oncogenic enhancers prime quiescent metastatic cells to escape NK immune surveillance by eliciting transcriptional memory

Metastasis arises from disseminated tumour cells (DTCs) that are characterized by intrinsic phenotypic plasticity and the capability of seeding to secondary organs. DTCs can remain latent for years before giving rise to symptomatic overt metastasis. In this context, DTCs fluctuate between a quiescent and proliferative state in response to systemic and microenvironmental signals including immune-mediated surveillance. Despite its relevance, how intrinsic mechanisms sustain DTCs plasticity has not been addressed. By interrogating the epigenetic state of metastatic cells, we find that tumour progression is coupled with the activation of oncogenic enhancers that are organized in variable interconnected chromatin domains. This spatial chromatin context leads to the activation of a robust transcriptional response upon repeated exposure to retinoic acid (RA). We show that this adaptive mechanism sustains the quiescence of DTCs through the activation of the master regulator SOX9. Finally, we determine that RA-stimulated transcriptional memory increases the fitness of metastatic cells by supporting the escape of quiescent DTCs from NK-mediated immune surveillance. Overall, these findings highlight the contribution of oncogenic enhancers in establishing transcriptional memories as an adaptive mechanism to reinforce cancer dormancy and immune escape, thus amenable for therapeutic intervention

Cosmological parameters from CMB and other data: a Monte-Carlo approach

We present a fast Markov Chain Monte-Carlo exploration of cosmological parameter space. We perform a joint analysis of results from recent CMB experiments and provide parameter constraints, including sigma_8, from the CMB independent of other data. We next combine data from the CMB, HST Key Project, 2dF galaxy redshift survey, supernovae Ia and big-bang nucleosynthesis. The Monte Carlo method allows the rapid investigation of a large number of parameters, and we present results from 6 and 9 parameter analyses of flat models, and an 11 parameter analysis of non-flat models. Our results include constraints on the neutrino mass (m_nu < 0.3eV), equation of state of the dark energy, and the tensor amplitude, as well as demonstrating the effect of additional parameters on the base parameter constraints. In a series of appendices we describe the many uses of importance sampling, including computing results from new data and accuracy correction of results generated from an approximate method. We also discuss the different ways of converting parameter samples to parameter constraints, the effect of the prior, assess the goodness of fit and consistency, and describe the use of analytic marginalization over normalization parameters.Comment: Quintessence results now include perturbations. Changes to match version accepted by PRD. MCMC code and data are available at http://cosmologist.info/cosmomc/ along with a B&W printer-friendly version of the pape