Zenghu Chang

Tailoring a 67 attosecond pulse through advantageous phase-mismatch

A single isolated attosecond pulse of 67 as was composed from an extreme UV supercontinuum covering 55-130 eV generated by the double optical gating technique. Phase mismatch was used to exclude the single-atom cutoff of the spectrum that possesses unfavorable attochirp, allowing the positive attochirp of the remaining spectrum to be compensated by the negative dispersion of a zirconium foil. Two algorithms, PROOF and FROG-CRAB, were employed to retrieve the pulse from the experimental spectrogram, yielding nearly identical results.

High contrast multi-terawatt pulse generation using chirped pulse amplification on the VULCAN laser facility

High power (8 TW), ultra-short (2.4 ps), pulses have been generated using chirped pulse amplification techniques on the VULCAN Nd: glass laser. A novel oscillator was developed as a driver producing ≈ ps pulses at 105 nm. The oscillator output was stretched prior to amplification and compressed at an aperture of 150 mm. The contrast ratio obtained was ≈ 106, which is suitable for laser plasma interaction studies.

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.

Characterizing ultrabroadband attosecond lasers.

Recent progress in sub-laser-cycle gating of high-order harmonic generation promises to push the limits on optical pulse durations below the atomic unit of time, 24 as, which corresponds to a bandwidth broader than 75 eV. However, the available techniques for attosecond pulse measurement are valid only for narrow-bandwidth spectra, due to one of the key approximations made in the phase retrieval. Here we report a new technique for characterizing attosecond pulses, whereby the spectral phase of the attosecond pulse is extracted from the oscillation component with the dressing laser frequency in the photoelectron spectrogram. This technique, termed PROOF (Phase Retrieval by Omega Oscillation Filtering), can be applied to characterizing attosecond pulses with ultrabroad bandwidths.

Sub-cycle Oscillations in Virtual States Brought to Light

Understanding and controlling the dynamic evolution of electrons in matter is among the most fundamental goals of attosecond science. While the most exotic behaviors can be found in complex systems, fast electron dynamics can be studied at the fundamental level in atomic systems, using moderately intense (≲103 W/cm2) lasers to control the electronic structure in proof-of-principle experiments. Here, we probe the transient changes in the absorption of an isolated attosecond extreme ultraviolet (XUV) pulse by helium atoms in the presence of a delayed, few-cycle near infrared (NIR) laser pulse, which uncovers absorption structures corresponding to laser-induced “virtual” intermediate states in the two-color two-photon (XUV+NIR) and three-photon (XUV+NIR+NIR) absorption process. These previously unobserved absorption structures are modulated on half-cycle (~1.3 fs) and quarter-cycle (~0.6 fs) timescales, resulting from quantum optical interference in the laser-driven atom.

Carrier-envelope phase shift caused by variation of grating separation

The effects of variation of the grating separation in a stretcher on the carrier-envelope (CE) phase of amplified pulses are investigated. By translating one of the telescope mirrors in the stretcher with a piezoelectric transducer, it is found that a 1 mum change of the distance causes a 3.7+/-1.2 rad shift of the CE phase, which is consistent with theoretical estimations. The results indicate that optical mounts used for gratings and telescope mirrors must be interferometrically stable; otherwise their vibration and thermal drift will cause significant phase error. The CE phase drift was corrected by feedback controlling the grating separation.

Carrier-envelope phase shift caused by grating-based stretchers and compressors

Simple analysis indicates that when the distance between gratings in optical stretchers and compressors varies by one half of the grating constant due to mechanical vibration or thermal motion, the change of the carrier-envelope phase is of the order of 2pi rad. To suppress the phase noise, one feedback loop is needed to stabilize the compressor while two loops are required for the stretcher. When the phase drift is measured with an f-to-2f interferometer, either the stretcher or the compressor can be feedback controlled to stabilize the carrier-envelope phase of the pulses from a chirped pulse amplifier.

Extreme ultraviolet supercontinua supporting pulse durations of less than one atomic unit of time

Double optical gating of high-harmonic generation was used to obtain supercontinuous spectra in the extreme UV (XUV) region including the water window. The spectra supported a 16 as pulse duration that is below one atomic unit of time (24 as). The dependence of the gated spectra on the carrier-envelope phase of the laser provided evidence that isolated attosecond pulses were generated. In addition, to ensure the temporal coherence of the XUV light, the pulse shape and phase of isolated 107 as XUV pulses using a portion of the spectrum were characterized by attosecond streaking.

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.

Precision control of carrier-envelope phase in grating based chirped pulse amplifiers.

It is demonstrated that the carrier-envelope (CE) phase of pulses from a high power ultrafast laser system with a grating-based stretcher and compressor can be stabilized to a root mean square (rms) value of 180 mrad over almost 2 hours, excluding a brief re-locking period. The stabilization was accomplished via feedback control of the grating separation in the stretcher. It shows that the long term CE phase stability of a grating based chirped pulse amplification system can be as good as that of lasers using a glass-block stretcher and a prism pair compressor. Moreover, by adjusting the grating separation to preset values, the relative CE phase could be locked to an arbitrary value in the range of 2pi. This method is better than using a pair of wedge plates to adjust the phase after the hollow-core fiber compressor. The CE phase stabilization after a hollow-core fiber compressor was confirmed by a CE-phase meter based on the measurement of the left-to-right asymmetry of electrons produced by above-threshold ionization.