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We demonstrate Yb laser (170fs) propagation in large a core hollow-core fiber (HCF) with 97.4% transmission. It serves as a pre-compression stage to achieve 1.3 cycle pulses in a second step with 70% overall efficiency

We developed a high transmission Hollow-core fiber (HCF) with 1mm ID that supports 97% absolute transmission. We were able to achieve about 30 times spectral broadening of 11mJ, 1.5ps pulses from a 275W, 25 kHz InnoSlab multi-pass amplifier with 90% fiber transmission. The spectrum achieved with Kr at 1 bar supports a 50fs TL bandwidth.

The results are based on a commercial Yb:KGW regenerative amplifier (Pharos, Light Conversion) delivering 1 mJ, 170 fs pulses at 1 to 6 kHz. This beam is precisely matched to the fundamental mode of a large core HCF ( few-cycle Inc.) of 1 mm inner diameter, by realizing a focal spot diameter of 660 μm at 1/e 2 of intensity (corresponding approximately to a Rayleigh range of 0.3 m).

Few cycle pulse damage of ultra-broad band chirped mirror was studied with 5 fs pulses showing dramatic damage morphology, which has been analyzed with cross-sectional scanning electron microscopy, showing delamination behavior of the top few layers.

Laser-induced damage mechanisms were investigated for an ultra-broadband chirped mirror, as part of a systematic study of few-cycle pulse laser-induced damage threshold (LIDT) of widely-used ultra-broadband optics, in vacuum and in air, for single and multi-pulse regimes (S-on-1). Microscopic analysis of damage morphology suggests that three different damage mechanisms occur across the fluence range 0.15-0.4J/cm2, while no ablation was yet observed. The three regimes resulted in shallow swelling (< 10 nm tall), tall blistering (~ 150 nm tall), and annular blistering (damage suppressed at highest intensity, forming a ring shape). Descriptions of the potential mechanisms are discussed.

We demonstrate a novel approach for the extension of self-referenced spectral interferometry to the temporal characterization of few-optical cycle pulses. The new experimental setup is characterized by low dispersion and a collinear geometry. 4-fs pulses have been characterized by performing single-shot measurements, with high dynamic range on a broad temporal region. An independent measurement of the pulse duration, obtained by using attosecond streaking, allowed us to cross-check the experimental technique.

A compact system for the generation of few-cycle multi-mJ Carrier Envelope Phase (CEP) stabilized pulses is presented. At the output 1.9 mJ, 5.7 fs pulses were achieved after hollow fiber compression (HFC) of 5 mJ, 25 fs circularly-polarized pulses from a Ti:sapphire multipass chirped pulse amplifier (CPA). Polarization control of the generated pulses was done using all reflective phase retarders which can be nearly arbitrarily scaled for increasing energies. The CEP noise from the amplifier system is shown to be 190 mrad rms over a period of more than 7 hours. The full system, i.e., oscillator, amplifier, CEP stabilization, and HFC is compact enough to fit on a standard optical table.

The implementation of multilayer optics for the compression and delivery of few-cycle femtosecond pulses will be discussed in the case of several cutting-edge applications.

High intensity contrast ratio, pre-pulse free pulses with pulse duration of 5 fs and pulse energy of 1 mJ were generated at a repetition rate of 1 kHz by compressing the output pulses of a multi-stage Ti:sapphire laser amplifier system in a Ne-gas-filled hollow fibre. The spatial and temporal properties of the output laser beam were fully characterised.

Ultra-broad and smooth spectra are generated directly from a mirror-dispersion-controlled Kerr-lens mode-locked Ti:sapphire oscillator. The full width at half maximum of the spectrum is 277 nm. The pulse width is evaluated as 6.5 fs by using both interferometric autocorrelation and spectral phase interferometry for electric-field reconstruction. This compact, user-friendly source opens the door to routine implementation of ultra-high-resolution as well as spectroscopic optical coherence tomography in a clinical environment.

Compressing pulses of a mode-locked extended-cavity Ti:sapphire laser using a standard single-mode fiber and tilted-front-interface chirped mirrors yields phase-stabilized 4-fs, 3-nJ light pulses at the full, 24-MHz, repetition rate. The demonstrated source paves the way towards exploring interactions sensitive to the carrier-envelope-offset phase, such as, for example, photoemission from solid targets at moderate intensities (∼1012 W/cm2).

We report stable self-starting near-transform-limited 26-fs pulses at 250-mW output power from a Kerr-lens mode-locked (KLM) Cr4+:YAG laser using chirped mirrors in combination with prisms. The highest output power achieved in KLM regime was 600 mW at 55-fs pulse duration. The experimental results agree well with the results of theoretical analysis with respect to KLM self-starting ability and stability against continuous-wave and multi-pulse operation. Parameter ranges for stable transform-limited KLM pulses as well as the shortest achievable pulse durations are established. Using an InGaAs-InP semiconductor saturable absorber mirror we could obtain self-starting 57-fs pulses at the average output power of 200 mW. Two-photon absorption was found to be the main mechanism favoring the multiple-pulse operation.

We demonstrate a simple and robust single-shot interferometric technique that allows the in situ measurement of intensity-dependent phase changes experienced by ultrashort laser pulses upon nonlinear propagation. The technique is applied to the characterization of carrier-envelope phase noise in hollow fiber compressors both in the pressure gradient and in the static cell configuration. Measurements performed simultaneously with conventional f-to-2f interferometers before and after compression indicate that the noise emerging in the waveguide adds up arithmetically to the phase noise of the amplifier, thus being strongly correlated to the phase noise of the pulses coupled into the compressor.

We report on a single-shot, in-situ interferometric method for measuring intensity-dependent phase changes in laser pulse propagation. With this method, the impact of hollow fiber compressors on phase stability was characterized.

We present a single-shot, in-situ interferometric method for measuring phase changes experienced by laser pulses upon nonlinear propagation. With this method we characterized the intensity-dependent phase fluctuations emerging in hollow fiber compressors.

Single soft-x-ray pulses of ∼90–electron volt (eV) photon energy are produced by high-order harmonic generation with 7-femtosecond (fs), 770-nanometer (1.6 eV) laser pulses and are characterized by photoionizing krypton in the presence of the driver laser pulse. By detecting photoelectrons ejected perpendicularly to the laser polarization, broadening of the photoelectron spectrum due to absorption and emission of laser photons is suppressed, permitting the observation of a laser-induced downshift of the energy spectrum with sub-laser-cycle resolution in a cross correlation measurement. We measure isolated x-ray pulses of 1.8 (+0.7/−1.2) fs in duration, which are shorter than the oscillation cycle of the driving laser light (2.6 fs). Our techniques for generation and measurement offer sub-femtosecond resolution over a wide range of x-ray wavelengths, paving the way to experimental attosecond science. Tracing atomic processes evolving faster than the exciting light field is within reach.

A systematic study of few-cycle pulse laser induced damage threshold (LIDT) determination was performed for commercially-available ultra-broadband optics, (i.e. chirped mirrors, silver mirrors, beamsplitters, etc.) in vacuum and in air, for single and multi-pulse regime (S-on-1). Multi-pulse damage morphology at fluences below the single-pulse LIDT was studied in order to investigate the mechanisms leading to the onset of damage. Stark morphological contrast was observed between multi-pulse damage sites formed in air versus those in vacuum. One effect of vacuum testing compared to air included suppression of laser-induced periodic surface structures (LIPSS) formation, possibly influenced by a reduced presence of damage debris. Another effect of vacuum was occasional lowering of LIDT, which appears to be due to the stress-strain performance of the coating design during laser irradiation and under the external stress of vacuum ambience. A fused silica substrate is also examined, and a non-LIPSS nanostructuring is observed on the surface. Possible mechanisms are discussed.

The dependence of the high-harmonic conversion efficiency in noble gases on experimental parameters is analyzed. The results presented here supply a guideline for the optimization of HHG. The most striking finding of our analysis is the dependence of the harmonic yield on the laser pulse duration. It is generally accepted that the harmonic yield increases for shorter pulse durations. Calculations in neon show that this is only true for harmonics close to the cutoff. Away from the cutoff however, the harmonic yield increases approximately linearly with the pulse duration.

We report on a sub-10-fs optical parametric amplifier in the visible, using noncollinear phase matching in β-barium borate, with a pulse compressor made exclusively of chirped dielectric mirrors. We employ mirrors of novel design, providing negative group delay dispersion for bandwidths of over 200 THz, up to the blue-green spectral region. The resulting setup, compact, easy to operate and reliable, is an ideal tool for ultrafast spectroscopy with extremely high time resolution. We use these pulses to observe in the time domain coherent oscillations in organic molecules with period as short as 16 fs.

Our theoretical analysis reveals that tunnel ionization significantly modifies the electric field of few-cycle laser pulses within a single oscillation period. This subcycle self-modulation is predicted to result in phase matching, making high harmonic generation in the x-ray regime possible for the first time. Such a radiation source opens novel possibilities in the investigation of matter with x-ray techniques, such as time resolved x-ray diffraction and absorption.

A novel equation for light propagation in underdense field-ionizing media is derived by extending concepts of nonlinear optics into the strong-field domain. The equation is first order in the propagation coordinate and is valid for arbitrarily short pulse durations. Solutions of the first-order wave equation are found to be in excellent agreement with solutions of the scalar wave equation. Furthermore, the polarization response of a field-ionizing medium is derived from semiclassical considerations. The polarization reveals, in agreement with experiments, a previously unrecognized contribution that is shown to significantly affect the absorption loss in the presence of ionization.

Coherent growth of high-order harmonic radiation has been extended to propagation lengths comparable to the XUV absorption length at wavelengths as short as 10 nm range for the first time. Absorption-limited production of high harmonics of sub-10-fs near-infrared laser pulses in gas jets gives rise to a harmonic conversion efficiency of $(3--4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ in the range of 10--13 nm in neon and some 2 orders of magnitude higher at about 30 nm in argon. This kHz-repetition-rate ultrafast soft-x-ray source emits approximately ${10}^{10}$ photons per sec within a 0.9-nm bandwidth of a typical Mo:Si mirror at 13.4 nm in a near-diffraction-limited beam and opens up the way to soft-x-ray nonlinear optics and the measurement of sub-fs soft-x-ray pulses.

W/cm2. The high resistance of fused silica to damage in the sub-10-fs regime allows stable reproducible operation without degradation of the capillary waveguide. In preliminary experiments, we demonstrate kHz-repetition-rate guided-wave high-harmonic generation in helium down to the 10-nm range. The reported experiments open up the way to realizing high-field interactions with plane-wave excitation at intensity levels in excess of 1015 W/cm2 under well-controlled conditions.

Powerful techniques for spectral broadening and ultrabroad-band dispersion control, which allow compression of high-energy femtosecond pulses to a duration of a few optical cycles, are analyzed. Spectral broadening in a gas-filled hollow fiber and compression by chirped mirrors with high-energy 20-fs input pulses are presented. Using 1-mJ seed pulses we have demonstrated the generation of 0.5-mJ 5-fs pulses at 0.8-/spl mu/m and 1-kHz repetition rate. General design criteria to scale the compression technique toward the terawatt level are presented.

Ultrabroad-band chirped multilayer dielectric mirrors providing nearly constant negative group delay dispersion over the wavelength range of 640-950 nm and high reflectance between 590 and 970 nm are demonstrated. A key to this performance has been an improved design method, which also substantially reduces the computing time needed for ultimate optimization. The presented devices constitute an enabling technology for producing high-quality terawatt pulses in the sub-10-fs regime. The generation of 5-fs 0.1-TW pulses by using exclusively these mirrors as negative delay line demonstrates this potential.

A fast and simple algorithm enables the accurate retrieval of the wavelength-dependent group delay introduced by multilayer coatings upon reflection directly from white light spectral interferograms, without making use of the Fourier transform.

The feed-forward technique has recently revolutionized carrier-envelope phase stabilization, enabling unprecedented values of residual phase jitter. Nevertheless, in its original demonstrations the stabilized beam exhibited angular and temporal dispersion. We demonstrate that these problems can be solved, resulting in few-cycle pulses with good beam quality. This in turn enables the use of monolithic interferometers, providing excellent long-term stability of the system. Out-of-loop RMS phase noise of less than 80 mrad over 33 minutes (0.5 mHz to 5 kHz) is measured, i.e., a value that has previously been reported for a few seconds integration time. The current method promises to enable reliable operation of CEP-stable systems over several days.

By use of a novel ultrashort pulse fiber delivery laser pulses at 770 nm are delivered over 19 meters without pre-chirping. The fiber delivery module is described revealing its potential for remote distance THz applications.

A compact system for the generation of few-cycle multi-mJ carrier envelope phase (CEP) stabilized pulses is presented. The output 5.4 fs, 1.9 mJ pulses have CEP noise of only 190 mrad rms over seven hours.

Typical femtosecond pulse compression of deep ultraviolet radiation consists of prism or diffraction grating pair chirp compensation but, both techniques introduce higher-order dispersion, spatial-spectral beam distortion and poor transmission. While negatively chirped dielectric mirrors have been used to compress near infrared and visible pulses to

220-nJ, 42-fs, 5.25-MHz pulses from a long-cavity Ti:Sapphire chirped pulse oscillator were spectrally broadened by nonlinear propagation in a Sapphire plate. The chirp was subsequently compensated with dispersive mirrors. After farfield spatial filtering the compressor delivered 80-nJ, sub-15-fs pulses at 5.25 MHz. A novel 500-nJ Oscillator has been developed in order to investigate the energy-scaling potential of this compression scheme. 16-fs 130-nJ compressed pulses were obtained with this source. A second compression stage has been calculated and designed in order to reduce the pulse duration down to < 10 fs.

Circularly polarized, 25 fs 5 mJ pulses generated at a repetition rate of 1 kHz from a two-stage chirped pulse amplifier were spectrally broadened by means of nonlinear propagation in a Ne-filled hollow fiber. Subsequent compression with dispersive mirrors resulted in 5.2 fs, 1.7 mJ pulses. After recompression an all-reflective achromatic phase retarder was used to obtain linear polarization.

We report generation of 400 microJ, 13.1 fs, 1425 nm optical parametric amplifier laser pulses. Spectral broadening of a 100 Hz optical parametric amplifier laser source is achieved by self-phase modulation in an argon-filled hollow-core fiber, and dispersion compensation is performed using chirped mirrors. This laser source will be useful for ultrafast time-resolved molecular orbital tomography.

Sub-5fs pulses are generated at the mJ level through a hollow-core fiber seeded with circularly polarized pulses. The preservation of carrier-envelope phase of the hollow-core fiber compressor is also demonstrated.

International audience; Carrier-envelope phase (CEP) stabilization of a femtosecond chirped-pulse amplification system featuring a compact transmission grating compressor is demonstrated. The system includes two amplification stages and routinely generates phase-stable (~250 mrad rms) 2 mJ, 25 fs pulses at 1 kHz. Minimizing the optical pathway in the compressor enables phase stabilization without feedback control of the grating separation or beam pointing. We also demonstrate for the first time to the best of our knowledge, out-of-loop control of the CEP using an acousto-optic programmable dispersive filter inside the laser chain.

Novel ultracompact multiphoton sub-20 femtosecond near infrared 85 MHz laser scanning microscopes and conventional 250 fs laser microscopes have been used to perform high spatial resolution two-photon imaging of stem cell clusters as well as selective intracellular nanoprocessing and knock out of living single stem cells within an 3D microenvironment without any collateral damage. Also lethal cell exposure of large parts of cell clusters was successfully probed while maintaining single cells of interest alive. The mean power could be kept in the milliwatt range for 3D nanoprocessing and even in the microwatt range for two-photon imaging. Ultracompact low power sub-20 fs laser systems may become interesting tools for optical nanobiotechnology such as optical cleaning of stem cell clusters as well as optical transfection. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Femtosecond laser pulses came of age and found applications in many fields of lifesciences resulting in an increased interest in the dispersion-managed guiding of very short optical pulses. We demonstrate the delivery of 25 fs, 1.1 nJ pulses from a Ti:Sapphire laser through 1.6 m optical fiber.

Stabilizing the Carrier-Phase-Envelope (CEP) in grating-based CPA systems is of prime interest since they can produce significantly higher pulse energies than lasers with prism-based compressors. It was recently shown that feedback control of the effective grating separation in either the stretcher or compressor could be used as a method for correcting for the slow phase drift of the amplified pulses [2]. Using small pulse stretching factors has the advantage of keeping minimal optical paths, thereby minimizing the sources of dispersion fluctuations and offering better prospects for long-term CEP stability.

It has long been considered that the advantages emerging from employing chirp pre-compensation in nonlinear microscopy were overweighed by the complexity of prism- or grating-based compressors. These concerns were refuted with the advent of dispersive-mirrors-based compressors that are compact, user-friendly and sufficiently accurate to support sub-20-fs pulse delivery. Recent advances in the design of dispersive multilayer mirrors resulted in improved bandwidth (covering now as much as half of the gain bandwidth of Ti:Sapphire) and increased dispersion per bounce (one reflection off a state-of-the-art dispersive mirror pre-compensates the dispersion corresponding to >10mm of glass). The compressor built with these mirrors is sufficiently compact to be integrated in the housing of a sub-12-fs Ti:Sapphire oscillator. A complete scanning nonlinear microscope (FemtOgene, JenLab GmbH) equipped with highly-dispersive, large-NA objectives (Zeiss EC Plan-Neofluoar 40x/1.3, Plan-Neofluar 63x/1,25 Oil) was directly seeded with this negatively chirped laser. The pulse duration was measured at the focus of the objectives by inserting a scanning autocorrelator in the beam path between the laser and the microscope and recording the second order interferometric autocorrelation traces with the detector integrated in the microscope. Pulse durations

A compact all-solid-state femtosecond Ti:sapphire oscillator–amplifier system using no grating-based pulse stretcher produces 20-fs, 1.5-mJ pulses at a 1-kHz repetition rate. The pulses are subsequently compressed in a hollow-fiber chirped-mirror compressor. The system delivers bandwidth-limited 5-fs, 0.5-mJ pulses at 780 nm in a diffraction-limited beam.

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.

We present a theoretical investigation of self-focusing in a hollow waveguide filled with noble gas. Our analysis was performed for a laser pulse that was predominantly in the fundamental mode and revealed the physical processes involved in self-focusing in a hollow waveguide. A critical power for self-focusing was obtained that was found to be substantially higher than the critical power for self-focusing in a bulk medium. Useful design criteria for pulse-compression systems are presented. We identify the parameter range for which the transverse variation of the pulse phase introduced by the Kerr nonlinearity is small.

Generation of sub-10-fs pulses with an average power of 1 W and a peak of 1.5 MW from a Kerr-lens mode-locked mirror-dispersion-controlled Ti:sapphire laser is demonstrated. A specially designed lens triplet focuses the output of this compact all-solid-state source to a peak intensity in excess of 5x10(13) W/cm (2) . Nonperturbative nonlinear optics is now becoming feasible by use of the output of a cw mode-locked laser.

Pulses from an optical parametric amplifier in the visible are compressed to sub-10-fs duration by a delay line made exclusively from chirped dielectric mirrors. We employ what we believe are the first ultrabroadband visible chirped mirrors, which provide negative group-delay dispersion up to the blue-green spectral region. The setup is compact and reliable, and we used it to observe, for what is to our knowledge the first time in organic molecules, coherent oscillations with periods as short as 16 fs.

Frequency-doubled pulses from a sub-40-fs, 1-kHz Ti:sapphire amplifier system are spectrally broadened in an argon-filled hollow waveguide. Compression of the self-phase-modulated pulses is implemented with chirped mirrors and a prism pair, yielding 8-fs, 15-muJ pulses in the violet spectral range.

We demonstrated that the dispersion of scanning microscope optics (including a Zeiss 40x/1.2 Apochromat objective) can be compensated by means of chirped mirrors over a bandwidth of 170 nm at 800 nm. The interferometric autocorrelation trace recorded at the focus of the microscope objective with a two-photon diode indicated a pulse duration of < 12 fs. The propagation time difference of the system can be minimized by proper choice of the components, enabling sub-12-fs pulse delivery with a completely filled 40x/0.8 Zeiss Achroplan water immersion objective.

Multiphoton microscopy (MPM) has become an important tool for high-resolution and non-invasive imaging in biological tissues. However, the efficiencies of two-photon excited fluorescence (TPEF) and second harmonic generation (SHG) are relatively low because of their nonlinear nature. Therefore, it is critical to optimize laser parameters for most efficient excitation of MPM. Reducing the pulse duration can increase the peak intensity of excitation and thus potentially increase the excitation efficiency. In this paper, a multiphoton microscopy system using a 12 fs Ti:Sapphire laser is reported. With adjustable dispersion pre-compensation, the pulse duration at the sample location can be varied from 400 fs to sub-20 fs. The efficiencies of TPEF and SHG are studied for the various pulse durations, respectively. Both TPEF and SHG are found to increase proportionally to the inverse of the pulse duration for the entire tested range. To transmit most of the SHG and TPEF signals, the spectral transmission widow of the detection optics needs to be carefully considered. Limitation from phase-matching in SHG generation is not significant because the effective interaction length for SHG is less than 10 μm at the focal depth of the objectives. These results are important in improving MPM excitation efficiency using ultrashort pulses. MPM images from human artery wall are also demonstrated.

A new scheme for stabilizing the carrier-envelope phase (CE phase) of a few-cycle pulse train and its integration in a high-power chirped-pulse Ti:Sapphire amplifier system is demonstrated.

Properties of 3 types of multilayer dielectric chirped mirrors manufactured with Helios Leybold system (magnetron sputtering with plasma/ion assisted technology) are discussed. The first type includes mirrors providing negative group delay dispersion (GDD) and high reflectance in a wavelength range 650-1150 nm. Such mirrors are used in a broadband Ti:Sapphire oscillator for generating sub-6 femtosecond pulses. The second type of mirrors has extremely low both GDD oscillations and losses in a range 740-840 nm. Such mirrors are of interest for both long-cavity high-energy Ti:Sapphire oscillators and so-called external cavities for storing energy from femtosecond oscillators. The lowest residual GDD fluctuations achieved for this type of mirrors are 5 fs 2 . The third type is high reflectors with very high reflectivity. Different coating materials: Nb 2 O 5 , TiO 2 , Ta 2 O 5 together with SiO 2 are compared for these 2 types of mirrors. The results of reproducibility of such mirrors and their characterization in terms of reflectivity, losses, surface quality are presented. Comparison of the design with the manufactured mirror is also done.