Coulomb potential determining terahertz polarization in a two-color laser field.

The orientation and ellipticity of terahertz (THz) polarization generated by a two-color strong field not only casts light on underlying mechanisms of laser-matter interaction, but also plays an important role for various applications. We develop the Coulomb-corrected classical trajectory Monte Carlo (CTMC) method to well reproduce the joint measurements, that the THz polarization generated by the linearly polarized 800 nm and circularly polarized 400 nm fields is independent on two-color phase delay. The trajectory analysis shows that the Coulomb potential twists the THz polarization by deflecting the orientation of asymptotic momentum of electron trajectories. Further, the CTMC calculations predict that, the two-color mid-infrared field can effectively accelerate the electron rapidly away from the parent core to relieve the disturbance of Coulomb potential, and simultaneously create large transverse acceleration of trajectories, leading to the circularly polarized THz radiation.

Direct Observation of Ultrafast Relaxation Dynamics of a Mixed Excimer State in Perylene Monoimide Dimer by Femtosecond Transient Absorption.

A J-type dimer PMI-2, two perylene monoimides linked by butadiynylene bridger was prepared, and its excited-state dynamics was studied using ultrafast femtosecond transient absorption spectroscopy, along with steady-state spectroscopy and quantum chemical calculations. It is evidently demonstrated that the symmetry-breaking charge separation (SB-CS) process in PMI-2 is positively mediated by an excimer, which is mixed by localized Frenkel excitation (LE) and an interunit charge transfer (CT) state. Kinetic studies show that, with the polarity increasing of the solvent, the transformation of excimer from a mixture to the CT state (SB-CS) is accelerated, and the recombination time of the CT state is reduced obviously. Theoretical calculations indicate that these are due to PMI-2 obtaining more negative free energy (ΔGcs) and lower CT state energy levels in highly polar solvents. Our work suggests that the mixed excimer can be formed in a J-type dimer with suitable structure, in which the charge separation the process is sensitive to the solvent environment.

Observation of carrier transfer in a vertical 0D-CsPbBr3/2D-MoS2 mixed-dimensional van der Waals heterojunction.

Two-dimensional transition metal dichalcogenides with outstanding properties open up a new way to develop optoelectronic devices such as phototransistors and light-emitting diodes. Heterostructure with light-harvesting materials can produce many photogenerated carriers via charge and/or energy transfer. In this paper, the ultrafast dynamics of charge transfer in zero-dimensional CsPbBr3 quantum dot/two-dimensional MoS2 van der Waals heterostructures are investigated through femtosecond time-resolved transient absorption spectroscopy. Hole and electron transfers in the ps and fs magnitude at the interfaces between MoS2 and CsPbBr3 are observed by modulating pump wavelengths of the pump-probe configurations. Our study highlights the opportunities for realizing the exciton devices based on quantum dot/two-dimensional semiconductor heterostructures.

Trajectory analysis for low-order harmonic generation in two-color strong laser fields.

Focusing two-color laser fields in gas-phase medium produces ultrashort ultra-broadband low-order harmonics spanning from terahertz to extreme ultraviolet regime. The low-order harmonic generation can be explained by both macroscopic photocurrent model and microscopic strong field approximation theory. Here, we analytically build a bridge between the macroscopic and microscopic theories by means of the trajectory method, which manifests correspondences between macroscopic and microscopic theories. And we demonstrate the trajectory analysis to explain phase-dependent terahertz and third-harmonic generations, and contribute the phase-dependent yields and spectral shapes to the coherent superposition of electron trajectories released at distinct ionization instants, reflecting electron interfering with itself in radiation process. The trajectory method readily connects the low-order harmonics characteristics and behaviors of electron wavepacket, which has potential for reconstructing ultrafast electron dynamics by means of low-harmonics observations.

Low amplified spontaneous emission threshold from 2-thiophenemethylammonium quasi-2D perovskites via phase engineering.

Quasi-2D Ruddlesden-Popper perovskites attract great attention as an optical gain media in lasing applications due to their excellent optoelectronic properties. Herein, a novel quasi-2D Ruddlesden-Popper perovskite based on 2-thiophenemethylammonium (ThMA) is synthesized by a facile solution-processed method. In addition, an anti-solvent treatment method is proposed to tune the phase distribution, and preferential orientation of quasi-2D (ThMA)2Csn-1PbnBr3n+1 thin films. The large-n-dominated narrow domain distribution improves the energy transfer efficiency from small-n to large-n phases. Also, the highly oriented nanocrystals facilitate the efficient Förster energy transfer, beneficial for the carrier population transfer. Furthermore, a green amplified spontaneous emission with a low threshold of 13.92 µJ/cm2 is obtained and a single-mode vertical-cavity laser with an 0.4 nm linewidth emission is fabricated. These findings provide insights into the design of the domain distribution to realize low-threshold multicolor continuous-wave or electrically driven quasi-2D perovskites laser.

Dissociative multi-ionization of N2O molecules in strong femtosecond laser field.

Multi-ionization and subsequent Coulomb explosion (CE) of the N2O molecule irradiated by a linearly polarized 800 nm laser field is investigated by a reaction microscope, where a number of CE channels of N2Oq+ with q ≤ 5 for two-body fragmentation and q ≤ 8 for three-body fragmentation were observed. For two-body CE, by analyzing the internuclear separations extracted from kinetic energy releases (KERs), dissociation branching fractions, and laser intensity dependence, interestingly, we found that fragmentation N2O5+ → N3+ + NO2+ is produced directly from dissociating N2O3+ via non-sequential stairstep ionization, whereas most of the others result from the sequential stairstep ionization. For three-body CE, 25 fragmentation channels of N2Oq+ (q = 3-8) are distinguished in the present charge-encoded multi-photoion coincidence plot, and the concerted fragmentation mechanism is nominated in a typical Dalitz plot. With the help of the numerical computation with the measured KERs and momentum correlation angles, the geometric structures of molecular ions prior to fragmentation are reconstructed, which display the bending motion and simultaneous two-bond stretching before the CE. Increasing of the bond length for high charged N2Oq+ indicates the dominating stairstep ionization in the three-body fragmentation.

Intensity-surged and bandwidth-extended terahertz radiation in two-foci cascading plasmas.

The two-color strong-field mixing in gas medium is a widely used approach to generate bright broadband terahertz (THz) radiation. Here, we present a new, to the best of our knowledge, and counterintuitive method to promote THz performance in the two-color scheme. Beyond our knowledge that the maximum THz generation occurs with two-color foci overlapped, we found that, when the foci of two-color beams are noticeably separated along the propagation axis resulting in cascading plasmas, the THz conversion efficiency is surged by one order of magnitude and the bandwidth is stretched by more than two times, achieving 10-3 conversion efficiency and >100 THz bandwidth under the condition of 800/400 nm, ∼35 fs driving lasers. With the help of the pulse propagation equation and photocurrent model, the observations can be partially understood by the compromise between THz generation and absorption due to the spatial redistribution of laser energy in cascading plasmas. The present method can be extended to a mid-infrared driving laser, and new records of THz peak power and conversion efficiency are expected.

Charge-encoded multi-photoion coincidence for three-body fragmentation of CO2 in the strong laser fields.

The photoion-photoion coincidence (PIPICO) is a simple and effective approach for the selection of correlated fragments in a specific dissociating channel in molecules. We propose here a charge-encoded multi-photoion coincidence (cMUPICO) method, in analogy to traditional PIPICO, however in which the charge of individual fragments is taken into account. The cMUPICO method allows for clearly displaying coincident channels for dissociation channels containing three more fragments with unequal charge states, invisible in the traditional PIPICO. As a demonstration, three-body fragmentation dynamics of CO2 in strong IR laser fields is analyzed, and 11 dissociation channels are effectively identified, five of which are first found with cMUPICO. The present results show that cMUPICO is a powerful and practical tool for distinguishing various dissociation channels with multiply charged multi-photoions.

Third-order harmonic generation in a bi-chromatic elliptical laser field.

The low-order harmonic generation induced by a strong laser field produces a bright, ultrashort, supercontinuum radiation ranging from the terahertz to ultraviolet band. By controlling the phase-delay and ellipticity of the bi-chromatic laser fields, the third harmonic generation is experimentally and theoretically investigated for elucidating the mechanism of the low-order harmonics. The third harmonic generation is found to be strongly suppressed in the counter-rotating bi-chromatic laser field due to the selection rule for harmonic emissions. The continuum-continuum transition in the strong field approximation is extended to explain the third harmonic generation as a function of the phase delay and ellipticity of the bi-chromatic laser fields. Compared with the semi-classical photocurrent model, the continuum-continuum transition on the basis of quantum-mechanical treatment achieves better agreement with the experimental observations. Our work indicates that the overlapping in continuum states via different quantum paths of a single electron plays a role in low-order harmonics generation under elliptical bi-chromatic laser fields.

Multiexciton dynamics in CsPbBr3nanocrystals: the dependence on pump fluence and temperature.

Multiexcitons generation is a process of generating electron-hole pairs in nanostructured semiconductors by absorbing a single high-energy photon. The multiexciton process is essential for the performance of optoelectronic devices based on perovskite nanomaterials. In this paper, ultrafast time-resolved transient absorption spectroscopy is used to study the ultrafast dynamics of CsPbBr3nanocrystals. It is found that the multiexcitons Auger recombination lifetime increases with the decrease of pump fluence, while it is on the contrary for the hot carrier cooling time. The increase in the number of photons absorbed by each nanocrystal under high pump fluence slows down the relaxation of hot carriers to the band edge. The hot carrier cooling lifetime increases from 0.25 to 0.85 ps when the pump fluence increases from 6 to 127µJ cm-2. Temperature-dependent transient absorption spectroscopy exhibits that the relaxation process of hot carriers slows down sharply when the lattice temperature decreases from 280 to 80 K. Moreover, the exciton binding energy 46 meV of CsPbBr3nanocrystals is obtained by temperature-dependent steady-state photoluminescence spectroscopy. These findings provide insights for applications such as solar cells and light-emitting devices based on CsPbBr3nanocrystals.

Identification of a bridge-specific intramolecular exciton dissociation pathway in donor-π-acceptor alternating conjugated polymers.

Intramolecular exciton dissociation is critical for high efficient mobile charge carrier generations in organic solar cells. Yet despite much attention, the effects of π bridges on exciton dissociation dynamics in donor-π-acceptor (D-π-A) alternating conjugated polymers remain still unclear. Here, using a combination of femtosecond time-resolved transient absorption (TA) spectroscopy and steady-state spectroscopy, we track ultrafast intramolecular exciton relaxation dynamics in three D-π-A alternating conjugated polymers which were synthesized by Qin's group and named HSD-A, HSD-B, HSD-C. It is found that the addition of thiophene unit as π bridges will lead to the red shift of steady-state absorption spectrum. Importantly, we reveal the existence of a new intramolecular exciton dissociation pathway mediated by a bridge-specific charge transfer (CT') state with the TA fingerprint peak at 1200 nm in π-bridged HSD-B and HSD-C. This CT' state results in higher electron capture rates for HSD-B and HSD-C as compared to HSD-A. Depending on the proportion of CT' state and nongeminate recombination are important step for the understanding of high power conversion efficiencies in HSD-B than in HSD-C. We propose that this bridge-specific exciton dissociation pathway plays an important role in ultrafast intramolecular exciton dissociation of organic photovoltaic material D-π-A alternating conjugated polymers.

Excitation Wavelength and Intensity-Dependent Multiexciton Dynamics in CsPbBr3 Nanocrystals.

CsPbBr3 has attracted great attention due to unique optical properties. The understanding of the multiexciton process is crucial for improving the performance of the photoelectric devices based on CsPbBr3 nanocrystals. In this paper, the ultrafast dynamics of CsPbBr3 nanocrystals is investigated by using femtosecond transient absorption spectroscopy. It is found that Auger recombination lifetime increases with the decrease of the excitation intensity, while the trend is opposite for the hot-exciton cooling time. The time of the hot-carriers cooling to the band edge is increased when the excitation energy is increased from 2.82 eV (440 nm) to 3.82 eV (325 nm). The lifetime of the Auger recombination reaches the value of 126 ps with the excitation wavelength of 440 nm. The recombination lifetime of the single exciton is about 7 ns in CsPbBr3 nanocrystals determined by nanosecond time-resolved photoluminescence spectroscopy. The exciton binding energy is 44 meV for CsPbBr3 nanocrystals measured by the temperature-dependent steady-state photoluminescence spectroscopy. These findings provide a favorable insight into applications such as solar cells and light-emitting devices based on CsPbBr3 nanocrystals.

Hot excitons cooling and multiexcitons Auger recombination in PbS quantum dots.

In the past few years, lead chalcogenide quantum dots (QDs) have attracted attention as a new system with a strong quantum confinement effect. In this paper, the hot-excitons cooling and Auger recombination of multiexcitons in PbS QDs are investigated by the femtosecond time-resolved transient absorption spectroscopy. The results show that the excitons dynamics in PbS QDs are closely related to the pump-photon energy and pump-pulse energy. Multiexcitons generate when the excess energy of the absorbed photons is larger than the bandgap energy in PbS QDs. The hot-excitons cooling lifetime increases but the Auger recombination lifetime decreases as the pump-photon energy and the pump-pulse energy increase. Besides, there is a competitive relation between multiple-excitons generation and hot-excitons cooling. The dynamics results of the formation and relaxation of multiexcitons in PbS QDs would shed light on the further understanding of the interaction between excitons and photons in the optoelectronic application based on PbS QDs.

Direct Observation of Competition between Amplified Spontaneous Emission and Auger Recombination in Quasi-Two-Dimensional Perovskites.

Quasi-two-dimensional (quasi-2D) Ruddlesden-Popper perovskites (RPPs) have been reported recently as laser media. The carrier dynamics of lasing in quasi-2D RPP films is still controversial. This work presents the amplified spontaneous emission (ASE) at room temperature based on the solution-processed quasi-2D (PEA)2Csn-1PbnBr3n+1 (⟨n⟩ = 3-5) thin films. Transient absorption spectroscopy was conducted to demonstrate the accumulation of excitons and population inversion for ASE caused by the energy transfer process along cascade quantum wells. The competition between the ASE and Auger recombination processes was observed via systematic and quantitative studies on the carrier dynamics. It is found that the contribution of Auger recombination is reduced by decreasing the temperature or adjusting the domain distribution of the quasi-2D RPP films, in order to achieve lower thresholds. This work re-emphasizes the importance of Auger recombination control for achieving low-threshold electrically or continuous-wave driven lasers and deepens the fundamental understanding of charge carrier dynamics of lasing in quasi-2D perovskite films.

Strong-Field Extreme-Ultraviolet Dressing of Atomic Double Excitation.

We report on the experimental observation of a strong-field dressing of an autoionizing two-electron state in helium with intense extreme-ultraviolet laser pulses from a free-electron laser. The asymmetric Fano line shape of this transition is spectrally resolved, and we observe modifications of the resonance asymmetry structure for increasing free-electron-laser pulse energy on the order of few tens of Microjoules. A quantum-mechanical calculation of the time-dependent dipole response of this autoionizing state, driven by classical extreme-ultraviolet (XUV) electric fields, evidences strong-field-induced energy and phase shifts of the doubly excited state, which are extracted from the Fano line-shape asymmetry. The experimental results obtained at the Free-Electron Laser in Hamburg (FLASH) thus correspond to transient energy shifts on the order of a few meV, induced by strong XUV fields. These results open up a new way of performing nonperturbative XUV nonlinear optics for the light-matter interaction of resonant electronic transitions in atoms at short wavelengths.

Imipramine Protects against Bone Loss by Inhibition of Osteoblast-Derived Microvesicles.

The maintenance of bone homeostasis is largely dependent upon cellular communication between osteoclasts and osteoblasts. Microvesicles (MVs) represent a novel mechanism for osteoblasts and osteoclasts communication, as has been demonstrated in our previous study. Sphingomyelinases catalyze the hydrolysis of sphingomyelin, which leads to increased membrane fluidity and facilitates MV generation. This effect can be inhibited by imipramine, an inhibitor of acid sphingomyelinase (ASM), which is also known as a member of tricyclic antidepressants (TCAs). A recent study has reported that in vitro treatment of imipramine blocked MVs release from glial cells. However, whether imipramine has this effect on osteoblast-derived MVs and whether it is involved in MV generation in vivo is unclear. Here, our investigations found that imipramine slightly reduced the expression of osteoblast differentiation of related genes, but did not impact parathyroid hormone (PTH) regulation for these genes and also did not affect receptor activator of nuclear factor-κB ligand (RANKL)-mediated osteoclast formation; however, imipramine treatment blocked MVs released from osteoblasts and inhibited MV-induced osteoclast formation. In vivo, mice administrated with imipramine were protected from ovariectomy-induced bone loss as evaluated by various bone structural parameters and serum levels of biochemical markers. Our results suggest that inhibiting the production of MVs containing RANKL in vivo is very important for preventing bone loss.

Genetic variants of the class A scavenger receptor gene are associated with essential hypertension in Chinese.

BACKGROUND: The class A scavenger receptor, which is encoded by the macrophage scavenger receptor 1 (MSR1) gene, is a pattern recognition receptor (PPR) primarily expressed in macrophages. It has been reported that genetic polymorphisms of MSR1 are significantly associated with many cardiovascular events. However, whether it links genetically to essential hypertension (EH) in Chinese is not defined. METHODS: We performed an independent case-control study in a Chinese population consisting of 617 EH cases and 620 controls by genotyping three single nucleotide polymorphisms (SNPs) of MSR1. RESULTS: We found that rs13306541 and rs3747531 were significantly associated with an increased risk of EH with per allele odds ratio (OR) of 1.63 [95% confidence interval (CI): 1.27-2.09; P<0.001] and 1.29 (95% CI: 1.09-1.52; P=0.003), respectively. Individuals with 2-4 risk alleles had a 2.03-fold (95% CI: 1.48-2.78) increased risk of EH compared with those having none of the risk alleles (P for trend <0.001). CONCLUSIONS: Our results indicate that genetic variants of MSR1 may serve as predictive markers for the risk of EH in combination with traditional risk factors of EH in Chinese population.

Charge and Nuclear Dynamics Induced by Deep Inner-Shell Multiphoton Ionization of CH3I Molecules by Intense X-ray Free-Electron Laser Pulses.

In recent years, free-electron lasers operating in the true X-ray regime have opened up access to the femtosecond-scale dynamics induced by deep inner-shell ionization. We have investigated charge creation and transfer dynamics in the context of molecular Coulomb explosion of a single molecule, exposed to sequential deep inner-shell ionization within an ultrashort (10 fs) X-ray pulse. The target molecule was CH3I, methane sensitized to X-rays by halogenization with a heavy element, iodine. Time-of-flight ion spectroscopy and coincident ion analysis was employed to investigate, via the properties of the atomic fragments, single-molecule charge states of up to +22. Experimental findings have been compared with a parametric model of simultaneous Coulomb explosion and charge transfer in the molecule. The study demonstrates that including realistic charge dynamics is imperative when molecular Coulomb explosion experiments using short-pulse facilities are performed.

Osteoblast-derived microvesicles: A novel mechanism for communication between osteoblasts and osteoclasts.

The maintenance of bone homeostasis is largely dependent upon cellular communication between osteoclasts and osteoblasts. Microvesicles (MVs) have received a good deal of attention and are increasingly considered as mediators of intercellular communication due to their capacity to merge with and transfer a repertoire of bioactive molecular content (cargo) to recipient cells, triggering a variety of biologic responses. Here, we demonstrated that MVs shed from osteoblasts contain RANKL protein and can transfer it to osteoclast precursors through receptor ligand (RANKL-RANK), leading to stimulation of RANKL-RANK signaling to facilitate osteoclast formation. Such MV-mediated intercellular communication between osteoblasts and osteoclasts may represent a novel mechanism of bone modeling and remodeling. It may be worthwhile to further explore MVs as tools to modify the biological responses of bone cells or develop an alternative drug to treat bone diseases.