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    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.
    S. Deliwala, J. R. Goldman, K. Hsien Chen, C. Lu, and E. Mazur. 1994. “Coherent Anti-Stokes Raman Spectroscopy of Infrared Multiphoton Excited Molecules.” J. Chem. Phys., 101, Pp. 8517–8528.Abstract
    Time-resolved coherent anti-Stokes Raman spectroscopy is 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.
    Y. Siegal, E. N. Glezer, L. Huang, and E. Mazur. 1994. “Laser-induced bandgap collapse in GaAs.” In . Ultrafast Phenomena in Semiconductors. Publisher's VersionAbstract
    We present recent time-resolved measurements of the linear dielectric constant of GaAs at 2.2 eV and 4.4 eV following femtosecond laser pulse excitation. In sharp contrast to predictions based on the widely-used Drude model, the data show an interband absorption peak coming into resonance first with the 4.4-eV probe photon energy and then with the 2.2-eV probe photon energy, indicating major changes in the band structure. The time scale for these changes ranges from within 100 fs to a few picoseconds, depending on the incident pump pulse fluence.
    Q. Yu Wang, A. Feder, and E. Mazur. 1994. “Capillary wave damping in heterogeneous monolayers.” J. Phys. Chem., 98, Pp. 12720–12726.Abstract
    We studied capillary wave damping in heterogeneous monolayers of triglyceride at the air-water interface over a range of surface wavelengths (70-300 m) using a light scattering technique. In addition, we studied the monolayer morphology using a Brewster angle microscope. We found that the morphology has a strong effect on the capillary wave damping. In the gas-liquid expanded (G/LE) coexistence region the monolayer forms a two-dimensional foam structure, where 'bubbles' of gas phase are separated by regions of liquid expanded phase. If the width of the LE regions is smaller than the wavelength of the capillary wave, the monolayer has no measurable effect on the damping of the capillary wave. When the width of the LE regions is larger than the wavelength, the capillary wave damping constant increases from its value for a clean water surface. We attribute this increase to a rise in the dynamic dilational elasticity of the heterogeneous monolayer.
    Y. Siegal, E. N. Glezer, and E. Mazur. 1994. “Dielectric constant of GaAs During Subpicosecond Laser-Induced Phase Transition.” Phys. Rev. B, 49, Pp. 16403–16406. Publisher's VersionAbstract
    We measured the time evolution of the real and imaginary parts of the dielectric constant of GaAs following femtosecond laser pulse excitation. The data show a collapse of the average optical gap, or average bonding anti-bonding energy level separation. The rate of collapse increases with pump fluence. The decrease in the gap indicates that the pump beam induces a structural transformation from a covalent, tetrahedrally-coordinated crystal to a phase with metallic cohesive properties.
    Y. Siegal. 1994. “Time-resolved studies of laser-induced phase transitions in GaAs”. Publisher's VersionAbstract
    This thesis describes a series of time-resolved experiments of the linear and nonlinear optical properties of GaAs during laser-induced phase transitions. The first set of experiments consists of a direct determination of the behavior of the linear dielectric constant at photon energies of 2.2 eV and 4.4 eV following excitation of the sample with 1.9-eV, 70-fs laser pulses spanning a fluence range from 0 to 2.5 kJ/m2. The results from this set of experiments were used to extract the behavior of the second-order optical susceptibility from second-harmonic generation measurements made under identical excitation conditions. These experiments are unique because they provide explicit information on the behavior of intrinsic material properties - the linear and nonlinear optical susceptibilities - during laser- induced phase transitions in semiconductors without the ambiguities in interpretation that are generally inherent in reflectivity and second-harmonic generation measurements. The dielectric constant data indicate a drop in the average bonding- antibonding splitting of GaAs following the laser pulse excitation. This behavior leads to a collapse of the band-gap on a picosecond time scale for excitation at fluences near the damage threshold of 1.0 kJ/m 2 and even faster at higher excitation fluences. The changes in the electronic band structure result from a combination of electronic screening by the excited free carriers and structural deformation of the lattice caused by the destabilization of the covalent bonds. The behavior of the second- order susceptibility shows that the material loses long-range order before the average bonding-antibonding splitting, which is more sensitive to short- range structure, changes significantly. Loss of long-range order and a drop of more than 2 eV in the average bonding-antibonding splitting are seen even at fluences below the damage threshold, a regime in which the laser- induced changes are reversible.
    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.
    K. Y. Christina Lee, T. Chou, D. S. Chung, and E. Mazur. 1993. “Direct measurement of the spatial damping of capillary waves at liquid-vapor interfaces.” J. Phys. Chem., 97, Pp. 12876–12878.Abstract
    We measured the spatial damping of low-frequency surface waves at air-water interfaces using a novel heterodyne light scattering technique. For pure water the measured damping agrees well with linear hydrodynamic theory. For interfaces covered with a monolayer of pentadecanoic acid, we find a five-fold increase in damping at a surface concentration of 2.2 molecules nm-2, near the high-density end of the gas/liquid-expanded coexistence region. The behavior of the damping as a function of surface concentration cannot be explained by existing theories.
    J. Wang and E. Mazur. 1993. “Rejection of Stochastic background noise in low-level light pulsed light scattering experiments.” Rev. Sci. Instrum., 64, Pp. 2550–2551. Publisher's VersionAbstract
    We present an electronic scheme for reducing stochastic noise in the detection of low-level light signals in experiments where the signal coincides in time with a probe laser pulse. This scheme has been applied successfully in Raman experiments to reject the noise generated by afterpulsing in photomultiplier tubes as well as by unwanted laser-induced-fluorescence.
    J. Kai Wang, Y. Siegal, P. N. Saeta, N. Bloembergen, and E. Mazur. 1993. “Femtosecond, Electronically-Induced Disordering of GaAs.” In Ultrafast Phenomena VIII, edited by G. A. Mourou J.-L. Martin, A. Migus and A. H. Zewail, Pp. 420–421. Springer-Verlag. Publisher's VersionAbstract
    Excitation of GaAs with a femtosecond laser pulse leads to disordering in less than 100 femtoseconds, before appreciable lattice heating through phonon emission.
    J. Kai Wang. 1992. “Femtosecond nonlinear optics in gases and solids”. Publisher's VersionAbstract
    This thesis describes three experiments on nonlinear optical interactions in materials using high-energy, femtosecond laser pulses as well as several applications of these experiments. The first one make use of linear and nonlinear optical techniques to study ultrafast laser-induced disordering in gallium arsenide. The pump-probe experiment is performed on both the (100) and (110) GaAs crystalline surfaces with 165-fs, 620-nm optical pulses. The second-harmonic generation monitors the electronic disordering induced by the high-energy pump pulses. The linear reflectivity, on the other hand, gives information on the variation of the dielectric constant in the highly- excited region. Experimental results show that the second-harmonic signal vanishes with a decay time of 90 fs, indicating that a centrosymmetric structure is established within the pulse width. The linear reflectivity rises to a metallic value with a rise time of about 200 fs. Theoretical study shows that nonlinear optical absorption processes are important for laser pulses shorter than the electron-phonon interaction time. When a critical free carrier density is excited, the average bonding force is weakened and cannot maintain the crystal structure. The dielectric constant extracted from the high reflectivity value indicates a less conducting liquid phase than equilibrium liquid GaAs. Second-harmonic generation is also used to study Auger recombination at high carrier density for pump pulse fluence below the disordering threshold. The Auger recombination time is measured to be 400 fs, which requires a screening model for explanation. The long-time lattice heating by the pump pulse can be investigated by both linear and nonlinear optical techniques, which give the same lattice heating time of 60 ps in GaAs. In the second experiment, self-phase modulation in a single-mode fiber is used to provide a synchronized 200-nm supercontinuum source generated from the 165-fs laser pulses. Because of the Gaussian spatial profile from the single-mode fiber and the profile-preserving amplifier cells, high-quality, high-energy laser pulses are produced by a 10-Hz dye amplifier. A grating pair then compresses the pulse width to 45 fs. This design provides synchronized, tunable, femtosecond laser pulses with a Gaussian profile, suitable for high-energy two-color pump-probe experiment and ultrafast nonlinear laser spectroscopy. The third experiment demonstrates the effects of self-phase modulation and self-focusing on stimulated Raman scattering in high-pressure hydrogen using subpicosecond laser pulses. Theoretical study of transient stimulated Raman scattering is performed to take into account pump depletion due to self-phase modulation. The experimental results show that the behavior of the Raman gain falls into three input energy regimes. The Stokes radiation production in the low input energy region can be predicted by the theory of transient stimulated Raman scattering without pump depletion. In the medium input energy region, strong self-phase modulation suppresses the Stokes radiation output. This behavior is further confirmed by the addition of argon gas. Strong self-focusing, on the other hand, breaks up the laser beam and suppress self-phase modulation in the high input energy region. The Stokes radiation, therefore, recovers partially.
    E. Mazur. 1992. “Qualitative versus quantitative reasoning: Are we teaching the right thing?” Optics and Photonics News. Publisher's VersionAbstract
    For the past eight years I have been teaching an introductory physics course for engineering and science concentrators at Harvard University. Teaching this class, which does not include any physics majors, is a challenging experience because the students take this course as a concentration requirement, not because of a genuine interest in physics. At the same time it can be a very rewarding experience when, at the end of the semester, students show much more appreciation for the subject matter. I used to teach a fairly traditional course in an equally traditional lecture-type of presentation, enlivened by classroom demonstrations. I was generally satisfied with my teaching during these years my students did well on what I considered pretty difficult problems and the feedback I received from them was positive. About a year ago, however, I came across a series of articles by David Hestenes of Arizona State University, which completely and permanently changed my views on teaching.
    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.
    E. Mazur. 1991. “Can We Teach Computers to Teach?” In Computers in Physics, 5: Pp. 31–38. Publisher's VersionAbstract
    The computer has become a mandatory tool in academia and business. A walk around a university campus is likely to show that there are as many computers as there are students, faculty and staff. Outside the campus many of our daily activities have to do with computers: banking, reservations, check-out registers at supermarkets, not to mention all the computer-generated mail we receive every day.Surprisingly, in education the computer is still not a very much appreciated newcomer. One reason for this is that until not so long ago computers were text-oriented, accepting only commands in the forms of words. Such "educational" software usually emulated multiple-choice exams. Naturally, such programs could not keep anyone's attention very long. Another reason for the small inroads computers have made in education is that computers usually excel at doing routine tasks while education normally is all but routine. I don't think computers will replace teachers, but I am confident, however, that computers will play an important role in improving teaching. To illustrate this I will discuss several projects pertaining to computers in education in which I am involved, which, naturally, are in my own discipline, physics.
    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.
    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.
    P. N. Saeta, J. Kai Wang, Y. Siegal, N. Bloembergen, and E. Mazur. 1991. “Ultrafast Electronic Disordering During Femtosecond Laser Melting of GaAs.” Phys. Rev. Lett., 67, Pp. 1023–1026. Publisher's VersionAbstract
    We have observed an ultrarapid electronic phase transformation to a centrosymmetric electronic state during laser excitation of GaAs with intense femtosecond pulses. Reflection second-harmonic intensity from the upper 90 atomic layers vanishes within 100 fs; reflectivity rises within 0.5 ps to a steady value characteristic of a metallic molten phase, long before phonon emission can heat the lattice to the melting temperature.
    D. S. Chung. 1991. “Fourier transform heterodyne spectroscopy of liquid interfaces”. Publisher's VersionAbstract
    This thesis describes the application of a novel Fourier transform heterodyne spectroscopy technique with an ultrahigh resolution of 200 mHz to the study of capillary waves at liquid-vapor interfaces. The apparatus uses a frequency-shifted local oscillator to separate signals from counter- propagating capillary waves of identical frequency. The main beam and local oscillator are aligned in such a way as to select capillary waves of a given, continuously adjustable frequency. This capability to separate counter-propagating waves was used to study the spectral asymmetry of light scattered from capillary waves at a nonequilibrium water surface in the presence of a temperature gradient. The observed asymmetries agree, in sign and order of magnitude, with the one predicted by linearized fluctuating hydrodynamics. This apparatus was also used to measure the spatial damping coefficients of capillary waves at a clean water surface and a water surface covered with a monolayer of pentadecanoic acid. For these measurements a double-beam heterodyne technique, which requires no calibration or deconvolution of instrumental functions, was used. The spatial damping coefficient of a clean water is in good agreement with the hydrodynamics theory. A sharp maximum in the spatial damping was observed at the end of the coexistence region of two phases of the pentadecanoic acid monolayer.

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