M. Shen, J. E. Carey, C. H. Crouch, M. Kandyla, H.A. Stone, and E. Mazur. 2008. “High-density regular arrays of nanometer-scale rods formed on silicon surfaces via femtosecond laser irradiation in water.” Nano Leters, 8, Pp. 2087–2091. Publisher's VersionAbstract
    We report on the formation of high-density regular arrays of nanometer-scale rods using femtosecond laser irradiation of a silicon surface immersed in water. The resulting surface exhibits both micrometer-scale and nanometer-scale structures. The micrometer-scale structure consists of spikes of 5-10 µm width, which are entirely covered by nanometer-scale rods that are roughly 50 nm wide and that protrude perpendicularly from the micrometer-scale spikes. The formation of the nanometer-scale rods involves several processes: refraction of laser light in highly excited silicon, interference of scattered and refracted light, rapid cooling in water, and capillary instabilities.
    A. Serpengüzel, A. Kurt, I. Inanc, J. E. Carey, and E. Mazur. 2008. “Luminescence of black silicon.” J. Nanophoton., 2, Pp. 021770–9. Publisher's VersionAbstract
    Room temperature visible and near-infrared photoluminescence from black silicon has been observed. The black silicon is manufactured by shining femtosecond laser pulses on silicon wafers in air, which were later annealed in vacuum. The photoluminescence is quenched above 120 K due to thermalization and competing nonradiative recombination of the carriers. The photoluminescence intensity at 10K depends sublinearly on the excitation laser intensity confirming band tail recombination at the defect sites.
    N. Lasry. 2008. “Clickers or Flashcards: Is There Really a Difference?” Phys. Teacher, 46, Pp. 242–244. Publisher's VersionAbstract
    A growing number of physics teachers are currently turning to instructional technologies such as wireless handheld response systemscolloquially called clickers. Two possible rationales may explain the growing interest in these devices. The first is the presumption that clickers are more effective instructional instruments. The second rationale is somewhat reminiscent of Martin Davis' declaration when purchasing the Oakland Athletics: As men get older, the toys get more expensive. Although personally motivated by both of these rationales, the effectiveness of clickers over inexpensive low-tech flashcards remains questionable. Thus, the first half of this paper presents findings of a classroom study comparing the differences in student learning between a Peer Instruction group using clickers and a Peer Instruction group using flashcards. Having assessed student learning differences, the second half of the paper describes differences in teaching effectiveness between clickers and flashcards.
    N. Lasry, E. Mazur, and J. Watkins. 2008. “Peer Instruction: From Harvard to Community Colleges.” Am. J. Phys., 76, Pp. 1066–1069. Publisher's VersionAbstract
    We compare the effectiveness of a first implementation of peer instruction (PI) in a two-year college with the first PI implementation at a top-tier four-year research institution. We show how effective PI is for students with less background knowledge and what the impact of PI methodology is on student attrition in the course. Results concerning the effectiveness of PI in the college setting replicate earlier findings: PI-taught students demonstrate better conceptual learning and similar problem-solving abilities than traditionally taught students. However, not previously reported are the following two findings: First, although students with more background knowledge benefit most from either type of instruction, PI students with less background knowledge gain as much as students with more background knowledge in traditional instruction. Second, PI methodology is found to decrease student attrition in introductory physics courses at both four-year and two-year institutions.
    R. R. Gattass and E. Mazur. 2008. “Femtosecond laser micromachining in transparent materials.” Nat. Phot., 2, Pp. 219–225. Publisher's VersionAbstract
    Femtosecond laser micromachining can be used either to remove materials or to change a material's properties, and can be applied to both absorptive and transparent substances. Over the past decade, this technique has been used in a broad range of applications, from waveguide fabrication to cell ablation. This review describes the physical mechanisms and the main experimental parameters involved in the femtosecond laser micromachining of transparent materials, and important emerging applications of the technology.
    C. R. Mendonca, D. S. Correa, T. Baldacchini, P. Tayalia, and E. Mazur. 2008. “Two-photon absorption spectrum of Lucirin TPO-L.” Appl. Phys. A, 90, Pp. 633–636. Publisher's VersionAbstract
    Two-photon absorption induced polymerization provides a powerful method for the fabrication of intricate three-dimensional microstructures. Recently, Lucirin TPO-L was shown to be a photoinitiator with several advantageous properties for two-photon induced polymerization. Here we measure the two-photon absorption cross-section spectrum of Lucirin TPO-L, which presents a maximum of 1.2 GM at 610 nm. Moreover, from the two-photon absorption spectrum we determined that Lucirin TPO-L radical quantum yield is independent of the wavelength. Despite its small two-photon absorption cross- section, it is possible to fabricated excellent microstructures by two-photon polymerization microfabrication due to the high polymerization quantum yield (0.99) of Lucirin TPO-L. These results show that optimization of the two-photon absorption cross-section is not the only factor to be considered when searching for new photoinitiators for microfabrication via two-photon absorption.
    P. Tayalia, C. R. Mendonca, T. Baldacchini, D. J. Mooney, and E. Mazur. 2008. “3D cell migration studies using two-photon engineered polymer scaffolds.” Advanced Materials, 20, Pp. 4494–4498. Publisher's VersionAbstract
    We use two-photon polymerization to fabricate 3D scaffolds with precise control over pore size and shape for studying cell migration in 3D. These scaffolds allow movement of cells in all directions. The fabrication, imaging, and quantitative analysis method developed here can be used to do systematic cell studies in 3D.
    G. Thomas Svacha. 2008. “Nanoscale nonlinear optics using silica nanowires”. Publisher's VersionAbstract
    We have fabricated silica fibers with diameters less than one micrometer and molecularly smooth side walls. We modeled the waveguide properties and demonstrated low-loss light propagation over tens of millimeters, sufficient for microphotonic device applications. We manipulated silica nanowires into geometries to demonstrate waveguiding around tight bends as small as 5 micrometers, evanescent coupling, and wavelength filtering as a ring resonator. The linear waveguiding properties produced by the large index contrast between silica and air yield a tight confinement of the mode, which, when combined with the high electric field intensity in an ultrashort laser pulse, can produce significant nonlinear effects over lengths of about one millimeter. We studied nonlinear optical properties by observing the spectral broadening as a probe for the diameter-dependent nonlinearity of the silica nanowire. Our measurements confirmed the dependence of the generated spectra on the theoretically calculated effective nonlinearity and diameter-dependent dispersion. Our results reveal a diameter range for silica nanowires with an enhanced nonlinearity which may be employed in nonlinear devices. We fabricate a nonlinear Sagnac interferometer using silica nanowires for optical switching and discuss possibilities for optical logic. Our results confirm light-by-light modulation, with pulse energies less than a couple of nJ. Combining top-down and bottom-up fabrication techniques, we use tapered silica fibers to couple light directly into the waveguiding modes of ZnO nanowires. We experimentally confirm simulations for the coupling efficiencies and propagation of higher order modes. We also excite ZnO nanowires with ultrashort laser pulses and compare the spectrum transmitted along the nanowire waveguide with the spectrum at the excitation spot. Our results show a shifting of the band edge that indicates a local heating of the nanowire by a few hundred degrees, which is confirmed by finite-element simulation. Finally, we discuss the outlook for nanoscale nonlinear optical devices.
    C. R. Mendonca, T. Shih, and E. Mazur. 2008. “Femtosecond laser waveguide micromachining of PMMA films with azoaromatic chromophores.” Opt. Express, 16, Pp. 200–206. Publisher's VersionAbstract
    We report on the femtosecond-laser micromachining of poly(methyl methacrylate) (PMMA) films doped with nonlinear azoaromatic chromophores: Disperse Red 1, Disperse Red 13 and Disperse Orange 3. We study the conditions for controlling chromophore degradation during the micromachining of PMMA doped with each chromophore. Furthermore, we successfully used fs-micromachining to fabricate optical waveguides within a bulk sample of PMMA doped with these azochromophores.
    S. Kudryashov, M. Kandyla, C. A. D. Roeser, and E. Mazur. 2007. “Intraband and interband optical deformation potentials in femtosecond-laser excited alpha-Te.” Phys. Rev. B, 75, Pp. 085207–. Publisher's VersionAbstract
    We calculated the intraband and interband optical deformation potentials for semiconducting alpha-Te from experimental data of the band gap shrinkage and softening of the A1-optical phonon mode in response to femtosecond laser excitation. These potentials were obtained by applying first- and second-order perturbation theory to the Frlich Hamiltonian describing the carrier-phonon interaction. The intraband optical deformation potential is considerably smaller than previously estimated values; there are no previously reported values for the interband optical deformation potential.
    A. Heisterkamp, J. Baumgart, I. Zaharieva Maxwell, A. Ngezahayo, E. Mazur, and H. Lubatschowski. 2007. “Fs-Laser Scissors for Photobleaching, Ablation in Fixed Samples and Living Cells, and Studies of Cell Mechanics.” In Laser Manipulation of Cells and Tissues, edited by Michael Berns and Karl Greulich, Pp. 293–307. Academic Press. Publisher's VersionAbstract
    The use of ultrashort laser pulses for microscopy has steadily increased over the past years. In this so-called multiphoton microscopy, laser pulses with pulse duration around 100 femtoseconds (fs) are used to excite fluorescence within the samples. Due to the high peak powers of fs lasers, the absorption mechanism of the laser light is based on nonlinear absorption. Therefore, the fluorescence signal is highly localized within the bulk of biological materials, similar to a confocal microscope. However, this nonlinear absorption mechanism can not only be used for imaging but for selective alteration of the material at the laser focus: The absorption can on one hand lead to the excitation of fluorescent molecules of fluorescently tagged cells by the simultaneous absorption of two or three photons or on the other hand, in case of higher order processes, to the creation of free-electron plasmas and, consequently, plasma-mediated ablation. Typical imaging powers are in the range of tens of milliwatts using 100-fs pulses at a repetition rate of 80-90 MHz, while pulse energies needed for ablation powers are as low as a few nanojoules when using high numerical aperture microscope objectives for focusing the laser radiation into the sample. Since the first demonstration of this technique, numerous applications of fs lasers have emerged within the field of cellular biology and microscopy. As the typical wavelengths of ultrashort laser systems lie in the near infrared between 800 and 1000 nm, high penetration depth can be achieved and can provide the possibility of imaging and manipulating the biological samples with one single laser system.
    B. R. Tull. 2007. “Femtosecond Laser Ablation of Silicon: Nanoparticles, Doping and Photovoltaics”. Publisher's VersionAbstract
    In this thesis, we investigate the irradiation of silicon, in a background gas of near atmospheric pressure, with intense femtosecond laser pulses at energy densities exceeding the threshold for ablation (the macroscopic removal of material). We study the resulting structure and properties of the material ejected in the ablation plume as well as the laser irradiated surface itself. The material collected from the ablation plume is a mixture of single crystal silicon nanoparticles and a highly porous network of amorphous silicon. The crystalline nanoparticles form by nucleation and growth; the amorphous material has smaller features and forms at a higher cooling rate than the crystalline particles. The size distribution of the crystalline particles suggests that particle formation after ablation is fundamentally different in a background gas than in vacuum. We also observe interesting structures of coagulated particles such as straight lines and bridges. The laser irradiated surface exhibits enhanced visible and infrared absorption of light when laser ablation is performed in the presence of certain elements–-either in the background gas or in a film on the silicon surface. To determine the origin of this enhanced absorption, we perform a comprehensive annealing study of silicon samples irradiated in the presence of three different elements (sulfur, selenium and tellurium). Our results support that the enhanced infrared absorption is caused by a high concentration of dopants dissolved in the lattice. Thermal annealing reduces the infrared absorptance of each doped sample at the same rate that dopants diffuse from within the polycrystalline grains in the laser irradiated surface layer to the grain boundaries. Lastly, we measure the photovoltaic properties of the laser irradiated silicon as a function of several parameters: annealing temperature, laser fluence, background gas, surface morphology and chemical etching. We explore the concept of using thin silicon films as the irradiation substrate and successfully enhance the visible and infrared absorption of films < 2 micrometers thick. Our results suggest that the incorporation of a femtosecond laser modified region into a thin film silicon device could greatly enhance its photovoltaic efficiency.
    T. R. Polte, M. Shen, J. Karavitis, M. Montoya, J. Pendse, S. Xia, E. Mazur, and D. E. Ingber. 2007. “Nanostructured magnetizable materials that switch cells between life and death.” Biomaterials, 28, Pp. 2783–2790. Publisher's VersionAbstract
    Development of biochips containing living cells for biodetection, drug screening and tissue engineering applications is limited by a lack of reconfigurable material interfaces and actuators. Here we describe a new class of nanostructured magnetizable materials created with a femtosecond laser surface etching technique that function as multiplexed magnetic field gradient concentrators. When combined with magnetic microbeads coated with cell adhesion ligands, a microarray of virtual adhesive islands that can support cell attachment, resist cell traction forces and maintain cell viability. A cell death (apoptosis) response can then be actuated on command by removing the applied magnetic field, thereby causing cell retraction, rounding and detachment. This simple technology may be used to create reconfigurable interfaces that allow users to selectively discard contaminated or exhausted cellular sensor elements, and to replace them with new living cellular components for continued operation in future biomedical microdevices and biodetectors. Keywords: magnetic particles, magnetic gradient concentrator, culture substrate, apoptosis, mechanical force, cell shape
    C. R. Mendonca, S. Orlando, G. Cosendey, M. T. Winkler, and E. Mazur. 2007. “Femtosecond laser micromaching in the conjugated polymer MEH- PPV.” Applied Surface Science, 254, Pp. 1135–1139. Publisher's VersionAbstract
    Femtosecond-laser micromachining of poly[2-methoxy-5-(2'1/2- ethylhexyloxy)- p-phenylene vinylene] (MEH-PPV) films is investigated using 130-fs pulses at 800-nm from a laser oscillator operating at 76-MHz repetition rate. We investigate the effect of pulse energy and translation speed on the depth and morphology of the micromachined regions. We quantified the MEH- PPV photobleaching induced by the fs-laser, and the conditions in which the emission of MEH-PPV is preserved after the micromaching.
    M. Kandyla, T. Shih, and E. Mazur. 2007. “Femtosecond dynamics of the laser-induced solid-to-liquid phase transition in aluminum.” Phys. Rev. B, 75, Pp. 214107-1–7. Publisher's VersionAbstract
    We present femtosecond time-resolved measurements of the reflectivity of aluminum during the laser-induced solid-to-liquid phase transition over the spectral range 1.73.5 eV. Previous optical and electron diffraction studies have shown discrepancies on the order of picoseconds in the timescale of the solid- to-liquid phase change. As a result, it is not clear if the transition mechanism is thermal or non-thermal. Our experiments conclusively show that this transition is a thermal process mediated through the transfer of heat from the photoexcited electronic population to the lattice. Our findings agree with the results of the electron diffraction study and rule out the non-thermal mechanism proposed by the optical study.
    M. Kandyla, T. Shih, and E. Mazur. 2007. “Femtosecond dynamics of the laser-induced solid-to-liquid phase transition in aluminum.” In . (CLEO) Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Phototnics Applications Systems Technologies 2007 Technical Digest. Publisher's VersionAbstract
    We present femtosecond time-resolved broadband measurements of the reflectivity of aluminum during the laser-induced solid-to-liquid phase transition. Our experiments show that this transition is a thermal process, settling an existing controversy.
    S. Kudryashov, M. Kandyla, C. A. D. Roeser, and E. Mazur. 2007. “Transient picometer atomic displacements in a-Te photoexcited by femtosecond laser pulses.” In . International conference on coherent and nonlinear optics, Proceedings of SPIE Vol. 6727. Publisher's VersionAbstract
    Subpicosecond, picometer atomic displacements in α-Те photoexcited by single femtosecond laser pulses have been measured by means of time-resolved optical reflectometry revealing threshold- like coherent quantum emission of single softened fully symmetrical optical A1-phonons and demonstrating absolute detection capability of this technique in studies of coherent phonon dynamics in solids.
    M. Kandyla, T. Shih, and E. Mazur. 2007. “Turning Aluminum Liquid in Picoseconds.” In Optics and Photonics News, 18: Pp. 44–. Publisher's VersionAbstract
    The interactions between the electrons and the lattice in solids define many basic properties of these materials. Exciting the electrons in a solid with intense ultrashort laser pulses is one way to induce non-equilibrium processes, observe the resulting dynamics and obtain direct information on electron-lattice interactions. An ultrashort laser pulse can rapidly heat electrons to temperatures on the order of 103 K, while leaving the lattice near room temperature.
    C. R. Mendonca, T. Baldacchini, P. Tayalia, and E. Mazur. 2007. “Reversible birefringence in microstructures fabricated by two-photon absorption polymerization.” J. Appl. Phys., 103, Pp. 013109-1–013109-4. Publisher's VersionAbstract
    This paper reports the fabrication of birefringent microstructures using two- photon absorption polymerization. The birefringence is caused by a light-driven molecular orientation of azoaromatic molecules (Disperse Red 13) upon excitation with an Ar+ laser at 514.5 nm. For an azoaromatic dye content of 1% by weight we obtain a birefringence of 5x10-5. This birefringence can be completely erased by overwriting the test spot with circularly polarized laser light or by heating the sample. Our results open the door to the development of new applications in optical data storage, wave guiding, and optical circuitry.
    T. Shih, R. R. Gattass, C. R. Mendonca, and E. Mazur. 2007. “Faraday rotation in femtosecond laser micromachined waveguides.” Opt Express, 15, Pp. 5809–5814. Publisher's VersionAbstract
    We demonstrate magneto-optic switching in femtosecond-laser micromachined waveguides written inside bulk terbium-doped Faraday glass. By measuring the polarization phase shift of the light as a function of the applied magnetic field, we find that there is a slight reduction in the effective Verdet constant of the waveguide compared to that of bulk Faraday glass. Electron Paramagnetic Resonance (EPR) measurements confirm that the micromachining leaves the concentration of the terbium ions that are responsible for the Faraday effect virtually unchanged.