Deep-subwavelength single grooves prepared by femtosecond laser direct writing on Si

It is well known that femtosecond laser pulses can easily spontaneously induce deep-subwavelength periodic surface structures on transparent dielectrics but not on non-transparent semiconductors.Nevertheless,in this study,we demonstrate that using high-numerical-aperture 800 nm femtosecond laser direct writing with controlled pulse energy and scanning speed in the near-damage-threshold regime,polarization-dependent deep-subwavelength single grooves with linewidths of~180 nm can be controllably prepared on Si.Generally,the single-groove linewidth increases slightly with increase in the pulse energy and decrease in the scanning speed,whereas the single-groove depth significantly increases from~300 nm to~600 nm with decrease in the scanning speed,or even to over 1μm with multi-processing,indicating the characteristics of transverse clamping and longitudinal growth of such deep-subwavelength single grooves.Energy dispersive spectroscopy composition analysis of the near-groove region confirms that single-groove formation tends to be an ultrafast,non-thermal ablation process,and the oxidized deposits near the grooves are easy to clean up.Furthermore,the results,showing both the strong dependence of groove orientation on laser polarization and the occurrence of double-groove structures due to the interference of pre-formed orthogonal grooves,indicate that the extraordinary field enhancement of strong polarization sensitivity in the deep-subwavelength groove plays an important role in single-groove growth with high stability and collimation.

Laser direct writing of Ga_(2)O_(3)/liquid metal-based flexible humidity sensors

Flexible and wearable humidity sensors play a vital role in daily point-of-care diagnosis and noncontact human-machine interactions.However,achieving a facile and high-speed fabrication approach to realizing flexible humidity sensors remains a challenge.In this work,a wearable capacitive-type Ga_(2)O_(3)/liquid metal-based humidity sensor is demonstrated by a one-step laser direct writing technique.Owing to the photothermal effect of laser,the Ga_(2)O_(3)-wrapped liquid metal particles can be selectively sintered and converted from insulative to conductive traces with a resistivity of 0.19Ω·cm,while the untreated regions serve as active sensing layers in response to moisture changes.Under 95%relative humidity,the humidity sensor displays a highly stable performance along with rapid response and recover time.Utilizing these superior properties,the Ga_(2)O_(3)/liquid metal-based humidity sensor is able to monitor human respiration rate,as well as skin moisture of the palm under different physiological states for healthcare monitoring.

3D direct writing and micro detonation of CL-20 based explosive ink containing O/W emulsion binder

The booming development of DIW technology present an unprecedented prospect in energetic materials field and has attracted great interest due to its relative simplicity and high flexibility of manufacturing.Herein,a novel CL-20 based explosive ink formulation have been developed successfully for MEMS initiation systems via DIW technology.We designed PVA/GAP into an oil-in-water(O/W)emulsion,in the way that the aqueous solution of PVA as water phase,the ethyl acetate solution of GAP as oil phase,the combination of Tween 80 and SDS as emulsifier,BPS as a curing agent of GAP.The ideal formulation with good shear-thinning rheology properties and clear gel point was prepared using only 10 wt%emulsion.The dual-cured network formed during the curing process made the printed sample have good mechanical properties.The printed samples had satisfactory molding effect without cracks or fractures,the crystal form of CL-20 not changed and the thermal stability have improved.Deposition of explosive inks via DIW in micro-scale grooves had excellent detonation performances,which critical detonation size was 1×0.045 mm,detonation velocity was 7129 m/s and when the corner reaching 150°can still detonated stably.This study may open new avenues for developing binder systems in explosive ink formulations.

Femtosecond laser direct writing of functional stimulus-responsive structures and applications

Diverse natural organisms possess stimulus-responsive structures to adapt to the surrounding environment.Inspired by nature,researchers have developed various smart stimulus-responsive structures with adjustable properties and functions to address the demands of ever-changing application environments that are becoming more intricate.Among many fabrication methods for stimulus-responsive structures,femtosecond laser direct writing(FsLDW)has received increasing attention because of its high precision,simplicity,true three-dimensional machining ability,and wide applicability to almost all materials.This paper systematically outlines state-of-the-art research on stimulus-responsive structures prepared by FsLDW.Based on the introduction of femtosecond laser-matter interaction and mainstream FsLDW-based manufacturing strategies,different stimulating factors that can trigger structural responses of prepared intelligent structures,such as magnetic field,light,temperature,pH,and humidity,are emphatically summarized.Various applications of functional structures with stimuli-responsive dynamic behaviors fabricated by FsLDW,as well as the present obstacles and forthcoming development opportunities,are discussed.

Femtosecond-laser direct writing 3D micro/nano-lithography using VIS-light oscillator

Here we report a femtosecond laser direct writing(a precise 3D printing also known as two-photon polymerization lithography) of hybrid organic-inorganic SZ2080^(TM)pre-polymer without using any photo-initiator and applying ~100 fs oscillator operating at 517 nm wavelength and 76 MHz repetition rate. The proof of concept was experimentally demonstrated and benchmarking 3D woodpile nanostructures, micro-scaffolds, free-form micro-object “Benchy” and bulk micro-cubes are successfully produced. The essential novelty underlies the fact that non-amplified laser systems delivering just 40-500 p J individual pulses are sufficient for inducing localized cross-linking reactions within hundreds of nanometers in cross sections. And it is opposed to the prejudice that higher pulse energies and lower repetition rates of amplified lasers are necessary for structuring non-photosensitized polymers. The experimental work is of high importance for fundamental understanding of laser enabled nanoscale 3D additive manufacturing and widens technology’ s field of applications where the avoidance of photo-initiator is preferable or is even a necessity, such as micro-optics, nano-photonics, and biomedicine.

Bioinspired micro/nanostructured surfaces prepared by femtosecond laser direct writing for multi-functional applications

manufacturing of biomimetic micro/nanostructures due to its specific advantages including high precision,simplicity,and compatibility for diverse materials in comparison with other methods(e.g.ion etching,sol-gel process,chemical vapor deposition,template method,and self-assembly).These biomimetic micro/nanostructured surfaces are of significant interest for academic and industrial research due to their wide range of potential applications,including self-cleaning surfaces,oil-water separation,and fog collection.This review presents the inherent relationship between natural organisms,fabrication methods,micro/nanostructures and their potential applications.Thereafter,we throw a list of current fabrication strategies so as to highlight the advantages of FLDW in manufacturing bioinspired microstructured surfaces.Subsequently,we summarize a variety of typical bioinspired designs(e.g.lotus leaf,pitcher plant,rice leaf,butterfly wings,etc)for diverse multifunctional micro/nanostructures through extreme femtosecond laser processing technology.Based on the principle of interfacial chemistry and geometrical optics,we discuss the potential applications of these functional micro/nanostructures and assess the underlying challenges and opportunities in the extreme fabrication of bioinspired micro/nanostructures by FLDW.This review concludes with a follow up and an outlook of femtosecond laser processing in biomimetic domains.

Laser direct writing derived robust carbon nitride films with efficient photon-to-electron conversion for multifunctional photoelectrical applications

Carbon nitride,an emerging polymeric semiconductor,has attracted attention in research ranging from photocatalysis to photodetection due to its favorable visible light response and high physicochemical stability.For its practical device application,the fabrication of high-quality carbon nitride films on substrates is essential.However,conventional methodologies to achieve high polymerization of carbon nitride are often accompanied by its decomposition,significantly compromising the film quality.Herein,we report an ultrafast fabrication of carbon nitride film by laser direct writing(LDW).The instantaneous high temperature and pressure during LDW can efficiently boost the polymerization of carbon nitride and suppress its decomposition,resulting in high-quality carbon nitride film with excellent mechanical stability with the substrate.Due to the efficient photon-to-electron conversion,it exhibits an outstanding photoelectrochemical water splitting and optoelectronic detection capability,even under strong acid/alkaline conditions.This study thus offers a facile and efficient LDW strategy for the rapid fabrication of carbon nitride film photoelectrodes,demonstrating its great feasibility in multifunctional photoelectrical applications,including but not limited to photoelectrochemical water splitting and optoelectronic detection.

Periodic transparent nanowires in ITO film fabricated via femtosecond laser direct writing

This paper reports the fabrication of regular large-area laser-induced periodic surface structures(LIPSSs)in indium tin oxide(ITO)films via femtosecond laser direct writing focused by a cylindrical lens.The regular LIPSSs exhibited good properties as nanowires,with a resistivity almost equal to that of the initial ITO film.By changing the laser fluence,the nanowire resistances could be tuned from 15 to 73 kΩ/mm with a consistency of±10%.Furthermore,the average transmittance of the ITO films with regular LIPSSs in the range of 1200-2000 nm was improved from 21%to 60%.The regular LIPSS is promising for transparent electrodes of nano-optoelectronic devices-particularly in the near-infrared band.

Laser Direct Writing of Ag Films from Solution on Si Substrate

Pulsed Nd:YAG laser was used to irradiate Si substrate immersed in AgNO3 ethylene glycol solution to deposit Ag films along the lines scanned by laser on the substrate, which is a photo-thermal decomposing process. The decomposed Ag atoms congregate and form polycrystalline Ag particles. The Ag concentration changes greatly with the total laser energyA absorbed by substrate. Transmission electron microscopy (TEM) observation shows the Ag particles are inlaid in the Si substrate. Auger electron spectrum (AES) shows that the Ag concentration decreases with the increase of the sputtering depth, and there is no oxygen element on the surface of the deposited Ag films.

Self-limiting laser crystallization and direct writing of 2D materials

Direct growth and patterning of atomically thin two-dimensional(2D)materials on various substrates are essential steps towards enabling their potential for use in the next generation of electronic and optoelectronic devices.The conventional gas-phase growth techniques,however,are not compatible with direct patterning processes.Similarly,the condensed-phase methods,based on metal oxide deposition and chalcogenization processes,require lengthy processing times and high temperatures.Here,a novel self-limiting laser crystallization process for direct crystallization and patterning of 2D materials is demonstrated.It takes advantage of significant differences between the optical properties of the amorphous and crystalline phases.Pulsed laser deposition is used to deposit a thin layer of stoichiometric amorphous molybdenum disulfide(MoS2)film(∼3 nm)onto the fused silica substrates.A tunable nanosecond infrared(IR)laser(1064 nm)is then employed to couple a precise amount of power and number of pulses into the amorphous materials for controlled crystallization and direct writing processes.The IR laser interaction with the amorphous layer results in fast heating,crystallization,and/or evaporation of the materials within a narrow processing window.However,reduction of the midgap and defect states in the as crystallized layers decreases the laser coupling efficiency leading to higher tolerance to process parameters.The deliberate design of such laser 2D material interactions allows the selflimiting crystallization phenomena to occur with increased quality and a much broader processing window.This unique laser processing approach allows high-quality crystallization,direct writing,patterning,and the integration of various 2D materials into future functional devices.

Laser direct writing of flexible multifunctional airflow sensors on the Kevlar fabric

The growing interest in flexible devices has emerged as a global trend due to their advantages in flexibility,lightweight structure,and wearability,addressing the limitations of traditional devices.While wearable airflow sensors have been previously reported,the development of flexible fabric-based airflow sensors capable of functioning in environments with open flames—critical for fire rescue operations—has yet to be explored,largely due to the poor fire resistance of conventional fabrics.In this work,we first present a flexible,wearable,and multifunctional airflow sensor with excellent fire-resistant properties,fabricated through a simple direct laser writing process.This sensor maintains airflow detection capabilities even in the presence of open flames.Typically,the fabrication of fabric-based sensors involves complex procedures such as carbon materials doping or vapor-phase deposition,leading to lengthy preparation cycles and high costs.Furthermore,fabric-based devices are inherently prone to flammability.To address these challenges,we introduce twice-vertical laser-induced graphene(TVLIG)as a sensitive and reliable component for fire-resistant airflow sensors.The resulting TVLIG/Kevlar fabric can be integrated into various garments,particularly protective suits,to form sensitive and fire-resistant airflow sensors capable of detecting airflow velocity and direction in both two-dimensional(2D)and three-dimensional(3D)spaces during fire incidents.Additionally,the TVLIG patterns can be expanded to multifunctional platforms,such as glucose detection for injured individuals,offering further applications in rescue operations.This functional expansion reduces the burden on rescue personnel and streamlines device preparation.With its outstanding sensing capabilities,fire resistance,and expandability,the developed flexible airflow sensor shows great potential for various real-world rescue scenarios,promising advancements in wearable sensing technology for rescue engineering.

Compact 1×3 waveguide beam splitters with variable splitting ratios in LiNbO_(3) crystal fabricated by femtosecond laser direct writing

In this Letter,waveguide beam splitters(1×3)with type I modifications are fabricated in a LiNbO_(3) crystal by femtosecond laser direct writing.The influence of the relative positions of three sub-waveguides on power splitting ratios are investigated in detail and the corresponding output intensities as functions of the relative positions in the numerical simulation are plotted,which are in good accordance with the experimental results.In addition,the waveguide beam splitter with a 1:1:1 splitting ratio is fabricated by changing the relative widths of the three branch-waveguides.Guiding performances at 532 nm are measured and analyzed by a typical end-face coupling system.The simulation and experimental results demonstrate that the beam splitting ratio of the waveguide splitter can be precisely regulated by the positions and widths of the sub-waveguides.

Transient features of graphitization and nitrogen-vacancy color centers in a diamond fabricated by localization femtosecond laser direct writing

Femtosecond laser direct writing provides an efficient approach to fabricating single nitrogen vacancy(NV) color centers with a relatively high yield. Different from previously reported NV color centers with a random distribution in a bulk diamond or nanocrystals, this gives an opportunity to study the photophysical properties of single NV color centers with precise numbers and positions. However, ultrafast studies on single NV color centers prepared by localization femtosecond laser direct writing are still rare, especially for the graphitization inside a diamond and its relationship with single NV color centers. Here, we report the broadband transient absorption(TA) spectroscopic features of the graphitization and NV color centers in a diamond fabricated by localization femtosecond laser direct writing at room temperature under 400 nm excitation. In comparison with the graphene oxide film, the bleaching features of the graphitization point array in a diamond are similar to reduced graphene oxide,accompanied by excited state absorption signals from local carbon atom vacancy defects in graphene-like structures induced by laser writing. On the other hand, transient features of laser processing array containing single NV color centers with a yield of~50% are different from those of the graphitization point array. Our findings suggest that for ultrashort pulse processing of diamonds, broadband TA spectral signals are sensitive to the surrounding atomic environment of processing sites, which could be applied to laser writing point defects in other materials used as solid-state single photon sources.

All laser direct writing process for temperature sensor based on graphene and silver

A highly sensitive temperature sensing array is prepared by all laser direct writing(LDW)method,using laser induced silver(LIS)as electrodes and laser induced graphene(LIG)as temperature sensing layer.A finite element analysis(FEA)photothermal model incorporating a phase transition mechanism is developed to investigate the relationship between laser parameters and LIG properties,providing guidance for laser processing parameters selection with laser power of 1–5 W and laser scanning speed(greater than 50 mm/s).The deviation of simulation and experimental data for widths and thickness of LIG are less than 5%and 9%,respectively.The electrical properties and temperature responsiveness of LIG are also studied.By changing the laser process parameters,the thickness of the LIG ablation grooves can be in the range of 30–120μm and the resistivity of LIG can be regulated within the range of 0.031–67.2Ω・m.The percentage temperature coefficient of resistance(TCR)is calculated as−0.58%/°C.Furthermore,the FEA photothermal model is studied through experiments and simulations data regarding LIS,and the average deviation between experiment and simulation is less than 5%.The LIS sensing samples have a thickness of about 14μm,an electrical resistivity of 0.0001–100Ω・m is insensitive to temperature and pressure stimuli.Moreover,for a LIS-LIG based temperature sensing array,a correction factor is introduced to compensate for the LIG temperature sensing being disturbed by pressure stimuli,the temperature measurement difference is decreased from 11.2 to 2.6°C,indicating good accuracy for temperature measurement.

Supercritical metalens at h-line for high-resolution direct laser writing

Supercritical lens(SCL)can break the diffraction limit in the far field and has been demonstrated for high-resolution scanning confocal imaging.Its capability in sharper focusing and needle-like long focal depth should allow high-resolution lithography at violet or ultraviolet(UV)wavelength,however,this has never been experimentally demonstrated.As a proof of concept,in this paper SCLs operating at 405 nm(h-line)wavelength with smaller full-width-at-half-maximum focal spot and longer depth of focus than conventional Fresnel zone lens while maintaining controlled side lobes are designed for direct laser writing(DLW)lithography.Aluminum nitride(AlN)with a high refractive index and low loss in UVvisible range is used to fabricate nanopillar-based metasurfaces structure for the metalens.Grating arrays with improved pitch resolution are fabricated using the SCLs with sub-diffraction-limit focusing capability.The AlN-based metasurface for SCLs at short wavelength for DLW could extend further to UV or deep UV lithography and might be of great interest to both the research and industry applications.

Direct writing of electronics based on alloy and metal （DREAM） ink： A newly emerging area and its impact on energy, environment and health sciences

Electronics, such as printed circuit board （PCB）, transistor, radio frequency identification （RFID）, organic light emitting diode （OLED）, solar cells, electronic display, lab on a chip （LOC）, sensor, actuator, and transducer etc. are playing increasingly important roles in people＇s daily life. Conventional fabrication strategy towards integrated circuit working steps, generally （IC）, requesting at least six consumes too much energy, material and water, and is not environmentally friendly. During the etching process, a large amount of raw materials have to be abandoned. Besides, lithography and microfabrication are typically carried out in ＂Clean room＂ which restricts the location of IC fabrication and leads to high production costs. As an alternative, the newly emerging inkjet printing electronics are gradually shaping modem electronic industry and its related areas, owing to the invention of a series of conductive inks composed of polymer matrix, conductive fillers, solvents and additives. Nevertheless, the currently available methods also encoun ter some technical troubles due to the low electroconduc tivity, complex sythesis and sintering process of the inks. As an alternative, a fundamentally different strategy was recently proposed by the authors＇ lab towards truly direct writing of electronics through introduction of a new class of conductive inks made of low melting point liquid metal or its alloy. The method has been named as direct writingof electronics based on alloy series of functional circuits, and metal （DREAM） ink. A sensors, electronic elements and devices can thus be easily written on various either soft or rigid substrates in a moment. With more and more technical progresses and fundamental discoveries being kept made along this category, it was found that a new area enabled by the DREAM ink electronics is emerging, which would have tremendous impacts on future energy and environmental sciences. In order to promote the research and development along this direction, the present paper is dedicated to draft a comprehensive picture on the DREAM ink technology by summarizing its most basic features and principles. Some important low melting point metal ink candidates, especially the room temperature liquid metals such as gallium and its alloy, were collected, listed and analyzed. The merits and demerits between conventional printed electronics and the new direct writing methods were comparatively evaluated. Important scientific issues and technical strategies to modify the DREAM ink were suggested and potential application areas were proposed. Further, digestions on the impacts of the new technology among energy, health, and environmental sciences were presented. Meanwhile, some practical challenges, such as security, environmentfriendly feature, steady usability, package, etc. were summarized. It is expected that the DREAM ink technology will initiate a series of unconven tional applications in modem society, and even enter into peoples＇ daily life in the near future.

Protein-based soft micro-optics fabricated by femtosecond laser direct writing

In this work,we report a novel soft diffractive micro-optics,called‘microscale kinoform phase-type lens(micro-KPL)’,which is fabricated by femtosecond laser direct writing(FsLDW)using bovine serum albumin(BSA)as building blocks and flexible polydimethylsiloxane(PDMS)slices as substrates.By carefully optimizing various process parameters of FsLDW(e.g.,average laser power density,scanning step,exposure time on a single point and protein concentration),the as-formed protein micro-KPLs exhibit excellent surface quality,well-defined three-dimensional(3D)geometry and distinctive optical properties,even in relatively harsh operation environments(for instance,in strong acid or base).Laser shaping,imaging and other optical performances can be easily achieved.More importantly,micro-KPLs also have unique flexible and stretchable properties as well as good biocompatibility and biodegradability.Therefore,such protein hydrogel-based micro-optics may have great potential applications,such as in flexible and stretchable photonics and optics,soft integrated optical microsystems and bioimplantable devices.

Direct writing of graphene patterns and devices on graphene oxide films by inkjet reduction

Direct writing of graphene patterns and devices may significantly facilitate the application of graphene-based flexible electronics. In terms of scalability and cost efficiency, inkjet printing is very competitive over other existing direct- writing methods. However, it has been challenging to obtain highly stable and clog-free graphene-based ink. Here, we report an alternative and highly efficient technique to directly print a reducing reagent on graphene oxide film to form conductive graphene patterns. By this ＂inkjet reduction＂ method, without using any other microfabrication technique, conductive graphene patterns and devices for various applications are obtained. The ionic nature of the reductant ink makes it clog-free and stable for continuous and large-area printing. The method shows self-limited reduction feature, which enables electrical conductivity of graphene patterns to be tuned within 5 orders of magnitude, reaching as high as 8,000 S.m-1. Furthermore, this method can be extended to produce noble metal/graphene composite patterns. The devices, including transistors, biosensors, and surface- enhanced Raman scattering substrates, demonstrate excellent functionalities. This work provides a new strategy to prepare large-area graphene-based devices that is low-cost and highly efficient, promising to advance research on graphene- based flexible electronics.

Direct writing of silver microfiber with precise control on patterning for robust and flexible ultrahigh-performance transparent conductor

Ultrafine silver fiber is an alternative to commercial indium tin oxide(ITO) as a new-generation flexible transparent conductor that can be used in flexible electronics.However,its primary limitation is the unrepeatable optoelectronic properties due to the disordered distribution of silver fibers.In this work,we report the in-situ direct writing of the silver microfiber pattern with high conductivity and transparency to attain a flexible transparent conductor.The silver network is composed of silver microfibers,which can be artificially designed and regularly patterned under the precise control of the fiber position and shape;this is crucial for regulating its optoelectronic properties.Herein,a high-performance conductor is achieved in the silver network with high stability.This novel conductor has a sheet resistance of 2 Ω sq-1at 90% transparency,which corre sponds to a high Figure of merit σdc/σopt=1742.The in-situ direct writing technique developed here is distinct from other fabrication methods because it requires no transfer steps,templates or heating.Further,this silver network is integrated into a light-printable rewritable device,and can be used as a wearable heater;this heater when driven by a 1.5 V battery attains a temperature of up to 55.6℃.Therefore,in-situ direct writing is expected to offer a new platform for facile,scalable,and ultralow-cost production of high-performance metal networks for flexible transparent conductors.

Subwavelength-resolution direct writing using submicron-diameter fibers

A novel direct writing technique using submicron-diameter fibers is presented. This technique adopts contact mode in the process of writing, and submicron lines with different widths have been obtained. Experimental results demonstrate that the resolution of this technique can be smaller than the exposure wavelength of 442 nm, and 380-nm-wide line is achieved. In addition, the distribution of light fields in the photoresist layer is analyzed by finite-difference time-domain method.