Focusing Optics Research Articles

A Single Sub-Millimetric Metasurface-Based Optical Element for Lattice Bessel Beam Excitation Enabling Brain Activity Recordings In Vivo.

Bessel beams (BBs) are propagation-invariant optical fields that retain a narrow central intensity profile over longer propagation lengths than Gaussian beams (GBs). Due to this property, they have been adopted in fluorescence-based light sheet microscopy (LSM) to obtain 2D longitudinally-extended light-sheets. Yet, current approaches for generating BB lattices in LSM focus on regular excitation patterns and involve complex and bulky optics, limiting integration capability and versatility. Here, a flexible method is presented to obtain BB-arrays with arbitrary geometries by encoding on a single sub-millimetric surface all the optical transformations required. This method is applied using a single metasurface to encode the generation of a linear array of BBs, avoiding the use of conjugation and focusing optics. With respect to the current strategies, this approach, allowing for the independent design of each beamlet of the array, increases the degrees of freedom while making optimal use of the available light with no rejection, thus facilitating its integration into optical systems. According to this method, we fabricated a metasurface-based optical element for generating a linear BB-array of excitation in an LSM configuration and recorded neuronal activity at cellular resolution from the zebrafish larva brain. Thus, the proposed approach greatly extends the BB-array versatility and the application scenarios.

Rapid focusing of light through turbid water based on a digital micromirror device

The strong scattering and time variability of dynamic scattering media pose a major challenge to optical focusing and imaging applications. In this paper, a feedback wavefront shaping technology based on digital micromirror devices is proposed, which combines the hardware design and algorithm optimization. Through a new hardware-triggered image acquisition system and dynamic subgroup genetic algorithm, the image acquisition rate and focusing efficiency are significantly improved, and the light is quickly focused in dynamic turbid water. The imaging speed is about 34.89 times that of the traditional method based on a software-triggered image acquisition system and genetic algorithm. The experimental results of focusing through a milk solution show that this technique has a remarkable effect in improving the efficiency of dynamic focusing and the light intensity of focusing region, which provides an effective optimization strategy for optical imaging technology through a dynamic scattering medium in the future.

Enhancing the efficiency of a wavelength-dispersive spectrometer based on a slitless design using a single-bounce monocapillary.

This paper introduces a novel slit-less wavelength-dispersive spectrometer design that incorporates a single-bounce monocapillary with the goal of positioning the sample directly on the Rowland circle, thereby eliminating the need for a traditional entrance slit. This configuration enhances photon throughput while preserving energy resolution, demonstrated in comparative measurements on boron nitride and different lithium nickel manganese cobalt oxide cathodes. A common alternative to an entrance slit for limiting the source size on the Rowland circle is a customized design of the beamline involving a focusing optics unit consisting of two Kirkpatrick-Baez mirrors close to the end station. The new slit-less design does not rely on specialized beamlines and can be considered, thanks to the increased efficiency, for spectrometers using laboratory based sources equipped with equivalent optics. The comparative measurements found that the resolving power achieved was E/ΔE = 1085 at 401.5 eV incident energy, and the enhancement in detection efficiency was a factor of 3.7 due to more effective utilization of the X-ray beam.

Optical-resolution parallel ultraviolet photoacoustic microscopy for slide-free histology.

Intraoperative imaging of slide-free specimens is crucial for oncology surgeries, allowing surgeons to quickly identify tumor margins for precise surgical guidance. While high-resolution ultraviolet photoacoustic microscopy has been demonstrated for slide-free histology, the imaging speed is insufficient, due to the low laser repetition rate and the limited depth of field. To address these challenges, we present parallel ultraviolet photoacoustic microscopy (PUV-PAM) with simultaneous scanning of eight optical foci to acquire histology-like images of slide-free fresh specimens, improving the ultraviolet PAM imaging speed limited by low laser repetition rates. The PUV-PAM has achieved an imaging speed of 0.4 square millimeters per second (i.e., 4.2 minutes per square centimeter) at 1.3-micrometer resolution using a 50-kilohertz laser. In addition, we demonstrated the PUV-PAM with eight needle-shaped beams for an extended depth of field, allowing fast imaging of slide-free tissues with irregular surfaces. We believe that the PUV-PAM approach will enable rapid intraoperative photoacoustic histology and provide prospects for ultrafast optical-resolution PAM.

Effect of depressions on the three-dimensional propagation of longitudinal surface crack in continuous casting strands of hypo-peritectic steel

Surface depressions are common defects in the continuous casting process of hypo-peritectic steel, categorised into transverse and longitudinal depressions. While the formation processes of these depressions are well understood, their influence on the propagation of longitudinal surface cracks remains unclear. This study developed a new method for large-area surface depth measurement of the strand based on optical focusing. Utilising this method, we investigated the impact of surface depressions on the three-dimensional propagation of longitudinal surface crack in terms of width, depth and casting direction through macrostructure characteristics and finite element static analysis. The results indicated that transverse and longitudinal depressions affect crack propagation differently. Specifically, the depth of longitudinal depressions increases crack width, while transverse depressions can inhibit it to some extent. Macrostructure analysis of the cross-section revealed a continuous segregation line at the bottom of the depression, closely related to heat transfer, thermal stress, and the flow of solute-enriched liquid steel. Statistical analysis indicated that the depth of longitudinal depressions significantly affects the length of the segregation line, which in turn influences crack depth. However, no surface cracks were found at the bottom of transverse depressions adjacent to longitudinal depressions. This may be due to the banded grains at the bottom of transverse depressions resisting tensile thermal stress parallel to the strand's width direction. Along the casting direction, crack propagation primarily occurs along the bottom of longitudinal depressions due to continuous thermal disturbances at the meniscus. Meanwhile, the transverse depression can restrict the propagation of the existing crack while promoting the formation of the new crack at the other site. Finally, the study proposed mechanisms by which depressions influence the propagation of the longitudinal surface crack. This research further elucidates the impact patterns of surface depressions on macrostructure and stress distribution and how these factors subsequently influence crack propagation in three dimensions. It underscores the importance of controlling surface depressions to mitigate the longitudinal surface crack.

In-situ reaction monitoring and kinetics study of photochemical reactions by optical focusing inductive electrospray mass spectrometry

In-situ reaction monitoring and kinetics study of photochemical reactions by optical focusing inductive electrospray mass spectrometry

Compound meta-atoms enabling ultra-compact multiband optical manipulation.

Metasurfaces formed by subwavelength meta-atoms have continuously attracted interest owing to their flexible optical modulation, which offers unprecedented capability for the miniaturization of optical devices. Recently, broadband achromatic metalenses have been extensively reported for visible and mid-infrared light. However, it is a huge challenge to simultaneously manipulate the visible and mid-infrared light due to a large wavelength range. Here, a compound meta-atom is proposed to realize the multiband optical manipulation, which comprises two-layer decoupled meta-atoms. The lower-layer meta-atom and upper-layer ones are employed to modulate the mid-infrared and visible light, respectively. As a result, a compound metalens is designed to accomplish optical focusing at wavelengths of 0.65 µm and 3.7-4.8 µm. Numerical results demonstrate that the compound metalens can realize diffraction-limited focusing of visible light and broadband achromatic focusing of the mid-infrared light at the same focal plane. The compound metalens exhibits achromatic focusing for multiband light in a large wavelength range. Additionally, the design methodology of our meta-atoms is also applicable to other multiband optical modulation.

A study of structural effects on the focusing and imaging performance of hard X-rays with 20-30 nm zone plates.

Hard X-ray microscopes with 20-30 nm spatial resolution ranges are an advanced tool for the inspection of materials at the nanoscale. However, the limited efficiency of the focusing optics, for example, a Fresnel zone plate (ZP) lens, can significantly reduce the power of a nanoprobe. Despite several reports on ZP lenses that focus hard X-rays with 20 nm resolution -mainly constructed by zone-doubling techniques -a systematic investigation into the limiting factors has not been reported. We report the structural effects on the focusing and imaging efficiency of 20-30 nm-resolution ZPs, employing a modified beam-propagation method. The zone width and the duty cycle (zone width/ring pitch) were optimized to achieve maximum efficiency, and a comparative analysis of the zone materials was conducted. The optimized zone structures were used in the fabrication of Pt-hydrogen silsesquioxane (HSQ) ZPs. The highest focusing efficiency of the Pt-HSQ-ZP with a resolution of 30 nm was 10% at 7 keV and >5% in the range 6-10 keV, whereas the highest efficiency of the Pt-HSQ-ZP with a resolution of 20 nm was realized at 7 keV with an efficiency of 7.6%. Optical characterization conducted at X-ray beamlines demonstrated significant enhancement of the focusing and imaging efficiency in a broader range of hard X-rays from 5 keV to 10 keV, demonstrating the potential application in hard X-ray focusing and imaging.

Tunable Imaging Distance Focusing Module Directly Driven by a Thin‐plate Piezoelectric Actuator

AbstractWith the advancement of imaging technology, the performance of optical focusing modules becomes crucial for determining imaging quality. Conventional focusing modules frequently face challenges of excessive bulk and weight, complicating the integrated design and limiting application fields. Herein, this study presents a piezo‐actuated optical focusing module (POFM) and the construction strategy to alleviate these issues. A novel radial‐bending resonant‐type piezoelectric actuator (RB‐RPA) is developed to directly drive the POFM. The RB‐RPA, which features a hollow structure (27 × 27 × 7 mm3), utilizes both radial and bending vibration modes of a thin metal plate. It achieves a speed of 122.93 mm s−1, a thrust force of 0.14 N, and a displacement resolution of 0.52 µm. Employing the proposed construction strategy, RB‐RPA is integrated with imaging components to fabricate a POFM prototype. Furthermore, the micrometer‐level displacement resolution of POFM facilitates optical focusing on imaging targets, allowing the acquisition of sharp images within 70 ms. It achieves an image resolution of 119.57 lp mm−1 within a 42 mm segment of the captured image. Finally, a QT‐based interface for the POFM, enables real‐time image capture and sharpness evaluation, thereby enhancing imaging efficiency and integration. This work provides a potential candidate for next‐generation imaging devices.

Raman Measurements in the Hydroformylation of Decene with Laser Powers of 1 mW in Ex Zones

AbstractA custom‐made Raman setup is improved and adjusted to allow its use in Ex zones by using a 1‐mW laser and no optical focusing elements. The application under consideration is the hydroformylation of decene to undecanal, which must take place under explosion protection. To investigate the dispersive mixture, a separation takes place in a phase separation cell in which the Raman probe performs measurements. It can be shown that, with the use of highly sensitive detectors, even very low excitation powers can lead to a processable signal, and thus the application of Raman measurement technology far below the requirements of explosion protection is possible.

Generation of an Ultra‐Long Transverse Optical Needle Focus Using a Monolayer MoS2 Based Metalens

AbstractLine‐scan mode enables rapid and high‐throughput imaging through the development of an appropriate optical transverse needle focus. Diffraction gratings allow the generation of a line focus, but they face the challenge of low light power utilization due to multiple high‐order diffractions. In addition, designing the focus requires the selection of functional materials. Atomically thin transition metal dichalcogenides with high dielectric constants provide significant phase shifts to incident light by utilizing phase singularity at zero reflection. However, at zero reflection, no light power is available for utilization, necessitating a balance between phase and amplitude modulation. In this work, the aforementioned issues are addressed by designing a monolayer MoS2 based Fresnel strip metalens. An optical needle primary focus with a transverse length of 40 µm (≈80 λ, the longest value reported to date), a sub‐diffraction‐limited lateral spot, and a broad range of working wavelengths are achieved using the metalens. The metalens not only concentrate light power in the primary diffraction order by overcoming the constraint of momentum conservation but also maintains a constant phase modulation between distinct strips. The novel method for optical manipulation present here holds great promise for applications in biology, oncology, nanofabrication, energy harvesting, and optical information processing.

Controllable three-dimensional helical energy backflow in an optical focusing field.

The energy of light generally moves forward along the light propagation direction. However, in recent years, a counter-intuitive effect has been discovered that energy backflow can occur in certain special light fields. Presently, controlling the energy backflow of light for application requirements is becoming a significant challenge. Here we propose a method to generate and control a three-dimensional helical energy backflow (HEB) optical field. By designing an exponential helical conical phase mask, the energy backflow region in a common tightly focused field can be stretched into a three-dimensional helical shape with an ultra-long longitudinal length. The length of the HEB region can be flexibly extended up to 47.6λ by varying the exponential phase index and the initial topological charge. Furthermore, we demonstrate that nanoparticles within the HEB field are subjected to optical pulling forces, thereby drawing the nanoparticles back to the light source. This work presents a new, to the best of our knowledge, approach for the three-dimensional manipulation of the energy backflow field with potential applications in nanoparticle manipulation and detection.

Spatial resolution limit for a solid immersion lens

The solid immersion (SI) effect is widely used to increase the spatial resolution of optical focusing systems and even overcome the Abbe diffraction limit. Resolution enhancement offered by a SI lens is mostly a function of its geometry and refractive index nSI. While SI lenses are relatively well understood, the scaling of the resolution enhancement by such lenses is still a subject of debate, with some works reporting ≃nSI and ≃nSI2 dependencies for the hemispherical and hyperhemispherical SI lens configurations, respectively. In this paper, we offer a general argument for a resolution limit for SI optics and, then, verify it via the numerical analysis of the hemispherical and hyperhemispherical silicon SI lenses designed for the terahertz (THz) range. In fact, we find that there is no contradiction in the reported resolution enhancements ≃nSI and ≃nSI2; however, they happen in different operation regimes. We then demonstrate that the resolution values reported for the different SI lens arrangements in the visible (VIS), near-, and middle-infrared (NIR and MIR), as well as THz bands obey the derived limit. Our findings will be useful for the further design and applications of SI optics.

Fabrication of high-quality microlens arrays on fused silica based on combined rapid evaporative ablation and precision melting polishing of CO2 lasers

Microlens arrays (MLAs) made from fused silica possess broad applications in wavefront sensing, optical focusing, beam shaping, etc. However, it is challenging to fabricate the hard-brittle silica MLAs with high accuracy and low cost by conventional manufacturing techniques efficiently. In this work, a novel two-step method, which consists of rapid evaporative ablation and precision melting polishing by CO2 lasers, is proposed to fabricate high-quality MLAs. The first step is to rapidly ablate and engrave micro-pillar arrays (MPAs) with flat groove bottom by high-energy density CO2 lasers. The second step is to precisely polish and shape the flat-topped MPAs into the dome-topped MLAs with good surface roughness (Sa) by low-energy density CO2 lasers. The whole preparation process could be achieved through a set of machining system. Firstly, the geometric dimension (period, p and height, h) of the microlens is determined by the Monte Carlo ray tracing method. Secondly, to diminish the groove depth inconsistency caused by heat accumulation, the multiple ablation compensation strategy is developed. The MPAs (p = 200–400 μm), of which maximum height error is 7.1%, are prepared in 1 s by the proposed compensation strategy. Then, the MPAs are smoothed into the MLAs by low-energy density CO2 lasers, and the average Sa is improved from 960 nm to 32.56 nm. Finally, the optical performances of the MLAs are verified from the effectiveness of imaging, focusing and homogenization. The proposed two-step processing method paves a new way to fabricate high-performance MLAs on hard-brittle fused silica with low cost and high efficiency.

Lights, camera, vaporization: Coupling acoustics and microscopy to study acoustic droplet vaporization

Within the Department of Radiology at the University of Michigan, we have integrated mechanics, material science, and biomedical acoustics to develop ultrasound-assisted therapies and diagnostic techniques. What is our approach? Using ultrasound-responsive droplets. These droplets phase-transition from liquid to gas bubbles when exposed to ultrasound, a process termed acoustic droplet vaporization (ADV). We harness this transformation for a wide array of biomedical applications, such as delivering therapeutic payloads, modulating hydrogel properties, and characterizing material properties. In this video, we will demonstrate how to make monodisperse droplets using microfluidic techniques and prepare tissue-mimicking hydrogels containing these droplets for optical characterization. Resolving both the ADV-bubble response in an ultrasound field at clinically relevant frequencies, as well as the associated cellular and biological responses in real-time, requires high temporal and spatial resolution. To achieve this, we couple an ultra-high-speed camera (10 million frames per second) and time-lapse confocal fluorescence microcopy (1 frame per second). We will walk you through the process: aligning optical and ultrasound foci, coupling the transducer with samples on the microscope stage, aligning laser for high-intensity back illumination, synchronizing ultrasound and the camera, and finally, triggering ultrasound to generate ADV-bubbles

Evaluation of Focus Measures for Hyperspectral Imaging Microscopy Using Principal Component Analysis.

An automatic focusing system is a crucial component of automated microscopes, adjusting the lens-to-object distance to find the optimal focus by maximizing the focus measure (FM) value. This study develops reliable autofocus methods for hyperspectral imaging microscope systems, essential for extracting accurate chemical and spatial information from hyperspectral datacubes. Since FMs are domain- and application-specific, commonly, their performance is evaluated using verified focus positions. For example, in optical microscopy, the sharpness/contrast of visual peculiarities of a sample under testing typically guides as an anchor to determine the best focus position, but this approach is challenging in hyperspectral imaging systems (HSISs), where instant two-dimensional hyperspectral images do not always possess human-comprehensible visual information. To address this, a principal component analysis (PCA) was used to define the optimal ("ideal") optical focus position in HSIS, providing a benchmark for assessing 22 FMs commonly used in other imaging fields. Evaluations utilized hyperspectral images from visible (400-1100 nm) and near-infrared (900-1700 nm) bands across four different HSIS setups with varying magnifications. Results indicate that gradient-based FMs are the fastest and most reliable operators in this context.

High-precision alignment of optoelectronic devices for optical phase conjugation

Abstract Digital optical phase conjugation (DOPC) is considered as a promising solution to achieve optical focusing against scattering. The implementation of DOPC based on the digital micromirror device (DMD) has been proven to have great potential, supporting a large number of modulation modes and a high modulation rate. However, the accuracy of optical alignment seriously affects the focusing contrast, limiting the applications of DMD-based DOPC systems. Here we demonstrate a simple alignment protocol including a marker-assisted tuning and an embedded compensation. Our approach can realize an exact pixelwise optical conjugation between the DMD and detector, as well as a rapid compensation for aberrations and minor misalignment. Experimental results show that the proposed alignment protocol improves the focusing contrast to 66% of the highest value predicted in the theory.

X-ray nano-holotomography reconstruction with simultaneous probe retrieval.

In conventional tomographic reconstruction, the pre-processing step includes flat-field correction, where each sample projection on the detector is divided by a reference image taken without the sample. When using coherent X-rays as a probe, this approach overlooks the phase component of the illumination field (probe), leading to artifacts in phase-retrieved projection images, which are then propagated to the reconstructed 3D sample representation. The problem intensifies in nano-holotomography with focusing optics, which, due to various imperfections creates high-frequency components in the probe function. Here, we present a new iterative reconstruction scheme for holotomography, simultaneously retrieving the complex-valued probe function. Implemented on GPUs, this algorithm results in 3D reconstruction resolving twice thinner layers in a 3D ALD standard sample measured using nano-holotomography.

Real‐Time Wavefront Control of Multimode Fibers under Dynamic Perturbation

AbstractMultimode fibers (MMFs), which transmit multiple spatial modes simultaneously, are essential in imaging, communication, and sensing. However, mode crosstalk significantly impairs the clarity of transmitted signals. Wavefront shaping has emerged as an effective strategy to minimize these distortions. Given the dynamic environmental conditions under which MMFs operate, rapid technological adaptation is crucial. A high‐speed full‐field wavefront shaping system designed for real‐time MMF control is developed. This system leverages probabilistic phase shaping, superpixel modulation, and a digital micromirror device (DMD) to achieve operational speeds of 38 ms per cycle for 400 spatial modes, translating to an average mode time of 95 µs. This rate sets a new record for DMD‐based systems, pushing hardware limits. The system supports continuous operation at 11 Hz and maintains high‐quality optical focus through MMFs under varying environmental conditions, with a focusing efficiency exceeding 50% of the theoretical maximum. Its compatibility with fluorescent guide stars enables transmission matrix characterization when direct access is unfeasible, broadening its applications. This high‐speed full‐field wavefront shaping system represents a significant breakthrough, enhancing the functionality and versatility of MMF‐based applications.

Conformal Ordered Solid–Liquid Coupled Piezoelectric Units for Programmable Adaptive Optics

AbstractOrdered piezoelectric units are designed to achieve unnaturally large strains and artificial vibrational modes, owing to the synergistic effects of multiple units. However, when interfacing with another physical field, the challenge for ordered piezoelectric units is to achieve precise control of individual unit independently while maintaining a large manipulation range. In this study, conformal ordered solid–liquid coupled piezoelectric units are introduced, which effectively reduce stress transfer by as much as 84% between units via incorporating a liquid phase into each unit. This design allows for the generation of considerable strain levels (up to ɛZ = 1.13%) when used with a flexible layer and optimized structure. Hence, these conformal ordered piezoelectric units achieve simultaneous production of large strains and independent electromechanical control for each individual unit. As a practical illustration, a biomimetic piezoelectric adaptive lens and a lens array based on solid–liquid coupled piezoelectric units are designed and fabricated. They offer an extensive zoom range from 100.3 mm to infinity, ultra‐high diopter sensitivity of 70 D mV−1, ultra‐rapid response time of 50 µs, and programmable optical focusing through individual manipulation of each unit. This research paves the way for inspiring new designs for piezoelectric metamaterials and devices for intelligent electromechanical systems.