Publications

    T. Her. 1998. “Femtochemistry at gas/solid interfaces”. Publisher's VersionAbstract
    This thesis studies the chemical reactions on surfaces induced by ultrashort laser pulses. Two sets of experiments are presented. In the first experiment, we study the desorption of O2 and the oxidation of CO from CO/O2/Pt(111) induced by 300-fs laser pulses. We observe the transition between linear and nonlinear fluence dependence of the photoyield. In the nonlinear regime, the yield of O2 is enhanced by a factor of 1000 compared to that in the linear regime, and is a few decades more than the yield of CO2 . The decay of the photoexcitation is found to be around 1 ps. We verify that the nascent photoexcited electrons in platinum to be responsible for the chemical reactions on the surface and the transition in the fluence dependence indicates a change from single to cooperative action of photo-excited electrons. We also study the reaction pathway leading to the desorption and oxidation by isotope labeling. Our data show conclusively that the O2 desorbs molecularly and put constraints on the formation of CO2 . Results from this work elucidate the mechanism of the metal photocatalysis and expand the understanding of electron-mediated pro-cesses at surfaces and interfaces. In the second experiment, we discover that silicon surfaces develop an array of sharp conical spikes when irradiated with 500 laser pulses of 100-fs duration, 10-kJ/m2 fluence in the atmosphere of 500-torr SF6 or Cl2 gas. The spikes, which are up to 40-microm tall, have a cross section of about 6 x 10 microm2 near the base and taper down to a diameter of about 0.8-microm near the tip. The tops of the spikes are approximately at the same level as the surrounding surface of the wafer and are capped by a 1.5-microm ball. Sharp spikes are formed only in the presence of a halogen-containing gas; irradiation in vacuum or in the presence of N2 or He produces blunt spikes with irregular sides and rounded tops. We also find that the size of spikes depends strongly on pulse duration and laser fluence. At low fluence or long pulse-duration, the spikes become smaller and denser. Preliminary study suggests that the spike formation is seeded by the growth of silicon islands on the surface, followed by differential removal of substrate materials around the islands via laser ablation and laser-induced halogen gas-assisted etching.
    C. B. Schaffer, N. Nishimura, and E. Mazur. 1998. “Thresholds for femtosecond laser-induced breakdown in bulk transparent solids and water.” In . SPIE Annual Meeting. Publisher's VersionAbstract
    We present thresholds for optical breakdown in bulk transparent solids and water with 100-fs laser pulses. In solids, we used microscopy and scattering techniques to determine thresholds for plasma formation and permanent damage in a wide variety of materials. Transmission measurements show that damage occurs at energies where there is little absorption of the laser pulse. In water, we used scattering and acoustic techniques to measure the breakdown threshold for 100-fs pulses. In contrast to solids, transmission measurements in water indicate that there is no plasma or bubble formation unless there is significant absorption. For comparison, we also measured breakdown thresholds for 200- ps pulses.
    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
    A. Feder. 1997. “Optical studies of monolayers at the air/water interface”. Publisher's VersionAbstract
    Monolayers of amphiphilic molecules at the air/water interface, Langmuir monolayers, provide an experimentally accessible system for the study of physics in two dimensions. As a function of temperature and density, Langmuir monolayers display a series of phases and phase transitions; this thesis describes several optical experiments which clarify the macroscopic properties of some of these monolayer phases. In one experiment a combination of two techniques, laser light scattering and Brewster angle microscopy, is used to study the effect of inhomogeneous monolayers on the damping of capillary waves. It is shown that the monolayer contributes to the capillary wave damping when the size of a typical monolayer domain is equal to or larger than the capillary wavelength. Secondly, a new technique to measure the tilt angle of molecules from the surface normal is described. The validity of the technique is demonstrated by comparison to previously performed x-ray scattering experiments. The tilt angle of a monolayer of long-chain alcohol molecules is measured as a function of temperature and pressure near first- and second-order phase transitions from tilted to untilted monolayer phases. Finally, depolarized Brewster angle microscopy is used to study orientational fluctuations in a two-dimensional smectic-C liquid crystal. The results are in excellent agreement with theoretical predictions. In addition, the first measurements of orientational elasticity and viscosity in a liquid crystal system with variable density are presented.
    E. N. Glezer and E. Mazur. 1997. “Ultrafast-laser driven micro-explosions in transparent materials.” Appl. Phys. Lett., 71, Pp. 882–884. Publisher's VersionAbstract
    We initiate micro-explosions inside fused silica, quartz, sapphire, and other transparent materials using tightly-focused 100-fs laser pulses. In the micro-explosions, material is ejected from the center, forming a cavity surrounded by a region of compacted material. We examine the resulting structures with optical microscopy, diffraction, and atomic force microscopy of internal cross-sections. We find the structures have a diameter of only 200250 nm, which we attribute to strong self-focusing of the laser pulse. These experiments probe a unique regime of light propagation inside materials at intensities approaching 1021 W/m2, the electron ionization that accompanies it, and the material response to extreme pressure and temperature conditions. The micro-explosions also provide a novel technique for internal microstructuring of transparent materials.
    R. J. Finlay, S. Deliwala, J. R. Goldman, T. Her, W. D. Mieher, C. Wu, and E. Mazur. 1995. “Femtosecond laser activation of surface reactions.” In . Laser Techniques for Surface Science II. Publisher's VersionAbstract
    Laser induced formation of CO2 and desorption of O 2 are initiated with femtosecond and picosecond laser excitation of a Pt(111) surface prepared with coadsorbed CO and O2 at 90 K. The nonlinear fluence dependent reaction yields were measured for 267, 400, and 800 nm wavelengths, and for pulse durations from 80 fs to 3.6 ps. Two- pulse correlation experiments measuring total O2 desorption yield versus time delay between 80 fs pulses show a 0.9 ps HWHM central peak and a slower 0.1 ns time-scale. At 267 nm the relative yields of O2 and CO2 are found to depend on fluence. Comparison of results at different wavelengths and pulsewidths shows that nonthermalized surface electrons play a role in the laser-induced surface chemistry.
    S. Deliwala. 1995. “Time-resolved studies of molecular dynamics using nano- and femtosecond laser pulses”. Publisher's VersionAbstract
    This thesis presents the results of two experiments that measure the evolution of laser excited molecules. The experiment performed with 0.1-ps laser pulses elucidates the dynamics of desorption of O2 and formation of CO2 on a platinum surface. The experiment performed with nanosecond time resolution reveals the inter- and intra- molecular vibrational dynamics of infrared laser pumped molecules. Desorption of O2 and formation of CO2 were induced with subpicosecond laser pulses on a Pt(111) surface dosed with coadsorbed O2 and CO. Fluence dependent yields obtained over a range of laser wavelengths from 267 to 800 nm, and pulse durations from 80 fs to 3.6 ps are presented. We observe a dependence of the nonlinear desorption yield on wavelength. Two-pulse correlation measurements show two different time- scales relevant to the desorption. The results show that nonthermal electrons play a role in the surface chemistry, and that an equilibrated pre-heating of the surface modes leads to enhanced desorption. In the second set of experiments reported in this thesis, time- resolved coherent anti-Stokes Raman spectroscopy was used to obtain the rovibrational energy distributions in polyatomic molecules following infrared multiphoton excitation. In addition to presenting new results on SF6, we review previously obtained data on SO2 and OCS. The data yield new details about infrared multiphoton excitation and intramolecular vibrational energy relaxation. In particular they show the significance of collisions in redistributing vibrational energy following excitation. The results also clearly show stronger inter-mode coupling and higher excitation in systems with increasing numbers of atoms per molecule. In addition, a detailed description is provided of the Ti:Sapphire based ultrashort pulsed amplified laser system. Both, the principles and the design of the laser system are discussed to serve as a manual for the femtosecond laser system constructed for the study of molecules adsorbed on a metal surface.
    P. N. Saeta. 1991. “Optical studies of ultrafast carrier dynamics in semiconducting materials”. Publisher's VersionAbstract
    This thesis describes three experiments using pump-probe spectroscopy with picosecond and femtosecond laser pulses to study carrier dynamics in semiconductors. In the first experiment, highly excited gallium arsenide is studied with reflectivity and reflection second-harmonic probes using 160-fs pulses at 623 nm. Above a threshold incident fluence of  0.1 J/ cm2, the second-harmonic signal is observed to fall to zero in 100 fs. This drop shows that the valence electrons undergo a transformation to a centrosymmetric configuration and strongly suggests that the atomic lattice disorders before acquiring appreciable energy from the pulse. With an exponential time of 200 fs the reflectivity rises to a steady high value that is consistent with a metallic molten phase. Tens of picoseconds after excitation, the reflectivity drops considerably from the high value of the liquid and probe light is increasingly scattered out of the plane of incidence. The excitation produces  90-nm-deep pits in the wafer surface that are covered with a layer of solidified droplets. An estimate of the absorption and scattering caused by a cloud of liquid droplets ejected from the surface suggests that ablated material can account for the reflectivity drop. In the second experiment, femtosecond transient absorption spectroscopy is used to study the carrier dynamics of type II GaAs/ AlAs multiple quantum wells. The spectra show a rapid partial recovery in the pump-induced bleaching near the absorption edge that is produced by the rapid scattering of conduction electrons in the Gamma valley of the GaAs layers to the X valleys of the AlAs barrier layers. The scattering time is measured to be 100 fs for an 8-monolayer sample and 400 fs for an 11-monolayer sample. In the third experiment, the picosecond laser melting of silicon is studied using a streak camera to provide spatial and temporal resolution. The reflectivity near Brewsters angle shows the expected eightfold increase following melting. Images of the excited silicon surface emphasize the importance of spatial resolution in nearthreshold experiments.
    T. B. Simpson, E. Mazur, I. Burak, and N. Bloembergen. 1984. “Infrared multiphoton excitation of small molecules.” Israel J. Chem., 24, Pp. 179–186. Publisher's VersionAbstract
    The collisionless infrared multiphoton excitation of small molecules, OCS, SO2, NO2 and DN3 is discussed. Depite the small absorbance of IR photons by these molecules, evidence for the population of highly excited vibronic levels below the dissociation threshold is obtained. Multiphoton dissociation is observed, as well. The average number of photons absorbed and dissociation yields depend predominately on the laser intensity. High order multiphoton processes couple the ground vibrational level with the quasicontinuum of excited levels below and above the dissociation threshold.
    E. Mazur, G. W. t Hooft, L. J. F. Hermans, and H. F. P. Knaap. 1978. “Senftleben-Beenakker effects in diffusing and heat conducting gas mixtures.” In . Proc. 11th Symposium on Rarefied Gas Dynamics. Publisher's VersionAbstract
    The influence of an external field on the transport phenomena in polyatomic gases leads to detailed information about the kinetic theory of such gases. In particular, a magnetic field gives rise to transverse transport, perpendicular to both the macroscopic gradient and the field. The present experimental results, along with other recent results from our laboratory, comprise a complete set of data on such transverse effects occurring in heat conducting and diffusing gas mixtures, viz., simple heat conduction and diffusion and also their cross effects, thermal diffusion and the diffusion-thermo effect. Special emphasis is given to the phenomenology of these effects and to experimental techniques.