Search Results (1,163)

This work presents a refined theoretical and numerical framework for shaping the axial intensity of finite-energy Bessel–Gaussian beams through programmable nonlinear phase modulation. Starting from the scalar Fresnel diffraction integral, we reformulate the propagation of a Gaussian-apodized axicon beam using a dimensionally consistent stationary-phase method. This analysis directly relates the radial phase gradient to the saddle-point trajectory, phase curvature, and on-axis intensity distribution. A Gaussian phase modulation (GPM) serves as a reference design to achieve a flattop axial profile while preserving the characteristic transverse Bessel ring structure. This work is validated against beam propagation simulations and previously reported spatial light modulator (SLM) experiments, confirming its accuracy within the paraxial regime. A parametric study then clarifies the scaling of wavelength, beam waist, axicon angle, and refractive index for extended focusing. Beyond standard GPM, several alternative nonlinear phase functions are systematically compared. High-performing profiles must replicate not only the amplitude scale but, more importantly, the radial phase-gradient structure of the Gaussian reference, which governs energy redistribution from annular regions to the axis. The results identify smooth, localized nonlinear functions as promising candidates for stable flattop Bessel beam generation. The proposed framework offers a flexible optical design for applications such as through-glass via (TGV) micromachining and light-sheet illumination, while prospective high-intensity laser plasma uses remain beyond the present linear model. Full article

Silicon micromachining serves as the foundational enabling technology for high-precision MEMS inertial sensors. However, the relentless pursuit of enhanced sensitivity and multi-functionality in emerging applications encounters a fundamental bottleneck when confined to two-dimensional scaling. The evolution toward complex three-dimensional (3D) stacking architectures is an inevitable trajectory for devices including MEMS inertial sensors, yet performance is constrained by the limitations of conventional processes in fabricating and integrating intricate 3D hollow structures. Specifically, uniformity in large-area deep silicon etching, structural integrity of convex corners in wet etching, and residual stress induced by multi-layer wafer bonding have emerged as critical, shared challenges. To address these issues, this paper proposes a triple-layer wafer-level stacking scheme that synergistically combines wet/dry hybrid etching with low-temperature adhesive bonding. This stacking scheme incorporates an innovative linear compensation model for wet-etched convex corners, enabling high-precision fabrication of complex corner structures under deep etching conditions. Furthermore, a collaborative strategy involving temporary bonding and plasma flow-field optimization improves the uniformity and integrity of dry etching for large perforated structures. A low-temperature triple-layer wafer-level stacking process is developed, encompassing precise adhesive dispensing, optical alignment, and a stepped low-temperature curing profile, thereby achieving highly symmetric 3D integration with controlled adhesive distribution. The efficacy of this stacking scheme is validated through the fabrication of a symmetrically stacked triple-layer MOEMS accelerometer sensing element. Test results demonstrate a noise floor as low as 0.40 µg/√Hz and a bias instability of 1.81 µg over 10 min. Compared with a double-layer counterpart, improved performance is obtained. The wafer-level stacking scheme established in this work not only provides a viable pathway for pushing the manufacturing limits of high-precision inertial devices but also offers a generic methodology for tackling complex hollow structure formation and low-temperature integration, holding referential value for broader applications in high-precision 3D microsystems. Full article

Aiming at the problems of high friction coefficient, severe wear, and unsatisfactory service life and operational reliability of brass under complex working conditions such as dry friction, wet friction, and oil-lubricated friction, H62 brass was taken as the research object to improve its friction and wear properties via surface micro-texture technology. This study systematically compares the tribological performance of three typical geometric micro-textures under three coupled working conditions for the first time. Circular, rectangular, and hexagonal micro-dimple textures were fabricated on the brass surface using ultraviolet laser micromachining. The control variable method was adopted to systematically investigate the effects of micro-texture parameters including shape, size, and area density on the friction and wear properties of brass under the three typical working conditions, combined with reciprocating friction and wear tests and ultra-depth-of-field microscope characterization. The results show that the hexagonal micro-dimple texture (200 μm in size, 10% in area density) exhibits the optimal friction-reducing and anti-wear performance. Compared with the smooth surface, the friction coefficient decreases from 0.51 to 0.43, and the wear rate of the GCr15 steel ball is reduced by 2.8% under dry friction; the friction coefficient decreases from 0.43 to 0.12 with an 11.8% reduction in wear rate under wet friction; and the friction coefficient decreases from 0.29 to 0.24 with an 8.3% reduction in wear rate under oil lubrication. Relative to dry friction, the wear rates are further reduced by 16.7% and 8.3% under wet friction and oil lubrication, respectively. Different from most existing studies that only focus on a single texture type or a single friction condition, this paper systematically reveals the coupling regulation mechanism between texture parameters and working conditions, clarifies the optimal micro-texture design strategy for multi-working conditions, verifies that hexagonal micro-textures can significantly improve the wear resistance of brass, and provides technical support for the surface optimization design of brass workpieces under complex working conditions. Full article

Tube boilers are extensively employed in oil and gas refineries, as well as in petroleum, energy, and power generation industries, where they serve critical functions in local steam-generation units and combined-cycle gas turbine (CCGT) plants. However, these boilers are prone to defects arising from waterside corrosion (e.g., thinning of U-bend tubes), fireside corrosion, and material degradation caused by stress or creeping. Among these issues, wall thinning of tube bends is particularly severe, as it results in localized metal loss, reduced structural integrity, and an elevated risk of tube rupture or failure under high-temperature and high-pressure operating conditions. Such failures can significantly compromise boiler safety and efficiency, potentially leading to forced outages, costly unplanned repairs, or catastrophic damage if not detected in time. The current condition-monitoring policy for U-bends relies on scheduled preventive maintenance and unscheduled corrective interventions. In practice, this involves randomly checking approximately 10–20% of the tubes through spot scanning, partial scanning, or full scanning, with repairs typically carried out only after an undetected failure occurs. Such maintenance strategies generally require plant shutdowns, making the process time-consuming, labor-intensive, and ultimately not cost-effective. This paper reviews existing solutions, technologies, and research addressing the problem, and introduces femtosecond laser micromachined fiber optic sensors as a transformative approach for real-time monitoring of wall thickness reduction in U-bend boiler tubes, thereby opening pathways for further research. Full article

The use of dried matrix spots (DMSs) has recently re-emerged as a useful sample storage technique and analytical platform along with the increased adoption of and general preference for ambient ionization mass-spectrometric methods. However, challenges associated with precise liquid confinement and sample targeting persist. In this paper, we present a laser micromachining-based approach to prepare DMSs on hydrophobic paper substrates that include visual recognition elements, or reticles, around surface energy traps (SETs). This targeted DMS substrate is combined with direct mass spectrometric analyses, namely liquid microjunction–surface sampling probe–mass spectrometry (LMJ-SSP-MS) and flowing atmospheric-pressure afterglow–mass spectrometry (FAPA-MS). With the laser-based micromachining approach, DMSs flanked by crosshairs for enhanced visualization are prepared on SETs as small as 0.55 mm in diameter, which offers an approximately 12-fold reduction in size compared to traditional DMS preparations. The DMSs prepared on these targeting SETs are demonstrated with the detection of caffeine in model aqueous and artificial urine solutions using LMJ-SSP-MS and FAPA-MS, respectively. With further refinement, this approach could be automated using computer vision and robotics to broaden the scope of DMSs and improve the analytical workflow. Full article

Acoustic emission (AE) monitoring in metal additive manufacturing (AM) requires compact sensors capable of high-frequency operation, broad bandwidth, and high sensitivity. However, increasing structural stiffness to achieve high resonance frequencies typically reduces electromechanical sensitivity. This work presents a finite element study of a coupled-resonator capacitive micromachined ultrasonic transducer (CMUT) designed to address this trade-off. The proposed architecture integrates three mechanically coupled silicon membranes with a stepped capacitive cavity that increases capacitance while preserving structural stiffness, enabling enhanced sensitivity without compromising high-frequency operation. COMSOL Multiphysics simulations were used to evaluate modal characteristics and frequency response under DC pre-stressed conditions. Modal coupling produced closely spaced resonances that broadened the effective bandwidth, while the stepped cavity significantly increased voltage output through improved electromechanical coupling. Compared to a single-resonator flat-cavity design, the coupled stepped-cavity configuration demonstrated nearly a threefold enhancement in output voltage while maintaining operation near 100 kHz. Additionally, adjusting the central resonator length enabled controlled frequency tuning for scalable array implementation. These results establish a proof of concept for a high-frequency, high-sensitivity micro-electro-mechanical systems (MEMS) CMUT architecture suitable for distributed AE monitoring in advanced manufacturing environments. Full article

Ultrafast fiber lasers operating near 2 μm have emerged as a critical platform for advancing mid-infrared photonics due to their narrow pulse durations, high peak powers, and broad tunability. These sources exploit the rich energy-level structures of Tm³⁺ and Ho³⁺ doped fibers and reside within an atmospheric transmission window, enabling applications spanning nonlinear microscopy, precision micromachining, optical frequency metrology, biophotonics, and free-space optical communication. Recent progress in low-loss fiber fabrication, dispersion-engineered cavity design, and mode-locking technologies has significantly expanded the performance boundaries of 2 μm ultrafast fiber lasers. This review systematically examines the underlying pulse-formation mechanisms and categorizes state-of-the-art mode-locking approaches. Representative laser architectures are compared with respect to pulse duration, energy scalability, repetition-rate enhancement, spectral characteristics, and environmental stability. Key application pathways in high-resolution spectroscopy, biomedical diagnostics, and mid-IR supercontinuum generation are highlighted. Finally, the remaining challenges and prospective research directions are discussed to inform the development of next-generation ultrafast photonic sources in the 2 μm band. Full article

Deep Si trenches with vertical sidewalls are critical structures in advanced MEMS sensors and microfluidic devices. (110)-oriented Si is specifically required for this purpose, as its crystallographic geometry inherently provides the nearly 90° vertical {111} planes. However, achieving precise morphology on (110) Si remains challenging due to the formation of unwanted V-shaped footing profiles at the bottom. This study establishes a systematically coupled experimental and numerical framework to investigate the anisotropic wet etching mechanism of (110) Si, quantifying the effects of KOH concentration (10–50 wt.%) and temperature (50–90 °C) on profile evolution. Experimental results demonstrate that 10 wt.% KOH at 70 °C yielded the most favorable morphology within the investigated range, with a minimized footing ratio (<2%). Based on these results, a dual-parameter kinetic regulation mechanism is proposed. Low concentration of KOH can minimize the crystallographic etching rate disparity ($\gamma$) between fast-etching {100}/{110} and slow-etching {111} planes, while the selected temperature helps maintain interfacial hydrodynamic stability. Furthermore, an Arbitrary Lagrangian-Eulerian (ALE)-based multiphysics model calibrated with Arrhenius kinetics was developed, which captures the overall trend of trench evolution and the dependence of footing formation on KOH concentration and temperature. This work not only provides a recommended process window for suppressing footing defects but also offers a trend-predictive simulation framework for orientation-dependent Si micromachining. Full article

Silicon carbide (SiC) has emerged as a highly attractive material for microelectromechanical systems (MEMS) operating in harsh environments, owing to its outstanding mechanical, thermal, and chemical properties. This review provides a comprehensive overview of the advantages and limitations of SiC-based MEMS, with particular emphasis on the strong interdependence between material structure, mechanical properties, and epitaxial growth processes. The role of defects, residual stress, and crystal quality is discussed in relation to device performance and reliability. Special attention is devoted to cubic SiC grown on silicon substrates, highlighting how growth-induced features influence the mechanical response of micromachined structures. Furthermore, a detailed analysis of the quality factor (Q-factor) is presented for 3C-SiC (111)/Si resonators, including the development of analytical models and their validation through numerical simulations performed using COMSOL Multiphysics (Version 6.1). The necessity of incorporating anisotropic loss factors in numerical modeling is demonstrated to be essential for accurately describing the experimentally observed behavior. This review aims to provide design guidelines and modeling strategies for the optimization of SiC MEMS, supporting their further development for high-performance and extreme-environment applications, including pressure sensors, mechanical resonators and high-stress-tolerant sensors. Full article

A capacitive micromachined ultrasonic transducer (CMUT) was engineered and functionalized with either zeolitic imidazolate framework-8 (ZIF-8) dispersed in an AZ1512HS photoresist matrix or with graphene oxide (GOx) to operate as a gravimetric sensor for organic vapors. The sensor response was investigated under controlled humidity conditions during pulsed exposure to acetone, ethyl methyl ketone, isopropanol, kerosene, and diesel vapors. The impedance of the device was monitored by observing and tracking the resonance frequency shift as well as the resistance maximum shift, giving us the possibility to track two response parameters simultaneously. Different combinations of shifts in the sensor resonance frequency and the resistance maximum values were observed for the ZIF-8 functionalized device when exposed to the selected vapors, ranging from 12.4 kHz for ethyl methyl ketone to 2.4 kHz for diesel, and from 580 Ω for acetone to 20 Ω for isopropanol. Sensors functionalized with GOx did not demonstrate any significant response to either ethyl methyl ketone or isopropanol in the frequency domain. GOx-functionalized sensors were used for relative humidity monitoring in test gases. Besides the conventional response of the produced gravimetric sensing system, we also observed a strong relationship between the humidity of the gas mixture and the strength of the interaction of target gases with the functional film of the sensor. The results highlight the multidimensional nature of the sensor response and demonstrate how humidity influences the interaction between vapor molecules and the functional coating. This paper focuses on the characterization of the coupled behavior of resonance frequency and resistance shifts under controlled operating conditions. The presented experimental setup provides a basis for future concentration-dependent investigations and functional material comparisons in CMUT-based gravimetric sensing systems and provides a necessary foundation for accurate interpretation of future concentration-resolved measurements. Full article

Capacitive micromachined ultrasonic transducers (CMUTs) have been developed over the past 30 years and achieved practical applications in both medical imaging and industrial non-destructive testing. This article presents the fundamental principles of CMUTs and surveys fabrication technologies, offering a comprehensive review of major advances and challenges in medical ultrasound and photoacoustic imaging applications. The article further reviews and analyzes three primary challenges currently confronting CMUTs in medical imaging applications: lower output acoustic pressure, dielectric charging effects, and the need for high bias voltage. It also presents and discusses a potential combined approach to comprehensively address these challenges, with the aim of enhancing CMUT performance and broadening clinical adoption. Full article

Laser-induced microjet-assisted ablation is an emerging technology in the field of laser processing. However, the influence of solid boundaries on jet behavior and the associated material removal mechanism remains unclear after observing and analyzing the ablation process. To address this, the present study systematically investigates the effect of the incidence angle on the processing efficiency and material removal mechanism in laser-induced microjet ablation. By controlling the laser power and liquid layer thickness, the dynamic behavior of the microjet, material removal performance, and surface morphology evolution under different inclination angles were explored. Based on video analysis and OpenCV processing, the regulation of jet morphology and impact mode by the incidence angle was revealed. Combined with white light interferometry and ultra-depth-of-field three-dimensional microscopy, the ablation depth and material removal rate were quantitatively characterized. The results showed that under normal incidence, the maximum material removal rate of 0.092 mm³/s was achieved at 9 W, while further increases in power led to a decrease in removal rate due to bubble aggregation. When the sample was tilted to 15°, the material removal rate reached 0.163 mm³/s, representing a 106.30% improvement compared to that at 0°, and the ablation depth also peaked with an average maximum depth of 12.32 ± 0.58 μm and a single-point maximum of 54.36 μm. Furthermore, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were employed to elucidate the microstructural features and elemental distribution under different process parameters. Through multi-parameter experiments, this study achieved process parameter optimization and clarified the material removal mechanism influenced by different incidence angles, providing both a process reference and theoretical basis for efficient micro-machining of aerospace materials. Full article

Multifrequency operation in micromachined ultrasonic transducers, enabled by targeted excitation of specific vibrational modes, has emerged as an attractive approach for achieving tunable performance and configurability, well-suited for advanced ultrasound imaging and therapeutic applications. This paper presents a dual-electrode rectangular piezoelectric micromachined ultrasonic transducer (PMUT) designed for efficient dual-frequency operation through mode-selective actuation. The proposed architecture employs segmented electrodes that are spatially aligned with the strain distributions of two distinct flexural modes, enabling selective excitation of Mode 1 (fundamental) and Mode 3 (higher order) through appropriate electrode actuation. Finite element simulations and impedance analysis were used to guide the electrode configuration and validate the mode-selective behavior. The dual-mode PMUT was fabricated alongside a conventional single-electrode PMUT using identical membrane dimensions and material stack for direct comparison. Comprehensive electrical and underwater acoustic characterization confirmed that the conventional PMUT is limited to single-frequency operation at the fundamental resonance. In contrast, the proposed design achieved a substantial improvement in higher-order performance, with a threefold increase in acoustic pressure at Mode 3 compared to the conventional device. These results demonstrate that mode-aligned electrode segmentation enables efficient dual-mode operation without added fabrication complexity, making the design highly suitable for multifrequency ultrasonic applications such as biomedical imaging and sensing. Full article