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Publications

    J. B. Ashcom and E. Mazur. 2001. “Femtosecond laser-induced microexplosions in transparent materials.” In . LEOS 2001. Publisher's VersionAbstract
    By focusing femtosecond laser pulses with high numerical-aperture microscope objectives, we micromachine optical glass using energies that are in the range of modern laser oscillators. When a femtosecond laser pulse is tightly focused inside a transparent material, energy deposition occurs only at the focus, where the laser intensity is high enough to cause absorption through nonlinear processes. When enough energy is deposited, the material is damaged and a localized change in the index of refraction is produced. By scanning the focus through the sample, very precise, three-dimensional microstructuring can be achieved.
    C. B. Schaffer. 2001. “Interaction of Femtosecond Laser Pulses with Transparent Materials”.Abstract
    An intense femtosecond laser pulse can have an electric field strength which approaches or even exceeds the strength of the electric field that holds valence electrons in a transparent material to their ionic cores. In this regime, the interaction between the laser pulse and the material becomes highly nonlinear. Laser energy can be nonlinearly absorbed by the material, leading to permanent damage, and the materials nonlinear response to the laser field can, in turn, induce radical changes in the laser pulse itself. The nature of these nonlinear interactions, the changes produced in the material and to the laser pulse, as well as several practical applications are explored in this thesis. We measure the laser intensity required to damage bulk transparent materials and uncover the dominant nonlinear ionization mechanism for different laser wavelengths and material band gaps. Using optical and electron microscopy, we examine the morphology of the material changes induced by tightly-focused femtosecond laser pulses in bulk transparent materials, and identify several mechanisms by which material changes are produced. We show that a high repetition rate train of femtosecond laser pulses can provide a point source of heat located inside the bulk of a transparent material, an effect which no other technique can achieve. The mechanism for white-light continuum generation is uncovered through measurement of the laser wavelength, the material band gap, and the external focusing angle dependence of the continuum spectrum. Using a time-resolved imaging technique, we follow the dynamics of the laser-produced plasma over eight orders of magnitude in time, revealing picosecond time scale dynamics that have not been previously observed. Finally, we discuss applications in direct writing of optical waveguides and in sub-cellular laser surgery.
    C. B. Schaffer and E. Mazur. 2001. “Micromachining using Ultrashort Pulses from a Laser Oscillator.” In Optics and Photonics News, Vol. 12, No.4: Pp. 21–23.Abstract
    In recent years, femtosecond laser pulses have been used to micromachine a great variety of materials. Ultrashort pulses cleanly ablate virtually any material with a precision that meets or exceeds that of other laser-based techniques, making the femtosecond laser an attractive micromachining tool. In transparent materials, where micromachining relies on nonlinear absorption, femtosecond lasers allow three-dimensional microfabrication with sub-micrometer precision. These lasers can produce three-dimensionally localized refractive index changes in the bulk of a transparent material, opening the door to the fabrication of a wide variety of optical devices. Until now micromachining of transparent materials required amplified laser systems. We recently found that transparent materials can also be micromachined using tightly focused trains of femtosecond laser pulses from an unamplified laser oscillator. In addition to reducing the cost and complexity of the laser system, femtosecond laser oscillators enable micromachining using a multiple-shot cumulative effect. We have used this new technique to directly write single-mode optical waveguides into bulk glass.
    A. M.-T. Kim, J. Paul Callan, C. A. D. Roeser, E. Mazur, and J. Solis. 2000. “Ultrafast phase transition dynamics in GeSb alloys.” In . Nonlinear Optics: Materials, Fundamentals, and Applications, 2000. Publisher's VersionAbstract
    We measure the femtosecond time resolved dielectric function of a-GeSb after excitation with an ultrashort laser pulse. The results reveal an ultrafast transition to a new non-thermodynamic phase which is not c-GeSb as previously believed. We present the most thorough experimental study to date of laser induced ultrafast phase transitions in GeSb alloys. We investigate the changes of the material by directly monitoring the full dielectric function over a broad energy range (1.7 eV - 3.5 eV) with 100 fs time resolution.
    J. Paul Callan, A. M.-T. Kim, L. Huang, and E. Mazur. 2000. “Ultrafast Electron and Lattice Dynamics in Semiconductors at High Excited Carrier Densities.” Chemical Physics, 251, Pp. 167–179.Abstract
    We directly measure ultrafast changes in the dielectric function of GaAs over the spectral range from the near-IR to the near-UV caused by intense 70-fs laser excitation. The dielectric function reveals the nature of the ultrafast phase transformations generated in the material, including a semiconductor-to-metal transition for the strongest excitations. Although the electron and lattice dynamics are complex when large carrier densities are excited between 1 and 20% of the valence electrons the dominant processes and their timescales can be deduced.
    J. Paul Callan. 2000. “Ultrafast Dynamics and Phase Changes in Solids Excited by Femtosecond Laser Pulses”.Abstract
    This dissertation reports the response of crystalline GaAs, amorphous GaAs and thin films of amorphous GeSb when a femtosecond laser pulse excites 1-20% of the valence electrons. We developed a broadband pump-probe technique to measure the dielectric function from the near-infrared to the near-ultraviolet with a time resolution of about 100 femtoseconds. The dielectric function provides more information than ever before on the ultrafast electronic and structural dynamics and the phase changes that occur. The dynamics depend on the excitation strength. In crystalline and amorphous GaAs, electronic effects dominate during the first few picoseconds for weaker excitations. The excited carriers affect optical properties not only through free carrier absorption, as previous experiments suggested, but also through modifications to the band structure (or allowed energy states) and filling of conduction states. Excited carriers recombine through an Auger process in crystalline GaAs and, in both phases, transfer their energy to the lattice via phonon emission. The materials consequently heat, and the dielectric function tracks the rise in lattice temperature. For strong excitations, the dielectric function data contradict the suggestion, from reflectivity measurements at 620 nm, that GeSb films undergo a remarkable amorphous-to-crystalline transition in about 200 femtoseconds. The dielectric function we observe at this time does not match that of the thermodynamic crystalline phase. Instead the transition leads to a metal-like state that is likely to be disordered. We observe a similar ultrafast semiconductor-to-disordered-metal transition in all three materials when the excitation is sufficiently strong. The transition can take as little as 150 femtoseconds, but it always takes longer than the pulse duration. Thus the excited electrons do not cause the change directly; rather bonds are broken when electrons are excited, the ions move to new positions and a non-thermal structural transition takes place. In all three materials, the plasma frequency of the resulting metallic state falls over time, due either to diffusion of carriers into the material or ablation from the surface.
    C. B. Schaffer, A. Brodeur, J. F. Garcia, W. A. Leight, and E. Mazur. 2000. “Micromachining optical waveguides in bulk glass using a femtosecond laser oscillator.” In . Conference on Lasers and Electro Optics (CLEO).Abstract
    Using tightly-focused femtosecond laser pulses of only a few nanojoules we produce optical breakdown and damage in bulk transparent materials, enabling micromachining using unamplified lasers. As a demonstration, we fabricate single-mode optical waveguides inside bulk glass.
    C. B. Schaffer, A. Brodeur, J. F. Garcia, W. A. Leight, and E. Mazur. 2000. “Micromachining optical waveguides in bulk glass using a femtosecond laser oscillator.” In . Conference on Lasers and Electro Optics (CLEO).Abstract
    Using tightly-focused femtosecond laser pulses of only a few nanojoules we produce optical breakdown and damage in bulk transparent materials, enabling micromachining using unamplified lasers. As a demonstration, we fabricate single-mode optical waveguides inside bulk glass.
    D. Wheeler and E. Mazur. 2000. “The Great Thermometer Challenge.” Phys. Teach., 38, Pp. 235–235.Abstract
    A surprisingly effective measurement challenge involves simply the reading of three thermometers, one of which is deliberately broken so it gives the temperature of ice water as 38 C (which is precisely what most students record). Years later students report that they have never forgotten their first physics class when they so eagerly insisted that ice-water was much hotter than room-temperature water! They proudly tell how they learned to think when taking readings and no longer unequivocally trust instruments (or physics teachers!).
    T. Her, R. J. Finlay, C. Wu, and E. Mazur. 2000. “Femtosecond laser-induced formation of spikes on silicon.” Appl. Phys. A, 70, Pp. 383–385.Abstract
    We find that silicon surfaces develop arrays of sharp conical spikes when irradiated with 500 femtosecond laser pulses in SF6. The height of the spikes decreases with increasing pulse duration or decreasing laser fluence, and scales nonlinearly with the average separation between spikes. The spikes have the same crystallographic orientation as bulk silicon and always point along the incident direction of laser pulses. The base of the spikes has an asymmetric shape and its orientation is determined by the laser polarization. Our data suggest that both laser ablation and laser-induced chemical etching of silicon are involved in the formation of the spikes.
    C. Wu. 2000. “Femtosecond laser-gas-solid interactions”.Abstract
    This dissertation discusses two sets of experiments. The first set of experiments investigates the interaction of femtosecond laser pulses with silicon in the environment of a halogen-containing gas. We find that upon irradiation, sharp spikes are formed on the surface. Spike formation strongly depends on the laser and gas conditions. The spikes are crystalline and contain a high density of both structural and chemical defects. The spikes are very strong light absorbers and exhibit absorption exceeding A > 0.9 for ultraviolet (0.25 m) to near-infrared (2.5 m) wavelengths. Spiked avalanche photodiodes show a more than threefold increase in quantum efficiency upon illumination with 1.06 m and 1.31 m radiation. Unlike ordinary silicon, microstructured silicon luminesces strongly in the visible. The photoluminescence intensity and peak wavelength depend on the laser conditions used to produce the luminescent surfaces. In the second set of experiments, we study the oxidation of CO from CO/O2/Pt(111) using femtosecond laser pulses. We observe a nonlinear dependence of both the CO oxidation and O2-desorption yields on the laser fluence. The yields depend strongly on the laser wavelength. Nonthermal electrons must play an important role in the excitation mechanism leading to desorption of O2 and production of CO2. Oxidation of CO from CO/O2/Pt(111) induced with femtosecond laser pulses most likely proceeds via the formation of an intermediate CO3* complex.
    Y. Tabe, N. Shen, E. Mazur, and H. Yokoyama. 1999. “Simultaneous Observation of Molecular Tilt and Azimuthal Angle Distributions in Spontaneously Modulated Liquid-Crystalline Langmuir Monolayers.” Phys. Rev. Lett., 82, Pp. 759–762. Publisher's VersionAbstract
    We carried out the first quantitative measurements of correlated modulations of molecular tilt and azimuthal angles in two-dimensional (2D) smectic C Langmuir monolayers using simultaneous linear- and circular-polarized reflected light microscopy. For spontaneously formed stripes and higher-order point defects, the tilt angle varies nearly sinusoidally at twice the spatial frequency of the azimuthal rotation. The tilt modulation grows as the second power of the modulation wavenumber and leads to a large escaped core for the point defect. Our results can be explained by an extended Landau theory of tilted smectics.
    C. B. Schaffer, A. Brodeur, J. F. Garcia, and E. Mazur. 1999. “Micromachining bulk glass with tightly-focused femtosecond laser pulses.” In . IEEE LEOS Annual Meeting.Abstract
    By focusing femtosecond laser pulses with high numerical-aperture microscope objectives, we micromachine optical glass using energies that are in the range of modern laser oscillators. When a femtosecond laser pulse is tightly focused inside a transparent material, energy deposition occurs only at the focus, where the laser intensity is high enough to cause absorption through nonlinear processes. When enough energy is deposited, the material is damaged and a localized change in the index of refraction is produced. By scanning the focus through the sample, very precise, three-dimensional microstructuring can be achieved
    C. B. Schaffer, A. Brodeur, J. F. Garcia, and E. Mazur. 1999. “Micromachining bulk glass with tightly-focused femtosecond laser pulses.” In . IEEE LEOS Annual Meeting.Abstract
    By focusing femtosecond laser pulses with high numerical-aperture microscope objectives, we micromachine optical glass using energies that are in the range of modern laser oscillators. When a femtosecond laser pulse is tightly focused inside a transparent material, energy deposition occurs only at the focus, where the laser intensity is high enough to cause absorption through nonlinear processes. When enough energy is deposited, the material is damaged and a localized change in the index of refraction is produced. By scanning the focus through the sample, very precise, three-dimensional microstructuring can be achieved
    C. B. Schaffer, A. Brodeur, N. Nishimura, and E. Mazur. 1999. “Laser-induced microexplosions in transparent materials: microstructuring with nanojoules.” In . Photonics West. Publisher's VersionAbstract
    We tightly focus femtosecond laser pulses in the bulk of a transparent material. The high intensity at the focus causes nonlinear absorption of the laser energy, producing a microscopic plasma and damaging the material. The tight external focusing allows high intensity to be achieved with low energy, minimizing the effects of self-focusing. We report the thresholds for breakdown and critical self-focusing in fused silica using 110-fs pulses at both 400-nm and 800-nm wavelength. We find that permanent damage can be produced with only 10 nJ (25 nJ) for 400-nm (800-nm) pulses, and that the threshold for critical self-focusing is 140 nJ for the 400-nm pulses and 580 nJ for the 800-nm pulses. The critical self-focusing thresholds are more than an order of magnitude above the breakdown thresholds, confirming that self-focusing does not play a dominant role in the damage formation. This lack of self-focusing allows a straightforward interpretation of the wavelength and bandgap dependence of bulk breakdown thresholds. The energies necessary for material damage are well within the range of a cavity-dumped oscillator, allowing for precision microstructuring of dielectrics with a high repetition-rate laser that is roughly one-third the cost of an amplified system.
    C. B. Schaffer. 1999. “Non-perturbative up-conversion techniques: Ultrafast meets X-rays.” In Ultrafast Dynamics of Quantum Systems: Physical Processes and Spectroscopic Techniques, edited by B. Di Bartolo, Pp. 611–623. Plenum Press. Publisher's VersionAbstract
    Femtosecond laser pulses are now available from the ultraviolet (200 nm) all the way to the mid-infrared (12 m) in compact (often commercially available) systems. This tunability is achieved through perturbative nonlinear optical wavelength conversion techniques in crystals. To reach further into the ultraviolet and Xray regions of the spectrum, a new set of techniques becomes necessary. In this paper, we will review some of these nonperturbative nonlinear optical methods. Specifically, we will consider the high harmonic generation process in detail, and go through the essentials of the semi-classical theory. Next, we will review a new technique, based on Thomson scattering, which has produced 0.4-Angstrom, 300-fs radiation. Finally we will consider means of measuring femtosecond pulses in this short wavelength regime.
    N. Nishimura, C. B. Schaffer, E. Herbert Li, and E. Mazur. 1998. “Tissue ablation with 100-fs and 200-ps laser pulses.” In . IEEE Engineering in Medicine and Biology. Publisher's VersionAbstract
    We used water and human skin tissue to compare the surgical potential of 100-fs and 200-ps laser pulses. For investigation of threshold behavior of 100-fs and 200-ps pulses, we use water as a model for tissue. In addition to having a lower threshold, we find that energy deposition is much more consistent with 100-fs pulses. We also compared 100-fs and 200-ps laser pulse effects on the surface and in the bulk of human skin tissue. On the surface, pulses with 100-fs and 200-ps duration leave similar size ablation regions. In the bulk both 100-fs and 200-ps pulses produce cavities, however, 100-fs pulses result in a smaller cavity size. On both the surface and in the bulk 100-fs pulses show less collateral tissue damage than 200-ps pulses.
    T. Her, R. J. Finlay, C. Wu, and E. Mazur. 1998. “Surface femtochemistry of CO/O2/Pt(111): The importance of nonthermalized substrate electrons.” J. Chem. Phys., 108, Pp. 8595–8598.Abstract
    We studied the surface femtochemistry of CO/O2/Pt(111) induced with 0.3-ps laser pulses over a wide range of wavelength and fluence. Below 10 J/mm2, the yields depend linearly on fluence. Above 10 J/mm2, the yields scale nonlinearly in the fluence. From the dependence of the yields on wavelength, we determine that the nonlinear surface femtochemistry is influenced by nonthermal substrate electrons.
    C. B. Schaffer, E. N. Glezer, N. Nishimura, and E. Mazur. 1998. “Ultrafast laser induced microexplosions: explosive dynamics and sub-micrometer structures.” In . Photonics West. Publisher's VersionAbstract
    Tightly focused femtosecond laser pulses can be nonlinearly absorbed inside transparent materials, creating a highly excited electron ion plasma. These conditions exist only in a small volume at the laser focus. This tight confinement and extreme conditions lead to an explosive expansion a microexplosion. In solid materials, a microexplosion can result in permanent structural changes. We find that the damage produced by femtosecond pulses in this way is surprisingly small, with only a 200-nm diameter. Material left at the center of the microexplosion is either amorphous and less dense or entirely absent. The threshold for breakdown and structural change is nearly independent of material. Time-resolved measurements of microexplosions in water allow us to observe the dynamics of the explosive expansion. The structural changes in solids resulting from microexplosions allow for three-dimensional data storage and internal microstructuring of transpa

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