Shouyuan Chen

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 continuum high-order harmonics from carbon plasma using double optical gating

We demonstrated continuum high-order harmonics from carbon plasma using the double optical gating method. The extreme ultraviolet continuum covered 17‐25 eV. The observation of such continuum is the first step towards the generation of high-flux single attosecond pulses from plasma harmonics. (Some figures may appear in colour only in the online journal)

Practical issues of retrieving isolated attosecond pulses

The attosecond streaking technique is used for the characterization of isolated extreme ultraviolet (XUV) attosecond pulses. This type of measurement suffers from low photoelectron counts in the streaked spectrogram, and is thus susceptible to shot noise. For the retrieval of few- or mono-cycle attosecond pulses, high-intensity streaking laser fields are required, which cause the energy spectrum of above-threshold ionized (ATI) electrons to overlap with that of the streaked photoelectrons. It is found by using the principal component generalized projections algorithm that the XUV attosecond pulse can accurately be retrieved for simulated and experimental spectrograms with a peak value of 50 or more photoelectron counts. Also, the minimum streaking intensity is found to be more than 50 times smaller than that required by the classical streaking camera for retrieval of pulses with a spectral bandwidth supporting 90 as transform-limited pulse durations. Furthermore, spatial variation of the streaking laser intensity, collection angle of streaked electrons and time delay jitter between the XUV pulse and streaking field can degrade the quality of the streaked spectrogram. We find that even when the XUV and streaking laser focal spots are comparable in size, the streaking electrons are collected from a 4π solid angle, or the delay fluctuates by more than the attosecond pulse duration, the attosecond pulses can still be accurately retrieved. In order to explain the insusceptibility of the streaked spectrogram to these factors, the linearity of the streaked spectrogram with respect to the streaking field is derived under the saddle point approximation.

Delay control in attosecond pump-probe experiments.

The time delay between the pump and probe pulses in attosecond time-resolved experiments, such as attosecond streaking, is commonly introduced by splitting and recombining the two pulses in an interferometer. This technique suffers from instability in the optical path lengths of the two arms due to mechanical vibration of the optical elements and fluctuating environmental conditions. We present a technique with which the instability of the unconventional interferometer is suppressed while at the same time the time delay is controlled to within 20 as RMS using a feedback loop. Using this scheme, the streaked spectrogram of an attosecond pulse was measured.

Retrieval of satellite pulses of single isolated attosecond pulses

Synopsis: When isolated attosecond pulses are reconstructed from an ideal streaked spectrogram, the relative intensity of accompanying satellite pulses can be identified from interference. However, the interference pattern can be distorted by variation of the streaking laser intensity in the focal volume or by the use of large delay steps in acquiring the spectrogram. We investigate these effects on the reconstruction of satellite pulses with fulland halfcycle separations and find that satellite pulses with full-cycle separation are largely unaffected by these issues.

Carrier-envelope phase stabilization and control of 1 kHz, 6 mJ, 30 fs laser pulses from a Ti:sapphire regenerative amplifier.

Carrier-envelope (CE) phase stabilization of a two-stage chirped pulse amplifier laser system with regenerative amplification as the preamplifier is demonstrated. The CE phase stability of this laser system is found to have a 90 mrad rms error averaged over 50 laser shots for a locking period of 4.5 h. The CE phase locking was confirmed unambiguously by experimental observation of the 2pi periodicity of the high-order harmonic spectrum generated by double optical gating.

Temperature feedback control for long-term carrier-envelope phase locking

By combining control of the Ti: Sapphire crystal temperature with the modulation of the pump laser power, the carrier-envelope offset frequency f<inf>CEO</inf> was locked for more than 19 hours.

Attosecond time-resolved autoionization

Autoionization in argon atoms was studied by transient absorption spectroscopy with isolated attosecond XUV pulses. The peak position, line shape and population of the resonant states were modified by intense near infrared laser pulses.

Attosecond Absorption Spectroscopy

Photoabsorption spectroscopy with isolated attosecond pulses provides a means for all-optical time-domain measurement of ultrafast dynamics in bound and quasi-bound states of atoms and molecules, as well as near absorption edges in condensed matter systems. Due to the strong oscillator strength in atoms and molecules in the extreme ultraviolet and soft X-ray spectral regions as well as the high detection efficiency of the transmitted photon signal, attosecond transient absorption spectroscopy is an attractive alternative to traditional photoelectron spectroscopy techniques. We present measurements of the laser-perturbed photoabsorption of helium 1snp singly excited states and quasi-bound autoionizing states of argon with a few-femtosecond autoionization lifetime. In both systems, we observe laser-induced changes in the photoabsorption spectrum which evolve on the single-femtosecond timescale, faster than the laser light oscillation period.

Intense high-order harmonics from carbon plasma for intense attosecond pulse generation

Summary form only given. Generation of intense isolated attosecond pulses is of significant importance to explore new domains in attosecond science. Recently, we have demonstrated high-order harmonic generation (HHG) with high conversion efficiency using low-density plasma as the nonlinear medium [1,2], instead of gas. However, isolated attosecond pulses have yet to be demonstrated from plasma. In this work, we apply the Double Optical Gating (DOG) [3] method to generate continuum high-order harmonics from lowly ionized carbon plasma, which is the first step toward the generation of intense attosecond pulses.