Publications

    E. Mazur. 1996. “Science Lectures: A relic of the past?” In Physics World, 9: Pp. 13–14.Abstract
    In most introductory science courses we require the students to buy textbooks of encyclopedic dimensions and then we use lecture time to present what is printed in the text. We write the material on the blackboard and students copy it into their notebooks. If we are lucky they can follow the first fifteen minutes of lecture. If they lose the thread somewhere – and this is bound to happen sooner rather than later – note taking becomes completely blind: "I'll think about it later." Unfortunately the thinking is not always happening, and many students resort to memorization of the equations and algorithms copied in their notebooks. Many bad study habits are a direct result of the lecture system. I believe the days of straight lecturing in introductory science courses are numbered – we can no longer afford to ignore the inefficiency of the traditional lecture method, regardless of how lucid or inspiring our lectures are. The time has come to offer our students in introductory science classes more than a mere regurgitation of printed material.
    E. N. Glezer, M. Milosavljevic, L. Huang, R. J. Finlay, T. Her, J. Paul Callan, and E. Mazur. 1996. “3-D Optical data storage and engraving in transparent materials.” In Ultrafast Phenomena X, edited by W.H. Knox P.F. Barbara, J.G. Fujimoto and W. Zinth, Pp. 157–158. Springer Verlag. Publisher's VersionAbstract
    We present a novel method for 3-D optical data storage and internal engraving that has sub-micron resolution, provides a large contrast in index of refraction and is applicable to a wide range of transparent materials.
    E. N. Glezer, M. Milosavljevic, L. Huang, R. J. Finlay, T. Her, J. Paul Callan, and E. Mazur. 1996. “Three-dimensional optical storage inside transparent materials.” Opt. Lett., 21, Pp. 2023–2025. Publisher's VersionAbstract
    We present a novel method for 3-D optical data storage that has submicron-size resolution, provides a large contrast in index of refraction, and is applicable to a wide range of transparent materials. Bits are recorded by focusing 100-fs laser pulses inside the material using a 0.65 NA objective. The laser pulse produces a submicron-diameter structurally altered region with high contrast in index of refraction. Binary information can be recorded by writing such bits in multiple planes, and read out with a microscope objective with a short depth of field. We demonstrate data storage and retrieval with 2-m in-plane bit spacing and 15-m inter-plane spacing (17 Gbits/cm3). Scanning electron microscopy and atomic force microscopy show structural changes confined to an area 200 nm in diameter.
    E. N. Glezer. 1996. “Techniques of ultrafast spectroscopy.” In Spectroscopy and Dynamics of Collective Excitations in Solids, edited by B. Di Bartolo, Pp. 375–416. Plenum. Publisher's VersionAbstract
    This chapter begins with a general introduction to ultrafast spectroscopy, considers the limits of time and frequency resolution, and reviews the linear and nonlinear propagation of light pulses in a dispersive medium. Next, the basic elements of ultrashort laser pulse generation are described, including gain medium requirements, mode-locking mechanisms, compensation for group velocity dispersion, and pulse amplification. The last section deals with the measurement of ultrashort pulses, including joint time-frequency techniques, and also describes pulse-shaping in the frequency domain.
    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. Mazur, E. N. Glezer, L. Huang, and J. Paul Callan. 1996. “Ultrafast laser-induced structural changes in semiconductors.” In . 28th Annual Boulder Damage Symposium. Publisher's VersionAbstract
    We present experimentally determined values of dielectric function of GaAs following fs laser excitation. The data at photon energies of 2.2 and 4.4 eV show that the response of the dielectric function to the excitation is dominated by changes in the electronic band structure and not by the optical susceptibility of the excited free carriers. The behavior of the dielectric function indicates a drop in the average bonding-antibonding splitting of GaAs following the excitation, which leads to a collapse of the band gap. The changes in the electronic band structure result from a combination of electronic screening as well as structural deformation of the lattice caused by the destabilization of the covalent bond. The broadband measurement dielectric function from 1.5-3.5 eV reveals the ultrafast laser-induced heating, disordering and semiconductor to metal transition on a picosecond time scale.
    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.
    R. J. Finlay, T. Her, S. Deliwala, C. Wu, W. D. Mieher, and E. Mazur. 1996. “Surface femtochemistry: What is the role of substrate electrons?” In FEMTOCHEMISTRY: Ultrafast Chemical and Physical Processes in Molecular Systems, edited by M. Chergui, Pp. 457–464. World Scientific. Publisher's VersionAbstract
    The desorption of O2 from O2/Pt(111) and the formation of CO2 from CO/O2/Pt(111) are measured following excitation at various laser wavelengths with pulse durations from 80-fs to 3.6-ps. The trends in the reaction yield reflect a reaction mechanism in which substrate electrons out of thermal equilibrium interact with the adsorbates. We also demonstrate a rudimentary control of branching ratio using subpicosecond ultraviolet laser pulses.
    E. Mazur. 1996. “Interaction of ultrashort laser pulses with solids.” In Spectroscopy and Dynamics of Collective Excitations in Solids, edited by B. Di Bartolo, Pp. 417–468. Plenum. Publisher's VersionAbstract
    Beginning with some basic considerations in electromagnetic theory and solid state physics, I hope, in these four lectures, to develop some appreciation for the wonderfully rich electronic and optical properties of solids and to present an overview of current research in the area of the interaction of ultrashort laser pulses with solids. The first two lectures are tutorials on the electronic and optical properties of solids and on energy transfer and relaxation in semiconductors. While some of the introductory material is treated in undergraduate courses, I will try to paint a broad picture and connect a number of facts that often remain disconnected. This introduction is followed by a survey of optical measurements of carrier and phonon dynamics in solids. I will conclude with an overview of recent experiments on electronic and structural changes induced by intense short laser pulses, including work done in my own research group.
    E. Mazur. 1996. “The Problem with Problems.” In Optics and Photonics News, 6: Pp. 59–60. Publisher's VersionAbstract
    Standard, end-of-chapter textbook problems can generally be solved by rote memorization of sets of formulas and so-called 'problem-solving techniques.' Often, students solve problems by identifying quivalent problems that they have solved before. Don't we want our students to be able to tackle more challenging problems?Enrico Fermi was well known for his legendary ability to solve seemingly intractable problems in subjects entirely unfamiliar to him (e.g., How many piano tuners in Chicago?). Such 'Fermi problems' cannot be solved by deduction alone and require assumptions, models, order-of-magnitude estimates, and a great deal of self-confidence. We often use back-of-the-envelope estimates to familiarize ourselves with new problems. So why do we keep testing our students with conventional problems? Problems that contain the same number of unknowns and givens and frequently require nothing but mathematical skills. What distinguishes the successful scientist is not the ability to solve an integral, a differential equation, or a set of coupled equations but rather the ability to develop models, to make assumptions, to estimate magnitudes, the very skills developed in Fermi problems.
    E. N. Glezer, Y. Siegal, L. Huang, and E. Mazur. 1995. “Laser-induced bandgap collapse in GaAs.” Phys. Rev. B, 51, Pp. 6959–6970. Publisher's VersionAbstract
    We present experimentally determined values of the dielectric constant of GaAs 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 data show that the response of the dielectric constant to the excitation is dominated by changes in the electronic band structure and not by the optical susceptibility of the excited free carriers. The behavior of the dielectric constant indicates a drop in the average bonding-antibonding splitting of GaAs following the laser pulse excitation. This drop in the average splitting leads to a collapse of the bandgap on a picosecond time scale for excitation at fluences near the damage threshold of 1.0 kJ/m2 and on a subpicosecond time scale at higher excitation fluences. The changes in the electronic band structure result from a combination of electronic screening of the ionic potential as well as structural deformation of the lattice caused by the destabilization of the covalent bonds.
    S. Deliwala, R. J. Finlay, J. R. Goldman, T. Her, W. D. Mieher, and E. Mazur. 1995. “Surface femtochemistry of O2 and CO on Pt (111).” Chem. Phys. Lett., 242, Pp. 617–622.Abstract
    Desorption of O2 and formation of CO2 were induced with subpicosecond laser pulses on a Pt(111) surface dosed with coadsorbed O2 and CO. We report the 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. The nonlinear dependence of the yield on fluence is different at different wavelengths. Two-pulse correlation measurements show two different time-scales relevant to the desorption. The results show that nonthermalized electrons play a role in the surface chemistry, and that an equilibrated pre-heating of the surface modes leads to enhanced desorption.
    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.
    E. N. Glezer, L. Huang, Y. Siegal, J. Paul Callan, and E. Mazur. 1995. “Phase transitions induced by femtosecond laser pulses.” In . Materials Research Society Symposium, Vol. 397.Abstract
    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. Direct optical measurements of the dielectric constant and second-order nonlinear susceptibility are used to determine the time evolution of the phase
    Y. Siegal, E. N. Glezer, and E. Mazur. 1995. “Laser-Induced Phase Transitions in GaAs.” In Femtosecond Chemistry, edited by J. Manz and L. Wste, Pp. 581–602. Verlag Chemie.Abstract
    For over two decades the subject of laser-induced phase transitions in semiconductors has generated considerable interest. This field arose originally in the context of semiconductor annealing, a technologically important process aimed at repairing the damage to semiconductor crystals caused by dopant atom implantation. The conventional method for this process is thermal annealing the slow baking of a semiconductor in an oven. In the heated sample the increased mobility allows defects and dislocations to diffuse to the surface. In the 1970s, a similar effect was produced by irradiating a doped semiconductor with a short laser pulse. Although laser annealing of semiconductors has not replaced thermal annealing in industrial semiconductor processing, the discovery of laser annealing opened up a new and exciting chapter in the study of light-matter interactions: the use of light to alter the structure of matter.
    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. 1995. “Laser-Induced Phase Transitions in GaAs.” In Femtosecond Chemistry, edited by J. Manz and L. Wste, Pp. 581–602. Verlag Chemie.Abstract
    For over two decades the subject of laser-induced phase transitions in semiconductors has generated considerable interest. This field arose originally in the context of semiconductor annealing, a technologically important process aimed at repairing the damage to semiconductor crystals caused by dopant atom implantation. The conventional method for this process is thermal annealing the slow baking of a semiconductor in an oven. In the heated sample the increased mobility allows defects and dislocations to diffuse to the surface. In the 1970s, a similar effect was produced by irradiating a doped semiconductor with a short laser pulse. Although laser annealing of semiconductors has not replaced thermal annealing in industrial semiconductor processing, the discovery of laser annealing opened up a new and exciting chapter in the study of light-matter interactions: the use of light to alter the structure of matter.
    E. N. Glezer, Y. Siegal, L. Huang, and E. Mazur. 1995. “The behavior of chi(2) during laser-induced phase transtions in GaAs.” Phys. Rev. B, 51, Pp. 9589–9596. Publisher's VersionAbstract
    We explicitly determine the second-order optical susceptibility of GaAs following intense femtosecond laser-pulse excitation from second-harmonic generation measurements. To separate the dependence of the 4.4-eV second-harmonic signal on the second-order susceptibility from its dependence on the linear dielectric constant, we use experimentally determined values for the dielectric constant of GaAs at 2.2 and 4.4 eV. The results show that the excitation of electrons and the resulting changes in the lattice affect the behavior of the second-order susceptibility. At pump fluences of 0.6 kJ/m2 and higher, the material loses long-range order on a time scale ranging from 100 femtoseconds to tens of picoseconds, depending on the pump fluence. A recovery of the second-order susceptibility to its initial value at pump fluences between 0.6 and 1.0 kJ/m2 shows that the loss of long-range order is reversible in this fluence regime.
    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.

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