Few-cycle laser pulse characterization on-target using high-harmonic generation from nano-scale solids.

We demonstrate high-harmonic generation for the time-domain observation of the electric field (HHG-TOE) and use it to measure the waveform of ultrashort mid-infrared (MIR) laser pulses interacting with ZnO thin-films or WS2 monolayers. The working principle relies on perturbing HHG in solids with a weak replica of the pump pulse. We measure the duration of few-cycle pulses at 3200 nm, in reasonable agreement with the results of established pulse characterization techniques. Our method provides a straightforward approach to accurately characterize femtosecond laser pulses used for HHG experiments right at the point of interaction.

Dynamical Franz-Keldysh Effect in Diamond in the Deep Ultraviolet Probed by Transient Absorption and Dispersion Spectroscopy Using a Miniature Beamline.

Here, we introduce a miniature beamline for transient absorption and dispersion spectroscopy, using a tailored deep ultraviolet field immediately after the noncollinear generation without subsequent optical elements. We explore the near-band-gap region in diamond in the presence of a few-femtosecond pump pulse where the delayed dynamical Franz-Keldysh effect and the almost instantaneous optical Kerr effect coexist.

Onset of Bloch oscillations in the almost-strong-field regime.

In the field of high-order harmonic generation from solids, the electron motion typically exceeds the edge of the first Brillouin zone. In conventional nonlinear optics, on the other hand, the excursion of band electrons is negligible. Here, we investigate the transition from conventional nonlinear optics to the regime where the crystal electrons begin to explore the first Brillouin zone. It is found that the nonlinear optical response changes abruptly already before intraband currents due to ionization become dominant. This is observed by an interference structure in the third-order harmonic generation of few-cycle pulses in a non-collinear geometry. Although approaching Keldysh parameter γ = 1, this is not a strong-field effect in the original sense, because the iterative series still converges and reproduces the interference structure. The change of the nonlinear interband response is attributed to Bloch motion of the reversible (or transient or virtual) population, similar to the Bloch motion of the irreversible (or real) population which affects the intraband currents that have been observed in high-order harmonic generation.

Resonant Perfect Absorption Yielded by Zero-Area Pulses.

We propose and study the manipulation of the macroscopic transient absorption of an ensemble of open two-level systems via temporal engineering. The key idea is to impose an ultrashort temporal gate on the polarization decay of the system by transient absorption spectroscopy, thus confining its free evolution and the natural reshaping of the excitation pulse. The numerical and analytical results demonstrate that even at moderate optical depths, the resonant absorption of light can be reduced or significantly enhanced by more than 5 orders of magnitude relative to that without laser manipulation. The achievement of the quasicomplete extinction of light at the resonant frequency, here referred to as resonant perfect absorption, arises from the full destructive interference between the excitation pulse and its subpulses developed and tailored during propagation, and is revealed to be connected with the formation of zero-area pulses in the time domain.

Characterization of weak deep ultraviolet pulses using cross-phase modulation scans.

Temporal pulse characterization methods can often not be applied to deep ultraviolet (DUV) pulses due to the lack of suitable nonlinear crystals and very low pulse energies. Here, a method is introduced for the characterization of two unknown and independent laser pulses. The applicability is broad, but the method is especially useful for pulses in the DUV, because pulse energies on the picojoule scale suffice. The basis is a spectral analysis of the two interfering DUV pulses, while one of the pulses is phase shifted by an unknown visible-infrared (VIS-IR) pulse via cross-phase modulation. The pulse retrieval is analytic, and the fidelity can be checked by comparing the complex-valued data trace with the retrieved trace.

Characterization of two ultrashort laser pulses using interferometric imaging of self-diffraction.

Noncollinear pulse characterization methods can be applied to over-octave spanning waveforms, but geometrical effects in the nonlinear medium such as beam smearing and critical sensitivity to beam alignment hinder their accurate application. Here, a method is introduced for the temporal and spatial characterization of two pulses by interferometric, spectrally resolved imaging of self-diffraction. Geometrical effects are resolved by the method and, therefore, do not limit the accuracy. Two methods for quantitative pulse retrieval are presented. One method is analytical and very fast; the other method is iterative and more robust if applied to noisy data.

Strong-field induced XUV transmission and multiplet splitting in 4d(-1)6p core-excited Xe studied by femtosecond XUV transient absorption spectroscopy.

Light-induced coupling of core-excited states of Xe atoms is investigated by femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy with photon energies ranging from 50 eV to 72 eV. Coupling of the 4d(-1)((2)D(5/2))6p((2)P(3/2)) (65.1 eV) and 4d(-1)((2)D(3/2))6p((2)P(1/2)) (67.0 eV) core-excited states to nearby states by a strong infrared laser field leads to a threefold enhancement of XUV transmission. The transmission at 65.1 eV (67.0 eV) changes from 3.2 ± 0.4% (5.9 ± 0.5%) without the coupling laser to 9 ± 2% (22 ± 5%) at the maximum of the laser field. A strong-field induced broad XUV absorption feature between 60 eV and 65 eV is ascribed to splitting of the field-free absorption lines into multiple branches when the Rabi frequencies of the coupling transitions exceed the infrared laser frequency. This picture is supported by a comparison of the strong-field induced absorption spectrum with a numerical integration of the von Neumann equation for a few-level quantum system. The valence hole-alignment of strong-field ionized Xe is revisited, confirming the previously observed reduced alignment compared to theoretical predictions.