Publications

    T. Voss, G. Thomas Svacha, E. Mazur, S. Mueller, C. Ronning, D. Konjhodzic, and F. Marlow. 2007. “High Order Waveguide Modes in ZnO Nanowires.” Nanoletters, 7, Pp. 3675–3680. Publisher's VersionAbstract
    We use tapered silica fibers to inject laser light into ZnO nanowires with diameters around 250 nm to study their waveguiding properties. We find that high-order waveguide modes are frequently excited and carry significant intensity at the wire surface. Numerical simulations reproduce the experimental observations and indicate a coupling efficiency between silica and ZnO nanowires of 50%. Experimentally, we find an emission angle from the ZnO nanowires of about 90, which is in agreement with the simulations.
    A. Samuel, S. H. Chung, D. A. Clark, C. V. Gabel, C. Chang, V. Murthy, and E. Mazur. 2006. “Femtosecond laser dissection in C. elegans neural circuits.” In . SPIE Photonics West. Publisher's VersionAbstract
    The nematode C. elegans, a millimeter-long roundworm, is a well-established model organism for studies of neural development and behavior, however physiological methods to manipulate and monitor the activity of its neural network have lagged behind the development of powerful methods in genetics and molecular biology. The small size and transparency of C. elegans make the worm an ideal test-bed for the development of physiological methods derived from optics and microscopy. We present the development and application of a new physiological tool: femtosecond laser dissection, which allows us to selectively ablate segments of individual neural fibers within live C. elegans. Femtosecond laser dissection provides a scalpel with submicrometer resolution, and we discuss its application in studies of neural growth, regenerative growth, and the neural basis of behavior.
    R. A. Myers, R. Farrell, A. Karger, J. E. Carey, and E. Mazur. 2006. “Enhancing near-infrared avalanche photodiode performance by femtosecond laser microstructuring.” Appl. Opt., 45, Pp. 8825–8831. Publisher's VersionAbstract
    A processing technique using femtosecond laser pulses to microstructure the surface of a silicon ava- lanche photodiode (APD) has been used to enhance its near-infrared (near-IR) response. Experiments were performed on a series of APDs and APD arrays using various structuring parameters and post- structuring annealing sequences. Following thermal annealing, we were able to fabricate APD arrays with quantum efficiencies as high as 58% at 1064 nm without degradation of their noise or gain performance. Experimental results provided evidence to suggest that the improvement in charge collec- tion is a result of increased absorption in the near-IR.
    J. L. Rosenberg, M. Lorenzo, and E. Mazur. 2006. “Peer Instruction: Making Science Engaging.” In Handbook of College Science Teaching, edited by Joel J. Mintzes and William H. Leonard, Pp. 77–85. NSTA Press. Publisher's VersionAbstract
    Science is a creative process where the synthesis of new ideas requires discussion and debate. However, the traditional model for teaching assumes that all information presented to students is automatically learned. As a result, most students leave their introductory science courses frustrated and without a solid conceptual understanding. At the same time, instructors feel that students have not lived up to their expectations, yet they cannot identify the problem. Peer Instruction is an interactive approach that was designed to improve the learning process. This approach provides students with greater opportunity for synthesizing the concepts while instructors get timely feedback that can help focus the instruction on the points that are the most difficult for the students. Peer Instruction is flexible and easy to use on its own or in conjunction with other teaching methods. This chapter discusses the motivation for using Peer Instruction and the mechanics of implementing it in the classroom.
    Z. Huang, J. E. Carey, M. Liu, X. Guo, E. Mazur, and J. C. Campbell. 2006. “Microstructured silicon photodetector.” Appl. Phys. Lett., 89, Pp. 033506–033508. Publisher's VersionAbstract
    Photodetectors fabricated on microstructured silicon are reported. The photodetectors exhibited high photoresponse; at 3 V bias, the responsivities were 92 A/W at 850 nm and 119 A/W at 960 nm. At wavelengths longer than 1.1 m, the photodetectors still showed strong photoresponse. A generation- recombination gain mechanism has been proposed to explain the photoresponse of these photodiodes. From measurements of the noise current density, the calculated gain was approximately 1200 at 3 V bias.
    I. Zaharieva Maxwell. 2006. “Application of femtosecond lasers for subcellular nanosurgery”. Publisher's VersionAbstract
    This dissertation o ers a study of femtosecond laser disruption in single cells. Cells and tissues do not ordinarily absorb light in the near-IR wavelength range of femtosecond lasers. However, the peak intensity of a femtosecond laser pulse is very high and material disruption is possible through nonlinear absorption and plasma generation. Because the pulse duration is very short, it is possible to reach the intensity of optical breakdown at only nanojoules of energy per pulse. The low energy deposition and the high spatial localization of the nonlinear absorption, make femtosecond laser pulses an ideal tool for minimally disruptive subcellular nanosurgery. We show definitively that there can be bulk ablation within a single cell by studying the disrupted region under a transmission electron microscope. The width of the ablated area can be as small as 250 nm in diameter at energies near the ablation threshold. We also studied the e ect of the laser repetition rate on the subcellular disruption threshold. We compared the pulse energies for kHz and MHz pulse trains, and found that in the MHz regime heat accumulation in the focal volume needs to be accounted for. For this repetition rate the minimum pulse energy necessary for disruption depends on the laser irradiation time. We used femtosecond laser nanosurgery to probe tension in actin stress fibers in living endothelial cells. By severing an individual stress fiber and visualizing its retraction, we showed that actin carries prestress in adherent, non-contractile cells. By plating the cells on softer, more compliant substrates, we measured the deflection of the substrate and extrapolated the force contribution of a stress filament on total amount of force exerted by the cell.
    J. B. Ashcom, R. R. Gattass, C. B. Schaffer, and E. Mazur. 2006. “Numerical aperture dependence of damage and supercontinuum generation from femtosecond laser pulses in bulk fused silica.” J. Opt. Soc. Am. B, 23, Pp. 2317–2322. Publisher's VersionAbstract
    Competing nonlinear optical effects are involved in the interaction of femtosecond laser pulses with transparent dielectrics: supercontinuum generation and multiphoton-induced bulk damage. We measured the threshold energy for supercontinuum generation and bulk damage in fused silica using numerical apertures ranging from 0.01 to 0.65. The threshold for supercontinuum generation exhibits a minimum near 0.05 NA, and increases quickly above 0.1 NA. For numerical apertures greater than 0.25, we observe no supercontinuum generation. The extent of the blue broadening of the supercontinuum spectrum decreases significantly as the numerical aperture is increased from 0.01 to 0.08, showing that loose focusing is important for generating the broadest supercontinuum spectrum. Using a light scattering technique to detect the onset of bulk damage, we confirmed bulk damage at all numerical apertures studied. At high numerical aperture, the damage threshold is well below the critical power for self-focusing.
    R. R. Gattass, G. Thomas Svacha, L. Tong, and E. Mazur. 2006. “Supercontinuum generation in submicrometer diameter silica fibers.” Opt. Exp., 14, Pp. 9408–9414. Publisher's VersionAbstract
    Silica nanowires provide strong mode confinement in a cylindrical silica-core/ air-cladding geometry and serve a model system for studying nonlinear propagation of short optical pulses inside fibers. We report on the fiber diameter dependence of the supercontinuum generated by femtosecond laser pulses in silica fiber tapers with average diameters in the range of 200 nm to 1200 nm. We observe a strong diameter-dependence of the spectral broadening, which can be attributed to the fibers diameter-dependent dispersion and nonlinearity. The short interaction length (less than 20 mm) and the low energy threshold for supercontinuum generation (about 1 nJ) make tapered fibers with diameters between 400 nm and 800 nm an ideal source of coherent white-light source in nanophotonics.
    T. Baldacchini, J. E. Carey, M. Zhou, and E. Mazur. 2006. “Superhydrophobic surfaces prepared by microstructuring of silicon using a femtosecond laser.” Langmuir, 22, Pp. 4917–4919. Publisher's VersionAbstract
    Superhydrophobic surfaces exhibit contact angles with water that are larger than 150 and negligible difference in contact angle between the advancing and receding contact angles, the so-called contact angle hysteresis. In this paper, we present a novel and simple structuring process that uses intense femtosecond- laser pulses to create microstructured superhydrophobic surfaces with remarkable wetting characteristics.
    B. R. Tull, J. E. Carey, E. Mazur, J. McDonald, and S. M. Yalisove. 2006. “Surface morphologies of silicon surfaces after femtosecond laser irradiation.” Mat. Res. Soc. Bull., 31, Pp. 626–633. Publisher's VersionAbstract
    In this article, we present summaries of the evolution of surface morphology resulting from the irradiation of single-crystal silicon with femtosecond laser pulses. In the first section, we discuss the development of micrometer-sized cones on a silicon surface irradiated with hundreds of femtosecond laser pulses in the presence of sulfur hexafluoride and other gases. We propose a general formation mechanism for the surface spikes. In the second section, we discuss the formation of blisters or bubbles at the interface between a thermal silicon oxide and a silicon surface after irradiation with one or more femtosecond laser pulses. We discuss the physical mechanism for blister formation and its potential use as channels in microfluidic devices.
    M. Lorenzo, C. H. Crouch, and E. Mazur. 2006. “Reducing the gender gap in the physics classroom.” Am. J. Phys., 74, Pp. 118–122. Publisher's VersionAbstract
    We investigate if the gender gap in conceptual understanding in an introductory university physics course can be reduced by using interactive engagement methods that promote in-class interaction, reduce competition, foster collaboration, and emphasize conceptual understanding. To this end we analyzed data from the introductory calculus-based physics course for non- majors at Harvard University taught traditionally or using different degrees of interactive engagement. Our results show that teaching with certain interactive strategies not only yields significantly increased understanding for both males and females, but also reduces the gender gap. In the most interactively taught courses, the pre-instruction gender gap was gone by the end of the semester.
    C. R. Mendonca, P. Tayalia, T. Baldacchini, and E. Mazur. 2006. “Three-dimensional microfabrication for photonics and biomedical applications.” In . Macro 2006 - 41st International Symposium on Macromolecules Proceedings. Publisher's VersionAbstract
    We use two-photon absorption polymerization to fabricate microstructures containing compounds with interesting properties for optical and biomedical applications. Our investigations open the door to new applications in data storage, waveguides manufacturing, organic LEDs, optical circuitry and scaffold for bio-applications.
    M. Kandyla. 2006. “Ultrafast dynamics of the laser-induced solid-to-liquid phase transition in aluminum”. Publisher's VersionAbstract
    This dissertation reports the ultrafast dynamics of aluminum during the solid-toliquid phase transition of melting after excitation by an intense femtosecond laser pulse. Photoexcitation with intense femtosecond laser pulses is known to create a novel melting mechanism called non-thermal melting. This mechanism has been observed repeatedly in semiconductors, but not yet in metals. We investigate the melting mechanism of aluminum by monitoring the reflectivity response following excitation by an intense laser pulse. We employ an optical pumpprobe technique designed to measure broadband reflectivity across the visible spectrum with femtosecond time resolution. A non-thermal melting mechanism was proposed for aluminum by optical experiments that demonstrated transition of the optical properties from solid to liquid values within 500 fs after phototexcitation. This result was later challenged by electron diffraction experiments, which showed that the lattice loses long range order within 3.5 ps during photoinduced melting. This time scale implies conventional thermal melting. We find that the broadband optical properties during the solid-to-liquid phase transition in aluminum agree with the results obtained by the electron diffraction experiments. The transition of the broadband reflectivity from solid to liquid values is complete within 1.5 2 ps in our experiments, which is compatible with thermal melting. We dont observe time scales on the order of 500 fs. All the experimental evidence in this dissertation lead to the conclusion that the laser-induced, solid-to-liquid phase transition in aluminum is a thermal process.
    S. H. Chung, D. A. Clark, C. V. Gabel, E. Mazur, and A. Samuel. 2006. “The role of the AFD neuron in C. elegans thermotaxis analyzed using femtosecond laser ablation.” BMC Neuroscience, 7, Pp. 30–. Publisher's VersionAbstract
    Background: Caenorhabditis elegans actively crawls down thermal gradients until it reaches the temperature of its cultivation, exhibiting what is called cryophilic movement. Implicit in the worms ability to actively bias its movements down thermal gradients is an ability to detect thermal gradients, and implicit in regulating the display of cryophilic bias is the ability to compare current ambient temperature with a stored memory of cultivation temperature. Several lines of evidence link the AFD sensory neuron to thermotactic behavior, but its exact role is not yet known. A current model contends that AFD is part of a thermophilic mechanism which biases movement up thermal gradients that counterbalances a cryophilic mechanism which biases movement down thermal gradients. Results: We used tightly-focused femtosecond laser pulses to dissect the AFD neuronal cell bodies and the AFD sensory dendrites in C. elegans to investigate their contribution to biased cryophilic movement. We establish that femtosecond laser ablation can exhibit submicrometer precision allowing the severing of individual AFD nerve fibers without causing collateral damage. Severing AFD dendrites in young adult worms permanently abolishes their sensory contribution without functional regeneration. We show that thermosensory input to the AFD neuron is required to activate a mechanism for generating cryophilic bias, but we find no evidence that AFD laser surgery reduces a putative ability to generate thermophilic bias. In addition, although disruption of the AIY interneuron causes worms to exhibit cryophilic bias at all temperatures, we find no evidence that disruption of the AIZ interneuron causes thermophilic bias at any temperature. Conclusions: We conclude that laser surgical analysis of the thermotactic circuit does not support a current model in which AFD opposes cryophilic bias by generating thermophilic bias. Our data supports a model in which a mechanism for generating cryophilic bias is gated by the AFD neurons.
    D. Souza Correa, P. Tayalia, E. Mazur, and C. R. Mendonca. 2006. “Complex microstructures fabricated via two-photon absorption polymerization.” In . Macro 2006 - 41st Symposium on Macromolecules. Publisher's VersionAbstract
    Using acrylic resin and Lucirin TPO-L as photoinitiator, we fabricated complex microstructures via the process of two photon absorption (2PA) polymerization. We measured the 2PA cross-section of Lucirin TPO-L, which is the parameter responsible for the nonlinear process, and the value found is among the ones reported in the literature for common photoinitiators. We also carried out quantum chemistry calculation in order to correlate the nonlinear optical properties of this photoinitiator to its molecular structure.
    C. R. Mendonca, D. S. Correa, T. Baldacchini, P. Tayalia, and E. Mazur. 2006. “A novel photoinitiator for microfabrication via two-photon polymerization.” In . CLEO 2006. Publisher's VersionAbstract
    We measured the two-photon absorption cross-section of the photoinitiator Lucirin TPO-L and fabricated complex microstructures using this photoinitiator in an acrylate resin. Using quantum chemistry calculations, we relate the nonlinear optical properties of the photoinitiator it to its molecular structure.
    M. A. Sheehy, B. R. Tull, C. M. Friend, and E. Mazur. 2006. “Chalcogen doping of silicon via intense femtosecond-laser irradiation.” Mat. Sci. Eng. B, 137, Pp. 289–294. Publisher's VersionAbstract
    We have previously shown that doping silicon with sulfur via femtosecond- laser irradiation leads to near-unity absorption of radiation from ultraviolet wavelengths to below band gap short-wave infrared wavelengths. Here, we demonstrate that doping silicon with two other group VI elements (chalcogens), selenium and tellurium, also leads to near-unity broadband absorption. A powder of the chalcogen dopant is spread on the silicon substrate and irradiated with femtosecond-laser pulses. We examine and compare the resulting morphology, optical properties, and chemical composition for each chalcogen-doped substrate before and after thermal annealing. Thermal annealing reduces the absorption of below-band gap radiation by an amount that correlates with the diffusivity of the chalcogen dopant used to make the sample. We propose a mechanism for the absorption of below band gap radiation based on defects in the lattice brought about by the femtosecond laser irradiation and the presence of a supersaturated concentration of chalcogen dopant atoms. The selenium and tellurium doped samples show particular promise for use in infrared photodetectors as they retain most of their infrared absorptance even after thermal annealinga necessary step in many semiconductor device manufacturing processes.

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