Optimized holographic femtosecond laser patterning method towards rapid integration of high-quality functional devices in microchannels

Rapid fabrication of highly uniform polygons by femtosecond laser patterning based on free lens modulation

Structured light featuring multiple customizable degrees of freedom has become a powerful tool for femtosecond laser processing, enabling much higher throughput and considerable finesse and flexibility. A non-iterative beam shaping technique avoids solving inversion problems of light propagation, but the types of available beam profiles are finite. Here, a phase-only method that can prescribe the beam intensity along an arbitrary two-dimensional curve, called free lens modulation, is applied in femtosecond laser patterned exposure. Single polygonal microstructures with diverse morphology and high surface quality can be fabricated in less than 1 s while outperforming common iterative algorithms in contour fidelity. Moreover, a microfluidic device with a filtering function is designed and demonstrated by integrating microtrap arrays composed of polygons into a microchannel based on the holographic approach. The method offers new inspiration for the rapid construction of large-area microfluidic devices and integrated microsystems with customizable functional applications.

High-axial-resolution speckle-free holographic reconstruction via cylindrical quadratic phase method and temporal focusing

Holographic techniques enable precise laser manipulation, but suffer from two considerable limitations: speckle and deterioration of axial distribution. Here, we propose a cylindrical quadratic phase (CQP) method with temporal focusing (TF) to generate speckle-free holographic illumination with high axial resolution. TF-CQP utilizes a superposed cylindrical phase as the initial guess to iteratively optimize phase hologram, realizing speckle-free holographic reconstruction on the target focal plane and eliminating secondary focus on the defocused planes. TF-CQP further disperses defocused beams symmetrically by a blazed grating, placed conjugate to the focal plane, which enhances axial confinement. Simulation and experimental results show that TF-CQP reconstructs speckle-free illumination with arbitrary shapes and <10 µm axial resolution. Compared to TF-GS (Gerchberg-Saxton algorithm), widely used in holographic optogenetics, TF-CQP shows increased uniformity of 200% and improved modulation efficiency of 32.33% for parallel holographic illumination, as well as a 10% increment in axial resolution.

Speckle suppression in arbitrary parallel holographic illumination by the spatial frequency regaining method

This Letter proposes a spatial frequency regaining method for parallel holographic illumination (SFR-PHI) to suppress speckle noise in phase-only computer-generated holography (CGH). Based on the accurate calculation of the beam bandwidth, this method uses the bandwidth-limited quadratic initial phase and weighted constraint iteration to generate the optimized phase hologram, which can provide the accurate spatial frequency of multiple illumination patterns. The results show that SFR-PHI performs superiorly in speckle suppression for generating dozens of illumination patterns in parallel and with arbitrary shapes and numbers. Compared with other speckle-suppression methods, it exhibits significant advantages in terms of accuracy and modulation efficiency.

Caustic networks with customized intensity statistics

Controlling random light is a key enabling technology that pioneered statistical imaging methods like speckle microscopy. Such low-intensity illumination is especially useful for bio-medical applications where photobleaching is crucial. Since the Rayleigh intensity statistics of speckles do not always meet the requirements of applications, considerable effort has been dedicated to tailoring their intensity statistics. A special random light distribution that naturally comes with radically different intensity structures to speckles are caustic networks. Their intensity statistics support low intensities while allowing sample illumination with rare rouge-wave-like intensity spikes. However, the control over such light structures is often very limited, resulting in patterns with inadequate ratios of bright and dark areas. Here, we show how to generate light fields with desired intensity statistics based on caustic networks. We develop an algorithm to calculate initial phase fronts for light fields so that they smoothly evolve into caustic networks with the desired intensity statistics during propagation. In an experimental demonstration, we exemplarily realize various networks with a constant, linearly decreasing and mono-exponential probability density function.

Rapid trapping and tagging of microparticles in controlled flow by in situ digital projection lithography

Real-time and fast trapping and tagging of microfeatures, such as microparticles and cells, are of great significance for biomedical research. In this work, we propose a novel in situ digital projection lithography technology that integrates real-time, in situ generation of digital masks for particle processing and fluid control into conventional DMD-based projection lithography. With the help of image recognition technology, we rapidly resolve the information of the microparticle profile or channel location, combining the selection of existing masks of different shapes, thus enabling in situ generation of user-customized micro-trap arrays and microfilter arrays for particle trapping and tagging. The success in trapping and filtering single particles, particle arrays, and cells has indicated the promising prospects of this novel technology for broad applications in microfluidics, single-cell analysis, and early-stage disease diagnostics.

A novel approach for audible acoustic quick response codes

Compared to image-based quick response (QR) codes, acoustic QR codes have some advantages. However, an acoustic QR scanner cannot recognize an acoustic QR code at a distance of more than two meters from an acoustic QR announcer. To this end, we propose a new sort of acoustic QR code, called an audible acoustic QR code (AAQRC), which employs humanly audible sound to carry users’ information directly. First, a user’s string of characters is translated into a string of pitches. Then, the related algorithms convert the string of pitches into a playable audio file. As a result, an AAQRC is generated, consisting of the audio itself. AAQRC recognition is the opposite process of AAQRC generation. Compared with the existing approach for acoustic QR codes, the new method can recognize acoustic QR codes at a longer distance, even if there are obstacles between the AAQRC announcer and AAQRC scanner.

An open source three-mirror laser scanning holographic two-photon lithography system

Two-photon polymerization is a widely adopted technique for direct fabrication of 3D and 2D structures with sub-diffraction-limit features. Here we present an open-hardware, open-software custom design for a holographic multibeam two-photon polymerization system based on a phase-only spatial light modulator and a three-mirror scanhead. The use of three reflective surfaces, two of which scanning the phase-modulated image along the same axis, allows to overcome the loss of virtual conjugation within the large galvanometric mirrors pair needed to accommodate the holographic projection. This extends the writing field of view among which the hologram can be employed for multi-beam two-photon polymerization by a factor of ~2 on one axis (i.e. from ~200μm to ~400μm), with a voxel size of ~250nm × ~1050nm (lateral × axial size), and writing speed of three simultaneous beams of 2000 voxels/s, making our system a powerful and reliable tool for advanced micro and nano-fabrications on large area.

Rapid, continuous projection multi-photon 3D printing enabled by spatiotemporal focusing of femtosecond pulses

There is demand for scaling up 3D printing throughput, especially for the multi-photon 3D printing process that provides sub-micrometer structuring capabilities required in diverse fields. In this work, high-speed projection multi-photon printing is combined with spatiotemporal focusing for fabrication of 3D structures in a rapid, layer-by-layer, and continuous manner. Spatiotemporal focusing confines printing to thin layers, thereby achieving print thicknesses on the micron and sub-micron scale. Through projection of dynamically varying patterns with no pause between patterns, a continuous fabrication process is established. A numerical model for computing spatiotemporal focusing and imaging is also presented which is verified by optical imaging and printing results. Complex 3D structures with smooth features are fabricated, with millimeter scale printing realized at a rate above 10^-3 mm³ s^-1. This method is further scalable, indicating its potential to make fabrications of 3D structures with micro/nanoscale features in a practical time scale a reality.

Tethered and Untethered 3D Microactuators Fabricated by Two-Photon Polymerization: A Review

Microactuators, which can transform external stimuli into mechanical motion at microscale, have attracted extensive attention because they can be used to construct microelectromechanical systems (MEMS) and/or microrobots, resulting in extensive applications in a large number of fields such as noninvasive surgery, targeted delivery, and biomedical machines. In contrast to classical 2D MEMS devices, 3D microactuators provide a new platform for the research of stimuli-responsive functional devices. However, traditional planar processing techniques based on photolithography are inadequate in the construction of 3D microstructures. To solve this issue, researchers have proposed many strategies, among which 3D laser printing is becoming a prospective technique to create smart devices at the microscale because of its versatility, adjustability, and flexibility. Here, we review the recent progress in stimulus-responsive 3D microactuators fabricated with 3D laser printing depending on different stimuli. Then, an outlook of the design, fabrication, control, and applications of 3D laser-printed microactuators is propounded with the goal of providing a reference for related research.

Inhibition and enhancement of linear and nonlinear optical effects by conical phase front shaping for femtosecond laser material processing

The emergence of high-powered femtosecond lasers presents the opportunity for large volume processing inside of transparent materials, wherein a myriad of nonlinear optical and aberration effects typically convolves to distort the focused beam shape. In this paper, convex and concave conical phase fronts were imposed on femtosecond laser beams and focussed into wide-bandgap glass to generate a vortex beam with tuneable Gaussian-Bessel features offset from the focal plane. The influence of Kerr lensing, plasma defocussing, and surface aberration on the conical phase front shaping were examined over low to high pulse energy delivery and for shallow to deep processing tested to 2.5 mm focussing depth. By isolating the underlying processes, the results demonstrate how conical beams can systematically manipulate the degree of nonlinear interaction and surface aberration to facilitate a controllable inhibition or enhancement of Kerr lensing, plasma defocussing, and surface aberration effects. In this way, long and uniform filament tracks have been generated over shallow to deep focussing by harnessing surface aberration and conical beam shaping without the destabilizing Kerr lensing and plasma defocussing effects. A facile means for compressing and stretching of the focal interaction volume is presented for controlling the three-dimensional micro- and nano-structuring of transparent materials.

Laser photonic-reduction stamping for graphene-based micro-supercapacitors ultrafast fabrication

Micro-supercapacitors are promising miniaturized energy storage devices that have attracted considerable research interest. However, their widespread use is limited by inefficient microfabrication technologies and their low energy density. Here, a flexible, designable micro-supercapacitor can be fabricated by a single pulse laser photonic-reduction stamping. A thousand spatially shaped laser pulses can be generated in one second, and over 30,000 micro-supercapacitors are produced within 10 minutes. The micro-supercapacitor and narrow gaps were dozens of microns and 500 nm, respectively. With the unique three-dimensional structure of laser-induced graphene based electrode, a single micro-supercapacitor exhibits an ultra-high energy density (0.23 Wh cm⁻³), an ultra-small time constant (0.01 ms), outstanding specific capacitance (128 mF cm⁻² and 426.7 F cm⁻³) and a long-term cyclability. The unique technique is desirable for a broad range of applications, which surmounts current limitations of high-throughput fabrication and low energy density of micro-supercapacitors.

Microfabrication for cost-effective miniaturized energy storage devices remains a challenge. Here, the authors propose a spatially shaped femtosecond laser method, which is ultrafast, one-step, high resolution and large-scale, for use in patterning flexible high-performance micro-supercapacitors.

Rapid Fabrication of Continuous Surface Fresnel Microlens Array by Femtosecond Laser Focal Field Engineering

Femtosecond laser direct writing through two-photon polymerization has been widely used in precision fabrication of three-dimensional microstructures but is usually time consuming. In this article, we report the rapid fabrication of continuous surface Fresnel lens array through femtosecond laser three-dimensional focal field engineering. Each Fresnel lens is formed by continuous two-photon polymerization of the two-dimensional slices of the whole structure with one-dimensional scan of the corresponding two-dimensional engineered intensity distribution. Moreover, we anneal the lens array to improve its focusing and imaging performance.

Adaptive optics in laser processing

Adaptive optics are becoming a valuable tool for laser processing, providing enhanced functionality and flexibility for a range of systems. Using a single adaptive element, it is possible to correct for aberrations introduced when focusing inside the workpiece, tailor the focal intensity distribution for the particular fabrication task and/or provide parallelisation to reduce processing times. This is particularly promising for applications using ultrafast lasers for three-dimensional fabrication. We review recent developments in adaptive laser processing, including methods and applications, before discussing prospects for the future.

Multi-foci laser microfabrication of 3D polymeric scaffolds for stem cell expansion in regenerative medicine

High quality large scale fabrication of cellular scaffolds, with three-dimensional resolution comparable to cell size, is an important task to enable regenerative medicine applications with stem cells. We are using two-photon polymerization to produce our stem cell culture substrate called Nichoid, which we already demonstrated capable of stimulating cell proliferation while maintaining their stemness, without the need of dangerous additives. Parallelization of this technique can be achieved with the use of a spatial light modulator: here we show the results obtained combining this device with fast linear stages to produce Nichoid-covered substrates by two-photon polymerization. The well-polymerized structures confirm that this approach is particularly convenient for porous structures, and allows a significant time saving by a factor of almost five, with minor design adjustments. A Live & Dead assay was performed on mesenchymal stem cells cultured into the Nichoid microstructures in order to verify that no difference in cell viability is present, compared to microstructures fabricated by a single focus. This parallel setup opens the possibility to obtain a much larger number of microstructured substrates, that are essential to test new stem cell-based therapies. This approach can be also used for the fast fabrication of other kinds of cell culture devices.

Predictive capabilities for laser machining via a neural network

The interaction between light and matter during laser machining is particularly challenging to model via analytical approaches. Here, we show the application of a statistical approach that constructs a model of the machining process directly from experimental images of the laser machined sample, and hence negating the need for understanding the underlying physical processes. Specifically, we use a neural network to transform a laser spatial intensity profile into an equivalent scanning electron microscope image of the laser-machined target. This approach enables the simulated visualization of the result of laser machining with any laser spatial intensity profile, and hence demonstrates predictive capabilities for laser machining. The trained neural network was found to have encoded functionality that was consistent with the laws of diffraction, hence showing the potential of this approach for discovering physical laws directly from experimental data.

3D nanofabrication inside rapid prototyped microfluidic channels showcased by wet-spinning of single micrometre fibres

Microfluidics is an established multidisciplinary research domain with widespread applications in the fields of medicine, biotechnology and engineering. Conventional production methods of microfluidic chips have been limited to planar structures, preventing the exploitation of truly three-dimensional architectures for applications such as multi-phase droplet preparation or wet-phase fibre spinning. Here the challenge of nanofabrication inside a microfluidic chip is tackled for the showcase of a spider-inspired spinneret. Multiphoton lithography, an additive manufacturing method, was used to produce free-form microfluidic masters, subsequently replicated by soft lithography. Into the resulting microfluidic device, a three-dimensional spider-inspired spinneret was directly fabricated in-chip via multiphoton lithography. Applying this unprecedented fabrication strategy, the to date smallest printed spinneret nozzle is produced. This spinneret resides tightly sealed, connecting it to the macroscopic world. Its functionality is demonstrated by wet-spinning of single-digit micron fibres through a polyacrylonitrile coagulation process induced by a water sheath layer. The methodology developed here demonstrates fabrication strategies to interface complex architectures into classical microfluidic platforms. Using multiphoton lithography for in-chip fabrication adopts a high spatial resolution technology for improving geometry and thus flow control inside microfluidic chips. The showcased fabrication methodology is generic and will be applicable to multiple challenges in fluid control and beyond.

Spatial light modulator based laser microfabrication of volume optics inside solar modules

Ultrashort pulse laser systems enable new approaches of material processing and manufacturing with enhanced precision and productivity. Time- and cost-effectiveness in the context of the industrialization of ultrashort laser pulse processes require an improvement of processing speed, which is of key importance for strengthening industrial photonics based manufacturing and extending its field of applications. This article presents results on improving the speed of a laser process by parallelization for creating light deflecting volume optics. Diffractive optical elements are fabricated directly inside the encapsulant of solar modules by utilizing a spatial light modulator based parallel laser microfabrication method. The fabricated volume optical elements effectively deflect light away from front side electrodes and significantly reduce the corresponding optical losses.

Single-pulse writing of a concave microlens array

This work developed a method of femtosecond laser (fs-laser) parallel processing assisted by wet etching to fabricate 3D micro-optical components. A 2D fs-laser spot array with designed spatial distribution was generated by a spatial light modulator. A single-pulse exposure of the entire array was used for parallel processing. By subsequent wet etching, a close-packed hexagonal arrangement, 3D concave microlens array on a curved surface with a radius of approximately 120 μm was fabricated, each unit lens of which has designable spatial distribution. Characterization of imaging was carried out by a microscope and showed a unique imaging property in multi-planes. This method provides a parallel and efficient technique to fabricate 3D micro-optical devices for applications in optofluidics, optical communication, and integrated optics.