Coherent surface plasmon polariton amplification via free-electron pumping.

Surface plasmonics with its unique confinement of light1,2 is expected to be a cornerstone for future compact radiation sources and integrated photonics devices. The energy transfer between light and matter is a defining aspect that underlies recent studies on optical surface-wave-mediated spontaneous emissions3-5. However, coherent stimulated emission of free electrons, which is essential for free-electron light sources, and its dynamical amplification process remain to be disclosed in a clear, unambiguous and calibrated manner. Here we present the coherent amplification of terahertz surface plasmon polaritons via free-electron-stimulated emission: a femtosecond optical pulse creates an in-phase free-electron pulse with an initial terahertz surface wave, and their ensuing interactions intensify the terahertz surface wave coherently. The underlying dynamics of the amplification, including a twofold redshift in the radiation frequency over a one-millimetre interaction length, are resolved as electromagnetic-field-profile evolutions using an optical pump-probe method. By extending the approach to a properly phase-matched electron bunch, our theoretical analysis predicts a super-radiant surface-wave growth, which lays the ground for a stimulated surface-wave light source and may facilitate capable means for matter manipulation, especially in the terahertz band.

Free-electron lasing at 27 nanometres based on a laser wakefield accelerator.

X-ray free-electron lasers can generate intense and coherent radiation at wavelengths down to the sub-ångström region1-5, and have become indispensable tools for applications in structural biology and chemistry, among other disciplines6. Several X-ray free-electron laser facilities are in operation2-5; however, their requirement for large, high-cost, state-of-the-art radio-frequency accelerators has led to great interest in the development of compact and economical accelerators. Laser wakefield accelerators can sustain accelerating gradients more than three orders of magnitude higher than those of radio-frequency accelerators7-10, and are regarded as an attractive option for driving compact X-ray free-electron lasers11. However, the realization of such devices remains a challenge owing to the relatively poor quality of electron beams that are based on a laser wakefield accelerator. Here we present an experimental demonstration of undulator radiation amplification in the exponential-gain regime by using electron beams based on a laser wakefield accelerator. The amplified undulator radiation, which is typically centred at 27 nanometres and has a maximum photon number of around 1010 per shot, yields a maximum radiation energy of about 150 nanojoules. In the third of three undulators in the device, the maximum gain of the radiation power is approximately 100-fold, confirming a successful operation in the exponential-gain regime. Our results constitute a proof-of-principle demonstration of free-electron lasing using a laser wakefield accelerator, and pave the way towards the development of compact X-ray free-electron lasers based on this technology with broad applications.

Reconfigurable Ag/HfO2/NiO/Pt Memristors with Stable Synchronous Synaptic and Neuronal Functions for Renewable Homogeneous Neuromorphic Computing System.

Artificial synapses and bionic neurons offer great potential in highly efficient computing paradigms. However, complex requirements for specific electronic devices in neuromorphic computing have made memristors face the challenge of process simplification and universality. Herein, reconfigurable Ag/HfO2/NiO/Pt memristors are designed for feasible switching between volatile and nonvolatile modes by compliance current controlled Ag filaments, which enables stable and reconfigurable synaptic and neuronal functions. A neuromorphic computing system effectively replicates the biological synaptic weight alteration and continuously accomplishes excitation and reset of artificial neurons, which consist of bionic synapses and artificial neurons based on isotype Ag/HfO2/NiO/Pt memristors. This reconfigurable electrical performance of the Ag/HfO2/NiO/Pt memristors takes advantage of simplified hardware design and delivers integrated circuits with high density, which exhibits great potency for future neural networks.

Phase-matching of high-order harmonic generation in the extreme ultraviolet region with orbital angular momentum.

High-order harmonics can generate vortex beams with orbital angular momentum (OAM) in the extreme ultraviolet region. However, experimental research on their phase-matching (PM) characteristics is limited. In this study, vortex high-order harmonic generation (HHG) in the extreme ultraviolet region was generated with Ar gas. Phase-matched HHG with OAM was obtained by optimizing the focus position, laser energy, and gas pressure. The dependence of the PM characteristics on these parameters was analyzed. In addition, we conducted an experimental analysis of the dimensional properties of vortex harmonics under PM conditions. This study is a contribution towards the intense vortex high-order harmonic light sources and their applications.

Characteristics of broadband OPCPA based on DKDP crystals with different deuterations for the SEL-100 PW laser system.

We present the performances of a broadband optical parametric chirped pulse amplification (OPCPA) using partially deuterated potassium dihydrogen phosphate (DKDP) crystals with deuteration levels of 70% and 98%. When pumped by a Nd:glass double frequency laser, the OPCPA system using the 98% deuterated DKDP crystal achieves a broad bandwidth of 189 nm (full width at 1/e2 maximum) from 836 nm to 1025 nm. For the DKDP crystal with length of 43 mm, the pump-to-signal conversion efficiency reaches 28.4% and the compressed pulse duration is 13.7 fs. For a 70% deuterated DKDP crystal with a length of 30 mm, the amplified spectrum ranges from 846-1021 nm, the compressed pulse duration is 15.7 fs, and the conversion efficiency is 25.5%. These results demonstrate the potential of DKDP crystals with higher deuteration as promising nonlinear crystals for use as final amplifiers in 100 Petawatt (PW) laser systems, supporting compression pulse duration shorter than 15 fs.

Gigahertz electromagnetic pulse emission from femtosecond relativistic laser-irradiated solid targets.

The interactions between high-intensity laser and matter produce particle flux and electromagnetic radiation over a wide energy range. The generation of extremely intense transient fields in the radio frequency-microwave regime has been observed in femtosecond-to-nanosecond laser pulses with 1011-1020-W/cm2 intensity on both conductive and dielectric targets. These fields typically cause saturation and damage to electronic equipment inside and near an experimental chamber; nevertheless, they can also be effectively used as diagnostic tools. Accordingly, the characterization of electromagnetic pulses (EMPs) is extremely important and currently a popular topic for present and future laser facilities intended for laser-matter interaction. The picosecond and sub-picosecond laser pulses are considerably shorter than the characteristic electron discharge time (∼0.1 ns) and can be efficient in generating GHz EMPs. The EMP characterization study of femtosecond laser-driven solid targets is currently mainly in the order of 100 mJ laser energy, in this study, the EMP generated by intense (Joule class) femtosecond laser irradiation of solid targets has been measured as a function of laser energy, laser pulse duration, focal spot size, and target materials. And a maximum electric field of the EMP reaching up to 105 V/m was measured. Analyses of experimental results confirm a direct correlation between measured EMP energy and laser parameters in the ultrashort pulse duration regime. The EMP signals generated by femtosecond laser irradiation of solid targets mainly originate from the return current inside the target after hot electron excitation. Numerical simulations of EMP are performed according to the target charging model, which agree well with the experimental results.

Spatial mode cleaning and efficient nonlinear pulse compression to sub-50†fs in a gas-filled multipass cell.

We demonstrate a gas-filled multipass cell (MPC) that cleaned the spatial mode of a spatial-filter-free 250 W, 100 kHz, 445 fs driven source based on an Innoslab amplifier and compressed the pulse duration to 41 fs simultaneously. The multipass cell acted as a spatial filter and benefited from its discrete waveguide nature, in which the input beam quality factor M2 was improved from 1.53 to a near-diffraction-limited value of 1.21 at 96% transmission.

Terahertz-triggered ultrafast nonlinear optical activities in two-dimensional centrosymmetric PtSe2.

The nonlinear mechanisms of polarization and optical fields can induce extensive responses in materials. In this study, we report on two kinds of nonlinear mechanisms in the topological semimetal PtSe2 crystal under the excitation of intense terahertz (THz) pulses, which are manipulated by the real and imaginary parts of the nonlinear susceptibility of PtSe2. Regarding the real part, the broken inversion symmetry of PtSe2 is achieved through a THz-electric-field polarization approach, which is characterized by second harmonic generation (SHG) measurements. The transient THz-laser-induced SHG signal occurs within 100 fs and recombines to the equilibrium state within 1 ps, along with a high signal-to-noise ratio (∼51 dB) and a high on/off ratio (∼102). Regarding the imaginary part, a nonlinear absorption change can be generated in the media. We reveal a THz-induced absorption enhancement in PtSe2 via nonlinear transmittance measurements, and the sheet conductivity can be modulated up to 42% by THz electric fields in our experiment. Therefore, the THz-induced ultrafast nonlinear photoresponse reveals the application potential of PtSe2 in photonic and optoelectronic devices in the THz technology.

A comprehensive study on three typical photoacid generators using photoelectron spectroscopy and ab initio calculations.

Conducting a comprehensive molecular-level evaluation of a photoacid generator (PAG) and its subsequent impact on lithography performance can facilitate the rational design of a promising 193 nm photoresist tailored to specific requirements. In this study, we integrated spectroscopy and computational techniques to meticulously investigate the pivotal factors of three prototypical PAG anions, p-toluenesulfonate (pTS-), 2-(trifluoromethyl)benzene-1-sulfonate (TFMBS-), and triflate (TF-), in the lithography process. Our findings reveal a significant redshift in the absorption spectra caused by specific PAG anions, attributed to their involvement in electronic transition processes, thereby enhancing the transparency of the standard PAG cation, triphenylsulfonium (TPS+), particularly at ∼193 nm. Furthermore, the electronic stability of PAG anions can be enhanced by solvent effects with varying degrees of strength. We observed the lowest vertical detachment energy of 6.6 eV of pTS- in PGMEA solution based on the polarizable continuum model, which prevents anion loss at 193 nm lithography. In addition, our findings indicate gas-phase proton affinity values of 316.4 kcal/mol for pTS-, 308.1 kcal/mol for TFMBS-, and 303.2 kcal/mol for TF-, which suggest the increasing acidity strength, yet even the weakest acid pTS- is still stronger than strong acid HBr. The photolysis of TPS+-based PAG, TPS+·pTS-, generated an excited state leading to homolysis bond cleavage with the lowest reaction energy of 83 kcal/mol. Overall, the PAG anion pTS- displayed moderate acidity, possessed the lowest photolysis reaction energy, and demonstrated an appropriate redshift. These properties collectively render it a promising candidate for an effective acid producer.

Annexin-A1 tripeptide enhances functional recovery and mitigates brain damage in traumatic brain injury by inhibiting neuroinflammation and preventing ANXA1 nuclear translocation in mice.

This study explores the role and mechanism of Annexin-A1 Tripeptide (ANXA1sp) in mitigating neuronal damage and promoting functional recovery in a mouse model of traumatic brain injury (TBI). Our goal is to identify ANXA1sp as a potential therapeutic drug candidate for TBI treatment. Adult male C57BL/6J mice were subjected to controlled cortical impact (CCI) to simulate TBI, supplemented by an in vitro model of glutamate-induced TBI in HT22 cells.  We assessed neurological deficits using the Modified Neurological Severity Score (mNSS), tested sensorimotor functions with beam balance and rotarod tests, and evaluated cognitive performance via the Morris water maze. Neuronal damage was quantified using Nissl and TUNEL staining, while microglial activation and inflammatory responses were measured through immunostaining, quantitative PCR (qPCR), Western blotting, and ELISA. Additionally, we evaluated cell viability in response to glutamate toxicity using the Cell Counting Kit-8 (CCK-8) assay and lactate dehydrogenase (LDH) release. Intraperitoneal administration of ANXA1sp significantly enhanced neurological outcomes, markedly reducing sensorimotor and cognitive impairments caused by TBI. This treatment resulted in a significant reduction in lesion volume and decreased neuronal cell death in the ipsilateral cortex. Moreover, ANXA1sp effectively diminished microglial activation around the brain lesion and decreased the levels of pro-inflammatory markers such as IL-6, IL-1ß, TNF-α, and TGF-ß in the cortex, indicating a significant reduction in neuroinflammation post-TBI. ANXA1sp also offered protection against neuronal cell death induced by glutamate toxicity, primarily by inhibiting the nuclear translocation of ANXA1, highlighting its potential as a neuroprotective strategy in TBI management. Administration of ANXA1sp significantly reduced neuroinflammation and neuronal cell death, primarily by blocking the nuclear translocation of ANXA1. This treatment substantially reduced brain damage and improved neurological functional recovery after TBI. Consequently, ANXA1sp stands out as a promising neuroprotective agent for TBI therapy.

Frequency shift of even-order high harmonic generation in monolayer MoS2.

Sub-optical-cycle electron dynamics in materials driven by intense laser fields can be investigated by high harmonic generation. We observed frequency shift of high harmonic spectrum near the band gap of monolayer MoS2 experimentally. Through semi-classical quantum trajectory analysis, we demonstrated that the phase of transition dipole moment varies according to the recombination timing and momentum of tunneled electrons. It results in either blue- or red-shift of harmonic frequencies, determined by the modulated energy gap by transition dipole phases (TDPs) and Berry connections. Our finding reveals the effect of TDPs on high harmonic frequency in non-central symmetric materials.

417†W, 2.38†mJ Innoslab amplifier compressible to a high pulse quality of 406†fs.

We demonstrate a 417 W, 175 kHz Innoslab chirped pulse amplification laser compressible to short and clean 406 fs pulse duration. A spectral bandwidth (full width at half maximum, FWHM) of ∼3 nm was maintained at full pump power, and the pulses exhibited good pulse quality in a wide tunable pulse energy range from 1.7 mJ to a maximum of 2.38 mJ. At the maximum output power, the compressed pulses were nearly pedestal free. The comprehensive effects of residual high-order dispersion from the front end, the gain shaping effects of the amplifier, and the slight mismatch of third-order dispersion (TOD) between the stretcher (CFBG) and the gating compressor, along with the small nonlinear phase shift accumulated in the amplifier, could have facilitated the high pulse quality. To the best of our knowledge, this is the shortest pulse duration from the Innoslab amplifiers at hundreds of watts average power in the millijoule energy regime.

Dynamic range enhancement of self-referenced spectral interferometry with extended time excursion method by the cascaded Kerr lens process.

Self-referenced spectral interferometry with extended time excursion (SRSI-ETE) is a powerful method for single-shot characterization of the temporal contrast of a high peak power laser, which has high temporal resolution but a low dynamic range. Here, a temporal contrast reduction method is proposed that uses the cascaded Kerr lens process in two thin glass plates. Combined with the SRSI-ETE method, the measurement dynamic range of the method is increased about two orders of magnitude while having a 20 fs temporal resolution and a 40 ps time window in single shot.

Simulating spatiotemporal dynamics of ultra-intense ultrashort lasers through imperfect grating compressors.

The upcoming 100 Petawatt (PW) laser is going to provide a possibility to experimentally study vacuum physics. Pulse compression and beam focusing, which can be affected by the spatiotemporal coupling, are two key processes of generating a 100 PW laser and then determine whether its physical objective can be achieved or not. We improved our previous model of the spatiotemporal coupling where only the grating wavefront error and the output optical field of a common compressor configuration were included, and in the improved model, the grating amplitude modulation, the spatio-spectral clipping, and the optical field inside the compressor were added. By using it, we theoretically investigated the spatiotemporal dynamics of an ultra-intense ultrashort laser passing through an imperfect grating compressor for different cases, especially the spatio-temporal/spectral coupling and the on-target intensity variation induced by the phase and amplitude modulation at different grating positions in two different compressor configurations. This study is of importance for both engineering development and physical application of the upcoming Exawatt-class laser.

Measurements of microjoule-level, few-femtosecond ultraviolet dispersive-wave pulses generated in gas-filled hollow capillary fibers.

To the best of our knowledge, we demonstrate the first time-domain measurement of µJ-level, few-fs ultraviolet dispersive-wave (DW) pulses generated in gas-filled hollow capillary fibers (HCFs) in an atmosphere environment using several chirped mirrors. The pulse temporal profiles, measured using a self-diffraction frequency-resolved optical gating setup, exhibit full width at half maximum pulse widths of 9.6 fs at 384 nm and 9.4 fs at 430 nm, close to the Fourier-transform limits. Moreover, theoretical and experimental studies reveal the strong influences of driving pulse energy and HCF length on temporal width and shape of the measured DW pulses. The ultraviolet pulses obtained in an atmosphere environment with µJ-level pulse energy, few-fs pulse width, and broadband wavelength tunability are ready to be used in many applications.

Investigation and suppression of pre-pulses on nanosecond time scale in the SULF-1PW laser.

It is of crucial significance to investigate and suppress pre-pulses on nanosecond time scale because the intense pre-plasma generated by them may have enough time to expand and, thus, cause fatal impact on laser-matter interactions. In this research, we analyze the potential origins of pre-pulses on nanosecond time scale in a typical Ti:sapphire chirped pulse amplification laser system. Based on the analysis, the initial status of these generated pre-pulses in the SULF-1PW laser is measured and investigated. Then different measures, including fine control on the time synchronization and the replacement for the Ti:sapphire, are adopted in the SULF-1PW laser to suppress these pre-pulses with respective origins, which can promote the energy ratio between the main pulse and these pre-pulses by 2-3 orders of magnitude. This research not only improves the temporal contrast of the SULF-1PW laser on nanosecond time scale but also provides beneficial guidance for the design and construction of similar laser facilities.

Multistep pulse compressor for 10s to 100s PW lasers.

High-energy tens (10s) to hundreds (100s) petawatt (PW) lasers are key tools for exploring frontier fundamental researches such as strong-field quantum electrodynamics (QED), and the generation of positron-electron pair from vacuum. Recently, pulse compressor became the main obstacle on achieving higher peak power due to the limitation of damage threshold and size of diffraction gratings. Here, we propose a feasible multistep pulse compressor (MPC) to increase the maximum bearable input and output pulse energies through modifying their spatiotemporal properties. Typically, the new MPC including a prism pair for pre-compression, a four-grating compressor (FGC) for main compression, and a spatiotemporal focusing based self-compressor for post-compression. The prism pair can induce spatial dispersion to smooth and enlarge the laser beam, which increase the maximum input and output pulse energies. As a result, as high as 100 PW laser with single beam or more than 150 PW through combining two beams can be obtained by using MPC and current available optics. This new optical design will simplify the compressor, improve the stability, and save expensive gratings/optics simultaneously. Theoretically, the output pulse energy can be increased by about 4 times using the MPC method in comparison to a typical FGC. Together with the multi-beam tiled-aperture combining method, the proposed tiled-grating based tiled-aperture method, larger gratings, or negative chirp pulse based self-compression method, several 100s PW laser beam is expected to be obtained by using this MPC method in the future, which will further extend the ultra-intense laser physics research fields.

Controlling of the harmonic generation induced by the Berry curvature.

High-order harmonic generation in solid state has attracted a lot of attentions. The Berry curvature (BC), a geometrical property of the Bloch energy band, plays an important role for the harmonic generation in crystal. As we all know, the influence of BC on the harmonic emission has been investigated before and BC is simplified as a 1D structure. However, many other materials including MoS2 are 2D materials. In this work, we extend the investigation for BC to 2D structure and get a generalized equation, which not only gives a new method to control the harmonic emission with BC, but also gives a deeper understanding for the influence of the BC. We show the ability to control the harmonic emission related to the BC using the orthogonal two-color (OTC) laser field. By tuning the delay of OTC laser field, one can steer the trajectory of electrons and modulate the emission of harmonics. This study can provide us a deeper insight into the role of the BC which is difficult to be measured experimentally.

Inter-half-cycle spectral interference in high-order harmonic generation from monolayer MoS2.

The enhancement of even-order harmonics near the cut-off of high-order harmonic spectra from monolayer MoS2 has been experimentally observed recently by several groups. Here we demonstrate that this enhancement can be interpreted as a result of spectral interference between half-cycles with opposite polarity by adopting a fully quantum mechanical calculation. We found that, due to the energy modulation induced by Berry connections, only half-cycles with the same polarity can generate high-order harmonics near the cut-off frequency, thus the lack of destructive interference leads to the enhanced intensity of the corresponding even-order harmonics. The explanation is supported by the frequency shift of the measured harmonic peaks. Our finding revealed the role of inter-half-cycle interference in high-harmonic generation (HHG) from non-centrosymmetric materials.

Effect of the interface on femtosecond laser damage of a metal-dielectric low dispersion mirror.

Metal-dielectric low dispersion mirrors (MLDM) have a promising application prospect in petawatt (PW) laser systems. We studied the damage characteristics of MLDM and found that the damage source of MLDM (Ag + Al2O3+SiO2) is located at the metal-dielectric interface. We present the effect of the interface on the femtosecond laser damage of MLDM. Finite element analysis shows that thermal stress is distributed at the interface, causing stress damage which is consistent with the damage morphology. After enhancing the interface adhesion and reducing the residual stress, the damage source transfers from the interface to a surface SiO2 layer, and the damage threshold can be increased from 0.60 J/cm2 to 0.73 J/cm2. This work contributes to the search for new techniques to improve the damage threshold of MLDM used in PW laser systems.