Publications

    Y. Siegal, E. N. Glezer, L. Huang, and E. Mazur. 1995. “Laser-induced phase transitions in semiconductors.” Ann. Rev. Mat. Sci., 25, Pp. 223–247. Publisher's VersionAbstract
    Optical studies of semiconductors under intense femtosecond laser pulse excitation suggest that an ultrafast phase transition takes places before the electronic system has time to thermally equilibrate with the lattice. The excitation of a critical density of valence band electrons destabilizes the covalent bonding in the crystal, resulting in a structural phase transition. The deformation of the lattice leads to a decrease in the average bonding- antibonding splitting and a collapse of the band-gap. We review the relationship between structural, electronic and optical properties, as well as the timescales for electron recombination, diffusion, and energy relaxation. Direct optical measurements of the dielectric constant and second-order nonlinear susceptibility are used to determine the time evolution of the phase transition.
    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. 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.
    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.
    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.
    D. S. Chung, K. Y. Christina Lee, and E. Mazur. 1990. “Spectral asymmetry in the light scattered from a nonequilibrium liquid interface.” Phys. Lett. A, 145, Pp. 348–352. Publisher's VersionAbstract
    An asymmetry of the two capillary wave peaks in the light scattered from a water surface subject to a temperature gradient parallel to the surface has been observed using a Fourier transform heterodyne technique. The sign and order of magnitude of the effect agree with linear fluctuating hydrodynamic theory.
    K. Hsien Chen. 1989. “Spontaneous Raman and coherent anti-Stokes Raman spectroscopy of infrared multiphoton excited molecules”. Publisher's VersionAbstract
    This thesis is a study of infrared multiphoton excitation using spontaneous and coherent anti-Stokes Raman spectroscopy. The spontaneous Raman measurements provide information on the intramolecular vibrational energy distribution over the different modes. This information is complemented by the CARS measurements which make it possible to perform state-specific studies of the vibrational and rotational distribution. For SF6, the time-resolved spontaneous Raman measurements show complete equilibration of energy from the pump mode to other vibrational modes. In contrast, for smaller molecules such as CF2Cl2, a nonthermal energy distribution is observed after excitation. These measurements therefore disprove the general belief that the intramolecular energy distribution in infrared multiphoton molecules is always in equilibrium. The CARS measurements on bulk OCS provide values for the anharmonicities and for the energy transfer rates between modes. In addition the spectra show a very fast relaxation of the vibrational energy within the n2 mode. For SO2, the CARS measurements show that it is the n1 symmetric stretching mode and not the overtone excitation of the n2 bending mode that is pumped by the CO2 laser. Moreover, it is shown that the hot bands of SO2 have been incorrectly assigned up to now. Corrected values for the anharmonicities are given. In the second half of the thesis, a pulsed supersonic molecular beam is added to the infrared multiphoton excitation study. Combined with the state- specific CARS technique, the collisionless and internally cooled molecules in the beam open the door to a more detailed study of the excitation process. Pure rotational CARS is used to study the change in rotational distribution of ethylene due to infrared excitation in the beam. The appearance of rotational holes reveal which rotational states are pumped by the CO2 laser. For OCS the evolution of the overtone population into a thermal distribution is studied, providing a value for the intramode relaxation rate. Finally, the study of SF6 in the supersonic beam sheds new light on the energy distribution in SF6 after infrared multiphoton excitation. It is shown that the two-ensemble population distribution observed by other investigators after infrared multiphoton excitation involves a considerable amount of collisional relaxation.
    J. Wang, K. Hsien Chen, and E. Mazur. 1988. “Raman spectroscopy of infrared multiphoton excited molecules.” Laser Chem., 8, Pp. 97–122. Publisher's VersionAbstract
    This paper presents an overview of data obtained on the intramolecular vibrational energy distribution in infrared multiphoton excited CF2HCl, CF2Cl2, SF6 and CH3CHF2. All but CF2HCl show collisionless changes in the intensity of the spontaneous Raman signals after excitation, indicating that the excitation alters the population in the Raman active modes. A comparison of the spectrally integrated intensities of the Raman signals yields information on the distribution of vibrational energy over the modes of the molecule. The results for CF2Cl2 show a nonthermal distribution of energy after the excitation.
    K. Hsien Chen, J. Wang, and E. Mazur. 1987. “Nonthermal intramolecular vibrational energy distribution in infrared-multiphoton-excited CF2Cl2.” Phys. Rev. Lett., 59, Pp. 2728–2731. Publisher's VersionAbstract
    The intramolecular vibrational energy distribution of infrared multiphoton excited CF2Cl2 molecules is studied using time-resolved spontaneous Raman scattering. The time evolution of the signals from three vibrational modes is studied up to 600 ns after excitation and in the presence of N2 buffer gas. Following collisionless infrared multiphoton excitation a non-thermal distribution of vibrational energy is observed. The experimental result shows that equilibration of the intra-molecular vibrational energy distribution does not occur in this molecule up to at least 10,000 cm-1 of total internal vibrational energy. Note: also see Comment and Reply published separately.
    J. Wang, K. Hsien Chen, and E. Mazur. 1986. “Time-resolved spontaneous Raman spectroscopy of infrared-multiphoton-excited SF6.” Phys. Rev. A, 34, Pp. 3892–3901. Publisher's VersionAbstract
    Spontaneous Raman spectroscopy is used as a tool for studying the vibrational energy distribution of collisionless infrared multiphoton excited SF6. A collisionless increase in Stokes and anti-Stokes signals from the strong Raman-active n1-mode is observed after infrared multiphoton excitation by a high-power 500 ps CO2-laser pulse tuned to the infrared active n3-mode. Results are presented over a pressure range from 13 Pa (100 mtorr) to 270 Pa (2 torr). The pressure dependence clearly proves that the increase does not depend on collisions. The effects are studied as a function of time and of the infrared energy fluence, infrared wavelength and infrared pulse duration. The experimental data show that an intramolecular equilibrium of vibrational energy is established within the 20 ns time-resolution of the experimental setup. The multiphoton excitation shows a red shift and intensity broadening. A comparison with results from photoacoustic measurements is made.
    E. Mazur, K. Hsien Chen, and J. Wang. 1986. “The interaction of infrared radiation with isolated molecules: intramolecular nonequilibrium.” In . Int. Conf. on Lasers '86. Publisher's VersionAbstract
    Anti-Stokes signals from various modes of isolated, infrared multiple photon excited molecules are measured to determine the intramolecular distribution of vibrational energy. This paper presents results for CF2HCl, CF2Cl 2, SF6 and 1,1-p;C2H 4F2. All but CF2HCl exhibit collisionless changes in Raman spectrum after infrared multiphoton excitation. This shows that the excitation modifies the population of these modes. Even though the symmetric SF 6 molecule reaches an intramolecular equilibrium within the 20 ns time resolution of the experiment, the other molecules exhibit a distinct nonequilibrium intramolecular distribution of vibrational excitation energy.
    E. A. Mason and E. Mazur. 1985. “Theory of field effects on transport properties of polyatomic gases in the transition regime.” Physica, 130A, Pp. 437–464. Publisher's VersionAbstract
    A simple method is presented for describing the effects of external magnetic or electric fields on the transport properties of polyatomic gases over the entire range from the continuum to the Knudsen regime. Instead of treating bulk and boundary-layer effects separately, both molecular and surface scattering are included from the beginning in the collisional part of the Boltzmann equation, and the surface is treated as one component of a multicomponent mixture. A simple first-order solution of this problem is sufficient to account for the dependence of the transport coefficients on the Knudsen number in the presence of a field. Detailed results for the longitudinal and transverse viscomagnetic effects in a single gas are presented, and shown to be in good agreement with experimental data for CO and N2.
    E. Mazur and P. Mazur. 1984. “General expression for the matrix of saturated field effects.” Phys. Rev. A, 29, Pp. 991–993. Publisher's VersionAbstract
    A general expression for the change in transport coefficients of a dilute gas in the limit of an infinite external magnetic or electric field is derived without using a perturbation expansion of the collision operator. Previously derived properties of the saturated field effects are subsequently generalized and some implications are discussed.
    K. D. Van Den Hout, P. W. Hermans, E. Mazur, and H. F. P. Knaap. 1980. “The broadening and shift of the rotational Raman lines for hydrogen isotopes at low temperatures.” Physica, 104A, Pp. 509–547. Publisher's VersionAbstract
    Collisional broadening of rotational Raman lines has been investigated for the gases nH2, nD2 and HD between 20 and 300 K. For H2 and D2 the dependence of the linewidth on ortho-para composition has been investigated. For a number of systems the pressure shift of the Raman lines has also been determined. The experimental results are compared with theory.

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