Attosecond pulse characterization

Fast phase retrieval for broadband attosecond pulse characterization

Efficient characterization method for broadband attosecond pulses has become more and more essential, since attosecond pulses with bandwidth spanning few-hundreds electron-volts have been generated. Here we propose a fast phase retrieval algorithm for broadband attosecond pulse characterization with an omega oscillation filtering technique. We introduce a new error function to improve the accuracy of the retrieved phases. More importantly, it can be solved by the steepest descent methods with iterative algorithm, which is much faster than genetic algorithm adopted previously. An experimental spectrogram for isolated attosecond pulses with photon energy covering 52-127 eV and a pulse width of 71 as was successfully retrieved with this method as demonstrated. The proposed technique will help provide real-time feedback on atto-chirp compensation for ultrashort isolated attosecond pulse generation.

Strong-field coherent control of isolated attosecond pulse generation

Attosecond science promises to reveal the most fundamental electronic dynamics occurring in matter and it can develop further by meeting two linked technological goals related to high-order harmonic sources: improved spectral tunability (allowing selectivity in addressing electronic transitions) and higher photon flux (permitting to measure low cross-section processes). New developments come through parametric waveform synthesis, which provides control over the shape of field transients, enabling the creation of highly-tunable isolated attosecond pulses via high-harmonic generation. Here we demonstrate that the first goal is fulfilled since central energy, spectral bandwidth/shape and temporal duration of isolated attosecond pulses can be controlled by shaping the laser waveform via two key parameters: the relative-phase between two halves of the multi-octave spanning spectrum, and the overall carrier-envelope phase. These results not only promise to expand the experimental possibilities in attosecond science, but also demonstrate coherent strong-field control of free-electron trajectories using tailored optical waveforms.

Attosecond pulse generation needs improvements both in terms of tunability and photon flux for next level attosecond experiments. Here the authors show how to control the HHG emission and its spectral-temporal characteristics by driving the IAP generation with synthesized sub-cycle optical pulses.

Waveform retrieving of an isolated attosecond pulse using high-order harmonics generation of the superimposed infrared field

An all-optical method is suggested for the metrology of an isolated, pulse-to-pulse stabilized attosecond pulse. It is shown analytically that high-order harmonic generation (HHG) yield for an intense IR pulse and time-delayed attosecond pulse keeps encoded waveform of the attopulse, which can be decoded by the time delay measurements of the HHG yield. The retrieval method is demonstrated by modeling HHG from Ne atom within time-dependent Kohn-Sham equations. The application of the suggested method for monitoring the carrier-envelope phase of the attosecond pulse is discussed.

Design of an optically-locked interferometer for attosecond pump-probe setups

We present the design and performance of an active stabilization system for attosecond pump-probe setups based on a Mach-Zehnder interferometer configuration. The system employs a CW laser propagating coaxially with the pump and probe beams in the interferometer. The stabilization is achieved with a standalone feedback controller that adjusts the length of one of its arms to maintain a constant relative phase between the CW beams. With this system, the time delay between the pump and probe beams is stabilized within 10 as rms over several hours. The system is easy to operate and only requires a few minutes to set up before any pump/probe measurements.

Complete reconstruction of ultra-broadband isolated attosecond pulses including partial averaging over the angular distribution

Attosecond streaking is a powerful tool to investigate ultrafast electron dynamics on the attosecond time scale. To obtain the highest temporal resolution in a pump-probe experiment, soft-X-ray (SXR) and infrared (IR) pulses have to be carefully characterized. Here, we present a detailed description of our recent generalization of the Volkov-transform generalized projection algorithm (VTGPA) and its application to multiple overlapping photoelectron bands. This method allows for the complete temporal reconstruction of both IR and SXR pulses under the inclusion of accurate complex photoionization matrix elements (PMEs). In this article, we compare the performance of our new method with traditional algorithms. We particularly focus on the important role played by the photoelectron angular distribution (PAD) which needs to be taken into account for the highest fidelity of attosecond pulse reconstruction. For this purpose, we investigate numerically the influence of the finite collection angle of the electron spectrometer on the retrieval and the obtained pulse parameters. We further theoretically demonstrate the reliability of the reconstruction for pulse durations even shorter than the atomic unit of time.

Streaking of 43-attosecond soft-X-ray pulses generated by a passively CEP-stable mid-infrared driver

Attosecond metrology has so far largely remained limited to titanium:sapphire lasers combined with an active stabilization of the carrier-envelope phase (CEP). These sources limit the achievable photon energy to ∼100 eV which is too low to access X-ray absorption edges of most second- and third-row elements which are central to chemistry, biology and material science. Therefore, intense efforts are underway to extend attosecond metrology to the soft-X-ray (SXR) domain using mid-infrared (mid-IR) drivers. Here, we introduce and experimentally demonstrate a method that solves the long-standing problem of the complete temporal characterization of ultra-broadband (≫10 eV) attosecond pulses. We generalize the recently proposed Volkov-transform generalized projection algorithm (VTGPA) to the case of multiple overlapping photoelectron spectra and demonstrate its application to isolated attosecond pulses. This new approach overcomes all key limitations of previous attosecond-pulse reconstruction methods, in particular the central-momentum approximation (CMA), and it incorporates the physical, complex-valued and energy-dependent photoionization matrix elements. These properties make our approach general and particularly suitable for attosecond supercontinua of arbitrary bandwidth. We apply this method to attosecond SXR pulses generated from a two-cycle mid-IR driver, covering a bandwidth of ∼100 eV and reaching photon energies up to 180 eV. We extract an SXR pulse duration of (43±1) as from our streaking measurements, defining a new world record. Our results prove that the popular and broadly available scheme of post-compressing the output of white-light-seeded optical parametric amplifiers is adequate to produce high-contrast isolated attosecond pulses covering the L-edges of silicon, phosphorous and sulfur. Our new reconstruction method and experimental results open the path to the production and characterization of attosecond pulses lasting less than one atomic unit of time (24 as) and covering X-ray absorption edges of most light elements.