Publications

    K. Anne Miller, J. Schell, A. Ho, B. Lukoff, and E. Mazur. 2015. “Response switching and self-efficacy in Peer Instruction classrooms.” Physical Review Special Topics, 11, Pp. –. Publisher's VersionAbstract
    Peer Instruction, a well-known student-centered teaching method, engages students during class through structured, frequent questioning and is often facilitated by classroom response systems. The central feature of any Peer Instruction class is a conceptual question designed to help resolve student misconceptions about subject matter. We provide students two opportunities to answer each question—once after a round of individual reflection and then again after a discussion round with a peer. The second round provides students the choice to “switch” their original response to a different answer. The percentage of right answers typically increases after peer discussion: most students who answer incorrectly in the individual round switch to the correct answer after the peer discussion. However, for any given question there are also students who switch their initially right answer to a wrong answer and students who switch their initially wrong answer to a different wrong answer. In this study, we analyze response switching over one semester of an introductory electricity and magnetism course taught using Peer Instruction at Harvard University. Two key features emerge from our analysis: First, response switching correlates with academic self-efficacy. Students with low self-efficacy switch their responses more than students with high self-efficacy. Second, switching also correlates with the difficulty of the question; students switch to incorrect responses more often when the question is difficult. These findings indicate that instructors may need to provide greater support for difficult questions, such as supplying cues during lectures, increasing times for discussions, or ensuring effective pairing (such as having a student with one right answer in the pair). Additionally, the connection between response switching and self-efficacy motivates interventions to increase student self-efficacy at the beginning of the semester by helping students develop early mastery or to reduce stressful experiences (i.e., high-stakes testing) early in the semester, in the hope that this will improve student learning in Peer Instruction classrooms.
    C. C. Evans, K. Shtyrkova, O. Reshef, M. Gerhard Moebius, J. D.B. Bradley, S. Griesse-Nascimento, E. Ippen, and E. Mazur. 2015. “Multimode phase-matched third-harmonic generation in sub-micrometer-wide anatase TiO2 waveguides.” Optics Express, 23, Pp. 7832–7841. Publisher's VersionAbstract
    Third-harmonic generation (THG) has applications ranging from wavelength conversion to pulse characterization, and has important implications for quantum sources of entangled photons. However, on-chip THG devices are nearly unexplored because bulk techniques are difficult to adapt to integrated photonic circuits. Using sub- micrometer-wide polycrys- talline anatase TiO2 waveguides, we demonstrate third-harmonic generation on a CMOS-compatible platform. We correlate higher conversion effi- ciencies with phase-matching between the fundamental pump mode and higher-order signal modes. Using scattered light, we estimate conversion efficiencies as high as 2.5% using femtosecond pulses, and thus demonstrate that multimode TiO2 waveguides are promising for wideband wavelength conversion and new applications ranging from sensors to triplet-photon sources.
    S. Denis Courvoisier, N. Saklayen, M. Huber, J. Chen, E. D. Diebold, L. Bonacina, J. Wolf, and E. Mazur. 2015. “Plasmonic Tipless Pyramid Arrays for Cell Poration.” Nanoletters, 15, Pp. 4461–4466. Publisher's VersionAbstract
    Improving the efficiency, cell survival, and throughput of methods to modify and control the genetic expression of cells is of great benefit to biology and medicine. We investigate, both computationally and experimentally, a nanostructured substrate made of tipless pyramids for plasmonic-induced transfection. By optimizing the geometrical parameters for an excitation wavelength of 800 nm, we demonstrate a 100-fold intensity enhancement of the electric near field at the cell−substrate contact area, while the low absorption typical for gold is maintained. We demonstrate that such a substrate can induce transient poration of cells by a purely optically induced process.
    H. H. Gandhi, E. Mazur, K. Phillips, and S. K. Sundaram. 2015. “Ultrafast Laser Processing Of Materials: A Review.” Advances In Optics and Photonics, 7, Pp. 684–712. Publisher's VersionAbstract
    We present an overview of the different processes that can result from focusing an ultrafast laser light in the femtosecond–nanosecond time regime on a host of materials, e.g., metals, semiconductors, and insulators. We summarize the physical processes and surface and bulk applications and highlight how femtosecond lasers can be used to process various materials. Throughout this paper, we will show the advantages and disadvantages of using ultrafast lasers compared with lasers that operate in other regimes and demonstrate their potential for the ultrafast processing of materials and structures.
    F. Fabbri, Y. Lin, G. Bertoni, F. Rossi, M. J. Smith, S. Gradecak, E. Mazur, and G. Salviati. 2015. “Origin of the visible emission of black silicon microstructures.” Appl. Phys. Lett., 107, Pp. 021907-1–021907-4. Publisher's VersionAbstract
    Silicon, the mainstay semiconductor in microelectronics, is considered unsuitable for optoelectronic applications due to its indirect electronic band gap that limits its efficiency as light emitter. Here, we univocally determine at the nanoscale the origin of visible emission in microstructured black silicon by cathodoluminescence spectroscopy and imaging. We demonstrate the formation of amorphous silicon oxide microstructures with a white emission. The white emission is composed by four features peaking at 1.98 eV, 2.24 eV, 2.77 eV, and 3.05 eV. The origin of such emissions is related to SiOx intrinsic point defects and to the sulfur doping due to the laser processing. Similar results go in the direction of developing optoelectronic devices suitable for silicon-based circuitry.
    O. Reshef, K. Shtyrkova, M. Gerhard Moebius, S. Griesse-Nascimento, S. Spector, C. C. Evans, E. Ippen, and E. Mazur. 2015. “Polycrystalline Anatase Titanium Dioxide Micro-ring Resonators with Negative Thermo-optic Coefficient.” J. Opt. Soc. Am. B, 32, Pp. 2288–2293. Publisher's VersionAbstract
    We fabricate polycrystalline anatase TiO2 micro-ring resonators with loaded quality factors as high as 25,000 and average losses of 0.58 dB/mm in the telecommunications band. Additionally, we measure a negative thermo-optic coefficient dn/dT of −4.9 ± 0.5 × 10−5 K−1. The presented fabrication uses CMOS- compatible lithographic techniques that take advantage of substrate-independent, non-epitaxial growth. These properties make polycrystalline anatase a promising candidate for the implementation of athermal, vertically-integrated, CMOS- compatible nanophotonic devices for nonlinear applications.
    P. Muñoz, O. Reshef, G. England, and R. McClellan. 2015. “Inverse Transformation Optics with Realistic Material Parameters.” In . META Conference. Publisher's VersionAbstract
    We present a method to generate transformation functions based on a space of achievable material properties. To validate this approach, we consider the range of effective refractive index achievable using silver nanowires in a dielectric background. Given fabrication constraints, we generate a reduced cloaking transformation and confirm its performance using FDTD and FEM simulations. We explore conditions for finding appropriate mappings in restricted parameter spaces, and strategies for optimizing transformations to account for absorption and scattering.
    Y. Lin, N. Mangan, S. Marbach, T. M. Schneider, G. Deng, S. Zhou, M. Brenner, and E. Mazur. 2015. “Creating femtosecond-laser-hyperdoped silicon with a homogeneous doping profile.” Appl. Phys. Lett., 106, Pp. 062105–. Publisher's VersionAbstract
    Femtosecond-laser hyperdoping of sulfur in silicon typically produces a concentration gradient that results in undesirable inhomogeneous material properties. Using a mathematical model of the doping process, we design a fabrication method consisting of a sequence of laser pulses with varying sulfur concentrations in the atmosphere, which produces hyperdoped silicon with a uniform concentration depth profile. Our measurements of the evolution of the concentration profiles with each laser pulse are consistent with our mathematical model of the doping mechanism, based on classical heat and solute diffusion coupled to the far-from-equilibrium dopant incorporation. The use of optimization methods opens an avenue for creating controllable hyperdoped materials on demand.
    S. Kang, K. Vora, and E. Mazur. 2015. “One-step direct-laser metal writing of sub-100 nm 3D silver nanostructures in a gelatin matrix.” Nanotechnology, 26, Pp. 1–6. Publisher's VersionAbstract
    Developing an ability to fabricate high-resolution, 3D metal nanostructures in a stretchable 3D matrix is a critical step to realizing novel optoelectronic devices such as tunable bulk metaldielectric optical and THz metamaterial devices that are not feasible with alternate techniques. We report a new chemistry method to fabricate high-resolution, 3D silver nanostructures using a femtosecond-laser direct metal writing technique. Previously, only fabrication of 3D polymeric structures or single/few-layer metal structures was possible. Our method takes advantage of unique gelatin properties to overcome these previous limitations such as limited freedom in 3D material design and short sample lifetime. We fabricate more than 15 layers of 3D silver nanostructures with a resolution of less than 100 nm in a stable dielectric matrix that is flexible and has high large transparency well- matched for potential applications in the optical and THz metamaterial regimes. This is a single-step process that does not require any further processing. This work will be of interest to those interested in the fabrication methods that utilize nonlinear light-matter interactions and the realization of future metamaterials.
    M. Gerhard Moebius, K. Vora, S. Kang, P. Munoz, G. Deng, and E. Mazur. 2015. “Direct Laser Writing of 3D Gratings and Diffraction Optics.” In . CLEO: Science and Innovations Laser-Induced Structuring in Bulk Material (SW1K). Publisher's VersionAbstract
    We fabricate 3D gratings and diffraction optics using direct laser writing. Diffraction patterns of gratings agree with Laue theory. We demonstrate zone plates for visible wavelengths. Direct laser writing is promising for integrated diffraction optics.
    2015. “Integrated super-couplers based on zero-index metamaterials.” In . META Conference.Abstract
    Zero-refractive-index metamaterials have been proposed as potential candidates for super-coupling applications, where light is confined to sub- diffraction limited length scales on-chip. Such a device allows for efficient coupling between disparate modes and compact 90 degree bends, which are challenging to achieve using dielectric waveguides. We discuss the simulation and fabrication results of all-dielectric on-chip zero-index metamaterial-based couplers. We observe transmission normal to all faces, regardless of the structure's shape, highlighting an unexplored feature of zero index metamaterials for integrated photonics.
    D. Inna Vulis, O. Reshef, P. Muñoz, S. Kita, Y. Li, M. Loncar, and E. Mazur. 2015. “Integrated super-couplers based on zero-index metamaterials.” In . META Conference. Publisher's VersionAbstract
    Zero-refractive-index metamaterials have been proposed as potential candidates for super-coupling applications, where light is confined to sub-diffraction limited length scales on-chip. Such a device allows for efficient coupling between disparate modes and compact 90 degree bends, which are challenging to achieve using dielectric waveguides. We discuss the simulation and fabrication results of all-dielectric on- chip zero-index metamaterial-based couplers. We observe transmission normal to all faces, regardless of the structure’s shape, highlighting an unexplored feature of zero index metamaterials for integrated photonics.
    S. Kita, Y. Li, P. Muñoz, O. Reshef, D. Inna Vulis, R. Day, and C. M. Lieber. 2015. “On-chip Super-robust All-dielectric Zero-Index Material.” In . CLEO. Publisher's VersionAbstract
    The robustness of the modal degeneracy for photonic Dirac-cone can be engineered by designing all-dielectric pillar arrays giving on-chip platform of zero index material for any wavelength regime. We demonstrate this concept for telecom regime.
    Y. Li, S. Kita, P. Muñoz, O. Reshef, D. Inna Vulis, M. Yin, M. Loncar, and E. Mazur. 2015. “On-chip zero-index metamaterials.” Nat. Photonics, 9, Pp. 738–742. Publisher's VersionAbstract
    Metamaterials with a refractive index of zero exhibit physical properties such as infinite phase velocity and wavelength. However, there is no way to implement these materials on a photonic chip, restricting the investigation and application of zero-index phenomena to simple shapes and small scales. We designed and fabricated an on-chip integrated metamaterial with a refractive index of zero in the optical regime. Light refracts perpendicular to the facets of a prism made of this metamaterial, directly demonstrating that the index of refraction is zero. The metamaterial consists of low-aspect- ratio silicon pillar arrays embedded in a polymer matrix and clad by gold films. This structure can be fabricated using standard planar processes over a large area in arbitrary shapes and can efficiently couple to photonic integrated circuits and other optical elements. This novel on- chip metamaterial platform opens the door to exploring the physics of zero index and its applications in integrated optics.
    2015. “Integrated super-couplers based on zero-index metamaterials.” In . META Conference.Abstract
    Zero-refractive-index metamaterials have been proposed as potential candidates for super-coupling applications, where light is confined to sub- diffraction limited length scales on-chip. Such a device allows for efficient coupling between disparate modes and compact 90 degree bends, which are challenging to achieve using dielectric waveguides. We discuss the simulation and fabrication results of all-dielectric on-chip zero-index metamaterial-based couplers. We observe transmission normal to all faces, regardless of the structure's shape, highlighting an unexplored feature of zero index metamaterials for integrated photonics.
    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.
    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
    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.
    Y. Lin. 2014. “Femtosecond-laser hyperdoping and texturing of silicon for photovoltaic applications”. Publisher's VersionAbstract
    This dissertation explores strategies for improving photolvoltaic efficiency and reduc- ing cost using femtosecond-laser processing methods including surface texturing and hyperdoping. Our investigations focus on two aspects: 1) texturing the silicon sur- face to create efficient light-trapping for thin silicon solar cells, and 2) understanding the mechanism of hyperdoping to control the doping profiles for fabricating efficient intermediate band materials.> We first discuss the light-trapping properties in laser-textured silicon and its benefit to thin silicon heterojunction solar cells. We report a nearly 15% improvement in the short circuit current and device efficiency after surface texturing, which is attributed to the enhancement of absorption due to the formation of Lambertian surfaces. We next present studies on the hyperdoping mechanism using a pump-probe method. We measure in situ the change in surface reflectivity during hyperdoping and extract the dynamics of the melt front. Understanding the melt dynamics allows us to constrain the physical parameters in a numerical model, which we use to simulate the doping profile with a simplified classical picture. We then demonstrate the successful fabrication of homogeneously doped silicon by manipulating the hyperdoping process based on theoretically predicted design principles.

Pages