FLASH free electron laser pump-probe laser concept based on spectral broadening of high-power ytterbium picosecond systems in multi-pass cells

Within the FLASH2020+ upgrade, the pump-probe laser capabilities of the extreme ultraviolet and soft x-ray free-electron laser (XFEL) FLASH in Hamburg will be extended. In particular, providing wavelength tunability, shorter pulse durations, and reduced arrival time jitter will increase the scientific opportunities and the time resolution for the XFEL-optical laser pump-probe experiments. We present here a novel concept for the pump-probe laser at FLASH that is based on the post-compression of picosecond pulses emitted from high-power Ytterbium:YAG slab amplifiers. Flexible reduction of the pulse duration is facilitated by spectral broadening in pressure-tunable multi-pass cells. As an application, we show the pumping of a commercial optical parametric amplifier with 150 fs post-compressed pulses. By means of an additional difference frequency generation stage, tunable spectral coverage from 1.3 to 16 μm is reached with multi-μJ, sub-150 fs pulses. Finally, a modular reconfiguration approach to the optical setups close to the free-electron laser instruments is implemented. This enables fast installation of the nonlinear frequency converters at the end stations for user operation and flexibility between different instruments in the two experimental halls.

1-GHz dual-comb spectrometer with high mutual coherence for fast and broadband measurements

Dual-frequency comb spectroscopy permits broadband precision spectroscopy with high acquisition rate. The combs' repetition rates as well as the mutual coherence between the combs are key to fast and broadband measurements. Here, we demonstrate a 1-GHz high-repetition-rate dual-comb system with high mutual coherence (sub-Hz heterodyne beatnotes) based on mature, digitally controlled, low-noise erbium-doped mode-locked lasers. Two spectroscopy experiments are performed with acquisition parameters not attainable in a 100-MHz system: detection of water vapor absorption around 1375 nm, illustrating the potential for fast and ambiguity-free broadband operation, as well as acquisition of narrow gas absorption features across a spectral span of 0.6 THz (600 comb lines) in only 5 μs.

Synchronized beamline at FLASH2 based on high-order harmonic generation for two-color dynamics studies

We present the design, integration, and operation of the novel vacuum ultraviolet (VUV) beamline installed at the free-electron laser (FEL) FLASH. The VUV source is based on high-order harmonic generation (HHG) in gas and is driven by an optical laser system synchronized with the timing structure of the FEL. Ultrashort pulses in the spectral range from 10 to 40 eV are coupled with the FEL in the beamline FL26, which features a reaction microscope (REMI) permanent endstation for time-resolved studies of ultrafast dynamics in atomic and molecular targets. The connection of the high-pressure gas HHG source to the ultra-high vacuum FEL beamline requires a compact and reliable system, able to encounter the challenging vacuum requirements and coupling conditions. First commissioning results show the successful operation of the beamline, reaching a VUV focused beam size of about 20 µm at the REMI endstation. Proof-of-principle photo-electron momentum measurements in argon indicate the source capabilities for future two-color pump-probe experiments.

Suppression of the vacuum space-charge effect in fs-photoemission by a retarding electrostatic front lens

The performance of time-resolved photoemission experiments at fs-pulsed photon sources is ultimately limited by the e-e Coulomb interaction, downgrading energy and momentum resolution. Here, we present an approach to effectively suppress space-charge artifacts in momentum microscopes and photoemission microscopes. A retarding electrostatic field generated by a special objective lens repels slow electrons, retaining the k-image of the fast photoelectrons. The suppression of space-charge effects scales with the ratio of the photoelectron velocities of fast and slow electrons. Fields in the range from -20 to -1100 V/mm for E_kin = 100 eV to 4 keV direct secondaries and pump-induced slow electrons back to the sample surface. Ray tracing simulations reveal that this happens within the first 40 to 3 μm above the sample surface for E_kin = 100 eV to 4 keV. An optimized front-lens design allows switching between the conventional accelerating and the new retarding mode. Time-resolved experiments at E_kin = 107 eV using fs extreme ultraviolet probe pulses from the free-electron laser FLASH reveal that the width of the Fermi edge increases by just 30 meV at an incident pump fluence of 22 mJ/cm² (retarding field -21 V/mm). For an accelerating field of +2 kV/mm and a pump fluence of only 5 mJ/cm², it increases by 0.5 eV (pump wavelength 1030 nm). At the given conditions, the suppression mode permits increasing the slow-electron yield by three to four orders of magnitude. The feasibility of the method at high energies is demonstrated without a pump beam at E_kin = 3830 eV using hard x rays from the storage ring PETRA III. The approach opens up a previously inaccessible regime of pump fluences for photoemission experiments.

Shaping femtosecond laser pulses at short wavelength with grazing-incidence optics

We present the design of an extreme ultraviolet (XUV) pulse shaper relying on reflective optics. The instrument will allow tailoring of the time-frequency spectrum of femtosecond pulses generated by seeded free-electron lasers (FEL) and high-harmonic generation (HHG) sources down to a central wavelength of ~15 nm. The device is based on the geometry of a 4f grating compressor that is a standard concept in ultrafast laser science and technology. We apply it to shorter wavelengths using grazing-incidence optics operated under ultra-high vacuum conditions. The design blaze angle and the line density of the gratings allow the manipulation of all different harmonics typical for seeded FEL and HHG photon sources without the need of realignment of the instrument and even simultaneously in multi-color experiments. A proof-of-principle pulse shaping experiment using 266 nm laser light has been performed, demonstrating relative phase-control of femtosecond UV pulses.

Acceleration of sub-relativistic electrons with an evanescent optical wave at a planar interface

We report on a theoretical and experimental study of the energy transfer between an optical evanescent wave, propagating in vacuum along the planar boundary of a dielectric material, and a beam of sub-relativistic electrons. The evanescent wave is excited via total internal reflection in the dielectric by an infrared (λ = 2 μm) femtosecond laser pulse. By matching the electron propagation velocity to the phase velocity of the evanescent wave, energy modulation of the electron beam is achieved. A maximum energy gain of 800 eV is observed, corresponding to the absorption of more than 1000 photons by one electron. The maximum observed acceleration gradient is 19 ± 2 MeV/m. The striking advantage of this scheme is that a structuring of the acceleration element's surface is not required, enabling the use of materials with high laser damage thresholds that are difficult to nano-structure, such as SiC, Al₂O₃ or CaF₂.

Optical gating and streaking of free electrons with sub-optical cycle precision

The temporal resolution of ultrafast electron diffraction and microscopy experiments is currently limited by the available experimental techniques for the generation and characterization of electron bunches with single femtosecond or attosecond durations. Here, we present proof of principle experiments of an optical gating concept for free electrons via direct time-domain visualization of the sub-optical cycle energy and transverse momentum structure imprinted on the electron beam. We demonstrate a temporal resolution of 1.2±0.3 fs. The scheme is based on the synchronous interaction between electrons and the near-field mode of a dielectric nano-grating excited by a femtosecond laser pulse with an optical period duration of 6.5 fs. The sub-optical cycle resolution demonstrated here is promising for use in laser-driven streak cameras for attosecond temporal characterization of bunched particle beams as well as time-resolved experiments with free-electron beams.

Midinfrared frequency combs from coherent supercontinuum in chalcogenide and optical parametric oscillation

We observe the coherence of the supercontinuum generated in a nanospike chalcogenide-silica hybrid waveguide pumped at 2 μm. The supercontinuum is shown to be coherent with the pump by interfering it with a doubly resonant optical parametric oscillator (OPO) that is itself coherent with the shared pump laser. This enables coherent locking of the OPO to the optically referenced pump frequency comb, resulting in a composite frequency comb with wavelengths from 1 to 6 μm.

Mid-infrared supercontinuum generation in As2S3-silica "nano-spike" step-index waveguide

Efficient generation of a broad-band mid-infrared supercontinuum spectrum is reported in an arsenic trisulphide waveguide embedded in silica. A chalcogenide "nano-spike", designed to transform the incident light adiabatically into the fundamental mode of a 2-mm-long uniform section 1 µm in diameter, is used to achieve high launch efficiencies. The nano-spike is fully encapsulated in a fused silica cladding, protecting it from the environment. Nano-spikes provide a convenient means of launching light into sub-wavelength scale waveguides. Ultrashort (65 fs, repetition rate 100 MHz) pulses at wavelength 2 µm, delivered from a Tm-doped fiber laser, are launched with an efficiency ~12% into the sub-wavelength chalcogenide waveguide. Soliton fission and dispersive wave generation along the uniform section result in spectral broadening out to almost 4 µm for launched energies of only 18 pJ. The spectrum generated will have immediate uses in metrology and infrared spectroscopy.

Carrier envelope offset frequency of a doubly resonant, nondegenerate, mid-infrared GaAs optical parametric oscillator

We measure the carrier envelope offset (CEO) frequency of the mid-infrared frequency comb (wavelength tunable between 3 and 6 μm) from a doubly resonant nondegenerate synchronously pumped optical parametric oscillator (SPOPO) as a function of the CEO frequency of the Tm-fiber pump laser. We show that the CEO frequency of the SPOPO signal wave is a linear function of the CEO frequency of the pump laser, with a slope determined by the signal to pump center-frequency ratio.

Frequency comb stabilization with bandwidth beyond the limit of gain lifetime by an intracavity graphene electro-optic modulator

Intracavity loss modulation enables offset-frequency control with bandwidths beyond what is possible by pump power modulation. To demonstrate this new method, we use a subwavelength thick graphene electro-optic modulator to stabilize the offset frequency in a Tm:fiber frequency comb at 1.95 μm wavelength. Record-low residual phase noise of 144 mrads was achieved with this new locking scheme.

Widely tunable midinfrared difference frequency generation in orientation-patterned GaAs pumped with a femtosecond Tm-fiber system

We demonstrate a midinfrared source tunable from 6.7 to 12.7 μm via difference frequency generation (DFG) in orientation-patterned GaAs, with 1.3 mW average output power. The input pulses are generated via Raman self-frequency shift of a femtosecond Tm-doped-fiber laser system in a fluoride fiber. We numerically model the DFG process and show good agreement between simulations and experiments. We use this numerical model to show an improved design using longer pump pulses.

Full phase stabilization of a Yb:fiber femtosecond frequency comb via high-bandwidth transducers

We present full phase stabilization of an amplified Yb:fiber femtosecond frequency comb using an intracavity electro-optic modulator and an acousto-optic modulator. These transducers provide high servo bandwidths of 580 kHz and 250 kHz for f(rep) and f(ceo), producing a robust and low phase noise fiber frequency comb. The comb was self-referenced with an f-2f interferometer and phase locked to an ultrastable optical reference used for the JILA Sr optical clock at 698 nm, exhibiting 0.21 rad and 0.47 rad of integrated phase errors (over 1 mHz-1 MHz), respectively. Alternatively, the comb was locked to two optical references at 698 nm and 1064 nm, obtaining 0.43 rad and 0.14 rad of integrated phase errors, respectively.

Power optimization of XUV frequency combs for spectroscopy applications [Invited]

We address technical impediments to the generation of high-photon flux XUV frequency combs through cavity-enhanced high harmonic generation. These difficulties arise from mirror damage, cavity nonlinearity, the intracavity plasma generated during the HHG process, and imperfect phase-matching. By eliminating or minimizing each of these effects we have developed a system capable of generating > 200 μW and delivering ~20 μW of average power for each spectrally separated harmonic (wavelengths ranging from 50 nm - 120 nm), to actual comb-based spectroscopy experiments.

Supercontinuum generation in quasi-phase-matched LiNbO3 waveguide pumped by a Tm-doped fiber laser system

We demonstrate self-referencing of a Tm-doped fiber oscillator-amplifier system by performing octave-spanning supercontinuum generation in a periodically poled lithium niobate waveguide. We model the supercontinuum generation numerically and show good agreement with the experiment.

Supercontinuum generation in quasi-phasematched waveguides

We numerically investigate supercontinuum generation in quasi-phase-matched waveguides using a single-envelope approach to capture second and third order nonlinear processes involved in the generation of octave-spanning spectra. Simulations are shown to agree with experimental results in reverse-proton-exchanged lithium-niobate waveguides. The competition between χ((2)) and χ((3)) self phase modulation effects is discussed. Chirped quasi-phasematched gratings and stimulated Raman scattering are shown to enhance spectral broadening, and the pulse dynamics involved in the broadening processes are explained.

Broadband phase noise suppression in a Yb-fiber frequency comb

We report a simple technique to suppress high-frequency phase noise of a Yb-based fiber optical frequency comb using an active intensity noise servo. Out-of-loop measurements of the phase noise using an optical heterodyne beat with a cw laser show suppression of phase noise by ≥7 dB out to Fourier frequencies of 100 kHz with a unity-gain crossing of ∼700 kHz. These results are enabled by the strong correlation between the intensity and phase noise of the laser. Detailed measurements of intensity and phase noise spectra, as well as transfer functions, reveal that the dominant phase and intensity noise contribution above ∼100 kHz is due to amplified spontaneous emission or other quantum noise sources.

Cavity-enhanced similariton Yb-fiber laser frequency comb: 3 x 10(14) W/cm2 peak intensity at 136 MHz

We report on a passive cavity-enhanced Yb-fiber laser frequency comb generating 230 MW of peak power (3 kW of average power) at a 136 MHz pulse repetition rate. The intracativy peak intensity of 3 x 10(14) W/cm2 for the 95 fs pulse is sufficient to ionize noble gases, such as Xe, Kr, or Ar. The laser system is based on a mode-locked Yb-fiber similariton oscillator in conjunction with a cladding-pumped chirped-pulse fiber amplifier. After recompression, 75 fs duration pulses at a 13.1 W average power are obtained. These pulses are then coherently added inside a passive ring cavity by controlling the fiber oscillator's pulse repetition rate and carrier-envelope offset frequency. This system is well suited for studying high-field phenomena at very high pulse repetition rates.

Generation of octave-spanning spectra inside reverse-photon-exchanged periodically poled lithium niobate waveguides

We demonstrate simultaneous octave-level spectral broadening and carrier-envelope-offset sensing of mode-locked Er- and Yb-doped femtosecond fiber lasers using constant-period and chirped reverse-proton-exchanged periodically poled lithium niobate waveguides. Chirped quasi-phase-matching gratings greatly improve spectral broadening of Yb-fiber lasers.

Fiber-laser frequency combs with subhertz relative linewidths

We investigate the comb linewidths of self-referenced, fiber-laser-based frequency combs by measuring the heterodyne beat signal between two independent frequency combs that are phase locked to a common cw optical reference. We demonstrate that the optical comb lines can exhibit instrument-limited, subhertz relative linewidths across the comb spectra from 1200 to 1720 nm with a residual integrated optical phase jitter of approximately 1 rad in a 60 mHz to 500 kHz bandwidth. The projected relative pulse timing jitter is approximately 1 fs. This performance approaches that of Ti:sapphire frequency combs.

Integrated self-referenced frequency-comb laser based on a combination of fiber and waveguide technology

An optically integrated self-referenced frequency comb laser is demonstrated. The system consists of a passively-modelocked Er-fiber laser, a butt-coupled periodically poled lithium niobate (PPLN) waveguide phase-sensor and an electronic feedback loop for carrier-envelope-offset (CEO) phase stabilization. The fceo-beat-signal has a linewidth of 62 kHz and is detected with a S/N-ratio of 40 dB, with greatly reduced pulse energy requirements compared to bulk crystal phase-sensors. To our knowledge this is the first self-referenced frequency-comb system entirely based on guided-wave technology.

Library-based selection of retroviruses selectively spreading through matrix metalloprotease-positive cells

Viruses conditionally replicating in cancer cells form an attractive novel class of antitumoral agents. To engineer such viruses infectivity can be coupled with proteolytic activity of the target cell by modifying the envelope (Env) protein of murine leukaemia virus (MLV) with blocking domains that prevent cell entry unless they are cleaved off by tumour-associated proteases like the matrix metalloproteases (MMP). Here we show that MLV variants selectively spreading through MMP-positive cells can be evolved from virus libraries, in which a standard MMP-2 substrate peptide connecting the blocking domain CD40L with the Env protein was diversified. Passaging the virus library on human fibrosarcoma or glioma cell lines resulted in the selection of about 10 virus clones, of which the three most frequent ones were shown to become activated by MMPs and to be replication competent on MMP-positive cells only. On these cells, the selected linker peptides improved the spreading by several orders of magnitude in vitro, as well as in tumour xenografts in vivo, approaching the kinetic of the unmodified wild-type virus. The data suggest that retroviral protease substrate libraries form a potent tool for the engineering of viruses conditionally replicating in a given cancer cell type of interest.

An optimized Er gain band all-fiber chirped pulse amplification system

We demonstrate an all-fiber Er chirped pulse amplification (CPA) system based on compression in photonic band gap fiber (PBGF) that produces 570 fs pulses with 312 nJ pulse energy. The dispersion of the PBGF is measured precisely and used to design a dispersion-matched nonlinearly-chirped fiber Bragg grating stretcher. We analyze the trade-offs of such all-fiber CPA system design and compare different PBGFs in terms of the derived figure of merit. Such system architecture should be scalable to few micro-Joule level pulse energies close to the compressor nonlinearity limit when PBGFs with improved figure of merit become available.

Frequency metrology with a turnkey all-fiber system

The repetition rate and carrier-envelope phase offset frequencies of a turnkey, all-fiber-based continuum generator were phase locked to a hydrogen maser. The frequency of the hydrogen maser was calibrated with a highly stable cesium atomic clock, and therefore a fully phase-locked optical frequency comb with well-defined absolute frequencies was obtained. In contrast with the commonly used Ti:sapphire-laser-based systems, we have accomplished a fully turnkey system with no user-adjustable parts. To evaluate the performance of this novel system, we performed absolute frequency measurements in the telecommunications region and at 1064 nm and compared them with our traditional Ti:sapphire-based comb.

Chirped pulse amplification with a nonlinearly chirped fiber Bragg grating matched to the Treacy compressor

We demonstrate a fiber chirped pulse amplification system that uses an engineered nonlinearly chirped fiber Bragg grating stretcher dispersion matched to the Treacy compressor. The seed pulses at 1558 nm are stretched to 720 ps, amplified by more than 50 dB to 6.5-microJ energy, and recompressed to 940 fs. After almost 1000 times compression the pulses are within 30% of the bandwidth limit and have a contrast ratio of better than 30 dB.

Ultrahigh resolution real time OCT imaging using a compact femtosecond Nd:Glass laser and nonlinear fiber

Ultrahigh resolution, real time OCT imaging is demonstrated using a compact femtosecond Nd:Glass laser that is spectrally broadened in a high numerical aperture single mode fiber. A reflective grating phase delay scanner enables broad bandwidth, high-speed group delay scanning. We demonstrate in vivo, ultrahigh resolution, real time OCT imaging at 1 microm center wavelength with <5 microm axial resolution in free space (<4 microm in tissue). The light source is robust, portable, and well suited for in vivo imaging studies.

High-resolution optical coherence microscopy for high-speed, in vivo cellular imaging

Optical coherence microscopy (OCM) is demonstrated with a high-speed, broadband, reflective-grating phase modulator and a femtosecond Ti:Al2O3 laser. The novel system design permits high-resolution OCM imaging in a new operating regime in which a short coherence gate is used to relax the requirement for high-numerical-aperture confocal axial sectioning. In vivo cellular imaging is demonstrated in the Xenopus laevis tadpole and in human skin with a 3-microm coherence gate and a 30-microm confocal gate. The ability to achieve cellular imaging with a lower numerical aperture should facilitate the development of miniaturized probes for in vivo imaging applications.

Directed evolution of retroviruses activatable by tumour-associated matrix metalloproteases

Protease-activatable retroviral vectors offer the possibility of targeted gene transfer into cancer cells expressing a unique set of proteases as, for example, the matrix metalloproteases (MMPs). However, it is difficult to predict which substrate sequence will be optimally cleaved by a given tumour cell type. Therefore, we developed a novel approach that allows the selection of MMP-activatable retroviruses from libraries of viruses displaying combinatorially diversified protease substrates. Starting from a virus harbouring a standard MMP-2 substrate motif, after only two consecutive cycles of diversification and in vivo selection, MMP-activatable viruses were recovered. Biochemical characterization of the selected viruses revealed that their linker peptides showed a considerably increased sensitivity for MMP-2 cleavage, and interestingly also improved the particle incorporation rate of the Env protein. Owing to the optimized linker peptide, the selected viruses exhibited a greatly enhanced spreading efficiency through human fibrosarcoma cells, while having retained the dependency on MMP activation. Moreover, cell entry efficiency and virus titres were considerably improved as compared to the parental virus displaying the standard MMP-2 substrate. The results presented imply that retroviral protease substrate libraries allow the definition of MMP substrate specificities under in vivo conditions as well as the generation of optimally adapted tumour-specific viruses.

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

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

Ultrahigh-resolution optical coherence tomography using continuum generation in an air-silica microstructure optical fiber

We demonstrate ultrahigh-resolution optical coherence tomography (OCT) using continuum generation in an air-silica microstructure fiber as a low-coherence light source. A broadband OCT system was developed and imaging was performed with a bandwidth of 370 nm at a 1.3-mu;m center wavelength. Longitudinal resolutions of 2.5 microm in air and ~2 microm in tissue were achieved. Ultrahigh-resolution imaging in biological tissue in vivo was demonstrated.

Quantitative EEG before and after open heart surgery in children. A significant decrease in the beta and alpha 2 bands postoperatively

Quantitative EEGs of 30 patients undergoing open heart surgery were investigated before, 6 days, 11 days and 44 days after operation. The study was conducted in order to investigate whether quantitative EEGs can show postoperative changes in children after open heart surgery. In 28 children, no new neurological signs of cerebral involvement were seen postoperatively. The most striking feature in these children was a significant decrease in the beta and the alpha 2 bands 6 and 11 days postoperatively. The pattern in the delta band was dominated by an increase 6 days postoperatively. Except for a slight decrease in alpha 2 waves, all variables were restored to preoperative values at 44 days after the operation. We found a significant decrease in plasma sodium and chloride after surgery but children with no or slight declines (1-3 mmol/l) also showed increased slow activity postoperatively. Therefore we conclude that an organic brain syndrome, although usually mild and transient, is a general phenomenon after open heart surgery.

[The anion gap--screening for hyperlactatemia in critically ill children?]

Serum lactate provides important information about tissue perfusion in critically ill patients. Aim of this study is to evaluate anion gap as a screening method to detect hyperlactatemia at a pediatric intensive care unit. In infants and children of different ages and diagnosis we determined blood gases, electrolytes and lactate. Sensitivity of anion gap was increasing with increasing hyperlactatemia. For the total population the test exhibited a sensitivity of 64% and a specificity of 76%, respectively. The power of anion gap as a screening method to detect hyperlactatemia was similarly fair for all age groups. Therefore hyperlactatemia has also to be included in the differential diagnosis of non-anion gap metabolic acidosis.