We theoretically demonstrate a scheme to produce elliptically polarized isolated attosecond pulses, based on the high-order harmonic generation from the current-carrying state of an atom exposed in linearly polarized laser fields. It is shown that the nonzero ellipticity of the attosecond pulse generated at the single-atom level is attributed to the nonvanishing angular momentum of the state, which is further elucidated by the strong-field approximation model. With specific carrier envelope phases (CEPs), isolated attosecond pulses with large ellipticity can be obtained. Moreover, the ellipticity of high-order harmonics remains almost invariable versus the CEP of the few-cycle driving field. Hence, the temporal profile and ellipticity of the attosecond-pulse radiation can be manipulated separately, which is beneficial to shape the attosecond-pulse radiation without compromising the ellipticity. The obtained elliptical attosecond pulse may serve as a potential tool to explore and manipulate the ultrafast dynamics in magnetic materials and chiral media.

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 investigate the attosecond streaking spectrum of ${\mathrm{H}}_2{}^{+}$ by numerically solving the time-dependent Schrödinger equation. The results show that the streaking spectrum of ${\mathrm{H}}_2{}^{+}$ depends sensitively on the internuclear distance $R$. In addition to the regular streaking structure, some streaking spectra of ${\mathrm{H}}_2{}^{+}$ with the splitting and distorted structures are also observed as $R$ changes. The splitting and distorted streaking spectra of ${\mathrm{H}}_2{}^{+}$ are demonstrated to be related to the destructive two-center interference and resonant transition triggered by the IR field, respectively. Meanwhile, the mixture structure of split and distortion on the streaking spectrum of ${\mathrm{H}}_2{}^{+}$ is attributed to the coupling of the destructive two-center interference and resonant transition. The $R$-dependent streaking spectra might pave an accessible route toward monitoring the ultrafast nuclear dynamics of molecules.