We present a theoretical investigation of the influence of molecular alignment and orientation on elliptically polarized high-order harmonic generation (HHG) from CO molecules exposed to a linearly polarized pulse. The results show that the ellipticity of HHG observed in a partially aligned or oriented CO molecular ensemble significantly deviates from that of the individual response. Additionally, we find that the ellipticity of HHG in the CO molecular ensemble exhibits strong dependence on the degree of molecular alignment/orientation as well as the time delay between the alignment/orientation and probe pulses. More importantly, under the proper alignment/orientation angle, a large HHG ellipticity can be achieved from the partially aligned/oriented molecular ensemble at a moderate degree of alignment/orientation. These findings relax the experimental requirement of a high degree of molecular alignment/orientation for harmonic generation with large ellipticity.

We theoretically demonstrate a scheme to generate circularly polarized (CP) isolated attosecond pulses (IAPs) with tunable helicity using a polarization gating laser field interacting with the CO molecule. The results show that a broadband CP supercontinuum is produced from the oriented CO molecule, which supports the generation of an IAP with an ellipticity of 0.98 and a duration of 90 as. Furthermore, the helicity of the generated harmonics and IAP can be effectively controlled by modulating the laser field and the orientation angle of the CO molecule. Our method will advance research on chiral-specific dynamics and magnetic circular dichroism on the attosecond timescale.

Photoacoustic (PA) imaging is a non-invasive imaging technique with high optical resolution and acoustic penetration depth, which has been widely used in medical and clinical researches. As an important part in functional PA imaging, photoacoustic mechanical imaging (PAMI) has great potential in visualizing and understanding the local development of pathological process. This review provides lots of PA breakthroughs which have been made in elasticity detection, viscosity detection, and viscoelasticity detection through PAMI techniques. The current research problems, challenges and future development directions were discussed.

Photoionization dynamics of bounded electrons in the ground state, the first and second excited states of a hydrogen atom, triggered by ultrashort near-infrared laser pulses, have been investigated in a transition regime ($\gamma \sim 1$) that offers both multiphoton and tunneling features. Significant differences in spectral characteristics are found between the three low-energy states. The H($2s$) ionization probability is larger than the H($2p$) value with a special oscillating structure, but both are much greater than the ground state H($1s$) in a wide range of laser intensities. By comparing the momentum spectrum and angular distributions of low-energy photoelectrons released from these degenerate states, we find the H($2p$) state shows a stronger long-range Coulomb attraction force than the H($2s$) state on account of the difference in the initial electron wave packet. Furthermore, analysis of the photoelectron momentum distributions sheds light on both the first and second excited states with a symmetrical intercycle interference structure in a multicycle field but an intracycle interference of an asymmetric left-handed or right-handed rotating spectrum in a few-cycle field. By analyzing photoelectron spectroscopy, we identify the parity characteristics of photoelectrons in different energy intervals and their corresponding above-threshold single-photon ionization (ATSI) or above-threshold double-photon ionization (ATDI) processes. We finally present the momentum distributions of the electrons ionized by laser pulses with different profiles and find the carrier-envelope phase (CEP) is a strong factor in deciding the rotating structure of the emission spectrum, which provides a new method to distinguish the CEP of few-cycle pulses.