Modulation of High-Order Harmonic Generation from a Monolayer ZnO by Co-rotating Two-Color Circularly Polarized Laser Fields

By numerically solving the two-dimensional semiconductor Bloch equation,we study the high-order harmonic emission of a monolayer ZnO under the driving of co-rotating two-color circularly polarized laser pulses.By changing the relative phase between the fundamental frequency field and the second one,it is found that the harmonic intensity in the platform region can be significantly modulated.In the higher order,the harmonic intensity can be increased by about one order of magnitude.Through time-frequency analysis,it is demonstrated that the emission trajectory of monolayer ZnO can be controlled by the relative phase,and the harmonic enhancement is caused by the second quantum trajectory with the higher emission probability.In addition,near-circularly polarized harmonics can be generated in the co-rotating two-color circularly polarized fields.With the change of the relative phase,the harmonics in the platform region can be altered from left-handed near-circularly polarization to right-handed one.Our results can obtain high-intensity harmonic radiation with an adjustable ellipticity,which provides an opportunity for syntheses of circularly polarized attosecond pulses.

Conical Intersection versus Avoided Crossing:Geometric Phase Effect in Molecular High-Order Harmonics

Nonadiabatic dynamics around an avoided crossing or a conical intersection play a crucial role in the photoinduced processes of most polyatomic molecules.The present work shows that the topological phase in conical intersection makes the behavior of pump-probe high-order harmonic signals different from the case of avoided crossing.The coherence built up when the system crosses the avoided crossing will lead to the oscillatory behavior of the spectrum,while the geometric phase erodes these oscillations in the case of conical intersection.Additionally,the dynamical blueshift and the splitting of the time-resolved spectrum allow capturing the snapshot dynamics with the sub-femtosecond resolution.

Strain effect on the orientation-dependent harmonic spectrum of monolayer aluminum nitride

In this study, we theoretically investigate the strain effect on the orientation-dependent high-order harmonic generation(HHG) of monolayer aluminum nitride(Al N) by solving the multiband semiconductor Bloch equations in strong laser fields. Our simulations denote that the efficiency of the orientation-dependent HHG is considerably enhanced when a 15% biaxial tensile strain is applied to Al N, which is attributed to the downshifting energy level of the conduction band. Furthermore, the odd-even feature in the orientation-dependent high harmonic spectra owing to the strain is considerably different when compared with that in the case without strain. The enhanced quantum interference between different energy bands in strained Al N around the Γ-M direction is responsible for the observed odd-even distributions of the orientation-dependent HHG. This study helps to better understand the HHG in solids by tuning their electronic structures.

The relative and combined effects of herbivore assemblage and soil nitrogen on plant diversity

Plant diversity can be affected by both herbivore grazing and soil resources. However, it is unclear if the joint effects of herbivores and soil resources might vary with components of plant diversity. Here, we evaluated the relative and combined effects of herbivore assemblage and soil nitrogen(N) quantity and heterogeneity on the α and β components of plant diversity in a grassland that was subjected to four years of grazing under differing herbivore assemblages(no grazing, cattle grazing, sheep grazing, and mixed grazing). We found that herbivore assemblage combined with soil N quantity explained 41% of the variation in plant α-diversity, while herbivore assemblage combined with soil N heterogeneity explained 15% of the variation in plant β-diversity. The independent effects of herbivore assemblage explained more than those of soil N for both α-and β-diversity(α-diversity: 12% vs. 4%;β-diversity: 18% vs. 16%). We concluded that the effects of herbivores are stronger than those of soil N,and that grazing-induced changes in soil resources are important drivers of plant diversity change, especially α-diversity.Therefore, we suggest that managing herbivore species by accounting for the effects that their grazing can have on soil resources may be significant for plant diversity maintenance.

Effects of changes in precipitation on energy and water balance in a Eurasian meadow steppe

Introduction:Water such as precipitation is the most critical environment driver of ecosystem processes and functions in semi-arid regions.Frequency and intensity of drought and transient waterlogging are expected to increase in the meadow steppe in northeastern China.Using a 4-year dataset of eddy covariance flux measurements,ground measurements of biomass,phenology,and meteorological conditions,we investigated the changes in energy fluxes at multiple temporal scales and under different precipitation regimes.Results:The meadow steppe was latent heat(LE)dominated when soil water content was>0.3 m3 m−3,but switched to sensible heat(H)dominated status when soil water content fell below 0.3 m3 m−3.LE dominated the energy exchange of the meadow grasslands on a yearly basis.Intensive precipitation had a profound impact on water-energy balance that could reduce the damages of drought by elevating deep soil moisture.The influence of LE on waterlogging depended on timing,with increased LE at the beginning of growing season and decreased LE after waterlogging.Spring and summer droughts resulted in different energy partitioning between latent and sensible heat energies,with spring drought dramatically decreased the LE fraction due to the change in water.In contrast,summer drought had little impact on LE due to the sufficient water input from large precipitation events at the beginning of the growing season.Conclusions:There existed great seasonal and interannual variabilities in energy balance and partitioning in the meadow steppe over the 4-year study period,which were strongly influenced by changes in precipitation.The water loss through latent heat was more sensitive to spring drought than to summer drought,while summer drought had negligible impact on LE.Waterlogging contributed to LE by enhancing its values during and after the waterlogged periods at the beginning of the growing season in a dry year,but lowering its value after the waterlogged periods in growing season.