Ultrafast dynamics in solids

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.