Ivanka Grguraš

Synthesized Light Transients

Light spanning the near infrared to the ultraviolet has been confined in pulses shorter than a single optical cycle. Manipulation of electron dynamics calls for electromagnetic forces that can be confined to and controlled over sub-femtosecond time intervals. Tailored transients of light fields can provide these forces. We report on the generation of subcycle field transients spanning the infrared, visible, and ultraviolet frequency regimes with a 1.5-octave three-channel optical field synthesizer and their attosecond sampling. To demonstrate applicability, we field-ionized krypton atoms within a single wave crest and launched a valence-shell electron wavepacket with a well-defined initial phase. Half-cycle field excitation and attosecond probing revealed fine details of atomic-scale electron motion, such as the instantaneous rate of tunneling, the initial charge distribution of a valence-shell wavepacket, the attosecond dynamic shift (instantaneous ac Stark shift) of its energy levels, and its few-femtosecond coherent oscillations.

Femtosecond all-optical synchronization of an X-ray free-electron laser

Many advanced applications of X-ray free-electron lasers require pulse durations and time resolutions of only a few femtoseconds. To generate these pulses and to apply them in time-resolved experiments, synchronization techniques that can simultaneously lock all independent components, including all accelerator modules and all external optical lasers, to better than the delivered free-electron laser pulse duration, are needed. Here we achieve all-optical synchronization at the soft X-ray free-electron laser FLASH and demonstrate facility-wide timing to better than 30 fs r.m.s. for 90 fs X-ray photon pulses. Crucially, our analysis indicates that the performance of this optical synchronization is limited primarily by the free-electron laser pulse duration, and should naturally scale to the sub-10 femtosecond level with shorter X-ray pulses.

Invited Article: Attosecond photonics: Synthesis and control of light transients

Ultimate control over light entails the capability of crafting its field waveform. Here, we detail the technological advances that have recently permitted the synthesis of light transients confinable to less than a single oscillation of its carrier wave and the precise attosecond tailoring of their fields. Our work opens the door to light field based control of electrons on the atomic, molecular, and mesoscopic scales.

Coherent spectral enhancement of carrier-envelope-phase stable continua with dual-gas high harmonic generation

Attosecond science is enabled by the ability to convert femtosecond near-infrared laser light into coherent harmonics in the extreme ultraviolet spectral range. While attosecond sources have been utilized in experiments that have not demanded high intensities, substantially higher photon flux would provide a natural link to the next significant experimental breakthrough. Numerical simulations of dual-gas high harmonic generation indicate that the output in the cutoff spectral region can be selectively enhanced without disturbing the single-atom gating mechanism. Here, we summarize the results of these simulations and present first experimental findings to support these predictions.

Femtosecond profiling of shaped x-ray pulses

Arbitrary manipulation of the temporal and spectral properties of x-ray pulses at free-electron lasers would revolutionize many experimental applications. At the Linac Coherent Light Source at Stanford National Accelerator Laboratory, the momentum phase-space of the free-electron laser driving electron bunch can be tuned to emit a pair of x-ray pulses with independently variable photon energy and femtosecond delay. However, while accelerator parameters can easily be adjusted to tune the electron bunch phase-space, the final impact of these actuators on the x-ray pulse cannot be predicted with sufficient precision. Furthermore, shot-to-shot instabilities that distort the pulse shape unpredictably cannot be fully suppressed. Therefore, the ability to directly characterize the x-rays is essential to ensure precise and consistent control. In this work, we have generated x-ray pulse pairs via electron bunch shaping and characterized them on a single-shot basis with femtosecond resolution through time-resolved photoelectron streaking spectroscopy. This achievement completes an important step toward future x-ray pulse shaping techniques.

Attosecond sources for time-bandwidth balanced spectroscopy

Spectral bandwidth in attosecond pulses can span multiple energy levels simultaneously, complicating time-resolved photoemission applications. We seek to adapt attosecond sources to study condensed-matter dynamics with sub-eV resolution, unfolding on the corresponding femtosecond timescale.

Attosecond physics with Synthesized Transients of Light

We demonstrate synthesis of superoctave, intense, subcycle transients of light and their application to attosecond control of matter.

Sub-optical-cycle waveform synthesis of light

Electron dynamics in atoms [1] molecules and condensed matter are typically clocked in tens to thousands of attoseconds. Real-time control of these dynamics requires the exertion of electric or magnetic fields controllable in strength and in direction on that time scale. Such precisely controlled fields can become available via waveform synthesis of continuous (to warrant isolated structures in the time domain) light spectra that span over more than an optical octave.