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.
    N. Lasry, J. Watkins, E. Mazur, and A. Ibrahim. 2013. “Response Times to Conceptual Questions.” Am. J. Phys., 81, Pp. 703–706. Publisher's VersionAbstract
    We measured the time taken by students to respond to individual Force Concept Inventory (FCI) questions. We examine response time differences between correct and incorrect answers, both before and after instruction. We also determine the relation between response time and expressed confidence. Our data reveal three results of interest. First, response times are longer for incorrect answers than for correct ones, indicating that distractors are not automatic choices. Second, response times increase after instruction for both correct and incorrect answers, supporting the notion that instruction changes students' approach to conceptual questions. Third, response times are inversely related to students' expressed confidence; the lower their confidence, the longer it takes to respond.
    K. Anne Miller, N. Lasry, K. Chu, and E. Mazur. 2013. “Role of physics lecture demonstrations in conceptual learning.” PRST, 9(2), Pp. –. Publisher's VersionAbstract
    Previous research suggests that students’ prior knowledge can interfere with how they observe and remember lecture demonstrations. We measured students’ prior knowledge in introductory mechanics and electricity and magnetism at two large universities. Students were then asked to predict the outcome of lecture demonstrations. We compare students’ predictions before having seen the demonstration to what they report having seen both right after the demonstration and several weeks later. We report four main findings. First, roughly one out of every five observations of a demonstration is inconsistent with the actual outcome. Second, students who understand the underlying concepts before observing the demonstration are more likely to observe it and remember it correctly. Third, students are roughly 20% (23%) more likely to observe a demonstration correctly if they predict the outcome first, regardless of whether the prediction is correct or not. Last, conceptual learning is contingent on the student making a correct observation. This study represents an initial step towards understanding the disconnect reported between demonstrations and student learning.
    H. Shimizu, G. Obara, M. Terakawa, E. Mazur, and M. Obara. 2013. “Evolution of Femtosecond Laser-Induced Surface Ripples on Lithium Niobate Crystal Surfaces.” Appl. Physics Express, 6, Pp. 112701-1–3. Publisher's VersionAbstract
    We fabricated periodic ripple structures on the surface of a lithium niobate crystal by irradiation with femtosecond laser pulses and observed the evolution of these structures under irradiation with successive laser pulses. After just a few laser pulses we observed nanorod-shaped craters, aligned with each other but randomly distributed over the surface. The nanocraters are caused by nanoablation at defects in the crystal surface. With increasing pulse number, side-lobed nanocraters appear and light scattered from the initial nanorod- shaped craters at the crystal surface interferes with the incident light, causing the formation of periodic structures.
    G. Haberfehlner, M. J. Smith, J. Idrobo, G. Auvert, M. Sher, M. T. Winkler, E. Mazur, N. Gambacorti, S. Gradečak, and P. Bleuet. 2013. “Selenium segregation in femtosecond-laser hyperdoped silicon revealed by electron tomography.” Microscopy and Microanalysis, 19, Pp. 716–725. Publisher's VersionAbstract
    Doping of silicon with chalcogens (S, Se, Te) by femtosecond laser irradiation leads to nearunity optical absorptance in the visible and infrared range and is a promising route towards siliconbased infrared optoelectronics. However, open questions remain about the nature of the infrared absorptance and in particular about the impact of the dopant distribution and possible role of dopant diffusion. Here we use electron tomography using a high-angle annular dark field (HAADF) detector in a scanning transmission electron microscope (STEM) to extract information about the threedimensional distribution of selenium dopants in silicon and correlate these findings with the optical properties of selenium- doped silicon. We quantify the tomography results to extract information about the size distribution and density of selenium precipitates. Our results show correlation between nanoscale distribution of dopants and the observed sub- band gap optical absorptance, and demonstrate the feasibility of HAADF-STEM tomography for the investigation of dopant distribution in highly-doped semiconductors.
    G. Obara, H. Shimizu, T. Enami, E. Mazur, M. Terakawa, and M. Obara. 2013. “Growth of high spatial frequency periodic ripple structures on SiC crystal surfaces irradiated with successive femtosecond laser pulses.” Optics Express, 21, Pp. 26323–26334. Publisher's VersionAbstract
    We present experimentally and theoretically the evolution of high spatial frequency periodic ripples (HSFL) fabricated on SiC crystal surfaces by irradiation with femtosecond laser pulses in a vacuum chamber. At early stages the seed defects are mainly induced by laser pulse irradiation, leading to the reduction in the ablation threshold fluence. By observing the evolution of these surface structures under illumination with successive laser pulses, the nanocraters are made by nanoablation at defects in the SiC surface. The Mie scattering by the nanoablated craters grows the periodic ripples. The number of HSFL is enhanced with increasing pulse number. At the edge of the laser spot the Mie scattering process is still dominant, causing the fabrication of HSFL. On the periphery of the spot SiC substrate remains a semiconductor state because the electron density in the SiC induced by laser irradiation is kept low. The HSFL observed is very deep in the SiC surface by irradiating with many laser pulses. These experimental results are well explained by 3D FDTD (three-dimensional finite-difference time- domain) simulation.
    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.
    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.
    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.
    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.
    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.
    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.
    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.
    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.
    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.
    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.
    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.

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