In recent years, peculiar physical phenomena enabled by non-Hermitian systems, especially the parity-time (PT)-symmetric systems, have drawn tremendous research interests. Particularly, special spectral degeneracies known as exceptional points (EPs) and coherent perfect absorber-laser (CPAL) points where zero and infinite large eigenvalues coexist are the most popular topics to be studied. To date, the discussions of EPs that serve as transition boundaries between broken PT-symmetry phase and exact PT-symmetry phase have been intensively presented. However, the theoretical analysis and experimental validations of CPAL points are inadequate. Different from EPs, CPAL points, as a special solution of broken PT-symmetry phase, may exhibit even further counterintuitive physical features, which may have significant implications to study non-Hermitian physics. Here, we review some recent advances of CPAL phenomena in different sub-disciplines of physics, including optics, electronics and electromagnetics, and acoustics. Additionally, we also provide an envision of future directions and applications of CPAL systems.
Wetting condition of micro/nanostructured surface has received tremendous attention due to the potential applications in commercial, industrial, and military areas. Surfaces with extreme wetting properties, e. g., superhydrophobic or superhydrophilic, are extensively employed due to their superior anti-icing, drag reduction, enhanced boiling heat transfer, self-cleaning, and anti-bacterial properties depending on solid-liquid interfacial interactions. Laser-based techniques have gained popularity in recent years to create micro/nano-structured surface owing to their high flexibility, system precision, and ease for automation. These techniques create laser induced periodic surface structures (LIPSS) or hierarchical structures on substrate material. However, micro/nanostructures alone cannot attain the desired wettability. Subsequent modification of surface chemistry is essentially needed to achieve target extreme wettability. This review paper aims to provide a comprehensive review for both laser texturing techniques and the following chemistry modification methods. Recent research progress and fundamental mechanisms of surface structure generation via different types of lasers and various chemistry modification methods are discussed. The complex combination between the laser texturing and surface chemistry modification methods to decide the final wetting condition is presented. More importantly, surface functionalities of these surfaces with extreme wetting properties are discussed. Lastly, prospects for future research are proposed and discussed.
Multi-function, multiband, cost-effective, miniaturized reconfigurable radio frequency (RF) components are highly demanded in modern and future wireless communication systems. This paper discusses the needs and implementation of multiband reconfigurable RF components with microfabrication techniques and advanced materials. RF applications of fabrication methods such as surface and bulk micromachining techniques are reviewed, especially on the development of RF microelectromechanical systems (MEMS) and other tunable components. Works on the application of ferroelectric and ferromagnetic materials are investigated, which enables RF components with continuous tunability, reduced size, and enhanced performance. Methods and strategies with nano-patterning to improve high frequency characteristics of ferromagnetic thin film (e. g., ferromagnetic resonance frequency and losses) and their applications on the development of fully electrically tunable RF components are fully demonstrated.
Metal superhydrophobic surfaces with anisotropic wettability and adhesion have become more and more important due to their promising applications. Herein, we report a new fabrication strategy through a combination of pulsed laser ablation and low-temperature annealing post-processing. An inclined cone structure array is made on stainless steel surfaces, and then 120 °C low-temperature annealing is applied. Such surface displays excellent mechanical durability and anisotropic superhydrophobicity. It is demonstrated experimentally that the contact angle of water droplets on the surface is different along the parallel (167° ±2°) and perpendicular directions (157° ±2°) of the inclined cone structure. The sliding behaviors of water droplets and mechanical durability of the inclined cone structures are studied. These surfaces obtained in a short time with environmentally friendly fabrication can be applied in industries for water harvesting, droplet manipulation, and pipeline transportation.
Here we report a femtosecond laser direct writing (a precise 3D printing also known as two-photon polymerization lithography) of hybrid organic-inorganic SZ2080™ 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 pJ 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.
The model of heat source (MHS) which reflects the thermal interaction between materials and laser during processing determines the accuracy of simulation results. To acquire desirable simulations results, although various modifications of heat sources in the aspect of absorption process of laser by materials have been purposed, the distribution of laser power density (DLPD) in MHS is still modeled theoretically. However, in the actual situations of laser processing, the DLPD is definitely different from the ideal models. So, it is indispensable to build MHS using actual DLPD to improve the accuracy of simulation results. Besides, an automatic modeling method will be benefit to simplify the tedious pre-processing of simulations. This paper presents a modeling method and corresponding algorithm to model heat source using measured DLPD. This algorithm automatically processes original data to get modeling parameters and provides a step MHS combining with absorption models. Simulations and experiments of heat transfer in steel plates irradiated by laser prove the mothed and the step MHS. Moreover, the investigations of laser induced thermal-crack propagation in glass highlight the signification of modeling heat source based on actual DLPD and demonstrate the enormous application of this method in the simulation of laser processing.
Ablation threshold is an important concept in the study of femtosecond laser micro- and nano-machining. In this paper, the ablation experiments of three kinds of surface roughness 4H-SiC substrates irradiated by femtosecond laser were carried out. The feature thresholds were systematically measured for three surface roughness SiC substrates and found in the modification and annealing regions ranging from coincidence (Ra=0.5 nm) to a clear demarcation (Ra=5.5 nm), eventually being difficult to identify the presence of the former (Ra=89 nm). Under multi-pulse laser irradiation, oriented ripple structures were generated in the annealing region, where deep subwavelength ripples (about 110 nm, Λ ≈ 0.2λ) can be generated above substrates with surface roughness higher than 5.5 nm. We investigated the effect of surface roughness on the ablation morphology, ablation threshold, and periodic structures of femtosecond laser ablation of 4H-SiC substrates, while the ablation threshold was tended to decrease and stabilize with the increase of pulse number N⩾500.
The influence of the picosecond (ps) pulsed burst with a nanosecond scale of temporal separation (50 ns) on filamentary traces in sapphire substrate is investigated. The spatiotemporal evolution of the filamentary plasma string induced by sub-pulses of the burst-mode is revealed according to the analysis of the instantaneous photoluminescence images. Due to the presence of residual plasma, the energy loss of sub-pulse during the balancing of self-focusing effect is reduced, and thus refreshes the plasma via refocusing. The refreshed plasma peak generated by the subsequent subpulse appears at relatively low density positions in the formed filamentary plasma string, which results in more uniform densities and less spatial overlap among the plasma peaks. The continuity and uniformity of the filamentary trace in sapphire are enhanced by the burst-mode. Besides, the burst filamentary propagation can also remain effective when the sub-pulse energy is below the self-focusing threshold. Based on this uniform and precise energy propagation mode, the feasibility of its use for the laser lift-off (LLO) process is verified.
Surface-enhanced Raman scattering (SERS) has been widely used as an effective technique for low-concentration molecules detections in the past decades. This work proposes a rapid and accessible process to fabricate SERS-active substrates with high uniformity and controllability based on two-step laser ablation. Laser beams directly ablate the surface of Si, concurrently creating microstructures and ejecting molten materials caused by the thermal effect that nucleate in ambient air. The nuclei grow into nanoparticles and deposit over the surface. These nanoparticles, together with microstructures, improve the light collection efficiency of the SERS-active substrates. Especially after Au thin film deposition, these nanoparticles can provide nanogaps as hotspots for SERS. By orthogonal experiment design, laser processing parameters for better performances are determined. Compared with substrates fabricated by single 1064 nm master oscillator power amplifier (MOPA) laser ablation, substrates ablated by the primary 1064 nm MOPA laser and secondary UV pulsed laser show more uniform nanoparticles’ deposition over the surface. The optimized large-area substrate has a SERS detection limit of 10−8 mol/L for 4-aminothiophenol (4-ATP), indicating the potential real-world applications for trace detection.
Photonic nanojets (PNJs) have a wide range of applications in laser processing, nanolithography, optical high-density storage, super-resolution microscopy, and other fields due to their processing capacity to overcome the diffraction limit. Herein, we control static microsphere be developed into the motion state to fabricate vector graphics nano-grooves. The microspheres roll on the substrate while the laser is kept synchronously irradiated, and the overlapping PNJ ablated craters form patterned grooves on the indium-tin oxide (ITO) substrate. Thus, PNJ has been expanded from “point” processing to “line” processing. The fabricated nano grooves have high continuity and consistency. Whereas, the precise customization of critical groove dimension can be achieved via modulation in diameter and kinetics of dielectric microshperes. Furthermore, by etching vectographs on an ITO conductive glass substrate, we demonstrated the advantages and potential of the proposed method in nanopatterning. The proposed method effectively reduces the cost and complexity of photonic nanojets applied in nanopatterning. The proposed nanopatterning methodology will play a vital role in the fabrication of semiconductor materials, sensors, microfluidic devices, surface-enhanced Raman scattering (SERS), biomedicine, nanoscience and nanoengineering.
Electron beam lithography (EBL) is a key technology in the fabrication of nanoscale silicon optical waveguide. The influence of exposure dose, the main process parameter of EBL, on the structure profile of poly-methyl methacrylate (PMMA) after development was studied using a silicon on insulator (SOI) wafer with 220 nm top silicon as the substrate. The relationship between exposure dose and structure pattern width after development was analyzed according to the measurement results. The optimum exposure dose of 220 µC/cm2 was found to obtain a final structure consistent with the designed mask value through subsequent processes. At the same time, according to the image segmentation curve tracking technology, the contour extraction process of the dose test results was carried out, and the relationship among mask design value, exposure dose and two-dimensional roughness of boundary contour was analyzed, which can provide reference for the subsequent electron beam lithography of the same substrate material.
In this work, the nickel-based powder metallurgy superalloy FGH95 was selected as experimental material, and the experimental parameters in multiple overlap laser shock processing (LSP) treatment were selected based on orthogonal experimental design. The experimental data of residual stress and microhardness were measured in the same depth. The residual stress and microhardness laws were investigated and analyzed. Artificial neural network (ANN) with four layers (4-N-(N-1)-2) was applied to predict the residual stress and microhardness of FGH95 subjected to multiple overlap LSP. The experimental data were divided as training-testing sets in pairs. Laser energy, overlap rate, shocked times and depth were set as inputs, while residual stress and microhardness were set as outputs. The prediction performances with different network configuration of developed ANN models were compared and analyzed. The developed ANN model with network configuration of 4-7-6-2 showed the best predict performance. The predicted values showed a good agreement with the experimental values. In addition, the correlation coefficients among all the parameters and the effect of LSP parameters on materials response were studied. It can be concluded that ANN is a useful method to predict residual stress and microhardness of material subjected to LSP when with limited experimental data.
A micro-displacement sensor based on fiber Bragg grating (FBG) is proposed. The device consists of a pair of FBGs with different central wavelengths fabricated by femtosecond laser phase mask method and a metal substrate with lever structure. The displacement is amplified by lever structure and it converts into axial tension of FBG, which has a high displacement sensitivity. The amplification factors obtained by theoretical analysis and finite element simulation are 2.67 and 2.50, respectively. The experimental results show that in the range of 0–50 µm the shift of FBG center wavelength is linearly related to the displacement of measured object and displacement sensitivity reaches 121 pm/µm. In addition, the cascaded FBG is used to compensate the temperature.
Fog harvesting has been considered as a promising method for solving water crisis in underdeveloped regions. To mimic and optimize the alleged natural fog harvesting ability of the stenocara beetle, hybrid superhydrophobic (hydrophobic, superhydrophilic)/hydrophilic patterns are processed on stainless steel via picosecond laser direct writing. Basically, after laser processing, the surfaces of stainless steel change from hydrophilic to superhydrophilic. Then, after chemical and heat treatment, the superhydrophilic surfaces become superhydrophobic with ultra-low adhesion, and superhydrophobic (hydrophobic) with ultra-high adhesion, respectively. This work systematically examines the fog harvesting ability of picosecond laser treated surfaces (LTS), pristine surfaces (PS), laser and chemical treated surfaces (LCTS), laser and heat-treated surfaces (LHTS). Compared with the PS, the as-prepared surfaces enhanced the fog harvesting efficiency by 50%. This work provides a fast and simple method to fog collectors, which offer a great opportunity to develop water harvesters for real world applications.
Poly(ε-caprolactone) (PCL) holds unique bioresorbability and competent biomechanical properties for tissue-engineering application. However, PCL is hydrophobic intrinsically and poor in cell-biomaterial interaction. In this study, we prepared a composite based on PCL and bioactive tantalum (Ta) to understand the effects of direct laser micropatterning on composite surface properties. The PCL/Ta composite after preparation was surface-patterned by femtosecond laser and characterized with surface morphology, crystal structure, chemical composition, wettability and cellular response of fibroblast. It was found that laser micropatterning enlarged the difference of wetting properties (∼15°) on PCL and PCL/Ta surfaces. The wetting changes was dependent on both material composition and laser-machined geometry. The blending of Ta enhanced surface wettability with prolonged contact time on the laser-machined line and rectangle microarrays. In vitro culture results showed beneficial effects of laser micropatterning on cell morphology of the fibroblasts. On the PCL/Ta surfaces with line and rectangle microarrays, the cells were more likely to bridge the sidewalls of the microgrooves, showing adaptive 3D morphologies to the micro/nano topographies on the sidewalls. These findings are envisaged to facilitate surface design and micropattern optimization for favorable tuning the cell response to biomedical PCL/Ta composites.
Laser cleaning is a highly nonlinear physical process for solving poor single-modal (e.g., acoustic or vision) detection performance and low inter-information utilization. In this study, a multi-modal feature fusion network model was constructed based on a laser paint removal experiment. The alignment of heterogeneous data under different modals was solved by combining the piecewise aggregate approximation and gramian angular field. Moreover, the attention mechanism was introduced to optimize the dual-path network and dense connection network, enabling the sampling characteristics to be extracted and integrated. Consequently, the multi-modal discriminant detection of laser paint removal was realized. According to the experimental results, the verification accuracy of the constructed model on the experimental dataset was 99.17%, which is 5.77% higher than the optimal single-modal detection results of the laser paint removal. The feature extraction network was optimized by the attention mechanism, and the model accuracy was increased by 3.3%. Results verify the improved classification performance of the constructed multi-modal feature fusion model in detecting laser paint removal, the effective integration of acoustic data and visual image data, and the accurate detection of laser paint removal.
Laser processing provides highly-controlled modification and on-demand fabrication of plasmon metal nanostructures for light absorption and photothermal convention. We present the laser-induced forward tansfer (LIFT) fabrication of silver nanomembranes in control of light absorption. By varying the hatch distance, different morphologies of randomly distributed plasmon silver nanostructures were produced, leading to well-controlled light absorption levels from 11% to 81% over broadband. The anti-reflection features were maintained below 17%. Equilibrated and plain absorptions were obtained throughout all absorption levels with a maximum intensity fluctuation of ±8.5% for the 225 µJ cases. The 45 µJ pulse energy can offer a highly equilibrated absorption at a 60% absorption level with an intensity fluctuation of ±1%. Pattern transfer was also achieved on a thin tape surface. The laser-transferred characters and patterns demonstrate a localized temperature rise. A rapid temperature rising of roughly 15 °C can be achieved within 1 s. The LIFT process is highly efficiently fabricated with a typical speed value of 103 to 105 cm2/h. The results indicated that LIFT is a well-controlled and efficient method for the production of optical films with specific absorption levels.
Femtosecond pulsed lasers have been widely used over the past decades due to their capability to fabricate precise patterns at the micro- and nano-lengths scales. A key issue for efficient material processing is the determination of the laser parameters used in the experimental set ups. Despite a systematic investigation that has been performed to highlight the impact of every parameter independently, little attention has been drawn on the role of the substrate material on which the irradiated solid is placed. In this work, the influence of the substrate is emphasised for films of various thicknesses, which demonstrates that both the optical and thermophysical properties of the substrate affect the thermal fingerprint on the irradiated film while the impact is manifested to be higher at smaller film sizes. Two representative materials, silicon and fused silica, have been selected as typical substrates for thin films (gold and nickel) of different optical and thermophysical behaviour and the thermal response and damage thresholds are evaluated for the irradiated solids. The pronounced influence of the substrate is aimed to pave the way for new and more optimised designs of laser-based fabrication set ups and processing schemes.
In the current microwelding process using femtosecond (fs) laser between dissimilar materials, surface polishing and pressure assistance, so-called optical contact, are believed necessary. In this paper, direct welding of soda lime glass and Kovar alloy using a fs laser is investigated to overcome the limit of optical contact. The processing of fs laser welding is comprehensively studied by varying the laser power, welding velocity and the number of welding. The shear joining strength is as high as 2 MPa. The cross-section of glass-Kovar alloy joints, the elemental diffusion and the fracture behavior of welded joints were studied. The results show that the fs laser irradiates the surface of Kovar alloy, micron/nanometer-sized metal particles are generated. These particles perform the role as an adhesive part in the welding process. It is believed that the Si atoms diffuses to Kovar alloy from the glass and partially replaces the Fe2+ ions on the surface of Kovar alloy, indicating that the mixing and interdiffusion of materials have occurred during the welding process. Finally, the welded sample was tested and has excellent water resistance and sealing property. Furthermore, to justify that this method can be applied to other stack ups, the glass-copper, the glass-Al6063 and sapphire-ceramic are also welded together. This work greatly simplifies the fs laser microwelding process and promotes its industrial applications, such as optoelectronic devices, medical devices and MEMS.
In order to study the corrosion resistance of high-speed laser cladding (HLC) coating while improving production efficiency, a CoCrFeNiMo0.2 high-entropy alloy (HEA) coating was prepared by HLC. The optimized parameters of HLC are laser power of 880 W, scanning speed of 18 m/min, overlapping ratio of 60%, and powder feed speed of 3 r/min. Then, the surface roughness, microstructure, phase composition, element distribution, and electrochemical properties in 3.5 wt% NaCl solution of the coatings were analyzed, respectively. The local surface roughness of the CoCrFeNiMo0.2 HEA coating was found to be 15.53 µm. A distinct metallurgical bond could be observed between the coating and the substrate. Compared to the conventional laser cladding (CLC), the results of electrochemical tests showed that CoCrFeNiMo0.2 HEA coating exhibited a significant passivation. The corrosion current density of 5.4411×10−6 A·cm−2 and the corrosion potential of −0.7445 V for the HLC coating were calculated by the Tafel extrapolation method. The CLC coating’s corrosion current density and corrosion potential are 2.7083×10−5 A·cm−2 and −0.9685 V, respectively. The HLC coating shows a superior corrosion resistance, crucially due to the uniform and fine grains. Under various complex and harsh working conditions, this method can be widely used in the field of repairing and remanufacturing of corrosion-proof workpieces.
This experiment obtained different laser energy density (LED) by changing SLM molding process parameters. The surface morphology, surface quality, and microstructure of as-fabricated samples were studied. The effects of scanning speed, hatching space, and laser power on surface quality were analyzed, and the optimal LED range for surface quality was determined. The results show that pores and spherical particles appear on the sample’s surface when low LED is applied, while there are lamellar structures on the sides of the samples. Cracks appear on the sample’s surface, and the splash phenomenon increases when a high LED is taken. At the same time, a large amount of unmelted powder adhered to the side of the sample. The surface quality is the best when the LED is 150–170 J/mm3. The preferred hatch space is currently 0.05–0.09 mm, the laser power is 200–350 W, and the average surface roughness value is (15.1±3) µm. The average surface hardness reaches HV404±HV3, higher than the forging standard range of HV340–HV395. Increasing the LED within the experiment range can increase the surface hardness, yet an excessively high LED will not further increase the surface hardness. The microstructure is composed of needle-like α′-phases with a length of about 20 µm, in a crisscross ‘N’ shape, when the LED is low. The β-phase grain boundary is not obvious, and the secondary-phase volume fraction is high; when the LED is high, the α′-phase of the microstructure is in the form of coarse slats, and the secondary-phase is composed of a small amount of secondary α′-phase, the tertiary α′-phase and the fourth α′-phase disappear, and the volume fraction of the secondary-phase becomes low.
In this work, laser heat conduction lap welding (LHCLW) of AZ31B magnesium alloy sheet and DP780 galvanized steel sheet was carried out by the defocused laser beam. The effects of laser power on the microstructure and mechanical properties of the joint were studied. The pros and cons of the joint were identified and evaluated by measuring the tensile shear strength, microhardness and microstructure observation. The formation mechanism of various phases at the Mg/steel interface was analyzed. The results indicated that the galvanized layer could promote the metallurgical bonding between magnesium alloy and steel by improving the diffusion ability of molten magnesium alloy at the steel interface and reacting with Mg, so as to enhance the strength of the joint. A continuous dense layered eutectic structure (α-Mg+MgZn) was formed at the interface of the joint, while MgZn2 and MgZn phase was formed at the weld edge zone and heat affective zone (HAZ), whereas no reaction layer was generated between the uncoated steel and magnesium alloy. A sound joint could be obtained at 2.5 kW, and the corresponding tensile shear strength reached the maximum value of 42.9 N/mm. The strength was slightly reduced at 2.6 kW due to the existence of microcracks in the eutectic reaction layer.
This work aims to establish a suitable numerical simulation model for hybrid laser-electric arc heat source welding of dissimilar Mg alloys between AZ31 and AZ80. Based on the energy conservation law and Fourier’s law of heat conduction, the differential equations of the three-dimensional temperature field for nonlinear transient heat conduction are built. According to the analysis of nonlinear transient heat transfer, the equations representing initial conditions and boundary conditions are obtained. The “double ellipsoidal heat source + 3D Gaussian heat source” combination was chosen to construct the laser-electric arc hybrid heat source. The weld bead morphologies and the distribution of temperature, stress, displacement and plastic strains are numerically simulated. The actual welding experiments were performed by a hybrid laser-electric arc welding machine. The interaction mechanism between laser and electric arc in the hybrid welding of Mg alloys is discussed in detail. The hybrid heat source can promote the absorption of laser energy and electric arc in the molten pool, resulting in more uniform energy distribution in the molten pool and the corresponding improvement of welding parameters. This work can provide theoretical guidance and data supports for the optimization of the hybrid laser-electric arc welding processes for Mg alloys.
Laser cladding of powder mixture of TiN and SS304 is carried out on an SS304 substrate with the help of fibre laser. The experiments are performed on SS304, as per the Taguchi orthogonal array (L16) by different combinations of controllable parameters (microhardness and clad thickness). The microhardness and clad thickness are recorded at all the experimental runs and studied using Taguchi S/N ratio and the optimum controllable parametric combination is obtained. However, an artificial neural network (ANN) identifies different sets of optimal combinations from Taguchi method but they both got almost the same clad thickness and hardness values. The micro-hardness of cladded layer is found to be 6.22 times (HV0.5752) the SS304 hardness (HV0.5121). The presence of nitride ceramics results in a higher micro hardness. The cladded surface is free from cracks and pores. The average clad thickness is found to be around 0.6 mm.