Harnessing Atomically Dispersed Cobalt for the Reductive Catalytic Fractionation of Lignocellulose.

The reductive catalytic fractionation (RCF) of lignocellulose, considering lignin valorization at design time, has demonstrated the entire utilization of all lignocellulose components; however, such processes always require catalysts based on precious metals or high-loaded nonprecious metals. Herein, the study develops an ultra-low loaded, atomically dispersed cobalt catalyst, which displays an exceptional performance in the RCF of lignocellulose. An approximately theoretical maximum yield of phenolic monomers (48.3 wt.%) from lignin is realized, rivaling precious metal catalysts. High selectivity toward 4-propyl-substituted guaiacol/syringol facilitates their purification and follows syntheses of highly adhesive polyesters. Lignin nanoparticles (LNPs) are generated by simple treatment of the obtained phenolic dimers and oligomers. RCF-resulted carbohydrate pulp are more obedient to enzymatic hydrolysis. Experimental studies on lignin model compounds reveal the concerted cleavage of Cα-O and Cß-O pathway for the rupture of ß-O-4 structure. Overall, the approach involves valorizing products derived from lignin biopolymer, providing the opportunity for the comprehensive utilization of all components within lignocellulose.

Energy deposition in a telescopic laser filament for the control of fuel ignition.

The efficiency of energy coupled to plasma during femtosecond (fs) laser filamentation plays a decisive role in a variety of filament applications such as remote fabrication and spectroscopy. However, the energy deposition characterization in the fs laser filament formed by a telescope, which provides an efficient way to extend the filament distance, has not yet been revealed. In the present study, we show that when the distance between the two lenses in a telescope changes, i.e., the effective focal length changes, there exists an optimal plateau energy deposition region in which the energy deposited into the filament per unit length called the average lineic energy deposition (ALED) remains at high levels, exhibiting a remarkable difference from the monotonic change in a single-lens focusing system. As a proof of principle, we examined the influence of the energy deposition on the ignition of a lean methane/air mixture, and found that the use of the telescope can efficiently extend the ignition distance when compared with a single-lens focusing system under the same incident laser energy condition. Our results may help understand the energy deposition behaviors in a variety of telescopic filaments and provide more options to manipulating laser ignition at a desired distance.

Air-laser-based coherent Raman spectroscopy of atmospheric molecules in a filamentary plasma grating.

Coherent Raman spectroscopy (CRS) with air-laser-based hybrid femtosecond/picosecond (fs/ps) pulses has shown promising potential for remote detection and surveillance of atmospheric species with high temporal and frequency resolution. Here, to enhance the sensitivity and extend the detection distance, we generate the CRS spectra of air molecules in situ in a filamentary plasma grating, and show that the grating can efficiently enhance the intensities of the coherent vibrational Raman lines of N2, O2, and N2 + by 2-3 orders of magnitude at an extended distance. By examining the intensities of the Raman lines, fs-pulsed supercontinuum, and ps-pulsed air laser produced under different grating conditions, we reveal that the optimization of the Raman lines is achieved by the dynamic balance between the supercontinuum-induced vibrational coherence and air-laser-induced polarization of the air species.

Transition from triggered super-radiance to seed amplification in N2 + lasing.

Air lasing induced by laser filamentation opens a new route for research on atmospheric molecular physics and remote sensing. The generation of air lasing is composed of two processes, i.e., building up optical gain of air molecules in femtosecond time scale and emitting coherent radiation in picosecond time scale. Here, we focus on the emission mechanisms of N2 + air lasing and reveal, by examining the intensities and temporal profiles of N2 + lasing at 391 nm generated respectively in a time-varying polarization-modulated and a linearly polarized pump laser field under different nitrogen gas pressures, that the N2 + lasing can emit through either triggered super-radiance or seed amplification. We find that the two pressure-sensitive factors, i.e., the dipole dephasing time T2 and the population inversion density n, determine which of these two mechanisms dominates the N2 + lasing emission process, enabling manipulation of the transition from triggered super-radiance to seed amplification or vice versa. Our findings clarify the emission mechanism of N2 + lasing under different pressures and provide a deeper understanding of N2 + air lasing not only in the establishment of optical gain but also in the lasing emission process.

Sensing Trace-Level Metal Elements in Water Using Chirped Femtosecond Laser Pulses in the Filamentation Regime.

Femtosecond filament-induced breakdown spectroscopy (FIBS) is an efficient approach in remote and in situ detection of a variety of trace elements, but it was recently discovered that the FIBS of water is strongly dependent on the large-bandgap semiconductor property of water, making the FIBS signals sensitive to laser ionization mechanisms. Here, we show that the sensitivity of the FIBS technique in monitoring metal elements in water can be efficiently improved by using chirped femtosecond laser pulses, but an asymmetric enhancement of the FIBS intensity is observed for the negatively and positively chirped pulses. We attribute the asymmetric enhancement to their different ionization rates of water, in which the energy of the photons participating in the ionization process in the front part of the negatively chirped pulse is higher than that in the positively chirped pulse. By optimizing the pulse chirp, we show that the limit of detection of the FIBS technique for metal elements in water, e.g., aluminum, can reach to the sub-ppm level, which is about one order of magnitude better than that by the transform-limited pulse. We further examine the FIBS spectra of several representative water samples including commercial mineral water, tap water, and lake water taken from two different environmental zones, i.e., a national park and a downtown business district (Changchun, China), from which remarkably different concentrations of Ca, Na, and K elements of these samples are obtained. Our results provide a possibility of using FIBS for direct and fast metal elemental analysis of water in different field environments.

Rational highly dispersed ruthenium for reductive catalytic fractionation of lignocellulose.

Producing monomeric phenols from lignin biopolymer depolymerization in a detachable and efficient manner comes under the spotlight on the fullest utilization of sustainable lignocellulosic biomass. Here, we report a low-loaded and highly dispersed Ru anchored on a chitosan-derived N-doped carbon catalyst (RuN/ZnO/C), which exhibits outstanding performance in the reductive catalytic fractionation of lignocellulose. Nearly theoretical maximum yields of phenolic monomers from lignin are achieved, corresponding to TON as 431 molphenols molRu-1, 20 times higher than that from commercial Ru/C catalyst; high selectivity toward propyl end-chained guaiacol and syringol allow them to be readily purified. The RCF leave high retention of (hemi)cellulose amenable to enzymatic hydrolysis due to the successful breakdown of biomass recalcitrance. The RuN/ZnO/C catalyst shows good stability in recycling experiments as well as after a harsh hydrothermal treatment, benefiting from the coordination of Ru species with N atoms. Characterizations of the RuN/ZnO/C imply a transformation from Ru single atoms to nanoclusters under current reaction conditions. Time-course experiment, as well as reactivity screening of a series of lignin model compounds, offer insight into the mechanism of current RCF over RuN/ZnO/C. This work opens a new opportunity for achieving the valuable aromatic products from lignin and promoting the industrial economic feasibility of lignocellulosic biomass.

The upper bound of cumulative return of a trading series.

We present an upper bound of cumulative return in financial trading time series to formulate the most possible profit of many trades. The bound can be used to formally analyze the cumulative return varied by the number of trades, the mean return, and transaction cost rate. We also prove and show the validation of the upper bound, and verify the trend of cumulative return is consistent with that of the proposed bound via simulation experiments. Introducing a set of stochastic assessment methodology based on bootstrap into the organization of experimental data, we illustrate the influence on cumulative return from the relationship between the mean of return and transaction cost rate, technical trading rules, and stock indexes. To the best of our knowledge, this is the first to present and prove a bound of cumulative return of a stock trading series in theory. Both theoretical analyses and simulation experiments show the presented bound is a good mathematical tool to evaluate the trading risks and chances using given trading rules in stock trading markets.

Extremely enhanced N2+ lasing in a filamentary plasma grating in ambient air.

Cavity-free air lasing offers a promising route towards the realization of atmospheric lasers for various applications such as remote sensing and standoff spectroscopy; however, achieving efficient generation and control of air lasing in ambient air is still a challenge. Here we show the experimental realization of a giant lasing enhancement by three to four orders of magnitude in ambient air for the self-seeded N2+ lasing at 428 nm, assigned to the B2Σu+(ν'=0) and X2Σg+(ν''=1) emission, by modulating the spatiotemporal overlap of ultrashort near-infrared control-pump pulses in a filamentary plasma grating; meanwhile, the spontaneous emission from the same transition is only enhanced by three to four times. We find that this enhancement is sensitive to the relative polarization and interference time of the two pulses, and reveal that the formation of the plasma grating induces different population variations in the B2Σu+(ν'=0) and X2Σg+(ν''=1) levels, resulting in an enormous population inversion between the two levels, thereby a higher gain for the giant enhancement of N2+ lasing in ambient air.

Molybdenum-catalyzed hydrogenolysis of herbaceous biomass: A procedure integrated lignin fragmentation and components fractionation.

In this work, a low-cost MoO2/C catalyst was prepared for the reductive catalytic fractionation (RCF) of various herbaceous biomass feedstocks (Miscanthus, Triarrhena, Floridulus, Sorghum stem and Corncob). Phenolic monomers from the hydrogenolysis of lignin component were obtained in up to 26.4 wt%, with high selectivity towards methyl coumarate (33%) and methyl ferulate (24%). The RCF left solid carbohydrate pulps with high retentions (up to 87%), which were suitable for enzymatic hydrolysis. The reaction conditions, including temperature, time, H2 pressure, and sawdust size were examined in terms of monophenols yield, selectivity, delignification and sugar retention. This study showed that MoO2/C could function as an excellent catalyst for lignin fragmentation as well as the fractionation of herbaceous biomass components.

Robust and ultralow-energy-threshold ignition of a lean mixture by an ultrashort-pulsed laser in the filamentation regime.

Laser ignition (LI) allows for precise manipulation of ignition timing and location and is promising for green combustion of automobile and rocket engines and aero-turbines under lean-fuel conditions with improved emission efficiency; however, achieving completely effective and reliable ignition is still a challenge. Here, we report the realization of igniting a lean methane/air mixture with a 100% success rate by an ultrashort femtosecond laser, which has long been regarded as an unsuitable fuel ignition source. We demonstrate that the minimum ignition energy can decrease to the sub-mJ level depending on the laser filamentation formation, and reveal that the resultant early OH radical yield significantly increases as the laser energy reaches the ignition threshold, showing a clear boundary for misfire and fire cases. Potential mechanisms for robust ultrashort LI are the filamentation-induced heating effect followed by exothermal chemical reactions, in combination with the line ignition effect along the filament. Our results pave the way toward robust and efficient ignition of lean-fuel engines by ultrashort-pulsed lasers.

Selective hydrogenolysis of catechyl lignin into propenylcatechol over an atomically dispersed ruthenium catalyst.

C-lignin is a homo-biopolymer, being made up of caffeyl alcohol exclusively. There is significant interest in developing efficient and selective catalyst for depolymerization of C-lignin, as it represents an ideal feedstock for producing catechol derivatives. Here we report an atomically dispersed Ru catalyst, which can serve as an efficient catalyst for the hydrogenolysis of C-lignin via the cleavage of C-O bonds in benzodioxane linkages, giving catechols in high yields with TONs up to 345. A unique selectivity to propenylcatechol (77%) is obtained, which is otherwise hard to achieve, because this catalyst is capable of hydrogenolysis rather than hydrogenation. This catalyst also demonstrates good reusability in C-lignin depolymerization. Detailed investigations by model compounds concluded that the pathways involving dehydration and/or dehydrogenation reactions are incompatible routes; we deduced that caffeyl alcohol generated via concurrent C-O bonds cleavage of benzodioxane unit may act as an intermediate in the C-lignin hydrogenolysis. Current demonstration validates that atomically dispersed metals can not only catalyze small molecules reactions, but also drive the transformation of abundant and renewable biopolymer.

Asymmetric enhancement of N2+ lasing in intense, birefringence-modulating elliptical laser fields.

We experimentally demonstrate an asymmetric enhancement of the N2+ lasing at 391 nm for the transition between the B2Σu+ (v = 0) and X2Σg+ (v" = 0) states in an intense laser field with the ellipticity, ε, modulated by a 7-order quarter-wave plate (7-QWP). It is found that when the 7-QWP is rotated from α = 0 to 90°, where α is the angle between the polarization direction of the input laser and the slow axis of the 7-QWP, the intensity of the 391-nm lasing is optimized at ε ∼ 0.3 with α∼ 10°-20° and 70°-80° respectively, but the optimization intensity at α∼ 10°-20° is about 4 times smaller than that at α∼ 70°-80°. We interpret the asymmetric enhancement based on a post-ionization coupling model, in which the birefringence of the 7-QWP induces an opposite change in the relative amplitudes of the ordinary (Eo) and extraordinary (Ee) electric components under the two conditions, so that the same temporal separation of Eo and Ee leads to a remarkably different coupling strength for the population transfer from the X2Σg+ (v "=0) to A2Πu (v '=2) states.

Giant Enhancement of Air Lasing by Complete Population Inversion in N_{2}

A fine manipulation of population transfer among molecular quantum levels is a key technology for control of molecular processes. When a light field intensity is increased to the TW-PW cm^{-2} level, it becomes possible to transfer a population to specific excited levels through nonlinear light-molecule interaction, but it has been a challenge to control the extent of the population transfer. We deplete the population in the X^{2}Σ_{g}^{+}(v=0) state of N_{2}^{+} almost completely by focusing a dual-color (800 nm and 1.6 µm) intense femtosecond laser pulse in a nitrogen gas, and make the intensity of N_{2}^{+} lasing at 391 nm enhanced by 5-6 orders of magnitude. By solving a time-dependent Schrödinger equation describing the population transfer among the three lowest electronic states of N_{2}^{+}, we reveal that the X^{2}Σ_{g}^{+}(v=0) population is depleted by the vibrational Raman excitation followed by the electronic excitation A^{2}Π_{u}(v=2,3,4)←X^{2}Σ_{g}^{+}(v=1)←X^{2}Σ_{g}^{+}(v=0), resulting in the excessive population inversion between the B^{2}Σ_{u}^{+}(v=0) and X^{2}Σ_{g}^{+}(v=0) states. Our results offer a promising route to efficient population transfer among vibrational and electronic levels of molecules by a precisely designed intense laser field.

Rotational, Vibrational, and Electronic Modulations in N_{2}

We investigate lasing of a N_{2} gas induced by intense few-cycle near-IR laser pulses. By the pump-probe measurements, we reveal that the intensity of the B^{2}Σ_{u}^{+}-X^{2}Σ_{g}^{+} lasing emission of N_{2}^{+} oscillates at high (0.3-0.5 PHz), medium (50-75 THz), and low (∼3 THz) frequencies, corresponding to the energy separations between the rovibrational levels of the A^{2}Π_{u} and X^{2}Σ_{g}^{+} states. By solving the time-dependent Schrödinger equation, we reproduce the oscillations in the three different frequency ranges and show that the coherent population transfer among the three electronic states of N_{2}^{+} creates the population inversion between the B^{2}Σ_{u}^{+} and X^{2}Σ_{g}^{+} states, resulting in the lasing at 391 nm.

Robust Remote Sensing of Trace-Level Heavy-Metal Contaminants in Water Using Laser Filaments.

Water is the major natural resource that enables life on our planet. Rapid detection of water pollution that occurs due to both human activity and natural cataclysms is imperative for environmental protection. Analytical chemistry-based techniques are generally not suitable for rapid monitoring because they involve collection of water samples and analysis in a laboratory. Laser-based approaches such as laser-induced breakdown spectroscopy (LIBS) may offer a powerful alternative, yet conventional LIBS relies on the use of tightly focused laser beams, requiring a stable air-water interface in a controlled environment. Reported here is a proof-of-principle, quantitative, simultaneous measurement of several representative heavy-metal contaminants in water, at ppm-level concentrations, using ultraintense femtosecond laser pulses propagating in air in the filamentation regime. This approach is straightforwardly extendable to kilometer-scale standoff distances, under adverse atmospheric conditions and is insensitive to the movements of the water surface due to the topography and water waves.

Sequential utilization of bamboo biomass through reductive catalytic fractionation of lignin.

Reductive catalytic fractionation (RCF) has emerged as a new biorefinery paradigm for the fractionation and sequential utilization of entire components of biomass. Herein, we investigated the RCF of bamboo, a highly abundant herbaceous feedstock, in the presence of Pd/C catalyst. The lignin fraction in bamboo was preferentially depolymerized into well-defined low-molecular-weight phenols, with leaving carbohydrates pulp as a solid residue. In the soluble fraction, four major phenolic compounds, e.g., methyl coumarate/ferulate derived from hydroxycinnamic units and propanol guaiacol/syringol derived from ß-O-4 units, were generated up to 41.7 wt% yield based on original lignin content. In the insoluble fraction, the carbohydrates of bamboo were recovered with high retentions of cellulose (68%) and hemicellulose (49%), which upon treatment with enzyme gave glucose (90%) and xylose (85%). Overall, the three major components of bamboo could efficient to be fractionated and converted into useful platform chemicals on the basis of this study.

Significant Enhancement of N_{2}

We show that the intensity of self-seeded N_{2}^{+} lasing at 391 nm, assigned to the B^{2}Σ_{u}^{+}(v^{'}=0)→X^{2}Σ_{g}^{+}(v^{''}=0) emission, is enhanced by 2 orders of magnitude by modulating in time the polarization of an intense ultrashort near-IR (40 fs, 800 nm) laser pulse with which N_{2} is irradiated. We find that this dramatic enhancement of the 391 nm lasing is sensitive to the temporal variation of the polarization state within the laser pulse while the intensity of the spontaneous fluorescence emission at 391 nm is kept constant when the polarization state varies. We conclude that a postionization multiple-state coupling, through which the population can be transferred from the X^{2}Σ_{g}^{+} state of N_{2}^{+} to the first electronically excited A^{2}Π_{u} state, leads to the depletion of the population in the X^{2}Σ_{g}^{+} state, and consequently, to the population inversion between the X^{2}Σ_{g}^{+} state and the B^{2}Σ_{u}^{+} state.

Intense Femtosecond Laser-Mediated Electrical Discharge Enables Preparation of Amorphous Nickel Phosphide Nanoparticles.

Reported here is a high-efficiency preparation method of amorphous nickel phosphide (Ni-P) nanoparticles by intense femtosecond laser irradiation of nickel sulfate and sodium hypophosphite aqueous solution. The underlying mechanism of the laser-assisted preparation was discussed in terms of the breaking of chemical bond in reactants via highly intense electric field discharge generated by the intense femtosecond laser. The morphology and size of the nanoparticles can be tuned by varying the reaction parameters such as ion concentration, ion molar ratio, laser power, and irradiation time. X-ray diffraction and transmission electron microscopy results demonstrated that the nanoparticles were amorphous. Finally, the thermogravimetric-differential thermal analysis experiment verified that the as-synthesized noncrystalline Ni-P nanoparticles had an excellent catalytic capability toward thermal decomposition of ammonium perchlorate. This strategy of laser-mediated electrical discharge under such an extremely intense field may create new opportunities for the decomposition of molecules or chemical bonds that could further facilitate the recombination of new atoms or chemical groups, thus bringing about new possibilities for chemical reaction initiation and nanomaterial synthesis that may not be realized under normal conditions.

Polarization-orthogonal filament array induced by birefringent crystals in air.

We demonstrate the generation of filament array with orthogonal polarizations in air by using specifically designed wedge-type birefringent quartz plates. Experimental results show that the number of the generated filaments can be expressed as N = 2n wherenis the number of quartz plates inserted in the laser propagation path. By manipulating the optic axis of the quartz plates with respect to the polarization direction of the input laser pulse, the generated filaments can be separated into two parts with the polarization directions perpendicular with each other. The separation distance between the adjacent filaments is found to be linearly dependent on the focal length of external focusing lens. Our results provide a simple and efficient way to generate regular and reproductive femtosecond filament array in air.

Selective Fragmentation of Biorefinery Corncob Lignin into p-Hydroxycinnamic Esters with a Supported Zinc Molybdate Catalyst.

Lignin is the largest renewable resource of bioaromatics, and the catalytic fragmentation of lignin into phenolic monomers is increasingly recognized as an important starting point for lignin valorization. Herein, we report that ZnMoO4 supported on MCM-41 can catalyze the fragmentation of biorefinery technical lignin, enzymatic mild acidolysis lignin, and native lignin derived from corncob to yield lignin oily products that contain 15-37.8 wt % phenolic monomers, in which the high selectivities towards methyl coumarate (1) and methyl ferulate (2) were obtained (up to 78 %). The effects of some key parameters such as the influence of the solvent, reaction temperature, time, H2 pressure, and catalyst dosage were examined in view of activity and selectivity. The loss of Zn from the catalyst is discussed as the primary cause of deactivation, and the catalytic activity and selectivity can be well preserved in at least six runs by thermal calcination. The high selectivity to 1 and 2 leads to their easy separation and purification from lignin oily product to provide sustainable monomers for the preparation of functional polyether esters and polyesters.