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We investigate free-space lasing actions from molecular nitrogen ions (N2+) at the wavelengths of ~391 nm and ~428 nm. Our results show that pronounced gain can be measured at either 391 nm or 428 nm laser wavelength with a pump laser centered at 800 nm wavelength, whereas the gain at 391 nm laser wavelength completely disappears when the wavelength of the pump laser is tuned to 1500 nm. Our theoretical analysis reveals that the different gain behaviors can be attributed to the vibrational distribution of populations in X2Σg+(v=0) and X2Σg+(v=1) states as the N2+ ions are generated by photoionization in the laser fields, giving rise to more robust (i.e., less sensitive to the pump laser wavelength) population inversion for generating the 428 nm laser.

We carried out a joint theoretical and experimental study of the polarization of high-order harmonics generated from ZnO by intense infrared laser pulses. Experimentally we found that the dependence of parallel and perpendicular polarizations on the crystal orientation for all odd harmonics are nearly identical, but they are quite different from even harmonics which also show little order dependence. A one-dimensional two-band model, combined with a linear coupled excitation model, is shown to be able to explain the observed polarization behavior, including low-order harmonics. We further note that the same odd/even order contrast have been reported in a number of other crystals, despite that the harmonics were perceived to be generated via entirely different mechanisms. We demonstrated that this universality is governed by crystal symmetry, not by specific mechanisms. Thus, polarization measurements of harmonics offers a powerful pure optical method for determining the crystal axes as well as monitoring their ultrafast changes when crystals are undergoing deformation. In addition, the ellipticity of harmonic has been studied. It shows that ellipticity of high-order harmonics from solids can be tuned precisely by changing the bond structure of the sample., 17 pages, 10 figures, regular paper

High-order harmonic spectra from solid materials driven by single-color multicycle laser fields sometimes contain even harmonics. In this work we attribute the appearance of even harmonics to the nonzero transition dipole phase (TDP) when the solid system has broken symmetry. By calculating the harmonic efficiency from graphene and gapped graphene by using the semiconductor Bloch equations under the tight-binding approximation, we demonstrate the role of the TDP, which has been ignored for a long time. When the crystal has inversion symmetry, or reflection symmetry with the symmetry plane perpendicular to the laser polarization direction, the TDP can be neglected. Without such symmetry, however, the TDP will lead to the appearance of even harmonics. We further show that the TDP is sensitive to the crystal geometry. To extract the structure information from the harmonic spectra of a solid the TDP cannot be ignored.

Using quantum mechanics calculations, we theoretically study the dissociation and ionization dynamics of the hydrogen-molecule ion in strong laser fields. Having prepared the nuclear wave packet of H2+ in a specific vibrational state, a pump laser is used to produce a vibrational excitation, leading to quasi-periodical vibration without ionization. Then, a time-delayed few-cycle laser is applied to trigger the dissociation or ionization of H2+. Both the time delay and the intensity of the probe laser alter the competition between dissociation and ionization. We also explore the dependence of kinetic-energy release spectra of fragments on the time delay, showing that the channels of above-threshold dissociation and below-threshold dissociation are opened and closed periodically. Also, dissociation from different channels is influenced by nuclear motion. The dissociation mechanism has been described in detail using the Floquet picture. This work provides a useful method for steering the electronic and nuclear dynamics of diatomic molecules in intense laser fields.

We investigate the dependence of charge transfer on the intensity of driving laser field when SiO2 crystal is irradiated by an 800 nm laser. It is surprising that the direction of charge transfer undergoes a sudden reversal when the driving laser intensity exceeds critical values with different carrier envelope phases. By applying quantum-trajectory analysis, we find that the Bloch oscillation plays an important role in charge transfer in solid. Also, we study the interaction of strong laser with gallium nitride (GaN) that is widely used in optoelectronics. A pump-probe scheme is applied to control the quantum trajectories of the electrons in the conduction band. The signal of charge transfer is controlled successfully by means of theoretically proposed approach.

By solving the time-dependent Schrödinger equation both in simplified one-dimensional coordinate and three-dimensional cylindrical coordinate systems, the high-order harmonic generation from Hsub2/subsup+/supin spatially symmetric and asymmetric nonhomogeneous laser fields was studied. At large internuclear distances, minima were clearly observed in high energy part of harmonic spectra, which can be attributed to two-center interference in diatomic molecule. Compared with previous studies, the minima in nonhomogeneous laser field are more distinct. Remarkably, the positions of the minima are different in these two types of fields, which demonstrate that interference effects are greatly influenced by laser parameters. Besides, the asymmetric nonhomogeneous field leads to an asymmetric recollision of the ionized electron, and both odd and even order harmonics could be emitted, which is explained in detail based on quantum dynamics calculations.

High-order-harmonic generation in \ensuremath{\alpha}-quartz $\mathrm{Si}{\mathrm{O}}_{2}$ is theoretically investigated under a strong laser field by solving the extended semiconductor Bloch equations. The accurate band structures as well as dipole moments between different bands are obtained from state-of-the-art first-principles calculations. We find that the shapes of $k$-space-dependent dipole moments play an important role in harmonic generation. The calculated results show that harmonic conversion efficiency is significantly enhanced and the cutoff energy is distinctly increased when the dipole moments change greatly along a valley in the $k$ direction in the solid. Based on that dependence on the dipole moment, we also show that symmetry groups greatly affect the harmonic spectra from the solid materials. Moreover, a two-color synthesized field is used to achieve a supercontinuum harmonic spectrum near the cutoff region, and isolated attosecond pulses can be obtained directly by filtering out the harmonic radiation. We hope the contribution presented in this work provides a useful reference for future studies on laser-crystal interactions.

We carry out a combined theoretical and experimental investigation on the population distributions in the ground and excited states of tunnel ionized N2 molecules at various driver wavelengths in the near- and mid-infrared range. Our results reveal that efficient couplings (i.e., population exchanges) between the ground state and the excited states occur in strong laser fields. The couplings result in the population inversion between the ground and the excited states at the wavelengths near 800 nm, which is verified by our experiment by observing the amplification of a seed at ~391 nm. The result provides insight into the mechanism of free-space nitrogen ion lasers generated in remote air with strong femtosecond laser pulses., Comment: 18 pages, 4 figures

We investigate nuclear dynamics and population transfer among multiple electronic states in tunnel ionized nitrogen molecules, and clarify the underlying mechanism behind the population inversion responsible for free-space N 2 + lasers.

In the present study, differences in inorganic solute accumulation in shoots, radicles and cotyledons during the seedling stage of Vicia cracca Linn were evaluated in response to a range of sodium chloride (NaCl) concentrations. Seeds were sown in Petri dishes, germinated and grown with NaCl treatment for 10 days in a growth chamber, with a temperature of 20°C and a 12 h light/dark cycle. Results showed that percentage germination, germination rate, fresh weight and dry weight, and relative water content decreased as the NaCl concentration increased in shoots, radicles and cotyledons. There were no significant differences in dry weight/fresh weight ratios in shoots and radicles among treatments. However, the dry weight/fresh weight ratio in cotyledons was significantly higher at 200 mM NaCl compared to treatments with lower NaCl concentrations. Sodium+ and Cl− concentrations in shoots and radicles increased as the NaCl concentration increased. Sodium+ and Cl− concentrations in shoots and radicles were muc...

Background As a consequence of global change, intensity and frequency of extreme events such as heat waves (HW) have been increasing worldwide. Methods By using a combination of continuous 60-year meteorological and 6-year tower-based carbon dioxide (CO2) flux measurements, we constructed a clear picture of a HWs effect on the dynamics of carbon, water, and vegetation on the Eurasian Songnen meadow steppe. Results The number of HWs in the Songnen meadow steppe began increasing since the 1980s and the rate of occurrence has advanced since the 2010s to higher than ever before. HWs can reduce the grassland carbon flux, while net ecosystem carbon exchange (NEE) will regularly fluctuate for 4–5 days during the HW before decreasing. However, ecosystem respiration (Re) and gross ecosystem production (GEP) decline from the beginning of the HW until the end, where Re and GEP will decrease 30% and 50%, respectively. When HWs last five days, water-use efficiency (WUE) will decrease by 26%, soil water content (SWC) by 30% and soil water potential (SWP) will increase by 38%. In addition, the soil temperature will still remain high after the HW although the air temperature will recover to its previous state. Conclusions HWs, as an extreme weather event, have increased during the last two decades in the Songnen meadow steppe. HWs will reduce the carbon flux of the steppe and will cause a sustained impact. Drought may be the main reason why HWs decrease carbon flux. At the later stages of or after a HW, the ecosystem usually lacks water and the soil becomes so hot and dry that it prevents roots from absorbing enough water to maintain their metabolism. This is the main reason why this grassland carbon exchange decreases during and after HWs.

Amine-functionalized mesoporous and macroporous silica materials impregnated with transition-metal-monosubstituted polyoxometalate K6[Ni(H2O)SiW11O39] (SiW11Ni) clusters were prepared by coordination of nickel centers in the cluster with the amine surface groups in silica supports. The structures and compositions of the materials were characterized by UV–vis diffuse reflectance spectra (DR-UV–vis), infrared (IR) spectra, and 29Si MAS NMR, indicating that the SiW11Ni clusters were chemically attached to the silica supports, and the primary Keggin structure remained intact in the as-synthesized composites. The topologies and porosities of the materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and nitrogen adsorption determination. The heterogeneous photocatalytic behaviors of the materials were tested via degradation of dye rhodamine B (RB), and a RB degradation mechanism was proposed based on the detected intermediates and final products. That is, photocatalytic degradation of RB on the supported SiW11Ni underwent the successive steps of deethylation, deamination, decarboxylation, and cleavage of the chromophore ring structure, resulting in the final products of CO2, NO3−, and Cl− ions. A Drop of SiW11Ni clusters from the amine-modified silica matrix into the reaction system was hardly observed during the photocatalytic tests, attributed to the strong coordination interaction between the SiW11Ni molecule and the amine-modified silica support.

A new type of heterogeneous photocatalytic materials, microporous polyoxotungstates such as H 3 PW 12 O 40 /SiO 2 , H 4 SiW 12 O 40 /SiO 2 and Na 4 W 10 O 32 /SiO 2 , were synthesized by incorporating the polyoxotungstates into the silica matrix via a sol–gel technique, and some physico-chemical properties of as-synthesized composites were measured. The photocatalytic activity of the composites was tested via degradation of the model molecule, hydroxy-butanedioic acid (malic acid, (MA)). After 90 or 180 min irradiation in the presence of Na 4 W 10 O 32 /SiO 2 or H 3 PW 12 O 40 /SiO 2 (H 4 SiW 12 O 40 /SiO 2 ), an aqueous MA (100 mg/l) was totally degraded into the intermediates such as oxalic acid, glyceric acid, tartaric acid, butenedioic acid, acetic acid, formic acid and so on. After subsequent 90 or 60 min irradiation under the same conditions, these intermediates were totally mineralized into CO 2 and H 2 O. Disappearance of MA (in the range of 0–200 mg/l) followed Langmuir–Hinshelwood first-order kinetics, and the reaction rate constant and adsorption equilibrium constant were measured. Most of the intermediates, identified during the process of H 4 SiW 12 O 40 /SiO 2 -photocatalyzed MA degradation, are the same with those detected during the process of TiO 2 -photocatalyzed MA degradation. The mechanism studies showed that the desolubilized polyoxotungstates appears to photo-oxidative degradation of MA by both OH radical attack and excited state polyoxotungstate direct oxidation.

We present a theoretical investigation of high-order harmonic generation in spatially inhomogeneous two-color laser fields by solving three dimensional time dependent Schrodinger equation. The cutoff in the harmonic spectra can be significantly extended by means of our proposed method (i.e., from helium interacting with the plasmon-enhanced two-color laser fields), and an ultrabroad supercontinuum up to 1.5 keV is generated by selecting proper carrier-envelope phase of the controlling field. Moreover, classical trajectory extraction, time-dependent ionization and recombination rates, and time-frequency analyses are used to explain the generation of this ultrabroadband supercontinuum. As a result, an isolated 8.8 attosecond pulse can be generated directly by the superposition of the supercontinuum harmonics.

We theoretically investigate harmonic generation driven by intense midinfrared lasers. A valleylike structure is observed at very low-order harmonics because of a low-energy photoelectron suppression effect. Further, at the beginning of a broad supercontinuum, a convex structure appears that is distributed from the tail in a very narrow energy band in time-frequency maps. Surprisingly, our quantum dynamics calculations demonstrate the beneficial wavelength scaling of the harmonic yield to be ${\ensuremath{\lambda}}^{4.6}$ for He and ${\ensuremath{\lambda}}^{5.1}$ for Ne over selected energy windows. The bandwidth of the harmonic plateau with only a single quantum trajectory contribution can be further extended by adding a controlling laser field, and the harmonic efficiency is found to be significantly enhanced after macroscopic propagation. In addition, ultrashort isolated attosecond pulses can be obtained by properly superposing the harmonics in the plateaus of both the He and Ne systems.

We present a systematic study of high-order harmonic generation (HHG) from helium ion with the initial state prepared as a coherent superposition of electronic ground state and an excited state. As a result, the conversion efficiency of the harmonic spectrum is significantly enhanced. When we add a static electric field in fundamental field, the supercontinuum region of the harmonic spectrum is distinctly extended and an isolated 100 as pulse can be generated. Moreover, we use a spatial nonhomogeneous field to increase the cutoff energy in high-order harmonic generation spectrum, which can be extended to about 700 eV, and an isolated 50 as pulse can be obtained directly by the superposition of the supercontinuum harmonics.

By solving the time-dependent Schrodinger equation both in simplified one-dimensional coordinate and three-dimensional cylindrical coordinate systems, the high-order harmonic generation from H2 + in spatially symmetric and asymmetric nonhomogeneous laser fields was studied. At large internuclear distances, minima were clearly observed in high energy part of harmonic spectra, which can be attributed to two-center interference in diatomic molecule. Compared with previous studies, the minima in nonhomogeneous laser field are more distinct. Remarkably, the positions of the minima are different in these two types of fields, which demonstrate that interference effects are greatly influenced by laser parameters. Besides, the asymmetric nonhomogeneous field leads to an asymmetric recollision of the ionized electron, and both odd and even order harmonics could be emitted, which is explained in detail based on quantum dynamics calculations.

The use of iris loaded cavities to accelerate high current bunched beams makes it important to understand the impedance and wake fields caused by an iris in a beam pipe. In this paper we present a method to calculate both the longitudinal and transverse impedance of a circular iris of radius [ital b] and thickness [ital g] in a circular beam pipe of radius [ital a] for ultrarelativistic particles. An integral equation is derived for the transverse electric field at the junction between the iris and the beam pipe and a variational expression is obtained for the impedance, using the transverse field as a trial function. Accurate numerical results are obtained for the longitudinal and transverse impedances using a trial function with only a few adjustable parameters. By invoking causality we confirm the analytic behavior of the impedances in the complex frequency plane and obtain the corresponding wake functions. We particularly explore the limit [ital b][r arrow][infinity] to compare with previous studies of the impedance of a circular hole in a transverse metallic plane.

Based on the field experiment on Songnen grassland, a new method was established to ameliorate the secondary bare alkali-saline patches (SAP) through inserting cornstalk. The experiment was rested on the assumption that through inserting cornstalk in the secondary bare alkali-saline patches (SAP) to retain seeds moving over its surface, the necessary seed source could be gained; and these seeds should be able to germinate and survive successfully on the cornstalk itself or in its neighborhood, where should be more fit to grow than other sites in SAP, due to the decomposition of cornstalk and its special role, so that, the aim to restore vegetation of SAP could be achieved at a pretty low cost and rapid speed. The results showed that the seed bank in soil was increased significantly, owing to the inserted cornstalk and its operating processes. The seed number in ameliorated soil was 4020.0 +/- 1773.6 seeds x m(-2), while that in the secondary bare alkali-saline patches (SAP) was only 10.0 +/- 31.6 seeds x m(-2). Although the soil chemical and physical characters in ameliorated zone were improved to some extent, the overall situation of soil was still bad for plant growth, as the pH, soluble saline ion and organic matter were concerned. Most of Chloris virgata grew around or on the cornstalk, the plants around each cornstalk being 3.9 +/- 2.2, and the total being 48.64 +/- 38.72 g x m(-2). Therefore, this method demanded a few resources, and needed simple technology and low cost, which is potentially deserved to popularize.

In a recent paper, we constructed a variational form to calculate both the longitudinal and transverse impedance of a thick iris in a beam pipe. Implementation of this calculation led to rapidly converging and accurate values for these impedances for a circular beam pipe and iris. We have now constructed an analogous variational form for the longitudinal and transverse impedance of periodic irises in a beam pipe, with similar convergence and accuracy properties. In this case the numerically calculated impedance is imaginary, except for isolated narrow resonances corresponding to modes propagating with the velocity of light. The real part of impedance is obtained by using causality. Analysis and numerical results are discussed and presented.