Femtosecond Laser Pulse Excitation Research Articles

Coherent magnetization rotation induced by optical modulation in ferromagnetic/antiferromagnetic exchange-coupled bilayers

We have demonstrated recently that ultrafast (\ensuremath{\sim}100 ps) coherent magnetization rotation can be induced by optical modulation of the exchange coupling in an exchange coupled bilayer (NiFe/NiO) with short laser pulses [Phys. Rev. Lett. 82, 3705 (1999)]. In this paper, we acquire further support of the coherent magnetization model by studying the details of magnetization switching dynamics, including the dependence on the amplitude of exchange bias field. The exchange bias field is subject to large transient modulation by femtosecond pulsed laser excitation, providing the impulsive force for subsequent magnetization dynamics. Good agreement is obtained with a simple model of coherent magnetization processes based on the Landau-Lifshitz-Gilbert equations of motion. We have also measured a strong dependence of the phenomenological damping constant (\ensuremath{\alpha}) on the exchange bias in the coupled ferromagnetic/antiferromagnetic spin system.

Photocoloration of Spironaphthoxazine Microcrystalline Powder by Femtosecond Laser Pulse Excitation

Abstract Photocoloration of microcrystalline powder of a spirooxazine compound by femtosecond laser pulse excitation, depending on its intensity, is reported with transient absorption spectral measurements of the powder. The mechanism is discussed in terms of light induced local heating and succeeding cooling of a small volume in the microcrystal.

Subpicosecond electromagnetic pulse generation from high-Tc superconducting thin films by femtosecond laser pulse excitation

Ultrashort electromagnetic pulses with a halfwidth of 0.45 psec have been radiated from current biased superconducting YBCO films by exciting femtosecond laser pulses. Characteristics of the radiation are studied in detail.

Multiphoton ionization of molecules: A comparison between femtosecond and nanosecond laser pulse ionization efficiency

Multiphoton ionization mass spectra of nonvolatile molecules laser desorbed into a supersonic beam are recorded. It is shown by indirect measurements that the laser desorption of neutrals is not mass limited, but lead to the formation of neutrals with intesities large enough for intense signals. To investigate the efficiency of the multiphoton ionization process with varying laser pulse durations, simultaneous laser pulses of 500 fs and 5 ns or 100 fs and 5 ns have been applied to the neutral beam. The energies of both femtosecond and nanosecond laser pulses are held in a comparable magnitude, and thus produce, in the resulting ion intensity, very large differences up to 4 orders of magnitude. For larger evaporated molecules (> 500 u) the ionization efficiency from nanosecond laser pulses drops significantly in comparison to femtosecond laser pulse excitation. A variety of possible reasons for the different ionization and dissociation behavior in femtosecond and nanosecond laser pulse excitations are discussed in this paper. It is rationalized that even with very short laser pulses and large molecules the "ladder switching model" for ionization and fragmentation is valid.

Laser-Induced Phase Transitions in Semiconductors

Optical studies of semiconductors under intense femtosecond laser pulse excitation suggest that an ultrafast phase transition takes place 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 anti bonding splitting and a collapse of the bandgap. We review the relationship between structural, electronic, and optical properties, well as the timescales for electron recombination, diffusion, and energy relax­ ation. Direct optical measurements of the dielectric constant and second­ order nonlinear susceptibility are used to determine the time evolution of the phase transition.

Behavior of chi (2) during a laser-induced phase transition in GaAs.

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/${\mathrm{m}}^{2}$ 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/${\mathrm{m}}^{2}$ shows that the loss of long-range order is reversible in this fluence regime.

Phase Transitions Induced By Femtosecond Pulses

ABSTRACTOptical 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 transition.

Dielectric constant of GaAs during a subpicosecond laser-induced phase transition.

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.

Early stage of Coulomb screening and kinetics under femtosecond excitation within the non-equilibrium RPA

During and shortly after a femtosecond laser pulse excitation of a semiconductor, the carrier kinetics is drastically altered. At this early stage the Coulomb potential is still unscreened and there is essentially no energy conservation in the two-particle scattering events. This explains in a natural way the results of recent femtosecond spectroscopy experiments.