Dong Hyuk Ko

Attosecond chirp compensation over broadband high-order harmonics to generate near transform-limited 63 as pulses

By generating broadband high-harmonic pulses from neon and compensating for attosecond chirp by the material dispersion of argon, the generation of near transform-limited 63 as pulses was achieved. The spectral phase analysis showed that, without proper compensation, the attosecond chirp of the broadband harmonics caused splitting of attosecond high-harmonic pulses in addition to pulse broadening. Although it was attained only within a limited spectral range, the attosecond chirp compensation was successful in bringing out pulse compression over broad harmonics, which signifies the effectiveness of the attosecond chirp compensation by material dispersion.

Complete temporal reconstruction of attosecond high-harmonic pulse trains

The method of complete reconstruction of attosecond bursts has been demonstrated for attosecond high-harmonic pulse trains. The retrieved harmonic field provided detailed information about the envelope and the individual attosecond pulses contained in the attosecond pulse train. The time–frequency analysis revealed complicated spectral chirp structures and the contribution of different quantum paths to attosecond pulse formation.

Attosecond-chirp compensation with material dispersion to produce near transform-limited attosecond pulses

The duration of attosecond pulses, produced through high harmonic generation from gaseous atoms, was found to be much longer than the transform-limited duration. The inherent attosecond chirp existing in the temporal structure of the attosecond pulses was analysed to obtain a scaling factor in terms of laser intensity and wavelength. We explored the attosecond chirp compensation using various materials with negative group delay dispersion to cover different spectral ranges. From numerical simulations based on the time-dependent Schrodinger equation, we showed that 38 as isolated attosecond pulses could be produced by applying intense few-cycle mid-infrared laser pulses after compensating for the attosecond chirp in Xe.

Comparison of RABITT and FROG measurements in the temporal reconstruction of attosecond pulse trains

Attosecond high-harmonic pulses obtained from Ar were characterized by the two methods — RABITT and FROG CRAB. The comparison of the two results revealed the capabilities of the two methods well.

Transform-Limited Attosecond Pulse Generation Through Atto-Chirp Compensation by Material Dispersion

High harmonics are a unique light source with ultrashort duration and superb spatial coherence in the extreme ultraviolet and X-ray region. Though regularly spaced broad harmonic spectra are suitable for generating very short pulses, they suffer from the inherent chirp due to the harmonic generation process. Here we proposed and demonstrated the method to compensate for the attosecond chirp by propagating the harmonic pulses through a medium with negative group delay dispersion. In a single Ar gas cell, we showed that the chirp compensation could be achieved in the same harmonic generation cell, obtaining near transform-limited 206-as pulses. We could demonstrate also the generation of much shorter 63-as pulses by obtaining the harmonics from Ne and by compensating for the attosecond chirp in the second gas cell of Ar. The chirp compensation in gaseous media is, thus, very effective and powerful in obtaining near transform-limited attosecond pulses—very valuable for attosecond science.

Generation of Ultrashort Attosecond High-Harmonic Pulses from Chirp-compensated Ne Harmonics

Near transform-limited 63-as high-harmonic pulses were successfully generated by compensating positive attosecond chirp of harmonics using negative group delay dispersion of Ar in broad spectral region. Without proper chirp compensation, the harmonic pulses were split into multiple peaks due to the large phase variation, in addition to the broadening of pulse duration. The chirpcompensated broadband harmonic pulses, approaching the transform-limited duration, will be very valuable for exploring ultrafast dynamics in atoms and molecules with high temporal resolution.

Complete Temporal Reconstruction of Attosecond Harmonic Pulses

Using the FROG CRAB method, complete temporal reconstruction of attosecond harmonic pulses was achieved. The compensation of intrinsic attosecond chirp was also demonstrated in the harmonic generation medium itself, thereby achieving transform-limited attosecond pulses.

Compression of Attosecond Harmonic Pulses in the Harmonic Generation Medium Itself

The intrinsic chirp of attosecond harmonic pulse obtained from Ar was compensated by taking into account the material dispersion of the harmonic generation medium itself. The measured attosecond pulse duration was very close to the transform-limited value.

Complete Temporal Characterization of Attosecond High Harmonic Pulses using the FROG Technique

For complete temporal characterization of an attosecond harmonic pulse train the frequency-resolved optical gating technique is applied to the photoelectron spectra of He driven by high harmonic pulses and IR laser pulses with time delay, which shows the detailed temporal information of attosecond pulses in the attosecond pulse train.