Focusing Capability Research Articles

Optimization, analysis, and thermal performance enhancement of the cooling flow channel for a condenser mirror

Condenser mirrors with the ellipsoidal surface type are widely used in laser illuminating systems. Nonetheless, thermal absorption in optical components is unavoidable because of high-energy lasers, leading to a decline in surface shape accuracy. In extreme instances, this can shorten the operational longevity or impair the system’s focusing capabilities. Currently, water cooling is a major technical approach for controlling the thermal deformation of condenser mirrors. This paper proposes an efficient cooling flow channel based on topology optimization methods for high-gradient ellipsoidal optical mirrors. Firstly, topology optimization of the cooling flow channel was performed with the objective function of minimizing average temperature and entransy dissipation. Next, the influence of different design parameters on the optimization results was investigated, and the optimal topological structure was obtained. Then, a thermal–fluid–solid coupled simulation was carried out for a three-dimensional model of the water-cooled condenser mirror to verify the topology optimization design. The results show that when the inlet flow rate is 2.82×10−4m3/s, the surface shape accuracy of the water-cooled condenser mirror (RMS 213.79 nm) equipped with the topology optimization cooling flow channel is reduced by 22.96% compared to that of the water-cooled condenser mirror (RMS 277.66 nm) with the traditional spiral cooling flow channel. Finally, the manufacturability of the optimized model was verified by selective laser melting technology and x-ray detection.

Deep-Ultraviolet AlN Metalens with Imaging and Ultrafast Laser Microfabrication Applications.

Deep-ultraviolet (DUV) light is essential for applications including fabrication, molecular research, and biomedical imaging. Compact metalenses have the potential to drive further innovation in these fields, provided they utilize a material platform that is cost-effective, durable, and scalable. In this work, we present aluminum nitride (AlN) metalenses as an efficient solution for DUV applications. These metalenses, with a thickness of only 380 nm, deliver DUV focusing and imaging capabilities close to the theoretical diffraction limit. Leveraging their robustness to intense ultrafast laser irradiation, we demonstrate successful DUV ultrafast laser direct writing of microstructures on a polymer film and silicon substrate. These results underscore the significant promise of advancing photonic technologies in this critical wavelength regime.

Highly efficient fiber Bragg grating spectrometer fabricated with violet and near-infrared femtosecond laser pulses and the phase mask technique.

All-fiber visible spectrometers with a sub-nanometer resolution and record-high light outcoupling (70%) are fabricated using violet (400 nm) and near-infrared (800 nm) femtosecond laser pulses and the phase mask technique. The spectrometers are based on highly localized uniform Bragg gratings produced by tightly focusing the femtosecond pulses into the core of visible single-mode fibers. The unique nanoscale morphology of the resultant Bragg gratings ensures very strong outcoupling of light from the fiber, while bending of the fiber provides the focusing capability to define an efficient all-fiber spectrometer.

An Improved Detecting Algorithm of Moving Targets for Airborne Maritime Surveillance Radar.

The traditional method is capable of detecting and tracking stationary and slow-moving targets in a sea surface environment. However, the signal focusing capability of such a method could be greatly reduced especially for those variable-speed targets. To solve this problem, a novel tracking algorithm combining range envelope alignment and azimuth phase filtering is proposed. In this method, the motion of the airborne maritime surveillance radar platform is firstly compensated for target echoes. Secondly, range envelope alignment is performed to correct the unpredictable range migration of the target after pulse compression. The higher-order phase difference between the adjacent pulses is estimated and compensated. Ultimately, such pulse series are accumulated through azimuth Fourier transform. Traditional methods compensate only for platform motion, limiting their ability to handle variable-speed targets. The proposed algorithm addresses this limitation by compensating for both platform and target motion, significantly improving signal focusing and tracking accuracy. A detailed analysis shows that our algorithm can significantly increase the signal accumulating gain and improve the focusing effect. The simulation results are provided to demonstrate the effectiveness of the proposed algorithm.

Fabrication of Artificial Compound Eyes with Biplanar Focal Planes on a Curved Surface.

Inspired by ancient trilobites, novel curved microlens arrays (CMLAs) were designed. Direct, fast, and low-cost CMLAs with two focal planes were fabricated using ultraprecision machining technology and hot embossing technology. We designed four pairs of artificial compound eyes (ACEs) composed of large and small lenses with four different curvatures to achieve focusing and imaging on two focal planes. A test system was constructed to capture the first-order and second-order images for each level of the ACEs. Additionally, we analyzed the deformation patterns in the first-order and second-order images. The wide field of view (FOV) value was 68°, which aligns with the theoretical prediction. The focusing performance was also investigated, and the experimental results indicate that each lens achieves uniform focusing within the FOV range. These results confirm that microlens arrays with two focal planes possess advanced imaging and focusing capabilities, enabling a wide depth-of-field function. This opens new avenues for the development of advanced detectors and optical imaging devices.

Hot Nanogap Networks-In-Triangular Nanoframes: A Strategy for Positioning Adsorbates Near Hot Spots.

This study reports the synthesis of plasmonic hot nanogap networks-in-triangular nanoframes (NITNFs), featuring narrow intraparticle nanogap networks embedded within triangular nanoframes. Starting from Au nanotriangles, Pt NITNFs are synthesized through a cascade reaction involving simultaneous Pt deposition and Au etching in a one-pot process. The Pt NITNFs are then transformed into plasmonically active Au NITNFs via Au coating. The near-field focusing capabilities of the Au NITNFs are tailored by fine-tuning the void area fraction down to 3.9%, resulting in the formation of narrow nanogaps of ≈1nm. This optimization enables the successful implementation of single-particle surface-enhanced Raman scattering (SERS) measurements. Then, monolayer Au NITNFs films on Al substrates are prepared, which enabled weakly adsorbing species to be positioned close to the hot spots of the NITNFs by anchoring them to the underlying Al substrates. As a representative sensing application, the SERS-based detection of gas-phase dimethyl methylphosphonate (DMMP) using a film of plasmonic NITNFs on an Al substrate exhibits outstanding performances, achieving a limit of detection of 5ppm and a detection time of 120 s.

Comprehensive evaluation of lens and planar circular ultrasonic transducer combinations to enhance C-scan imaging: a phantom study at 10 and 15 MHz

Abstract Objective. To evaluate the imaging capabilities of four polydimethylsiloxane-based acoustic lenses with subsonic refractive indices, focusing on a spherical lens and three designed aspherical lenses with varying focus. Approach. We investigated a spherical lens with a radius curvature of 8 mm (SL-R8), two aspherical lenses with focal lengths of 15 mm and 10 mm (AL15 and AL10), and an aspherical folding lens with a focal length of 10 mm (AFL10). The equation cross-section profile of an aspherical lens is calculated using the assumption that all trajectories passing through the lens have the same flight duration when they reach the focal point. Furthermore, we observed that we can fold this aspherical lens without greatly diminishing its focusing capability. Utilizing circular planar ultrasonic transducers at frequencies of 10 MHz and 15 MHz, we examined the reflected waveforms via pulse-echo tests and measured focal points and beam widths using a needle hydrophone in the acoustic intensity measurement system. The target resolution was assessed using a C-Scan imaging system. Main results. Analysis of the axial and lateral beam profiles revealed that AFL10 and AL10 lenses exhibited narrower beam widths at the focal point. We evaluate the AFL10 and AL10 lenses combined with a 15 MHz transducer for preclinical testing using standard medical phantoms containing microparticles in gelatin- and agar-based matrices. In conclusion, using these twocombinations, C-Scan imaging successfully formed a 25 µm backscatter inside the phantom image. Significance. Due to their high-resolution capabilities, applying cross-sectional profile equations in the lateral plane of the lens surface in linear ultrasonic array probes offers promises for early-stage medical diagnostics.

Ultrasonic nonlinear frequency diverse array: principle and applications

In ultrasonic testing, omnidirectional wave propagation presents challenges of nonfocus energy loss and interface interference. Traditional phased array (PA) methods, despite their focusing capabilities, suffer from limitations in energy utilization and target detection accuracy. Frequency diverse array (FDA) technology, initially developed for radar, has demonstrated potential for improved range-angle-dependent ultrasonic beampatterns but still exhibits limitations due to S-shaped coupling. This study proposes an ultrasonic nonlinear frequency diverse array (ultrasonic-NFDA) with nonuniform frequency offsets to address these issues. The ultrasonic-NFDA employs modulated frequency offsets by introducing nonlinear carrying functions and incorporating additional parameters to manipulate beamforming and enhances energy concentration at focal points. Furthermore, the multi-objective differential evolution algorithm is used for parameter optimization. The proposed method decouples the range-angle beampattern and improves the signal-to-interference-plus-noise ratio. Numerical evaluations and simulations demonstrate that ultrasonic-NFDA outperforms the ultrasonic linear FDA and PA series, achieving better peak sidelobe level and half-power beam width. Experimental validations confirm the effectiveness of ultrasonic-NFDA in practical ultrasonic testing scenarios, showing significant improvements in defect detection and imaging clarity. The optimized beampatterns effectively reduce artifacts and enhance the precision of defect localization. The proposed integration of nonlinear frequency offsets and parameter optimization in array testing offers a robust solution for more accurate and efficient ultrasonic inspection methodologies.

Lightweight global adaptive feature enhancement network for underwater object detection with sonar image

Abstract Sonar target detection is widely used in various underwater detection tasks. However, sonar images often lack target information and blurry features because of the interference of seabed environmental noise and complex background information. This poses significant challenges for sonar target detection tasks. A new lightweight Global Adaptive Feature Enhancement Network (GAFE-Net) is proposed to enhance the ability to acquire target information and effectively suppress background information. This network utilizes lightweight convolutional calculations and adaptive feature extraction blocks with global feature extraction capabilities to capture multi-scale semantic features of sonar images. Specifically, GhostConv is used to maintain local feature extraction capability while reducing computation complexity. The adaptive feature extraction block (C2FC) is employed to capture deep semantic features. The partial self-attention (PSA) mechanism is adopted to enhance the capability of focusing on targets. The Slim-neck is deployed for fusing multi-scale information. Validation on the public sonar image dataset URPC2021 shows that, compared to other advanced sonar target detection algorithms, the proposed method improves accuracy while maintaining low computational complexity, demonstrating excellent performance.

A novel BP-GA based autofocus method for detection of circuit board components

Optical micro-inspection systems use different focusing methods depending on the inspection requirements of different scenarios. In practical industrial micro-inspection, grid samples have a wide variety of components, for example, electronic components on circuit boards and transistors on electronic chips. Changes in the surrounding environment (e.g., brightness of light, flatness of the platform, and temperature, etc.) during the inspection of these processed parts may lead to out-of-focus of the object under the microscope. Therefore, this paper proposes an autofocus algorithm to cope with the complex environment during inspection. The algorithm is based on feature vectors reflecting the external geometry of the spot and the internal energy distribution, and is combined with a back-propagation neural network with a genetic algorithm (GA) to enhance the focusing capability of the optical microscope. Preliminary numerical test results show that because of the bias problem in the focusing system, the accuracy of the neural network in calculating the defocused amount (DA) is significantly improved, despite the pitfalls of its generalization ability and the possibility of endless loops during the focusing process. In order to further solve the pitfalls of neural networks, this paper introduces a full reference image evaluation model into the optical microscope system and finally develops the autofocus software. Focusing tests using the developed software for the inspection of real components demonstrate that the introduced full reference image evaluation model not only expands the focusing distance of the inspection system, but also prevents the autofocus algorithm from falling into a dead loop.

Political conformity and digital transformation: Evidence from China

This study investigates the influence of political conformity on digital transformation in Chinese family businesses from 2011 to 2020. We find that political conformity significantly boosts digital transformation by strengthening strategic leadership, technology focus, and organizational capabilities. It also fosters risk-taking, reduces agency costs, and eases financial constraints. The impact is more pronounced in situations in contexts with high economic policy uncertainty, when executives have limited IT knowledge, family members hold positions in the party committee, and the firm is geographically proximate to the political center, Beijing. Additionally, aligning political conformity with technology strategies not only advances digital transformation but also enhances the businesses' Environmental, Social, and Governance (ESG) profile and firm value.

Narrow‐Gap‐Integrated Ring Arrays (GRA) as Ultrahigh Field‐Enhancement Optical Resonators

AbstractGold nanoring nanoantenna are increasingly being utilized in biological and chemical sensing owing to their favorable spatially expanded hotspot distribution and distinctive field focusing capabilities. However, it is extremely challenging to further elevate the local electric field of nanorings for higher sensitivity without trading off the two aforementioned features. Here, a new nanoantenna design integrates conventional nanorings with nanogaps is introduced: in conjunction with metal sputtering and ion milling on a nanopillar template, narrow‐gap‐integrated ring arrays (GRA) are successfully nanofabricated featuring monolithic integration of nanoring arrays with sub‐5 nm nanogaps on‐chip. This new GRA nanophotonic structure simultaneously preserves the intrinsic benefits of the nanoring and amplifies the local electric field, achieving a field enhancement factor of up to 104 magnitudes. Using both top‐down and bottom‐up thin‐film processes, the GRA shows high degree‐of‐freedom in nanoantenna geometry, leading to tunable resonances throughout visible and near‐infrared wavelengths. These results combined showcased the potential of using GRA's tunable resonance, high e‐field enhancement, and wide hotspot‐distribution features in the creation of a universal molecular sensing platform for enhanced fluorescence.

Magnetic actuator driver system for laser scanning capsule endoscopy

This paper focuses on designing and implementing a power and area-efficient magnetic actuator driver interface integrated circuit for laser scanning capsule endoscopy. The proposed system contains a 3D-printed focus-adjusting actuator embarking a lens, multiple magnets, an external coil, battery, laser, and actuator driver integrated circuit with off-chip components. The actuator features multiple pantograph springs connected to the lens, as well as multiple magnets, enabling precise focusing capability through electromagnetic actuation. A magnetic actuator driver integrated circuit implemented in a commercial 180 nm CMOS process drives the coil at 32 Hz, which is the mechanical resonance frequency of the actuator. A novel control methodology for the driver has been devised, aimed at enhancing driving efficiency and mitigating total harmonic distortion. Simulations and measurements substantiate that the actuator can induce a 3.22 mm focal point displacement while the driver circuit delivers 9.62 mA (RMS) current to the 7.7 mH coil. Under these conditions, the system exhibits an aggregate power consumption of 11.48 mW, thereby achieving a power efficiency of 85.5%.

Comparative analysis of microlens array formation in fused silica glass by laser: Femtosecond versus picosecond pulses

The growing demand for flexible, high-quality fabrication of free-form micro-optics drives the development of laser-based fabrication techniques for both the shape formation and surface polishing of optical elements. In this paper, we performed a thorough and systematic study on fused silica glass ablation using 10 ps and 320 fs duration pulses. Ablation processes for both pulse durations were optimized based on the measurements of the removed material layer thickness and surface roughness, and by analyzing the topographies of ablated cavities to remove material layers as thin as possible with minimum surface damage. Our findings demonstrate higher process resolution and surface quality for femtosecond pulses. Ablation of pre-roughened glass reduced the minimal removable glass layer thickness well below the 1 μm mark for both pulse durations, improving the process resolution. The minimal removable glass layer thickness was 14 times smaller for the femtosecond pulses, with up to 4.5 times lower surface roughness compared to samples processed with picosecond pulses. On the other hand, results revealed faster glass removal rates with picosecond pulses. In the end, arrays of microlenses were fabricated with both pulse durations and subsequently polished with a CO2 laser. Results revealed higher performance of microlenses fabricated with femtosecond pulses, providing better focusing capabilities and lesser beam scattering. Finally, this study demonstrated the successful fabrication of free-form optical elements with femtosecond and picosecond pulses, demonstrating the versatility and the potential of laser-based techniques.

Multilevel Diffractive Lenses: Recent Advances and Applications

Multilevel diffractive lenses (MDLs) has undergone considerable advancements, marked by their exceptional efficiency and diverse focusing capabilities, resulting in their widespread use in optical systems. In recent times, MDLs have consistently been juxtaposed with metalenses, which have experienced swift progress over the last decade. Concurrently, MDLs have continued to evolve, propelled by their distinct advantages, such as cost-effective production and adaptability for mass manufacturing. This article explores the evolution and foundational concepts of MDLs, highlighting the advantages of their circular symmetry in enhancing simulation and optimization efficiency. Furthermore, we present several innovative fabrication methods for MDLs that capitalize on the latest advancements in 3D printing technology. We also show the practical applications and potential future developments of MDLs.

Amorphous to Crystalline Transition in Nanoimprinted Sol-Gel Titanium Oxide Metasurfaces.

Crystalline titanium dioxide (TiO2) (anatase and rutile) possesses a higher refractive index than amorphous TiO2 and near-zero absorption in the visible region, making them an ideal material for visible metasurfaces. However, fabrication limitations hinder their implementation into flat optics. In this work, a wafer-scale manufacturing platform is proposed for crystalline TiO2 metasurfaces. Sol-gel TiO2 is developed as a printable material in which its material phase can be precisely controlled to produce amorphous, anatase, or rutile, depending on the crystallization temperature. Therefore, anatase or rutile metalenses can be fabricated on a wafer scale using thermal nanoimprint lithography and sintering process. The high refractive index of the crystalline TiO2 contributes to the enhanced conversion efficiency of the fabricated metalenses. The fabricated metalenses exhibit diffraction-limited focusing and imaging capabilities, comparable to the theoretically ideal lenses.

Terahertz switchable vortex beam generator based on vanadium dioxide reflective metasurface

Vortex beams exhibit significant potential for applications in diverse fields, such as optical communications, particle manipulation, and quantum information, due to the orbital angular momentum (OAM) they carry. While substantial progress has been made in the generation of vortex beams and the control of their wavefronts, further optimization is necessary to enhance the variability of OAM modes and the modulation of focusing. This study presents three tunable vortex beam generators based on vanadium dioxide metasurfaces. By leveraging the insulator-metal phase transition of vanadium dioxide (VO2), these metasurfaces can convert incident left-circularly polarized (LCP) light into a deflected vortex beam with adjustable OAM modes, a switchable focused vortex beam, or a focused vortex beam with tunable OAM modes in the terahertz band. Specifically, by varying the angle of the incident light, a vortex beam carrying variable topological charges can be deflected over a range from -34° to 90°. Vortex beams with a topological charge of 1 enable reversible switching between near-focus (0.85 mm) and far-focus (3.2 mm), demonstrating highly effective tunable focusing capabilities. Additionally, focused vortex generators with switchable topological charges are proposed. Focused vortex beams with a topological charge of -1 are generated in the insulating state, while a topological charge of -2 is observed in the metallic state. These vortex beam generators allow for different phase gradient distributions in the phase transition states, enabling diverse OAM and variable focuses. This research has potential applications in areas such as dynamic imaging and optical communications.

Method for improving sound projection performance based on linearly constrained minimum power algorithm

Sound projection is a technique that utilizes directional sound sources to project sound wave onto reflective surfaces, aiming to create auditory events in the direction of projected sound, which can be achieved via loudspeaker arrays beamforming. Being limited by the focusing capability of loudspeaker array, the direct sound that reaches the listener earlier than the projected sound can influence the localization perception of projected sound due to the precedence effect. Therefore, for sound projection, it is crucial to reduce the direct sound intensity relative to the projected sound. In this study, a beamforming method of weakening direct sound is proposed to improve the performance of sound projection. Based on the linearly constrained minimum power algorithm, where a beamforming null is set in the direction of direct sound, a beamformer for a 16-unit linear loudspeaker arrays is designed to reduce the level of direct sound. Then, the ratio of reflected sound to direct sound (R/D ratio) is used as a criterion to evaluate the performance of the proposed method by simulation. The results show that the proposed method has significantly higher R/D ratio compared to the traditional time delay method, which validates the effectiveness in reducing the level of direct sound.

Efficient fabrication of large-scale chalcogenide glass microlens arrays via precision molding method

Infrared microlens arrays (IR MLAs) are crucial for infrared imaging, target detection, and night vision, but creating high-quality large-scale IR MLAs is costly and challenging. This study presents a precision molding method to produce 500 × 500 microlenses on chalcogenide glass. The method involves creating concave silica molds with femtosecond laser-assisted chemical etching, which are then used to press convex MLAs onto the chalcogenide glass surface. By using single-pulse direct writing technology, a microlens array template containing 500 × 500 microlenses can be completed within 25 min. Meanwhile, the chemical etching method offers a highly efficient and low-cost way to prepare mold. The resulting MLAs exhibited smooth surfaces, uniform structures, and good imaging and focusing capabilities, indicating the effectiveness of this approach for large-scale MLA patterning and batch production.

Multimodal emotion recognition based on a fusion of audiovisual information with temporal dynamics

AbstractIn the Human-Machine Interactions (HMI) landscape, understanding user emotions is pivotal for elevating user experiences. This paper explores Facial Expression Recognition (FER) within HMI, employing a distinctive multimodal approach that integrates visual and auditory information. Recognizing the dynamic nature of HMI, where situations evolve, this study emphasizes continuous emotion analysis. This work assesses various fusion strategies that involve the addition to the main network of different architectures, such as autoencoders (AE) or an Embracement module, to combine the information of multiple biometric cues. In addition to the multimodal approach, this paper introduces a new architecture that prioritizes temporal dynamics by incorporating Long Short-Term Memory (LSTM) networks. The final proposal, which integrates different multimodal approaches with the temporal focus capabilities of the LSTM architecture, was tested across three public datasets: RAVDESS, SAVEE, and CREMA-D. It showcased state-of-the-art accuracy of 88.11%, 86.75%, and 80.27%, respectively, and outperformed other existing approaches.