Publications

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.
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.
M. Sher and E. Mazur. 2014. “Intermediate Band Conduction in Femtosecond-Laser Hyperdoped Silicon.” Appl. Phys. Lett., 105, Pp. 032103-1–032103-5. Publisher's VersionAbstract
We use femtosecond-laser hyperdoping to introduce non-equilibrium concentrations of sulfur into silicon and study the nature of the resulting intermediate band. With increasing dopant concentration, the sub-bandgap absorption increases. To better understand the dopant energetics, we perform temperature-dependent Hall and resistivity measurements. We analyze the carrier concentration and the energetics of the intermediate band using a two- band model. The temperature-dependence of the carrier concentration and resistivity suggests that the dopant concentration is below the insulator-to-metal transition and that the samples have a localized intermediate band at 70 meV below the conduction band edge.
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.
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.
M. J. Smith, M. Sher, B. Franta, Y. Lin, E. Mazur, and S. Gradecak. 2014. “Improving Dopant Incorporation During Femtosecond- Laser Doping of Si with a Se Thin-Film Dopant Precursor.” Appl. Phys. A.Abstract
We study the dopant incorporation processes during thin-film fs-laser doping of Si and tailor the dopant distribution through optimization of the fs-laser irradiation conditions. Scanning electron microscopy, transmission electron microscopy, and profilometry are used to study the interrelated dopant incorporation and surface texturing mechanisms during fs-laser irradiation of Si coated with a Se thin-film dopant precursor. We show that the crystallization of Se-doped Si and micrometer-scale surface texturing are closely coupled and produce a doped surface that is not conducive to device fabrication. Next, we use this understanding of the dopant incorporation process to decouple dopant crystallization from surface texturing by tailoring the irradiation conditions. A low-fluence regime is identified in which a continuous surface layer of doped material forms in parallel with laser-induced periodic surface structures over many laser pulses. This investigation demonstrates the ability to tailor the dopant distribution through a systematic investigation of the relationship between fs-laser irradiation conditions, microstructure, and dopant distribution.
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).
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.
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. 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.
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.
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.
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.
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.
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. 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.
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.
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.
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.

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