激光偏振方向依赖的氩晶体中孤立阿秒脉冲产生（特邀）

The ultrafast motion of electrons in atoms， molecules， and condensed matter can generally involve attosecond timescales. Attosecond light pulse can provide unusual functionalities for probing， initiating， driving， and controlling the ultrafast electronic dynamics with unprecedented high temporal and spatial resolutions simultaneously. The progress of attosecond science is closely linked to the improvement of attosecond light sources in terms of shorter and more intense attosecond pulses. Indeed， following its first synthesis and characterization， with the tendency towards reducing the pulse durations and increasing the pulse intensities， attosecond light pulses have and will continue to open up new venues for studying both fundamental and applied sciences， enabling a number of exciting possibilities. Over the last decade， people have conducted a lot of explorations on new methods of generating single attosecond pulses both experimentally and theoretically. In principle， an isolated attosecond light pulse can be generated via HHG originating from coherent electron motion in atoms， molecules， clusters and bulk crystals exposed to intense few-cycle femtosecond laser pulses. Theoretically， HHG in the atomic case can be well understood in the framework of a semi-classical model consisting of three steps. First， an electron is ionized into the continuum by tunneling through the potential barrier formed by the atomic Coulomb field and the driving laser field. Then， the ionized electron gains energy while being accelerated by the driving laser field. Finally， the electron recombines to the parent ion with an energy release in the form of harmonic photons. The generated harmonic radiation that occurs on successive half-cycles of the driving laser is coherent， leading to the emission of odd harmonics. Ultrashort attosecond pulse can be obtained only when the low-harmonic orders are filtered out. In the last two decades， almost all advances in isolated attosecond laser sources were based on HHG from atoms exposed to intense driving laser pulses. The main problem of isolated attosecond pulse generated by HHG in atoms is its weak intensity and low generation efficiency. To increase the strength of isolated pulses， laser-crystal interaction may be an alternative method worth investigating because in bulk crystals the existence of multiple ionization and recombination sites， the high density and periodic structure makes for richer dynamics allowing the possibility of higher conversion efficiency. At present， it is safe to say that while HHG in atomic gases has been explored extensively， much less has been done for bulk crystals. Interestingly enough， NDABASHIMIYE G et al. reported a direct comparison of HHG in the solid and gas phases of Ar. They found that the HHG spectra of the noble gas solids exhibit multiple platforms， well beyond the atomic limits of the corresponding gas phase harmonics measured under similar conditions， implying that shorter attosecond pulses could be realized in solids. What is most interesting to us is that the dependence of HHG on the laser polarization direction with respect to the Ar crystal， which are currently little studied. We theoretically investigated optimal control of isolated attosecond pulse generation in an Ar crystal irradiated by few-cycle femtosecond pulse， employing quantum time-dependent density-functional theory method. We explored systematically the effect of different laser polarization directions on isolated attosecond pulses generation， showing that the laser polarization direction with respect to the crystal is a sensitive control parameter for producing isolated attosecond pulses. The results indicate that for an Ar crystal， the intensity of isolated attosecond pulses is maximal at an optimal laser polarization direction with respect to the crystal， demonstrating about 11-fold intensity enhancement compared with that generated in an Ar atom under the same driving laser pulses. Our results also suggest opportunities for future investigations for the optimal control of isolated attosecond pulse generation in bulk crystal solids.