Showing papers on "Femtosecond published in 2001"

Femtosecond Structural Dynamics in VO2 during an Ultrafast Solid-Solid Phase Transition.

Abstract: Femtosecond x-ray and visible pulses were used to probe structural and electronic dynamics during an optically driven, solid-solid phase transition in VO(2). For high interband electronic excitation (approximately 5 x 10(21) cm(-3)), a subpicosecond transformation into the high-T, rutile phase of the material is observed, simultaneous with an insulator-to-metal transition. The fast time scale observed suggests that, in this regime, the structural transition may not be thermally initiated.

Micromachining bulk glass by use of femtosecond laser pulses with nanojoule energy

Abstract: Using tightly focused femtosecond laser pulses of just 5 nJ, we produce optical breakdown and structural change in bulk transparent materials and demonstrate micromachining of transparent materials by use of unamplified lasers. We present measurements of the threshold for structural change in Corning 0211 glass as well as a study of the morphology of the structures produced by single and multiple laser pulses. At a high repetition rate, multiple pulses produce a structural change dominated by cumulative heating of the material by successive laser pulses. Using this cumulative heating effect, we write single-mode optical waveguides inside bulk glass, using only a laser oscillator.

Laser-induced breakdown and damage in bulk transparent materials induced by tightly focused femtosecond laser pulses

Abstract: Laser-induced breakdown and damage to transparent materials has remained an active area of research for four decades. In this paper we review the basic mechanisms that lead to laser-induced breakdown and damage and present a summary of some open questions in the field. We present a method for measuring the threshold intensity required to produce breakdown and damage in the bulk, as opposed to on the surface, of the material. Using this technique, we measure the material band-gap and laser-wavelength dependence of the threshold intensity for bulk damage using femtosecond laser pulses. Based on these thresholds, we determine the relative role of different nonlinear ionization mechanisms for different laser and material parameters.

Femtosecond laser-assisted three-dimensional microfabrication in silica.

Abstract: We demonstrate direct three-dimensional (3-D) microfabrication inside a volume of silica glass. The whole fabrication process was carried out in two steps: (i) writing of the preprogrammed 3-D pattern inside silica glass by focused femtosecond (fs) laser pulses and (ii) etching of the written structure in a 5% aqueous solution of HF acid. This technique allows fabrication of 3-D channels as small as 10 μm in diameter inside the volume with any angle of interconnection and a high aspect ratio (10‐μm-diameter channels in a 100‐μm-thick silica slab).

Ultrafast Electron Dynamics and Optical Nonlinearities in Metal Nanoparticles

Abstract: The femtosecond optical response of noble metal nanoparticles and its connection to the ultrafast electron dynamics are discussed in light of the results of high-sensitivity femtosecond pump−probe experiments. The physical origins of the nonlinear responses in the vicinity of the surface plasmon resonance and interband transition threshold are analyzed using extension of the theoretical models used in the bulk materials. These responses contain information on the electron interaction processes (electron−electron and electron−phonon scattering) that can thus be directly investigated in the time domain. Their size and environment dependences are discussed, and the results are compared to the ones in the bulk materials. Time-resolved techniques also permit direct study of the vibrational modes of metal nanoparticles and, in particular, the determination of their damping, which is a sensitive probe of the nature of the surrounding matrix and of the interface quality.

Ablation of solids by femtosecond lasers: ablation mechanism and ablation thresholds for metals and dielectrics

Abstract: The mechanism of ablation of solids by intense femtosecond laser pulses is described in an explicit analytical form. It is shown that at high intensities when the ionization of the target material is complete before the end of the pulse, the ablation mechanism is the same for both metals and dielectrics. The physics of this new ablation regime involves ion acceleration in the electrostatic field caused by charge separation created by energetic electrons escaping from the target. The formulae for ablation thresholds and ablation rates for metals and dielectrics, combining the laser and target parameters, are derived and compared to experimental data. The calculated dependence of the ablation thresholds on the pulse duration is in agreement with the experimental data in a femtosecond range, and it is linked to the dependence for nanosecond pulses.

Direct imaging of transient molecular structures with ultrafast diffraction.

Abstract: Ultrafast electron diffraction (UED) has been developed to study transient structures in complex chemical reactions initiated with femtosecond laser pulses. This direct imaging of reactions was achieved using our third-generation apparatus equipped with an electron pulse (1.07 ± 0.27 picoseconds) source, a charge-coupled device camera, and a mass spectrometer. Two prototypical gas-phase reactions were studied: the nonconcerted elimination reaction of a haloethane, wherein the structure of the intermediate was determined, and the ring opening of a cyclic hydrocarbon containing no heavy atoms. These results demonstrate the vastly improved sensitivity, resolution, and versatility of UED for studying ultrafast structural dynamics in complex molecular systems.

Photoselective adaptive femtosecond quantum control in the liquid phase

Abstract: Coherent light sources can be used to manipulate the outcome of light–matter interactions by exploiting interference phenomena in the time and frequency domain. A powerful tool in this emerging field of ‘quantum control’1,2,3,4,5,6 is the adaptive shaping of femtosecond laser pulses7,8,9,10, resulting, for instance, in selective molecular excitation. The basis of this method is that the quantum system under investigation itself guides an automated search, via iteration loops, for coherent light fields best suited for achieving a control task designed by the experimenter11. The method is therefore ideal for the control of complex experiments7,12,13,14,15,16,17,18,19,20. To date, all demonstrations of this technique on molecular systems have focused on controlling the outcome of photo-induced reactions in identical molecules, and little attention has been paid to selectively controlling mixtures of different molecules. Here we report simultaneous but selective multi-photon excitation of two distinct electronically and structurally complex dye molecules in solution. Despite the failure of single parameter variations (wavelength, intensity, or linear chirp control), adaptive femtosecond pulse shaping can reveal complex laser fields to achieve chemically selective molecular excitation. Furthermore, our results prove that phase coherences of the solute molecule persist for more than 100 fs in the solvent environment.

Structural changes in fused silica after exposure to focused femtosecond laser pulses

Abstract: Using in situ Raman scattering in a confocal microscopy setup, we have observed changes in the network structure of fused silica after modifying regions inside the glass with tightly focused 800-nm 130-fs laser pulses at fluences of 5–200 J cm-2 The Raman spectra show a large increase in the peaks at 490 and 605 cm-1, owing to 4- and 3-membered ring structures in the silica network, indicating that densification occurs after exposure to the femtosecond laser pulses The results are consistent with the formation of a localized plasma by the laser pulse and a subsequent microexplosion inside the glass

Fabrication and analysis of a directional coupler written in glass by nanojoule femtosecond laser pulses.

Abstract: A directional coupler written in a glass sample by the focused 400-nm output from a 25-fs oscillator is reported. The coupler is single mode; the splitting ratio is 1.9 dB at 633 nm. A refractive-index profile of the waveguide with a magnitude of Δn=4.5×10-3 was retrieved from a near-field mode pattern.

Photonic device fabrication in glass by use of nonlinear materials processing with a femtosecond laser oscillator

Abstract: Single-mode X couplers and three-dimensional waveguides are fabricated in transparent glasses by use of an unamplified femtosecond laser generating energies of up to 100 nJ. Changing fabrication parameters such as power and scanning speed permits creation of waveguides with a wide range of structures and refractive-index difference. Optical coherence tomography shows large refractive-index changes of up to ∼10-2 in the waveguides; these changes are consistent with guided mode analysis.

Femtosecond polarization pulse shaping.

Abstract: We report computer-controlled femtosecond polarization pulse shaping where intensity, momentary frequency, and light polarization are varied as functions of time. For the first time to our knowledge, a pulse shaper is used to modulate the degree of ellipticity as well as the orientation of the elliptical principal axes within a single laser pulse by use of a 256-pixel two-layer liquid-crystal display inside a zero-dispersion compressor. Interferometric stability of the setup is not required. Complete pulse characterization is achieved by dual-channel spectral interferometry. This technology has a large range of applications, especially in the field of quantum control.

White-light supercontinuum generation with 60-ps pump pulses in a photonic crystal fiber

Abstract: The generation of a spatially single-mode white-light supercontinuum has been observed in a photonic crystal fiber pumped with 60-ps pulses of subkilowatt peak power. The spectral broadening is identified as being due to the combined action of stimulated Raman scattering and parametric four-wave-mixing generation, with a negligible contribution from the self-phase modulation of the pump pulses. The experimental results are in good agreement with detailed numerical simulations. These findings demonstrate that ultrafast femtosecond pulses are not needed for efficient supercontinuum generation in photonic crystal fibers.

Femtosecond laser interference technique with diffractive beam splitter for fabrication of three-dimensional photonic crystals

Abstract: A simple optical interference method to fabricate microperiodic structures was demonstrated. Femtosecond laser pulse was split by a diffractive beam splitter and overlapped with two lenses. Temporal overlap of the split femtosecond pulses, which requires 10 μm order accuracy in optical path lengths, was automatically achieved by this optical setup. One-, two-, and three-dimensional periodic microstructures with micrometer-order periods were fabricated using this method.

Nanodissection of human chromosomes with near-infrared femtosecond laser pulses.

Abstract: Near-infrared laser pulses of a compact 80-MHz femtosecond laser source at 800 nm, a mean power of 15-100 mW, 170-fs pulse width, and millisecond beam dwell times at the target have been used for multiphoton-mediated nanoprocessing of human chromosomes. By focusing of the laser beam with high-numerical-aperture objectives of a scanning microscope to diffraction-limited spots and with light intensities of terawatts per cubic centimeter, precise submicrometer holes and cuts in human chromosomes have been processed by single-point exposure and line scans. A minimum FWHM cut size of ~100 nm during a partial dissection of chromosome 1, which is below the diffraction-limited spot size, and a minimum material removal of ~0.003mum (3) were determined by a scanning-force microscope. The plasma-induced ablated material corresponds to ~1/400 of the chromosome 1 volume and to ~65x10(3) base pairs of chromosomal DNA. A complete dissection could be performed with FWHM cut sizes below 200 nm. High-repetition-frequency femtosecond lasers at low mean power in combination with high-numerical-aperture focusing optics appear therefore as appropriate noncontact tools for nanoprocessing of bulk and (or) surfaces of transparent materials such as chromosomes. In particular, the noninvasive inactivation of certain genomic regions on single chromosomes within living cells becomes possible.

Ultrafast Carrier Dynamics in CdSe Nanocrystals Determined by Femtosecond Fluorescence Upconversion Spectroscopy

Abstract: Femtosecond fluorescence upconversion has been utilized to study the band edge and deep trap emission dynamics of cadmium selenide (CdSe) nanocrystals (NC's) ranging in size from 27 to 72 A in diameter Both the band edge rise time and decay show a direct correlation to NC size, and a rise time that depends on excitation energy Surface-oxidized and non-oxidized NC's display the same band edge fluorescence decay kinetics, but the relative amplitudes of the short and long components differ The deep trap emission that appears within 2 ps is attributed to ultrafast relaxation of a surface selenium dangling bond electron to the valence band where it combines radiatively with the initial photogenerated hole By this process, the large amplitude of the band edge emission that is attributed to direct electron/hole recombination is attenuated within the initial 2−6 ps The long lifetime of the band edge emission originates from a triplet state, with an energy lying just below the lowest electronic level consiste

Three-dimensional hole drilling of silica glass from the rear surface with femtosecond laser pulses.

Abstract: By moving silica glass in a preprogrammed structure, we directly produced three-dimensional holes with femtosecond laser pulses in single step. When distilled water was introduced into a hole drilled from the rear surface of the glass, the effects of blocking and redeposition of ablated material were greatly reduced and the aspect ratio of the depth of the hole was increased. Straight holes of 4‐μm diameter were more than 200 μm deep. Three-dimensional channels can be micromachined inside transparent materials by use of this method, as we have demonstrated by drilling a square-wave-shaped hole inside silica glass.

Phase-Coherent Optical Pulse Synthesis from Separate Femtosecond Lasers

Abstract: We generated a coherently synthesized optical pulse from two independent mode-locked femtosecond lasers, providing a route to extend the coherent bandwidth available for ultrafast science. The two separate lasers (one centered at 760 nanometers wavelength, the other at 810 nanometers) are tightly synchronized and phase-locked. Coherence between the two lasers is demonstrated via spectral interferometry and second-order field cross-correlation. Measurements reveal a coherently synthesized pulse that has a temporally narrower second-order autocorrelation width and that exhibits a larger amplitude than the individual laser outputs. This work represents a new and flexible approach to the synthesis of coherent light.

Cooling dynamics of an optically excited molecular probe in solution from femtosecond broadband transient absorption spectroscopy

Abstract: The cooling of p-nitroaniline (PNA), dimethylamino-p-nitroaniline (DPNA) and trans-stilbene (t-stilbene) in solution is studied experimentally and theoretically. Using the pump–supercontinuum probe (PSCP) technique we observed the complete spectral evolution of hot absorption induced by femtosecond optical pumping. In t-stilbene the hot S1 state results from Sn→S1 internal conversion with 50 fs characteristic time. The time constant of intramolecular thermalization or intramolecular vibrational redistribution (IVR) in S1 is estimated as τIVR≪100 fs. In PNA and DPNA the hot ground state is prepared by S1→S0 relaxation with characteristic time 0.3–1.0 ps. The initial molecular temperature is 1300 K for PNA and 860 K for t-stilbene. The subsequent cooling dynamics (vibrational cooling) is deduced from the transient spectra by assuming: (i) a Gaussian shape for the hot absorption band, (ii) a linear dependence of its peak frequency νm and width square Γ2 on molecular temperature T. Within this framework we de...

Ultrafast Manipulation of Electron Spin Coherence

Abstract: A technique is developed with the potential for coherent all-optical control over electron spins in semiconductors on femtosecond time scales. The experiments show that optical “tipping” pulses can enact substantial rotations of electron spins through a mechanism dependent on the optical Stark effect. These rotations were measured as changes in the amplitude of spin precession after optical excitation in a transverse magnetic field and approach π/2 radians. A prototype sequence of two tipping pulses indicates that the rotation is reversible, a result that establishes the coherent nature of the tipping process.

Intensity clamping and re-focusing of intense femtosecond laser pulses in nitrogen molecular gas

Abstract: We present results of measurements of fluorescence spectra due to the interaction of a Ti:sapphire laser pulse with N2 molecules at different gas pressures and pulse energies. The analysis of the data together with the results of numerical simulations, using a propagation model, reveal signatures of the phenomena of intensity clamping and of re-focusing of the laser pulse at high gas pressure. The laser pulse energy for intensity clamping as a function of the gas pressure is determined.

Femtosecond X-Ray Measurement of Ultrafast Melting and Large Acoustic Transients

Abstract: (Received 29 June 2001; published 7 November 2001)Time-resolved x-ray diffraction with ultrashort 300 fs , multi-keV x-ray pulses has been used tostudy the femtosecond laser-induced solid-to-liquid phase transition in a thin crystalline layer of germa-nium. Nonthermal melting is observed to take place within 300–500 fs. Following ultrafast melting weobserve strong acoustic perturbations evolving on a picosecond time scale.

The relaxation pathways of CdSe nanoparticles monitored with femtosecond time-resolution from the visible to the IR: Assignment of the transient features by carrier quenching

Abstract: The relaxation dynamics of photoexcited CdSe nanoparticles (NPs) were studied with femtosecond pump−probe spectroscopy in the spectral range from 450 nm to 5 μm. Thus, the intraband relaxation of the electron and the hole were monitored with femtosecond time resolution. The transition from delocalized electronic to localized defect states and the slower relaxation through the trapping sites (trap hopping mechanism) were followed by time-resolved emission on time scales from picoseconds to milliseconds in the spectral range from the visible into the mid-infrared. The spectral dynamics in the visible, near-infrared (NIR), and infrared (IR) range give a mechanistic picture about the relaxation pathway of the excited charge carriers in CdSe NPs. In addition, by using electron and hole quenchers to the photoexcited nanoparticles, we could assign the observed dynamics to the electron or the hole. In the visible range, bleach features (negative signals in the differential transient absorption measurements) were ...

In situ observation of photoinduced refractive-index changes in filaments formed in glasses by femtosecond laser pulses.

Abstract: We investigated the relationship between the formation of filaments and local refractive-index changes induced by femtosecond laser pulses in silica glass. In situ observation revealed that the location of a filament coincided with that of the refractive-index change. Observation also showed that the region of refractive-index change was elongated toward the upstream direction of the laser pulses with increasing exposure time. The region of refractive-index change was several hundred micrometers long, and its diameter was smaller than 2 mum. The refractive-index change was confirmed by two of three different methods to be as large as 0.8 x 10(-2).

Evolutionary algorithms and their application to optimal control studies

Abstract: Coherent control of a physical or chemical process can be achieved by using phase and amplitude modulated femtosecond laser pulses. A self-learning loop, which connects a femtosecond pulse shaper, an optimization algorithm, and an experimental feedback signal, can automatically steer the interaction between system and electric field and allows control even without any knowledge of the Hamiltonian. The dependability of such a loop is essential to the significance of the optimization results, assigning the optimization algorithm an important role within these learning loops. In this paper, an evolutionary strategy is presented in detail that has successfully been applied to femtosecond pulse shaping in optimal control experiments. A general introduction to evolutionary algorithms is given and the specific adaptation for femtosecond pulse shaping is described. The stability and effectiveness of the algorithm is investigated both in experiments and simulations with an emphasis on the influence of steering parameters of the algorithm, number of configurations in search space, and noise. The algorithm optimizes a set of variables parametrizing the electric field. This particular mapping greatly facilitates the dissection of the optimization goal which is demonstrated by three possible parametrizations and associated applications: polynomial phase functions and adaptive femtosecond pulse compression, periodic phase functions and control of nonlinear photon transitions, multiple pulse structures and control of molecular dynamics.

Femtosecond optical ranging in biological systems

Abstract: Optical ranging using femtosecond laser pulses and nonlinear-optical cross correlation is demonstrated for the investigation of the microstructure of biological systems. By using pulses of 65-fsec duration generated by a colliding-pulse mode-locked ring dye laser, a spatial resolution of less than 15 μm is achieved with a detection sensitivity to remitted signals as small as 10 -7 of the incident pulse energy. This technique is applied to measure the cornea in rabbit eyes in vivo as well as to investigate the epidermal structure of human skin in vitro.

Breakup and fusion of self-guided femtosecond light pulses in air.

Abstract: We report experiments showing the breakup and the merging of filaments formed by the modulational instability of femtosecond optical pulses in air. For input powers as high as 25 times the self-focusing threshold, the beams are shown to split into two spots, which coalesce into a self-guided beam. This effect occurs in an optically Kerr regime and plays an important role in the guiding process. Numerical simulations and theoretical estimates both support the comparison with the experimental data.