Online single-shot characterization of ultrafast pulses from high-gain free-electron lasers

X-ray free-electron lasers(FELs)provide cutting-edge tools for fundamental researches to study nature down to the atomic level at a time-scale that fits this resolution.A precise knowledge of temporal information of FEL pulses is the central issue for its applications.Here we proposed and demonstrated a novel method to determine the FEL temporal profiles online.This robust method,designed for ultrafast FELs,allows researchers to acquire real-time longitudinal profiles of FEL pulses as well as their arrive times with respect to the external optical laser with a resolution better than 6 fs.Based on this method,we can also directly measure various properties of FEL pulses and correlations between them online.This helps us to further understand the FEL lasing processes and realize the generation of stable,nearly fully coherent soft X-ray laser pulses at the Shanghai Soft X-ray FEL facility.This method will enhance the experimental opportunities for ultrafast science in various areas.

Effect of ballistic electrons on ultrafast thermomechanical responses of a thin metal film

The ultrafast thermomechanical coupling problem in a thin gold film irradiated by ultrashort laser pulses with different electron ballistic depths is investigated via the ultrafast thermoelasticity model. The solution of the problem is obtained by solving finite element governing equations. The comparison between the results of ultrafast thermomechanical coupling responses with different electron ballistic depths is made to show the ballistic electron effect. It is found that the ballistic electrons have a significant influence on the ultrafast thermomechanical coupling behaviors of the gold thin film and the best laser micromachining results can be achieved by choosing the specific laser technology（large or small ballistic range）.In addition, the influence of simplification of the ultrashort laser pulse source on the results is studied, and it is found that the simplification has a great influence on the thermomechanical responses, which implies that care should be taken when the simplified form of the laser source term is applied as the Gaussian heat source.

Ultrafast Stretched-Pulse Fiber Laser Mode-Locked by Carbon Nanotubes

Ultrafast fiber sources having short pulses, broad bandwidth, high energy, and low amplitude fluctuations have widespread applications. Stretched-pulse fiber lasers, incorporating segments of normal and anomalous dispersion fibers, are a preferred means to generate such pulses. We realize a stretched-pulse fiber laser based on a nanotube saturable absorber, with 113 fs pulses, 33.5 nm spectral width and ~0.07% amplitude fluctuation, outperforming current nanotube-based designs.

Terahertz generation from laser-induced plasma

Interest of the research in terahertz(THz)wave has been strongly motivated by its wide applications in the fields of physics,chemistry,biology,and engineering.Developing efficient and reliable THz source is of uttermost priority in these researches.Numerous attempts have been made in fulfilling the THz generation.Greatly benefited from the progress of the ultrafast pulses,the laser-induced-plasma is one of the auspicious tools to provide desirable THz waves,owing to its superiorities in high power threshold,intense THz signal,and ultrawide THz spectrum.This paper reviews the physics and progress of the THz generation from the laser-induced plasmas,which are produced by gas,liquid,and solid.The characteristics of the emitted THz waves are also included.There are many complicated physical processes involved in the interactions of laser-plasma,making various laser-plasma scenarios in the THz generations.In view of this,we will only focus on the THz generation classified by physical mechanisms.Finally,we discuss a perspective on the future of THz generation from the laser-induced plasma,as well as its involved challenges.

High spatial-resolution biological tissue imaging in the second near-infrared region via optical parametric amplification pumped by an ultrafast vortex pulse [Invited]

Applying an ultrafast vortex laser as the pump,optical parametric amplification can be used for spiral phase-contrast imaging with high gain,wide spatial bandwidth,and high imaging contrast.Our experiments show that this design has realized the 1064 nm spiral phase-contrast idler imaging of biological tissues(frog egg cells and onion epidermis)with a spatial resolution at several microns level and a superior imaging contrast to both the traditional bright-or dark-field imaging under a weak illumination of 7 nW/cm^(2).This work provides a powerful way for biological tissue imaging in the second near-infrared region.

Forward acceleration and generation of femtosecond megaelectronvolt electron beams by an ultrafast intense laser pulse

We present a new mechanism of energy gain of electrons accelerated by a laser pulse. It is shown that when the intensity of an Tiltrafast intense laser pulse decreases rapidly along the direction of propagation, electrons leaving the pulse experience an action of ponderomotive deceleration at the descending part of a lower-intensity laser field than acceleration at the ascending part of a high-intensity field, thus gain net energy from the pulse and move directly forward. By means of such a mechanism, a megaelectronvolt electron beam with a bunch length shorter than 100 fs could be realized with an ultrafast (≤30 fs), intense (≥1019 W/cm2) laser pulse.

Magnetic skyrmions:Basic properties and potential applications

Magnetic skyrmions are particle-like topological magnetic textures that are potential information carriers in future spintronics.An enormous body of research confirms their existence in a broad range of magnetic materials since their first discovery in 2009.To date,magnetic skyrmions can not only be found in asymmetric systems but also in centrosymmetric ones.Notably,engineered magnetic multilayers are promising structures for skyrmion-based spintronics because they can stabilize small-sized skyrmions at room temperature and facilitate their electric manipulation.In this overview,we introduce the topological nature,their special properties,and nucleation methods of skyrmions,and show their potential for applications.Perspectives on skyrmionic devices and developments toward other,more three-dimensional particle-like magnetic nanostructures,are discussed at the end.