Simple and efficient high-energy few-cycle pulse generation through hybrid MPC and filamentation.

Recent developments in intense few-cycle laser pulse generation have triggered many breakthrough experiments in high-field science and have paved the way toward time-resolved spectroscopy on the attosecond (10−18 s) timescale []–[]. In particular, it was shown that intense driving pulses with only two to three reproducible field oscillations are a prerequisite for controlling the generation of isolated attosecond pulses by high-order harmonic generation in rare gases []. Until recently, intense few-cycle pulses could only be obtained through spectral broadening of amplified femtosecond laser pulses in a gas-filled, hollow-core fiber, followed by chirped mirror compression [],[]. Although successful, the hollow-fiber compression technique limits the pulse energy available for experiments to a few hundred microjoules. However, exploring laser-matter interactions in the relativistic intensity (> 1018 W · cm−2) regime requires significantly higher focused pulse energies []. When confined in space and time to a volume of a few λ3, femtosecond laser pulses can reach relativistic intensities with pulse energies close to 1 mJ []. This leads to strong nonlinear laser-plasma interactions such as relativistic reflection, deflection, and compression of a few-cycle femtosecond pulse down to the attosecond regime with an efficiency up to 10% []. One major requirement for this generation technique is the availability of carrier-envelope phase (CEP) stabilized few-cycle pulses with microjoule energy. We therefore explored a novel scheme for high-energy, CEP-stable few-cycle pulse generation, which is easy to handle and which overcomes the energy barrier of the hollow fiber approach.

Statistical size scaling of breakage strength of irregularly-shaped particles

We report on dispersion management in mid-IR optical parametric chirped pulse amplifiers (OPCPA) aiming for high-energy few-cycle pulses beyond 4 µm. The available pulse shapers in this spectral region limit the feasibility of sufficient higher-order phase control. Intending the generation of high energy pulses at 12 µm via DFG driven by the signal and idler pulses of a midwave-IR OPCPA, we introduce alternative approaches for mid-IR pulse shaping, namely a germanium-prism pair and a sapphire-prism Martinez compressor. Furthermore, we explore the limits of bulk compression in Si and Ge for multi-mJ pulse energies.

Calculation of the spallation target neutronic parameters in Accelerator Driven Subcritical TRIGA reactor

The cell seeding density and spatial distribution in a 3-D scaffold are critical to the morphogenetic development of an engineered tissue. A dynamic depth-filtration seeding method was developed to improve the initial cell seeding density and spatial distribution in 3-D nonwoven fibrous matrices commonly used as tissue scaffolds. In this work, trophoblast-like ED27 cells were seeded in poly(ethylene terephthalate) (PET) matrices with various porosities (0.85-0.93). The effects of the initial concentration of cells in the suspension used to seed the PET matrix and the pore size of the matrix on the resulting seeding density and subsequent cell proliferation and tissue development were studied. Compared to the conventional static seeding method, the dynamic depth-filtration seeding method gave a significantly higher initial seeding density (2-4 x 10(7) vs 4 x 10(6) cells/cm3), more uniform cell distribution, and a higher final cell density in the tissue scaffold. The more uniform initial cell spatial distribution from the filtration seeding method also led to more cells in S phase and a prolonged proliferation period. However, both uniform spatial cell distribution and the pore size of the matrices are important to cell proliferation and morphological development in the seeded tissue scaffold. Large-pore matrices led to the formation of cell aggregates and thus might reduce cell proliferation. The dynamic depth-filtration seeding method is better in providing a higher initial seeding density and more uniform cell distribution and is easier to apply to large tissue scaffolds. A depth-filtration model was also developed and can be used to simulate the seeding process and to predict the maximum initial seeding densities in matrices with different porosities.

A method for efficiently generating an isolated single-cycle attosecond pulse is proposed. It is shown that the ultraviolet (UV) attosecond (as) pulse can be utilized as a robust tool to control the dynamics of electron wave packets (EWPs). By adding a UV attosecond pulse to an infrared (IR) few-cycle pulse at a proper time, only one return of the EWP to the parent ion is selected to effectively contribute to the harmonics; then, an isolated two-cycle $130\text{\ensuremath{-}}\mathrm{as}$ pulse with a bandwidth of $45\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ is obtained. After complementing the chirp, an isolated single-cycle attosecond pulse with a duration less than $100\phantom{\rule{0.3em}{0ex}}\mathrm{as}$ seems achievable. In addition, the contribution of the quantum trajectories can be selected by adjusting the delay between the IR and UV fields. Using this method, the harmonic and attosecond pulse yields are efficiently enhanced in contrast to the scheme [G. Sansone et al., Science 314, 443 (2006)] using a few-cycle IR pulse in combination with the polarization gating technique.

Contemporary ultrafast science requires reliable sources of high-energy few-cycle light pulses. Currently two methods are capable of generating such pulses: post compression of short laser pulses and optical parametric chirped-pulse amplification (OPCPA). Here we give a comprehensive overview on the post-compression technology based on optical Kerr-effect or ionization, with particular emphasis on energy and power scaling. Relevant types of post compression techniques are discussed including free propagation in bulk materials, multiple-plate continuum generation, multi-pass cells, filaments, photonic-crystal fibers, hollow-core fibers and self-compression techniques. We provide a short theoretical overview of the physics as well as an in-depth description of existing experimental realizations of post compression, especially those that can provide few-cycle pulse duration with mJ-scale pulse energy. The achieved experimental performances of these methods are compared in terms of important figures of merit such as pulse energy, pulse duration, peak power and average power. We give some perspectives at the end to emphasize the expected future trends of this technology.

Controlled few-cycle light waveforms find numerous applications in attosecond science, most notably the production of isolated attosecond pulses in the XUV spectral region for studying ultrafast electronic processes in matter. Scaling up the pulse energy of few-cycle pulses could extend the scope of applications to even higher intensity processes, such as the generation of attosecond pulses with extreme brightness from relativistic plasma mirrors. Hollow-fiber compressors are widely used to produce few-cycle pulses with excellent spatiotemporal quality, whereby octave-spanning broadened spectra can be temporally compressed to near-single-cycle duration. In order to scale up the peak power of hollow-fiber compressors, the effective length and area mode of the fiber has to be increased proportionally, thereby requiring the use of longer waveguides with larger apertures. Thanks to an innovative design utilizing stretched flexible capillaries, we show that a stretched hollow-fiber compressor can generate pulses of TW peak power, the duration of which can be continuously tuned from the input seed laser pulse duration down to almost a single cycle (3.5fs at 750nm central wavelength) simply by increasing the gas pressure at the fiber end. The pulses are characterized online using an integrated d-scan device directly under vacuum. While the pulse duration and chirp are tuned, all other pulse characteristics, such as energy, pointing stability and focal distribution remain the same on target. This unique device makes it possible to explore the generation of high-energy attosecond XUV pulses from plasma mirrors using controllable relativistic-intensity light waveforms at 1kHz.

This study investigated the effects of various seasonal fitting techniques on the spatial distribution of the common mode errors taking the coordinate time series of the continuous GPS reference stations of the Crustal Movement Observation Network of China (CMONOC) as an example. First, the seasonal term of coordinate time series was calculated using constant amplitude harmonic fitting (CAF), continuous wavelet transform (CWT), and smoothing spline fitting (SPF). The seasonal term and linear trend were then removed to obtain the residual time series. Finally, to determine the common mode errors of residual time series, principal component analysis (PCA) was applied. The results indicate that 1) smoothing spline fitting is superior to constant amplitude harmonic fitting and continuous wavelet transform in its ability to fit short-term irregular seasonal signals. In comparison to constant amplitude harmonic fitting, N/E/U has root mean square error (RMSE) values of smoothing spline fitting that are lower by 25%, 20%, and 14.4%, respectively. Smoothing spline fitting also has a higher coefficient of determination than continuous wavelet transform and constant amplitude harmonic fitting. The coefficient of determination in the U direction is larger than that in the N and E directions. 2) Each order PC of the residual series fitted by smoothing spline fitting exhibits apparent spatial aggregation characteristics, with PC1 having a uniform spatial distribution and presenting a largely positive response. Nevertheless, the residual series obtained by constant amplitude harmonic fitting and continuous wavelet transform exhibits scattered spatial response distribution features in each order PC. Compared to N and E, U’s spatial response distribution is distinct. From north to south, the spatial response of PC1 in the U direction progressively diminishes. In addition to being much lower than that in other locations, the Sichuan–Yunnan region’s spatial response value of PC1 and PC3 also exhibits a clear negative reaction. The root mean square error value of the residual series after smoothing spline fitting is the least, and the filtering effect is the best when comparing the spatial filtering effect based on the three fitting methods. We also compared the root mean square error reduction ratio before and after spatial filtering, and the results showed that the root mean square error reduction ratio before and after the residual series obtained by smoothing spline fitting is slightly larger than that obtained by other methods.

Pesticide use in orchards creates drift-driven pesticide losses which contaminate the environment. Trunk injection of pesticides as a target-precise delivery system could greatly reduce pesticide losses. However, pesticide efficiency after trunk injection is associated with the underinvestigated spatial and temporal distribution of the pesticide within the tree crown. This study quantified the spatial and temporal distribution of trunk-injected imidacloprid within apple crowns after trunk injection using one, two, four or eight injection ports per tree. The spatial uniformity of imidacloprid distribution in apple crowns significantly increased with more injection ports. Four ports allowed uniform spatial distribution of imidacloprid in the crown. Uniform and non-uniform spatial distributions were established early and lasted throughout the experiment. The temporal distribution of imidacloprid was significantly non-uniform. Upper and lower crown positions did not significantly differ in compound concentration. Crown concentration patterns indicated that imidacloprid transport in the trunk occurred through radial diffusion and vertical uptake with a spiral pattern. By showing where and when a trunk-injected compound is distributed in the apple tree canopy, this study addresses a key knowledge gap in terms of explaining the efficiency of the compound in the crown. These findings allow the improvement of target-precise pesticide delivery for more sustainable tree-based agriculture.

These studies address critical technical issues involved in creating human mesenchymal stem cell (hMSC)/ scaffold implants for cartilage repair. These issues include obtaining a high cell density and uniform spatial cell distribution within the scaffold, factors that are critical in the initiation and homogeneity of chondrogenic differentiation. For any given scaffold, the initial seeding influences cell density, retention, and spatial distribution within the scaffold, which eventually will affect the function of the construct. Here, we discuss the development of a vacuum-aided seeding technique for HYAFF -11 sponges which we compared to passive infiltration. Our results show that, under the conditions tested, hMSCs were quantitatively and homogeneously loaded into the scaffolds with 90+% retention rates after 24 h in perfusion culture with no negative effect on cell viability or chondrogenic potential. The retention rates of the vacuum-seeded constructs were at least 2 times greater than those of passively seeded constructs at 72 h. Histomorphometric analysis revealed that the core of the vacuum-seeded constructs contained 240% more cells than the core of passively infiltrated scaffolds. The vacuum seeding technique is safe, rapid, reproducible, and results in controlled quantitative cell loading, high retention, and uniform distribution.

Despite extensive efforts to explore femtosecond lasers functionalized by nonlinear graphene (Gf) that relies on the traditional transfer process, maximizing the efficiency, customizing the nonlinear interaction, and minimizing the optical loss remain critical challenges, especially in high-energy pulse generation. We demonstrate an ultrafast nonlinear all-fiber device based on conformal Gf directly synthesized in three dimensions on the surface of an in-fiber microstructure. A femtosecond laser-induced selective etching process is used to fabricate a customized microstructure that ensures the minimum but efficient laser-Gf interaction as well as possesses excellent surface conditions to suppress absorption and scattering loss. Conformal Gf is prepared by a spatial diffusion-based atomic carbon spraying process that enables nanocrystals to be synthesized homogeneously even onto the complex surface of the microstructure. The demonstration of high-energy pulses from the Gf saturable absorber highlights its simple, process-efficient, adjustable, and robust performance. The resultant hyperbolic secant pulses display individual pulse energy and peak power of up to 13.2 nJ and 20.17 kW, respectively.

Chip-scale, high-energy optical pulse generation is becoming increasingly important as integrated optics expands into space and medical applications where miniaturization is needed. Q-switching of the laser cavity was historically the first technique to generate high-energy pulses, and typically such systems are in the realm of large bench-top solid-state lasers and fibre lasers, especially in the long wavelength range >1.8 µm, thanks to their large energy storage capacity. However, in integrated photonics, the very property of tight mode confinement that enables a small form factor becomes an impediment to high-energy applications owing to small optical mode cross-sections. Here we demonstrate a high-energy silicon photonics-based passively Q-switched laser with a compact footprint using a rare-earth gain-based large-mode-area waveguide. We demonstrate high on-chip output pulse energies of >150 nJ and 250 ns pulse duration in a single transverse fundamental mode in the retina-safe spectral region (1.9 µm), with a slope efficiency of ~40% in a footprint of ~9 mm2. The high-energy pulse generation demonstrated in this work is comparable to or in many cases exceeds that of Q-switched fibre lasers. This bodes well for field applications in medicine and space.

The generation of high-energy dual-wavelength domain wall pulse with a low repetition rate is demonstrated in a highly nonlinear fiber (HNLF)-based fiber ring laser. By introducing the intracavity birefringence-induced spectral filtering effect, the dual-wavelength lasing operation can be achieved. In order to enhance the cross coupling effect between the two lasing beams for domain wall pulse formation, a 215-m HNLF is incorporated into the laser cavity. Experimentally, it is found that the dual-wavelength domain wall pulse with a repetition rate of 77.67 kHz could be efficiently obtained through simply rotating the polarization controller (PC). At a maximum pump power of 322 mW, the 655-nJ single pulse energy in cavity is obtained. The proposed configuration provides a simpler and more efficient way to generate high energy pulse with a low repetition rate.

Abstract. This study investigated drought and flooding changes in West Africa between 1983–2012 and projected near future (2025–2054) periods. The datasets used are the CHIRTS and CHIRPS-2 for observed reanalysis and five (05) models of ISIMIP2b for Shared Socio-economic Pathways (SSP1.2-6 and SSP5-8.5). Extremely and very wet days total precipitation (R95pTOT; R99pTOT) and Standardized Precipitation Evapotranspiration Index (SPEI) were employed to investigate floods and drought spatial distribution using Sen Slope trend analysis method. The results showed that there is a variability in the spatial distribution of extreme indices with an upward and downward trend of dry and wet rainfall periods in West Africa in both historical and projected periods. This observation suggests that the study area is faced with rainfall variability marked by extreme events. A further examination on the spatial and temporal distribution of flood occurrence showed that more flood events were observed in the Gulf of Guinea and Savannah countries, followed by an increase in uniform spatial distribution and moderate wet days both under SSP1.2.6, and SSP 5.8.5. In addition, result showed that an upward trend in wet periods can cause the occurrence of extreme events associated with floods in the context of global warming. However, with these scenarios negative changes are not excluded in the East, the Sahel and some western part of the Gulf of Guinea in the study area for the SSP5.8.5 scenario. Thus, the results revealed that the spatio- temporal variability of extreme rainfall can have repercussions on the hydrological functioning of watersheds, water availability and water-dependent activities.