Showing papers on "Femtosecond published in 2004"
TL;DR: Femtosecond time-resolved or wave packet methods offer a view complementary to the usual spectroscopic approach and often yield a physically intuitive picture in discerning underlying dynamics.
Abstract: The development of femtosecond time-resolved methods for the study of gas-phase molecular dynamics is founded upon the seminal studies of Zewail and co-workers, as recognized in 1999 by the Nobel Prize in Chemistry.1 This methodology has been applied to chemical reactions ranging in complexity from bond-breaking in diatomic molecules to dynamics in larger organic and biological molecules, and has led to breakthroughs in our understanding of fundamental chemical processes. Photoexcited polyatomic molecules and anions often exhibit quite complex dynamics involving the redistribution of both charge and energy.2-6 These processes are the primary steps in the photochemistry of many polyatomic systems,7 are important in photobiological processes such as vision and photosynthesis,8 and underlie many concepts in molecular electronics.9 Femtosecond time-resolved methods involve a pump-probe configuration in which an ultrafast pump pulse initiates a reaction or, more generally, creates a nonstationary state or wave packet, the evolution of which is monitored as a function of time by means of a suitable probe pulse. Time-resolved or wave packet methods offer a view complementary to the usual spectroscopic approach and often yield a physically intuitive picture. Wave packets can behave as zeroth-order or even classical-like states and are therefore very helpful in discerning underlying dynamics. The information obtained from these experiments is very much dependent on the nature of the final state chosen in a given probe scheme. Transient absorption and nonlinear wave mixing are often the methods of choice in condensed-phase experiments because of their generality. In studies of molecules and clusters in the gas phase, the most popular methods, laser-induced fluorescence and resonant multiphoton ionization, usually require the probe laser to be resonant with an electronic transition in the species being monitored. However, as a chemical reaction initiated by the pump pulse evolves toward products, one expects that both the electronic and vibrational structures of the species under observation will change. Hence, these probe methods can be * To whom corresondence should be addressed. A.S.: telephone (613) 993-7388, fax (613) 991-3437, E-mail albert.stolow@nrc.ca. D.M.N.: telephone (510) 642-3505, fax (510) 642-3635, E-mail dan@radon.cchem.berkeley.edu. 1719 Chem. Rev. 2004, 104, 1719−1757
624 citations
TL;DR: The origin of phase stability and the precise calibration of excitation-pulse time delays using movable glass wedges are discussed and it is shown that correlations between different electronically excited states can be determined from the spectra.
Abstract: Two-dimensional (2D) spectroscopy is a powerful technique to study nuclear and electronic correlations between different transitions or initial and final states. Here we describe in detail our development of inherently phase-stabilized 2D Fourier-transform spectroscopy for electronic transitions. A diffractive-optic setup is used to realize heterodyne-detected femtosecond four-wave mixing in a phase-matched box geometry. Wavelength tunability in the visible range is accomplished by means of a 3 kHz repetition-rate laser system and optical parametric amplification. Nonlinear signals are fully characterized by spectral interferometry. Starting from fundamental principles, we discuss the origin of phase stability and the precise calibration of excitation-pulse time delays using movable glass wedges. Automated subtraction of undesired scattering terms removes experimental artifacts. On the theoretical side, the response-function formalism is extended to describe molecules with three electronic levels, and the shape of 2D spectral features is discussed. As an example for this technique, experimental 2D spectra are shown for the dye molecule Nile Blue in acetonitrile at 595 nm, recorded for a series of population times. Simulations explore the influence of different model parameters and qualitatively reproduce the experimental results. We show that correlations between different electronically excited states can be determined from the spectra. The technique described here can be used to measure the third-order response function of complex systems covering several electronic transitions.
487 citations
TL;DR: A volume sampling method to generate three-dimensional patterns is proposed and a systematic SEM-based analysis of the microstructure gives new insights toward a better understanding of the femtosecond laser interaction with fused silica glass.
Abstract: We present novel results obtained in the fabrication of high-aspect ratio micro-fluidic microstructures chemically etched from fused silica substrates locally exposed to femtosecond laser radiation. A volume sampling method to generate three-dimensional patterns is proposed and a systematic SEM-based analysis of the microstructure is presented. The results obtained gives new insights toward a better understanding of the femtosecond laser interaction with fused silica glass (a-SiO2).
445 citations
TL;DR: An overview of femtosecond laser interactions with dielectrics can be found in this article, where the focus is the dynamics of femto-laser-excited carriers and the propagation of femtecond laser pulses inside dielectric materials.
Abstract: Femtosecond laser pulses appear as an emerging and promising tool for processing wide bandgap dielectric materials for a variety of applications. This article aims to provide an overview of recent progress in understanding the fundamental physics of femtosecond laser interactions with dielectrics that may have the potential for innovative materials applications. The focus of the overview is the dynamics of femtosecond laser-excited carriers and the propagation of femtosecond laser pulses inside dielectric materials.
426 citations
TL;DR: In this paper, a direct, point-by-point inscription of fibre Bragg gratings by infrared femtosecond laser is reported for the first time, in a non-photosensitised, standard telecommunication fibre and dispersion shifted fibre.
Abstract: Direct, point-by-point inscription of fibre Bragg gratings by infrared femtosecond laser is reported for the first time. Gratings of first to third order have been produced in non-photosensitised, standard telecommunication fibre and dispersion shifted fibre.
419 citations
TL;DR: In this article, the optical properties, chemical composition, and crystallinity of silicon microstructures formed in the presence of SF6 by femtosecond laser irradiation and by nanosecond LIDAR irradiation were compared.
Abstract: We compare the optical properties, chemical composition, and crystallinity of silicon microstructures formed in the presence of SF6 by femtosecond laser irradiation and by nanosecond laser irradiation. In spite of very different morphology and crystallinity, the optical properties and chemical composition of the two types of microstructures are very similar. The structures formed with femtosecond (fs) pulses are covered with a disordered nanocrystalline surface layer less than 1 μm thick, while those formed with nanosecond (ns) pulses have very little disorder. Both ns-laser-formed and fs-laser-formed structures absorb near-infrared (1.1–2.5 μm) radiation strongly and have roughly 0.5% sulfur impurities.
403 citations
TL;DR: In this article, the authors give an overview of the development and current progress of femtosecond laser micro-nanofabrication based on multiphoton absorption, and particular emphasis is placed on two-photon photopolymerization.
Abstract: This chapter attempts to give an overview of the historical development and current progress of femtosecond laser micro-nanofabrication based on multiphoton absorption, and particular emphasis is placed on two-photon photopolymerization. Femtosecond laser interaction with matter differs essentially from those with longer pulses or CW lasers in its significant nonlinearity, ultrafast characteristics and the possibility of highly localization of reaction volume. These features enable three-dimensional (3D) micro-nanofabrication in solid and liquid media. In two-photon photopolymerization, when a near-infrared femtosecond laser is tightly focused into a photopolymerizable resin, 3D polymer micro-nanostructures are produced by pinpoint photopolymerization of liquid precursory resins. Using this direct laser writing scheme, various photonic, micro-optical components and micromechanical devices have been readily produced. The two-photon photopolymerization technology is expected to play a similar role to that played by lithography for planar semiconductor device processing, but for micro-nanofabrication of 3D polymer-based optoelectronic devices as well for microelectromechanical systems.
402 citations
TL;DR: In this article, a broadbandwidth femtosecond pulses are used to achieve high spectral resolution in nonlinear spectroscopy and microscopy, based on chirping the excitation pulses in order to focus their entire bandwidth into a narrow spectral region.
Abstract: In this work, we show how broad-bandwidth femtosecond pulses can be used to achieve high spectral resolution in nonlinear spectroscopy and microscopy. Our approach is based on chirping the excitation pulses in order to focus their entire bandwidth into a narrow spectral region. We show that spectral features which are 100 times narrower than the excitation light can be resolved with this simple spectral focusing. The gain in spectral selectivity and sensitivity makes its application to nonlinear microscopy very convenient. This is demonstrated with diffraction-limited coherent anti-Stokes Raman scattering microscopy.
337 citations
TL;DR: In this article, a semiconductor photoconductive switch with femtosecond laser illumination is used as a "pulse generator" to produce jitter-free, sub-100 ps rise time step function-like electrical pulses.
Abstract: We present an experimental approach to study the ultrafast polarization switching dynamics in thin-film ferroelectrics. A semiconductor photoconductive switch with femtosecond laser illumination is used as a “pulse generator” to produce jitter-free, sub-100 ps rise time step-function-like electrical pulses. Quantitative measurements yield a polarization switching time, ts, of ∼220 ps when measured with a 5 V, 68 ps rise time input electrical pulse. Modeling of the switching transients using the Merz–Ishibashi model and Merz–Shur model of switching kinetics yields a quantitative estimate of the characteristic switching time constant, t0, of ∼70–90 ps.
290 citations
TL;DR: The dependence of the stimulated Raman signal on experimental parameters is explored, demonstrating the expected exponential increase in Raman intensity with concentration, pathlength, and Raman pump power.
Abstract: The laser, detection system, and methods that enable femtosecond broadband stimulated Raman spectroscopy (FSRS) are presented in detail. FSRS is a unique tool for obtaining high time resolution (<100 fs) vibrational spectra with an instrument response limited frequency resolution of <10 cm(-1). A titanium:Sapphire-based laser system produces the three different pulses needed for FSRS: (1) A femtosecond visible actinic pump that initiates the photochemistry, (2) a narrow bandwidth picosecond Raman pump that provides the energy reservoir for amplification of the probe, and (3) a femtosecond continuum probe that is amplified at Raman resonances shifted from the Raman pump. The dependence of the stimulated Raman signal on experimental parameters is explored, demonstrating the expected exponential increase in Raman intensity with concentration, pathlength, and Raman pump power. Raman spectra collected under different electronic resonance conditions using highly fluorescent samples highlight the fluorescence rejection capabilities of FSRS. Data are also presented illustrating our ability: (i) To obtain spectra when there is a large transient absorption change by using a shifted excitation difference technique and (ii) to obtain high time resolution vibrational spectra of transient electronic states.
287 citations
TL;DR: A novel method to generate femTosecond and subfemtosecond photon pulses in a free-electron laser by selectively spoiling the transverse emittance of the electron beam, which can provide x-ray pulses the order of 1 fs in duration containing about 10 transversely coherent photons.
Abstract: We propose a novel method to generate femtosecond and subfemtosecond photon pulses in a free-electron laser by selectively spoiling the transverse emittance of the electron beam. Its merits are simplicity and ease of implementation. When the system is applied to the Linac Coherent Light Source, it can provide x-ray pulses the order of 1 fs in duration containing about 10(10) transversely coherent photons.
TL;DR: In this paper, the interaction of ultrashort laser pulses with materials involves a number of special features that are different from laser-matter interaction for longer pulse durations, and the fundamental physical processes such as energy deposition, melting and ablation are separated in time.
Abstract: The interaction of ultrashort laser pulses with materials involves a number of special features that are different from laser–matter interaction for longer pulse durations. For femtosecond laser excitation the fundamental physical processes such as energy deposition, melting, and ablation are separated in time. By choosing proper time windows, the various processes can be investigated separately. We present selected examples of theoretical studies of free electron excitation in metals, timescales of different melting processes, and peculiarities of near-threshold ablation. Depending on the timescales and intensity ranges, the discussed processes are combined in an overall picture of possible pathways of the material from excitation to ablation.
TL;DR: By combining simulation and experiments, the generation mechanism of the visible peak is explored and it is demonstrated that the blue peak is generated only when the input pulse is so strongly compressed that the short-wavelength tail of the spectrum includes the wavelength predicted for the dispersive wave.
Abstract: We study the nonlinear propagation of femtosecond pulses in the anomalous dispersion region of microstructured fibers, where soliton fission mechanisms play an important role. The experiment shows that the output spectrum contains, besides the infrared supercontinuum, a narrow-band 430-nm peak, carrying about one fourth of the input energy. By combining simulation and experiments, we explore the generation mechanism of the visible peak and describe its properties. The simulation demonstrates that the blue peak is generated only when the input pulse is so strongly compressed that the short-wavelength tail of the spectrum includes the wavelength predicted for the dispersive wave. In agreement with simulation, intensity-autocorrelation measurements show that the duration of the blue pulse is in the picosecond time range, and that, by increasing the input intensity, satellite pulses of lower intensity are generated.
TL;DR: In this article, a femtosecond laser-based optical frequency synthesizer is used to demonstrate the generation and control of the frequency of electromagnetic fields over 100 terahertz of bandwidth with fractional uncertainties approaching 1 part in 1019.
Abstract: A femtosecond laser–based optical frequency synthesizer is referenced to an optical standard, and we use it to demonstrate the generation and control of the frequency of electromagnetic fields over 100 terahertz of bandwidth with fractional uncertainties approaching 1 part in 1019. The reproducibility of this performance is verified by comparison of different types of femtosecond laser–based frequency synthesizers from three laboratories.
TL;DR: This work observed that in the intermediate regime there is a correlation among the negative sign of the effective index change, the presence of anisotropic reflection, and birefringence, and proposes a model that can explain all three principal characteristics.
Abstract: Although femtosecond lasers have proved to be of great utility for micromachining within bulk transparent materials, little is known about the fundamental physics that drive the process. Depending on the laser intensity delivered to the sample, any of three types of feature can be written into the glass. We observed that in the intermediate regime there is a correlation among the negative sign of the effective index change, the presence of anisotropic reflection, and birefringence. We propose a model that can explain all three principal characteristics. Results show that the local index change can be as high as 10-1.
01 Jan 2004
TL;DR: In this paper, the authors used femtosecond laser pulses at 800 nm and 120 fs to fabricate high-quality retroreflecting fiber Bragg gratings in standard Ge-doped telecom fiber (Corning SMF-28) and all-silica-core Fluorine doped cladding single-mode fiber using a deep-etch silica zero-order nulled phase mask.
Abstract: Femtosecond laser pulses at 800 nm and 120 fs were used to fabricate high-quality retroreflecting fiber Bragg gratings in standard Ge-doped telecom fiber (Corning SMF-28) and all-silica-core Fluorine doped cladding single-mode fiber using a deep-etch silica zero-order nulled phase mask. Induced index modulations of 1.9/spl times/10/sup -3/ were achieved with peak power intensities of 2.9/spl times/10/sup 12/ W/cm/sup 2/ without any fiber sensitization such as hydrogen loading. The fiber gratings have annealing characteristics similar to type II damage fiber gratings and demonstrate stable operation at temperatures as high as 950/spl deg/C. The grating devices exhibit low polarization dependence. The primary mechanism of induced index change results from a structural modification to the fiber core.
TL;DR: In this paper, a simple model describes theoretically the processes involved in the irradiation of solid targets by femtosecond laser pulses and predicts the optimal target and laser parameters for efficient nanoparticles synthesis.
Abstract: First, a simple model describes theoretically the processes involved in the irradiation of solid targets by femtosecond laser pulses and predicts the optimal target and laser parameters for efficient nanoparticles synthesis. Then, we show experimental evidence for successful synthesis of aluminum nanoparticles. Nanoparticles size distribution, morphology, atomic structure, and chemical composition are determined by various techniques, including x-ray diffraction, atomic force microscopy, scanning and transmission electron microscopy, and energy dispersive spectroscopy.
TL;DR: The phase and amplitude of the oscillatory XRD signal around a new equilibrium demonstrate that displacive excitation of the zone-folded acoustic phonons is the dominant mechanism for strong excitation.
Abstract: Reversible structural changes of a nanostructure were measured nondestructively with subpicometer spatial and subpicosecond temporal resolution via x-ray diffraction (XRD). The spatially periodic femtosecond excitation of a gallium arsenide/aluminum gallium arsenide superlattice results in coherent lattice motions with a 3.5-picosecond period, which was directly monitored by femtosecond x-ray pulses at a 1-kilohertz repetition rate. Small changes (DeltaR/R = 0.01) of weak Bragg reflexes (R = 0.005) were detected. The phase and amplitude of the oscillatory XRD signal around a new equilibrium demonstrate that displacive excitation of the zone-folded acoustic phonons is the dominant mechanism for strong excitation.
TL;DR: A wide-bandwidth, phase-stabilized femtosecond laser is used to monitor the real-time dynamic evolution of population transfer and the mechanical action of the optical frequency comb on the atomic sample is explored and controlled, leading to precision spectroscopy with an appreciable reduction in systematic errors.
Abstract: Ultrashort laser pulses have thus far been used in two distinct modes. In the time domain, the pulses have allowed probing and manipulation of dynamics on a subpicosecond time scale. More recently, phase stabilization has produced optical frequency combs with absolute frequency reference across a broad bandwidth. Here we combine these two applications in a spectroscopic study of rubidium atoms. A wide-bandwidth, phase-stabilized femtosecond laser is used to monitor the real-time dynamic evolution of population transfer. Coherent pulse accumulation and quantum interference effects are observed and well modeled by theory. At the same time, the narrow linewidth of individual comb lines permits a precise and efficient determination of the global energy-level structure, providing a direct connection among the optical, terahertz, and radio-frequency domains. The mechanical action of the optical frequency comb on the atomic sample is explored and controlled, leading to precision spectroscopy with an appreciable reduction in systematic errors.
TL;DR: A microfluidic twin laser that produces an array of two simultaneous laser emissions with one pump laser is built, believed to be the first time by integrating micro-optical and micro fluidic components by use of a femtosecond laser.
Abstract: Microfluidic dye lasers three-dimensionally embedded in glass have been fabricated for what is believed to be the first time by integrating micro-optical and microfluidic components by use of a femtosecond laser. By pumping the microfluidic laser, in which the microfluidic chamber was filled with the laser dye Rhodamine 6G dissolved in ethanol, with a frequency-doubled Nd:yttrium aluminum garnet laser, lasing action was confirmed by analysis of the emission spectra at different pump powers. In addition, by arranging two microfluidic chambers serially in the glass, we built a microfluidic twin laser that produces an array of two simultaneous laser emissions with one pump laser.
TL;DR: Using pulse-shaping methods, this work coherently control two-photon absorption in rubidium, demonstrating spectral and temporal resolutions that are 3-5 orders of magnitude below the actual bandwidth and temporal duration of the light itself.
Abstract: We experimentally demonstrate two-photon absorption with broadband down-converted light (squeezed vacuum). Although incoherent and exhibiting the statistics of a thermal noise, broadband down-converted light can induce two-photon absorption with the same sharp temporal behavior as femtosecond pulses, while exhibiting the high spectral resolution of the narrow band pump laser. Using pulse-shaping methods, we coherently control two-photon absorption in rubidium, demonstrating spectral and temporal resolutions that are 3--5 orders of magnitude below the actual bandwidth and temporal duration of the light itself. Such properties can be exploited in various applications such as spread-spectrum optical communications, tomography, and nonlinear microscopy.
TL;DR: The surface modification of single-crystalline silicon induced by single 130 femtosecond (fs) Ti:sapphire laser pulses (wavelength 800nm) in air is investigated by means of micro Raman spectroscopy (μ-RS), atomic force microscopy and scanning laser microscopy as mentioned in this paper.
TL;DR: In this article, femtosecond laser pulses were used to fabricate straight and bent through-channels, which had diameters on the order of tens of microns, high aspect ratios, and good wall-surface quality.
Abstract: Ultra-short-pulse lasers have proved to be effective tools for micromachining a wide range of materials. When the ultra-short laser pulse is focused inside the bulk of a transparent medium, nonlinear absorption occurs only near the focal volume that is subjected to high intensity. Three-dimensional structures can be fabricated inside transparent materials by taking advantage of this volumetric absorption. In this paper, femtosecond laser pulses were used to fabricate straight and bent through-channels. Drilling was initiated from the rear surface to preserve consistent absorption of the laser pulse. When the debris was not removed efficiently, variation of the channel diameter and occasional termination of the drilling process were observed. Machining in the presence of a liquid and additional use of ultrasonic wave agitation facilitated the debris ejection. The machined channels had diameters on the order of tens of microns, high aspect ratios, and good wall-surface quality.
TL;DR: Surprisingly, it is found from the exciton excited state absorption bands and multiphoton absorption resonances in the semiconducting nanotubes that transitions between subbands are allowed; this unravels the important role of electron-electron interaction in SWNT optics.
Abstract: We studied the femtosecond dynamics of photoexcitations in films containing semiconducting and metallic single-walled carbon nanotubes (SWNTs), using various pump-probe wavelengths and intensities We found that confined excitons and charge carriers with subpicosecond dynamics dominate the ultrafast response in semiconducting and metallic SWNTs, respectively Surprisingly, we also found from the exciton excited state absorption bands and multiphoton absorption resonances in the semiconducting nanotubes that transitions between subbands are allowed; this unravels the important role of electron-electron interaction in SWNT optics
TL;DR: In this paper, the authors report long-range self-channeling in air of multiterawatt femtosecond laser pulses with large negative initial chirps, and show that the peak intensity in the light channels is at least one order of magnitude lower than required for multiphoton ionization of air molecules.
Abstract: We report long-range self-channeling in air of multiterawatt femtosecond laser pulses with large negative initial chirps. The peak intensity in the light channels is at least one order of magnitude lower than required for multiphoton ionization of air molecules. A detailed comparison is made between experiments and realistic 3+1-dimensional numerical simulations. It reveals that the mechanism limiting the growth of intensity by filamentation is connected with broken revolution symmetry in the transverse diffraction plane.
TL;DR: In this paper, the energy and focusing angle dependence of structural changes induced in bulk glass by tightly focused femtosecond laser pulses was analyzed using optical and electron microscopy, and the role of various mechanisms for structural change was inferred from these morphological observations.
Abstract: Using optical and electron microscopy, we analyze the energy and focusing angle dependence of structural changes induced in bulk glass by tightly focused femtosecond laser pulses. We observe a transition from small density variations in the material to void formation with increasing laser energy. At energies close to the threshold for producing a structural change, the shape of the structurally changed region is determined by the focal volume of the objective used to focus the femtosecond pulse, while at higher energies, the structural change takes on a conical shape. From these morphological observations, we infer the role of various mechanisms for structural change.
TL;DR: It is shown that it is possible to organize regular filamentation patterns in air by imposing either strong field gradients or phase distortions in the input-beam profile of an intense femtosecond laser pulse, and for the first time that a control of the transport of high intensities over long distances may be achieved by forcing this well ordered propagation regime.
Abstract: We show that it is possible to organize regular filamentation patterns in air by imposing either strong field gradients or phase distortions in the input-beam profile of an intense femtosecond laser pulse. A comparison between experiments and 3+1 dimensional numerical simulations confirms this concept and shows for the first time that a control of the transport of high intensities over long distances may be achieved by forcing this well ordered propagation regime. In this case, deterministic effects prevail in multiple femtosecond filamentation, and no transition to the optical turbulence regime is obtained [Mlejnek et al., Phys. Rev. Lett. 83, 2938 (1999)].
TL;DR: Waveguides manufactured with the 520-nm radiation from a frequency-doubled, diode-pumped, cavity-dumped Yb:glass laser operating at a 166-KHz repetition rate, with a 300-fs pulse duration are demonstrated.
Abstract: Laser action is demonstrated in a 20-mm-long waveguide fabricated on an Er:Yb-doped phosphate glass by femtosecond laser pulses. An output power of 1.7 mW with approximately 300 mW of pump power coupled into the waveguide is obtained at 1533.5 nm. Waveguides are manufactured with the 520-nm radiation from a frequency-doubled, diode-pumped, cavity-dumped Yb:glass laser operating at a 166-KHz repetition rate, with a 300-fs pulse duration.
TL;DR: Femtosecond laser-induced sub-wavelength microstructuring of a thin gold layer coated onto a quartz glass substrate is investigated in this paper, where the formation of microbumps (microbubbles) and nanojets under single pulse laser irradiation is observed.
Abstract: Femtosecond laser-induced sub-wavelength microstructuring of a thin gold layer coated onto a quartz glass substrate is investigated. Formation of microbumps (microbubbles) and nanojets under single pulse laser irradiation is observed. Discussion of these effects and demonstration of their dependencies on the laser pulse energy and gold layer thickness are presented.
TL;DR: The dependence of the index differential on the peak intensity reveals the nonlinear nature of the photosensitivity in arsenic trisulfide below its bandgap energy, and the refractive-index change is correlated to the photoinduced structural changes inferred by Raman spectroscopy data.
Abstract: Single-channel waveguides and Y couplers were fabricated in chalcogenide thin films by use of femtosecond laser pulses from a 25-MHz repetition rate Ti:sapphire laser. Refractive-index differentials (delta n > 10(-2)) were measured through interferometric microscopy and are higher than the typical values reported for oxide glasses. The dependence of the index differential on the peak intensity reveals the nonlinear nature of the photosensitivity in arsenic trisulfide below its bandgap energy, and the refractive-index change is correlated to the photoinduced structural changes inferred by Raman spectroscopy data. A free-electron model to predict the parametric dependence of delta n is proposed.