Showing papers on "Femtosecond published in 2008"
TL;DR: This work demonstrates a method of high-harmonic generation that requires no extra cavities by exploiting the local field enhancement induced by resonant plasmons within a metallic nanostructure consisting of bow-tie-shaped gold elements on a sapphire substrate.
Abstract: High-harmonic generation by focusing a femtosecond laser onto a gas is a well-known method of producing coherent extreme-ultraviolet (EUV) light. This nonlinear conversion process requires high pulse intensities, greater than 10(13) W cm(-2), which are not directly attainable using only the output power of a femtosecond oscillator. Chirped-pulse amplification enables the pulse intensity to exceed this threshold by incorporating several regenerative and/or multi-pass amplifier cavities in tandem. Intracavity pulse amplification (designed not to reduce the pulse repetition rate) also requires a long cavity. Here we demonstrate a method of high-harmonic generation that requires no extra cavities. This is achieved by exploiting the local field enhancement induced by resonant plasmons within a metallic nanostructure consisting of bow-tie-shaped gold elements on a sapphire substrate. In our experiment, the output beam emitted from a modest femtosecond oscillator (100-kW peak power, 1.3-nJ pulse energy and 10-fs pulse duration) is directly focused onto the nanostructure with a pulse intensity of only 10(11) W cm(-2). The enhancement factor exceeds 20 dB, which is sufficient to produce EUV wavelengths down to 47 nm by injection with an argon gas jet. The method could form the basis for constructing laptop-sized EUV light sources for advanced lithography and high-resolution imaging applications.
1,320 citations
TL;DR: In this article, a femtosecond laser processing technique was used to create a variety of colors on a metal that ultimately leads to control its optical properties from UV to terahertz.
Abstract: For centuries, it had been the dream of alchemists to turn inexpensive metals into gold. Certainly, it is not enough from an alchemist’s point of view to transfer only the appearance of a metal to gold. However, the possibility of rendering a certain metal to a completely different color without coating can be very interesting in its own right. In this work, we demonstrate a femtosecond laser processing technique that allows us to create a variety of colors on a metal that ultimately leads us to control its optical properties from UV to terahertz.
507 citations
TL;DR: Waveguide morphology and thermal modeling indicate that strong thermal diffusion effects at 200 kHz give way to a weak heat accumulation effect at approximately 1 microJ pulse energy for generating low loss waveguides, while stronger heat accumulation effects above 1-MHz repetition rate offered overall superior guiding.
Abstract: A variable (0.2 to 5 MHz) repetition rate femtosecond laser was applied to delineate the role of thermal diffusion and heat accumulation effects in forming low-loss optical waveguides in borosilicate glass across a broad range of laser exposure conditions. For the first time, a smooth transition from diffusion-only transport at 200 kHz repetition rate to strong heat accumulation effects at 0.5 to 2 MHz was observed and shown to drive significant variations in waveguide morphology, with rapidly increasing waveguide diameter that accurately followed a simple thermal diffusion model over all exposure variables tested. Amongst these strong thermal trends, a common exposure window of 200 mW average power and approximately 15-mm/s scan speed was discovered across the range of 200 kHz to 2 MHz repetition rates for minimizing insertion loss despite a 10-fold drop in laser pulse energy. Waveguide morphology and thermal modeling indicate that strong thermal diffusion effects at 200 kHz give way to a weak heat accumulation effect at approximately 1 microJ pulse energy for generating low loss waveguides, while stronger heat accumulation effects above 1-MHz repetition rate offered overall superior guiding. A comprehensive characterization of waveguide properties is presented for laser writing in the thermal diffusion and heat accumulation regimes. The waveguides are shown to be thermally stable up to 800 degrees C and can be written in a convenient 520 microm depth range with low spherical aberration.
330 citations
TL;DR: In this paper, the femtosecond laser induced self-organized planar cracks inside fused silica glass are analyzed using form birefringence and shown that the high degree of control over these self-replicated periodic structures allows them to investigate applications in micro- and nanofluidics, porous capillaries for biofiltering and rewritable data storage for harsh environments.
Abstract: We review our recent experimental efforts towards developing photonic and biophotonic applications of femtosecond laser induced self-organized planar nanocracks inside fused silica glass. Our results show that sub-diffraction limited, periodic, planar cracks can be produced, organized, erased and rewritten and basically controlled inside fused silica glass where they can be diagnosed optically using form birefringence. The high degree of control over these self-replicated periodic structures allows us to investigate applications in micro- and nanofluidics, porous capillaries for biofiltering and rewritable data storage for harsh environments.
291 citations
TL;DR: The results show that the formation of periodicity can be attributed to the excitation of surface plasmon polaritons to induce the periodic enhancement of local fields in the surface layer.
Abstract: We report physical processes responsible for the periodic nanostructure formation in femtosecond-laser ablation of thin film surfaces. It has been found that an initial random distribution of nanoscale ablation traces is periodically structured with an increase in superimposed laser pulses or fluence on diamond-like carbon film used as the target. The results show that the formation of periodicity can be attributed to the excitation of surface plasmon polaritons to induce the periodic enhancement of local fields in the surface layer. The estimated field period is in good agreement with the observed size of nanostructures.
267 citations
TL;DR: A miniaturized inline Fabry-Perot interferometer directly fabricated on a single-mode optical fiber with a femtosecond laser is reported, attractive for sensing applications in high-temperature harsh environments.
Abstract: We report a miniaturized inline Fabry-Perot interferometer directly fabricated on a single-mode optical fiber with a femtosecond laser. The device had a loss of 16 dB and an interference visibility exceeding 14 dB. The device was tested and survived in high temperatures up to 1100°C. With an accessible cavity and all-glass structure, the new device is attractive for sensing applications in high-temperature harsh environments.
240 citations
TL;DR: In this article, a single 15 femtosecond soft-X-ray pulse was used to image a nanoscale object in free flight for the first time, an important step toward imaging uncrystallized biomolecules.
Abstract: In nanotechnology, strategies for the creation and manipulation of nanoparticles in the gas phase are critically important for surface modification and substrate-free characterization. Recent coherent diffractive imaging with intense femtosecond X-ray pulses has verified the capability of single-shot imaging of nanoscale objects at suboptical resolutions beyond the radiation-induced damage threshold. By intercepting electrospray-generated particles with a single 15 femtosecond soft-X-ray pulse, we demonstrate diffractive imaging of a nanoscale specimen in free flight for the first time, an important step toward imaging uncrystallized biomolecules.
232 citations
TL;DR: Using coherent diffraction imaging with femtosecond X-ray free-electron-laser pulses, this paper captured time-series snapshots of a solid as it evolves on the ultrafast timescale.
Abstract: The transient nanoscale dynamics of materials on femtosecond to picosecond timescales is of great interest in the study of condensed phase dynamics such as crack formation, phase separation and nucleation, and rapid fluctuations in the liquid state or in biologically relevant environments. The ability to take images in a single shot is the key to studying non-repetitive behaviour mechanisms, a capability that is of great importance in many of these problems. Using coherent diffraction imaging with femtosecond X-ray free-electron-laser pulses we capture time-series snapshots of a solid as it evolves on the ultrafast timescale. Artificial structures imprinted on a Si3N4 window are excited with an optical laser and undergo laser ablation, which is imaged with a spatial resolution of 50 nm and a temporal resolution of 10 ps. By using the shortest available free-electron-laser wavelengths1 and proven synchronization methods2 this technique could be extended to spatial resolutions of a few nanometres and temporal resolutions of a few tens of femtoseconds. This experiment opens the door to a new regime of time-resolved experiments in mesoscopic dynamics. High-speed imaging gives us a fascinating insight into ultrafast changes in materials. By combining the speed of optical pulses and the short wavelength of X-ray pulses, imaging with 50-nm spatial and 10-ps temporal resolution is possible, with scope to go much further.
224 citations
TL;DR: An axisymmetric computational model is presented to study the heating processes of gold nanoparticles, specifically nanorods, in aqueous medium by femtosecond laser pulses, using a two-temperature model for the particle, a heat diffusion equation for the surrounding water, and a thermal interface conductance to describe the coupling efficiency at the particle/water interface.
Abstract: We present an axisymmetric computational model to study the heating processes of gold nanoparticles, specifically nanorods, in aqueous medium by femtosecond laser pulses. We use a two-temperature model for the particle, a heat diffusion equation for the surrounding water to describe the heat transfer processes occurring in the system, and a thermal interface conductance to describe the coupling efficiency at the particle/water interface. We investigate the characteristic time scales of various fundamental processes, including lattice heating and thermal equilibration at the particle/surroundings interface, the effects of multiple laser pulses, and the influence of nanorod orientation relative to the beam polarization on energy absorption. Our results indicate that the thermal equilibration at the particle/water interface takes approximately 500 ps, while the electron-lattice coupling is achieved at approximately 50 ps when a 48×14 nm gold nanorod is heated to a maximum temperature of 1270 K with the application of a laser pulse having 4.70 J/m(2) average fluence. Irradiation by multiple pulses arriving at 12.5 ns time intervals (80 MHz repetition rate) causes a temperature increase of no more than 3 degrees during the first few pulses with no substantial changes during the subsequent pulses. We also analyze the degree of the nanorods' heating as a function of their orientation with respect to the polarization of the incident light. Lastly, it is shown that the temperature change of a nanorod can be modeled using its volume equivalent sphere for femtosecond laser heating within 5-15% accuracy.
213 citations
TL;DR: In this article, the absorbing structures for photovoltaic cells with different nanotexturization, obtained by means of a femtosecond laser, without the use of corrosive gas (under vacuum).
198 citations
TL;DR: This Account describes recent advances in laser technology both for the synthesis of organic nanoparticles and for their analysis by single nanoparticle spectroscopy and proposes "structural confinement" as an analogous paradigm fororganic nanoparticles.
Abstract: In working with nanoparticles, researchers still face two fundamental challenges: how to fabricate the nanoparticles with controlled size and shape and how to characterize them. In this Account, we describe recent advances in laser technology both for the synthesis of organic nanoparticles and for their analysis by single nanoparticle spectroscopy. Laser ablation of organic microcrystalline powders in a poor solvent has opened new horizons for the synthesis of nanoparticles because the powder sample is converted directly into a stable colloidal solution without additives and chemicals. By tuning laser wavelength, pulse width, laser fluence, and total shot number, we could control the size and phase of the nanoparticles. For example, we describe nanoparticle formation of quinacridone, a well-known red pigment, in water. By modifying the length of time that the sample is excited by the laser, we could control the particle size (30−120 nm) for nanosecond excitation down to 13 nm for femtosecond irradiation. ...
TL;DR: A simple approach for rejecting unwanted scattered light in two types of time-resolved pump-probe measurements, time-domain thermoreflectance (TDTR) and time- Resolved incoherent anti-Stokes Raman scattering (TRIARS).
Abstract: We describe a simple approach for rejecting unwanted scattered light in two types of time-resolved pump-probe measurements, time-domain thermoreflectance (TDTR) and time-resolved incoherent anti-Stokes Raman scattering (TRIARS). Sharp edged optical filters are used to create spectrally distinct pump and probe beams from the broad spectral output of a femtosecond Ti:sapphire laser oscillator. For TDTR, the diffusely scattered pump light is then blocked by a third optical filter. For TRIARS, depolarized scattering created by the pump is shifted in frequency by approximately 250 cm(-1) relative to the polarized scattering created by the probe; therefore, spectral features created by the pump and probe scattering can be easily distinguished.
TL;DR: In this paper, the authors review the state of the art of high-energy femtosecond laser oscillators, in particular mode-locked thin-disk lasers, and discuss their potential to drive high-field science experiments at multimegahertz repetition rates.
Abstract: Ultrafast laser oscillators have become ubiquitous in science and technology. For many years, however, their pulse energy has been limited to the nanojoule regime. Applications requiring more intense pulses relied on complex amplifier systems, which typically operate at low pulse repetition rates of the order of kilohertz. Recently, the pulse energy of femtosecond laser oscillators has greatly increased, such that some of these experiments can now be driven at multimegahertz repetition rates, which opens promising new avenues for many applications. We review the current state of the art of high-energy femtosecond laser oscillators, in particular mode-locked thin-disk lasers, and discuss their potential to drive high-field science experiments at multimegahertz repetition rates. Diode-pumped thin-disk lasers are now capable of generating femtosecond light pulses with a pulse energy in the microjoule regime at multi-megahertz repetition rates. This review describes the progress that has been made in scaling the performance of such lasers and the applications that may benefit as a result.
TL;DR: In this article, the theory of the transition-Cherenkov process was studied in detail and compared with recent experimental results for several gases, including CO 2, CO 2 and CO 2.
Abstract: A transition-Cherenkov electromagnetic emission by a femtosecond laser pulse propagating in a self-induced plasma channel in air has been very recently proposed as mechanism for production of terahertz (THz) radiation in the forward direction. In this paper, we study in detail the theory of the transition-Cherenkov process. The theoretical model is developed and compared with recent experimental results for several gases.
TL;DR: In this paper, the authors demonstrate that when the direction of the femtosecond laser beam is reversed from the +z to −z direction, the structures written in LiNbO3 crystal when translating the beam along the +y and −y directions are mirrored.
Abstract: Photosensitivity is a material property that is relevant to many phenomena and applications, from photosynthesis and photography to optical data storage and ultrafast laser writing. It was commonly thought that, in a homogeneous medium, photosensitivity and the corresponding light-induced material modifications do not change on reversing the direction of light propagation. Here we demonstrate that when the direction of the femtosecond laser beam is reversed from the +z to –z direction, the structures written in LiNbO3 crystal when translating the beam along the +y and –y directions are mirrored. In a non-centrosymmetric medium, modification of the material can therefore differ for light propagating in opposite directions. This is the first evidence of a new optical phenomenon of non-reciprocal photosensitivity. We interpret this effect in terms of light pressure and associated heat flow, resulting in a temperature gradient in homogeneous media without inversion symmetry under uniform intense irradiation.
TL;DR: Tunable femtosecond pump-near IR probe measurements on InAs/CdSe/ZnSe core/shell1/shell2 nanocrystal quantum dots were conducted to quantify spontaneous carrier multiplication, showing no signs of carrier multiplication within that range.
Abstract: Tunable femtosecond pump-near IR probe measurements on InAs/CdSe/ZnSe core/shell1/shell2 nanocrystal quantum dots were conducted to quantify spontaneous carrier multiplication previously reported in this system. Experimental conditions were chosen to eliminate the need for determining absolute wavelength dependent cross sections of the nanocrystals and allow direct comparison of band edge absorption bleach kinetics for different excitation energies up to 3.7 times the band gap. Results for two sample sizes show no signs of carrier multiplication within that range. This result is discussed in light of reports describing occurrence of this novel phenomenon in quantum dots based on this as well as numerous other semiconductor materials.
TL;DR: High-density regular arrays of nanometer-scale rods are formed using femtosecond laser irradiation of a silicon surface immersed in water using several processes: refraction of laser light in highly excited silicon, interference of scattered and refracted light, rapid cooling in water, roughness-enhanced optical absorptance, and capillary instabilities.
Abstract: We report on the formation of high-density regular arrays of nanometer-scale rods using femtosecond laser irradiation of a silicon surface immersed in water. The resulting surface exhibits both micrometer-scale and nanometer-scale structures. The micrometer-scale structure consists of spikes of 5-10 µm width, which are entirely covered by nanometer-scale rods that are roughly 50 nm wide and normal to the surface of the micrometer-scale spikes. The formation of the nanometer-scale rods involves several processes: refraction of laser light in highly excited silicon, interference of scattered and refracted light, rapid cooling in water, roughness-enhanced optical absorptance, and capillary instabilities. As semiconductor processing reaches smaller and smaller feature sizes, much research is dedicated to the development of new methods for producing regular, nanometer-scale structures on solid surfaces. Lithography and chemical etching are powerful methods that offer control over the shape and size of structures, but they are often complex and costly. 1–3 In contrast, pulsed laser-assisted etching 4,5 is a simple but effective method for fabricating small structures directly onto a substrate. However, most of the cross sections of such structures are typically larger than the laser wavelength. 4–14
TL;DR: It is found that the relaxation process along the modified potential is not inertial but rather involves multiple scattering towards the disordered state.
Abstract: The excitation of a high density of carriers in semiconductors can induce an order-to-disorder phase transition due to changes in the potential-energy landscape of the lattice. We report the first direct resolution of the structural details of this phenomenon in freestanding films of polycrystalline and (001)-oriented crystalline Si, using 200-fs electron pulses. At excitation levels greater than approximately 6% of the valence electron density, the crystalline structure of the lattice is lost in <500 fs, a time scale indicative of an electronically driven phase transition. We find that the relaxation process along the modified potential is not inertial but rather involves multiple scattering towards the disordered state.
TL;DR: In this article, a regular array of sharp nano-textured conical microstructures are formed on the titanium metal surface by irradiation with ultrafast laser pulses of 130 fs duration, 800 nm wavelength in vacuum (∼ 1 mbar) or in 100 mbar He.
Abstract: We report for the first time that a regular array of sharp nano-textured conical microstructures are formed on the titanium metal surface by irradiation with ultrafast laser pulses of 130 fs duration, 800 nm wavelength in vacuum (∼1 mbar) or in 100 mbar He. The microstructures are up to 25 μm tall, and taper to about 500 nm diameters at the tip. Irradiation in the presence of SF6, air or HCl creates a textured surface but does not create sharp conical microstructures. The surfaces of these microstructures exhibit periodic nano-texture of feature size comparable to the wavelength of light consistent with ripple formation. Contrary to pillar formation by femtosecond laser irradiation of silicon where the initial ripples evolve into the pillars and the ripples disappear, the ripples on titanium pillars have a much smaller periodicity than the pillars and remain on the surface of the pillars. The textured surface is pitch black compared to its original silver-grayish color, i.e, it exhibits greatly reduced reflectivity throughout the measured visible spectrum.
TL;DR: In this article, a large diameter hollow plasma waveguide was used to generate hundreds of filaments judiciously distributed in a cylindrical shape, creating a cylinrical plasma wall that acts as a microwave waveguide.
Abstract: Microwave guiding was demonstrated over 16cm in air using a large diameter hollow plasma waveguide. The waveguide was generated with the 100TW femtosecond laser system at the Advanced Laser Light Source facility. A deformable mirror was used to spatially shape the intense laser pulses in order to generate hundreds of filaments judiciously distributed in a cylindrical shape, creating a cylindrical plasma wall that acts as a microwave waveguide. The microwaves were confined for about 10ns, which corresponds to the free electron plasma wall recombination time. The characteristics of the plasma waveguide and the results of microwave guiding are presented.
TL;DR: The results set an important precedent for extremely rapid, formally forbidden (DeltaS = 2) nonradiative relaxation as well as defining the time scale for intramolecular optical switching between two electronic states possessing vastly different spectroscopic, geometric, and magnetic properties.
Abstract: A combination of femtosecond electronic absorption and stimulated Raman spectroscopies has been employed to determine the kinetics associated with low-spin to high-spin conversion following charge-transfer excitation of a FeII spin-crossover system in solution. A time constant of τ = 190 ± 50 fs for the formation of the 5T2 ligand-field state was assigned based on the establishment of two isosbestic points in the ultraviolet in conjunction with changes in ligand stretching frequencies and Raman scattering amplitudes; additional dynamics observed in both the electronic and vibrational spectra further indicate that vibrational relaxation in the high-spin state occurs with a time constant of ca. 10 ps. The results set an important precedent for extremely rapid, formally forbidden (ΔS = 2) nonradiative relaxation as well as defining the time scale for intramolecular optical switching between two electronic states possessing vastly different spectroscopic, geometric, and magnetic properties.
TL;DR: In this paper, the authors exploit the peak brilliance of the free-electron laser in Hamburg and establish X-ray- pulse-induced transient changes of the optical reflectivity in GaAs as a powerful tool for Xray/optical cross-correlation.
Abstract: For a fundamental understanding of ultrafast dynamics in chemistry, biology and materials science it has been a long-standing dream to record a molecular movie in which both the atomic trajectories and the chemical states of every atom in matter are followed in real time1. Free-electron lasers provide this perspective as they deliver brilliant femtosecond X-ray pulses spanning a wide photon energy range, which is necessary to gather element-specific and chemical-state-selective information with femtosecond time resolution. The key challenge lies in synchronizing the free-electron lasers with separate optical lasers. We exploit the peak brilliance of the free-electron laser in Hamburg2,3 (FLASH) and establish X-ray- pulse-induced transient changes of the optical reflectivity in GaAs as a powerful tool for X-ray/optical cross-correlation. This constitutes a breakthrough in the path towards recording a molecular movie and—equally importantly—opens up the field of femtosecond X-ray-induced dynamics, only accessible with high-brilliance X-ray free-electron lasers.
TL;DR: A compact nonlinear laser scanning microscope with sub-20 femtosecond 75 MHz near infrared laser pulses for nanosurgery of human stem cells and two-photon high-resolution imaging and optical stem cell manipulation is reported on.
Abstract: Multiphoton microscopes have become important tools for non-contact sub-wavelength three-dimensional nanoprocessing of living biological specimens based on multiphoton ionization and plasma formation. Ultrashort laser pulses are required, however, dispersive effects limit the shortest pulse duration achievable at the focal plane. We report on a compact nonlinear laser scanning microscope with sub-20 femtosecond 75 MHz near infrared laser pulses for nanosurgery of human stem cells and two-photon high-resolution imaging. Single point illumination of the cell membrane was performed to induce a transient nanopore for the delivery of extracellular green fluorescent protein plasmids. Mean powers of less than 7 mW (<93 pJ) and low millisecond exposure times were found to be sufficient to transfect human pancreatic and salivary gland stem cells in these preliminary studies. Ultracompact sub-20 femtosecond laser microscopes may become optical tools for nanobiotechnology and nanomedicine including optical stem cell manipulation.
TL;DR: In this article, the s-component of the incident field, not attenuated by the projection, determines length and orientation of the ordered ripples, resulting in curved structures bending from polarization-controlled to defect-controlled orientation.
Abstract: In self-organized nanostructure formation upon femtosecond laser ablation, the laser polarization is an important control parameter. Experiments on fluoride crystals, using circular and elliptical polarization, study this influence in more detail. For circular polarization, spherical nanoparticles of about 100 nm diameter are formed. With increasing ellipticity, longer and longer ordered chains and linear structures are generated, oriented perpendicular to the long axis of the polarization ellipse. A similar effect occurs when, for circular polarization, the angle of incidence is varied from normal to 45°: the s-component of the incident field, not attenuated by the projection, determines length and orientation of the ordered ripples. However, surface defects like scratches exert an even stronger influence on the ripples orientation than the polarization, resulting in curved structures bending from polarization-controlled to defect-controlled orientation. Since the structure formation takes place only long after the end of the laser pulse, a certain electrical field memorizer is required to account for this polarization dependence. A promising approach assumes directional atomic surface diffusion anisotropies, arising, e.g. from plasmon-coupled metal–colloid arrays.
TL;DR: A miniaturized probe which combines two-photon fluorescence microscopy and femtosecond laser microsurgery in a compact housing is demonstrated using live cancer cells.
Abstract: Combined two-photon fluorescence microscopy and femtosecond laser microsurgery has many potential biomedical applications as a powerful "seek-and-treat" tool. Towards developing such a tool, we demonstrate a miniaturized probe which combines these techniques in a compact housing. The device is 10 x 15 x 40 mm(3) in size and uses an aircore photonic crystal fiber to deliver femtosecond laser pulses at 80 MHz repetition rate for imaging and 1 kHz for microsurgery. A fast two-axis microelectromechanical system scanning mirror is driven at resonance to produce Lissajous beam scanning at 10 frames per second. Field of view is 310 microm in diameter and the lateral and axial resolutions are 1.64 microm and 16.4 microm, respectively. Combined imaging and microsurgery is demonstrated using live cancer cells.
TL;DR: Grazing-incidence x-ray diffraction is employed to characterize the coherent, femtosecond laser-induced lattice motion of a bismuth crystal as a function of depth from the surface with a temporal resolution of 193+/-8 fs.
Abstract: We employ grazing-incidence femtosecond x-ray diffraction to characterize the coherent, femtosecond laser-induced lattice motion of a bismuth crystal as a function of depth from the surface with a temporal resolution of 193 +/- 8 fs. The data show direct consequences on the lattice motion from carrier diffusion and electron-hole interaction, allowing us to estimate an effective diffusion rate of D=2.3 +/- 0.3 cm(2)/s for the highly excited carriers and an electron-hole interaction time of 260 +/- 20 fs.
TL;DR: The combination of fiber Bragg grating inscription with femtosecond laser sources and the usage of the Talbot interferometer setup not only gives access to the fabrication of Bragg gratings in new types of materials but also allows, at the same time, to keep the high flexibility of an interferometric setup in choosing the Bragggrating wavelength.
Abstract: The combination of fiber Bragg grating inscription with femtosecond laser sources and the usage of the Talbot interferometer setup not only gives access to the fabrication of Bragg gratings in new types of materials but also allows, at the same time, to keep the high flexibility of an interferometric setup in choosing the Bragg grating wavelength. Since the spatial and temporal coherence properties of the femtosecond laser source differ strongly from those of conventional laser sources, specific limits and tolerances in the interferometric setup have to be considered. Such limits are investigated on the basis of an analytical ray tracing model. The results are applied to tolerance measurements of fiber Bragg grating reflections recorded with a DUV sub-picosecond laser source at 262 nm. Additionally we demonstrate the wavelength versatility of the two-beam interferometer setup for femtosecond inscription over a 40 nm wavelength band. Inscription experiments in Al/Yb doped silica glasses are demonstrated as a prove for the access to non-photosensitive fibers.
TL;DR: A mode-locked Ti:sapphire femtosecond laser emitting 42 fs pulses at a 10 GHz repetition rate is reported, which permits identification of the mode index and measurement of the carrier envelope offset frequency of the comb.
Abstract: We report a mode-locked Ti:sapphire femtosecond laser emitting 42 fs pulses at a 10 GHz repetition rate. When operated with a spectrally integrated average power greater than 1 W, the associated femtosecond laser frequency comb contains ~500 modes, each with power exceeding 1 mW. Spectral broadening in nonlinear microstructured fiber yields comb elements with individual powers greater than 1 nW over ~250 nm of spectral bandwidth. The modes of the emitted comb are resolved and imaged with a simple grating spectrometer and digital camera. Combined with absorption spectroscopy of rubidium vapor, this approach permits identification of the mode index and measurement of the carrier envelope offset frequency of the comb.
TL;DR: The interaction of intense extreme ultraviolet femtosecond laser pulses from the FLASH free electron laser with clusters is investigated by means of photoelectron spectroscopy and modeled by Monte Carlo simulations and finds no evidence for electron emission from plasma heating processes.
Abstract: The interaction of intense extreme ultraviolet femtosecond laser pulses ({lambda}=32.8 nm) from the FLASH free electron laser (FEL) with clusters has been investigated by means of photoelectron spectroscopy and modeled by Monte Carlo simulations. For laser intensities up to 5x10{sup 13} W/cm{sup 2}, we find that the cluster ionization process is a sequence of direct electron emission events in a developing Coulomb field. A nanoplasma is formed only at the highest investigated power densities where ionization is frustrated due to the deep cluster potential. In contrast with earlier studies in the IR and vacuum ultraviolet spectral regime, we find no evidence for electron emission from plasma heating processes.
TL;DR: In this paper, a coherent anti-Stokes Raman microspectroscopy with high spectral resolution is achieved using femtosecond laser pulses chirped up to a few picoseconds by glass elements of known group-velocity dispersion without significant intensity losses.
Abstract: We demonstrate experimentally that coherent anti-Stokes Raman microspectroscopy with high spectral resolution is achieved using femtosecond laser pulses chirped up to a few picoseconds by glass elements of known group-velocity dispersion without significant intensity losses. By simply choosing the length of the glass, the chirp of Stokes and pump pulses is tailored to obtain a spectral resolution given by the Fourier limit of the chirped pulse duration. We show that for chirped pulse durations shorter than or comparable to the Raman coherence time, maximum signal occurs for a pump arriving after the Stokes pulse, a time-ordering effect confirmed by numerical simulations.