Corrosion and Fatigue Testing of Microsized 304 Stainless Steel Beams Fabricated by Femtosecond Laser

The 304 stainless steel (SS) microcantilever specimens with dimensions of 30 μm×30 μm×50 μm (thickness× width×length) were fabricated by femtosecond (fs) laser. The microsized cantilevers of good quality with struc- ture and dimensions according commendably with that of the designed cantilever were obtained. The result shows that fs laser micromachining is a promising method for directly fabricating metallic microcomponents. Corrosion and fatigue properties of microsized specimens were carried out on the microsized 304 SS cantilever beams by a newly developed fatigue testing machine. The results show that the microsized 304 SS specimens appear to have an improved resistance towards localized corrosion compared to ordinary-sized 304 SS speci- mens after the static corrosion testing. The testing result shows that the presence of corrosive solution reduces the fatigue lifetime of the 304 SS specimen by a factor of 10–100. The maximum bending loads measured by fatigue testing machine decrease rapidly at the terminal stage of environment assisted fatigue testing. Corrosion fracture first occurred at the range of notch with a higher tensile bending stress, and exhibited clear evidence of trans-columnar fracture detected by SEM (scanning electron microscopy).

260 fs,403 W coherently combined fiber laser with precise high‑order dispersion management

An ultrafast fiber laser system comprising two coherently combined amplifier channels is reported.Within this system,each channel incorporates a rod-type fiber power amplifier,with individual operations reaching approximately 233 W.The active-locking of these coherently combined channels,followed by compression using gratings,yields an output with a pulse energy of 504μJ and an average power of 403 W.Exceptional stability is maintained,with a 0.3%root mean square(RMS)deviation and a beam quality factor M^(2)<1.2.Notably,precise dispersion management of the front-end seed light effectively compensates for the accumulated high-order dispersion in subsequent amplification stages.This strategic approach results in a significant reduction in the final output pulse duration for the coherently combined laser beam,reducing it from 488 to 260 fs after the gratings compressor,while concurrently enhancing the energy of the primary peak from 65%to 92%.

Controllable alignment of elongated microorganisms in 3D microspace using electrofluidic devices manufactured by hybrid femtosecond laser microfabrication

This paper presents a simple technique to fabricate new electrofluidic devices for the three-dimensional(3D)manipulation of microorganisms by hybrid subtractive and additive femtosecond(fs)laser microfabrication(fs laser-assisted wet etching of glass followed by water-assisted fs laser modification combined with electroless metal plating).The technique enables the formation of patterned metal electrodes in arbitrary regions in closed glass microfluidic channels,which can spatially and temporally control the direction of electric fields in 3D microfluidic environments.The fabricated electrofluidic devices were applied to nanoaquariums to demonstrate the 3D electro-orientation of Euglena gracilis(an elongated unicellular microorganism)in microfluidics with high controllability and reliability.In particular,swimming Euglena cells can be oriented along the z-direction(perpendicular to the device surface)using electrodes with square outlines formed at the top and bottom of the channel,which is quite useful for observing the motions of cells parallel to their swimming directions.Specifically,z-directional electric field control ensured efficient observation of manipulated cells on the front side(45 cells were captured in a minute in an imaging area of~160×120μm),resulting in a reduction of the average time required to capture the images of five Euglena cells swimming continuously along the z-direction by a factor of~43 compared with the case of no electric field.In addition,the combination of the electrofluidic devices and dynamic imaging enabled observation of the flagella of Euglena cells,revealing that the swimming direction of each Euglena cell under the electric field application was determined by the initial body angle.

Attosecond probing and control of charge migration in carbon-chain molecule

In quantum mechanics,when an electron is quickly ripped off from a molecule,a superposition of new eigenstates of the cation creates an electron wave packet that governs the charge flow inside,which has been called charge migration(CM).Experimentally,extracting such dynamics at its natural(attosecond)timescale is quite difficult.We report the first such experiment in a linear carbon-chain molecule,butadiyne(C_(4)H_(2)),via high-harmonic spectroscopy(HHS).By employing advanced theoretical and computational tools,we showed that the wave packet and the CM of a single molecule are reconstructed from the harmonic spectra for each fixed-in-space angle of the molecule.For this onedimensional molecule,we calculate the center of charge <x>(t) to obtain v_(cm),to quantify the migration speed and how it depends on the orientation angle.The findings also uncover how the electron dynamics at the first few tens to hundreds of attoseconds depends on molecular structure.The method can be extended to other molecules where the HHS technique can be employed.

Structuring Nonlinear Wavefront Emitted from Monolayer Transition-Metal Dichalcogenides

The growing demand for tailored nonlinearity calls for a structure with unusual phase discontinuity that allows the realization of nonlinear optical chirality,holographic imaging,and nonlinear wavefront control.Transition-metal dichalcogenide(TMDC)monolayers offer giant optical nonlinearity within a few-angstrom thickness,but limitations in optical absorption and domain size impose restriction on wavefront control of nonlinear emissions using classical light sources.In contrast,noble metal-based plasmonic nanosieves support giant field enhancements and precise nonlinear phase control,with hundred-nanometer pixellevel resolution;however,they suffer from intrinsically weak nonlinear susceptibility.Here,we report a multifunctional nonlinear interface by integrating TMDC monolayers with plasmonic nanosieves,yielding drastically different nonlinear functionalities that cannot be accessed by either constituent.Such a hybrid nonlinear interface allows second-harmonic(SH)orbital angular momentum(OAM)generation,beam steering,versatile polarization control,and holograms,with an effective SH nonlinearityχ^((2))of~25 nm/V.This designer platform synergizes the TMDC monolayer and plasmonic nanosieves to empower tunable geometric phases and large field enhancement,paving the way toward multifunctional and ultracompact nonlinear optical devices.

Nonlinear Talbot self-healing in periodically poled LiNbO_3 crystal [Invited]

The nonlinear Talbot effect is a near-field nonlinear diffraction phenomenon in which the self-imaging of periodic objects is formed by the second harmonics of the incident laser beam. We demonstrate the first, to the best of our knowledge, example of nonlinear Talbot self-healing, i.e., the capability of creating defect-free images from faulty nonlinear optical structures. In particular, we employ the tightly focused femtosecond infrared optical pulses to fabricate LiNbO_(3) nonlinear photonic crystals and show that the defects in the form of the missing points of two-dimensional square and hexagonal periodic structures are restored in the second harmonic images at the first nonlinear Talbot plane. The observed nonlinear Talbot self-healing opens up new possibilities for defect-tolerant optical lithography and printing.

Resolving and weighing the quantum orbits in strong-field tunneling ionization

Tunneling ionization of atoms and molecules induced by intense laser pulses contains the contributions of numerous quantum orbits.Identifying the contributions of these orbits is crucial for exploring the application of tunneling and for understanding various tunneling-triggered strong-field phenomena.We perform a combined experimental and theoretical study to identify the relative contributions of the quantum orbits corresponding to the electrons tunneling ionized during the adjacent rising and falling quarter cycles of the electric field of the laser pulse.In our scheme,a perturbative second-harmonic field is added to the fundamental driving field.By analyzing the relative phase dependence of the signal in the photoelectron momentum distribution,the relative contributions of these two orbits are unambiguously determined.Our results show that their relative contributions sensitively depend on the longitudinal momentum and modulate with the transverse momentum of the photoelectron,which is attributed to the interference of the electron wave packets of the long orbit.The relative contributions of these orbits resolved here are important for the application of strong-field tunneling ionization as a photoelectron spectroscopy for attosecond time-resolved measurements.

Full experimental determination of tunneling time with attosecond-scale streaking method

Tunneling is one of the most fundamental and ubiquitous processes in the quantum world.The question of how long a particle takes to tunnel through a potential barrier has sparked a long-standing debate since the early days of quantum mechanics.Here,we propose and demonstrate a novel scheme to accurately determine the tunneling time of an electron.In this scheme,a weak laser field is used to streak the tunneling current produced by a strong elliptically polarized laser field in an attoclock configuration,allowing us to retrieve the tunneling ionization time relative to the field maximum with a precision of a few attoseconds.This overcomes the difficulties in previous attoclock measurements wherein the Coulomb effect on the photoelectron momentum distribution has to be removed with theoretical models and it requires accurate information of the driving laser fields.We demonstrate that the tunneling time of an electron from an atom is close to zero within our experimental accuracy.Our study represents a straightforward approach toward attosecond time-resolved imaging of electron motion in atoms and molecules.

Probing the launching position of the electron wave packet in molecule strong-field tunneling ionization

Strong-field tunneling ionization is the first step for a broad class of phenomena in intense laser-atom/molecule interactions. Accurate information about the electron wave packet from strong-field tunneling ionization of atoms and molecules is of essential importance for understanding various tunneling ionization triggered processes. Here, we survey the property of the electron wave packet in tunneling ionization of molecules with a method based on strong-field photoelectron holography. By solving the time-dependent Schr ¨odinger equation, it is shown that the holographic interference in the photoelectron momentum distribution exhibits the asymmetric behavior with respect to the laser polarization direction, when the molecule is aligned with a nonzero angle to the linearly polarized laser field. We demonstrate that this asymmetry is due to the nonzero initial transverse displacement of the electron wave packet at tunneling. By analyzing the holographic interference, this transverse displacement for the launching of electron wave packet tunneling from the molecules is accurately retrieved. This displacement is directly related to the electron density distribution in molecules, and thus our work developed a novel concept for probing electronic structure in molecules.

Real-time observation of frequency Bloch oscillations with fibre loop modulation

Bloch oscillations(BOs)were initially predicted for electrons in a solid lattice to which a static electric field is applied.The observation of BOs in solids remains challenging due to the collision scattering and barrier tunnelling of electrons.Nevertheless,analogies of electron BOs for photons,acoustic phonons and cold atoms have been experimentally demonstrated in various lattice systems.Recently,BOs in the frequency dimension have been proposed and studied by using an optical micro-resonator,which provides a unique approach to controlling the light frequency.However,the finite resonator lifetime and intrinsic loss hinder the effect from being observed practically.Here,we experimentally demonstrate BOs in a synthetic frequency lattice by employing a fibre-loop circuit with detuned phase modulation.We show that a detuning between the modulation period and the fibre-loop roundtrip time acts as an effective vector potential and hence a constant effective force that can yield BOs in the modulation-induced frequency lattices.With a dispersive Fourier transformation,the pulse spectrum can be mapped into the time dimension,and its transient evolution can be precisely measured.This study offers a promising approach to realising BOs in synthetic dimensions and may find applications in frequency manipulations in optical fibre communication systems.

Extracting the phase distribution of the electron wave packet ionized by an elliptically polarized laser pulse

We use an interferometic scheme to extract the phase distribution of the electron wave packet from above-threshold ionization in elliptically polarized laser fields.In this scheme,an electron wave packet released from a circularly polarized laser pulse acts as a reference wave and interferes with the electron wave packet ionized by a time-delayed counter-rotating elliptically polarized laser field.The generated vortex-shaped interference pattern in the photoelectron momentum distribution enables us to extract the phase distribution of the electron wave packet in the elliptically polarized laser pulse with high precision.By artificially screening the ionic potential at different ranges when solving the time-dependent Schördinger equation,we find that the angle-dependent phase distribution of the electron wave packet in the elliptically polarized laser field shows an obvious angular shift as compared to the strong-field approximation,whose value is the same as the attoclock shift.We also show that the amplitude of the angle-dependent phase distribution is sensitive to the ellipticity of the laser pulse,providing an alternative way to precisely calibrate the laser ellipticity in the attoclock measurement.

Direct Visualization of Deforming Atomic Wavefunction in Ultraintense High-Frequency Laser Pulses

Interaction of intense laser fields with atoms distorts the bound-state electron cloud.Tracing the temporal response of the electron cloud to the laser field is of fundamental importance for understanding the ultrafast dynamics of various nonlinear phenomena of matter,but it is particularly challenging.Here,we show that the ultrafast response of the atomic electron cloud to the intense high-frequency laser pulses can be probed with the attosecond time-resolved photoelectron holography.In this method,an infrared laser pulse is employed to trigger tunneling ionization of the deforming atom.The shape of the deforming electron cloud is encoded in the hologram of the photoelectron momentum distribution.As a demonstration,by solving the time-dependent Schrödinger equation,we show that the adiabatic deforming of the bound-state electron cloud,as well as the nonadiabatic transition among the distorted states,is successfully tracked with attosecond resolution.Our work films the formation process of the metastable Kramers-Henneberger states in the intense high-frequency laser pulses.This establishes a novel approach for time-resolved imaging of the ultrafast bound-state electron processes in intense laser fields.