Focusing Lens Research Articles

Numerical investigation of ultrasound focusing and bubble collapse

Ultrasound focusing and microbubble collapse are numerically investigated using a level-set interface tracking method for two-phase flows with multiple interfaces. The computations for ultrasound propagating through a spherical lens demonstrate the ultrasound refraction and pressure intensification at the rear of the lens. The focusing of the initial negative pressure wave through the lens induces a converging flow and the focusing of the subsequent positive pressure wave further intensifies the pressure at the lens. Computations are extended to bubble oscillations near the focusing lens and compared with the no-lens case. The lens not only amplifies the bubble expansion and contraction rates significantly but also generates a larger pressure gradient across the bubble. This ultrasound focusing effect contributes to the asymmetric collapse of the bubble and the formation of a liquid jet that penetrates the bubble. The effects of lens size, initial bubble radius and bubble-lens distance on bubble expansion and liquid jet are further investigated.

Water repellence of biomimetic structures fabricated via femtosecond laser direct writing

Femtosecond laser direct writing based on two-photon polymerization (TPP), which is a sub-classification of vat photopolymerization, has the advantages of unlimited geometrical freedom and high resolution, and has attracted significant interests from scientists and engineers in recent years. In the present study, TPP fabrication was performed using a laser equipment with wavelength of 514 nm. The collimated beam diameter before the focusing lens was measured as 5 mm and the focused laser spot diameter was calculated as 280 nm. Key TPP processing parameters were investigated for the fabrication of nanowires, and micro/ nanoscale biomimetic structures, including cone and pinecone structures. The finest nanowire has a diameter of 105 nm and is achieved at the laser power of 0.8 μW (near the laser polymerization threshold). For the three-dimensional (3D) structures, laser power, layer thickness and hatch spacing would influence the fabrication accuracy. Generation of a stiff 3D cone structure would require a minimum laser power of 1.4 μW when the hatch spacing and layer thickness are 200 nm. The optimum processing parameters (laser power 2.2 μW, hatch space and layer thickness 100 nm) can generate structures with dimensions close to the design values. For pinecones with nano features, the scanning strategies play a significant role in the nano fabrication accuracy. A row of pinecones and cones were fabricated and tested for their water repellence performance. The relationship between the performance and the nano cone density was investigated. The pinecone structures exhibit superior performance to the ones with no nano-scale structures, indicating that improved hydrophobicity can be achieved by designing the micro/nano hierarchical structures. The increased number density of the nano cones in a micro/nano hierarchical structure was found to profoundly enhance the water repellence of the structures. The mechanisms were discussed with Wenzel and Cassie models.

Influences of focusing conditions on optical poling in lithium niobate using a 1035 nm femtosecond fiber laser.

In this study, we demonstrate the optical induction of ferroelectric domain inversion on the -z surface of lithium niobate crystals using a 1035nm femtosecond fiber laser. We investigate the effects of laser power and focusing parameters on domain morphology. The results revealed that domains exhibit an irregular random distribution at low power, whereas regularly arranged domains were formed near the crystal damage threshold. Furthermore, with sufficiently high power, lens focusing enables the fabrication of uniformly sized domains exceeding 100µm. The results suggest potential applications for controlling domain morphology and the duty cycle, and fabricating ferroelectric functional devices.

Design of a point-focusing lens antenna based on 3D printing for a small-area plasma

The interaction mechanism between plasmas and electromagnetic waves has not been fully defined, and experiments are helpful for investigating the mechanism and verifying the theoretical analysis. This paper proposes a point-focusing lens antenna based on three-dimensional (3D) printing technology for the experimental study of the interaction between electromagnetic waves and a small-area plasma. First, CST simulation software was used for the simulation design of the horn antenna and focusing lens. Subsequently, 3D printing technology was used to print the lens combined with the horn antenna for experimental verification of the focusing performance. Finally, an electromagnetic wave passing through a plasma was experimentally investigated, and it was mutually verified with the propagation theory of electromagnetic waves in plasmas. The experimental and theoretical data indicated that the 3D-printed point-focusing lens antenna exhibited a good focusing effect on electromagnetic waves. This method may be used by employing a variety of materials and designs, thereby considerably improving the detection of small-area plasma. It may also solve the issue of the radial diffraction effect of narrow cylindrical plasma generated by inductively coupled plasma-generation equipment and reduce the error associated with experimental data. Therefore, this method may be useful for plasma propagation measurement, plasma-environment diffraction analysis, and parameter inversion of complex plasma environments.

Extreme wave formation past a submerged circular sill: The role of linear spatial focusing and resonant trapping

We investigate dispersive wave amplification past a submerged circular sill on an otherwise flat seabed. This phenomenon is important because it can generate large-amplitude waves near the sill, due to spatial focusing and resonant trapping in linear dispersive regime, endangering navigation. Based on the potential flow theory, the velocity potential is solved separately in the ocean region and in the sill region. Matching is then achieved by means of integral equations involving Galerkin expansion of the unknown velocity field at the border between the two regions. The model is successfully validated against known analytical expressions for long waves and a smoothed particle hydrodynamics numerical solution. Our results advance existing theories valid either for non-dispersive waves or for shallow submergence of the sill. We show that, for relatively short waves as compared to the ocean depth, the sill acts as a wave lens focusing energy behind it. Increasing the wavelength of the incident wave promotes transition from wave focusing behind the sill to partial trapping atop the sill. In intermediate water depth, the concurrence of focusing and partial trapping favors the emergence of extreme wave amplitudes that can exceed up to 6 times the amplitude of incident waves. We hypothesize that this phenomenon is the main cause of local peaks in skewness and kurtosis near a submerged circular shoal obtained in recent numerical simulations. Indications for further studies in the nonlinear regime are finally provided.

Drilling of cylindrical holes in Crown glass by a short-pulse flat-top CO2 laser beam

We investigated cylindrical hole drilling in a crown glass with a high thermal expansion coefficient of 94 × 10–7 K−1 and a low melting point of 724 °C using a short-pulse CO2 laser with a flat-top beam, and also examined the drilling characteristics. The short laser pulse consisted of a pulse spike with a pulse width of 276 ns and a pulse tail with a length of 56.9 µs at a repetition rate of 200 Hz. The laser beam had a flat-top profile with an estimated M2 parameter of 13.5 and a diameter of 12.5 mm before a focusing lens. The flat-top beam was focused by the focusing lens, which had a focal length of 12.7 mm, on the glass surface at a focus offset of −0.20 mm to +0.40 mm. The incident flat-top beam produced conical holes at focus offsets of −0.20 mm to 0.00 mm and produced cylindrical holes at focus offsets of +0.20 mm to +0.40 mm. The hole depth of the cylindrical holes was 109 μm to 434 μm, the surface hole diameter was 150 μm to 366 μm, and the aspect ratio, defined as the ratio of the hole depth to the surface hole diameter, was 0.30 to 2.89. The hole depth was influenced by the focus offset and the total irradiation fluence, whereas the surface hole diameter, the taper angle and the ratio of the surface hole diameter to the irradiation diameter were influenced by the focus offset only. The ratio of the surface HAZ (Heat affected zone) diameter to the irradiation diameter was influenced by both the focus offset and the total irradiation fluence.

Analysis and Experiment of Laser Energy Distribution of Laser Wireless Power Transmission Based on a Powersphere Receiver

Laser wireless power transmission (WPT) is one of the most important technologies in the field of long-range power transfer. This technique uses a laser as a transmission medium instead of conventional physical or electrical connections to perform WPT. It has the characteristics of long transmission distance and flexible operation. The existing laser wireless power transmission system uses photovoltaic cells as a receiver, which convert light into electricity. Due to the contradiction between the Gaussian distribution of laser and the uniform illumination requirements of photovoltaic cells, the laser wireless power transmission technology has problems such as low transmission efficiency and small output power. Therefore, understanding the energy distribution changes in the laser during transmission, especially the energy change after the laser is transmitted to each key device, and analyzing the influencing factors of the energy distribution state, are of great significance in improving the transmission efficiency and reducing the energy loss in the system. This article utilizes the optical software Lighttools as a tool to establish a laser wireless power transmission model based on a powersphere. This model is used to study the energy distribution changes in the laser as it passes through various components, and to analyze the corresponding influencing factors. To further validate the simulation results, an experimental platform was constructed using a semiconductor laser, beam expander, Fresnel lens, and powersphere as components. A beam quality analyzer was used to measure and analyze the laser energy distribution of each component except for the powersphere. The output voltage and current values of various regions of the powersphere were measured using a multimeter. The energy distribution of the powersphere was reflected based on the linear relationship between photo-generated current, voltage, and light intensity. The experimental results obtained were in good agreement with the simulation results. Simulations and experiments have shown that using a beam expander can reduce divergence angle and energy loss, while employing large-aperture focusing lens can enhance energy collection and output power, providing a basis for improving the efficiency of laser wireless power transfer.

Low-loss tunable beam collimator and expander assembly with no moving parts using anengineered diffuser and varifocal lenses.

In this paper, we present a novel design for a tunable beam collimator. A variable collimator assists in achieving an adaptive size of an output collimated beam. Alternatively, it can also provide an adjustable output beam divergence angle for a noncollimated beam output. Tunable collimators are highly desirable for various applications in testing, engineering, and measurements. Such devices are also useful in providing tunable illumination of samples or targets in microscopes and emulating different target distances for characterizing the performance of camera systems in laboratory settings. The proposed collimator has two distinct advantages: it is light-efficient compared with pinhole-based collimator designs, and it delivers a large range of output beam sizes without involving the mechanical motion of bulk components. These attributes are achieved via the use of an engineered diffuser (in the place of a pinhole) and a pair of large aperture tunable focus lenses, which deliver a tunable magnification to the output collimated beam. In laboratory experiments, we achieve an optical transmission efficiency of 90% for the proposed tunable collimator.

Flexible conductive patterns using electrohydrodynamic jet printing method based on high-voltage electrostatic focusing lens

Flexible conductive patterns using electrohydrodynamic jet printing method based on high-voltage electrostatic focusing lens

The PANDA Barrel DIRC

The PANDA experiment at the international accelerator Facility for Antiproton and Ion Research in Europe (FAIR), Darmstadt, Germany, will address fundamental questions of hadron physics using p̄p annihilations. Excellent Particle Identification (PID) over a large range of solid angles and particle momenta will be essential to meet the objectives of the rich physics program. Charged PID in the target region will be provided by a Barrel DIRC (Detection of Internally Reflected Cherenkov light) counter. The Barrel DIRC, covering the polar angle range of 22–140 degrees, will provide a π/K separation power of at least 3 standard deviations for charged particle momenta up to 3.5 GeV/c. The design of the Barrel DIRC features narrow radiator bars made from synthetic fused silica, an innovative multi-layer spherical lens focusing system, a prism-shaped synthetic fused silica expansion volume, and an array of lifetime-enhanced Microchannel Plate PMTs (MCP-PMTs) to detect the hit location and arrival time of the Cherenkov photons. Detailed Monte-Carlo simulations were performed, and reconstruction methods were developed to study the performance of the system. All critical aspects of the design and the performance were validated with system prototypes in a mixed hadron beam at the CERN PS. In 2020 the PANDA Barrel DIRC project advanced from the design stage to component fabrication. The series production of the fused silica bars was successfully completed in 2021 and delivery of the MCP-PMTs started in May 2022.

High transmittance Huygens' metasurface with 360° phase coverage and its metalens application

Huygens' metasurfaces, as a class of artificial electromagnetic surfaces, have garnered significant interest due to their ability to effectively manipulate electromagnetic wave fronts. In this article, a Huygens' element with a simple design is discussed. The structure comprises a dielectric substrate sandwiched between a reverse-opening resonant ring and a pair of single-opening resonant rings. The proposed structure enables independent modulation of the electrical and magnetic response, particularly delivering transmittance amplitude of at least 0.8 and 0°–360° phase control simultaneously in a single double layer without vias. Based on this structure, we create a Huygens' metasurface focusing lens that gathers plane waves at the focal point. Then, a patch antenna is mounted onto this metalens to transform the radiated spherical waves into plane waves, increasing the antenna gain by up to 11 dBi. The experimental and simulated results demonstrate excellent agreement, demonstrating the usefulness of the proposed metalens and the importance of the designed Huygens-atom for metalens applications.

Dynamic performance of a membrane-based variable focus lens with a large aperture.

Dynamic performance is one of the most important characteristics of a variable focus lens. However, there are few studies investigating the dynamic response of a membrane-based variable focus lens. In this paper, we present a mathematical model to describe spring-damping phenomena in theory. The first order natural frequencies with different scales were confirmed via finite element analysis. We also built a dynamic response experiment platform with changeable optical apertures, which was driven by a high-speed piezo stack actuator. A photodiode module was placed behind the lens to measure the variation of light luminance as the lens changed, and a laser displacement sensor was used to measure the deformation of the membrane. A series of data was collected with different optical apertures (20mm, 30mm, 50mm) and different pre-stretching ratios (200%, 300%) under different driving frequencies (from 5Hz to 25Hz in every 5Hz step). The experimental results were consistent with the mathematical model, which showed that the first order natural frequency increased as the aperture decreased or the membrane stiffness increased. This frequency-dependent characteristic of the variable focus lens provides a basis for further research on its dynamic performance.

A Fourier-invariant squared Laguerre-Gaussian vortex beam

It is shown that a squared Laguerre-Gaussian (LG) vortex beam is Fourier-invariant and retains its structure at the focus of a spherical lens. In the Fresnel diffraction zone, such a beam is transformed into superposition of conventional LG beams, the number of which is equal to the number of rings in the squared LG beam. If there is only one ring, then the beam is structurally stable. A more general beam, which is a “product” of two LG beams, is also considered. Such a beam will be Fourier-invariant if the number of rings in two LG beams in the “product” is the same. The considered beams complement the well-known family of LG beams, which are intensively studied as they remain stable during their propagation in turbulent media.

80‐1: Invited Paper: Tunable liquid crystal lens for dynamic Prescription (Rx) correction and Accommodation‐Convergence (AC) conflict correction in AR/VR/3D HMDs

While the near‐to‐eye HMDs are advancing in display resolution, there are active challenges related to accommodation‐convergence (AC) conflict correction as well as prescription correction (Rx). Many approaches are reported previously to solve AC conflict using variable focal lenses, however, very few of them included dynamic Rx correction. While AC correction requires only adjustment of focal plane, Rx correction requires simultaneous correction of focal length and astigmatism. In this report, we have shown concept, design, fabrication, and demonstration of liquid crystal based tunable lens that is capable of dynamic correction of both the issue without needing any mechanical moving parts. The device is lightweight, thin, flat, low voltage driven and has potential to replace the eyeglasses worn by the users with the HMDs.

COVID-19 vaccination procurement and contract management in a South African context

This article examines the lessons learnt and opportunities created to strengthen future procurement and contract management in the public health sector. In a rigid and highly regulated public sector procurement landscape, the COVID-19 vaccine procurement and vaccine roll-out activated not usually used regulations, policies and standards. The article views the COVID-19 vaccine rollout programme through a procurement and contract management lens focusing on two key segments in the COVID-19 vaccine programme – namely COVID-19 vaccine procurement and vaccine delivery into health facilities. To date, 60 million COVID-19 vaccine doses have been successfully procured and delivered to South Africa. The South African National Department of Health is integrating the COVID-19 vaccine programme into its national immunisation programme while also preparing to launch the National Health Insurance. The overall lessons learnt during the COVID-19 vaccine roll-out can be utilised to strengthen future procurement and contract management in the public health sector.

Design of an apertureless two-stage acceleration lens for a single-ion implantation system

A single-ion-implantation (SII) system with an implantation accuracy of several tens of nanometers and energy of several tens of kilo-electron volts (keV) is developed to produce an array of nitrogen-vacancy centers (NV centres). The device with the array is expected to be used for quantum information technologies. The system consists of a Paul-trap type laser cooling device (PTLCD) and an electrostatic focusing lens. The PTLCD generates a few single-nitrogen-ions via an ion-cooling procedure. The SII system needs the implantation of single-nitrogen ions with 100% probability, called “deterministic implantation,” to build the array. A focusing lens without an aperture and with a magnification of ≪ 1 must be developed to realize the deterministic implantation. In this study, an electrostatic focusing lens, namely a two-stage acceleration lens, was designed by mainly investigating a second acceleration lens that is suitable for the SII system based on the previously developed two-stage acceleration lens. A new two-stage acceleration lens with a magnification of 0.006 was designed and a numerical simulation based on the lens demonstrated that the beam width for a single-nitrogen ion with an energy of 14 keV was 4.5 nm including intrinsic aberrations at full width at half maximum.

Effects of primary aberrations on a dark-field microscopy imaging system with the sharper focusing lens

The effects of primary aberrations of the sharper focusing lens on the image quality is analyzed for a dark-field microscopy imaging system, which is illuminated by a left-circularly (LC) polarized Laguerre–Gaussian (LG) vortex beam. To confirm the aberration tolerance range of the sharper focusing lens for precise measurement, the analytical expression of the field distribution on the image plane is deduced. Then the dark-field image results corresponding to a defect with a scale of 6µm×6µm are simulated. The influence of the spherical error, coma, and astigmatism on the imaging quality is compared in detail. Then the verification test has been carried out to show the effects of different primary aberrations. Finally, both the simulation and experiment results help us to identify that the tolerance range of spherical aberration, astigmatism, and coma should be less than 1.5λ, 1λ, and 1.25λ, respectively.

Spectral preprocessing combined with feature selection improve model robustness for plastics samples classification by LIBS

Introduction: Nowadays, the widespread use of plastic products has significantly contributed towards environmental pollution caused by waste plastics. Laser-induced breakdown spectroscopy (LIBS), an emerging spectroscopic technology, has shown great potential for rapid sorting and recycling of plastics. However, the poor robustness of the classification model severely limits the large-scale application of LIBS technology in plastic sorting and recycling.Methods: In this research, we used spectral preprocessing combined with feature selection to improve the robustness of the support vector machine (SVM) classification model for four typical plastic samples (ABS, nylon, 3240, and its modified product FR-4). LIBS spectral data were collected under different experimental conditions, then we defined robustness over time (ROT), robustness over time and different focusing lenses (ROT&RFL), and robustness over time and different manufacturers (ROT&RDM) to assess model performance. The feature importance of the preprocessed spectra was evaluated using the Relief-F algorithm, and the maximum accuracy of the validation set was 92.6% when inputting the first 19 most important features. Eventually, the optimal model was used for the prediction of the test set.Results and discussion: The ROT of the original spectrum, spectrum preprocessing, and spectral preprocessing combined with feature selection were 58.4%, 79.1%, and 98.47%, respectively. Similarly, ROT&RFL for the same methods were 65.54%, 75%, and 95.25%, respectively. ROT&RDM were 65.5%, 67%, and 93.92%, respectively. The results demonstrate that spectral preprocessing combined with feature selection can significantly improve the robustness of the classification model, and the proposed method is feasible for plastic sorting and recycling.

High-precision laser focus positioning of rough surfaces by deep learning

This work presents a precise positioning detection based on a convolutional neural network (CNN) to control the laser focus in laser material processing systems. The images of the diffraction patterns measured at different positions of the laser focus concerning the workpiece are classified in the range of the Rayleigh length of the focusing lens with an increment of about 7% of it. The experiment was carried out on different materials with different levels of surface roughness, such as copper, silicon, and steel, and over 99% accuracy in the positioning detection was achieved. Considering surface roughness and camera noise, a theoretical model is established, and the effects of these parameters on the accuracy of focus detection are also presented. The proposed method exhibits a noise-robust focus detection system and the potential for many precise positioning detection systems in industry and biology.

Compact and scalable polarimetric self-coherent receiver using a dielectric metasurface

Polarimetric self-coherent systems using direct-detection-based Stokes-vector receivers (SVRs) are a promising technology to meet both the cost and capacity requirements of short-reach optical interconnects. However, conventional SVRs require a number of optical components to detect the state of polarization at high speed, resulting in substantially more complicated receiver configurations compared with the current intensity-modulation–direct-detection counterparts. Here, we demonstrate a simple and compact polarimetric self-coherent receiver based on a thin dielectric metasurface and a photodetector array (PDA). With a single 1.05-µm-thick metasurface device fabricated on a compact silicon-on-quartz chip, we implement functionalities of all the necessary passive components, including a 1×3 splitter, three polarization beam splitters with different polarization bases, and six focusing lenses. Combined with a high-speed PDA, we demonstrate self-coherent transmission of 20-GBd 16-ary quadrature amplitude modulation and 50-GBd quadrature phase-shift keying signals over a 25-km single-mode fiber. Owing to the surface-normal configuration, it can easily be scaled to receive spatially multiplexed channels from a multicore fiber or a fiber bundle, enabling compact and low-cost receiver modules for future highly parallelized self-coherent systems.