Ximao Feng

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.

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.

Direct measurement of an electric field in femtosecond Bessel-Gaussian beams

We demonstrated the mapping of the spatial oscillation of electric fields in the transverse plane of a femtosecond Bessel-Gaussian laser beam from the first principle of classical electrodynamics. An attosecond burst of electrons for probing the electric force was placed in the Bessel beam through photoemission with single isolated 276 as extreme ultraviolet pulses. The direction reversal of the electric field in adjacent Bessel rings was directly confirmed by observing the momentum shift of the probe electrons.

Calibration of electron spectrometer resolution in attosecond streak camera

We report a new method for determining the energy resolution of time-of-flight spectrometers for detecting photoelectrons produced with attosecond XUV pulses. By measuring the width of the 2s2p autoionization line of helium, we found the resolution of our spectrometer to be approximately 0.6 eV for electrons at 35.5 eV. Furthermore, the resolution in the 10 to 35 eV range was determined by applying a retarding potential at the entrance of the drift tube.

Probing laser disturbed doubly excited states with isolated attosecond pulses

Two-electron excitation and autoionization in helium atoms were studied experimentally using isolated attosecond pulses for the first time. The population of the resonance state was modified by intense near infrared laser pulses.

Single Isolated Attosecond Pulses Generation with Double Optical Gating

Isolated attosecond pulses are powerful tools for exploring electron dynamics in matter. We proposed and demonstrated a technique called generalized double optical gating for generating isolated attosecond pulses. The double optical gating scheme, relaxing the stringent requirement on laser pulse duration from ≤ 5 fs to pulse duration close to the amplifier output, would make attophysics more accessible to many laboratories that are capable of producing such multicycle laser pulses. The isolated attosecond pulses were measured by reconstructing the streaked photoelectron spectrogram. The same setup was used to trace the spatial profile of a femtosecond Bessel beam, which serves as a demonstration of attosecond pulse applications. We also review a technique for stabilizing the carrier-envelope phase of grating-based laser amplifiers, which controls the grating separation. The phase-stabilized lasers are required for generating isolated attosecond pulses with double optical gating as well as other schemes.