Showing papers on "Femtosecond published in 1998"

Control of Chemical Reactions by Feedback-Optimized Phase-Shaped Femtosecond Laser Pulses

Abstract: Tailored femtosecond laser pulses from a computer-controlled pulse shaper were used to optimize the branching ratios of different organometallic photodissociation reaction channels. The optimization procedure is based on the feedback from reaction product quantities in a learning evolutionary algorithm that iteratively improves the phase of the applied femtosecond laser pulse. In the case of CpFe(CO)2Cl, it is shown that two different bond-cleaving reactions can be selected, resulting in chemically different products. At least in this case, the method works automatically and finds optimal solutions without previous knowledge of the molecular system and the experimental environment.

Femtosecond Optical Breakdown in Dielectrics

Abstract: We report measurements of the optical breakdown threshold and ablation depth in dielectrics with different band gaps for laser pulse durations ranging from 5 ps to 5 fs at a carrier wavelength of 780 nm. For t, 100 fs, the dominant channel for free electron generation is found to be either impact or multiphoton ionization (MPI) depending on the size of the band gap. The observed MPI rates are substantially lower than those predicted by the Keldysh theory. We demonstrate that sub-10-fs laser pulses open up the way to reversible nonperturbative nonlinear optics (at intensities greater than 10 14 Wycm 2 slightly below damage threshold) and to nanometer-precision laser ablation (slightly above threshold) in dielectric materials. [S0031-9007(98)05969-9]

Microstructuring of silicon with femtosecond laser pulses

Abstract: We report that silicon surfaces develop an array of sharp conical spikes when irradiated with 500 laser pulses of 100-fs duration, 10-kJ/m2 fluence in 500-Torr SF6 or Cl2. The spikes are up to 40-μm tall, and taper to about 1-μm diam at the tip. Irradiation of silicon surfaces in N2, Ne, or vacuum creates structured surfaces, but does not create sharp conical spikes.

Coherent quantum control of two-photon transitions by a femtosecond laser pulse

Abstract: Coherent quantum control1,2,3 has attracted interest as a means to influence the outcome of a quantum-mechanical interaction. In principle, the quantum system can be steered towards a desired state by its interaction with light. For example, in photoinduced transitions between atomic energy levels, quantum interference effects can lead to enhancement or cancellation of the total transition probability. The interference depends on the spectral phase distribution of the incident beam; as this phase distribution can be tuned, the outcome of the interaction can in principle be controlled. Here we demonstrate that a femtosecond laser pulse can be tailored, using ultrashort pulse-shaping4,5,6,7 techniques, to control two-photon transitions in caesium. By varying the spectral phases of the pulse components, we observe the predicted cancellation of the transitions due to destructive quantum interference; the power spectrum and energy of these ‘dark pulses’ are unchanged. We also show that the pulse shape can be modified extensively without affecting the two-photon transition probability.

Femtosecond 1P-to-1S electron relaxation in strongly confined semiconductor nanocrystals

Abstract: High-sensitivity femtosecond transient absorption is applied to directly measure the population-depopulation dynamics of the lowest ( $1S$) and the first excited ( $1P$) electron states in CdSe nanocrystals (NC's) of different radii with $1S$--- $1P$ energy separation up to 16 longitudinal optical phonon energies. Instead of the drastic reduction of the energy relaxation rate expected due to a phonon bottleneck, we observe a fast subpicosecond $1P$-to- $1S$ relaxation, with the rate enhanced in NC's of smaller radius. This indicates the opening of new confinement-enhanced relaxation channels which likely involve Auger-type electron-hole energy transfer.

Exciton-migration and three-pulse femtosecond optical spectroscopies of photosynthetic antenna complexes

Abstract: A theory for four-wave-mixing signals from molecular aggregates, which includes effects of two-exciton states, static disorder, and coupling to a phonon bath with an arbitrary spectral density, is developed. The third-order polarization is rigorously partitioned into a coherent and a sequential contribution. The latter is given by a sum of an exciton-hopping and a ground state (bleaching) terms, both expressed using the doorway-window representation. Applications are made to photon-echo and pump-probe spectroscopies of the B850 system of the LH2 antenna in purple bacteria.

Dynamic spatial replenishment of femtosecond pulses propagating in air

Abstract: We present numerical simulations of nonlinear pulse propagation in air whereby an initial pulse is formed, absorbed by plasma generation, and subsequently replenished by power from the trailing edge of the pulse. This process can occur more than once for high-power input pulses and produce the illusion of long-distance propagation of one self-guided pulse.

Ultrafast dynamics of nonequilibrium electrons in a gold nanoparticle system

Abstract: Transient absorption spectra were measured to investigate the nonlinear response of a gold nanoparticle system by the femtosecond pump-probe method We obtained temporal changes of electron temperatures and effective damping constants by fitting transient absorption spectra with Mie scattering theory In the ultrafast region, the nonlinear response originates mainly from the hot electron system which is heated by the incident pump pulse It is noteworthy that the lattice temperature plays an important role even in the first step of the nonlinear response through the change of the effective damping constant In the long-time-scale region, over 10 ps, both the electron and the lattice temperatures contribute to the nonlinear response comparably The origin of the effective damping constant is also discussed with the surface scattering and the $e\ensuremath{-}p$scattering processes

Writing waveguides and gratings in silica and related materials by a femtosecond laser

Abstract: With the goal of creating various optical glass devices for the telecommunications industry, the effects of 810 nm, femtosecond laser radiation on various glasses were investigated. By focusing the laser beam via a microscope objective, transparent but visible, round-elliptical damage lines were successfully written inside high silica, borate, soda-lime-silicate, fluoride and chalcogenide glasses. Microscopic ellipsometric measurements of the damaged region in pure and Ge-doped silica glasses showed refractive index increases of 0.01 to 0.035. The formation of several types of defects, including Si E′ or Ge E′ centers, non-bridging oxygen hole centers, and peroxy radicals, was also detected in addition to the identification. These results suggest that multi-photon interactions occurs in the glasses and that it is possible to write three-dimensional optical circuits in bulk glasses via such a focused laser beam technique.

Lamellar refractive surgery with scanned intrastromal picosecond and femtosecond laser pulses in animal eyes.

Abstract: Purpose To evaluate the use of scanned intrastromal picosecond and femtosecond laser pulses in lamellar refractive surgical procedures. Methods Intrastromal corneal photodisruption was performed in fresh porcine and primate cadaver eyes with a solid-state femtosecond laser. Laser pulses were focused 150 to 200 microns below the epithelial surface and scanned in a spiral pattern to create a plane. A flap was made by scanning an arc pattern from the plane of the spiral to the surface of the cornea. Tissue plane separation was graded using a standard scale, while internal surfaces were analyzed by scanning electron microscopy. Comparison was made to a picosecond laser system using the same delivery system device. Creation of a stromal lenticule for in situ keratomileusis was also demonstrated and compared with both laser systems. Results For femtosecond pulses, tissue separation was achieved best with pulse energies from 4 to 8 microJ and spot separations from 10-15 microns. Picosecond pulses accomplished less complete separations with pulse energies of 25 microJ and spot separations from 10 to 20 microns. Surface quality corresponded to dissection results, with high-grade dissections resulting in a smooth surface appearance, versus a more irregular surface for low-grade dissections. Although high-grade dissections could be created with picosecond pulses (with optimal parameters) in ex vivo porcine eyes, only femtosecond parameters produced similar results in ex vivo primate eyes. Conclusion In contrast to previous attempts using picosecond lasers which require additional mechanical dissection, high precision lamellar refractive surgery may be practical with femtosecond laser pulses.

High average-power THz radiation from femtosecond laser-irradiated InAs in a magnetic field and its elliptical polarization characteristics

Abstract: The THz-radiation power from bulk InAs irradiated with femtosecond optical pulses is significantly enhanced and reaches 650 μW in a 1.7-T magnetic field with 1.5-W excitation power. The THz-radiation power is related almost quadratically both to the magnetic field and excitation laser power. We have also found that the power of the THz-radiation from an InAs sample in a magnetic field is over one order of magnitude higher than that from GaAs. Additionally, a dramatic change of ellipticity is observed, and the spectra of the horizontal and vertical polarization components are found to differ.

Dispersion pre-compensation of 15 femtosecond optical pulses for high-numerical-aperture objectives

Abstract: The excitation efficiency in two-photon absorption (TPA) microscopy depends strongly — owing to the square dependence of the TPA fluorescence on the excitation intensity — on the temporal width of the excitation pulse. Because of their inherently large frequency bandwidth, ultrashort optical pulses tend to broaden substantially because of dispersion from propagation through the dispersive elements in the microscope. In this paper, the dispersion characteristics of a wide range of microscope objectives are investigated. It is shown that the induced dispersion can be pre-compensated in all cases for pulses as short as 15 fs. Because of the excellent agreement between the results from theoretical modelling and the experimental data, predictions of the possibility of dispersion control for microscope objectives in general, as well as for even shorter pulses, can be inferred. Since for TPA imaging the background due to single photon absorption processes and scattering is independent of the pulse width, proper dispersion pre-compensation — which minimizes the pulse duration at the focal point and hence maximizes the excitation efficiency — provides optimal image contrast in TPA microscopy.

Influence of pulse duration on mechanical effects after laser-induced breakdown in water

Abstract: The influence of the pulse duration on the mechanical effects following laser-induced breakdown in water was studied at pulse durations between 100 fs and 100 ns. Breakdown was generated by focusing laser pulses into a cuvette containing distilled water. The pulse energy corresponded to 6-times breakdown threshold energy. Plasma formation and shock wave emission were studied photographically. The plasma photographs show a strong influence of self-focusing on the plasma geometry for femtosecond pulses. Streak photographic recording of the shock propagation in the immediate vicinity of the breakdown region allowed the measurement of the near-field shock pressure. At the plasma rim, shock pressures between 3 and 9 GPa were observed for most pulse durations. The shock pressure rapidly decays proportionally to r−(2⋯3) with increasing distance r from the optical axis. At a 6 mm distance of the shock pressure has dropped to (8.5±0.6) MPa for 76 ns and to <0.1 MPa for femtosecond pulses. The radius of the cavitation bubble is reduced from 2.5 mm (76 ns pulses) to less than 50 μm for femtosecond pulses. Mechanical effects such as shock wave emission and cavitation bubble expansion are greatly reduced for shorter laser pulses, because the energy required to produce breakdown decreases with decreasing pulse duration, and because a larger fraction of energy is required to overcome the heat of vaporization with femtosecond pulses.

Femtosecond dynamics of electron localization at interfaces

Abstract: The dynamics of two-dimensional small-polaron formation at ultrathin alkane layers on a silver(111) surface have been studied with femtosecond time- and angle-resolved two-photon photoemission spectroscopy. Optical excitation creates interfacial electrons in quasi-free states for motion parallel to the interface. These initially delocalized electrons self-trap as small polarons in a localized state within a few hundred femtoseconds. The localized electrons then decay back to the metal within picoseconds by tunneling through the adlayer potential barrier. The energy dependence of the self-trapping rate has been measured and modeled with a theory analogous to electron transfer theory. This analysis determines the inter- and intramolecular vibrational modes of the overlayer responsible for self-trapping as well as the relaxation energy of the overlayer molecular lattice. These results for a model interface contribute to the fundamental picture of electron behavior in weakly bonded solids and can lead to better understanding of carrier dynamics in many different systems, including organic light-emitting diodes.

A femtosecond code-division multiple-access communication system test bed

Abstract: This paper reports comprehensive experimental results on a femtosecond code-division multiple-access (CDMA) communication system test bed operating over optical fiber in the 1.5 /spl mu/m communication band. Our test bed integrates together several novel subsystems, including low-loss fiber-pigtailed pulse shapers for encoding-decoding, use of dispersion equalizing fibers in dispersion compensated links for femtosecond pulse transmission and also in femtosecond chirped pulse amplification (CPA) erbium doped fiber amplifiers (EDFAs), and high-contrast nonlinear fiber-optic thresholders. The individual subsystems are described, and single-user system level experimental results demonstrating the ability to transmit spectrally encoded femtosecond pulses over a 2.5-km dispersion compensated fiber link followed by decoding and high contrast nonlinear thresholding are presented.

Large third-order optical nonlinearity in Au:TiO2 composite films measured on a femtosecond time scale

Abstract: The wavelength dependence of the third-order nonlinear optical susceptibilities, χ(3), of the Au:TiO2 composite films with Au concentration varying from 15% to 60% (volume fraction), was measured by a degenerate four-wave mixing (DFWM) technique using a probe laser with a pulse width of 200 fs. It was found that, with the wavelength of the probe laser close to the surface plasmon resonance (∼680 nm), both the χ(3) and the figure of merit, χ(3)/α (α is optical absorption coefficient) were significantly enhanced. The maximum value of the χ(3) was 6×10−7 esu and occurred at an Au concentration of about 38%. Femtosecond time-resolved DFWM measurements revealed that the response time of the optical nonlinearity in the Au:TiO2 films is extremely fast. The time-resolved DFWM results suggest that the main physical mechanism involved in the optical nonlinearity in Au:TiO2 films on the femtoseconds time scale is the interband electric–dipole transition, and the hot electron excitation only partially contributes to ...

How to make femtosecond pulses overlap

Abstract: Normally, femtosecond light pulses that cross at a nonzero angle overlap over only a small region in space. This limitation can be overcome by the use of diffraction orders of a grating. We consider an arrangement in which, on diffraction of a femtosecond pulse by a grating, two beams that correspond to the first-order diffraction maxima are recombined at the image plane by a system of two confocal lenses. In this arrangement the beams overlap over the their full aperture, with the short duration of the pulses being preserved. We demonstrate the use of this setup as a simple autocorrelator and discuss a possible application to time-resolved vibrational spectroscopy.

Explosion of atomic clusters heated by high-intensity femtosecond laser pulses

Abstract: Atomic clusters have long been studied by chemists and physicists because of the unique position that clusters hold as an intermediate state between molecules and solids [1]. Many studies have traced the properties of materials from their monatomic characteristics to their bulk state characteristics through an examination of the material as it forms larger and larger clusters. Recently, there has been much activity in extending these studies to very high intensity, ultrashort laser pulses with peak laser intensities >1015 Wcm −2 and pulse widths of 0.1 to 10 ps [2–11]. There has also been some preliminary theoretical work in this area as well [6,12]. In this parameter regime the physics governing the laser cluster interaction is fundamentally different than in previous studies. At these intensities the laser interaction is non-perturbative and very high order multiphoton ionization and strong electric field tunnel ionization are possible. Consequently, highly charged ions can be produced [2,5,8,10]. Furthermore, the short pulses used are comparable to or shorter than the disassembly times of a cluster in the laser field [6] and, so, the entire laser pulse interacts with an inertially confined body of atoms.

Direct Observation of Ultrafast Electron Injection from Coumarin 343 to TiO2 Nanoparticles by Femtosecond Infrared Spectroscopy

Abstract: Transient infrared (IR) absorption of injected electrons in colloidal TiO2 nanoparticles in the 1900−2000 cm-1 region are measured by femtosecond IR spectroscopy. The direct detection of electrons in the nanoparticles with subpicosecond time resolution provides a new approach to study ultrafast interfacial electron transfer between semiconductor nanoparticles and molecular adsorbates. The dynamics of electron injection from sensitizers to nanoparticles and the subsequent back-transfer and relaxation dynamics of the injected electrons correspond to the rise and decay of the transient IR signal of injected electrons. Using this technique, the injection time for coumarin 343 sensitized TiO2 nanoparticles in D2O is determined to be 125 ± 25 fs. The subsequent decay dynamics of the injected electrons in nanoparticles are found to be different from conduction band electrons in a bulk TiO2 crystal.

Femtosecond Chirped Pulse Excitation of Vibrational Wave Packets in LD690 and Bacteriorhodopsin

Abstract: Chirped femtosecond pulses are used to selectively excite vibrationally coherent wave packets in the ground and excited states of molecules in solution. Femtosecond chirped pump/transform-limited p...

Femtosecond laser-induced three-dimensional bright and long-lasting phosphorescence inside calcium aluminosilicate glasses doped with rare earth ions

Abstract: We report on a novel phenomenon in calcium aluminosilicate glasses doped with Ce3+, Tb3+, and Pr3+. After irradiation by an 800 nm femtosecond pulsed laser, the focused part of the laser in the glasses emits bright and long-lasting phosphorescence able to be clearly seen with the naked eye in the dark even one hour after the removal of the activating laser. Moreover, by selecting appropriate glass compositions and species of rare earth ions, optional three-dimensional image patterns emitting long-lasting phosphorescence in various colors, including blue, green, and red, can be formed within glass samples by moving the focal point of the laser. Based on absorption spectra, the long-lasting phosphorescence is considered to be due to the thermostimulated recombination of holes and electrons at traps induced by the laser irradiation, which leave holes or electrons in a metastable excited state at room temperature.

Non-thermal ablation of neural tissue with femtosecond laser pulses

Abstract: Nonthermal in vitro ablation of bovine neural tis- sue by using laser-induced optical breakdown generated by ultrashort laser pulses, with durations from 100 fs to 35 ps and pulse energies of up to 165mJ, has been investigated. The experiments were performed at wavelengths ranging from 630 to 1053 nm by using a femtosecond Ti:Sapphire laser, a femtosecond dye laser, and a picosecond Nd:YLF laser sys- tem. Tissue ablations have been achieved by focusing the laser beam on the surface of the tissue, to a spot diameter of 5- 20mm, resulting in the generation of a microplasma. Laser pulses from the Ti:Sapphire laser with 140 fs duration showed a two times higher efficiency of ablation than the longer 30 ps pulses from a Nd:YLF laser with an identical pulse energy. At pulse energies of 140mJ, single pass excisions deeper than 200mm were generated by the 140 fs pulses. In addition, the fluence at threshold of the ablation was found to be reduced for shorter pulse durations. For 3p slaser pulses at 630 nm, we measured the fluence at threshold to be about 5: 3J =cm 2 ; for 100 fs pulses from the same laser the experimental thresh- old was at 1: 5J =cm 2 . Histopathological examinations and scanning electron micrographs confirm the high quality of the excisions. No sign of significant thermal damage was ob- served.

Breakdown of the Area Theorem: Carrier-Wave Rabi Flopping of Femtosecond Optical Pulses

Abstract: By solving Maxwell's curl equations coupled to a two-level atom, a theoretical study of carrier-wave Rabi flopping of femtosecond optical pulses of only several carrier-cycles time duration is reported. For pulse areas of $2\ensuremath{\pi}$, the usual self-induced transparency regime is essentially recovered. However, for larger pulse areas, carrier-wave Rabi flopping occurs that manifests in local carrier reshaping and subsequently to the production of significantly higher spectral components on the propagating pulse. These new features are not predicted by employing the area theorem or a slowly-varying-envelope approximation for the amplitude and phase terms\char22{}which is the usual approach.

Thermal and nonthermal melting of gallium arsenide after femtosecond laser excitation

Abstract: Thermal- and nonthermal melting in gallium arsenide after femtosecond laser excitation has been investigated by means of time resolved microscopy. Electronic melting within a few hundred femtoseconds is observed for rather strong excitation and the data reveal a distinct threshold fluence of $150{\mathrm{m}\mathrm{J}/\mathrm{c}\mathrm{m}}^{2}$ for this nonthermal process. Below that threshold melting occurs on a 100 ps time scale and is of thermal nature. Using a simple numerical model we describe this type of the phase transition as heterogeneous melting under strongly overheated conditions.

Femtosecond activation of reactions and the concept of nonergodic molecules

Abstract: The description of chemical reaction dynamics often assumes that vibrational modes are well coupled (ergodic) and redistribute energy rapidly with respect to the course of the reaction. To experimentally probe nonergodic, nonstatistical behavior, studies of a series of reactions induced by femtosecond activation for molecules of varying size but having the same reaction coordinates [CH_2 − (CH_2)_(n – 2) − C = O† → products, with n = 4, 5, 6, and 10] were performed. Comparison of the experimental results with theoretical electronic structure and rate calculations showed a two to four orders of magnitude difference, indicating that the basic assumption of statistical energy redistribution is invalid. These results suggest that chemical selectivity can be achieved with femtosecond activation even at very high energies.

Noncollinearly phase-matched femtosecond optical parametric amplification with a 2000 cm -1 bandwidth

Abstract: An optical parametric amplifier generating as short as 14 fs pulses in a visible region has been constructed in a noncollinear phase-matching configuration. The group-velocity matching between the signal and idler enormously broadens the gain bandwidth up to 2000 cm−1, which is mainly limited by the chirp of the seed pulses. Pulses shorter than 20 fs are tunable from 550 to 690 nm by scanning the delay-line of the pump beam.