Frequency-resolved optical gating of femtosecond pulses in the extreme ultraviolet

Phase-matching-induced near-chirp-free solitons in normal-dispersion fiber lasers

Direct generation of chirp-free solitons without external compression in normal-dispersion fiber lasers is a long-term challenge in ultrafast optics. We demonstrate near-chirp-free solitons with distinct spectral sidebands in normal-dispersion hybrid-structure fiber lasers containing a few meters of polarization-maintaining fiber. The bandwidth and duration of the typical mode-locked pulse are 0.74 nm and 1.95 ps, respectively, giving the time-bandwidth product of 0.41 and confirming the near-chirp-free property. Numerical results and theoretical analyses fully reproduce and interpret the experimental observations, and show that the fiber birefringence, normal-dispersion, and nonlinear effect follow a phase-matching principle, enabling the formation of the near-chirp-free soliton. Specifically, the phase-matching effect confines the spectrum broadened by self-phase modulation and the saturable absorption effect slims the pulse stretched by normal dispersion. Such pulse is termed as birefringence-managed soliton because its two orthogonal-polarized components propagate in an unsymmetrical "X" manner inside the polarization-maintaining fiber, partially compensating the group delay difference induced by the chromatic dispersion and resulting in the self-consistent evolution. The property and formation mechanism of birefringence-managed soliton fundamentally differ from other types of pulses in mode-locked fiber lasers, which will open new research branches in laser physics, soliton mathematics, and their related applications.

Aurore: A platform for ultrafast sciences

We present the Aurore platform for ultrafast sciences. This platform is based on a unique 20 W, 1 kHz, 26 fs Ti:sapphire laser system designed for reliable operation and high intensity temporal contrast. The specific design ensures the high stability in terms of pulse duration, energy, and beam pointing necessary for extended experimental campaigns. The laser supplies 5 different beamlines, all dedicated to a specific field: attosecond science (Aurore 1), ultrafast phase transitions in solids (Aurore 2 and 3), ultrafast luminescence in solids (Aurore 4), and femtochemistry (Aurore 5). The technical specifications of these five beamlines are described in detail, and examples of the recent results are given.

Time-Resolved Photoelectron Spectroscopy of Dissociating 1,2-Butadiene Molecules by High Harmonic Pulses

Using 42 nm high harmonic pulses, the dissociation dynamics of 1,2-butadiene was investigated by time-resolved photoelectron spectroscopy (TRPES), enabling us to observe dynamical changes of multiple molecular orbitals (MOs) with higher temporal resolution than conventional light sources. Because each lower-lying occupied MO has particular spatial electron distribution, the structural dynamics of photochemical reaction can be revealed. On the femtosecond time scale, a short-lived excited state with a lifetime of 37 ± 15 fs and the coherent oscillation of the photoelectron yield stimulated by Hertzberg-Teller coupling were observed. Ab initio molecular dynamics simulations in the electronically excited state find three relaxation pathways from the vertically excited structure in S1 to the ground state, and one of them is the dominant relaxation pathway, observed as the short-lived excited state. On the picosecond time scale, the photoelectron yields related to the C-C bond decreased upon photoexcitation, indicating C-C bond cleavage.

Interferometric characterization of ultrashort deep ultraviolet pulses using a multiphoton ionization mass spectrometer

The temporal characterization of a femtosecond laser pulse in the deep ultraviolet region using an interferometric autocorrelation scheme is demonstrated. Two-photon ionization of a molecule in a time-of-flight mass spectrometer was used as a nonlinear detector to obtain an autocorrelation trace. This setup proved useful in not only providing a temporal characterization of a pulse but also investigating the ultrafast dynamics of photochemical processes.

Two-photon absorption cross-sections and time-resolved fluorescence imaging using porphyrin photosensitisers

Three porphyrin systems have been characterised for use in two-photon fluorescence imaging of biological samples. We have determined the two-photon absorption cross sections (sigma(2)) of the di-cation, free-base and metallated forms of hematoporphyrin derivative (HpD), hematoporphyrin IX (Hp9) and a boronated protoporphyrin (BOPP) using the open-aperture Z-scan and the two-photon induced fluorescence (TPIF) techniques at an excitation wavelength of 800 nm. The insertion of either protons or a metal ion into the macrocycle is shown not to significantly influence the sigma(2) of the porphyrins. Two-photon time-resolved fluorescence images of C6 glioma cells transfected with a free-base form of the BOPP have been obtained as a function of the porphyrin concentration. These studies reveal a maximum useful porphyrin concentration for fluorescence imaging purposes of approximately 30 microg mL(-1).

Frequency-resolved optical gating of isolated attosecond pulses in the extreme ultraviolet

The pulse shape and phase of isolated attosecond extreme ultraviolet (XUV) pulses with a duration of 860 asec have been determined simultaneously by using frequency-resolved optical gating based on two-photon above-threshold ionization with 28-eV photons in He. From the detailed characterization, we succeeded in shaping isolated XUV pulses on an attosecond time scale by precise dispersion control with Ar gas density or by changing the driving pulse width. These results offer a novel way to excite and observe an electron motion in atoms and molecules.

Direct interferometric measurement of the atomic dipole phase in high-order harmonic generation

We report the first direct measurements of the atomic dipole phase in the process of high-order harmonic generation. Differently from previously reported studies based on frequency chirp measurements, we use extreme ultraviolet interferometry as the most natural and direct way to measure phase shifts. Our approach has the important advantage of allowing us to investigate the effects associated to both the main quantum paths involved in the emission of a particular harmonic, thus offering a particularly clear and simple picture of the underlying electronic dynamics.

High harmonic XUV spectral phase interferometry for direct electric-field reconstruction

We demonstrate the first experimental complete temporal characterization of high-harmonic XUV pulses by spectral phase interferometry, with an all-optical setup. This method allows us to perform single-shot measurements of the harmonic temporal profile and phase, revealing a remarkable shot-to-shot stability. We characterize harmonics generated in argon by a 50 fs 800 nm laser pulse. The 11th harmonic is found to be 22 fs long with a negative chirp rate of -4.8 x 10(27) s(-2). This duration can be reduced to 13 fs by modulating the polarization of the generating laser. The technique is easy to implement and could be routinely used in femtosecond XUV pump-probe experiments with harmonics.

Production of doubly charged helium ions by two-photon absorption of an intense sub-10-fs soft x-ray pulse at 42 eV photon energy

We report on the observation of doubly charged helium ions produced by a nonlinear interaction between a helium atom and photons with a photon energy of 42 eV which are generated with the 27th harmonic of a femtosecond pulse from a Ti:sapphire laser. The number of ions is proportional to the square of the intensity of the 27th harmonic pulse, and thus two-photon double ionization should be dominantly induced as compared with other nonlinear processes accompanying sequential ionization via a singly charged ion. This phenomenon is utilized to measure the pulse duration of the 27th harmonic pulse by using an autocorrelation technique, for the first time to our knowledge, and as a result a duration of 8 fs is found.

Generation of terawatt 10-fs blue pulses by compensation for pulse-front distortion in broadband frequency doubling

Pulse-front distortion as a result of broadband frequency doubling (BFD) has been compensated for over a 3-cm beam diameter by use of a telescope image relay system. As a result, distortion-free blue pulses with a peak power of 1.4 TW and a pulse width of 10 fs have been generated with a 10-Hz Ti:sapphire laser by use of this modified BFD configuration.

Nonlinear optics in the extreme ultraviolet

Nonlinear responses to an optical field are universal in nature but have been difficult to observe in the extreme ultraviolet (XUV) and soft X-ray regions owing to a lack of coherent intense light sources. High harmonic generation is a well-known nonlinear optical phenomenon and is now drawing much attention in attosecond pulse generation. For the application of high harmonics to nonlinear optics in the XUV and soft X-ray regime, optical pulses should have both large pulse energy and short pulse duration to achieve a high optical electric field. Here we show the generation of intense isolated pulses from a single harmonic (photon energy 27.9 eV) by using a sub-10-femtosecond blue laser pulse, producing a large dipole moment at the relatively low (ninth) harmonic order nonadiabatically. The XUV pulses with pulse durations of 950 attoseconds and 1.3 femtoseconds were characterized by an autocorrelation technique, based on two-photon above-threshold ionization of helium atoms. Because of the small cross-section for above-threshold ionization, such an autocorrelation measurement of XUV pulses with photon energy larger than the ionization energy of helium has not hitherto been demonstrated. The technique can be extended to the characterization of higher harmonics at shorter wavelengths.

Observation of two-photon above-threshold ionization of rare gases by xuv harmonic photons

We have successfully observed two-photon above-threshold ionization in rare gas atoms (Ar, Xe, and He) by the fifth harmonic (25 eV photon energy) of a KrF laser. Use of the energy-resolved photoelectron counting system together with our laser, providing strong 25 eV radiation at 40-100 Hz, enabled us to detect the very weak single-color two-photon above-threshold ionization signals. Experimental data are in good agreement with our theoretical calculations newly developed along the line of multichannel quantum defect theory.