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Recht D.,Harvard University | Hutchinson D.,Rensselaer Polytechnic Institute | Cruson T.,Rensselaer Polytechnic Institute | DiFranzo A.,Rensselaer Polytechnic Institute | And 5 more authors.
Applied Physics Express | Year: 2012

Photoconductivity in silicon hyperdoped with sulfur and selenium above the insulator-to-metal transition was measured via photoinduced changes in the microwave reflectivity of hyperdoped layers formed on p-type silicon. Despite these materials' strong subgap optical absorption, exposing them to 1310 and 1550nm light results in a change in conductivity per photon 10,000 times smaller than what is observed in untreated silicon exposed to 980nm light. A similar bound applies for 405nm light, which is absorbed entirely in the hyperdoped layer. We use these results to deduce that the photocarrier lifetime in the hyperdoped material is ≤100 ns. © 2012 The Japan Society of Applied Physics. Source


Said A.J.,Harvard University | Recht D.,Harvard University | Sullivan J.T.,Massachusetts Institute of Technology | Warrender J.M.,Benet Laboratories | And 3 more authors.
Applied Physics Letters | Year: 2011

Highly supersaturated solid solutions of selenium or sulfur in silicon were formed by ion implantation followed by nanosecond pulsed laser melting. n +p photodiodes fabricated from these materials exhibit gain (external quantum efficiency >3000%) at 12 V of reverse bias and substantial optoelectronic response to light of wavelengths as long as 1250 nm. The amount of gain and the strength of the extended response both decrease with decreasing magnitude of bias voltage, but >100% external quantum efficiency is observed even at 2 V of reverse bias. The behavior is inconsistent with our expectations for avalanche gain or photoconductive gain. © 2011 American Institute of Physics. Source


Ananthasayanam B.,Clemson University | Joseph P.F.,Clemson University | Joshi D.,Clemson University | Gaylord S.,Clemson University | And 7 more authors.
Journal of Thermal Stresses | Year: 2012

Coupled thermomechanical finite element models were developed in ABAQUS to simulate the precision glass lens molding process, including the stages of heating, soaking, pressing, cooling and release. The aim of the models was the prediction of the deviation of the final lens profile from that of the mold, which was accomplished to within one-half of a micron. The molding glass was modeled as viscoelastic in shear and volume using an n-term, prony series; temperature dependence of the material behavior was taken into account using the assumption of thermal rheological simplicity (TRS); structural relaxation as described by the Tool-Narayanaswamy-Moynihan (TNM)-model was used to account for temperature history dependent expansion and contraction, and the molds were modeled as elastic taking into account both mechanical and thermal strain. In Part I of this two-part series, the computational approach and material definitions are presented. Furthermore, in preparation for the sensitivity analysis presented in Part II, this study includes both a bi-convex lens and a steep meniscus lens, which reveals a fundamental difference in how the deviation evolves for these different lens geometries. This study, therefore, motivates the inclusion of both lens types in the validations and sensitivity analysis of Part II. It is shown that the deviation of the steep meniscus lens is more sensitive to the mechanical behavior of the glass, due to the strain response of the newly formed lens that occurs when the pressing force is removed. © 2012 Copyright Taylor and Francis Group, LLC. Source


Ananthasayanam B.,Clemson University | Joseph P.F.,Clemson University | Joshi D.,Clemson University | Gaylord S.,Clemson University | And 7 more authors.
Journal of Thermal Stresses | Year: 2012

In Part I of this study a coupled thermo-mechanical finite element model for the simulation of the entire precision glass lens molding process was presented. That study addressed the material definitions for the molding glass, L-BAL35, computational convergence, and how the final deviation of the lens shape from the mold shape is achieved for both a bi-convex lens and a steep meniscus lens. In the current study, after validating the computational approach for both lens types, an extensive sensitivity analysis is performed to quantify the importance of several material and process parameters that affect deviation for both lens shapes. Such a computational mechanics approach has the potential to replace the current trial-and-error, iterative process of mold profile design to produce glass optics of required geometry, provided all the input parameters are known to sufficient accuracy. Some of the critical contributors to deviation include structural relaxation of the glass, thermal expansion of the molds, TRS and viscoelastic behavior of the glass and friction between glass and mold. The results indicate, for example, the degree of accuracy to which key material properties should be determined to support such modeling. In addition to providing extensive sensitivity results, this computational model also helps lens molders/machine designers to understand the evolution of lens profile deviation for different lens shapes during the course of the process. © 2012 Copyright Taylor and Francis Group, LLC. Source


Hussain M.A.,Benet Laboratories | Johnson M.A.,Benet Laboratories
AIP Conference Proceedings | Year: 2010

A new method for detecting cracked, thick-walled, cylindrical geometries using statistical pattern recognition is described. A defective cylinder is identified by monitoring the fundamental modes of vibration that arise following an impulse load. A new model was developed to illustrate the effects of a crack on these modes using the dispersion characteristics of tangentially guided and standing waves. These effects are inherently incorporated in a feature that is used by a statistical pattern classification algorithm to identify a defective sample. © 2010 American Institute of Physics. Source

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