Publications

    J. C. Hwang. 2002. “Femtosecond Laser-Induced Damage for Micromachining of Transparent Materials”.Abstract
    Femtosecond laser pulses have extremely short temporal profiles. When tightly focused with a microscope objective, one micron spatial profiles can be achieved. Even with a very modest per-pulse energy, ranging from ten nanojoules to several microjoules, one can easily reach intensities over ten terawatts per square centimeter. Under such conditions, highly intensity-dependent nonlinear absorption processes such as multiphoton absorption and avalanche ionization take place, leading to permanent structural and chemical changes in transparent materials. Since transparent materials have electronic band gaps too large to bridge with a single photon, linear absorption of laser light does not occur. This allows beams to be focused into the bulk of a transparent material without damaging the surface. The nature of such nonlinear interactions, the physics of focusing in the bulk, the chemical changes induced, as well as a few practical applications are explored in this thesis. I first derive an equation relating the threshold intensity for damage in the bulk of a transparent material to physical quantities like the threshold energy, numerical aperture, and the critical power for self-focusing. Then I investigate the potential of bulk micromachining in transparent polymers to have applications in micromechanical devices and microcircuitry, and I study surface patterning of polydimethylsiloxane and its applications in microcontact printing. Finally, taking advantage of the derived equation, I explore the basic physics and chemistry of making damage in the bulk of diamond, a remarkably hard material with an extremely high refractive index.
    A. M.-T. Kim, J. Paul Callan, C. A. D. Roeser, and E. Mazur. 2002. “Ultrafast dynamics and phase changes in crystalline and amorphous GaAs.” Phys. Rev. B, 66, Pp. 245203–245203.Abstract
    The femtosecond time-resolved response of the spectral dielectric function provides a great wealth of information on carrier and lattice dynamics in highly photo-excited semiconductors. We present measurements of the dielectric function of crystalline and amorphous GaAs over a broad spectral energy range (1.7–3.4 eV), with sub- 100 fs time resolution. A detailed analysis of the data reveals many new insights into the dynamics and phase changes in semiconductors at high excitation fluences up to and beyond the damage threshold.
    C. A. D. Roeser, A. M.-T. Kim, and E. Mazur. 2002. “Ultrafast lattice-bonding dynamics of tellurium.” In . CLEO/QELS.Abstract
    We measure the ordinary and extraordinary dielectric function of Te with femtosecond time resolution after coherent phonons are generated with an ultrashort laser pulse. The results reveal oscillatory behavior in the bonding-antibonding split at   3 THz.
    C. A. D. Roeser, A. M.-T. Kim, and E. Mazur. 2002. “Ultrafast lattice-bonding dynamics of tellurium.” In . CLEO/QELS.Abstract
    We measure the ordinary and extraordinary dielectric function of Te with femtosecond time resolution after coherent phonons are generated with an ultrashort laser pulse. The results reveal oscillatory behavior in the bonding-antibonding split at   3 THz.
    S. K. Sundaram and E. Mazur. 2002. “Inducing and probing non-thermal transitions in semiconductors using femtosecond laser pulses.” Nature Materials, 1, Pp. 217–224. Publisher's VersionAbstract
    Soon after it was discovered that intense laser pulses of nanosecond duration from a ruby laser could anneal the lattice of silicon, it was established that this so-called pulsed laser annealing is a thermal process. Although the radiation energy is transferred to the electrons, the electrons transfer their energy to the lattice on the timescale of the excitation. The electrons and the lattice remain in equilibrium and the laser simply heats the solid to the melting temperature within the duration of the laser pulse. For ultrashort laser pulses in the femtosecond regime, however, thermal processes (which take several picoseconds) and equilibrium thermodynamics cannot account for the experimental data. On excitation with femtosecond laser pulses, the electrons and the lattice are driven far out of equilibrium and disordering of the lattice can occur because the interatomic forces are modified due to the excitation of a large (10% or more) fraction of the valence electrons to the conduction band. This review focuses on the nature of the non-thermal transitions in semiconductors under femtosecond laser excitation.
    C. B. Schaffer, J. Aus der Au, E. Mazur, and J. A. Squier. 2002. “Micromachining and material change characterization using femtosecond laser oscillators.” In . Photonics West. Publisher's VersionAbstract
    We use third harmonic generation (THG) microscopy to image waveguides and single-shot structural modifications produced in bulk glass using femtosecond laser pulses. THG microscopy reveals the internal structure of waveguides written with a femtosecond laser oscillator, and gives a three-dimensional view of the complicated morphology of the structural changes produced with single, above-threshold femtosecond pulses. We find that THG microscopy is as sensitive to refractive index change as differential interference contrast microscopy, while also offering the three-dimensional sectioning capabilities of a nonlinear microscopy technique. It is now possible to micromachine three- dimensional optical devices and to image these structures in three dimensions, all with a single femtosecond laser oscillator.
    C. Wu, C. H. Crouch, L. Zhao, and E. Mazur. 2002. “Visible luminescence from silicon surfaces microstructured in air.” Appl. Phys. Lett., 81, Pp. 1999–2001.Abstract
    We report visible luminescence from SiOx formed by microstructuring silicon surfaces with femtosecond laser pulses in air. Incorporation of oxygen into the silicon lattice occurs only where the laser beam strikes the surface. Laser microstructuring therefore offers the possibility of writing submicrometer luminescent features without lithographic masks. The amount of oxygen incorporated into the silicon surface depends on the laser fluence; the peak wavelength of the primary luminescence band varies between 540 and 630 nm and depends on the number of laser shots. Upon annealing, the intensity of the primary luminescence band increases significantly without any change in the luminescence peak wavelength, suggesting that the luminescence comes from defects rather than quantum confinement.
    C. B. Schaffer, N. Nishimura, E. N. Glezer, A. M.-T. Kim, and E. Mazur. 2002. “Dynamics of femtosecond laser-induced breakdown in water from femtoseconds to microseconds.” Opt. Express, 10, Pp. 196–203.Abstract
    Using time-resolved imaging and scattering techniques, we directly and indirectly monitor the breakdown dynamics induced in water by femtosecond laser pulses over eight orders of magnitude in time. We resolve, for the first time, the picosecond plasma dynamics and observe a 20 ps delay before the laser-produced plasma expands. We attribute this delay to the electron-ion energy transfer time.
    J. B. Ashcom, C. B. Schaffer, and E. Mazur. 2002. “Numerical aperture dependence of damage and white light generation from femtosecond laser pulses in bulk fused silica.” In . Photonics West. Publisher's VersionAbstract
    The femtosecond laser has become an important tool in the micromachining of transparent materials. In particular, focusing at high numerical aperture enables structuring the bulk of materials. At low numerical aperture and comparable energy, focused femtosecond pulses result in white light or continuum generation. It has proven difficult to damage transparent materials in the bulk at low NA. We have measured the threshold energy for continuum generation and for bulk damage in fused silica for numerical apertures between 0.01 and 0.65. The threshold for continuum generation exhibits a minimum near 0.05 NA, and increases quickly near 0.1 NA. Greater than 0.25 NA, no continuum is observed. The extent of the anti-stokes pedestal in the continuum spectrum decreases strongly as the numerical aperture is increased to 0.1, emphasizing that slow focusing is important for the broadest white light spectrum. We use a sensitive light scattering technique to detect the onset of damage. We are able to produce bulk damage at all numerical apertures studied. At high numerical aperture, the damae threshold is well below the critical power for self-focusing, which allows the breakdown intensity to be determined. Below 0.25 NA, the numerical aperture dependence suggests a possible change in damage mechanism.
    L. Tong, R. R. Gattass, J. Lou, J. B. Ashcom, M. Shen, and E. Mazur. 2002. “Submicron and nano-diameter silica wires for optical wave guiding.” In . Proceedings of the Asia-Pacific Optical and Wireless Communications 2002 SPIE Meeting. Publisher's VersionAbstract
    Based on the exact solutions of Maxwells equations, we have studied the basic theoretical properties of submicron and nano-diameter air-cladding silica-wire waveguides. The single-mode condition and the modal field of the fundamental modes have been obtained. Silica wires with diameters of 100-1000nm and lengths ranging from hundreds of micrometer to over 1 millimeter have been fabricated. SEM examination shows that these wires have uniform diameters and smooth surfaces, which are favorable for optical wave guiding. Light has been sent into these wires by optical coupling, and guiding light through a bent wire has also been demonstrated. These wires are promising for assembling photonic devices on a micron or submicron scale.
    D. Datta. 2002. “Tissue surgery and subcellular photodisruption with femtosecond laser pulses”.Abstract
    The short duration of femtosecond laser pulses allows high laser intensities to be reached with low pulse energies. Focusing pulses through high numerical aperture microscope objectives leads to intensities high enough to induce plasma formation and photodisruption of matter at the laser focus through nonlinear mechanisms. These studies investigate the potential for using femtosecond lasers for photodisruptive surgery on the surface and in the bulk of turbid tissue. As our tissue model, we use mouse skin tissue. Our experiments demonstrate that incisions are made on the tissue surface by translating the laser focus across the sample while irradiating with a continuous train of pulses. Subsurface cavities are made in the tissue bulk by focusing the laser beneath the surface. Images of histological sections of samples show that damage microstructures are created with high precision and minimal collateral damage outside the focal region. We find that there is a maximum depth at which subsurface cavities can be made; placing the laser focus below this depth results in filament formation through the nonlinear optical phenomenon of selffocusing. Studies of photodisruption in fixed cell samples show that damage morphologies are made on the size scale of subcellular structures. We demonstrate that cells can be fluorescently labeled for specific cellular structures and that photodisruption of these samples can be effectively discerned from photobleaching effects. Subcellular damage is also shown to be localized completely within the cell. We demonstrate the feasibility of using femtosecond laser pulses for manipulation of subcellular structures in living cells, and we show that this can be done without compromising cell membrane integrity.
    C. B. Schaffer, A. O. Jamison, J. F. Garcia, and E. Mazur. 2002. “Structural changes induced in transparent materials with ultrashort laser pulses.” In Ultrafast lasers: technology and applications, edited by M. E. Fermann, A. Galvanauskas and G. D. Sucha, Pp. 395–417. Marcel Dekker, Inc.Abstract
    In recent years, femtosecond lasers have been used for a multitude of micromachining tasks. Several groups have shown that femtosecond laser pulses cleanly ablate virtually any material with a precision that consistently meets or exceeds that of other laser-based techniques, making the femtosecond laser an extremely versatile surface micromachining tool. For large bandgap materials, where laser machining relies on nonlinear absorption of high intensity pulses for energy deposition, femtosecond lasers offer even greater benefit. Because the absorption in a transparent material is nonlinear, it can be confined to a very small volume by tight focusing, and the absorbing volume can be located in the bulk of the material, allowing three-dimensional micromachining. The extent of the structural change produced by femtosecond laser pulses can be as small as or even smaller than the focal volume. Recent demonstrations of three-dimensional micromaching of glass using femtosecond lasers include three-dimensional binary data storage, and the direct writing of optical waveguides and waveguide splitters. The growing interest in femtosecond laser micromachining of bulk transparent materials makes it more important than ever to uncover the mechanisms responsible for producing permanent structural changes.
    C. B. Schaffer, A. O. Jamison, J. F. Garcia, and E. Mazur. 2002. “Structural changes induced in transparent materials with ultrashort laser pulses.” In Ultrafast lasers: technology and applications, edited by M. E. Fermann, A. Galvanauskas and G. D. Sucha, Pp. 395–417. Marcel Dekker, Inc.Abstract
    In recent years, femtosecond lasers have been used for a multitude of micromachining tasks. Several groups have shown that femtosecond laser pulses cleanly ablate virtually any material with a precision that consistently meets or exceeds that of other laser-based techniques, making the femtosecond laser an extremely versatile surface micromachining tool. For large bandgap materials, where laser machining relies on nonlinear absorption of high intensity pulses for energy deposition, femtosecond lasers offer even greater benefit. Because the absorption in a transparent material is nonlinear, it can be confined to a very small volume by tight focusing, and the absorbing volume can be located in the bulk of the material, allowing three-dimensional micromachining. The extent of the structural change produced by femtosecond laser pulses can be as small as or even smaller than the focal volume. Recent demonstrations of three-dimensional micromaching of glass using femtosecond lasers include three-dimensional binary data storage, and the direct writing of optical waveguides and waveguide splitters. The growing interest in femtosecond laser micromachining of bulk transparent materials makes it more important than ever to uncover the mechanisms responsible for producing permanent structural changes.
    L. Tong, L. Ye, J. Lou, Z. Shen, Y. Shen, and E. Mazur. 2002. “Improved Y2O3-ZrO2 waveguide fiber optic sensor for measuring gas-flow temperature above 2000 degrees Celsius.” In . Fiber Optic Sensor Technology and Applications 2001. Publisher's VersionAbstract
    AlthoughY2O3-ZrO2 fiber-optic sensor has been developed for contact measurement of temperature higher than 2000 degree(s)C, its performance is not as good as that of sapphire fiber-optic sensor below 1900 degree(s)C due to the large optical loss of the Y2O3-ZrO2 fiber. In order to improve the Y2O3- ZrO2 fiber-optic sensor for ultra-high-temperature applications, in this work, based on a newly developed rectangular Y2O3-ZrO2 single-crystal waveguide with much lower optical loss, an improved Y2O3- ZrO2 waveguide- fiber-optic sensor has been developed. The sensor has been tested up to near 2300 degree(s)C, we estimate that, the improved sensor has similar performance as the sapphire fiber-optic sensor in accuracy and resolution, except the disadvantage of relatively short waveguide. In addition, in this work, instead of the previous volatile and toxic BeO-coated probe, we use a multi-ions-doped sensor head, which is much stable and safe.
    L. Tong, J. Lou, and E. Mazur. 2002. “High-quality rectangular Y2O3- ZrO2 single-crystal optical waveguides for high- temperature fiber optic sensors.” In . Fiber Optic Sensor Technology and Applications 2001. Publisher's VersionAbstract
    High quality Y2O3-ZrO2 single crystal rectangular waveguides had been developed for high-temperature sensing applications. The waveguides were fabricated from bulky Y2O3 stabilized ZrO2 single crystal by precise cut and fine polish. Three rectangular waveguides with cross-section larger than 1mmx1mm and length of 45mm 65mm were obtained. They showed much better optical properties than Y2O3-ZrO2 single crystal fibers grown for fiber-optic sensing in previous work, optical losses of these waveguides were lower than 0.03dB/cm at wavelength of 900nm, and they were able to endure temperature as higher as 2300 degree(s)C. All of them survived a 10g vibration test with average STF (strain to failure) of about 0.25%. Experimental results show that, these waveguides are promising for fiber-optic sensing for temperature above 2000 degree(s)C.
    C. Wu, C. H. Crouch, L. Zhao, J. E. Carey, R. J. Younkin, J. A. Levinson, E. Mazur, R. M. Farrel, P. Gothoskar, and A. Karger. 2001. “Near-unity below-band gap absorption by microstructured silicon.” Appl. Phys. Lett., 78, Pp. 1850–1852. Publisher's VersionAbstract
    We increased the absorptance of light by silicon to approximately 90% from the near ultraviolet (0.25 m) to the near infrared (2.5 m) by surface microstructuring using laser-chemical etching. The remarkable absorptance most likely comes from a high density of impurities and structural defects in the silicon lattice, enhanced by surface texturing. Microstructured avalanche photodiodes show significant enhancement of below-band gap photocurrent generation at 1.06 and 1.31 m, indicating promise for use in infrared photodetectors.
    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.

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