S. Kumar, I. Zaharieva Maxwell, A. Heisterkamp, T. R. Polte, T. Lele, M. Salanga, E. Mazur, and D. E. Ingber. 2006. “Viscoelastic retraction of single living stress fibers and its impact on cell shape, cytoskeletal organization, and extracellular matrix mechanics.” Biophys. J., 90, Pp. 3762–3773. Publisher's VersionAbstract
    Cells change their form and function by assembling actin stress fibers at their base and exerting traction forces on their extracellular matrix (ECM) adhesions. Individual stress fibers are thought to be actively tensed by the action of actomyosin motors and to function as elastic cables that structurally reinforce the basal portion of the cytoskeleton; however, these principles have not been directly tested in living cells, and their significance for overall cell shape control is poorly understood. Here we combine a laser nanoscissor, traction force microscopy, and fluorescence photobleaching methods to confirm that stress fibers in living cells behave as viscoelastic cables that are tensed through the action of actomyosin motors, to quantify their retraction kinetics in situ, and to explore their contribution to overall mechanical stability of the cell and interconnected ECM. These studies reveal that viscoelastic recoil of individual stress fibers after laser severing is partially slowed by inhibition of Rho-associated kinase and virtually abolished by direct inhibition of myosin light chain kinase. Importantly, cells cultured on stiff ECM substrates can tolerate disruption of multiple stress fibers with negligible overall change in cell shape, whereas disruption of a single stress fiber in cells anchored to compliant ECM substrates compromises the entire cellular force balance, induces cytoskeletal rearrangements, and produces ECM retraction many microns away from the site of incision; this results in large-scale changes of cell shape (> 5% elongation). In addition to revealing fundamental insight into the mechanical properties and cell shape contributions of individual stress fibers and confirming that the ECM is effectively a physical extension of the cell and cytoskeleton, the technologies described here offer a novel approach to spatially map the cytoskeletal mechanics of living cells on the nanoscale.
    B. R. Tull, J. E. Carey, M. A. Sheehy, C. M. Friend, and E. Mazur. 2006. “Formation of silicon nanoparticles and web-like aggregates by femtosecond laser ablation in a background gas.” Appl. Phys. A, 83, Pp. 341–346. Publisher's VersionAbstract
    We show that the mechanism of nanoparticle formation during femtosecond laser ablation of silicon is affected by the presence of a background gas. Femtosecond laser ablation of silicon in a H2 or H2S background gas yields a mixture of crystalline and amorphous nanoparticles. The crystalline nanoparticles form via a thermal mechanism of nucleation and growth. The amorphous material has smaller features and forms at a higher cooling rate than the crystalline nanoparticles. The background gas also results in the suspension of plume material in the gas for extended periods, resulting in the formation (on a thin film carbon substrate) of unusual aggregated structures including nanoscale webs that span tears in the film. The presence of a background gas provides additional control of the structure and composition of the nanoparticles during short pulse laser ablation. PACS 81.16.-c
    R. R. Gattass, L. R. Cerami, and E. Mazur. 2006. “Micromachining of bulk glass with bursts of femtosecond laser pulses at variable repetition rates.” Opt. Exp., 14, Pp. 5279–5284. Publisher's VersionAbstract
    Oscillator-only femtosecond laser micromachining enables the manufacturing of integrated optical components with circular transverse profiles in transparent materials. The circular profile is due to diffusion of heat accumulating at the focus. We control the heat diffusion by focusing bursts of femtosecond laser pulses at various repetition rates into sodalime glass. We investigate the effect the repetition rate and number of pulses have on the size of the resulting structures. We identify the combinations of burst repetition rate and number of pulses within a burst for which accumulation of heat occurs. The threshold for heat accumulation depends on the number of pulses within a burst. The burst repetition rate and the number of pulses within a burst provide convenient control of the morphology of structures generated with high repetition rate femtosecond micromachining.
    R. R. Gattass. 2006. “Femtosecond-laser interactions with transparent materials: applications in micromachining and supercontinuum generation”. Publisher's VersionAbstract
    Femtosecond-lasers represent a source for electric field pulses which can have field intensities approaching and even exceeding the atomic binding field. For an electric field of this order, the polarization response of the medium changes from linear to nonlinear. For transparent media, depending on the field intensity, the laser pulse is either nonlinearly absorbed or, at lower field intensities, modifies the medium as it propagates, modulating its own spectrum. Nonlinear absorption has direct applications to the micromachining of photonic devices. We discuss the effect of different laser parameters such as the repetition rate and number of pulses in the femtosecond-laser generated structures. Additionally, we investigate the transmission losses, bending loss, supported electromagnetic modes and index of refraction profiles of optical interconnects fabricated through femtosecond micromachining. This dissertation also covers experiments on the propagation of femtosecond pulses confined in structures whose diameter is below the wavelength of the incident light, silica based nanowires. We demonstrate the possibility of making sub-micrometer diameter silica fibers and discuss the effects of their diameter-dependent dispersion and enhanced nonlinearity for femtosecond laser pulse propagation. The nonlinearity and dispersion are presented as a function of the nanowire diameter and our results confirm the theoretical predictions for the enhancement of the nonlinearity and the effect of high dispersion. Both technologies, nanowires and femtosecond manufactured waveguides, represent alternatives for photonic circuits interconnects, but at nanometer and micrometer scales, respectively.
    L. Tong, R. R. Gattass, I. Zaharieva Maxwell, J. B. Ashcom, and E. Mazur. 2006. “Optical loss measurements in femtosecond laser written waveguides in glass.” Opt. Commun., 259, Pp. 626–630. Publisher's VersionAbstract
    The optical loss is an important parameter for waveguides used in integrated optics. We measured the optical loss in waveguides written in silicate glass slides with high repetition-rate (MHz) femtosecond laser pulses. The average transmission loss of straight waveguides is about 0.3 dB/mm at a wavelength of 633 nm and 0.05 dB/mm at a wavelength of 1.55 m. The loss is not polarization dependent and the waveguides allow a minimum bending radius of 36 mm without additional loss. The average numerical aperture (NA) of the waveguides is 0.065 at a wavelength of 633 nm and 0.045 at a wavelength of 1.55 m. In straight waveguides more than 90% of the transmission loss is due to scattering.
    G. Thomas Svacha, E. Mazur, and L. Tong. 2005. “Nanowiring Light.” In . Optical Fiber Communication Conference 2005. Publisher's VersionAbstract
    Recent advances in the fabrication and manipulation of sub-wavelength optical fibers provide new methods for building chemical and biological sensors, generating supercontinuum light by nonlinear pulse propagation, and constructing microphotonic components and devices.
    L. Tong, J. Lou, R. R. Gattass, S. He, X. Chen, L. Liu, and E. Mazur. 2005. “Assembly of silica nanowires on silica aerogels for microphotonic devices.” Nano Lett., 5, Pp. 259–262. Publisher's VersionAbstract
    We report on the assembly of low-loss silica nanowires into functional microphotonics devices on a low-index non-dissipative silica aerogel substrate. Using this all-silica technique, we fabricated linear waveguides, waveguide bends and branch couplers. The devices are significantly smaller than existing comparable devices and have low optical loss, indicating that the all-silica technique presented here has great potential for future applications in optical communication, optical sensing, and high-density optical integration.
    A. Heisterkamp, I. Zaharieva Maxwell, E. Mazur, J. M. Underwood, J. A. Nickerson, S. Kumar, and D. E. Ingber. 2005. “Pulse energy dependence of subcellular dissection by femtosecond laser pulses.” Opt. Express, 13, Pp. 3690–3696. Publisher's VersionAbstract
    Precise dissection of cells with ultrashort laser pulses requires a clear understanding of how the onset and extent of ablation (i.e., the removal of material) depends on pulse energy. We carried out a systematic study of the energy dependence of the plasma-mediated ablation of fluorescently-labeled subcellular structures in the cytoskeleton and nuclei of fixed endothelial cells using femtosecond, near-infrared laser pulses focused through a high- numerical aperture objective lens (1.4 NA). We find that the energy threshold for photobleaching lies between 0.9 and 1.7 nJ. By comparing the changes in fluorescence with the actual material loss determined by electron microscopy, we find that the threshold for true material ablation is about 20% higher than the photobleaching threshold. This information makes it possible to use the fluorescence to determine the onset of true material ablation without resorting to electron microscopy. We confirm the precision of this technique by severing a single microtubule without disrupting the neighboring microtubules, less than 1 m away.
    L. Tong, J. Lou, Z. Ye, G. Thomas Svacha, and E. Mazur. 2005. “Self-modulated taper drawing of silica nanowires.” Nanotechnology, 16, Pp. 1445–1448. Publisher's VersionAbstract
    We report a self-modulated taper-drawing process for fabricating silica nanowires with diameters down to 20 nm. Long amorphous silica nanowires obtained with this top-down approach present extraordinary uniformities that have not been achieved by any other means. The measured sidewall roughness of the wires goes down to the intrinsic value of 0.2 nm, along with a diameter uniformity better than 0.1%. The wires also show high strength and pliability for patterning under optical microscopes. The ability to prepare and manipulate highly uniform silica nanowires map open up new opportunities for studying and using low-dimensional silica material on a nanometer scale.
    G. A. Morris, L. Branum-Martin, N. Harshman, S. D. Baker, E. Mazur, S. Nath Dutta, T. Mzoughi, and V. McCauley. 2005. “Testing the test: Item response curves and test quality.” Am. J. Phys., 74, Pp. 449–453. Publisher's VersionAbstract
    We present a simple technique for evaluating multiple-choice questions and their answer choices beyond the usual measures of difficulty and the effectiveness of distractors. The technique involves the construction and qualitative consideration of item response curves (IRCs) and is based upon Item Response Theory from the field of education measurement. Item response curves relate the percentage of students who select each possible answer choice to overall ability level. To demonstrate the technique, we apply IRC analysis to three questions from the Force Concept Inventory (FCI). IRC analysis allows us to characterize qualitatively whether these questions are efficient, where efficient is defined in terms of the construction, performance, and discrimination of a question and its answer choices. Such analysis can be useful both in the development of future multiple-choice examination questions and in the development of a better understanding of results from existing diagnostic instruments such as the FCI.
    M. A. Sheehy, L. Winston, J. E. Carey, C. M. Friend, and E. Mazur. 2005. “Role of the background gas in the morphology and optical properties of laser-microstructured silicon.” Chem. Mater., 17, Pp. 3582–3586. Publisher's VersionAbstract
    We irradiate silicon with a train of femtosecond pulses in the presence of SF6, H2S, H2, SiH4, and a mixture of Ar and SF6 in order to analyze the role of the background gas in determining the morphology and the optical properties of the resultant surfaces. We discuss factors that affect the surface morphology created during irradiation and show that the presence of sulfur in these gases is important in creating sharp microstructures. We also show that the presence of sulfur is necessary to create the near-unity absorptance for both above-band and below-band gap radiation (0.25 2.5 micrometer) by silicon; only samples with sulfur concentrations higher than 0.6% absorb 95% for above-band gap radiation and have a flat, featureless absorptance of 90% for below-band gap radiation. KEYWORDS silicon, infrared absorptance, laser materials processing, microstructures, sulfur doping, femtosecond laser irradiation, RBS, elemental semiconductors
    C. A. D. Roeser and E. Mazur. 2005. “Light-matter interactions on the femtosecond time scale.” In Frontiers of Optical Spectroscopy: Investigating Extreme Physical Conditions with Advanced Optical Techniques, edited by B. Di Bartolo and O. Forte, Pp. 29–54. Kluwer Academic Publishers. Publisher's VersionAbstract
    The subject of electromagnetism in the presence of matter is both extensively studied and rich in diverse phenomena. It spans such topics as the quantization of the electromagnetic field to the semiclassical treatment of lightmatter interactions to the derivation of the Fresnel reflectivity formulas. Interest in femtosecond optics is rooted in nonlinear optical phenomena and in the complex electron and lattice dynamics that occur in a material following intense ultrashort-pulse irradiation. The experiments we discuss are concerned mainly with the latter and lie at the crossroad of femtosecond optics and materials science, so-called ultrafast materials science.
    M. Vogt and E. Mazur. 2005. “The Interactive Learning Toolkit.” Phys. Teach., 43, Pp. 398–398. Publisher's VersionAbstract
    Peer Instruction (PI) and Just-in-Time-Teaching (JITT) have been adopted widely in introductory science teaching because they have shown to increase both conceptual understanding of the material and problem solving skills. One of the main implementation hurdles, however, is the effort that goes into the preparation of questions and the management of student responses. To overcome this hurdle we have developed a number of Web-based resources.
    A. Heisterkamp, I. Zaharieva Maxwell, S. Kumar, J. M. Underwood, J. A. Nickerson, D. E. Ingber, and E. Mazur. 2005. “Nanosurgery in live cells using ultrashort laser pulses.” In . SPIE Photonics West. Publisher's VersionAbstract
    We selectively disrupted the cytoskeletal network of fixed and live bovine capillary endothelial cell using ultrashort laser pulses. We image the microtubules in the cytoskeleton of the cultured cells using green fluorescent protein. The cells are placed on a custom-built inverted fluorescence microscope setup, using a 1.4 NA oil-immersion objective to both image the cell and focus the laser radiation into the cell samples. The laser delivers 100-fs laser pulses centered at 800 nm at a repetition rate of 1 kHz; the typical energy delivered at the sample is 15nJ. The fluorescent image of the cell is captured with a CCD-camera at one frame per second. To determine the spatial discrimination of the laser cutting we ablated microtubules and actin fibers in fixed cells. At pulse energies below 2 nJ we obtain an ablation size of 200 nm. This low pulse energy and high spatial discrimination enable the application of this technique to live cells. We severed a single microtubule inside the live cells without affecting the cells viability. The targeted microtubule snaps and depolymerizes after the cutting. This nanosurgery technique will further the understanding and modeling of stress and compression in the cytoskeletal network of live cells.
    I. Zaharieva Maxwell, S. H. Chung, and E. Mazur. 2005. “Nanoprocessing of subcellular targets using femtosecond laser pulses.” Med. Laser Appl., 20, Pp. 193–200. Publisher's VersionAbstract
    In this paper we review the work done in our laboratory on femtosecond laser dissection within single cells and living organisms. Precise dissection of biological material with ultrashort laser pulses requires a clear understanding of the pulse-energy dependence of the onset and extent of plasma-mediated ablation (i.e., the removal of material). We carried out a systematic study of the energy dependence of the plasma-mediated ablation of fluorescently-labeled subcellular structures in the cytoskeleton and in nuclei of fixed endothelial cells using femtosecond, near- infrared laser pulses focused through a high- numerical aperture objective lens (1.4 NA). We performed laser nanosurgery in live cells, where we ablated a single mitochondrion and severed cytoskeletal filaments without compromising the cell membrane or the cells viability. We also cut dendrites in living C. elegans without affecting the neighboring neurons. This nanoprocessing technique enables non-invasive manipulation of the structural machinery of cells and tissues down to several-hundred- nanometer resolution.
    N. Shen, D. Datta, C. B. Schaffer, P. LeDuc, D. E. Ingber, and E. Mazur. 2005. “Ablation of cytoskeletal filaments and mitochondria in cells using a femtosecond laser nanoscissor.” Mechanics and Chemistry of Biosystems, 2, Pp. 17–26. Publisher's VersionAbstract
    Analysis of cell regulation requires methods for perturbing molecular processes within living cells with spatial discrimination on the nanometer- scale. We present a technique for ablating molecular structures in living cells using low-repetition rate, low-energy femtosecond laser pulses. By tightly focusing these pulses beneath the cell membrane, we ablate cellular material inside the cell through nonlinear processes. We selectively removed sub- micrometer regions of the cytoskeleton and individual mitochondria without altering neighboring structures or compromising cell viability. This nanoscissor technique enables non-invasive manipulation of the structural machinery of living cells with several-hundred-nanometer resolution. Using this approach, we unequivocally demonstrate that mitochondria are structurally independent functional units, and do not form a continuous network as suggested by some past studies. Keywords: Nanoscissor; nanosurgery; femtosecond laser; photodisruption; cytoskeleton; mitochondria
    M. Kamata, M. Obara, R. R. Gattass, L. R. Cerami, and E. Mazur. 2005. “Optical vibration sensor fabricated by femtosecond laser micromachining.” Appl. Phys. Lett., 87, Pp. 051106-1–051106-3. Publisher's VersionAbstract
    We fabricated an optical vibration sensor using a high-repetition rate femtosecond laser oscillator. The sensor consists of a single straight waveguide written across a series of three pieces of glass. The central piece is mounted on a suspended beam to make it sensitive to mechanical vibration, acceleration, or external forces. Displacement of the central piece is detected by measuring the change in optical transmission through the waveguide. The resulting sensor is small, simple, and requires no alignment. The sensor has a linear response over the frequency range 20 Hz 2 kHz, can detect accelerations as small as 0.01 m/s2, and is nearly temperature independent.
    J. E. Carey, C. H. Crouch, M. Shen, and E. Mazur. 2005. “Visible and near-infrared responsivity of femtosecond-laser microstructured silicon photodiodes.” Opt. Lett., 30, Pp. 1773–1775. Publisher's VersionAbstract
    We investigated the current-voltage characteristics and responsivity of photodiodes fabricated with silicon that was microstructured using femtosecond-laser pulses in a sulfur-containing atmosphere. The photodiodes we fabricated have a broad spectral response ranging from the visible to the near-infrared (4001600 nm). The responsivity depends on substrate doping, microstructuring fluence, and annealing temperature. We obtained room- temperature responsivities as high as 100 A/W at 1064 nm, two orders of magnitude higher than standard silicon photodiodes. For wavelengths below the band gap, we obtained responsivities as high as 50 mA/W at 1330 nm and 35 mA/W at 1550 nm.
    R. R. Gattass, L. R. Cerami, and E. Mazur. 2005. “Optical waveguide fabrication for integrated photonic devices.” In . Nano photonics and functional device technology. Publisher's VersionAbstract
    The dynamic nature of future optical networks requires high levels of integration, fast response times, and adaptability of the optical components. Laser micromachining circumvents the limitations of planar integration, allowing both three-dimensional integration and dense packaging of optical devices without alignment requirements. Femtosecond micromachining enables the analog of circuit printing by wiring light between photonic devices in addition to printing the actual photonic device into a single or multiple substrates. Femtosecond laser oscillator-only micromachining has several advantages over amplified femtosecond laser micromachining: easy control over the size of the structures without changing focusing, polarization-independent structures, lower initial investment cost and higher-speed manufacturing. In this paper we review recent results obtained in the field of femtosecond micromachining. Keywords: Femtosecond, micromachining, nonlinear absorption.