Focal Spot Research Articles

An on-machine measurement and calibration method for incident laser error in dual-swing laser heads

The dual-swing laser head is essential for five-axis laser machining, yet its precision is greatly affected by the incident laser beam. Any positional or angular deviation in the laser can cause the focus spot position of the head to change continuously during rotation, thereby severely compromise the manufacturing performance of the head. However, the current calibration methods for the incident beam of dual-swing laser heads have issues with low accuracy and insufficient engineering applicability. This paper proposes an on-machine measurement and calibration method for incident laser error in dual-swing laser heads. An error model for the incident beam with a dual-swing laser head was established, from which the law of spot position changes caused by incident beam errors during the head's rotation was derived. Subsequently, following this law, a precision calibration method for the laser head's incident beam error was proposed, based on the theory of optical image height. Afterwards, an on-machine error measurement system was established on the dual-swing laser head, and the calibration method was verified through experiments. The results show that the use of this calibration method can improve the accuracy of the incident beam for dual-swing laser head to 0.071 mm, which is approximately 3–4 times better than traditional calibration methods, thereby significantly enhancing the manufacturing precision of the laser head.

MATHEMATICAL MODELING OF THE NON-STATIONARY THERMAL STATE OF A COMPOSITE COATING OF CLOSE TO EQUIMOLAR COMPOSITION DURING LASER REMELTING

Based on assumptions and limitations, a mathematical model of the non-stationary thermal state of a composite coating during laser remelting on a metal substrate to obtain an alloy of equimolar or close to equimolar composition has been developed. The model includes the boundary value problem of calculating the nonstationary thermal state of the temperature distribution in the solid, two-phase and liquid regions, considering the heat release of crystallization according to the theory of a quasi-equilibrium two-phase zone in the presence of porosity of a mixture of metal powders. Based on the mathematical model, the computer program “Composite coating thermal state” was created, which allows to determine the non-stationary temperature distribution in the formed coating and substrate, the depth of penetration of the high-entropy coating layer, to evaluate the shrinkage of the molten coating under different technological conditions: laser power, the diameter of its focal spot, as well as the speed of movement along the surface of the processed material. The created program also allows to determine the dynamics of the position of two-dimensional liquidus and solidus lines for each metal included in the mixture. Using the created program, a computer simulation of the process of unsteady heating of a mixture of three metal powders in the same mass fractions: nickel, chromium and iron was carried out. Non-stationary temperature fields in the composite coating are obtained, considering its penetration and heating of the substrate. The depth of penetration of all metals, including the most low-melting one, was determined during laser remelting on a metal substrate in the production of a high-entropy alloy. The practical significance of the developed computer model lies in its use to predict the penetrationdepth of a composite coating under specified technological modes of laser remelting. At the same time, its adaptation is necessary, based on experimental data on the porosity and thickness of the coating before and after melting of simple metal systems with similar thermophysical properties.

Focused Electromagnetic Acoustic Transducers for Lamb Wave Inspection

Abstract Ultrasonic inspection of conductive samples can be performed using electromagnetic acoustic transducers (EMATs). These allow generation of guided waves, such as Lamb waves, in thin plates. Lamb waves are used for defect screening as they can travel long distances with relatively little attenuation. However, large wavelengths are used, which results in poor sensitivity to subwavelength sized defects. This work presents methods for using Lamb waves to detect smaller defects. Curved, geometrically focused EMATs were used to generate and detect Lamb waves in a 1 mm thick aluminum plate. The focal spot of these EMATs is investigated and the dependence of the focal spot dimensions and locations on the wavelength of the generation signal are presented. An array of geometrically focused detection EMATs was used to measure a 10×10×0.5mm square machined defect, simulating wall thinning. Perturbations in the pulse transmission in the region of the defect were detected, along with reflections, with each detector in the array sensitive to different defect features. B-scans and C-scans are presented utilizing these perturbations and reflections to locate, size, and image the machined defect. Analysis of data from the B-scan is shown to accurately size the defect width to within ±0.73mm. Very good agreement is shown between the C-scan images and the known location and orientation of the defect. Data fusion techniques are presented to combine data sets from different receiver EMATs and increase the defect detection capability, accuracy, and precision, with the potential for full defect sizing demonstrated.

Thermal fields induced by oscillating laser beams versus non-oscillating equivalent laser beams — A numerical study

In high power laser material processing, like laser transformation hardening, laser welding or laser cladding, tailoring the laser intensity profile is a method to improve stability of the laser material process and/or improve the processing results. A method to tailor the laser intensity profile in real-time is using a Galvanometer Scanner to quickly and iteratively oscillate the focal spot of the laser beam over a predefined path. Due to the oscillating nature of this beam shaping method, the temporal heat input will differ from the temporal heat input by an equivalent non-oscillated laser intensity distribution. Mathematically this equivalent non-oscillated beam profile is the convolution of the laser intensity distribution of the oscillated focal spot and the predefined oscillation path. This work presents simulation results of laser induced thermal fields. The thermal fields induced by oscillated laser beams at different frequencies are compared with the thermal field induced by the equivalent non-oscillated beam. These comparisons show that even at the highest oscillation frequencies the oscillated and equivalent non-oscillated laser beams induce significantly different in temperature fields (e.g. up to 325 K peak temperature difference) and different cooling rates (105 K/s and 5500 K/s respectively).

Adhesion of retinal cells to gold surfaces by biomimetic molecules.

Neural cell-electrode coupling is crucial for effective neural and retinal prostheses. Enhancing this coupling can be achieved through surface modification and geometrical design to increase neuron-electrode proximity. In the current research, we focused on designing and studying various biomolecules as a method to elicit neural cell-electrode adhesion via cell-specific integrin mechanisms. We designed extracellular matrix biomimetic molecules with different head sequences (RGD or YIGSR), structures (linear or cyclic), and spacer lengths (short or long). These molecules, anchored by a thiol (SH) group, were deposited onto gold surfaces at various concentrations. We assessed the modifications using contact angle measurements, fluorescence imaging, and X-ray Photoelectron Spectroscopy (XPS). We then analyzed the adhesion of retinal cells and HEK293 cells to the modified surfaces by measuring cell density, surface area, and focal adhesion spots, and examined changes in adhesion-related gene and integrin expression. Results showed that YIGSR biomolecules significantly enhanced retinal cell adhesion, regardless of spacer length. For HEK293 cells, RGD biomolecules were more effective, especially with cyclic RGD and long spacers. Both cell types showed increased expression of specific adhesion integrins and proteins like vinculin and PTK2; these results were in agreement with the adhesion studies, confirming the cell-specific interactions with modified surfaces. This study highlights the importance of tailored biomolecules for improving neural cell adhesion to electrodes. By customizing biomolecules to foster specific and effective interactions with adhesion integrins, our study provides valuable insights for enhancing the integration and functionality of retinal prostheses and other neural implants.

Analysis of micro-hole formation in titanium nanofilms using a low power CW laser

This study investigates the mechanisms and dynamics governing microhole formation in titanium nanofilms under the influence of a continuous-wave low-power laser emitting at 1070 nm. Two distinct configurations were explored: one in which the laser interacted with the titanium film in an air environment (referred to as configuration AM), and the other in which the laser beam passed through glass before reaching the titanium film (referred to as configuration SM). Our experiments demonstrated that microhole formation is possible in both configurations. However, the threshold powers, hole characteristics, and time scales involved differ significantly between AM and SM configurations. Temperature distribution simulations, based on a linear absorption model, revealed notable differences in peak temperatures between these two configurations, primarily attributed to variations in focal spot size and Fresnel losses. These temperature variations resulted in significantly different mechanisms of material removal in both configurations. In the AM configuration, the boiling temperature was not achieved, even at the highest power; however, a melting pool was formed at powers exceeding 50 mW. Thus, the primary driver of microhole formation was the Marangoni effect, which caused molten material to flow tangentially towards the edges of the molten pool. Furthermore, titanium oxidation was identified as a competing process contributing to changes in optical transparency and potentially inhibiting the complete formation of holes. Conversely, within the SM configuration, boiling temperatures are easily achieved, leading to localized boiling and the formation of gas bubbles near the glass–titanium interface. As a result, the rupture of the bubble through the capping molten layer led to a vigorous expulsion of both gas and material, allowing for the creation of microholes even at remarkably low laser power levels (10 mW). Notably, in this configuration, the influence of titanium oxidation is negligible due to the relatively short time scale involved (hundreds of microseconds).

Experimental visualization of optical spatial sensitivity through combination of diffuse correlation spectroscopy and acoustic radiation force

In field of diffuse optics for biomedical applications, the spatial sensitivity (SS) is a key parameter to evaluate or optimize the adopted modalities, such as penetration depth, signal-to-noise ratio as well as sensor distribution. Nevertheless, SS is usually estimated via computer simulations (e.g., photon Monte Carlo simulation), rather than being quantified experimentally, due to the technical difficulty. In this study, we report the experimental measurement and visualization of optical SS through combination of acoustic radiation force (ARF) and the scanning diffuse correlation spectroscopy (DCS). By spatially varying the location of ARF focal spot within liquid phantom, the enhanced particle flow, which represents the most spatial sensitive location, was identified by DCS. The experimental outcomes were cross-validated with the photon Monte Carlo simulation, thus demonstrating its accuracy, feasibility, and potential for guiding clinical usage.

Microstructuring of fused silica by laser-induced microplasma using a pyrographite target for phase optical elements fabrication

In this paper, we purpose to use laser-induced microplasma occurred from a pyrographite target by laser ablation for microstruturing of fused silica plate to fabricate phase optical elements. Laser parameters for erosive plasma torch formation and subsequent etching of fused silica were determined based on preliminary experimental studies and estimated energy calculations. The influence of the overlap coefficients in the focal spot on the reproducibility of etching depths results and the deviation accuracy of etching depths with the found laser parameters was experimentally studied. The best results in terms of accuracy (no more than 0.1 μm) in deviations of etching depths are achieved with the overlap coefficient of 0.5. Moreover, the paper presents estimated calculations of the speed and efficiency of ablation, as well as the coefficient of energy conversion from laser radiation to the ablation energy of fused silica, which was ∼ 0.25. Based on achieved results an attempt was made to fabricate the 6-sector SPP for the wavelength of 355 nm.

Improving the working distance for near-field lithography with supercell photonic crystal

Near-field lithography technology plays a vital role in chip manufacturing. However, near-field lithography devices with acceptable working distance (WD) have become a long-term pursuit for researchers. Here, a one-dimensional supercell photonic crystal (SPC) is proposed for near-field lithography. The simulation results show that the PCs could provide an air WD of over 100 nm in interference lithography and produce patterns with a resolution reach ∼67 nm (∼λ/6). The analyses indicate that using SPCs in interference lithography can make it more suitable for practical manufacturing. Furthermore, this type of PC can also be used in direct writing lithography. The simulation results show that the direct writing lithography could be realized with a WD of 100 nm, and the full width at half maximum of the focal spot is ∼137 nm. The principle presents a new method for controlling the evanescent waves, and the method would greatly promote the development and application of near field lithography.

Study of Neutron-, Proton-, and Gamma-Irradiated Silicon Detectors Using the Two-Photon Absorption-Transient Current Technique.

The Two-Photon Absorption-Transient Current Technique (TPA-TCT) is a device characterisation technique that enables three-dimensional spatial resolution. Laser light in the quadratic absorption regime is employed to generate excess charge carriers only in a small volume around the focal spot. The drift of the excess charge carriers is studied to obtain information about the device under test. Neutron-, proton-, and gamma-irradiated p-type pad silicon detectors up to equivalent fluences of about 7 × 1015 neq/cm2 and a dose of 186 Mrad are investigated to study irradiation-induced effects on the TPA-TCT. Neutron and proton irradiation lead to additional linear absorption, which does not occur in gamma-irradiated detectors. The additional absorption is related to cluster damage, and the absorption scales according to the non-ionising energy loss. The influence of irradiation on the two-photon absorption coefficient is investigated, as well as potential laser beam depletion by the irradiation-induced linear absorption. Further, the electric field in neutron- and proton-irradiated pad detectors at an equivalent fluence of about 7 × 1015 neq/cm2 is investigated, where the space charge of the proton-irradiated devices appears inverted compared to the neutron-irradiated device.

Fabrication of a microfluidic system with in situ-integrated microlens arrays using electrohydrodynamic jet printing

Microfluidic systems with integrated microlenses enhance cellular analysis and observation and have gained significant attention in the biomedical field. However, the integration and design of microlenses in microfluidic systems remain challenging. In this study, we utilize a high-precision electrohydrodynamic jet (E-jet) printing system for in-situ integration of microlens. By incorporating a tilting illumination observation module into the E-jet printing system, the print nozzle can be accurately positioned 10 μm above the microchannel bottom, thereby mitigating the edge effect of the microfluidic channels. We have developed a microfluidic system embedding microlens arrays (MLAs) that enable parallel, multichannel cell counting. The microfluidic system comprises nine channels, each featuring a 50 μm diameter microlens printed along the central axis. Using microlenses enables non-contact cell counting by detecting light-intensity changes. Simulating and optimizing the microfluidic channel size enable cells to align on the centerline of the channel and pass over the focal region of the microlens via inertial forces. As the cells flow through the microlens, the intensity of the focal spots decreases by approximately 50 %. The microfluidic cell-counting system can function independently or be integrated with other microfluidic systems as a unit, thus enhancing the single-cell operational and analytical capabilities of microfluidic systems.

Design and fabrication of a Fresnel zone plate with an enhanced depth of focus

A Fresnel zone plate (EFZP) with an extended depth of focus can maintain focused monochromatic light at different distances compared to a general Fresnel zone plate (FZP). The focal distances are determined by dividing the zone plate into multiple areas based on the desired order. The EFZP has potential applications in various research fields such as microscopy, direct laser lithography, and optical coherence tomography. However, manufacturing an EFZP is challenging due to the high precision requirements and difficulties associated with the calculation and simulation processes. In this research, a complete process is presented to design, simulate, and fabricate an EFZP using a Fourier optics design, simulations, and a direct laser lithographic machine. The resulting EFZP has an increased depth of focus of about nine times compared to a general Fresnel zone plate with similar parameters, while maintaining the focal spot diameter. The performance of this EFZP is evaluated through optical verification and mathematical simulation methods.

Topological Structures of Energy Flow: Poynting Vector Skyrmions.

Topological properties of energy flow of light are fundamentally interesting and may introduce novel physical phenomena associated with directional light scattering and optical trapping. In this Letter, skyrmionlike structures formed by Poynting vectors are unveiled in the focal region of two pairs of counterpropagating cylindrical vector vortex beams in free space. The appearance of local phase singularities, and the distinct traveling and standing wave modes of different field components passing through the focal spot lead to a Néel-Bloch-Néel transition of Poynting vector skyrmion textures along the light propagating direction. By shaping the wave front of the incident beams with patterned amplitudes, the topological invariant of the Poynting vector skyrmions can be further tuned in a prospective area. This work expands the family of optical skyrmions and holds great potential in energy flow associated applications.

Meta Shack–Hartmann wavefront sensor with large sampling density and large angular field of view: phase imaging of complex objects

Shack–Hartmann wavefront sensors measure the local slopes of an incoming wavefront based on the displacement of focal spots created by a lenslet array, serving as key components for adaptive optics for astronomical and biomedical imaging. Traditionally, the challenges in increasing the density and the curvature of the lenslet have limited the use of such wavefront sensors in characterizing slowly varying wavefront structures. Here, we develop a metasurface-enhanced Shack–Hartmann wavefront sensor (meta SHWFS) to break this limit, considering the interplay between the lenslet parameters and the performance of SHWFS. We experimentally validate the meta SHWFS with a sampling density of 5963 per mm2 and a maximum acceptance angle of 8° which outperforms the traditional SFWFS by an order of magnitude. Furthermore, to the best of our knowledge, we demonstrate the first use of a wavefront sensing scheme in single-shot phase imaging of highly complex patterns, including biological tissue patterns. The proposed approach opens up new opportunities in incorporating exceptional light manipulation capabilities of the metasurface platform in complex wavefront characterization.

A design of compact plasmonic lens consisting of high index dielectric gratings and metal nano-film

In this paper, a hybrid ultra-thin planar subwavelength focusing structure consisting of a high refractive index dielectric grating and a nano metal film was designed. The thickness of this structure is only 200 nm. Surface plasmon polaritons (SPPs) were excited from the nano metal film, propagated along the metal-air interface, and were then converted into a radiation field by the dielectric grating, forming a focused spot in free space. By adjusting the grating position and width parameters, the shape of the focused optical field could be controlled. The simulation results showed that, under 532 nm light irradiation, the lens could produce a 270 nm (full width at half-maximum, FWHM) spot size at a focal length of 2.46λ. Moreover, under the illumination of 633 nm and 780 nm light, the designed lenses were found to produce focal spot sizes of 304 nm (0.48λ) and 364 nm (0.47λ), respectively, which were smaller than the diffraction limit. The simplicity of this plasmonic lens design, coupled with its reduced thickness and minimal absorption loss, offered significant advantages in terms of cost-effectiveness and ease of fabrication.

Ultrasound Delta CBE Imaging: A New Approach Based on Local Energy Subtraction to Localization of the HIFU Focal Spot Using Changes in Backscattered Energy

Ultrasound Delta CBE Imaging: A New Approach Based on Local Energy Subtraction to Localization of the HIFU Focal Spot Using Changes in Backscattered Energy

Broadband achromatic thermal metalens with a wide field of view based on wafer-level monolithic processes

We present a monolithic metalens free of chromatic aberration over the 8–12 μm wavelength range for thermal imaging. The metalens consists of nano-donut-pillars for dispersion engineering. The proposed metalens design is based on a telecentric optical system, which effectively eliminates off-focus distortion and aberration, enhancing overall imaging quality. Offering a 90° field of view, the metalens ensures uniform focal spot sizes within a 45° field angle across the working wavelength. This enables the capture of high-quality thermal images with sharp images and minimal distortion. With a diameter of 5.75 mm, the metalens is suitable for integration into commercial thermal imaging cameras. The nano-donut-pillar structure of the metalens allows for relatively straightforward mass production, involving i-line stepper lithography and silicon deep etching processes.

Single-unit metalens integrated micro light-emitting diodes

Metalenses, characterized by their discontinuous local phase shifts, are optically analogous to macroscopic curved lenses but are flat and scaled down to micrometer dimensions. Their compactness and compatibility with semiconductor manufacturing processes facilitate monolithic integration into existing optical devices. Here, we report on the integration of metalenses with micro light-emitting diodes (μ-LEDs), resulting in enhanced extraction efficiency and directionality. The metalenses were composed of identical nanohole units, each 150 nm in diameter, strategically arranged to induce desired local phase shifts at specific coordinates by varying the density of these units. Both simulated and experimental results demonstrated that these easy-configuration metalenses effectively focused a plane wave at a predetermined spot and collimated a diverging light source at the focal spot, exemplifying optical reciprocity. When integrated with ultraviolet (λ = 390 nm) 60 μm-sized μ-LEDs, the metalenses significantly improved device performance, exhibiting a 338% enhancement in peak intensity and a ±8° reduction in beam divergence compared to an unpatterned μ-LED. We believe that metalens-integrated μ-LEDs with high brightness, directionality, and resolution are optimally suited for near-eye applications, including virtual reality and augmented reality displays.

A Light-Driven Carbon Nanocoil Microrobot

Mobile microrobots are of great scientific significance. However, external actuation and control methods are still challenging to conduct. We present a single carbon nanocoil (CNC) microrobot induced by an NIR laser beam, capable of light-driven locomotion and photothermal actuation. This research demonstrates that CNC-based microrobots rolls away from the focal spot when the laser beam is focused near the CNC. The maximum translational distance of a CNC microrobot increases with an increase in laser power, and the direction of motion is guided by controlling the focusing position of NIR. CNC-based microrobots can load and transport multiple cells under NIR light irradiation, resulting from the temperature gradient generated by photothermal conversion, which causes thermophoresis. The hydrophobic surface and unique helical structure of CNCs are beneficial to the underwater drag reduction in CNC microrobots’ motion and the adhesion of cells on CNC microrobots. Therefore, CNC microrobots, as cell vectors driven by a laser beam, may find applications in a wide range of biomedical applications. In addition, the rotation of a CNC powered by a laser beam provides promising prospects for the future of nanomechanical devices using a carbon nanocoil as a micro/nanomotor.

In-silico study of the impact of system design parameters on microcalcification detection in wide-angle digital breast tomosynthesis.

Accurate detection of microcalcifications ( ) is crucial for the early detection of breast cancer. Some clinical studies have indicated that digital breast tomosynthesis (DBT) systems with a wide angular range have inferior detectability compared with those with a narrow angular range. This study aims to (1)provide guidance for optimizing wide-angle (WA) DBT for improving detectability and (2)prioritize key optimization factors. An in-silico DBT pipeline was constructed to evaluate detectability of a WA DBT system under various imaging conditions: focal spot motion (FSM), angular dose distribution (ADS), detector pixel pitch, and detector electronic noise (EN). Images were simulated using a digital anthropomorphic breast phantom inserted with clusters. Evaluation metrics included the signal-to-noise ratio (SNR) of the filtered channel observer and the area under the receiver operator curve (AUC) of multiple-reader multiple-case analysis. Results showed that FSM degraded sharpness and decreased the SNR and AUC by 5.2% and 1.8%, respectively. Non-uniform ADS increased the SNR by 62.8% and the AUC by 10.2% for filtered backprojection reconstruction with a typical clinical filter setting. When EN decreased from 2000 to 200 electrons, the SNR and AUC increased by 21.6% and 5.0%, respectively. Decreasing the detector pixel pitch from 85 to improved the SNR and AUC by 55.6% and 7.5%, respectively. The combined improvement of a pixel pitch and EN200 was 89.2% in the SNR and 12.8% in the AUC. Based on the magnitude of impact, the priority for enhancing detectability in WA DBT is as follows: (1)utilizing detectors with a small pixel pitch and low EN level, (2)allocating a higher dose to central projections, and (3)reducing FSM. The results from this study can potentially provide guidance for DBT system optimization in the future.