B. Shan

Demonstration of a Sub-Picosecond X-Ray Streak Camera

A novel design, magnetically focused, x‐ray streak camera was designed and tested using sub‐20 fs soft‐x‐ray pulses generated by high harmonic emission in a gas. The temporal resolution of the camera was demonstrated to be under 0.9 ps throughout the ultraviolet to soft‐x‐ray wavelength region. Our streak camera represents the fastest x‐ray detector developed to date.

Ultrafast x-ray diffraction using a streak-camera detector in averaging mode

We demonstrate an apparatus for measuring time-dependent x-ray diffraction. X-ray pulses from a synchrotron are diffracted by a pair of Si(111) crystals and detected with an x-ray streak camera that has single-shot resolution of better than 1 ps. The streak camera is driven by a photoconductive switch, which is triggered by 100-fs laser pulses at a repetition rate of 1 kHz. The laser and the streak camera are synchronized with the synchrotron pulses. In the averaging mode, trigger jitter results in 2-ps temporal resolution. We measured the duration of 5-keV pulses from the Advanced Light Source synchrotron to be 70ps.

Generation of the attosecond extreme ultraviolet supercontinuum by a polarization gating

We report the first demonstration of extreme ultraviolet (XUV) supercontinuum generation in the plateau region of the high-order harmonic spectrum indicative of a single attosecond pulse. It was accomplished by combining the generation of sub-10 fs laser pulses with the polarization gating of high harmonic generation. When an 8 fs pulse centred at 750 nm was split and delayed with a quartz plate and then recombined with a quarter waveplate, a laser pulse was created whose polarization varied rapidly from circular to linear and back to circular. The near-linearly polarized portion of the resultant pulse was only 1.3 fs long, which was much shorter than the laser pulse duration. Since the high-order harmonic generation process is susceptible to the ellipticity of the driving field, the pulse with a time-dependent ellipticity behaved like a half-cycle linearly polarized pulse for generating XUV radiation. By exciting argon gas with the pulse, a supercontinuum that covered 25–45 nm was produced, which corresponds to an estimated single 200 as pulse.

An accumulative x-ray streak camera with sub-600-fs temporal resolution and 50-fs timing jitter

We demonstrated that the shot-to-shot timing jitter of an x-ray streak camera was reduced to close to 50 fs when it was triggered by a standard kilohertz laser with 1.2% rms pulse energy fluctuation. This was achieved by improving the response time of deflection plates and the rise time of the ramp pulse generated by a photoconductive switch, and by operating the photoconductive switch at optimum conditions. Furthermore, after reducing the angular distribution of electron bunch, the temporal resolution of the x-ray streak camera operating in accumulation mode was measured to be better than 600 fs.

Probing the intrinsic conductivity of multiwalled carbon nanotubes

The authors report the two-photon electron emission study of quantum transport parameters in multiwalled carbon nanotubes. The present experimental approach is based on measurements of the electron-phonon scattering dynamics and does not employ any electrical contacts, which dramatically reduces the uncertainty in the determination of an electron mean-free path l. It is found that near the Fermi level (<0.1eV) the ballistic travel of electrons averages around 4μm, which is comparable to the nanotube length, whereas in high-current regime (⩾0.3eV) l decreases to less than 1μm.

Ionization suppression of Cl 2 molecules in intense laser fields

The strong field ionization of Cl{sub 2} molecules is investigated by using an ultrashort pulse Ti:sapphire laser. A spatial imaging technique is used in such measurements to reduce the effect of spatial integration. Cl{sub 2} shows strong ionization suppression as do other diatomic molecules having valence orbitals with antibonding symmetry (O{sub 2},S{sub 2}) when compared with the field ionization of atoms with nearly identical ionization potential. A more general molecular tunneling ionization model is proposed, and the calculations are in reasonable agreement with the measurements. Our results support that antibonding leads to ionization suppression, a trend that only F{sub 2} goes against and that needs to be further investigated.

X-ray streak camera with 30-fs timing jitter

We demonstrated that the shot-to-shot timing jitter of a streak camera is reduced to 30 fs when it is triggered by a standard kilohertz laser with 1.2% RMS fluctuation. Such small jitter was obtained by improving the response time of deflection plates and the rise-time of a ramp pulse generated by a photoconductive switch, and by operating the photoconductive switch at the optimum working condition. The temporal resolution of the x-ray streak camera operating in accumulation mode is better than 600 femtosecond that is not limited by the timing jitter.

Generation of XUV supercontinuum and single attosecond pulses

Ultrafast duplicity dependent pulses were created by polarization shaping of few-cycle pulses with birefringence optics. When the pulses were used for high harmonic generation, a supercontinuum spectrum extending 25-45 nm was produced in the plateau and cutoff region that corresponds to single attosecond pulses

Photon-ion collisions and molecular clocks

The timing of molecular rearrangements can be followed in the time domain on a femtosecond scale by using momentum imaging techniques. Three examples are discussed in this paper: first, the diffraction of electrons ejected from the K-shell of one of the atomic constituents of the molecule takes a ‘picture’ of the molecule, and the correlation between the momentum vector of the photoelectron and the subsequent fragmentation pattern is used to estimate the time delay which accompanies the latter process. Second, the kinetic energy release of proton pairs from the double ionization of hydrogen by fast laser pulses is timed using the optical cycle as a clock. The mechanisms of rescattering, sequential and enhanced ionization are clearly identified in the momentum spectra. Third, the operation of rescattering double ionization in the case of nitrogen and oxygen molecules is discussed.

Ionization suppression of Cl{sub 2} molecules in intense laser fields

The strong field ionization of Cl{sub 2} molecules is investigated by using an ultrashort pulse Ti:sapphire laser. A spatial imaging technique is used in such measurements to reduce the effect of spatial integration. Cl{sub 2} shows strong ionization suppression as do other diatomic molecules having valence orbitals with antibonding symmetry (O{sub 2},S{sub 2}) when compared with the field ionization of atoms with nearly identical ionization potential. A more general molecular tunneling ionization model is proposed, and the calculations are in reasonable agreement with the measurements. Our results support that antibonding leads to ionization suppression, a trend that only F{sub 2} goes against and that needs to be further investigated.