Publications

2014
O. Reshef, K. Shtyrkova, M. Gerhard Moebius, C. C. Evans, S. Griesse-Nascimento, E. Ippen, and E. Mazur. 2014. “Polycrystalline Anatase Micro-Ring Resonators at Telecommunication Wavelengths”. Publisher's VersionAbstract
We fabricate and characterize integrated polycrystalline anatase TiO2 micro-ring resonators at around λ = 1550 nm. We obtain quality factors of 1.5×10^4 and calculate a propagation loss of 8.0 ± 1.3 dB/cm.
E. Mazur. 2014. Principles & Practice of Physics, Pp. 1275. Pearson. Publisher's VersionAbstract
The Principles and Practice of Physics is a new calculus-based introductory physics textbook that uses a unique organization and pedagogy to allow students to develop a true conceptual understanding of physics alongside the quantitative skills needed in the course. The book organizes introductory physics around the conservation principles and provides a unified contemporary view of introductory physics. The result of this reorganization is a groundbreaking new book that puts principles first, thereby making it more accessible to students and easier for instructors to teach. To request an examination copy of the complete book, please visit the book web site ISBN-13: 9780136150930
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. Shtyrkova, C. C. Evans, O. Reshef, J. D.B. Bradley, M. Gerhard Moebius, E. Mazur, and E. Ippen. 2014. “Third Harmonic Generation in Polycrystalline Anatase Titanium Dioxide Nanowaveguides”. Publisher's VersionAbstract
We experimentally demonstrate third-harmonic generation in polycrystalline anatase titanium dioxide nano-waveguides, using ultrashort optical pulses centered around 1550 nm. Phase matching is achieved using higher order optical modes at the third harmonic wavelength.
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).
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.
2013
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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, 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.

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