Showing papers on "Femtosecond published in 2003"
TL;DR: Semiconductor lasers for optical pumping and fast optical saturable absorbers, based on either semiconductor devices or the optical nonlinear Kerr effect, have dramatically improved these lasers and opened up new frontiers for applications with extremely short temporal resolution, extremely high peak optical intensities and extremely fast pulse repetition rates.
Abstract: Ultrafast lasers, which generate optical pulses in the picosecond and femtosecond range, have progressed over the past decade from complicated and specialized laboratory systems to compact, reliable instruments. Semiconductor lasers for optical pumping and fast optical saturable absorbers, based on either semiconductor devices or the optical nonlinear Kerr effect, have dramatically improved these lasers and opened up new frontiers for applications with extremely short temporal resolution (much smaller than 10 fs), extremely high peak optical intensities (greater than 10 TW/cm2) and extremely fast pulse repetition rates (greater than 100 GHz).
1,914 citations
TL;DR: Periodic nanostructures are observed inside silica glass after irradiation by a focused beam of a femtosecond Ti:sapphire laser, resulting in the periodic modulation of electron plasma concentration and the structural changes in glass.
Abstract: Periodic nanostructures are observed inside silica glass after irradiation by a focused beam of a femtosecond Ti:sapphire laser Backscattering electron images of the irradiated spot reveal a periodic structure of stripelike regions of ~20 nm width with a low oxygen concentration, which are aligned perpendicular to the laser polarization direction These are the smallest embedded structures ever created by light The period of self-organized grating structures can be controlled from ~140 to 320 nm by the pulse energy and the number of irradiated pulses The phenomenon is interpreted in terms of interference between the incident light field and the electric field of the bulk electron plasma wave, resulting in the periodic modulation of electron plasma concentration and the structural changes in glass
1,115 citations
TL;DR: This review provides an introduction to two-dimensional Fourier transform experiments exploiting second- and third-order vibrational and electronic nonlinearities.
Abstract: The simplest two-dimensional (2D) spectra show how excitation with one (variable) frequency affects the spectrum at all other frequencies, thus revealing the molecular connections between transitions. Femtosecond 2D Fourier transform (2D FT) spectra are more flexible and share some of the remarkable properties of their conceptual parent, 2D FT nuclear magnetic resonance. When 2D FT spectra are experimentally separated into real absorptive and imaginary refractive parts, the time resolution and frequency resolution can both reach the uncertainty limit set for each resonance by the sample itself. Coherent four-level contributions to the signal provide new molecular phase information, such as relative signs of transition dipoles. The nonlinear response can be picked apart by selecting a single coherence pathway (e.g., specifying the relative signs of energy level difference frequencies during different time intervals as in the photon echo). Because molecules are frozen on the femtosecond timescale, femtosecond 2D FT experiments can separate a distribution of instantaneous molecular environments and intramolecular geometries as inhomogeneous broadening. This review provides an introduction to two-dimensional Fourier transform experiments exploiting second- and third-order vibrational and electronic nonlinearities.
1,080 citations
TL;DR: Observations of the structural evolution of aluminum as it underwent an ultrafast laser–induced solid-liquid phase transition provide an atomic-level description of the melting process, which is best understood as a thermal phase transition under strongly driven conditions.
Abstract: We used 600-femtosecond electron pulses to study the structural evolution of aluminum as it underwent an ultrafast laser–induced solid-liquid phase transition. Real-time observations showed the loss of long-range order that was present in the crystalline phase and the emergence of the liquid structure where only short-range atomic correlations were present; this transition occurred in 3.5picoseconds for thin-film aluminum with an excitation fluence of 70 millijoules per square centimeter. The sensitivity and time resolution were sufficient to capture the time-dependent pair correlation function as the system evolved from the solid to the liquid state. These observations provide an atomic-level description of the melting process, in which the dynamics are best understood as a thermal phase transition under strongly driven conditions.
853 citations
TL;DR: In this article, the two-photon absorption coefficient and Kerr coefficient of bulk crystalline silicon are determined near the telecommunication wavelengths of 1.3 and 1.55 μm using femtosecond pulses and a balanced Z-scan technique.
Abstract: The two-photon absorption coefficient and Kerr coefficient of bulk crystalline silicon are determined near the telecommunication wavelengths of 1.3 and 1.55 μm using femtosecond pulses and a balanced Z-scan technique. A phase shift sensitivity of the order of 1 mrad is achieved, enabling the accurate measurement of third-order nonlinear coefficients at fluences smaller than 100 μJ/cm2. From the two-photon absorption coefficient (β∼0.8 cm/GW) and the Kerr coefficient (n2∼4×10−14 cm2/W) at a wavelength λ=1.54 μm, a value F∼0.35 for the nonlinear figure of merit for all-optical switching is determined.
666 citations
TL;DR: It is shown that two-photon fluorescence images can be obtained throughout almost the entire gray matter of the mouse neocortex by using optically amplified femtosecond pulses.
Abstract: It is shown that two-photon fluorescence images can be obtained throughout almost the entire gray matter of the mouse neocortex by using optically amplified femtosecond pulses The achieved imaging depth approaches the theoretical limit set by excitation of out-of-focus fluorescence
660 citations
Journal Article•
TL;DR: In this article, a near-field fluorescence imaging using femtosecond laser pulses of proper polarization is presented. But the technique is not suitable for near field imaging, and the spatial resolution is limited to 20 nm.
Abstract: We present a new scheme for near-field fluorescence imaging using a metal tip illuminated with femtosecond laser pulses of proper polarization. The strongly enhanced electric field at the metal tip ( $\ensuremath{\approx}15\mathrm{nm}$ end diameter) results in a localized excitation source for molecular fluorescence. Excitation of the sample via two-photon absorption provides good image contrast due to the quadratic intensity dependence. The spatial resolution is shown to be better than that of the conventional aperture technique. We used the technique to image fragments of photosynthetic membranes, as well as $J$-aggregates with spatial resolutions on the order of 20 nm.
594 citations
TL;DR: In this paper, the first true three-dimensional integrated optical device, a 1×3 splitter fabricated in pure fused silica, is presented, and the optical properties of this device and possibilities for the fabrication of complex high-density integrated optical elements are discussed.
Abstract: Using tightly focussed femtosecond laser pulses, waveguides can be fabricated inside various glasses and crystals. This technique has the potential to generate not only planar but three-dimensional photonic devices. In this paper we present, to the best of our knowledge, the first true three-dimensional integrated optical device, a 1×3 splitter fabricated in pure fused silica. The optical properties of this device and possibilities for the fabrication of complex high-density integrated optical elements are discussed.
575 citations
TL;DR: First applications of the two-photon polymerization technique for the fabrication of three-dimensional microstructures and photonic crystals in inorganic-organic hybrid polymers with a structure size down to 200 nm and a periodicity of 450 nm are discussed.
Abstract: Investigations of two-photon polymerization of inorganic-organic hybrid materials initiated by femtosecond Ti:sapphire laser pulses are performed. First applications of this technique for the fabrication of three-dimensional microstructures and photonic crystals in inorganic-organic hybrid polymers with a structure size down to 200 nm and a periodicity of 450 nm are discussed.
550 citations
TL;DR: Time-resolved X-ray diffraction measurements of the coherent atomic displacement of the lattice atoms in photoexcited bismuth close to a phase transition are reported, which leads to a subsequent loss of long-range order.
Abstract: The study of phase-transition dynamics in solids beyond a time-averaged kinetic description requires direct measurement of the changes in the atomic configuration along the physical pathways leading to the new phase. The timescale of interest is in the range 10(-14) to 10(-12) s. Until recently, only optical techniques were capable of providing adequate time resolution, albeit with indirect sensitivity to structural arrangement. Ultrafast laser-induced changes of long-range order have recently been directly established for some materials using time-resolved X-ray diffraction. However, the measurement of the atomic displacements within the unit cell, as well as their relationship with the stability limit of a structural phase, has to date remained obscure. Here we report time-resolved X-ray diffraction measurements of the coherent atomic displacement of the lattice atoms in photoexcited bismuth close to a phase transition. Excitation of large-amplitude coherent optical phonons gives rise to a periodic modulation of the X-ray diffraction efficiency. Stronger excitation corresponding to atomic displacements exceeding 10 per cent of the nearest-neighbour distance-near the Lindemann limit-leads to a subsequent loss of long-range order, which is most probably due to melting of the material.
548 citations
TL;DR: In this article, high-spatial-frequency periodic structures on the surfaces of InP, GaP, and GaAs have been observed after multiple-pulse femtosecond laser irradiation at wavelengths in the transparency regions of the respective solids.
Abstract: High-spatial-frequency periodic structures on the surfaces of InP, GaP, and GaAs have been observed after multiple-pulse femtosecond laser irradiation at wavelengths in the transparency regions of the respective solids. The periods of the structures are substantially shorter than the wavelengths of the incident laser fields in the bulk materials. In contrast, high-frequency structures were not observed for laser photon energies above the band gaps of the target materials.
TL;DR: In this article, the authors report evidence for two different mechanisms of material ablation in the liquid environment, whose relative contributions determine the size distribution of the produced colloidal gold nanoparticles.
Abstract: Femtosecond laser radiation has been used to ablate a gold target in pure deionized water to produce colloidal gold nanoparticles. We report evidence for two different mechanisms of material ablation in the liquid environment, whose relative contributions determine the size distribution of the produced particles. The first mechanism, associated with thermal-free femtosecond ablation, manifests itself at relatively low laser fluences F<400 J/cm2 and leads to very small (3–10 nm) and almost monodispersed gold colloids. The second one, attributed to the plasma-induced heating and ablation of the target, takes place at high fluences and gives rise to a much larger particle size and broad size distribution. The fabricated nanoparticles exhibit plasmon-related optical absorption peak and are of significance for biosensing applications.
TL;DR: Femtosecond phase-coherent two-dimensional spectroscopy has been experimentally demonstrated as the direct optical analog of 2D nuclear magnetic resonance and the desired nonlinear polarization was selected by phase cycling.
Abstract: Femtosecond phase-coherent two-dimensional (2D) spectroscopy has been experimentally demonstrated as the direct optical analog of 2D nuclear magnetic resonance. An acousto-optic pulse shaper created a collinear three-pulse sequence with well-controlled and variable interpulse delays and phases,which interacted with a model atomic system of rubidium vapor. The desired nonlinear polarization was selected by phase cycling (coadding experimental results obtained with different interpulse phases). This method may enhance our ability to probe the femtosecond structural dynamics of macromolecules.
TL;DR: In this article, a technique for inducing refractive-index changes in bulk material using a high-repetition-rate femtosecond oscillator is described, which is caused by a localized melting of the material, which results from an accumulation of thermal energy due to nonlinear absorption of the high repetition rate train of laser pulses.
Abstract: Femtosecond laser pulses can locally induce structural and chemical changes in the bulk of transparent materials, opening the door to the three-dimensional fabrication of optical devices. We review the laser and focusing parameters that have been applied to induce these changes and discuss the different physical mechanisms that play a role in forming them. We then describe a new technique for inducing refractive-index changes in bulk material using a high-repetition-rate femtosecond oscillator. The changes are caused by a localized melting of the material, which results from an accumulation of thermal energy due to nonlinear absorption of the high-repetition-rate train of laser pulses.
TL;DR: The neutralization reaction between an acid and a base in water, triggered after optical excitation, was studied by femtosecond vibrational spectroscopy and calls for a refinement of the traditional Eigen-Weller picture of acid-base reactions.
Abstract: The neutralization reaction between an acid and a base in water, triggered after optical excitation, was studied by femtosecond vibrational spectroscopy. Bimodal dynamics were observed. In hydrogen-bonded acid-base complexes, the proton transfer proceeds extremely fast (within 150 femtoseconds). In encounter pairs formed by diffusion of uncomplexed photoacid and base molecules, the reaction upon contact was an order of magnitude slower, in agreement with earlier reported values. These results call for a refinement of the traditional Eigen-Weller picture of acid-base reactions: A three-stage model is introduced to account for all observed dynamics.
TL;DR: Initial observations in the application of femtosecond time-resolved absorption techniques to the study of the photophysics of transition metal complexes are described and examples detailing these dynamics as they occur in the charge-transfer excited states of transition-metal polypyridyl chromophores are included.
Abstract: Our research is concerned with the application of femtosecond time-resolved absorption techniques to the study of the photophysics of transition metal complexes. The focus is to understand the events that characterize the process of excited-state evolution from the time a photon is absorbed by a molecule to the formation of the lowest-energy excited state of the system. This Account describes our initial observations in this area and includes examples detailing these dynamics as they occur in the charge-transfer excited states of transition-metal polypyridyl chromophores.
TL;DR: FemTosecond high-resolution pump-probe experiments have been used together with theoretical ab initio quantum calculations and wave packet dynamics simulations to decode an optimal femtosecond pulse that is generated from adaptive learning algorithms.
Abstract: Femtosecond high-resolution pump-probe experiments have been used together with theoretical ab initio quantum calculations and wave packet dynamics simulations to decode an optimal femtosecond pulse that is generated from adaptive learning algorithms. This pulse is designed to maximize the yield of the organometallic ion CpMn(CO) 5 while hindering the competing fragmentation. The sequential excitation and ionization of the target ion are accomplished by an optimized field consisting of two dominant subpulses with optimal frequencies and time delays.
TL;DR: In this article, an approach for the fabrication of optical waveguides by focused low-repetition-rate femtosecond laser pulses is described. But the main limitation of this approach is the strong asymmetry of the waveguide profile.
Abstract: We describe a novel approach for the fabrication of optical waveguides by focused low-repetition-rate femtosecond laser pulses. This approach overcomes the main limitation of the technique, i.e., the strong asymmetry of the waveguide profile. By use of an astigmatic beam and suitably controlling both beam waist and focal position in tangential and sagittal planes, it is possible to shape the focal volume in such a way as to obtain waveguides with a circular transverse profile and of the desired size. This technique is applied to the fabrication of active waveguides in Er:Yb-doped glass substrates. The waveguides are single mode at 1.5 μm and exhibit propagation losses of ∼0.25 dB/cm and an internal gain of 1.4 dB at 1534 nm.
TL;DR: In this paper, the authors studied the ablation of solids under femtosecond laser pulses using a two-dimensional molecular-dynamics model and showed that different expansion regimes develop as a function of the injected energy.
Abstract: The ablation of solids under femtosecond laser pulses is studied using a two-dimensional molecular-dynamics model. The simulations show that different expansion regimes develop as a function of the injected energy. The origin of these regimes lies in changes of the thermodynamical relaxation path the material follows when the intensity of the laser increases. The shape of the pressure waves generated as a result of the absorption of the pulse is shown to vary from bipolar at low fluence to unipolar at high fluence, as a result of the decrease of the tensile strength of the material with temperature. By combining these results with an analysis of the thermodynamical trajectories for different portions of the target, we show that four different mechanisms can account for ablation at fluences below the threshold for plasma formation, namely spallation, phase explosion, fragmentation, and vaporization. These mechanisms are characterized in detail; it is demonstrated that they can occur simultaneously in different parts of the target.
Journal Article•
TL;DR: Femtosecond as well as picosecond pulses provide approximately the same relative optical window for safe two-photon fluorescence microscopy, indicating that cell destruction is likely based on a two- photon excitation process rather than a one- or a three-ph photon event.
Abstract: The influence of the pulse length, ?, of ultrashort laser pulses at 780 and 920??nm on cell vitality and cellular reproduction has been studied. A total of 2400 nonlabeled cells were exposed to a highly focused scanning beam from a mode-locked 80-MHz Ti:sapphire laser with 60??s pixel dwell time. For the same pulse energy, destructive effects were more pronounced for shorter pulses. The damage behavior was found to follow approximately a P2/? dependence (P, mean power), indicating that cell destruction is likely based on a two-photon excitation process rather than a one- or a three-photon event. Therefore, femtosecond as well as picosecond pulses provide approximately the same relative optical window for safe two-photon fluorescence microscopy.
TL;DR: Femtosecond pump-probe experiments performed in the low-perturbation regime demonstrate a strong increase of the intrinsic electron-phonon interaction for nanoparticles smaller than 10 nm due to a confinement effect.
Abstract: Electron-lattice energy exchanges are investigated in gold and silver nanoparticles with sizes ranging from 30 to 2.2 nm embedded in different environments. Femtosecond pump-probe experiments performed in the low-perturbation regime demonstrate a strong increase of the intrinsic electron-phonon interaction for nanoparticles smaller than 10 nm due to a confinement effect.
TL;DR: It is demonstrated that one can greatly reduce the aspect ratio merely by inserting a slit, which is oriented parallel to the laser's scanning direction, before the focusing lens.
Abstract: Theoretical and experimental investigations have been made of the three-dimensional microchannel fabrication of photostructurable glass by use of a femtosecond (fs) laser. Generally, a microchannel fabricated inside glass by the scanning focal spot of a fs laser perpendicular to the direction of laser propagation assumes an elliptical shape with a cross section of large aspect ratio. We demonstrate that one can greatly reduce the aspect ratio merely by inserting a slit, which is oriented parallel to the laser’s scanning direction, before the focusing lens. Computer simulations show that a more symmetrical pattern is obtained in the vicinity of the focal point with the help of such a slit, owing essentially to a diffraction effect.
TL;DR: In this paper, a modulated hollow-core waveguide is used to periodically vary the intensity of the laser light driving the conversion, which can be used to generate high-energy extreme-ultraviolet (EUV) light.
Abstract: High-harmonic generation is a well-known method of producing coherent extreme-ultraviolet (EUV) light, with photon energies up to about 0.5 keV (refs 1, 2). This is achieved by focusing a femtosecond laser into a gas, and high harmonics of the fundamental laser frequency are radiated in the forward direction. However, although this process can generate high-energy photons, efficient high-harmonic generation has been demonstrated only for photon energies of the order 50-100 eV (ref. 5). Ionization of the gas prevents the laser and the EUV light from propagating at the same speed, which severely limits the conversion efficiency. Here we report a technique to overcome this problem, and demonstrate quasi-phase-matched frequency conversion of laser light into EUV. Using a modulated hollow-core waveguide to periodically vary the intensity of the laser light driving the conversion, we efficiently generate EUV light even in the presence of substantial ionization. The use of a modulated fibre shifts the energy spectrum of the high-harmonic light to significantly higher photon energies than would otherwise be possible. We expect that this technique could form the basis of coherent EUV sources for advanced lithography and high-resolution imaging applications. In future work, it might also be possible to generate isolated attosecond pulses.
TL;DR: The ability to control both the spectral phase and the spectral polarization enables measurement of a specific off-diagonal susceptibility tensor element while exploiting the different spectral response of the resonant Raman signal and the nonresonant background to achieve maximal background suppression.
Abstract: Phase-and-polarization coherent control is applied to control the nonlinear response of a quantum system We use it to obtain high-resolution background-free single-pulse coherent anti-Stokes Raman spectra The ability to control both the spectral phase and the spectral polarization enables measurement of a specific off-diagonal susceptibility tensor element while exploiting the different spectral response of the resonant Raman signal and the nonresonant background to achieve maximal background suppression
TL;DR: In this paper, the authors show that the near-unity infrared absorptance of conical microstructures fabricated by irradiating a Si(111) surface with 100 fs laser pulses depends on the ambient gas in which the structures are formed.
Abstract: We show that the near-unity infrared absorptance of conical microstructures fabricated by irradiating a Si(111) surface with 100 fs laser pulses depends on the ambient gas in which the structures are formed. SF6 produces an absorptance of 0.9 for radiation in the 1.2–2.5 μm wavelength range, higher than any of the other gases. Use of Cl2, N2, or air produces surfaces with absorptances intermediate between that for microstructures formed in SF6 and that for flat crystalline silicon, for which the absorptance is roughly 0.05–0.2 for a 260 μm thick sample. Secondary ion mass spectrometry shows that elements from the ambient gas are incorporated into the silicon surface in high concentration.
TL;DR: In this article, it was shown that the high spatio-temporal localization of the light field in the filament, which enables the supercontinuum generation, is sustained due to the dynamic transformation of light field on the whole transverse scale of the beam, including its edges.
Abstract: We present experimental and theoretical results on white-light generation in the filamentation of a high-power femtosecond laser pulse in water and atmospheric air. We have shown that the high spatio-temporal localization of the light field in the filament, which enables the supercontinuum generation, is sustained due to the dynamic transformation of the light field on the whole transverse scale of the beam, including its edges. We found that the sources of the supercontinuum blue wing are in the rings, surrounding the filament, as well as at the back of the pulse, where shock-wave formation enhanced by self-steepening takes place. We report on the first observation and demonstration of the interference of the supercontinuum spectral components arising in the course of multiple filamentation in a terawatt laser pulse. We demonstrate that the conversion efficiency of an initially narrow laser pulse spectrum into the supercontinuum depends on the length of the filament with high intensity gradients and can be increased by introducing an initial chirp.
TL;DR: In this article, the laser-induced periodic surface structure (LIPSS) with periodicity about a quarter of the laser wavelength on unpolished diamond film treated by a P-polarized femtosecond laser was observed.
Abstract: We report the laser-induced periodic surface structure (LIPSS) with periodicity about a quarter of the laser wavelength on unpolished diamond film treated by a P-polarized femtosecond laser. The short period LIPSS is parallel to the laser polarization and independent on the incidence angle. The LIPSS perpendicular to the laser polarization with periodicity shorter than a third of the laser wavelength slightly dependent on the incidence angle is also observed as well as the LIPSS perpendicular to the laser polarization with periodicity dependent on the incidence angle. The results are explained by interference of the incident laser and surface scattered wave related to the excited electrons during laser interactions with diamond, being in excellent agreement with a previously developed theory.
TL;DR: In this paper, the femtosecond laser-based nanofabrication of metal layers and transparent materials is investigated, and the authors show that two-photon polymerization of hybrid polymers can be used to construct two-and three-dimensional nanostructures with structure sizes on the order of several hundred nanometers.
Abstract: Detailed investigations of the possibilities for using femtosecond lasers for the nanostructuring of metal layers and transparent materials are reported. The aim is to develop a simple laser-based technology for fabricating two- and three-dimensional nanostructures with structure sizes on the order of several hundred nanometers. This is required for many applications in photonics, for the fabrication of photonic crystals and microoptical devices, for data storage, displays, etc. Measurements of thermionic electron emission from metal targets, which provide valuable information on the dynamics of femtosecond laser ablation, are discussed. Sub-wavelength microstructuring of metals is performed and the minimum structure size that can be fabricated in transparent materials is identified. Two-photon polymerization of hybrid polymers is demonstrated as a promising femtosecond laser-based nanofabrication technology.
TL;DR: In this paper, the authors examined the damage threshold as a function of polarization and concluded that the fundamental mechanism is self-terminated Zener-impact ionization, and that the deterministic and uniform damage threshold throughout the sample threshold stems from the uniform valence-electron density found in good quality optical materials.
Abstract: A remarkable feature of material damage induced by short-pulsed lasers is that the energy threshold becomes deterministic for sub-picosecond pulses. This effect, coupled with the advent of kHz and higher repetition rate chirped pulse amplification systems, has opened the field of femtosecond machining. Yet the mechanism of optical breakdown remains unclear. By examining the damage threshold as a function of polarization, we find that, contrary to established belief, multiphoton ionization plays an insignificant role in optical breakdown. The polarization independence, combined with the observed precise and uniform dielectric breakdown threshold even for nanometer-scale features, leads us to conclude that the fundamental mechanism is ‘self-terminated’ Zener-impact ionization, and that the deterministic and uniform damage threshold throughout the sample threshold stems from the uniform valence-electron density found in good-quality optical materials. By systematically exploring optical breakdown near threshold, we find that we can consistently machine features as small as 20 nm, demonstrating great promise for applications ranging from Micro ElectroMechanical Systems (MEMS) construction and microelectronics, to targeted disruption of cellular structures and genetic material.
TL;DR: In this article, the authors observed structural changes in the glass network of fused silica after modification by tightly focused 800-nm, 130-fs laser pulses at fluences between 5 and 200 cm-2.
Abstract: Atomic-scale structural changes have been observed in the glass network of fused silica after modification by tightly focused 800-nm, 130-fs laser pulses at fluences between 5 and 200 J cm-2. Raman spectroscopy of the modified glass shows an increase in the 490 and 605-cm-1 peaks, indicating an increase in the number of 4- and 3-membered ring structures in the silica network. These results provide evidence that densification of the glass occurs after exposure to fs pulses. Fluorescence spectroscopy of the modified glass shows a broad fluorescence band at 630 nm, indicating the formation of non-bridging oxygen hole centers (NBOHC) by fs pulses. Waveguides that support the fundamental mode at 633 nm have been fabricated inside fused silica by scanning the glass along the fs laser beam axis. The index changes are estimated to be approximately 0.07×10-3.