G. England, M. Kolle, P. Kim, M. Khan, P. Muñoz, E. Mazur, and J. Aizenberg. 2014. “Bioinspired micrograting arrays mimicking the reverse color diffraction elements evolved by the butterfly Pierella luna.” PNAS, Pp. –. Publisher's VersionAbstract
    Recently, diffraction elements that reverse the color sequence normally observed in planar diffraction gratings have been found in the wing scales of the butterfly Pierella luna. Here, we describe the creation of an artificial photonic material mimicking this reverse color-order diffraction effect. The bioinspired system consists of ordered arrays of vertically oriented microdiffraction gratings. We present a detailed analysis and modeling of the coupling of diffraction resulting from individual structural components and demonstrate its strong dependence on the orientation of the individual miniature gratings. This photonic material could provide a basis for novel developments in biosensing, anticounterfeiting, and efficient light management in photovoltaic systems and light-emitting diodes.
    J. M. Fraser, A. L. Timan, K. Anne Miller, J. Edward Dowd, L. Tucker, and E. Mazur. 2014. “Teaching and physics education research: bridging the gap.” Reports on Progress in Physics, 77, Pp. 032401–032417. Publisher's VersionAbstract
    Physics faculty, experts in evidence-based research, often rely on anecdotal experience to guide their teaching practices. Adoption of research-based instructional strategies is surprisingly low, despite the large body of physics education research (PER) and strong dissemination effort of PER researchers and innovators. Evidence-based PER has validated specific non-traditional teaching practices, but many faculty raise valuable concerns toward their applicability. We address these concerns and identify future studies required to overcome the gap between research and practice.
    K. Vora. 2014. “Three-dimensional nanofabrication of silver structures in polymer with direct laser writing”. Publisher's VersionAbstract
    This dissertation describes methodology that significantly improves the state of femtosecond laser writing of metals. The developments address two major shortcomings: poor material quality, and limited 3D patterning capabilities. In two dimensions, we grow monocrystalline silver prisms through femtosecond laser irradiation. We thus demonstrate the ability to create high quality material (with limited number of domains), unlike published reports of 2D structures composed of nanoparticle aggregates. This development has broader implications beyond metal writing, as it demonstrates a one-step fabrication process to localize bottom-up growth of high quality monocrystalline material on a substrate. In three dimensions, we direct laser write fully disconnected 3D silver structures in a polymer matrix. Since the silver structures are embedded in a stable matrix, they are not required to be self-supported, enabling the one-step fabrication of 3D patterns of 3D metal structures that need-not be connected. We demonstrate sub- 100-nm silver structures. This latter development addresses a broader limitation in fabrication technologies, where 3D patterning of metal structures is difficult. We demonstrate several 3D silver patterns that cannot be obtained through any other fabrication technique known to us. We expect these advances to contribute to the development of new devices in optics, plasmonics, and metamaterials. With further improvements in the fabrication methods, the list of potential applications broadens to include electronics (e.g. 3D microelectronic circuits), chemistry (e.g. catalysis), and biology (e.g. plasmonic biosensing).
    K. Anne Miller, N. Lasry, B. Lukoff, J. Schell, and E. Mazur. 2014. “Conceptual question response times in Peer Instruction classrooms.” PRST, 10(2), Pp. –. Publisher's VersionAbstract
    Classroom response systems are widely used in interactive teaching environments as a way to engage students by asking them questions. Previous research on the time taken by students to respond to conceptual questions has yielded insights on how students think and change conceptions. We measure the amount of time students take to respond to in- class, conceptual questions [ConcepTests (CTs)] in two introductory physics courses taught using Peer Instruction and use item response theory to determine the difficulty of the CTs. We examine response time differences between correct and incorrect answers both before and after the peer discussion for CTs of varying difficulty. We also determine the relationship between response time and student performance on a standardized test of incoming physics knowledge, precourse self-efficacy, and gender. Our data reveal three results of interest. First, response time for correct answers is significantly faster than for incorrect answers, both before and after peer discussion, especially for easy CTs. Second, students with greater incoming physics knowledge and higher self-efficacy respond faster in both rounds. Third, there is no gender difference in response rate after controlling for incoming physics knowledge scores, although males register significantly more attempts before committing to a final answer than do female students. These results provide insight into effective CT pacing during Peer Instruction. In particular, in order to maintain a pace that keeps everyone engaged, students should not be given too much time to respond. When around 80% of the answers are in, the ratio of correct to incorrect responses rapidly approaches levels indicating random guessing and instructors should close the poll.
    N. Lasry, J. Guillemette, and E. Mazur. 2014. “Two steps forward, one step back.” Nature Physics, 10, Pp. 402–403. Publisher's VersionAbstract
    Among physics students there exists a wide variety of misconceptions, generally thought to be robust and resistant to change. But our analysis of the path of progress has changed our conception of how students learn physics.
    K. Vora, S. Kang, M. Gerhard Moebius, and E. Mazur. 2014. “Femtosecond laser direct writing of monocrystalline hexagonal silver prisms.” Appl. Phys. Lett., 105, Pp. 141114–. Publisher's VersionAbstract
    *Kevin Vora and SeungYeon Kang have made equal contributions to the present work. Bottom-up growth methods and top-down patterning techniques are both used to fabricate metal nanostructures, each with a distinct advantage: One creates crystalline structures and the other offers precise positioning. Here, we present a technique that localizes the growth of metal crystals to the focal volume of a laser beam, combining advantages from both approaches. We report the fabrication of silver nanoprisms— hexagonal nanoscale silver crystals—through irradiation with focused femtosecond laser pulses. The growth of these nanoprisms is due to a nonlinear optical interaction between femtosecond laser pulses and a polyvinylpyrrolidone film doped with silver nitrate. The hexagonal nanoprisms have bases hundreds of nanometers in size and the crystal growth occurs over exposure times of less than 1 ms (8 orders of magnitude faster than traditional chemical techniques). Electron backscatter diffraction analysis shows that the hexagonal nanoprisms are monocrystalline. The fabrication method combines advantages from both wet chemistry and femtosecond laser direct-writing to grow silver crystals in targeted locations. The results presented in this letter offer an approach to directly positioning and growing silver crystals on a substrate, which can be used for plasmonic devices.
    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.
    S. H. Chung, A. Schmalz, R. Clarissa Ruiz, C. V. Gabel, and E. Mazur. 2013. “Femtosecond Laser Ablation Reveals Antagonistic Sensory and Neuroendocrine Signaling that Underlie C. elegans Behavior and Development.” Cell Reports, 4, Pp. 316–326. Publisher's VersionAbstract
    The specific roles of neuronal subcellular compo- nents in behavior and development remain largely unknown, even though advances in molecular biology and conventional whole-cell laser ablation have greatly accelerated the identification of contrib- utors at the molecular and cellular levels. We system- atically applied femtosecond laser ablation, which has submicrometer resolution in vivo, to dissect the cell bodies, dendrites, or axons of a sensory neuron (ASJ) in Caenorhabditis elegans to determine their roles in modulating locomotion and the develop- mental decisions for dauer, a facultative, stress- resistant life stage. Our results indicate that the cell body sends out axonally mediated and hormonal sig- nals in order to mediate these functions. Further- more, our results suggest that antagonistic sensory dendritic signals primarily drive and switch polarity between the decisions to enter and exit dauer. Thus, the improved resolution of femtosecond laser ablation reveals a rich complexity of neuronal signaling at the subcellular level, including multiple neurite and hormonally mediated pathways depen- dent on life stage.
    M. Sher. 2013. “Intermediate Band Properties of Femtosecond-Laser Hyperdoped Silicon”. Publisher's VersionAbstract
    This thesis explores using femtosecond-laser pulses to hyperdope silicon with chalcogen dopants at concentrations above the maximum equilibrium solubility. Hyperdoped silicon is promising for improving efficiencies of solar cells: the material exhibits broad-band light absorption to wavelengths deep below the corresponding bandgap energy of silicon. The high concentration of dopants forms an intermediate band (IB), instead of discrete energy levels, and the IB enables sub-bandgap light absorption. This thesis is divided into two primary studies: the dopant incorporation and the IB properties. First, we study dopant incorporation with a gas- phase dopant precursor (SF6) using secondary ion mass spectrometry. By varying the pressure of SF6, we find that the surface adsorbed molecules are the dominant source of the dopant. Furthermore, we show the hyperdoped layer is single crystalline. The results demonstrate that the dopant incorporation depth, concentration, and crystallinity are controlled respectively by the number of laser pulses, pressure of the dopant precursor, and laser fluence. Second, we study the IB properties of hyperdoped silicon using optical and electronic measurements. We use Fourier transform infrared spectroscopy to study light absorption. The absorption extends to wavelengths as far as 6 µm before thermal annealing and we find the upper bound of the IB location at 0.2 eV below the conduction band edge. For electronic measurements, we anneal the samples to form a diode between the hyperdoped layer and the substrate, allowing us to probe the IB using temperature-dependent electronic transport measurements. The measurement data indicate that these samples form a localized IB at concentrations below the insulator-to-metal transition. Using a two-band model, we obtain the location of the localized IB at >0.07 eV below the conduction band edge. After femtosecond-laser hyperdoping, annealing is necessary to reduce the laser-induced defects; however annealing decreases the sub-bandgap absorption. As we are interested in the IB that contributes to sub-bandgap absorption, we explore methods to reactivate the sub-bandgap absorption. We show that the sub-bandgap absorption is reactivated by annealing at high temperatures between 1350 and 1550 K followed by fast cooling (>50 K/s). Our results demonstrate an ability to control sub-bandgap absorption using thermal processing.
    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.
    A. Hu, G. Deng, S. Denis Courvoisier, O. Reshef, C. C. Evans, E. Mazur, and Y. Norman. Zhou. 2013. “Femtosecond laser induced surface melting and nanojoining for plasmonic circuits”. Publisher's VersionAbstract
    Femtosecond laser induced nonthermal processing is an emerging nanofabrication technique for delicate plasmonic devices. In this work we present a detailed investigation on the interaction between ultra-short pulses and silver nanomaterials, both experimentally and theoretically. We systematically study the laser-silver interaction at a laser fluent from 1 J/m2 to 1 MJ/m2. The optimal processing window for welding of silver nanowires occurs at fluences of 200-450 J/m2. The femtosecond laser-induced surface melting allows precise welding of silver nanowires for "T” and “X” shape circuits. These welded plasmonic circuits are successfully applied for routining light propagation.
    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.
    J. Watkins and E. Mazur. 2013. “Retaining students in science, technology, engineering, and mathematics (STEM) majors.” J. Coll. Sci. Teach., 42, Pp. 36–41. Publisher's VersionAbstract
    In this paper we present results relating undergraduate student retention in STEM majors to the use of Peer Instruction in an introductory physics course at a highly- selective research institution. We compare the percentages of students who switch out of a STEM major after taking a physics course taught using traditional lectures only or one using Peer Instruction, finding that nearly twice the percentage of students switch after the lecture-based course. By examining these results in light of the literature on STEM retention, we propose that providing opportunities for students to think, respond, and interact in class may have a substantial impact on the retention of students in STEM disciplines.
    M. Sher, K. Charles Hammond, L. Christakis, and E. Mazur. 2013. “The photovoltaic potential of femtosecond-laser textured amorphous silicon.” In . SPIE 2013 Photonics West. Publisher's VersionAbstract
    Femtosecond laser texturing of silicon yields micrometer scale surface roughness that reduces reflection and enhances light absorption. In this work, we study the potential of using this technique to improve efficiencies of amorphous silicon-based solar cells by laser texturing thin amorphous silicon films. We use a Ti:Sapphire femtosecond laser system to texture amorphous silicon, and we also study the effect of laser texturing the substrate before depositing amorphous silicon. We report on the material properties including surface morphology, light absorption, crystallinity, as well as solar cell efficiencies before and after laser texturing.
    D. Rioux, S. Vallières, S. b. Besner, P. Muñoz, M. Meunier, and E. Mazur. 2013. “An Analytic Model for the Dielectric Function of Au, Ag, and their Alloys.” Advanced Optical Materials, Pp. –. Publisher's VersionAbstract
    An analytical model for the prediction of the dielectric properties of gold– silver alloys is developed. This multi-parametric model is a modification of the usual Drude–Lorentz model that takes into account the band structure of the metals. It is fitted by a genetic algorithm to the dielectric function of thin alloy films of different gold–silver ratio obtained by ellipsometry. The model is validated for arbitrary alloy compositions by comparing the experimental extinction spectra of alloy nanoparticles with the spectra predicted by Mie theory.
    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.
    C. Lindstrøm and J. Schell. 2013. “Leveraging technology to enhance evidence-based pedagogy: A case study of Peer Instruction in Norway”. Publisher's VersionAbstract
    Peer Instruction (PI) is a research-based instructional strategy developed by Eric Mazur at Harvard University in the 1990s. Instructors across the disciplines, in every institutional type, and in classrooms throughout the world have adopted PI. The method relies on classroom response systems (CRSs) – or systems which allow instructors to collect student responses to questions. While PI can be and often is implemented using low-tech CRSs (e.g. flashcards), it is enhanced when paired with higher-tech tools (e.g. clickers). In this paper, we address the following research problem: Moving from flashcards to clickers in PI has advantages, however there is a lack of clarity about the practical aspects of this transition for individual instructors. We pose the following research questions: What is involved in the transition from a low-tech CRS (e.g. flashcards) to a high- tech CRS (e.g. clickers) for the instructor and students in a PI environment? What are student perceptions about the value of using clickers when they have previously used flashcards? What are the instructor perceptions of the value of using clickers when she has previously used flashcards? The purpose of this paper is to address the research problem and questions by presenting a case study of one instructor’s transition from flashcards to clickers in one university classroom. The paper also provides recommendations for instructors wishing to implement clickers to improve ease of implementation. We found that the transition from flashcards to clickers involves primarily familiarizing the instructor and students with the new technology. We also found that both students and the instructor prefer clickers to flashcards. Most importantly, we found that of the pre-service teachers in our sample (N=21) who filled out post- course surveys (n=19), 95% indicated that they intend to use PI, versus more traditional approaches, in their own teaching.** NOTE THIS IS A CORRECTION TO THE ABSTRACT IN THE PUBLISHED PAPER.
    T. Sarnet, T. J. - Derrien, R. Torres, P. Delaporte, F. Torregrosa, M. Sher, Y. Lin, B. Franta, G. Deng, and E. Mazur. 2013. “Black silicon for photovoltaic cells: towards a high-efficiency silicon solar cell.” In . EU PVSEC 2013, 28th European Photovoltaic Solar Energy Conference and Exhibition. Publisher's VersionAbstract
    Laser-created Black Silicon has been developed since 1998 at Harvard University. The unique optical and semiconducting properties of black silicon first led to interesting applications for sensors (photodetectors, thermal imaging cameras, etc.) 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 short review on recent research on the use of black silicon for photovoltaic cells.