Van-Hung Hoang

The concept of using photoelectron interferometry in short laser fields to probe electron dynamics and target structures was introduced more than two decades ago. However, the quality of experimental data has remained insufficient for quantitative analysis, largely due to the instability of few-cycle Ti:sapphire laser pulses-the current workhorse of short pulses. Here, we report the first systematic strong-field ionization experiments performed with industrial-grade, carrier-envelope-phase stabilized, near-single-cycle Yb lasers. By measuring photoelectron momentum distributions in the direct-ionization regime, we show that single-cycle cosine-shaped pulses can separate and enhance both spider-leg and fishbone holographic structures. The spider-leg structure enables extraction of the electron scattering phase from the Ar atomic potential-information typically accessible through attosecond metrology-while the fishbone structure reveals the orbital-parity contrast between Ar atoms and nitrogen molecules. Our measurements are quantitatively reproduced by both semiclassical Herman-Kluk-propagator and ab initio simulations, paving the way for precision studies of electron-molecule scattering with widely accessible industrial-grade lasers.

We present a theoretical investigation of high-order-harmonic generation (HHG) from impurity-doped materials. Based on the analysis of the exact numerical solutions of the time-dependent Schrödinger equation (TDSE) for one-dimensional models, we demonstrate the factorization of HHG yields as a product of an electron wave packet and the recombination cross section, in analogy to HHG from atoms and molecules in the gas phase. Furthermore, we show that the quantitative rescattering model based on this factorization accurately reproduces the TDSE results. This opens up new possibilities to study impurities in materials using the available techniques from strong-field physics.

Enhancement of the conversion efficiency of high-order harmonics (HHG) generated from an atom in the superposition of two quantum states instead of the ground state is essential because of the recent application in the generation of ultrashort isolated attosecond pulses. In this study, we first confirm the enhancement for a more real system with full three-dimensional geometry and then investigate an interesting effect—the influence of the depletion on the HHG cutoff. For this purpose, we prepare a hydrogen atom initially in the coherent superposition of the ground and the first excited states and calculate the HHG by numerically solving the three-dimensional time-dependent Schrödinger equation (TDSE)—the TDSE method. We find that the plateau of HHG spectra with the coherent superposition state ends earlier than that of the ground state, which can be attributed to the depletion of the excited state. We justify this link by adopting the classical simulation and time-dependent ionization rate. Besides, we show that the conversion efficiency is sensitive to the existence of the excited state. Particularly, for some values of laser field intensity, the excited state population of about 1% increases HHG intensity by more than 5 orders. However, after a critical excited state population, the HHG intensity is stable with continuously increasing initial population of the excited state that is meaningful for experimental setups.

We present a theoretical investigation of high-harmonic generation (HHG) from impurity-doped materials using the time-dependent Schr\"odinger equation (TDSE) approach. We demonstrate the factorization of HHG yields as a product of an electron wave packet and the recombination cross section, in analogy to HHG from atoms and molecules in the gas phase. Furthermore, we show that the quantitative rescattering model based on this factorization accurately reproduces the TDSE results. This opens up new possibilities to study impurities in materials using the available techniques from strong-field physics.

We present a detailed theoretical study of the odd–even harmonics generated from the polar molecule CO by the method based on numerically solving the time-dependent Schrödinger equation within the single-active-electron approximation. First, we reproduce the pure even harmonic generation of CO predicted theoretically by Hu et al. using the time-dependent density functional theory [H. Hu et al., Phys. Rev. Lett., 2017, 119, 173201]. Then, based on the Floquet approach, we are able to attribute this behavior to the half-cycle mirror symmetry of the molecule–field system when the polar molecule is perpendicular to the laser polarization. By numerical simulations, we show that this symmetry is broken at orientation angles other than 90° resulting in the odd–even harmonic generation and a non-trivial even-to-odd harmonics ratio strongly dependent on the molecular orientation. Furthermore, we investigate the influence of the dynamic core-electron polarization (DCeP) on the odd–even behavior near the cutoff of the high-order harmonic spectra. We emphasize that the DCeP effect is noticeable for the odd harmonics only.