Publications

    C. B. Schaffer, A. O. Jamison, J. F. Garcia, J. B. Ashcom, and E. Mazur. 2001. “Morphology and mechanisms of femtosecond laser-induced structural change in bulk transparent materials.” In . Conference on Lasers and Electro-Optics.Abstract
    Tightly-focused femtosecond laser pulses are used to produce localized structural changes in bulk glass. The morphology of these structures is studied using optical and electron microscopy, and several mechanisms for producing structural changes are identified.
    A. M.-T. Kim, J. Solis, J. Paul Callan, C. A. D. Roeser, and E. Mazur. 2001. “GeSb thin films: read-write optical data storage on the subnanosecond time scale.” In . Ultrafast Electronics and Optoelectronics. Publisher's VersionAbstract
    The transition from the low reflectivity amorphous to the highly reflective crystalline phase of a-GeSb films is studied with fs time-resolution. The results reveal an ultrafast transition to a new non-thermodynamic phase which is not c-GeSb as previously believed.
    C. B. Schaffer, A. Brodeur, and E. Mazur. 2001. “Laser-induced breakdown and damage in bulk transparent materials induced by tightly-focused femtosecond laser pulses.” Meas. Sci. Technol., 12, Pp. 1784–1794.Abstract
    Laser-induced breakdown and damage to transparent materials has remained an active area of research for four decades. In this paper we review the basic mechanisms that lead to laser-induced breakdown and damage and present a summary of some open questions in the field. We present a method for measuring the threshold intensity required to produce breakdown and damage in the bulk, as opposed to on the surface, of the material. Using this technique, we measure the material band-gap and laser-wavelength dependence of the threshold intensity for bulk damage using femtosecond laser pulses. Based on these thresholds, we determine the relative role of different nonlinear ionization mechanisms for different laser and material parameters.
    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.
    D. N. Fittinghoff, C. B. Schaffer, E. Mazur, and J. A. Squier. 2001. “Time-decorrelated multifocal micromachining and trapping.” IEEE J. Select. Topics Quantum Electron., 7, Pp. 559–566.Abstract
    Temporally decorrelated multifocal arrays eliminate spatial interference between adjacent foci and allow multifocal imaging with the diffraction-limited resolution of a single focus, even for foci spaced by less than the focal diameter. In this work, we demonstrate a high-efficiency cascaded-beamsplitter array for producing temporally decorrelated beamlets. These beamlets are used to produce a multifocal microscope with which we have demonstrated two-photon fluorescence imaging, multifocal micromachining of optical waveguides, and multifocal optical trapping.
    J. Paul Callan, A. M.-T. Kim, C. A. D. Roeser, E. Mazur, J. Solis, J. Siegel, C. N. Alfonso, and J. C. G. De Sande. 2001. “Ultrafast Laser-Induced Phase Transitions in Amorphous GeSb Films.” Phys. Rev. Lett., 86, Pp. 3650–3653.Abstract
    Time-resolved measurements of the spectral dielectric function reveal new information about ultrafast phase transitions induced by femtosecond laser pulses in Sb-rich amorphous GeSb films. The excitation generates a non-thermal phase within 200 fs. The dielectric function of this phase differs from that of the crystalline phase, contrary to previous suggestions of a disorder-to-order transition. The observed dielectric function is close to that of the liquid phase, indicating an ultrafast disorder-to-disorder transition from the amorphous phase to a glassy (liquid-like disordered) state.
    A. M.-T. Kim. 2001. “Ultrafast Carrier and Lattice Dynamics in Highly Photo-Excited Solids”.Abstract
    In this dissertation we report femtosecond time-resolved measurements of the spectra dielectric function of amourphous GaAs,GeSb thin films and single-crystalline Te. In all materials we measured the evolution of the dielectric function over a broad energy range (1.7 3.4 eV), following an impulsive excitation by an ultrashort laser pulse. The dielectric function data on a-GaAs show evidence of a nonthermal, structurally driven semiconductor-to-metal transition. A comparison to previously taken dielectric function data on c-GaAs is especially illuminating in terms of the influence of the initial structure on the phase transition. Our results on GeSb thin films reveal a new nonthermal, metallic phase.The dielectric function data provide a detailed picture of the transition from the amorphous phase to the crystalline phase of these thin films.Furthermore we refute a previous claim on an ultrafast disorder-to-order transformation in these materials. The time-resolved dielectric function data on Te reveal a great wealth of new information on impulsively driven coherent phonons, including their in fluence on the electronic bandstructure. We find evidence indicative of a new nonthermal phase of matter which we call a frustrated metal .
    C. A. D. Roeser, A. M.-T. Kim, J. Paul Callan, E. Mazur, and J. Solis. 2001. “Ultrafast phase transition dynamics in GeSb alloys.” In . CLEO/QELS 2001.Abstract
    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 that is not c-GeSb, as suggested elsewhere.
    C. A. D. Roeser, A. M.-T. Kim, J. Paul Callan, E. Mazur, and J. Solis. 2001. “Ultrafast phase transition dynamics in GeSb alloys.” In . CLEO/QELS 2001.Abstract
    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 that is not c-GeSb, as suggested elsewhere.
    J. Paul Callan, A. M.-T. Kim, C. A. D. Roeser, and E. Mazur. 2001. “Universal dynamics during and after ultrafast laser-induced semiconductor-to-metal transitions.” Phys. Rev. B, 64, Pp. 073201–4.Abstract
    We observe common features in semiconductor-to-metal transitions induced by femtosecond laser pulses in crystalline GaAs, amorphous GaAs and Sb-rich films of amorphous GeSb, by tracking ultrafast changes in the spectral dielectric function. The dielectric function of the metal-like state reveals a decay in the plasma frequency with time after the transition. In addition, the plasma frequency roughly decreases with increasing excitation fluence.
    J. E. Carey, C. H. Crouch, R. J. Younkin, E. Mazur, M. A. Sheehy, and C. M. Friend. 2001. “Fabrication of Micrometer-Sized Conical Field Emitters Using Femtosecond Laser-Assisted Etching of Silicon.” In . International Vacuum Microelectronics Conference 2001.Abstract
    Arrays of sharp, conical microstructures were obtained by structuring the surface of a silicon wafer using femtosecond laser-assisted etching. Analysis of the arrays shows high, stable field emission without any further processing steps and turn-on fields as low as 1.3 V/um
    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.
    R. J. Younkin. 2001. “Surface studies and microstructure fabrication using femtosecond laser pulses”.Abstract
    In the first of the two experiments described in this thesis, we constructed and characterized an apparatus for performing ultrafast surface spectroscopy on Pt (111). A combination of an optical parametric amplifier and a difference frequency generation stage allow us to create optical pulses with wavelengths in the range 550 nm-7.3 m and pulsewidths of approximately 100 fs. We also developed methods for using the infrared output of this apparatus in infrared-visible sum-frequency generation at a Pt-vacuum interface. Our attempts to time-resolve a chemical reaction taking place at this interface were unsuccessful because of power limitations, but the techniques that we developed should be applicable in related experiments. In the second series of experiments, we created arrays of conical microstructures on the surface of silicon by irradiating a silicon wafer with trains of ultrashort pulses in the presence of an ambient gas. The microstructures are typically tens of micrometers high and separated by several micrometers. We showed that the morphology of the structures is highly dependent on the species of ambient gas, the gas pressure, the laser fluence, and the number of laser pulses used.The sharpest structures are made in halogen-containing gases,such as SF6 or Cl 2 .The use of N2 or air results in blunt, rounded structures. In this thesis, I discuss the application of these silicon microstructures in optoelectronics and vacuum microelectronics. I note that surfaces covered with these microstructures have striking optical properties:structures made in SF6 absorb approximately 90% of incident light with wavelengths between 250 nm and 2.5 m.The sharp conical structures also show high field-emission current. The remarkable and potentially useful optical and field-emitting properties of the structures are the result of the conical morphology of the laser-induced microstructures and the impurities introduced into the silicon.
    J. E. Carey, C. H. Crouch, R. J. Younkin, E. Mazur, M. A. Sheehy, and C. M. Friend. 2001. “Fabrication of Micrometer-Sized Conical Field Emitters Using Femtosecond Laser-Assisted Etching of Silicon.” In . International Vacuum Microelectronics Conference 2001.Abstract
    Arrays of sharp, conical microstructures were obtained by structuring the surface of a silicon wafer using femtosecond laser-assisted etching. Analysis of the arrays shows high, stable field emission without any further processing steps and turn-on fields as low as 1.3 V/um
    A. M.-T. Kim, C. B. Schaffer, C. A. D. Roeser, and E. Mazur. 2001. “Two-Photon-Absorption FROG: measuring white-light continuum pulses.” In . Conference on Lasers and Electro-Optics.Abstract
    We demonstrate a new type of frequency resolved optical gating using the two-photon-absorption nonlinearity. The resonant enhancement and lack of phasematching requirements enable the FROG measurement of ultrashort white-light continuum laser pulses.
    A. M.-T. Kim, C. B. Schaffer, C. A. D. Roeser, and E. Mazur. 2001. “Two-Photon-Absorption FROG: measuring white-light continuum pulses.” In . Conference on Lasers and Electro-Optics.Abstract
    We demonstrate a new type of frequency resolved optical gating using the two-photon-absorption nonlinearity. The resonant enhancement and lack of phasematching requirements enable the FROG measurement of ultrashort white-light continuum laser pulses.

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