Publications

    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.
    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.
    J. Paul Callan, A. M.-T. Kim, L. Huang, E. N. Glezer, and E. Mazur. 1997. “From semiconductor to metal in a flash: observing ultrafast laser-induced phase transformations.” In . Materials Research Society Fall Meeting. Publisher's VersionAbstract
    We use a new broadband spectroscopic technique to measure ultrafast changes in the dielectric function of a material over the spectral range 1.53.5 eV following intense 70-fs laser excitation. The results reveal the nature of the phase transformations which occur in the material following excitation. We studied the response of GaAs and Si. For GaAs, there are three distinct regimes of behavior as the pump fluence is increased lattice heating, lattice disordering, and a semiconductor-to-metal transition.
    E. N. Glezer, C. B. Schaffer, N. Nishimura, and E. Mazur. 1997. “Minimally disruptive laser-induced breakdown in water.” In . Conference on Lasers and Electro-Optics. Publisher's VersionAbstract
    Using tightly focused 100-fs, 800-nm laser pulses we produce breakdown in water using less than 1 J of energy. By imaging the cavitation and pressure wave propagation we find that the supersonic expansion is limited to an 11-m diameter for 1-J pulses, increasing to a 20-m diameter for 30-J pulses.
    E. N. Glezer. 1996. “Ultrafast electronic and structural dynamics in solids”. Publisher's VersionAbstract
    This thesis investigates the dynamics of electrons and atoms in solids driven by intense, ultrashort laser pulses. Results of two series of experiments are presented. In the first set, the changes in the electronic properties of the semiconductor GaAs are determined by measuring the changes in its optical properties in response to 70-fs laser pulses. A fluence range of up to, and above, the damage threshold is examined. The experiments differ from previous work in the field, in that they are direct time-resolved measurements of the dielectric function and second- order optical susceptibility fundamental quantities that characterize the optical state of the material. The dielectric function is measured from 1.5 to 3.5 eV, and at 4.4 eV, while the second-order susceptibility is measured at a single frequency of 2.2 eV. The results suggest a new view of the underlying electronic and structural changes. Three regimes of behavior are observed: at low excitation, rapid bandstructure changes are followed by lattice heating for about 10 ps; at medium excitation, stronger bandstructure changes are followed by a loss of long-range order in the crystal within several picoseconds; and at high excitation, an increasingly rapid transition to a metallic state is seen. In the second set of experiments, the effect of ultrafast excitation inside the bulk of a solid is studied. It is shown that submicron-diameter voxels can be produced inside many transparent materials by tightly focusing 100-fs laser pulses. The use of such voxels for high-density 3D optical data storage is demonstrated. Scanning electron microscopy and atomic force microscopy are used to examine 200-nm diameter voxels. The results suggest that extreme temperatures and pressures create a micro-explosion, leading to the formation of a void surrounded by densified material. Permanent structural changes are produced even in such hard materials as quartz and sapphire.
    E. N. Glezer, L. Huang, R. J. Finlay, T. Her, J. Paul Callan, C. B. Schaffer, and E. Mazur. 1996. “Ultrafast-laser-induced microexplosions in transparent materials.” In . 28th Annual Boulder Damage Symposium. Publisher's VersionAbstract
    Submicron-diameter structures can be produced inside many transparent materials by tightly focused 100-fs laser pulses. The ultrafast energy deposition creates very high temperature and pressure inside the region, initiating a microexplosion. Material is ejected from the center and forced into the surrounding volume, forming a void surrounded by densified material. Scanning electron microscopy and atomic force microscopy show structural changes confined to an area 200 nm in diameter.
    J. R. Goldman. 1994. “Laser studies of energy- and charge-transfer dynamics”. Publisher's VersionAbstract
    This thesis presents the results of three experiments which use lasers to investigate energy-transfer and charge-transfer dynamics. The dynamical processes studied include nanosecond vibrational energy transfer in molecules, subpicosecond electron relaxation in semiconductors, and subpicosecond initiation of surface bimolecular reactions on a metal crystal. In experiments using time-resolved coherent Raman spectroscopy to probe infrared multiphoton excited molecules, we study CO 2-laser excited SO2 and SF6. In SO2 we observe direct n1-mode excitation and distinguish between this process and excitation of the nearly resonant n2-mode overtone. In SF6, we directly observe n3-mode excitation followed by collisional energy redistribution to a heat bath of non-pumped modes. Quantitative modeling of the SF6 spectra yields excited vibrational population distributions and resolves some long-standing inconsistencies between different previously published reports. In an experiment using time-resolved photoelectron spectroscopy, we observe the subpicosecond evolution of an optically-excited nonequilibrium electron distribution in silicon. We observe an electron thermalization time of less than 120 fs, electron equilibration with the lattice in 1 ps, and an energy-dependent electron cooling rate consistent with published calculations of the electron-phonon scattering rate. The results indicate the formation, in 1 ps, of a surface space-charge electron layer with an electron density two orders of magnitude greater than the bulk electron density. In an experiment using 100-fs laser pulses to induce desorption of O2 and reaction of O2+CO to form CO2 on a Pt(111) surface, we present desorption and reaction data obtained over an absorbed fluence range of 1- 20 mJ/ cm2 at wavelengths of 800, 400, and 266 nm. We observe a highly nonlinear desorption and reaction yield fluence dependence; the data are fit by a power law model in which the yield is proportional to fluence to the power p = 5.9 and 3.8 for the 800 nm and 400 nm data respectively. The ratio of O2 to CO2 desorption is found to be 14:1, 12:1 and 3:1 at 800, 400, and 266 nm respectively. At 800 nm, the desorption and reaction are independent of laser pulsewidth in the range 100 fs to 3.6 ps. Finally, this thesis describes the design, development and operation of new equipment used for the surface reaction experiment: a state-of-the- art amplified femtosecond Ti:sapphire laser, and an ultrahigh-vacuum surface- science chamber.
    J. Kai Wang, Y. Siegal, C. Lu, E. Mazur, and J. Reintjes. 1994. “Subpicosecond stimulated Raman scattering in high-pressure hydrogen.” J. Opt. Soc. Am. B, 11, Pp. 1031–1037. Publisher's VersionAbstract
    We studied the effect of self-phase modulation and self-focusing on transient stimulated Raman scattering in high-pressure hydrogen by using high-energy, subpicosecond laser pulses. Adding argon to the hydrogen emphasizes the effect of self-phase modulation on stimulated Raman scattering by increasing the former effect without affecting the latter. The behavior of the observed stimulated Raman scattering falls into three regimes depending on input energy: normal stimulated Raman scattering at low energies, suppression by self-phase modulation at medium energies, and a recovery at high energies because strong self-focusing limits self-phase modulation.
    J. Kai Wang, Y. Siegal, C. Lu, and E. Mazur. 1992. “Generation of dual-wavelength, synchronized, tunable, high-energy, femtosecond laser pulses with nearly perfect Gaussian spatial profile.” Opt. Commun., 91, Pp. 77–81. Publisher's VersionAbstract
    We use self-phase modulation in a single-mode fiber to produce broadband femtosecond laser pulses. Subsequent amplification through two Bethune cells yields high-energy, tunable, pulses synchronized with the output of an amplified colliding-pulse-modelocked (CPM) laser. We routinely obtain tunable 200-J pulses of 42-fs (FWHM) duration with a nearly perfect Gaussian spatial profile. Although self-phase modulation in a single-mode fiber is widely used in femtosecond laser systems, amplification of a figer-generated supercontinuum in a Bethune cell amplifier is a new feature which maintains the high-quality spatial profile while providing high gain. This laser system is particularly well suited for high energy dual-wavelength pump-probe experiments and time-resolved four-wave mixing spectroscopy.
    C. Lu, J. R. Goldman, S. Deliwala, K. Hsien Chen, and E. Mazur. 1991. “Direct evidence for nu1-mode excitation in the infrared multiphoton excitation of SO2.” Chem. Phys. Lett., 176, Pp. 355–360. Publisher's VersionAbstract
    We investigated the infrared multiphoton excitation of SO2 in bulk samplesand in a supersonic jet with the 9R(22), 9R(32), and 9P(32) CO2-laser lines. Coherent anti-Stokes Raman spectra reveal unambiguously that only the nu1-mode at 1151.3 cm-1 is actually pumped; no 2nu2 overtone pumping at 1035.2 cm-1 is observed. From the spectra we directly determine the anharmonic constants chi11 = -3.65 ± 0.06 cm-1 and chi12 = -3.3 ± 0.3 cm-1.
    E. Mazur and C. Lu. 1990. “Nonlinear spectroscopy of infrared multiphoton excited molecules.” In Resonances, edited by P. Pershan M. Levenson, E. Mazur and Y. R. Shen, Pp. 165–174. World Scientific. Publisher's VersionAbstract
    This paper presents an overview of recently obtained results on infrared multiphoton excited molecules using coherent anti-Stokes Raman Spectroscopy. The data underline the important role of collisions in the excitation process.
    K. Hsien Chen, C. Lu, E. Mazur, and N. Bloembergen. 1990. “Multiplex pure rotational coherent anti-Stokes Raman spectroscopy in a molecular beam.” J. Raman Spectroscopy, 21, Pp. 819–825. Publisher's VersionAbstract
    Pure rotational coherent anti-Stokes Raman spectroscopy using a single broadband dye-laser has been applied to study rotational energy distribution of molecules in a pulsed supersonic beam. The multiplex BOXCARS configuration allows accurate determination of rotational constants and rotational temperatures, and offers a great number of advantages over other methods. The technique was applied to calibrate the cooling effect of a pulsed molecular beam of N2 and to study the rotational energy distributions of infrared multiphoton excited C2H4.
    K. Hsien Chen, C. Lu, L. Anibal Avils, E. Mazur, N. Bloembergen, and M. J. Shultz. 1989. “Multiplex coherent anti-Stokes Raman spectroscopy study of infrared-multiphoton-excited OCS.” J. Chem. Phys., 91, Pp. 1462–1468. Publisher's VersionAbstract
    The vibrational energy distribution following v2 overtone excitation of OCS by a pulsed CO2 laser is studied by monitoring the coherent anti-Stokes Raman spectrum of the v1 mode. Because of the slow energy transfer from the pumped mode to other modes, and because the anharmonicity of the v2 mode is small, OCS is an ideal system for studying the interaction of an intense infrared laser field with a single, nearly harmonic, oscillator. From the spectra the cross-anharmonicities of the of the n1 mode are determined to be x12 = -6.0 cm-1 and x13 = -2.7 cm-1, respectively. The time-dependence of the spectra provides information on V-V energy transfer rates. In particular, the measurements put a lower limit of kv2->v2 = 1 ms-1 torr-1 on the vibrational relaxation rate within n2 mode. At high excitation, the temperature of the v2 mode rises up to 2000 K, and hot bands are observed up to the v = 4 level. This fourth overtone peak is split because of either a Fermi resonance or vibrational angular momentum splitting.
    K. Hsien Chen, J. Wang, and E. Mazur. 1989. “Chen, Wang, and Mazur Reply.” Phys. Rev. Lett., 63, Pp. 1534–1534. Publisher's VersionAbstract
    A reply to a Comment by A.L. Malinovsky and E.A. Ryabov on an article in the Physical Review Letters by Chen, Wang and Mazur. Their comment is attached to this reply.
    J. Wang, K. Hsien Chen, and E. Mazur. 1987. “Highly nonthermal intramolecular energy distribution in isolated infrared multiphoton excited CF2Cl2 molecules.” In Laser Spectroscopy VIII, edited by S. Svanberg, Pp. 236–238. Springer-Verlag. Publisher's VersionAbstract
    When a polyatomic molecule with a strong vibrational absorption band is irradiated with an intense resonant infrared laser pulse it can absorb many (10 to 40) infrared photons. If some initial energy deposition is localizedpreferably in one vibrational mode or in a subset of modesit may become possible to induce mode-selective reactions by infrared multiphoton excitation. The intramolecular dynamics of infrared multiphoton excited molecules has been studied by a variety of spectroscopic techniques. One of these techniques is spontaneous Raman spectroscopy. In the past five years this technique has been successfully applied to monitor the vibrational energy in infrared multiphoton excited molecules. In this work we present experimental results of recent time-resolved spontaneous Raman experiments on collisionless infrared multiphoton excited CF2Cl2 molecules. The experiments show that the intramolecular energy distribution is highly nonthermal, and that a large part of the vibrational energy remains localized in the pump mode for a period of time long compared to the mean free time of the molecules.
    E. Mazur. 1987. “Fourier transform heterodyne spectroscopy: a simple novel technique with ultrahigh (150 mHz) resolution.” In Laser Spectroscopy VIII, edited by S. Svanberg, Pp. 390–392. Springer-Verlag. Publisher's VersionAbstract
    Light beating spectroscopy has been used from the early days of the laser to study light scattering. By detecting the beating signal between the scattered light and a 'local oscillator' field derived from the same laser, resolving powers of 10^14 have been achieved. The Fourier transform heterodyne spectroscopy presented here is simpler and more direct than the conventional heterodyne techniques using autocorrelators or spectrum analyzers.

Pages