T. Shih, M. T. Winkler, T. Voss, and E. Mazur. 2009. “Dielectric function dynamics during femtosecond laser excitation of bulk ZnO.” Appl. Phys. A, 96, Pp. 363–367. Publisher's VersionAbstract
    Using a broadband dual-angle pump-probe reflectometry technique, we obtained the ultrafast dielectric function dynamics of bulk ZnO under femtosecond laser excitation. We determined that multiphoton absorption of the 800-nm femtosecond-laser excitation creates a large population of excited carriers with excess energy. Screening of the Coulomb interaction by the excited free carriers, causes damping of the exciton resonance and renormalization of the bandgap, causing broadband (2.33.5 eV) changes in the dielectric function of ZnO. From the dielectric function, many transient material properties, such as the index of refraction of ZnO under excitation, can be determined to optimize ZnO-based devices.
    C. R. Mendonca, M. Kandyla, T. Shih, R. F. Aroca, C. J.L. Constantino, and E. Mazur. 2009. “Ultrafast dynamics of bis (n-butylimido) perylene thin films excited by two-photon absorption.” Appl. Phys. A, 86, Pp. 369–372. Publisher's VersionAbstract
    We report a pump-probe study of the two-photon induced reflectivity changes in bis (n-butylimido) perylene thin films. To enhance the two-photon excitation we deposited bis (n-butylimido) perylene films on top of gold nano-islands. The observed transient response in the reflectivity spectrum of bis (n-butylimido) perylene is due to a depletion of the molecule�s ground state and excited state absorption.
    S. Kudryashov, M. Kandyla, C. A. D. Roeser, and E. Mazur. 2007. “Intraband and interband optical deformation potentials in femtosecond-laser excited alpha-Te.” Phys. Rev. B, 75, Pp. 085207–. Publisher's VersionAbstract
    We calculated the intraband and interband optical deformation potentials for semiconducting alpha-Te from experimental data of the band gap shrinkage and softening of the A1-optical phonon mode in response to femtosecond laser excitation. These potentials were obtained by applying first- and second-order perturbation theory to the Frlich Hamiltonian describing the carrier-phonon interaction. The intraband optical deformation potential is considerably smaller than previously estimated values; there are no previously reported values for the interband optical deformation potential.
    M. Kandyla, T. Shih, and E. Mazur. 2007. “Femtosecond dynamics of the laser-induced solid-to-liquid phase transition in aluminum.” Phys. Rev. B, 75, Pp. 214107-1–7. Publisher's VersionAbstract
    We present femtosecond time-resolved measurements of the reflectivity of aluminum during the laser-induced solid-to-liquid phase transition over the spectral range 1.73.5 eV. Previous optical and electron diffraction studies have shown discrepancies on the order of picoseconds in the timescale of the solid- to-liquid phase change. As a result, it is not clear if the transition mechanism is thermal or non-thermal. Our experiments conclusively show that this transition is a thermal process mediated through the transfer of heat from the photoexcited electronic population to the lattice. Our findings agree with the results of the electron diffraction study and rule out the non-thermal mechanism proposed by the optical study.
    M. Kandyla, T. Shih, and E. Mazur. 2007. “Femtosecond dynamics of the laser-induced solid-to-liquid phase transition in aluminum.” In . (CLEO) Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Phototnics Applications Systems Technologies 2007 Technical Digest. Publisher's VersionAbstract
    We present femtosecond time-resolved broadband measurements of the reflectivity of aluminum during the laser-induced solid-to-liquid phase transition. Our experiments show that this transition is a thermal process, settling an existing controversy.
    S. Kudryashov, M. Kandyla, C. A. D. Roeser, and E. Mazur. 2007. “Transient picometer atomic displacements in a-Te photoexcited by femtosecond laser pulses.” In . International conference on coherent and nonlinear optics, Proceedings of SPIE Vol. 6727. Publisher's VersionAbstract
    Subpicosecond, picometer atomic displacements in α-Те photoexcited by single femtosecond laser pulses have been measured by means of time-resolved optical reflectometry revealing threshold- like coherent quantum emission of single softened fully symmetrical optical A1-phonons and demonstrating absolute detection capability of this technique in studies of coherent phonon dynamics in solids.
    M. Kandyla, T. Shih, and E. Mazur. 2007. “Turning Aluminum Liquid in Picoseconds.” In Optics and Photonics News, 18: Pp. 44–. Publisher's VersionAbstract
    The interactions between the electrons and the lattice in solids define many basic properties of these materials. Exciting the electrons in a solid with intense ultrashort laser pulses is one way to induce non-equilibrium processes, observe the resulting dynamics and obtain direct information on electron-lattice interactions. An ultrashort laser pulse can rapidly heat electrons to temperatures on the order of 103 K, while leaving the lattice near room temperature.
    M. Kandyla. 2006. “Ultrafast dynamics of the laser-induced solid-to-liquid phase transition in aluminum”. Publisher's VersionAbstract
    This dissertation reports the ultrafast dynamics of aluminum during the solid-toliquid phase transition of melting after excitation by an intense femtosecond laser pulse. Photoexcitation with intense femtosecond laser pulses is known to create a novel melting mechanism called non-thermal melting. This mechanism has been observed repeatedly in semiconductors, but not yet in metals. We investigate the melting mechanism of aluminum by monitoring the reflectivity response following excitation by an intense laser pulse. We employ an optical pumpprobe technique designed to measure broadband reflectivity across the visible spectrum with femtosecond time resolution. A non-thermal melting mechanism was proposed for aluminum by optical experiments that demonstrated transition of the optical properties from solid to liquid values within 500 fs after phototexcitation. This result was later challenged by electron diffraction experiments, which showed that the lattice loses long range order within 3.5 ps during photoinduced melting. This time scale implies conventional thermal melting. We find that the broadband optical properties during the solid-to-liquid phase transition in aluminum agree with the results obtained by the electron diffraction experiments. The transition of the broadband reflectivity from solid to liquid values is complete within 1.5 2 ps in our experiments, which is compatible with thermal melting. We dont observe time scales on the order of 500 fs. All the experimental evidence in this dissertation lead to the conclusion that the laser-induced, solid-to-liquid phase transition in aluminum is a thermal process.
    C. A. D. Roeser and E. Mazur. 2005. “Light-matter interactions on the femtosecond time scale.” In Frontiers of Optical Spectroscopy: Investigating Extreme Physical Conditions with Advanced Optical Techniques, edited by B. Di Bartolo and O. Forte, Pp. 29–54. Kluwer Academic Publishers. Publisher's VersionAbstract
    The subject of electromagnetism in the presence of matter is both extensively studied and rich in diverse phenomena. It spans such topics as the quantization of the electromagnetic field to the semiclassical treatment of lightmatter interactions to the derivation of the Fresnel reflectivity formulas. Interest in femtosecond optics is rooted in nonlinear optical phenomena and in the complex electron and lattice dynamics that occur in a material following intense ultrashort-pulse irradiation. The experiments we discuss are concerned mainly with the latter and lie at the crossroad of femtosecond optics and materials science, so-called ultrafast materials science.
    C. A. D. Roeser, M. Kandyla, A. Mendioroz, and E. Mazur. 2004. “Optical control of coherent lattice vibrations in tellurium.” Phys. Rev. B, 70, Pp. 212302–212305. Publisher's VersionAbstract
    We present femtosecond time-resolved measurements of the dielectric tensor of tellurium under single and double pulse excitation. We demonstrate the ability to both enhance and cancel coherent lattice vibrations for large lattice shifts under near-damage threshold excitation. The excitation conditions for which cancellation is achieved in tellurium reveal a departure from the low-excitation strength behavior of similar materials.
    A. M.-T. Kim, C. A. D. Roeser, and E. Mazur. 2003. “Modulation of the Bonding-Antibonding Splitting in Te by Coherent Phonons.” Phys. Rev. B, 68, Pp. 012301–012304. Publisher's VersionAbstract
    We present femtosecond time-resolved measurements of the dielectric tensor of tellurium following intense photoexcitation. Strong impulsive photoexcitation of crystalline tellurium weakens the covalent bonds between atoms, which undergo coherent oscillations (at > 3 THz) as they relax to new equilibrium positions. As this photoexcitation drives the lattice toward a band-crossing transition, we track the decrease and oscillation of the bonding-antibonding splitting. The reduction of the bonding-antibonding splitting exceeds the band gap for 100 fs, indicating a transient state with crossed bands.
    C. A. D. Roeser, A. M.-T. Kim, J. Paul Callan, L. Huang, E. N. Glezer, Y. Siegal, and E. Mazur. 2003. “Femtosecond time-resolved dielectric function measurements by dual-angle reflectometry.” Rev. Sci. Instrum., 74, Pp. 3413–3422. Publisher's VersionAbstract
    We present a technique to measure the dielectric function of a material with femtosecond time resolution over a broad photon energy range. The absolute reflectivity is measured at two angles of incidence, and the dielectric function is calculated by numerical inversion of Fresnel-like formulas. Using white-light generation, the single-color probe is broadened from the near IR to the near UV, but femtosecond time resolution is maintained. Calibration of the apparatus and error analysis are discussed. Finally, measurements of isotropic, thin film, and uniaxial materials are presented and compared to reflectivity-only studies to illustrate the merit of the technique.
    C. A. D. Roeser, A. M.-T. Kim, and E. Mazur. 2003. “Ultrafast Lattice-Bonding Dynamics of Tellurium.” In . Ultrafast Electronics and Optoelectronics. Publisher's VersionAbstract
    A pump-probe technique measuring the dielectric function is presented and applied to the ultrafast dynamics of coherent phonons in Te. Oscillations in the bonding-antibonding splitting are revealed, allowing for THz modulation of a semiconductor-semimetal transition.
    C. A. D. Roeser. 2003. “Ultrafast Dynamics and Optical Control of Coherent Phonons in Tellurium”. Publisher's VersionAbstract
    This dissertation reports the ultrafast dynamics of tellurium after excitation by one or more intense femtosecond laser pulses. Irradiation of tellurium by femtosecond pulses is known to excite coherent phonons, but the nature of the excitation process and the details of the material dynamics under intense excitation are, as of yet, not precisely determined. We investigate these dynamics by monitoring the response of tellurium using an optical pump– probe technique designed to measure the dielectric tensor across the visible spectrum with femtosecond time resolution. The observed dynamics are similar to the ultrafast dynamics of molecules, where photoexcitation of electrons establishes a new potential surface on which the nuclei move. The time-resolved dielectric tensor measurements provide a "snapshot" of the material in a particular lattice configuration. From the observed changes in the optical properties, we infer the underlying changes in the lattice, and thereby develop a picture of the nuclear motion. We find that the main resonance for interband electronic transitions in tellurium shifts to lower photon energy due to the lattice displacement that results from photoexcitation. Under single pulse excitation, a rapid change in the equilibrium lattice configuration leads to a long-lived shift in the resonance energy along with fast oscillations around this value. Under double pulse excitation, the lattice dynamics can be controlled; we achieve both enhancement and cancellation of coherent phonons for excitation strengths up to the damage threshold.
    S. K. Sundaram and E. Mazur. 2002. “Inducing and probing non-thermal transitions in semiconductors using femtosecond laser pulses.” Nature Materials, 1, Pp. 217–224. Publisher's VersionAbstract
    Soon after it was discovered that intense laser pulses of nanosecond duration from a ruby laser could anneal the lattice of silicon, it was established that this so-called pulsed laser annealing is a thermal process. Although the radiation energy is transferred to the electrons, the electrons transfer their energy to the lattice on the timescale of the excitation. The electrons and the lattice remain in equilibrium and the laser simply heats the solid to the melting temperature within the duration of the laser pulse. For ultrashort laser pulses in the femtosecond regime, however, thermal processes (which take several picoseconds) and equilibrium thermodynamics cannot account for the experimental data. On excitation with femtosecond laser pulses, the electrons and the lattice are driven far out of equilibrium and disordering of the lattice can occur because the interatomic forces are modified due to the excitation of a large (10% or more) fraction of the valence electrons to the conduction band. This review focuses on the nature of the non-thermal transitions in semiconductors under femtosecond laser excitation.
    A. M.-T. Kim, J. Solis, J. Paul Callan, C. A. D. Roeser, and E. Mazur. 2001. “GeSb thin films: read-write optical data storage on the subnanosecond time scale.” In . Ultrafast Electronics and Optoelectronics. Publisher's VersionAbstract
    The transition from the low reflectivity amorphous to the highly reflective crystalline phase of a-GeSb films is studied with fs time-resolution. The results reveal an ultrafast transition to a new non-thermodynamic phase which is not c-GeSb as previously believed.
    A. M.-T. Kim, J. Paul Callan, C. A. D. Roeser, E. Mazur, and J. Solis. 2000. “Ultrafast phase transition dynamics in GeSb alloys.” In . Nonlinear Optics: Materials, Fundamentals, and Applications, 2000. Publisher's VersionAbstract
    We measure the femtosecond time resolved dielectric function of a-GeSb after excitation with an ultrashort laser pulse. The results reveal an ultrafast transition to a new non-thermodynamic phase which is not c-GeSb as previously believed. We present the most thorough experimental study to date of laser induced ultrafast phase transitions in GeSb alloys. We investigate the changes of the material by directly monitoring the full dielectric function over a broad energy range (1.7 eV - 3.5 eV) with 100 fs time resolution.
    L. Huang. 1997. “Semiconductor under Ultrafast Laser Excitation: Optical Studies of the Dynamics”. Publisher's VersionAbstract
    This thesis presents studies of semiconductors under intense femtosecond laser irradiation. In order to investigate the nature of the electronic and structural changes induced by laser pulses, a novel broadband technique is developed to measure the linear optical property of semiconductors dielectric function over the entire visible spectrum (1.53.5 eV) with femtosecond time resolution. By employing this broadband spectroscopic technique, the response of the dielectric function of GaAs following an intense 70-fs, 1.9 eV pump pulse is measured. The results provide the most detailed information thus far on the electron and lattice dynamics both above and below the fluence threshold for permanent damage. It is shown that electronic effects, manifested in changes in the band structure, dominate during the first few hundred fs following the excitation. After a few picoseconds, three distinct structural changes are observed depending upon the excitation strength: At low pump fluences, the dielectric function shows heating of the lattice caused by carrier relaxation. At intermediate fluences, the dielectric function reveals a temporary disordering of the lattice. At even higher fluences, a semiconductor-to-metal transition occurs even below the damage threshold. The latter two effects are attributed to the lattice instability caused by the destabilization of the covalent bonds. The time-integrated photoluminescence is also measured to investigate the dynamics of GaAs following fs laser excitation. The luminescence images reveal a reduction of emission due to the structural changes in GaAs. The spectral measurements provide new insight in the carrier dynamics. In addition, a series of II-VI semiconductors are also studied using similar techniques. The response of crystalline Si following fs laser excitation is also explored using the broadband spectroscopic technique. The dielectric function measurements show that lattice heating and semiconductor-to-metal transitions take place within a few picoseconds. The long time (up to 400 ps) behavior is investigated with both reflectivity and dielectric function measurements, providing detailed information on the relaxation of both electronic and structural changes following the excitation.
    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, 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.