Publications

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.

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.

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.

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.

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.

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.

We observe the first nonlinear spectral broadening of femtosecond pulses in single-mode anatase TiO2 waveguides at 793 and 1565 nm. The broad applicability and low two-photon absorption of TiO2 makes it a promising material for integrated photonics.

Turn to Your Neighbor is the Official Peer Instruction Blog, authored by Julie Schell.

We explore the chemical contribution to surface-enhanced Raman scattering (SERS) in mixed-metal substrates, both experimentally and by computer simulation. These substrates are composed of a chemically active, transition- metal overlayer deposited on an effective SERS substrate. We report improved analytical enhancement factors obtained by using a small surface coverage of palladium or platinum over nanostructured silver substrates. Theoretical predictions of the chemical contribution to the surface enhancement using density functional theory support the experimental results. In addition, these approaches show that the increased enhancement is due not only to an increase in surface coverage of the analyte but also to a higher Raman scattering cross section per molecule. The additional chemical enhancement in mixed-metal SERS substrates correlates with the binding energy of the analyte on the surface and includes both static and dynamical effects. SERS using mixed-metal substrates has the potential to improve sensing for a large group of analyte molecules and to aid the development of chemically specific SERS-based sensors.

We present a simple, one-step technique for direct-writing of a structured nanocomposite material with disconnected silver nanostructures in a polymer matrix. A nonlinear optical interaction between femtosecond laser pulses and a composite material creates silver structures that are embedded inside a polymer with submicrometer resolution (300 nm). We create complex patterns of silver nanostructures in three dimensions. The key to the process is the chemical composition of the sample that provides both a support matrix and controlled growth. The technique presented in this letter may offer a cost-effective approach for the fabrication of bulk optical devices with engineered dispersion. Copyright (2012) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.

We use femtosecond laser irradiation of titanium metal to create nanometer scale laser-induced periodic surface structures and study the influence of atmospheric composition on these surface structures. We find that gas composition and pressure affect the chemical composition of the films, but not the surface morphology. We demonstrate that irradiation of titanium in oxygen containing atmospheres forms a highly stable surface layer of nanostructured amorphous titanium dioxide.

Laser created Black Silicon has been developed since 1998 at Harvard University. The unique optical and semiconducting properties of the black silicon first lead to interesting applications for sensors (photodetectors, thermal imaging cameras…). Other applications like Photovoltaic solar cells have been rapidly identified, but it took more than ten years of research and development before demonstrating a real improvement of the photovoltaic efficiency on an industrial multi-crystalline solar cell. This paper is a brief review of the use of black silicon for photovoltaic cells.

The most powerful tool we have at our disposal in our quest to cultivate effective learning is teaching. This article provides an overview of Peer Instruction, an innovative pedagogy known to significantly improve student learning across various subjects and institutional types. (Article correction - author=Julie Schell, EdD).

We demonstrate waveguide-coupled titanium dioxide (TiO2) racetrack resonators with loaded quality factors of 2.2 × 104 for the visible wavelengths. The structures were fabricated in sputtered TiO2 thin films on oxidized silicon substrates using standard top-down nanofabrication techniques, and passively probed in transmission measure- ments using a tunable red laser.

Assessment is the primary means of feedback between students and instructors. However, to effectively use assessment, the ability to interpret collected information is essential. We present insights into three unique, important avenues of assessment in the physics classroom and laboratory. First, we examine students’ performance on conceptual surveys. The goal of this research project is to better utilize the information collected by instructors when they administer the Force Concept Inventory (FCI) to students as a pre-test and post-test of their conceptual understanding of Newtonian mechanics. We find that ambiguities in the use of the normalized gain, g, may influence comparisons among individual classes. Therefore, we propose using stratagrams, graphical summaries of the fraction of students who exhibit “Newtonian thinking,” as a clearer, more informative method of both assessing a single class and comparing performance among classes. Next, we examine students’ expressions of confusion when they initially encounter new material. The goal of this research project is to better understand what such confusion actually conveys to instructors about students’ performance and engagement. We investigate the relationship between students’ self-assessment of their confusion over material and their performance, confidence in reasoning, pre-course self-efficacy and several other measurable characteristics of engagement. We find that students’ expressions of confusion are negatively related to initial performance, confidence and self-efficacy, but positively related to final performance when all factors are considered together. Finally, we examine students’ exhibition of scientific reasoning abilities in the instructional laboratory. The goal of this research project is to explore two inquiry- based curricula, each of which proposes a different degree of scaffolding. Students engage in sequences of these laboratory activities during one semester of an introductory physics course. We find that students who participate in the less scaffolded activities exhibit marginally stronger scientific reasoning abilities in distinct exercises throughout the semester, but exhibit no differences in the final, common exercises. Overall, we find that, although students demonstrate some enhanced scientific reasoning skills, they fail to exhibit or retain even some of the most strongly emphasized skills.

A Method to Fabricate Disconnected Silver Nanostructures in 3D The standard nanofabrication toolkit includes techniques primarily aimed at creating 2D patterns in dielectric media. Creating metal patterns on a submicron scale requires a combination of nanofabrication tools and several material processing steps. For example, steps to create planar metal structures using ultraviolet photolithography and electron-beam lithography can include sample exposure, sample development, metal deposition, and metal liftoff. To create 3D metal structures, the sequence is repeated multiple times. The complexity and difficulty of stacking and aligning multiple layers limits practical implementations of 3D metal structuring using standard nanofabrication tools. Femtosecond-laser direct-writing has emerged as a pre-eminent technique for 3D nanofabrication. Femtosecond lasers are frequently used to create 3D patterns in polymers and glasses. However, 3D metal direct-writing remains a challenge. Here, we describe a method to fabricate silver nanostructures embedded inside a polymer matrix using a femtosecond laser centered at 800 nm. The method enables the fabrication of patterns not feasible using other techniques, such as 3D arrays of disconnected silver voxels.8 Disconnected 3D metal patterns are useful for metamaterials where unit cells are not in contact with each other, such as coupled metal dot or coupled metal rod resonators. Potential applications include negative index metamaterials, invisibility cloaks, and perfect lenses. In femtosecond-laser direct-writing, the laser wavelength is chosen such that photons are not linearly absorbed in the target medium. When the laser pulse duration is compressed to the femtosecond time scale and the radiation is tightly focused inside the target, the extremely high intensity induces nonlinear absorption. Multiple photons are absorbed simultaneously to cause electronic transitions that lead to material modification within the focused region. Using this approach, one can form structures in the bulk of a material rather than on its surface. Most work on 3D direct metal writing has focused on creating self-supported metal structures. The method described here yields submicrometer silver structures that do not need to be self-supported because they are embedded inside a matrix. A doped polymer matrix is prepared using a mixture of silver nitrate (AgNO3), polyvinylpyrrolidone (PVP) and water (H2O). Samples are then patterned by irradiation with an 11-MHz femtosecond laser producing 50-fs pulses. During irradiation, photoreduction of silver ions is induced through nonlinear absorption, creating an aggregate of silver nanoparticles in the focal region. Using this approach we create silver patterns embedded in a doped PVP matrix. Adding 3D translation of the sample extends the patterning to three dimensions.

We observe mixed two- and three-photon absorption in bulk rutile (TiO2) around 800 nm using the open aperture Z-scan technique. We fit the data with an extended model that includes multiphoton absorption, beam quality, and ellipticity. The extracted two- and three-photon absorption coefficients are below 1 mm/GW and 2 mm3/GW2, respectively. We observe negligible two-photon absorption for 813-nm light polarized along the extraordinary axis. We measure the nonlinear index of refraction and obtain two-photon nonlinear figures of merit greater than 1.1 at 774 nm and greater than 12 at 813 nm. Similarly, we obtain three-photon figures of merit that allow operational intensities up to 0.57 GW/mm2. We conclude that rutile is a promising material for all-optical switching applications around 800 nm.

Spindles are arrays of microtubules that segregate chromosomes during cell division. It has been difficult to validate models of spindle assembly due to a lack of information on the organization of microtubules in these structures. Here we present a method, based on femtosecond laser ablation, capable of measuring the detailed architecture of spindles. We used this method to study the metaphase spindle in Xenopus laevis egg extracts and find that microtubules are shortest near poles and become progressively longer towards the center of the spindle. These data, in combination with mathematical modeling, imaging, and biochemical perturbations, are sufficient to reject previously proposed mechanisms of spindle assembly. Our results support a model of spindle assembly in which microtubule polymerization dynamics are not spatially regulated, and the proper organization of microtubules in the spindle is determined by non-uniform microtubule nucleation and the local sorting of microtubules by transport.

Surface texturing of silicon using femtosecond (fs) laser irradiation can reduce the surface reflectivity to less than 5%, enables control over the resulting surface morphology, and uses little material. The laser-induced damage that occurs in parallel with surface texturing, however, can result in increased recombination currents that inhibit device performance. In this work, we investigate the light- material interaction during the texturing of silicon by directly correlating the formation of pressure-induced silicon polymorphs, fs-laser irradiation conditions, and the resulting morphology and microstructure using scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy. We identify resolidification-induced stresses as the mechanism responsible for driving sub- surface phase transformations during the surface texturing of silicon, the understanding of which is an important first step towards reducing laser-induced damage during the texturing of silicon with fs-laser irradiation.

We observe the first nonlinear spectral broadening of femtosecond pulses in single-mode anatase TiO2 waveguides at 793 and 1565 nm. The broad applicability and low two-photon absorption of TiO2 makes it a promising material for integrated photonics.