Laser-induced stretchable bioelectronic interfaces by frozen exfoliation

Highly stretchable laser-induced graphene-hydrogel film interfaces in flexible electronic materials are fabricated by frozen exfoliation,and exhibit high stretchability,durability,and design flexibility.This technology offers an advanced technological pathway for manufacturing highly flexible substrates.They can be utilized in numerous complex surface applications,providing an advanced technological pathway for manufacturing highly flexible substrates in the future.

Self-propelled Leidenfrost droplets on femtosecond-laser-induced surface with periodic hydrophobicity gradient

The controllable transfer of droplets on the surface of objects has a wide application prospect in the fields of microfluidic devices,fog collection and so on.The Leidenfrost effect can be utilized to significantly reduce motion resistance.However,the use of 3D structures limits the widespread application of self-propulsion based on Leidenfrost droplets in microelectromechanical system.To manipulate Leidenfrost droplets,it is necessary to create 2D or quasi-2D geometries.In this study,femtosecond laser is applied to fabricate a surface with periodic hydrophobicity gradient(SPHG),enabling directional self-propulsion of Leidenfrost droplets.Flow field analysis within the Leidenfrost droplets reveals that the vapor layer between the droplets and the hot surface can be modulated by the SPHG,resulting in directional propulsion of the inner gas.The viscous force between the gas and liquid then drives the droplet to move.

Photonic crystal fiber Mach-Zehnder interferometer with microholes ablated by a femtosecond laser for refractive index sensing

A new structure of the photonic crystal fiber（PCF）based Mach-Zednder interferometer（MZI）is fabricated and presented.The structure has microholes ablated by a femtosecond laser.The fringe visibility can be enhanced more than 10 dB compared with the interferometer without a microhole.The interferometer is characterized by sodium chloride solutions for refractive index（RI）sensing.The RI sensitivities are greatly increased by the hole fabrication since it directly changes the cladding modes of the PCF.For the interferometer sensor with two holes,the RI sensitivity is 157.74 nm/RIU,which is 5 times than that of the sensor without a microhole.Microholes ablation with a femtosecond laser on PCF can increase the sensor＇s sensitivity dramatically.Femtosecond laser has a wide application prospect in the field of performance improvement of the sensors.

Hybrid superhydrophilic-superhydrophobic micro/nanostructures fabricated by femtosecond laserinduced forward transfer for sub-femtomolar Raman detection

Raman spectroscopy plays a crucial role in biochemical analysis.Recently,superhydrophobic surface-enhanced Raman scattering(SERS)substrates have enhanced detection limits by concentrating target molecules into small areas.However,due to the wet transition phenomenon,further reduction of the droplet contact area is prevented,and the detection limit is restricted.This paper proposes a simple method involving femtosecond laser-induced forward transfer for preparing a hybrid superhydrophilic–superhydrophobic SERS(HS-SERS)substrate by introducing a superhydrophilic pattern to promote the target molecules to concentrate on it for ultratrace detection.Furthermore,the HS-SERS substrate is heated to promote a smaller concentrated area.The water vapor film formed by the contact of the solution with the substrate overcomes droplet collapse,and the target molecules are completely concentrated into the superhydrophilic region without loss during evaporation.Finally,the concentrated region is successfully reduced,and the detection limit is enhanced.The HS-SERS substrate achieved a final contact area of 0.013mm2,a 12.1-fold decrease from the unheated case.The reduction of the contact area led to a detection limit concentration as low as 10−16 M for a Rhodamine 6G solution.In addition,the HS-SERS substrate accurately controlled the size of the concentrated areas through the superhydrophilic pattern,which can be attributed to the favorable repeatability of the droplet concentration results.In addition,the preparation method is flexible and has the potential for fluid mixing,fluid transport,and biochemical sensors,etc.

Ultrafast dynamics observation during femtosecond laser-material interaction

Femtosecond laser technology has attracted significant attention from the viewpoints of fundamental and application;especially femtosecond laser processing materials present the unique mechanism of laser-material interaction.Under the extreme nonequilibrium conditions imposed by femtosecond laser irradiation,many fundamental questions concerning the physical origin of the material removal process remain unanswered.In this review,cutting-edge ultrafast dynamic observation techniques for investigating the fundamental questions,including timeresolved pump-probe shadowgraphy,ultrafast continuous optical imaging,and four-dimensional ultrafast scanning electron microscopy,are comprehensively surveyed.Each technique is described in depth,beginning with its basic principle,followed by a description of its representative applications in laser-material interaction and its strengths and limitations.The consideration of temporal and spatial resolutions and panoramic measurement at different scales are two major challenges.Hence,the prospects for technical advancement in this field are discussed finally.

Ultrafast quasi-three-dimensional imaging

Understanding laser induced ultrafast processes with complex three-dimensional(3D)geometries and extreme property evolution offers a unique opportunity to explore novel physical phenomena and to overcome the manufacturing limitations.Ultrafast imaging offers exceptional spatiotemporal resolution and thus has been considered an effective tool.However,in conventional single-view imaging techniques,3D information is projected on a two-dimensional plane,which leads to significant information loss that is detrimental to understanding the full ultrafast process.Here,we propose a quasi-3D imaging method to describe the ultrafast process and further analyze spatial asymmetries of laser induced plasma.Orthogonally polarized laser pulses are adopted to illuminate reflection-transmission views,and binarization techniques are employed to extract contours,forming the corresponding two-dimensional matrix.By rotating and multiplying the two-dimensional contour matrices obtained from the dual views,a quasi-3D image can be reconstructed.This successfully reveals dual-phase transition mechanisms and elucidates the diffraction phenomena occurring outside the plasma.Furthermore,the quasi-3D image confirms the spatial asymmetries of the picosecond plasma,which is difficult to achieve with two-dimensional images.Our findings demonstrate that quasi-3D imaging not only offers a more comprehensive understanding of plasma dynamics than previous imaging methods,but also has wide potential in revealing various complex ultrafast phenomena in related fields including strong-field physics,fluid dynamics,and cutting-edge manufacturing.

Micro/nano processing of natural silk fibers with near-field enhanced ultrafast laser

Silkworm silk fiber is an attractive material owing to its remarkable mechanical characteristics,excellent optical properties,and good biocompatibility and biodegradability.However,nano-processing of the silk fiber is still a challenge limiting its applications in nanoengineering and related fields.Herein,we report localized near-field enhancement-assisted ablation with an ultrafast laser to break this bottleneck.Localized processing of silk fiber,including nano-holing,nano-grooving,and cutting could retain the key molecular structure building blocks and the pristine functionality of the silk fiber.An extremely narrow nanohole with a width of^64 nm was successfully achieved.The processed silk fiber can be used to transfer micro/nanoparticles and drugs,showing potential for biomedical engineering.The processing strategy developed in this study can also be extended to other materials,paving a new way for fabricating functional nanostructures with precisely controlled size and morphology.

MoS_(2)core-shell nanoparticles prepared through liquid-phase ablation and light exfoliation of femtosecond laser for chemical sensing

Molybdenum disulfide(MoS_(2))-based nanostructures are highly desirable for applications such as chemical and biological sensing,photo/electrochemical catalysis,and energy storage due to their unique physical and chemical properties.In this work,MoS_(2)core-shell nanoparticles were first prepared through the liquid-phase processing of bulk MoS2by a femtosecond laser.The core of prepared nanoparticles was incompletely and weakly crystalline MoS_(2);the shell of prepared nanoparticles was highly crystalline MoS_(2),which wrapped around the core layer by layer.The femtosecond laser simultaneously achieved liquid-phase ablation and light exfoliation.The formation mechanism of the core-shell nanoparticles is to prepare the nanonuclei first by laser liquid-phase ablation and then the nanosheets by light exfoliation;the nanosheets will wrap the nanonuclei layer by layer through van der Waals forces to form core-shell nanoparticles.The MoS_(2)core-shell nanoparticles,because of Mo-S bond breakage and recombination,have high chemical activity for chemical catalysis.Afterward,the nanoparticles were used as a reducing agent to directly prepare three-dimensional(3D)Au-MoS_(2)micro/nanostructures,which were applied as surface-enhanced Raman spectroscopy(SERS)substrates to explore chemical sensing activity.The ultrahigh enhancement factor(1.06×10^(11)),ultralow detection limit(10-13M),and good SERS adaptability demonstrate highly sensitive SERS activity,great ability of ultralow concentration detection,and ability to detect diverse analytes,respectively.This work reveals the tremendous potential of 3D Au-MoS_(2)composite structures as excellent SERS substrates for chemical and biological sensing.

Femtosecond laser-induced graphene for temperature and ultrasensitive flexible strain sensing

Flexible sensors with high sensitivity and stability are essential components of electronic skin,applicable to detecting human movement,monitoring physiological health,preventing diseases,and other domains.In this study,we utilized a straightforward and efficient femtosecond laser direct writing technique using phenolic resin(PR)as a carbon precursor to produce high-quality laser-induced graphene(LiG)characterized by high crystallinity and low defect density.The fabricated LIG underwent comprehensive characterization using SEM,Raman spectroscopy,XPS,and XRD.Subsequently,we developed strain sensors with a hexagonal honeycomb pattern and temperature sensors with a line pattern based on PR-derived LIG.The strain sensor exhibited an outstanding measurement factor of 4.16×104 with a rapid response time of 32 ms,which is applied to detect various movements like finger movements and human pulse.Meanwhile,the temperature sensor demonstrated a sensitivity of 1.49%/°C with a linear response range of 20-5o C.The PR-derived LIG shows promising potential for applications in human physiological health monitoring and other advanced wearable technologies.

Refractive index sensing based on a twisted nano-kirigami metasurface

Plasmonic sensing technology has attracted considerable attention for high sensitivity due to the ability to effectively localize and manipulate light.In this study,we demonstrate a refractive index(RI)sensing scheme based on open-loop twisted meta-molecule arrays using the versatile nano-kirigami principle.RI sensing has the features of a small footprint,flexible control,and simple preparation.By engineering the morphology of meta-molecules or the RI of the ambient medium,the chiral surface lattice resonances can be significantly enhanced,and the wavelength,intensity,and sign of circular dichroism(CD)can be flexibly tailored.Utilizing the relation between the wavelength of the CD peak and the RI of the superstrate,the RI sensor achieves a sensitivity of 1133 nm/RIU.Additionally,we analyze these chiroptical responses by performing electromagnetic multipolar decomposition and electric field distributions.Our study may serve as an ideal platform for applications of RI measurement and provide new insights into the manipulation of chiral light–matter interactions.

Bottom-up scalable temporally-shaped femtosecond laser deposition of hierarchical porous carbon for ultrahigh-rate micro-supercapacitor

With the accelerated development of electronic devices,micro-supercapacitors(MSCs),as energy storage devices that can charge and discharge quickly,have attracted considerable attention.To improve the rate capability of MSCs with consideration of the energy density remains a challenge.We demonstrated a facile method for the preparation of thin films through bottom-up femtosecond pulsed laser deposition.The femtosecond laser irradiated the polyimide film through a transparent substrate to uniformly sputter the electrode material onto the lower surface of the substrate.We successfully deposited porous amorphous carbon,graphene,and carbon quantum dots with controllable properties by temporally shaping the femtosecond laser.The resulting MSC exhibited an ultrahigh frequency response and good performance at scan rates up to 10,000 V s^(-1).The characteristic frequency f_(0) of the MSC was as high as 42,000 Hz,and the relaxation time constant τ_(0) was 0.0238 ms.The MSC reached an impedance phase angle of -82.6° at a frequency of 120 Hz,an ultrahigh power density of more than 30 kW cm^(-3),and an energy density of 0.068 W h cm-3.This method provides a novel perspective for the preparation of ultrahigh frequency filters for future miniaturized portable electronic devices.

Betavoltaic Battery Conversion Efficiency Improvement Based on Interlayer Structures

Significant differences among the doping densities of PN junctions in semiconductors cause lattice mismatch and lattice defects that increase the recombination current of betavoltaic batteries.This extensively decreases the open circuit voltage and the short current,which results in low conversion efficiency.This study proposes P+PINN+-structure based betavoltaic batteries by adding an interlayer to typical PIN structures to improve conversion efficiency.Numerical simulations are conducted for the energy deposition of beta particles along the thickness direction in semiconductors.Based on this,63 Ni-radiation GaAs batteries with PIN and P+PINN+ structures are designed and fabricated to experimentally verify the proposed design.It turns out that the conversion efficiency of the betavoltaic battery with the proposed P+ PINN+ structure is about 1.45 times higher than that with the traditional PIN structure.

Highly sensitive and homogeneous SERS substrate fabricated by a femtosecond laser combined with dewetting

We report a simple, cost-effective and repeatable method for fabricating a large area and uniform substrate for surface-enhanced Raman scattering(SERS). The silicon, micromachined by a femtosecond laser, is coated with gold film and then treated through the dewetting process. The morphology shows a higher electric field enhancement due to light trapping. The enhancement factor of the SERS substrate is 9.2 × 107 with a 5 nm-thick film coated. Moreover, it also exhibits a uniform signal through Raman mapping and chemical stability with the greatest intensity deviation of 6% after a month. The proposed technique provides an opportunity to equip microchips with the SERS capabilities of high sensitivity, chemical stability, and homogeneous signals.

Probing electron and lattice dynamics by ultrafast electron microscopy: Principles and applications

Microscale charge and energy transfer is an ultrafast process that can determine the photoelectrochemical performance of devices.However,nonlinear and nonequilibrium properties hinder our understanding of ultrafast processes;thus,the direct imaging strategy has become an effective means to uncover ultrafast charge and energy transfer processes.Due to diffraction limits of optical imaging,the obtained optical image has insufficient spatial resolution.Therefore,electron beam imaging combined with a pulse laser showing high spatial–temporal resolution has become a popular area of research,and numerous breakthroughs have been achieved in recent years.In this review,we cover three typical ultrafast electron beam imaging techniques,namely,time-resolved photoemission electron microscopy,scanning ultrafast electron microscopy,and ultrafast transmission electron microscopy,in addition to the principles and characteristics of these three techniques.Some outstanding results related to photon–electron interactions,charge carrier transport and relaxation,electron–lattice coupling,and lattice oscillation are also reviewed.In summary,ultrafast electron beam imaging with high spatial–temporal resolution and multidimensional imaging abilities can promote the fundamental under-standing of physics,chemistry,and optics,as well as guide the development of advanced semiconductors and electronics.

High resolution angular-displacement sensor based on whispering gallery mode resonance in bent optical fibers

A simple fiber sensor to measure angular displacement with high resolution, which is based on whispering gallery mode （WGM） resonance in bent optical fibers,is proposed. The sensor is composed of a single loop formed by loosely tying a knot using single mode fiber. To measure the transmission spectra, a tunable laser and an optic power meter are connected to the two ends of fi- ber loop, respectively. Significant WGM resonances occur over the investigated wavelength range for all the sensors with different bend radius. The angular-displacement sensitivity is studied in the range from -0. 1°to 0. 1°. The detection limit of 1.49 × 10 ^-7 rad can be achieved for the detecting system with the resolution of lpm. The simple loop-structure fiber sensor has potential application prospect in the field of architecture or bridge building with low detection limit and low cost.

Local environment regulation of transition metal dichalcogenide-based single-atom catalysts

Single-atom catalysts have risen significant attention in the realm of green electrocatalytic energy conversion to address energy and environmental sustainability challenges.Transition metal dichalcogenide(TMD)-based single-atom catalysts are considered highly effective in electrocatalysis due to the TMDs'notable specific surface area,tunable elemental species and efficient utilization of single atoms.In order to enhance electrocatalytic performance,it is imperative to elaborately engineer the local environment surrounding the active sites of single atoms within TMDs.In this review,we initially explore the effects of synthesis methods on single-atom active sites and the influence of loading of single atoms on catalytic performance for TMDs.The modulation strategies of the local environment surrounding single-atom sites in TMDs are elaborated,including substitution engineering,surface adsorption,vacancies,spatial confinement and dual-atom site strategies.For each modulation strategy,the effects of diverse local environments on various electrocatalytic applications are presented,such as the oxygen evolution reaction,oxygen reduction reaction,nitrogen reduction reaction,CO_(2)reduction reaction and CO oxidation.Ultimately,this study presents a comprehensive overview of the challenges encountered and the potential directions for the advancement of single-atom catalysts based on TMDs in the realm of electrocatalysis.

High period frequency LIPSS emerging on 304 stainless steel under the irradiation of femtosecond laser double-pulse trains

In this work,we used femtosecond laser double-pulse trains to produce laser-induced periodic surface structures(LIPSS)on 304 stainless steel.Surprisingly,a novel type of periodic structure was discovered,which,to the best of our knowledge,is the first in literature.We surmised that the cause for this novel LIPSS was related to the weak energy coupling of subpulses when the intrapulse delay was longer than the thermal relaxation time of stainless steel.Furthermore,we found that the fluence combination and arrival sequence of subpulses in a double-pulse train also influenced LIPSS morphology.

Enhancing the expansion of a plasma shockwave by crater-induced laser refocusing in femtosecond laser ablation of fused silica

The dynamics of plasma and shockwave expansion during two femtosecond laser pulse ablation of fused silica are studied using a time-resolved shadowgraph imaging technique. The experimental results reveal that during the second pulse irradiation on the crater induced by the first pulse, the expansion of the plasma and shockwave is enhanced in the longitudinal direction. The plasma model and Fresnel diffraction theory are combined to calculate the laser intensity distribution by considering the change in surface morphology and transient material properties. The theoretical results show that after the free electron density induced by the rising edge of the pulse reaches the critical density, the originally transparent surface is transformed into a transient high-reflectivity surface(metallic state). Thus, the crater with a concave-lens-like morphology can tremendously reflect and refocus the latter part of the laser pulse, leading to a strong laser field with an intensity even higher than the incident intensity. This strong refocused laser pulse results in a stronger laser-induced air breakdown and enhances the subsequent expansion of the plasma and shockwave. In addition, similar shadowgraphs are also recorded in the single-pulse ablation of a concave microlens, providing experimental evidence for the enhancement mechanism.

Structure-Mediated Excitation of Air Plasma and Silicon Plasma Expansion in Femtosecond Laser Pulses Ablation

Femtosecond laser-induced surface structures upon multiple pulses irradiation are strongly correlated with the pulse number,which in turn signifcantly afects successive laser-material interactions.By recording the dynamics of femtosecond laser ablation of silicon using time-resolved shadowgraphy,here we present direct visualization of the excitation of air plasma induced by the refected laser during the second pulse irradiation.Te interaction of the air plasma and silicon plasma is found to enhance the shockwave expansion induced by silicon ablation in the longitudinal direction,showing anisotropic expansion dynamics in diferent directions.We further demonstrate the vanishing of air plasma as the pulse number increases because of the generation of a rough surface without light focusing ability.In the scenario,the interaction of air plasma and silicon plasma disappears;the expansion of the silicon plasma and shockwave restores its original characteristic that is dominated by the laser-material coupling.Te results show that the excitation of air plasma and the laser-material coupling involved in laser-induced plasma and shockwave expansion are structure mediated and dependent on the pulse number,which is of fundamental importance for deep insight into the nature of laser-material interactions during multiple pulses ablation.

Morphology control of laser-induced periodic surface structure on the surface of nickel by femtosecond laser

An interesting transition between low spatial frequency laser-induced periodic surface structure （LIPSS） and high spatial frequency LIPSS （HSFL） on the surface of aickel is revealed by changing the scanning speed and the laser fluence. The experimental results show the proportion of HSFL area in the overall LIPSS （i.e., K） presents a quasi-parabola function trend with the polarization orientation under a femtoseeond （fs） laser single-pulse train. Moreover, an obvious fluctuation dependence of Kon the pulse delay is observed under a fs laser dual-pulse train. The peak value of the fluctuation is found to be determined by the polarization orientation of the dual-pulse train.