Massively parallel femtosecond laser processing

Fast non-iterative algorithm for 3D point-cloud holography

Recently developed iterative and deep learning-based approaches to computer-generated holography (CGH) have been shown to achieve high-quality photorealistic 3D images with spatial light modulators. However, such approaches remain overly cumbersome for patterning sparse collections of target points across a photoresponsive volume in applications including biological microscopy and material processing. Specifically, in addition to requiring heavy computation that cannot accommodate real-time operation in mobile or hardware-light settings, existing sampling-dependent 3D CGH methods preclude the ability to place target points with arbitrary precision, limiting accessible depths to a handful of planes. Accordingly, we present a non-iterative point cloud holography algorithm that employs fast deterministic calculations in order to efficiently allocate patches of SLM pixels to different target points in the 3D volume and spread the patterning of all points across multiple time frames. Compared to a matched-performance implementation of the iterative Gerchberg-Saxton algorithm, our algorithm's relative computation speed advantage was found to increase with SLM pixel count, reaching >100,000x at 512 × 512 array format.

High-Resolution Patterning of Perovskite Quantum Dots via Femtosecond Laser-Induced Forward Transfer

High-resolution patterning of perovskite quantum dots (PQDs) is of significant importance for satisfying various practical applications, including high-resolution displays and image sensing. However, due to the limitation of the instability of PQDs, the existing patterning strategy always involves chemical reagent treatment or mask contact that is not suitable for PQDs. Therefore, it is still a challenge to fabricate high-resolution full-color PQD arrays. Here, we present a femtosecond laser-induced forward transfer (FsLIFT) technology, which enables the programmable fabrication of high-resolution full-color PQD arrays and arbitrary micropatterns. The FsLIFT process integrates transfer, deposition, patterning, and alignment in one step without involving a mask and chemical reagent treatment, guaranteeing the preservation of the photophysical properties of PQDs. A full-color PQD array with a high resolution of 2 μm has been successfully achieved. We anticipate that our facile and flexible FsLIFT technology can facilitate the development of diverse practical applications based on patterned PQDs.

Multi-focal laser processing in transparent materials using an ultrafast tunable acoustic lens

Fast and versatile alteration of focal positions is critical for applications including selective volumetric modification and parallel laser processing. In this Letter, we implement and characterize an ultrafast, variable focal system using a tunable acoustic gradient of index lens to achieve multi-focal laser processing. We apply our method to the femtosecond laser-induced intra-volumetric modification in glass to show the flexibility in controlling focal positions. Based on this understanding, we exploit the multi-focal nature of the system to demonstrate laser machining on both surfaces of a transparent glass slide in a single lateral scan.

3D Microvascularized Tissue Models by Laser-Based Cavitation Molding of Collagen

3D tissue models recapitulating human physiology are important for fundamental biomedical research, and they hold promise to become a new tool in drug development. An integrated and defined microvasculature in 3D tissue models is necessary for optimal cell functions. However, conventional bioprinting only allows the fabrication of hydrogel scaffolds containing vessel-like structures with large diameters (>100 µm) and simple geometries. Recent developments in laser photoablation enable the generation of this type of structure with higher resolution and complexity, but the photo-thermal process can compromise cell viability and hydrogel integrity. To address these limitations, the present work reports in situ 3D patterning of collagen hydrogels by femtosecond laser irradiation to create channels and cavities with diameters ranging from 20 to 60 µm. In this process, laser irradiation of the hydrogel generates cavitation gas bubbles that rearrange the collagen fibers, thereby creating stable microchannels. Such 3D channels can be formed in cell- and organoid-laden hydrogel without affecting the viability outside the lumen and can enable the formation of artificial microvasculature by the culture of endothelial cells and cell media perfusion. Thus, this method enables organs-on-a-chip and 3D tissue models featuring complex microvasculature.

Laser induced forward transfer isolating complex-shaped cell by beam shaping

Beam shaping techniques have been widely used in holographic optical tweezers to accurately manipulate tiny particles and hologram optimization algorithms have also been widely reported to improve the optical trapping performance. In this paper, we presented a beam shaping laser induced forward transfer (BS-LIFT) technique to isolate complex-shaped cells. To do this, we built up a BS-LIFT instrument which combined beam shaping methods and laser induced forward transfer using liquid-crystal-on-silicon spatial light modulator. The laser beam was modulated into multiple desired points at the focal plane employing the Gerchberg-Saxton (GS) algorithm. Feasibility was verified through transferring various samples. To our knowledge, this is the first demonstration of BS-LIFT applied to the transfer complex-shaped cells. We successfully transferred cells whose size ranged from 1 µm to 100 µm. Our design will provide a novel approach for the application of this beam shaping technique and the isolation of single cells with variable shapes.

Nanoparticle Deposition of Fluoropolymer CYTOP via Holographic Femtosecond Laser Processing and Its Biochip Application

The fundamental characteristics of nanoparticle (NP) deposition of the fluoropolymer CYTOP using a femtosecond (fs) laser were investigated. In previous studies, we have demonstrated the microfluidic fabrication of CYTOP, which enables clear microscopic observation of the fluid boundary because of its low refractive index, as well as that of water. In the present work, we generated CYTOP NPs using holographic fs laser processing with a spatial light modulator to demonstrate the capabilities of this functional polymer. We established a deposition technique via five-dot parallel fs laser beam irradiation for fibrous network and monolayer structures composed of CYTOP NPs on the surface of glass slides by manipulating the various fundamental laser processing parameters. The network structure on the glass surface exhibits superhydrophobic behavior, while the monolayer structure performs almost the same wettability as that of CYTOP thin film. After an investigation of the surface features of the NPs deposited onto the glass, the combination of the holographic fs laser deposition and the removal of CYTOP NPs was used to selectively pattern CYTOP NPs on the glass slide for HeLa cell culturing. Consequently, cells were selectively cultured on the glass surface where the laser removal of deposited NPs was carried out.

Bactericidal surfaces prepared by femtosecond laser patterning and layer-by-layer polyelectrolyte coating

Antimicrobial surfaces are important in medical, clinical, and industrial applications, where bacterial infection and biofouling may constitute a serious threat to human health. Conventional approaches against bacteria involve coating the surface with antibiotics, cytotoxic polymers, or metal particles. However, these types of functionalization have a limited lifetime and pose concerns in terms of leaching and degradation of the coating. Thus, there is a great interest in developing long-lasting and non-leaching bactericidal surfaces. To obtain a bactericidal surface, we combine micro and nanoscale patterning of borosilicate glass surfaces by ultrashort pulsed laser irradiation and a non-leaching layer-by-layer polyelectrolyte modification of the surface. The combination of surface structure and surface charge results in an enhanced bactericidal effect against both Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli bacteria. The laser patterning and the layer-by-layer modification are environmentally friendly processes that are applicable to a wide variety of materials, which makes this method uniquely suited for fundamental studies of bacteria-surface interactions and paves the way for its applications in a variety of fields, such as in hygiene products and medical devices.

In-system optimization of a hologram for high-stability parallel laser processing

A method for optimizing a computer-generated hologram (CGH) for high-stability laser processing is proposed. The CGH is optimized during laser processing; therefore, unpredicted dynamic changes in the laser processing system, in addition to its static imperfections, are automatically compensated for by exploiting the rewritable capability of the spatial light modulator. Consequently, the short-term and long-term stability are improved, which will contribute to the realization of high-speed, high-precision laser processing. A CGH that generated 36 parallel beams was continuously optimized, and the maximum uniformity reached 0.98, which is higher than reported in previous research. To the best of our knowledge, this is the first demonstration of gradual improvement of parallel laser processing with in-process optimization of the CGH. Furthermore, it was also demonstrated that the performance of the laser processing system against unexpected disturbances was improved.

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.

Large-scale waterproof and stretchable textile-integrated laser- printed graphene energy storages

Textile integrable large-scale on-chip energy storages and solar energy storages take a significant role in the realization of next-generation primary wearable devices for sensing, wireless communication, and health tracking. In general, these energy storages require major features like mechanical robustness, environmental friendliness, high-temperature tolerance, inexplosive nature, and long-term storage duration. Here we report on large-scale laser-printed graphene supercapacitors of dimension 100 cm² fabricated in 3 minutes on textiles with excellent water stability, an areal capacitance, 49 mF cm⁻², energy density, 6.73 mWh/cm⁻², power density, 2.5 mW/cm⁻², and stretchability up to 200%. Further, a demonstration is given for the textile integrated solar energy storage with stable performance for up to 20 days to reach half of the maximum output potential. These cost-effective self-reliant on-chip charging units can become an integral part for the future electronic and optoelectronic textiles.

Symmetry-wise nanopatterning and plasmonic excitation of ring-like gold nanoholes by structured femtosecond laser pulses with different polarizations

Low- and ultralow-energy tightly focused 200 fs, 515 nm donut-shaped laser pulses at 0.25 and 0.65 NA focusing were used for single-shot ablative pulse-energy scalable nanopatterning of 50 nm thick gold film and the following plasmonic excitation of dye monolayer photoluminescence (PL) in the fabricated nanostructures, respectively. The same pulses at much lower, non-ablative nanojoule energies, and the same focusing and linear, azimuthal, or radial polarizations provided efficient spectrally and symmetry-matched excitation of both localized and delocalized surface electromagnetic modes in the separate, ring-like through holes and their arrays in the film envisioned by our modeling, thus resulting in a polarization-sensitive yield of rhodamine 6G dye PL. The demonstrated consistency between the symmetries of the donut-shaped low-energy photo-exciting laser beam, its polarization state, and the donut-shaped gold nanostructures, produced by the same beam at high, ablative pulse energies, paves the way to smart, self-consistent nanofabrication and plasmonic sensing, when the structured light interacts with the consistently structured matter.

The mechanism of controlled integration of ZnO nanowires using pulsed-laser-induced chemical deposition

Laser-induced chemical deposition is an economical "grow-in-place" approach to produce functional materials. The lack of precise control over the component density and other properties hinders the development of the method towards an efficient nanomanufacturing technology. In this paper, we provide a mechanism of direct pulsed-laser integration of ZnO nanowire seeding and growth on silicon wafers toward controlled density. Investigation of laser-induced ZnO nucleation directly deposited on a substrate suggested that the coverage percentage of nucleus particles was a function of instantly available area, supplementing the classical nucleation theory for confined area deposition. A processing window was found in which ZnO nanowires only grew from the early deposited nucleated particles as seeds. A study on ZnO nanowire growth showed that the process became transport limited over time, which was important for density-controlled nanowire growth integrated on nucleated seeds. The proposed mechanism provided guidance to integrate nanomaterials using laser-induced chemical deposition with a controlled density and morphology.

10-million-elements-per-second printing of infrared-resonant plasmonic arrays by multiplexed laser pulses

We report on high-quality infrared (IR)-resonant plasmonic nanoantenna arrays fabricated on a thin gold film by tightly focused femtosecond (fs) laser pulses coming at submegahertz repetition rates at a printing rate of 10 million elements per second. To achieve this, the laser pulses were spatially multiplexed by fused silica diffractive optical elements into 51 identical submicrometer-sized laser spots arranged into a linear array at periodicity down to 1 μm. The demonstrated high-throughput nanopatterning modality indicates fs laser maskless microablation as an emerging robust, flexible, and competitive lithographic tool for advanced fabrication of IR-range plasmonic sensors for environmental sensing, chemosensing, and biosensing.

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.

Four-dimensional light shaping: manipulating ultrafast spatiotemporal foci in space and time

The spectral dispersion of ultrashort pulses allows the simultaneous focusing of light in both space and time, which creates so-called spatiotemporal foci. Such space-time coupling may be combined with the existing holographic techniques to give a further dimension of control when generating focal light fields. In the present study, it is shown that a phase-only hologram placed in the pupil plane of an objective and illuminated by a spatially chirped ultrashort pulse can be used to generate three-dimensional arrays of spatio-temporally focused spots. By exploiting the pulse front tilt generated at focus when applying simultaneous spatial and temporal focusing (SSTF), it is possible to overlap neighboring foci in time to create a smooth intensity distribution. The resulting light field displays a high level of axial confinement, with experimental demonstrations given through two-photon microscopy and the non-linear laser fabrication of glass.

Multi-beam pulsed-laser patterning of plasmonic films using broadband diffractive optical elements

Multi-sector broadband diffractive optical elements (DOEs) were designed and fabricated from fused silica for high-efficiency multiplexing of femtosecond and nanosecond Gaussian laser beams into multiple (up to one 100) optically tunable microbeams with increased high-numerical aperture (NA) focal depths. Various DOE-related issues, such as high-NA laser focusing, laser pulsewidth, and DOE symmetry-dependent heat conduction effects, as well as the corresponding spatial resolution, were discussed in the context of high-throughput laser patterning. The increased focal depths provided by such DOEs, their high multiplexing efficiency and damage threshold, as well as easy-to-implement optical shaping of output microbeams provide advanced opportunities for direct, mask-free, and vacuum-free high-throughput subtractive (ablative) and displacive pulsed-laser patterning of various nanoplasmonic films for surface-enhanced spectroscopy, sensing, and light control.