Doan-An Trieu

Multiorbital involvement in high-order harmonic generations (HHG) emitted from molecules exposed to an intense ultrashort laser pulse is essential for probing the structures and dynamics of molecules. A comprehensive understanding of how lower-lying orbitals contribute to HHG under varying laser parameters is needed; however, current research has primarily focused on variations in laser intensity for specific molecules, while investigations into other laser parameters remain desirable. In this study, we systematically investigate the effects of laser parameters, including pulse duration, intensity, and wavelength, on the lower-lying orbitals’ fingerprints in HHG emitted from hydrogen cyanide (HCN) molecules. By solving the time-dependent Schrödinger equation when HCN is aligned with the laser field, we find that the relative contribution of HOMO and HOMO-1 to HHG is not fixed but rather depends sensitively on the laser conditions. In particular, the competition between HOMO and HOMO-1 varies with laser intensity and pulse duration but remains relatively stable across different wavelengths. Notably, as pulse duration and intensity increase, HOMO-1 rapidly suppresses HOMO, controlling total HHG in both spectral shape and HHG yield. By analyzing the harmonic time-frequency profiles, we attribute these dependencies to differences in the growth rates of the ionization probability and the recombination transition dipole as we vary the laser parameters. Interestingly, the competition in HHG differs from that in ionization probability: while HOMO dominates for short pulses, HOMO-1 becomes more favorable for longer pulses. Our findings highlight the critical importance of selecting appropriate laser conditions to investigate the ultrafast dynamics of inner-shell electrons through HHG, particularly concerning multiorbital coupling.

Studying the effects caused by core electrons of highly polarizable atoms and molecules is essential since interactions between external fields and matter primarily occur in the outer electron shell, but the contribution of the core electrons has been revealed in recent studies. Therefore, investigating the core-electron polarization effect (MEP) in the high-order harmonic generation (HHG) spectrum of matter when interacting with intense, ultrashort laser pulses has attracted significant attention. In our latest work, we have shown that when interacting with a few-cycle laser pulse, MEP induces significant shifts in the HHG peaks of the CO molecules. However, the laser's parameter ranges of the laser pulse, allowing the observation of these peak shifts, have not yet been thoroughly examined. In this paper, by simulating the HHG emitted from the CO molecule under interactions with few-cycle laser pulses of different parameters, we demonstrate that the laser pulse duration should be greater than three cycles and less than six cycles. Furthermore, the carrier-envelope phase of the laser pulse should be within the ranges of ($2\pi/3$,~$7\pi/6$) and ($5\pi/3$,~$2\pi$). These optimal regions guide experiments in choosing laser settings to observe core-electron effects on the HHG spectrum via peak shifts.

Comprehensively and accurately characterizing matter information is the ultimate goal of physics and chemistry, which can be achieved by conventional electron diffraction (CED) and, recently, laser-induced electron diffraction (LIED). Remarkably, the latter method allows the retrieval of molecular structures during its rapid dynamical processes; however, the nuclear vibrational effect is still understudied. Moreover, if the vibrational effect in CED and LIED is substantial, a natural question is whether it is possible to reconstruct anharmonic interatomic potential from their measurements. The goal of this paper is to study the vibrational effect in LIED images, and then to examine the possibility of retrieving anharmonic interatomic potential from CED and LIED. To achieve this goal, we simulate the diffraction images incorporating the nuclear vibrations induced either by thermal vibration in CED or by an intense laser interaction in LIED. We show that the vibrational effect in LIED is significantly enhanced compared to that in the CED due to the presence of intense laser interaction. As a consequence, the diffraction images in LIED are much more sensitive to the potential shape; thus, it is possible to retrieve anharmonic interatomic potential. We also indicate that in CED, this extraction is also possible but requires far more stringent measurements. Although the retrieval is performed for simple Morse potential in this study, it can be generalized to retrieve other parameterized anharmonic interatomic potentials. In our study, the LIED images are simulated numerically but can be experimentally measured as well.

We develop an approach to directly control the phase of an electron's sub-cycle motion in an intense laser field. The method is established by tuning a low-frequency electric field applied on a centrosymmetric gaseous target during its interaction with a few-cycle infrared laser pulse. We find a universal analytical relation between the low-frequency electric field and its induced harmonic frequency shift, derived by the strong-field approximation. This simple relation and its universality are confirmed numerically by directly solving the time-dependent Schrödinger equation. Moreover, we demonstrate the benefits of the discovered relation in in situ applications, including continuously and precisely tuning XUV waves and developing a method of comprehensively sampling the THz pulse.

Odd-even harmonic generation contains the system's dynamical symmetry breaking, and its decoding is desirable not only in understanding physics but also for applications of dynamics extraction. In this work we discover a simple universal relation between the odd-even harmonics and the asymmetry of the terahertz (THz)-assisted laser-atomic system: atoms in a primary midinfrared laser pulse combined with a THz laser. We demonstrate numerically and then derive analytically the dependence of the harmonic even-to-odd ratio on the THz electric field. Notably, the functional form of this dependency reveals a universal scaling independent of the parameters of both the primary pulse and atomic target. This universality inspires us to propose a pump-probe scheme for THz waveform sampling from the even-to-odd ratio, accessible from conventional compact setups.