Light-field synthesizer based on multidimensional solitary states in hollow-core fibers.

Few-cycle, long-wavelength sources for generating isolated attosecond soft x ray pulses typically rely upon complex laser architectures. Here, we demonstrate a comparatively simple setup for generating sub-two-cycle pulses in the short-wave infrared based on multidimensional solitary states in an N2O-filled hollow-core fiber and a two-channel light-field synthesizer. Due to the temporal phase imprinted by the rotational nonlinearity of the molecular gas, the redshifted (from 1.03 to 1.36 µm central wavelength) supercontinuum pulses generated from a Yb-doped laser amplifier are compressed from 280 to 7 fs using only bulk materials for dispersion compensation.

Thermal effects in molecular gas-filled hollow-core fibers.

Few-cycle sources with high average powers are required for applications to attosecond science. Raman-enhanced spectral broadening of Yb-doped laser amplifiers in molecular gases can yield few-cycle pulses, but thermal excitation of vibrational and rotational degrees of freedom may preclude high-power operation. Here we investigate changes in the spectral broadening associated with repetitive laser interactions in an ${{\rm{N}}_2}{\rm{O}}$-filled hollow-core fiber. By comparing experimental measurements of the spectrum associated with each laser pulse to simulations based on a density matrix model, we find that losses in a spectral bandwidth and transmission are largely dominated by thermal excitation of the gas.

Laser waveform control of extreme ultraviolet high harmonics from solids.

Solid-state high-harmonic sources offer the possibility of compact, high-repetition-rate attosecond light emitters. However, the time structure of high harmonics must be characterized at the sub-cycle level. We use strong two-cycle laser pulses to directly control the time-dependent nonlinear current in single-crystal MgO, leading to the generation of extreme ultraviolet harmonics. We find that harmonics are delayed with respect to each other, yielding an atto-chirp, the value of which depends on the laser field strength. Our results provide the foundation for attosecond pulse metrology based on solid-state harmonics and a new approach to studying sub-cycle dynamics in solids.

Divided-pulse amplification to the joule level.

Divided-pulse amplification (DPA) has proven to be a valuable tool in scaling the peak power of diode-pumped ytterbium-doped amplifiers to beyond the single-pulse threshold for parasitic nonlinear effects. DPA enables the amplification of picosecond pulses in solid-state amplifiers with limited bandwidth beyond the single-pulse damage threshold. In this Letter, we demonstrate DPA of picosecond pulses in a flashlamp-pumped Nd:YAG amplifier for the first time, to the best of our knowledge, yielding a combined pulse energy of 167 mJ.

Suppression of driving laser in high harmonic generation with a microchannel plate.

Separating the infrared driving laser from the extreme ultraviolet (XUV) pulses after high-order harmonic generation has been a long-standing difficulty. In this Letter, we propose and demonstrate that the driving laser can be blocked by simply installing a microchannel plate (MCP) into the beam line. In addition to its high damage threshold, the MCP filter also transmits photons over the entire XUV region. This paves the way for attosecond pulse generation with unprecedented bandwidth.

In situ calibration of an extreme ultraviolet spectrometer for attosecond transient absorption experiments.

We report a method for calibrating an extreme ultraviolet spectrometer based on a flat-field grazing incidence spherical grating in the energy range of 20-30 eV. By measuring absorption lines corresponding to singly excited states in helium atoms and autoionizing states in argon atoms, the photon energy of the detected light was determined. The spectral resolution of the spectrometer, 60 meV, was obtained by deconvolving the Fano resonance profile of argon autoionizing states from the measured absorption line profiles.

Tailoring a 67 attosecond pulse through advantageous phase-mismatch.

A single isolated attosecond pulse of 67 as was composed from an extreme UV supercontinuum covering 55-130 eV generated by the double optical gating technique. Phase mismatch was used to exclude the single-atom cutoff of the spectrum that possesses unfavorable attochirp, allowing the positive attochirp of the remaining spectrum to be compensated by the negative dispersion of a zirconium foil. Two algorithms, PROOF and FROG-CRAB, were employed to retrieve the pulse from the experimental spectrogram, yielding nearly identical results.

Subcycle ac stark shift of helium excited states probed with isolated attosecond pulses.

Recent advances in attosecond science have relied upon the nearly instantaneous response of free electrons to an external field. However, it is still not clear whether bound electrons are able to rearrange on sublaser cycle time scales. Here, we probe the optical Stark shifts induced by a few-cycle near infrared laser field in helium bound states using isolated attosecond pulses in a transient absorption scheme and uncover a subcycle laser-induced energy level shift of the laser-dressed 1s3p state.

Characterizing ultrabroadband attosecond lasers.

Recent progress in sub-laser-cycle gating of high-order harmonic generation promises to push the limits on optical pulse durations below the atomic unit of time, 24 as, which corresponds to a bandwidth broader than 75 eV. However, the available techniques for attosecond pulse measurement are valid only for narrow-bandwidth spectra, due to one of the key approximations made in the phase retrieval. Here we report a new technique for characterizing attosecond pulses, whereby the spectral phase of the attosecond pulse is extracted from the oscillation component with the dressing laser frequency in the photoelectron spectrogram. This technique, termed PROOF (Phase Retrieval by Omega Oscillation Filtering), can be applied to characterizing attosecond pulses with ultrabroad bandwidths.

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.

Monitoring and controlling the electron dynamics in helium with isolated attosecond pulses.

Helium atoms in the presence of extreme ultraviolet light pulses can lose electrons through direct photoionization or through two-electron excitation followed by autoionization. Here we demonstrate that, by combining attosecond extreme ultraviolet pulses with near infrared femtosecond lasers, electron dynamics in helium autoionization can be not only monitored but also controlled. Furthermore, the interference between the two ionization channels was modified by the intense near infrared laser pulse. To the best of our knowledge, this is the first time that double excitation and autoionization were studied experimentally by using isolated attosecond pulses.

Isolated attosecond pulse generation without the need to stabilize the carrier-envelope phase of driving lasers.

Single isolated attosecond pulses can be extracted from a pulse train with an ultrafast gate in the generation target. By setting the gate width sufficiently narrow with the generalized double optical gating, we demonstrate that single isolated attosecond pulses can be generated with any arbitrary carrier-envelope phase value of the driving laser. The carrier-envelope phase only affects the photon flux, not the pulse duration or contrast. Our results show that isolated attosecond pulses can be generated using carrier-envelope phase unstabilized 23 fs pulses directly from chirped pulse amplifiers.

Attosecond time-resolved autoionization of argon.

Autoionization of argon atoms was studied experimentally by transient absorption spectroscopy with isolated attosecond pulses. The peak position, intensity, linewidth, and shape of the 3s3p6np ¹P Fano resonance series (26.6-29.2 eV) were modified by intense few-cycle near infrared laser pulses, while the delay between the attosecond pulse and the laser pulse was changed by a few femtoseconds. Numerical simulations revealed that the experimentally observed splitting of the 3s3p64p ¹P line is caused by the coupling between two short-lived highly excited states in the strong laser field.

Electron-electron correlations and structural, spectral and polarization properties of tetragonal BaTiO3.

To analyze the role of electron-electron correlation effects in structural (local-geometry), spectral and polarization properties of tetragonal BaTiO3 we apply DFT+U approach. We demonstrate that the system properties drastically change when the value of the local Coulomb repulsion U crosses the critical value U_c ≈ 7 eV. In particular, the correlation effects cause a change of the ratio of the in-plane and inter-plane Ti-O bond lengths, which results in a flip of the order of the Ti d-bands and change of the polarizability of the system. Since the exact value of U in BaTiO3 is unknown, we discuss how the obtained results may be revealed in experimental data, especially in the optical response and ultrafast charge dynamics, where effective U is dynamically tuned.

Multioctave supercontinuum generation and frequency conversion based on rotational nonlinearity.

The field of attosecond science was first enabled by nonlinear compression of intense laser pulses to a duration below two optical cycles. Twenty years later, creating such short pulses still requires state-of-the-art few-cycle laser amplifiers to most efficiently exploit "instantaneous" optical nonlinearities in noble gases for spectral broadening and parametric frequency conversion. Here, we show that nonlinear compression can be much more efficient when driven in molecular gases by pulses substantially longer than a few cycles because of enhanced optical nonlinearity associated with rotational alignment. We use 80-cycle pulses from an industrial-grade laser amplifier to simultaneously drive molecular alignment and supercontinuum generation in a gas-filled capillary, producing more than two octaves of coherent bandwidth and achieving >45-fold compression to a duration of 1.6 cycles. As the enhanced nonlinearity is linked to rotational motion, the dynamics can be exploited for long-wavelength frequency conversion and compressing picosecond lasers.

Extreme ultraviolet time- and angle-resolved photoemission setup with 21.5 meV resolution using high-order harmonic generation from a turn-key Yb:KGW amplifier.

Characterizing and controlling electronic properties of quantum materials require direct measurements of nonequilibrium electronic band structures over large regions of momentum space. Here, we demonstrate an experimental apparatus for time- and angle-resolved photoemission spectroscopy using high-order harmonic probe pulses generated by a robust, moderately high power (20 W) Yb:KGW amplifier with a tunable repetition rate between 50 and 150 kHz. By driving high-order harmonic generation (HHG) with the second harmonic of the fundamental 1025 nm laser pulses, we show that single-harmonic probe pulses at 21.8 eV photon energy can be effectively isolated without the use of a monochromator. The on-target photon flux can reach 5 × 1010 photons/s at 50 kHz, and the time resolution is measured to be 320 fs. The relatively long pulse duration of the Yb-driven HHG source allows us to reach an excellent energy resolution of 21.5 meV, which is achieved by suppressing the space-charge broadening using a low photon flux of 1.5 × 108 photons/s at a higher repetition rate of 150 kHz. The capabilities of the setup are demonstrated through measurements in the topological semimetal ZrSiS and the topological insulator Sb2-xGdxTe3.

Coupling between energy and phase in hollow-core fiber based f-to-2f interferometers.

The dependence of the carrier-envelope (CE) phase of the pulses from a hollow-core fiber on the input laser energy was studied using two f-to- 2f interferometers. The CE phase in the in-loop f-to-2f interferometer was measured with the octave spanning white-light spectrum from the hollow-core fiber, whereas the out-of-loop interferometer was based on a sapphire plate. By modulating the input power of the in-loop interferometer and measuring the out-of-loop CE phase at the same time, the coupling coefficient between the measured CE phase and the laser energy for the hollow-core fiber was determined to be 128 mrad per 1% energy change .

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.

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.