Publications

    I. Solano Araujo and E. Mazur. 2013. “Instrução pelos colegas e ensino sob medida: uma proposta para o engajamento dos alunos no processo de ensino-aprendizagem de Física (Peer Instruction and Just-in-Time Teaching: engaging students in physics learning).” Caderno Brasileiro de Ensino de Física, 30(2), Pp. 362–384. Publisher's VersionAbstract
    Melhorar a formação profissional e acadêmica dos indivíduos nos mais diversos níveis, passa por repensar o papel das estratégias formais de ensino. Em termos educacionais, pesquisa após pesquisa tem mostrado os problemas de se investir quase exclusivamente na apresentação oral dos conteúdos como estratégia didática. Seja por falta de infraestrutura para implementar novas soluções, inércia do sistema escolar ou mesmo desconhecimento de alternativas viáveis de mudança, essa estratégia quase milenar ainda hoje é onipresente no ambiente escolar. Em sua face mais visível, o chamado ensino tradicional está fortemente associado com a evasão escolar, a aprendizagem mecânica e a desmotivação para aprender, por parte dos estudantes. Diversas são as recomendações abstratas e gerais de cunho pedagógico feitas aos professores para reverter esse quadro. Contudo, poucas são as alternativas concretas apresentadas, em especial no Ensino de Física em nível médio e nas disciplinas básicas de nível superior. Tendo em vista esse cenário, o presente artigo tem como objetivos divulgar as potencialidades do uso combinado de dois métodos de ensino, focados na aprendizagem significativa de conceitos e procedimentos; e também fornecer conselhos práticos para favorecer a implementação deles em sala de aula.
    J. Schell, B. Lukoff, and E. Mazur. 2013. “Catalyzing Learner Engagement Using Cutting-Edge Classroom Response Systems in Higher Education.” Edited by Charles Wankel. Increasing Student Engagement and Retention Using Classroom Technologies Classroom Response Systems and Mediated Discourse Technologies. Publisher's VersionAbstract
    In this chapter, we introduce a new technology for facilitating and measuring learner engagement. The system creates a learning experience for students based on frequent feedback, which is critical to learning. We open by problematizing traditional approaches to learner engagement that do not maximize the potential of feedback and offer a research-based solution in a new classroom response system (CRS) two of the authors developed at Harvard University – Learning Catalytics. The chapter includes an overview of cognitive science principles linked to student learning and how those principles are tied to Learning Catalytics. We then provide an overview of the limitations of existing CRSs and describe how Learning Catalytics addresses those limitations. Finally, we describe how we used Learning Catalytics to facilitate and measure learner engagement in novel ways, through a pilot implementation in an undergraduate physics classroom at Harvard University. This pilot was guided by two questions: How can we use Learning Catalytics to help students engage with subject matter in ways that will help them learn? And how can we measure student engagement in new ways using the analytics built into the system? The objective of this chapter is to introduce Learning Catalytics as a new instructional tool and respond to these questions.
    C. C. Evans, K. Shtyrkova, J. D.B. Bradley, O. Reshef, E. Ippen, and E. Mazur. 2013. “Spectral broadening in anatase titanium dioxide waveguides at telecommunication and near-visible wavelengths.” Optics Express, 21, Pp. 18582–18591. Publisher's VersionAbstract
    We observe spectral broadening of femtosecond pulses in single-mode anatase- titanium dioxide (TiO2) waveguides at telecommunication and near-visible wavelengths (1565 and 794 nm). By fitting our data to nonlinear pulse propagation simulations, we quantify nonlinear optical parameters around 1565 nm. Our fitting yields a nonlinear refractive index of 0.16 × 10−18 m2/W, no two-photon absorption, and stimulated Raman scattering from the 144 cm−1 Raman line of anatase with a gain coefficient of 6.6 × 10−12 m/W. Additionally, we report on asymmetric spectral broadening around 794 nm. The wide wavelength applicability and negligible two-photon absorption of TiO2 make it a promising material for integrated photonics.
    L. Jiang, C. C. Evans, O. Reshef, and E. Mazur. 2013. “Optimizing anatase-TiO2 deposition for low-loss planar waveguides”. Publisher's VersionAbstract
    Polycrystalline anatase-TiO2 thin film possesses desirable properties for on-chip photonic devices that can be used for optic computing, communication, and sensing. Low-loss anatase-TiO2 thin films are necessary for fabricating high quality optical devices. We studied anatase-TiO2 by reactively sputtering titanium metal in an oxygen environment and annealing. By correlating key deposition parameters, including oxygen flow rate, deposition pressure, RF power, and temperature to film morphology and planar waveguiding losses, we aim to understand the dominant source of propagation losses in TiO2 thin films and achieve higher quality, lower-loss films.
    M. Sher, Y. Lin, M. T. Winkler, E. Mazur, C. Pruner, and A. Asenbaum. 2013. “Mid-infrared absorptance of silicon hyperdoped with chalcogen via fs-laser irradiation.” J. Appl. Phys., 113, Pp. 063520–. Publisher's VersionAbstract
    Silicon hyperdoped with heavy chalcogen atoms via femtosecond- laser irradiation exhibits strong broadband, sub-bandgap light absorption. Understanding the origin of this absorption could enable applications for hyperdoped-silicon based optoelectronic devices. In this work, we measure absorption to wavelengths up to 14 μm using Fourier transform infrared spectroscopy and study sulfur-, selenium- and tellurium- hyperdoped Si before and after annealing. We find that absorption in the samples extends to wavelengths as far as 6 μm. After annealing, the absorption spectrum exhibits features that are consistent with free-carrier absorption. Although the surface morphology influences the shape of the absorption curves, the data permit us to place an upper bound on the position of the chalcogen dopant energy levels.
    B. K. Newman, E. Ertekin, J. Timothy Sullivan, M. T. Winkler, M. A. Marcus, S. Fakra, M. Sher, E. Mazur, J. C. Grossman, and T. Buonassisi. 2013. “Extended X-ray absorption fine structure spectroscopy of selenium-hyperdoped silicon.” J. Appl. Phys., 114, Pp. 133507–133507-8. Publisher's VersionAbstract
    Silicon doped with an atomic percent of chalcogens exhibits strong, uniform sub-bandgap optical absorptance and is of interest for photovoltaic and infrared detector applications. This sub-bandgap absorptance is reduced with subsequent thermal annealing indicative of a diffusion mediated chemical change. However, the precise atomistic origin of absorptance and its deactivation is unclear. Herein, we apply Se K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy to probe the chemical states of selenium dopants in selenium-hyperdoped silicon annealed to varying degrees. We observe a smooth and continuous selenium chemical state change with increased annealing temperature, highly correlated to the decrease in sub-bandgap optical absorptance. In samples exhibiting strong sub-bandgap absorptance, EXAFS analysis reveals that the atoms nearest to the Se atom are Si at distances consistent with length scales in energetically favorable Se substitutional-type point defect complexes as calculated by density functional theory. As the sub- bandgap absorptance increases, EXAFS data indicate an increase in the Se-Si bond distance. In specimens annealed at 1225 K exhibiting minimal sub- bandgap absorptance, fitting of the EXAFS spectra indicates that Se is predominantly in a silicon diselenide (SiSe2) precipitate state. The EXAFS study supports a model of highly optically absorbing point defects that precipitate during annealing into structures with no sub-bandgap absorptance.
    M. T. Winkler, M. Sher, Y. Lin, M. J. Smith, H. Zhang, S. Gradečak, and E. Mazur. 2012. “Studying femtosecond-laser hyperdoping by controlling surface morphology.” Journal of Applied Physics, 111, Pp. 093511–. Publisher's VersionAbstract
    We study the fundamental properties of femtosecond-laser (fs-laser) hyperdoping by developing techniques to control the surface morphology following laser irradiation. By decoupling the formation of surface roughness from the doping process, we study the structural and electronic properties of fs-laser doped silicon. These experiments are a necessary step toward developing predictive models of the doping process. We use a single fs-laser pulse to dope silicon with sulfur, enabling quantitative secondary ion mass spectrometry, transmission electron microscopy, and Hall effect measurements. These measurements indicate that at laser fluences at or above 4 kJ m-2, a single laser pulse yields a sulfur dose > (3 ± 1) x 1013 cm–2 and results in a 45-nm thick amorphous surface layer. Based on these results, we demonstrate a method for hyperdoping large areas of silicon without producing the surface roughness.
    J. Edward Dowd. 2012. “Interpreting Assessments of Student Learning in the Introductory Physics Classroom and Laboratory”. Publisher's VersionAbstract
    Assessment is the primary means of feedback between students and instructors. However, to effectively use assessment, the ability to interpret collected information is essential. We present insights into three unique, important avenues of assessment in the physics classroom and laboratory. First, we examine students’ performance on conceptual surveys. The goal of this research project is to better utilize the information collected by instructors when they administer the Force Concept Inventory (FCI) to students as a pre-test and post-test of their conceptual understanding of Newtonian mechanics. We find that ambiguities in the use of the normalized gain, g, may influence comparisons among individual classes. Therefore, we propose using stratagrams, graphical summaries of the fraction of students who exhibit “Newtonian thinking,” as a clearer, more informative method of both assessing a single class and comparing performance among classes. Next, we examine students’ expressions of confusion when they initially encounter new material. The goal of this research project is to better understand what such confusion actually conveys to instructors about students’ performance and engagement. We investigate the relationship between students’ self-assessment of their confusion over material and their performance, confidence in reasoning, pre-course self-efficacy and several other measurable characteristics of engagement. We find that students’ expressions of confusion are negatively related to initial performance, confidence and self-efficacy, but positively related to final performance when all factors are considered together. Finally, we examine students’ exhibition of scientific reasoning abilities in the instructional laboratory. The goal of this research project is to explore two inquiry- based curricula, each of which proposes a different degree of scaffolding. Students engage in sequences of these laboratory activities during one semester of an introductory physics course. We find that students who participate in the less scaffolded activities exhibit marginally stronger scientific reasoning abilities in distinct exercises throughout the semester, but exhibit no differences in the final, common exercises. Overall, we find that, although students demonstrate some enhanced scientific reasoning skills, they fail to exhibit or retain even some of the most strongly emphasized skills.
    K. Vora, S. Kang, S. Shukla, and E. Mazur. 2012. “Fabrication of disconnected three-dimensional silver nanostructures in a polymer matrix.” Appl. Phys. Lett., 100, Pp. 063120–063120-3. Publisher's VersionAbstract
    We present a simple, one-step technique for direct-writing of a structured nanocomposite material with disconnected silver nanostructures in a polymer matrix. A nonlinear optical interaction between femtosecond laser pulses and a composite material creates silver structures that are embedded inside a polymer with submicrometer resolution (300 nm). We create complex patterns of silver nanostructures in three dimensions. The key to the process is the chemical composition of the sample that provides both a support matrix and controlled growth. The technique presented in this letter may offer a cost-effective approach for the fabrication of bulk optical devices with engineered dispersion. Copyright (2012) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.
    K. Vora, S. Kang, S. Shukla, and E. Mazur. 2012. “Three-dimensional silver nanostructure fabrication through multiphoton photoreduction.” In . Proceedings of the SPIE. Publisher's VersionAbstract
    Metal nanofabrication techniques have become increasingly important for photonic applications with rapid developments in plasmonics, nanophotonics and metamaterials. While two-dimensional (2D) techniques to create high resolution metal patterns are readily available, it is more difficult to fabricate 3D metal structures that are required for new applications in these fields. We present a femtosecond laser technique for 3D direct-writing silver nanostructures embedded inside a polymer. We induce the photoreduction of silver ions through non-linear absorption in a sample doped with a silver salt. Utilizing nonlinear optical interactions between the chemical precursors and femtosecond pulses, we limit silver-ion photoreduction processes to a focused volume smaller than that of the diffraction-limit. The focal volume is scanned rapidly in 3D by means of a computer-controlled translation stage to produce complex patterns. Our technique creates dielectric-supported silver structures, enabling the nanofabrication of silver patterns with disconnected features in 3D. We obtain 300 nm resolution. © 2012 COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
    E. Landis, K. Phillips, E. Mazur, and C. M. Friend. 2012. “Formation of nanostructured TiO2 by femtosecond laser irradiation of titanium in O2.” J. Appl. Phy., 112, Pp. –. Publisher's VersionAbstract
    We use femtosecond laser irradiation of titanium metal to create nanometer scale laser-induced periodic surface structures and study the influence of atmospheric composition on these surface structures. We find that gas composition and pressure affect the chemical composition of the films, but not the surface morphology. We demonstrate that irradiation of titanium in oxygen containing atmospheres forms a highly stable surface layer of nanostructured amorphous titanium dioxide.
    K. Vora, S. Kang, and E. Mazur. 2012. “A Method to Fabricate Disconnected Silver Nanostructures in 3D.” J. Vis. Exp., 69, Pp. e4399–. Publisher's VersionAbstract
    A Method to Fabricate Disconnected Silver Nanostructures in 3D The standard nanofabrication toolkit includes techniques primarily aimed at creating 2D patterns in dielectric media. Creating metal patterns on a submicron scale requires a combination of nanofabrication tools and several material processing steps. For example, steps to create planar metal structures using ultraviolet photolithography and electron-beam lithography can include sample exposure, sample development, metal deposition, and metal liftoff. To create 3D metal structures, the sequence is repeated multiple times. The complexity and difficulty of stacking and aligning multiple layers limits practical implementations of 3D metal structuring using standard nanofabrication tools. Femtosecond-laser direct-writing has emerged as a pre-eminent technique for 3D nanofabrication. Femtosecond lasers are frequently used to create 3D patterns in polymers and glasses. However, 3D metal direct-writing remains a challenge. Here, we describe a method to fabricate silver nanostructures embedded inside a polymer matrix using a femtosecond laser centered at 800 nm. The method enables the fabrication of patterns not feasible using other techniques, such as 3D arrays of disconnected silver voxels.8 Disconnected 3D metal patterns are useful for metamaterials where unit cells are not in contact with each other, such as coupled metal dot or coupled metal rod resonators. Potential applications include negative index metamaterials, invisibility cloaks, and perfect lenses. In femtosecond-laser direct-writing, the laser wavelength is chosen such that photons are not linearly absorbed in the target medium. When the laser pulse duration is compressed to the femtosecond time scale and the radiation is tightly focused inside the target, the extremely high intensity induces nonlinear absorption. Multiple photons are absorbed simultaneously to cause electronic transitions that lead to material modification within the focused region. Using this approach, one can form structures in the bulk of a material rather than on its surface. Most work on 3D direct metal writing has focused on creating self-supported metal structures. The method described here yields submicrometer silver structures that do not need to be self-supported because they are embedded inside a matrix. A doped polymer matrix is prepared using a mixture of silver nitrate (AgNO3), polyvinylpyrrolidone (PVP) and water (H2O). Samples are then patterned by irradiation with an 11-MHz femtosecond laser producing 50-fs pulses. During irradiation, photoreduction of silver ions is induced through nonlinear absorption, creating an aggregate of silver nanoparticles in the focal region. Using this approach we create silver patterns embedded in a doped PVP matrix. Adding 3D translation of the sample extends the patterning to three dimensions.
    R. Olivares-Amaya, D. Rappoport, P. Muñoz, P. Peng, E. Mazur, and A. n. Aspuru-Guzik. 2012. “Can Mixed-Metal Surfaces Provide an Additional Enhancement to SERS?” J. Phys. Chem., 116, Pp. 15568–15575. Publisher's VersionAbstract
    We explore the chemical contribution to surface-enhanced Raman scattering (SERS) in mixed-metal substrates, both experimentally and by computer simulation. These substrates are composed of a chemically active, transition- metal overlayer deposited on an effective SERS substrate. We report improved analytical enhancement factors obtained by using a small surface coverage of palladium or platinum over nanostructured silver substrates. Theoretical predictions of the chemical contribution to the surface enhancement using density functional theory support the experimental results. In addition, these approaches show that the increased enhancement is due not only to an increase in surface coverage of the analyte but also to a higher Raman scattering cross section per molecule. The additional chemical enhancement in mixed-metal SERS substrates correlates with the binding energy of the analyte on the surface and includes both static and dynamical effects. SERS using mixed-metal substrates has the potential to improve sensing for a large group of analyte molecules and to aid the development of chemically specific SERS-based sensors.
    M. J. Smith, M. Sher, B. Franta, Y. Lin, E. Mazur, and S. Gradečak. 2012. “The origins of pressure-induced phase transformations during the surface texturing of silicon using femtosecond laser irradiation.” J. Appl. Phys., 112, Pp. 083518–. Publisher's VersionAbstract
    Surface texturing of silicon using femtosecond (fs) laser irradiation can reduce the surface reflectivity to less than 5%, enables control over the resulting surface morphology, and uses little material. The laser-induced damage that occurs in parallel with surface texturing, however, can result in increased recombination currents that inhibit device performance. In this work, we investigate the light- material interaction during the texturing of silicon by directly correlating the formation of pressure-induced silicon polymorphs, fs-laser irradiation conditions, and the resulting morphology and microstructure using scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy. We identify resolidification-induced stresses as the mechanism responsible for driving sub- surface phase transformations during the surface texturing of silicon, the understanding of which is an important first step towards reducing laser-induced damage during the texturing of silicon with fs-laser irradiation.
    J. Brugués, V. Nuzzo, E. Mazur, and D. Needleman. 2012. “Nucleation and Transport Organize Microtubules in Metaphase Spindles.” Cell, 149(3):554-64, Pp. –. Publisher's VersionAbstract
    Spindles are arrays of microtubules that segregate chromosomes during cell division. It has been difficult to validate models of spindle assembly due to a lack of information on the organization of microtubules in these structures. Here we present a method, based on femtosecond laser ablation, capable of measuring the detailed architecture of spindles. We used this method to study the metaphase spindle in Xenopus laevis egg extracts and find that microtubules are shortest near poles and become progressively longer towards the center of the spindle. These data, in combination with mathematical modeling, imaging, and biochemical perturbations, are sufficient to reject previously proposed mechanisms of spindle assembly. Our results support a model of spindle assembly in which microtubule polymerization dynamics are not spatially regulated, and the proper organization of microtubules in the spindle is determined by non-uniform microtubule nucleation and the local sorting of microtubules by transport.
    C. C. Evans, J. D.B. Bradley, E. Armando Marti, and E. Mazur. 2012. “Mixed two- and three-photon absorption in bulk rutile (TiO2) around 800 nm.” Optics Express, 20, Pp. 3118–3128. Publisher's VersionAbstract
    We observe mixed two- and three-photon absorption in bulk rutile (TiO2) around 800 nm using the open aperture Z-scan technique. We fit the data with an extended model that includes multiphoton absorption, beam quality, and ellipticity. The extracted two- and three-photon absorption coefficients are below 1 mm/GW and 2 mm3/GW2, respectively. We observe negligible two-photon absorption for 813-nm light polarized along the extraordinary axis. We measure the nonlinear index of refraction and obtain two-photon nonlinear figures of merit greater than 1.1 at 774 nm and greater than 12 at 813 nm. Similarly, we obtain three-photon figures of merit that allow operational intensities up to 0.57 GW/mm2. We conclude that rutile is a promising material for all-optical switching applications around 800 nm.
    J. Schell. 2012. “Student-Centered University Learning: Turning Traditional Education Models Upside Down.” In ReVista: Harvard Review of Latin America, Fall: Pp. 20–23. Publisher's VersionAbstract
    Teachers teach. Students learn. This is the dominant paradigm of university education in Latin America. But is this age-old model sufficient to prepare students for tomorrow in a rapidly evolving region clamoring for innovation? More and more, educational reformers are emphasizing that no, it is not. However, student-centered teaching methods, such as Peer Instruction, are gaining popularity in Latin American universities as the region seeks to improve the quality of higher education.
    T. Sarnet, J. E. Carey, and E. Mazur. 2012. “From black silicon to photovoltaic cells, using short pulse lasers.” In . International Symposium on High Power Laser Ablation 2012. Publisher's VersionAbstract
    Laser created Black Silicon has been developed since 1998 at Harvard University. The unique optical and semiconducting properties of the black silicon first lead to interesting applications for sensors (photodetectors, thermal imaging cameras…). Other applications like Photovoltaic solar cells have been rapidly identified, but it took more than ten years of research and development before demonstrating a real improvement of the photovoltaic efficiency on an industrial multi-crystalline solar cell. This paper is a brief review of the use of black silicon for photovoltaic cells.

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