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Publications

    J. Choy, J. D.B. Bradley, P. Deotare, I. B. Burgess, C. C. Evans, E. Mazur, and M. Loncar. 2012. “Integrated TiO2 resonators for visible photonics.” Opt. Lett., 37, Pp. 539–541. Publisher's VersionAbstract
    We demonstrate waveguide-coupled titanium dioxide (TiO2) racetrack resonators with loaded quality factors of 2.2 × 104 for the visible wavelengths. The structures were fabricated in sputtered TiO2 thin films on oxidized silicon substrates using standard top-down nanofabrication techniques, and passively probed in transmission measure- ments using a tunable red laser.
    J. D.B. Bradley, C. C. Evans, J. Choy, O. Reshef, P. Deotare, F. Parsy, K. Phillips, M. Loncar, and E. Mazur. 2012. “Submicrometer-wide amorphous and polycrystalline anatase TiO2 waveguides for microphotonic devices.” Optics Express, 20, Pp. 23821–23831. Publisher's VersionAbstract
    We demonstrate amorphous and polycrystalline anatase TiO2 thin films and submicrometer-wide waveguides with promising optical properties for microphotonic devices. We deposit both amorphous and polycrystalline anatase TiO2 using reactive sputtering and define waveguides using electron-beam lithography and reactive ion etching. For the amorphous TiO2, we obtain propagation losses of 0.12 dB/mm at 633 nm and 0.04 dB/mm at 1550 nm in thin films and 3 dB/mm at 633 nm and 0.4 ± 0.2 dB/mm at 1550 nm in waveguides. Using single-mode amorphous TiO2 waveguides, we characterize microphotonic features including microbends and optical couplers. We show transmission of 780-nm light through microbends having radii down to 2 μm and variable signal splitting in microphotonic couplers with coupling lengths of 10 μm.
    C. C. Evans, J. D.B. Bradley, J. Choy, O. Reshef, P. Deotare, M. Loncar, and E. Mazur. 2012. “Submicrometer-width TiO2 waveguides.” In . CLEO: Science and Innovations Waveguides and Passive Components (CM3M). Publisher's VersionAbstract
    We fabricate submicrometer-width TiO2 strip waveguides and measure optical losses at 633, 780, and 1550 nm. Losses of 30, 13, and 4 dB/cm (respectively) demonstrate that TiO2 is suitable for visible-to-infrared on-chip microphotonic devices.
    J. Schell. 2012. “How to Transform Learning - With Teaching.” In Leaders of Learners, 5: Pp. 3–5. Publisher's VersionAbstract
    The most powerful tool we have at our disposal in our quest to cultivate effective learning is teaching. This article provides an overview of Peer Instruction, an innovative pedagogy known to significantly improve student learning across various subjects and institutional types. (Article correction - author=Julie Schell, EdD).
    P. Tayalia, E. Mazur, and D. J. Mooney. 2011. “Controlled architectural and chemotactic studies of 3D cell migration.” Biomaterials, 32, Pp. 2634–2641. Publisher's VersionAbstract
    Chemotaxis plays a critical role in tissue development and wound repair, and is widely studied using ex vivo model systems in applications such as immunotherapy. However, typical chemotactic models employ 2D systems that are less physiologically relevant or use end-point assays, that reveal little about the stepwise dynamics of the migration process. To overcome these limitations, we developed a new model system using microfabrication techniques, sustained drug delivery approaches, and theoretical modeling of chemotactic agent diffusion. This model system allows us to study the effects of 3D architecture and chemotactic agent gradient on immune cell migration in real time. We find that dendritic cell migration is characterized by a strong interplay between matrix architecture and chemotactic gradients, and migration is also influenced dramatically by the cell activation state. Our results indicate that Lipopolysaccharide-activated dendritic cells studied in a traditional transwell system actually exhibit anomalous migration behavior. Such a 3D ex vivo system lends itself for analyzing cell migratory behavior in response to single or multiple competitive cues and could prove useful in vaccine development.
    B. K. Newman, M. Sher, E. Mazur, and T. Buonassisi. 2011. “Reactivation of sub-bandgap absorption in chalcogen-hyperdoped silicon.” Appl. Phys. Lett., 98, Pp. 251905–. Publisher's VersionAbstract
    Silicon doped with non-equilibrium concentrations of chalcogens using a femtosecond laser exhibits near-unity absorption of sub-bandgap photons to wavelengths of at least 2500 nm. Previous studies have shown that sub-bandgap absorptance decreases with thermal annealing up to 1175 K, and that the absorption deactivation correlates with chalcogen diffusivity. In this work, we show that sub-bandgap absorptance can be reactivated by annealing at temperatures between 1350 K and 1550 K followed by fast cooling (> 50 K/s). Our results suggest that the defects responsible for sub-bandgap absorptance are in equilibrium at high-temperatures in hyperdoped Si:chalcogen systems.
    M. Sher, M. T. Winkler, and E. Mazur. 2011. “Pulsed-laser hyperdoping and surface texturing for photovoltaics.” MRS Bulletin, 36, Pp. 439–445. Publisher's VersionAbstract
    We describe two ways in which pulsed lasers can be used to increase efficiency in photovoltaic devices. First, pulsed-laser hyperdoping can introduce dopants into a semiconductor at non-equilibrium concentrations, which creates an intermediate band within the bandgap of the material and modifies the absorption coefficient. Second, pulsed-laser irradiation can enhance geometric light trapping by increasing surface roughness. Hyperdoping in silicon enables absorption of photons to wavelengths of at least 2.5 μm, while texturing enhances the absorptance to near unity at all absorbing wavelengths. In this paper, we review both effects and comment on outstanding questions and challenges in applying each to increasing the efficiency of photovoltaic devices.
    N. Lasry, S. Rosenfield, H. Dedic, A. Dahan, and O. Reshef. 2011. “The puzzling reliability of the Force Concept Inventory.” Am. J. Phys., 79, Pp. 909–. Publisher's VersionAbstract
    The Force Concept Inventory (FCI) has influenced the development of many research-based pedagogies. However, no data exists on the FCI’s internal consistency or test-retest reliability. The FCI was administered twice to one hundred students during the first week of classes in an electricity and magnetism course with no review of mechanics between test administrations. High Kuder–Richardson reliability coefficient values, which estimate the average correlation of scores obtained on all possible halves of the test, suggest strong internal consistency. However, 31% of the responses changed from test to retest, suggesting weak reliability for individual questions. A chi-square analysis shows that change in responses was neither consistent nor completely random. The puzzling conclusion is that although individual FCI responses are not reliable, the FCI total score is highly reliable.
    M. J. Smith, M. T. Winkler, M. Sher, Y. Lin, E. Mazur, and S. Gradečak. 2011. “The Effects of a Thin Film Dopant Precursor on the Structure and Properties of Femtosecond-laser Irradiated Silicon.” Appl. Phys. A, 105, Pp. 795–800. Publisher's VersionAbstract
    Femtosecond (fs) laser irradiation of a silicon substrate coated with a thin film is a flexible approach to producing metastable alloys with unique properties, including near-unity sub-band gap absorptance extending into the infrared. However, dopant incorporation from a thin film during fs-laser irradiation is not well understood. We study the thin film femtosecond-laser doping process through optical and structural characterization of silicon fs-laser doped using a selenium thin film, and compare the resulting microstructure and dopant distribution to fs-laser doping with sulfur from a gaseous precursor. We show that a thin film dopant precursor significantly changes the laser-material interactions, modifying both the surface structuring and dopant incorporation processes and in turn affecting p-n diode behavior.
    M. J. Smith, Y. Lin, M. Sher, M. T. Winkler, E. Mazur, and S. Gradecak. 2011. “Pressure-induced phase transformations during femtosecond-laser doping of silicon.” J. Appl. Phys., 110, Pp. 053524–. Publisher's VersionAbstract
    Silicon hyperdoped with chalcogens via femtosecond-laser irradiation exhibits unique near-unity sub-bandgap absorptance extending into the infrared region. The intense light-matter interactions that occur during femtosecond-laser doping produce pressure waves sufficient to induce phase transformations in silicon, resulting in the formation of metastable polymorphic phases, but their exact formation mechanism and influence on the doping process are still unknown. We report direct observations of these phases, describe their formation and distribution, and consider their potential impact on sub-bandgap absorptance. Specifically, the transformation from diamond cubic Si-I to pressure-induced polymorphic crystal structures (amorphous Si, Si-XII, and Si-III) during femtosecond-laser irradiation was investigated using scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy. Amorphous Si, Si-XII, and Si-III were found to form in femtosecond-laser doped silicon regardless of the presence of a gaseous or thin-film dopant precursor. The rate of pressure loading and unloading induced by femtosecond-laser irradiation kinetically limits the formation of pressure-induced phases, producing regions of amorphous Si 20 to 200 nm in size and nanocrystals of Si- XII and Si-III. The surface texturing that occurs during femtosecond-laser irradiation produces inhomogeneous pressure distributions across the surface and causes delayed development of high-pressure silicon polymorphs over many laser pulses. Finally, we find that the polymorph phases disappear during annealing more rapidly than the sub-bandgap absorptance decreases, enabling us to decouple these two processes through post-treatment annealing.
    M. T. Winkler, D. Recht, M. Sher, A. J. Said, E. Mazur, and M. J. Aziz. 2011. “Insulator-to-metal transition in sulfur-doped silicon.” Phys. Rev. Lett., 106, Pp. 178701–. Publisher's VersionAbstract
    We observe an insulator-to-metal (I–M) transition in crystalline silicon doped with sulfur to non-equilibrium concentrations using ion implantation followed by pulsed laser melting and rapid resolidification. This I–M transition is due to a dopant known to produce only deep levels at equilibrium concentrations. Temperature-dependent conductivity and Hall effect data measured for temperatures T > 1.7 K both indicate that a transition from insulating to metallic conduction occurs at a peak sulfur concentration between 1.8 and 4.3 × 1020 cm–3. Conduction in insulating samples is consistent with variable range hopping with a Coulomb gap. The capacity for deep states to effect metallic conduction by delocalization is the only known route to bulk intermediate band photovoltaics in silicon.
    E. D. Diebold. 2010. “Plasmon-enhanced nonlinear optics for applications in sensing and biology”. Publisher's VersionAbstract
    In this thesis, we present the results of three experiments that combine techniques from the elds of ultrafast nonlinear optics and plasmonics, with the aim of developing tools for improved surface-enhanced Raman spectroscopy and biological cell transfection. We fi rst describe the use of femtosecond laser pulses to generate large areas of a nanostructured silicon surface which is used as a new type of substrate for surface-enhanced Raman scattering (SERS). We perform spectroscopic characterization of this substrate and nd its Raman cross-section enhancement factor to be on the order of 10^7. This large, spatially-uniform, and reproducible enhancement factor is nearly constant across the near-infrared spectral region. In a second experiment, we develop a technique to spatially isolate the \hot spots" on SERS substrates. This technique leverages the plasmonic near eld enhancement of metallic nanostructures to preferentially expose a commercial photoresist using femtosecond laser pulses. By isolating the hot spots, analyte molecules adsorb only to the regions of largest electromagnetic enhancement. Compared to an unprocessed substrate covered with a sub-monolayer of benzenethiol molecules, a processed substrate shows a 27-fold im- provement in its average Raman cross-section enhancement factor. Finally, we present a proof-of-principle experiment which demonstrates high-throughput ultrafast laser transfection of biological cells using large-area plasmonic substrates. Utilizing the fi eld localization properties of a substrate fabricated using photolithography, wet etching, and template stripping, we demonstrate the introduction of silence RNA (siRNA) molecules into cells with an efficiency of approximately 50% after exposure to femtosecond laser pulses.
    K. Anne Miller, N. Lasry, O. Reshef, and E. Mazur. 2010. “Losing it: The Influence of Losses on Individuals' Normalized Gains.” In . PERC. Publisher's VersionAbstract
    Researchers and practitioners routinely use the normalized gain (Hake, 1998) to evaluate the effectiveness of instruction. Normalized gain (g) has been useful in distinguishing active engagement from traditional instruction. Recently, concerns were raised about normalized gain because it implicitly neglects retention (or, equivalently, "losses"). That is to say, g assumes no right answers become wrong after instruction. We analyze individual standardized gain (G) and loss (L) in data collected at Harvard University during the first five years that Peer Instruction was developed. We find that losses are non-zero, and that losses are larger among students with lower pre-test performances. These preliminary results warrant further research, particularly with different student populations, to establish whether the failure to address loss changes the conclusions drawn from g.
    J. Watkins. 2010. “Examining issues of underrepresented minority students in introductory physics”. Publisher's VersionAbstract
    In this dissertation we examine several issues related to the retention of underrepresented minority students in physics and science. In the first section, we show that in calculus-based introductory physics courses, the gender gap on the FCI is diminished through the use of interactive techniques, but in lower-level introductory courses, the gap persists, similar to reports published at other institutions. We find that under-represented racial minorities perform similar to their peers with comparable academic preparation on conceptual surveys, but their average exam grades and course grades are lower. We also examine student persistence in science majors; finding a significant relationship between pedagogy in an introductory physics course and persistence in science. In the second section, we look at student end-of-semester evaluations and find that female students rate interactive teaching methods a full point lower than their male peers. Looking more deeply at student interview data, we find that female students report more social issues related to the discussions in class and both male and female students cite feeling pressure to obtain the correct answer to clicker questions. Finally, we take a look an often-cited claim for gender differences in STEM participation: cognitive differences explain achievement differences in physics. We examine specifically the role of mental rotations in physics achievement and problem-solving, viewing mental rotations as a tool that students can use on physics problems. We first look at student survey results for lower-level introductory students, finding a low, but significant correlation between performance on a mental rotations test and performance in introductory physics courses. In contrast, we did not find a significant relationship for students in the upper-level introductory course. We also examine student problem-solving interviews to investigate the role of mental rotations on introductory problems.
    E. D. Diebold, P. Peng, and E. Mazur. 2010. “Surface-enhanced Raman scattering hot spot isolation using surface-enhanced multiphoton lithography.” In . Photonics West. Publisher's VersionAbstract
    In this Manuscript, we present the fabrication and spectroscopic characterization of a large-area surfaceenhanced Raman scattering (SERS) substrate, as well as a method for improving femtomole-level trace detection (109 molecules) using this substrate. Using multiphoton-induced exposure of a commercial photoresist, we physically limit the available molecular adsorption sites to only the electromagnetic "hot spots" on the substrate. This process prevents molecules from adsorbing to sites of weak SERS enhancement, while permitting adsorption to sites of extraordinary SERS enhancement. For a randomly adsorbed submonolayer of benzenethiol molecules the average Raman scattering cross-section of the processed sample is 27 times larger than that of an unprocessed SERS substrate.
    P. Zhang and E. Mazur. 2010. “Peer-Instruction—哈佛大学物理课程教学新方法.” 中国大学教学, Pp. 69–71. Publisher's VersionAbstract
    Peer-Instruction 教学方法是哈佛大学著名教授 Eric Mazur 创立,使用专门设计的用于 揭示学 生错误概念和引导学生深入探究的概念测试题(ConcepTests),借助计算机投票系统 (Computerized Voting System),组织大班课堂教学,变传统单一的讲授为基于问题的自主 学习和协作探究,有效地改变了传统课 堂教学手段、教学模式。在大班课堂教学中实现学 生自主学习、合作学习、师生互动、生生互动。本文旨 在介绍这种教学方法,阐明其教育 意义和促进学生学习方面的作用,综述了 PI 教学方法的广泛应用和相 关研究结果。
    T. Shih, M. T. Winkler, T. Voss, and E. Mazur. 2009. “Dielectric function dynamics during femtosecond laser excitation of bulk ZnO.” Appl. Phys. A, 96, Pp. 363–367. Publisher's VersionAbstract
    Using a broadband dual-angle pump-probe reflectometry technique, we obtained the ultrafast dielectric function dynamics of bulk ZnO under femtosecond laser excitation. We determined that multiphoton absorption of the 800-nm femtosecond-laser excitation creates a large population of excited carriers with excess energy. Screening of the Coulomb interaction by the excited free carriers, causes damping of the exciton resonance and renormalization of the bandgap, causing broadband (2.33.5 eV) changes in the dielectric function of ZnO. From the dielectric function, many transient material properties, such as the index of refraction of ZnO under excitation, can be determined to optimize ZnO-based devices.

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