Generating a tunable narrow electron beam comb via laser-driven plasma grating

We propose a novel approach for generating a high-density,spatially periodic narrow electron beam comb(EBC)from a plasma grating induced by the interference of two intense laser pulses in subcritical-density plasma.We employ particle-in-cell(PIC)simulations to investigate the effects of cross-propagating laser pulses with specific angles overlapping in a subcritical plasma.This overlap results in the formation of a transverse standing wave,leading to a spatially periodic high-density modulation known as a plasma grating.The electron density peak within the grating can reach several times the background plasma density.The charge imbalance between electrons and ions in the electron density peaks causes mutual repulsion among the electrons,resulting in Coulomb expansion and acceleration of the electrons.As a result,some electrons expand into vacuum,forming a periodic narrow EBC with an individual beam width in the nanoscale range.To further explore the formation of the nanoscale EBC,we conduct additional PIC simulations to study the dependence on various laser parameters.Overall,our proposed method offers a promising and controlled approach to generate tunable narrow EBCs with high density.

Photon polarization effects in polarized electron–positron pair production in a strong laser field

Deep understanding of the impact of photon polarization on pair production is essential for the efficient generation of laser-driven polarized positron beams and demands a complete description of polarization effects in strong-field QED processes.Employing fully polarization-resolved Monte Carlo simulations,we investigate correlated photon and electron(positron)polarization effects in the multiphoton Breit–Wheeler pair production process during the interaction of an ultrarelativistic electron beam with a counterpropagating elliptically polarized laser pulse.We show that the polarization of e−e+pairs is degraded by 35%when the polarization of the intermediate photon is resolved,accompanied by an∼13%decrease in the pair yield.Moreover,in this case,the polarization direction of energetic positrons at small deflection angles can even be reversed when high-energy photons with polarization parallel to the laser electric field are involved.

Generation and application of high-contrast laser pulses using plasma mirror in the SULF-1PW beamline

The plasma mirror system was installed on the 1 PW laser beamline of Shanghai Superintense Ultrafast Laser Facility[SULF]for enhancing the temporal contrast of the laser pulse.About 2 orders of magnitude improvement on pulse contrast was measured on picosecond and nanosecond time scales.The experiments show that high-contrast laser pulses can significantly improve the cutoff energy and quantity of proton beams.Then different target distributions are assumed in particles in cell simulations,which can qualitatively assume the expansion of nanometer-scale foil.The high-contrast laser enables the SULF-1PW beamline to generally be of benefit for many potential applications.

Ultra-broadband pulse generation via hollow-core fiber compression and frequency doubling for ultra-intense lasers

We demonstrate an ultra-broadband high temporal contrast infrared laser source based on cascaded optical parametric amplification,hollow-core fiber(HCF)and second harmonic generation processes.In this setup,the spectrum of an approximately 1.8μm laser pulse has near 1μm full bandwidth by employing an argon gas-filled HCF.Subsequently,after frequency doubling with cascaded crystals and dispersion compensation by a fused silica wedge pair,9.6 fs(~3cycles)and 150μJ pulses centered at 910 nm with full bandwidth of over 300 nm can be generated.The energy stability of the output laser pulse is excellent with 0.8%(root mean square)over 20 min,and the temporal contrast is>10^(12)at-10 ps before the main pulse.The excellent temporal and spatial characteristics and stability make this laser able to be used as a good seed source for ultra-intense and ultrafast laser systems.

Erbium-ytterbium co-doped lithium niobate single-mode microdisk laser with an ultralow threshold of 1μW

We demonstrate single-mode microdisk lasers in the telecom band with ultralow thresholds on erbium-ytterbium co-doped thin-film lithium niobate(TFLN).The active microdisk was fabricated with high-Q factors by photolithography-assisted chemomechanical etching.Thanks to the erbium-ytterbium co-doping providing high optical gain,the ultralow loss nanostructuring,and the excitation of high-Q coherent polygon modes,which suppresses multimode lasing and allows high spatial mode overlap between pump and lasing modes,single-mode laser emission operating at 1530 nm wavelength was observed with an ultralow threshold,under a 980-nm-band optical pump.The threshold was measured as low as 1μW,which is one order of magnitude smaller than the best results previously reported in single-mode active TFLN microlasers.The conversion efficiency reaches 4.06×10^(-3),which is also the highest value reported in single-mode active TFLN microlasers.

Collimated gamma beams with high peak flux driven by laser-accelerated electrons

Laser-accelerated electrons are promising in producing gamma-photon beams of high peak flux for the study of nuclear photonics,obtaining copious positrons and exploring photon–photon interaction in vacuum.We report on the experimental generation of brilliant gamma-ray beams with not only high photon yield but also low divergence,based on picosecond laser-accelerated electrons.The 120 J 1 ps laser pulse drives self-modulated wakefield acceleration in a high-density gas jet and generates tens-of-MeV electrons with 26 nC and divergence as small as 1.51◦.These collimated electrons produce gamma-ray photons through bremsstrahlung radiation when transversing a high-Z solid target.We design a high-energy-resolution Compton-scattering spectrometer and find that a total photon number of 2.2×10^(9)is captured within an acceptance angle of 1.1°for photon energies up to 16 MeV.Comparison between the experimental results and Monte Carlo simulations illustrates that the photon beam inherits the small divergence from electrons,corresponding to a total photon number of 2.2×10^(11)and a divergence of 7.73°.

High-performance 800-1050 nm seed pulses based on spectral broadening and filtering for petawatt lasers

High-performance 86μJ,11.2 fs pulses with a spectrum range of 800-1050 nm are generated based on 1030 nm,190 fs Yb femtosecond pulses by using multi-plate-based spectral broadening and filtering.Taking advantage of single beam configuration,the obtained pulses have excellent power and spectral stabilities.Since the output spectrum is obtained by spectrally filtering the broadened components,the temporal contrast of the output pulses is enhanced by at least four orders of magnitude.Together with the robust and simple setup,the proposed method is expected to be a competitive option for the generation of seed pulses for 10s-100s petawatt lasers.

Carrier-Envelope Phase-Dependent Phase Matching of Terahertz Emission in Laser Plasma

We investigate off-axis phase-matched terahertz(THz)radiation in laser plasma pumped by few-cycle laser pulses.We find that the THz amplitude and angular distributions in the far field are sensitively dependent on the pump pulse’s focal carrier-envelope phase(CEP).Ring-like profiles of THz radiation are obtained at CEP values of 0.5πand 1.5π,due to the inversely symmetric local THz waveforms emitted before and after laser focus.Off-axis phase-matched THz radiation offers a tool to accurately measure the CEP of few-cycle pulses at the center of a medium.

High-gradient modulation of microbunchings using a minimized system driven by a vortex laser

This study entailed the development of a high-gradient modulation of microbunching for traditional radiation frequency accelerators using a minimized system driven by a relativistic Laguerre–Gaussian(LG)laser in three-dimensional particlein-cell(PIC)simulations.It was observed that the LG laser could compress the transverse dimension of the beam to within a 0.7μm radius(divergence≈4.3 mrad),which is considerably lower than the case tuned by a Gaussian laser.In addition,the electron beam could be efficiently modulated to a high degree of bunching effect(>0.5)within~21 fs(~7μm)in the longitudinal direction.Such a high-gradient density modulation driven by an LG laser for pre-bunched,low-divergence,and stable electron beams provides a potential technology for the system minimization of X-ray free-electron lasers(XFELs)and ultrashort-scale(attosecond)electron diffraction research.

Controlling the collective radiative decay of molecular ions in strong laser fields

Molecular ions,produced via ultrafast ionization,can be quantum emitters with the aid of resonant electronic couplings,which makes them the ideal candidates to study strong-field quantum optics.In this work,we experimentally and numerically investigate the necessary condition for observing a collective emission arising from macroscopic quantum polarization in a population-inverted N_(2)^(+)gain system,uncovering how the individual ionic emitters proceed into a coherent collection within hundreds of femtoseconds.Our results show that for a relatively high-gain case,the collective emission behaviors can be readily initiated for all the employed triggering pulse area.However,for a low-gain case,the superradiant amplification is quenched since the building time of macroscopic interionic quantum coherence exceeds the dipole dephasing time,in which situation the seed amplification and free induction decay play an essential role.These findings not only clarify the contentious key issue regarding to the amplification mechanism of N_(2)^(+)lasing but also show the unique characteristics of ultrashort laser-induced amplification in a molecular ion system where both the microscopic and macroscopic quantum coherence might be present.

Polaron mobility modulation by bandgap engineering in black phaseα-FAPbI_(3)

Lead halide hybrid perovskites(LHP)have emerged as one of the most promising photovoltaic materials for their remarkable solar energy conversion ability.The transportation of the photoinduced carriers in LHP could screen the defect recombination with the help of the large polaron formation.However,the physical insight of the relationship between the superior optical-electronic performance of perovskite and its polaron dynamics related to the electron-lattice strong coupling induced by the substitution engineering is still lack of investigation.Here,the bandgap modulated thin films ofα-FAPbI_(3)with different element substitution is investigated by the time resolved Terahertz spectroscopy.We find the polaron recombination dynamics could be prolonged in LHP with a relatively smaller bandgap,even though the formation of polaron will not be affected apparently.Intuitively,the large polaron mobility in(FAPb I_(3))0.95(MAPbI_(3))0.05thin film is~30%larger than that in(FAPb I_(3))0.85(MAPbBr_(3))0.15.The larger mobility in(FAPb I_(3))0.95(MAPb I_(3))0.05could be assigned to the slowing down of the carrier scattering time.Therefore,the physical origin of the higher carrier mobility in the(FAPb I_(3))0.95(MAPbI_(3))0.05should be related with the lattice distortion and enhanced electron–phonon coupling induced by the substitution.In addition,(FAPbI_(3))0.95(MAPbI_(3))0.05will lose fewer active carriers during the polaron cooling process than that in(FAPb I_(3))0.85(MAPbBr_(3)),indicating lower thermal dissipation in(FAPbI_(3))0.95(MAPbI_(3))0.05.Our results suggest that besides the smaller bandgap,the higher polaron mobility improved by the substitution engineering inα-FAPbI_(3)can also be an important factor for the high PCE of the black phaseα-FAPbI_(3)based solar cell devices.

Heat accumulation effects in femtosecond laser-induced subwavelength periodic surface structures on silicon

High-repetition rate femtosecond lasers are shown to drive heat accumulation processes that are attractive for femtosecond laser-induced subwavelength periodic surface structures on silicon.Femtosecond laser micromachining is no longer a nonthermal process,as long as the repetition rate reaches up to 100 kHz due to heat accumulation.Moreover,a higher repetition rate generates much better defined ripple structures on the silicon surface,based on the fact that accumulated heat raises lattice temperature to the melting point of silicon(1687 K),with more intense surface plasmons excited simultaneously.Comparison of the surface morphology on repetition rate and on the overlapping rate confirms that repetition rate and pulse overlapping rate are two competing factors that are responsible for the period of ripple structures.Ripple period drifts longer because of a higher repetition rate due to increasing electron density;however,the period of laser structured surface is significantly reduced with the pulse overlapping rate.The Maxwell–Garnett effect is confirmed to account for the ripple period-decreasing trend with the pulse overlapping rate.

Intense ultraviolet-visible-infrared full-spectrum laser

A high-brightness ultrabroadband supercontinuum white laser is desirable for various fields of modern science.Here,we present an intense ultraviolet-visible-infrared full-spectrum femtosecond laser source(with 300–5000 nm 25 dB bandwidth)with 0.54 mJ per pulse.The laser is obtained by sending a 3.9μm,3.3 mJ mid-infrared pump pulse into a cascaded architecture of gas-filled hollow-core fiber,a bare lithium niobate crystal plate,and a specially designed chirped periodically poled lithium niobate crystal,under the synergic action of second and third order nonlinearities such as high harmonic generation and self-phase modulation.This full-spectrum femtosecond laser source can provide a revolutionary tool for optical spectroscopy and find potential applications in physics,chemistry,biology,material science,industrial processing,and environment monitoring.

Electro-optically tunable microdisk laser on Er^(3+)-doped lithium niobate thin film

We report an electro-optically(EO)tunable microdisk laser fabricated on the erbium(Er^(3+))-doped lithium niobate on insulator(LNOI) substrate.By applying a variable voltage on a pair of integrated chromium(Cr) microelectrodes fabricated near the LNOI microdisk,electro-optic modulation with an effective resonance-frequency tuning rate of 2.6 GHz/100 V has been achieved.This gives rise to a tuning range of 45 pm when the electric voltage is varied between-200 V and 200 V.

Femtosecond infrared optical vortex lasers based on optical parametric amplification

Infrared femtosecond optical vortices open up many new research fields,such as optical micro–nano manipulation,time-resolved nonlocal spectroscopy in solids,vortex secondary radiation and particle generations.In this article,we demonstrate a femtosecond optical vortex laser system based on a two-stage optical parametric amplifier.In our experiment,1.45µm vortex signal pulses with energy of 190µJ and 1.8µm vortex idler pulses with energy of 158µJ have been obtained,and the pulse durations are 51 and 48 fs,respectively.Both the energy fluctuations of the signal and idler pulses are less than 0.5%(root mean square),and the spectral fluctuations are less than 1.5%within 1 hour.This type of highly stable femtosecond optical vortex laser has a wide range of applications for vortex strong-field physics.

High-energy,high-repetition-rate ultraviolet pulses from an efficiency-enhanced,frequency-tripled laser

In this study,a high-energy,temporally shaped picosecond ultraviolet(UV)laser running at 100 Hz is demonstrated,with its pulses boosted to 120 mJ by cascaded regenerative and double-pass amplifiers,resulting in a gain of more than 10^(8).With precise manipulation and optimization,the amplified laser pulses were flat-top in the temporal and spatial domains to maintain high filling factors,which significantly improved the conversion efficiency of the subsequent third harmonic generation(THG).Finally,91 mJ,470 ps pulses were obtained at 355 nm,corresponding to a conversion efficiency as high as 76%,which,as far as we are aware of,is the highest THG efficiency for a high-repetition-rate picosecond laser.In addition,the energy stability of the UV laser is better than 1.07%(root mean square),which makes this laser an attractive source for a variety of fields including laser conditioning and micro-fabrication.

Automated synthesis of gadopentetate dimeglumine through solid-liquid reaction in femtosecond laser fabricated microfluidic chips

Despite the continuously increased requirement on automated synthesis of medicines for distributed manufacturing and personal care, it remains a challenge to realize automated synthesis which requires solid-liquid phase reactions. In this work, we demonstrated an automated solid-liquid synthesis for gadopentetate dimeglumine, the most widely used magnetic resonance imaging(MRI) contrast agent. The high-efficiency reaction was performed in a 3D microfluidic chip which was fabricated by femtosecond laser micromachining. The structure of the chip realized 3D shear flow which was essential for highly efficient mixing and movement of the solid-liquid mixtures. Ultraviolet visible(UV-vis) spectrometer was employed for in-line analysis to help automation of this system. Comparing with the round-bottom flask system, this synthetic system showed significantly higher reaction rate, indicating the advantage of the3D microfluidic technology in micro chemical engineering.

Femtosecond-laser-induced backward transfer of fluorinated ethylene propylene for fabrication of "lotus effect"surfaces

“Lotus effect” glass surfaces with fluorinated ethylene propylene were successfully fabricated by using a femtosecond laser-induced backward transfer(LIBT) method. By space-selectively modifying both the surface morphology and surface chemistry in a single step, LIBT provides a convenient and flexible route to fabricate superhydrophobic surfaces with ultralow adhesion. A systematic mechanism responsible for the anisotropic wetting behaviors and adhesion modulation was proposed with a combination of the Cassie and Wenzel models. X-ray photoelectron spectroscopy revealed that oxidation and defluorination were induced by laser radiation. LIBT is proved to be a promising method for programmable manipulations of functional surfaces with diverse wettability.

Electro-optic tuning of a single-frequencyultranarrow linewidth microdisk laser

Single-frequency ultranarrow linewidth on-chip microlasers with a fast wavelength tunability play a game-changing role in a broad spectrum of applications ranging from coherent communication,light detection and ranging,to metrology and sensing.Design and fabrication of such light sources remain a challenge due to the difficulties in making a laser cavity that has an ultrahigh optical quality(Q)factor and supports only a single lasing frequency simultaneously.Here,we demonstrate a unique single-frequency ultranarrow linewidth lasing mechanism on an erbium ion-doped lithium niobate(LN)microdisk through simultaneous excitation of high-Q polygon modes at both pump and laser wavelengths.As the polygon modes are sparse within the optical gain bandwidth compared with the whispering gallery mode counterpart,while their Q factors(above 10 million)are even higher due to the significantly reduced scattering on their propagation paths,single-frequency lasing with a linewidth as narrow as 322 Hz is observed.The measured linewidth is three orders of magnitude narrower than the previous record in on-chip LN microlasers.Finally,enabled by the strong linear electro-optic effect of LN,real-time electro-optical tuning of the microlaser with a high tuning efficiency of∼50 pm∕100 V is demonstrated.

Advances in metal halide perovskite lasers: synthetic strategies, morphology control, and lasing emission

.In the past decade,lead halide perovskites have emerged as potential optoelectronic materials in the fields of light-emitting diode,solar cell,photodetector,and laser,due to their low-cost synthesis method,tunable bandgap,high quantum yield,large absorption,gain coefficient,and low trap-state densities.In this review,we present a comprehensive discussion of lead halide perovskite applications,with an emphasis on recent advances in synthetic strategies,morphology control,and lasing performance.In particular,the synthetic strategies of solution and vapor progress and the morphology control of perovskite nanocrystals are reviewed.Furthermore,we systematically discuss the latest development of perovskite laser with various fundamental performances,which are highly dependent on the dimension and size of nanocrystals.Finally,considering current challenges and perspectives on the development of lead halide perovskite nanocrystals,we provide an outlook on achieving high-quality lead perovskite lasers and expanding their practical applications.