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Abstract: Using active and programmable focal-spot shaping, we demonstrate direct femtosecond laser micro-structuring of materials. Surface marking as well as drilling with user-defined geometrical shapes of small dimensions (20μm) are performed, without moving the laser beam nor the material target. [Copyright &y& Elsevier]

Abstract: Investigation of the process of nanohole formation on silicon surface mediated with near electromagnetic field enhancement in vicinity of gold particles is described. Gold nanospheres with diameters of 40, 80 and 200nm are used. Irradiation of the samples with laser pulse at fluences below the ablation threshold for native Si surface, results in a nanosized surface modification. The nanostructure formation is investigated for the fundamental (λ =800nm, 100fs) and the second harmonic (λ =400nm, 250fs) of the laser radiation generated by ultrashort Ti:sapphire laser system. The near electric field distribution is analyzed by an Finite Difference Time Domain (FDTD) simulation code. The properties of the produced morphological changes on the Si surface are found to depend strongly on the polarization and the wavelength of the laser irradiation. When the laser pulse is linearly polarized the produced nanohole shape is elongated in the E-direction of the polarization. The shape of the hole becomes symmetrical when the laser radiation is circularly polarized. The size of the ablated holes depends on the size of the gold particles, as the smallest holes are produced with the smallest particles. The variation of the laser fluence and the particle size gives possibility of fabricating structures with lateral dimensions ranging from 200nm to below 40nm. Explanation of the obtained results is given on the basis simulations of the near field properties using FDTD model and Mie''s theory. [Copyright &y& Elsevier]

Abstract: We compare the foaming characteristics induced by irradiation with single ns and fs laser pulses of UV, VIS and IR wavelengths on gelatines differing in gel strength (bloom values 75 and 225) and in crosslinking degree. We have observed that while laser irradiation with nanoseconds leads to the formation of a microfoam layer at 266nm, and melting and crater formation at longer wavelengths (532 and 1064nm), fs pulse irradiation leads to submicron foaming at all wavelengths studied (266, 400 and 800nm). These results show the possibility of controlling the submicrometric foam structure in this biomaterial and can shed light into the working mechanisms of fs laser nanoprocessing in biomaterials, where increase of temperature, thermoelastic stress generation, and stress-induced bubble formation are mediated by the generated plasma. [Copyright &y& Elsevier]

Abstract: We report that plasma generated during processing of materials with ultrashort pulse lasers and the associated high intensity optical beam have both favourable and unfavourable impact on the machined surface quality. Intensity of the optical beam propagating through ambient air medium enhanced further by self-focusing is sufficiently high to cause gas breakdown forming air plasma. The generated plasma reduces the effect of self-focusing but also distorts the beam profile. Duration of the pulse being too short for thermal equilibrium to establish, ablation occurs largely by direct removal of the material forming another plasma plume. Normally, the scattering effect of plasma results in distortions of the fabricated features. However, for certain parameter ranges, the competing self-focusing and gas plasma plumes supplemented with the material plasma can combine to cause filamentation, eliminating the distortions. Filament of hot plasma also acts as a well-shaped energy source. In the present study, brass is taken as an example for the investigation. Experiments were conducted to capture the spectrum of the light scattered by plasma using a spectrometer. Analysis was done to estimate the material plasma. Theoretical calculation on the intensity distribution in an optical beam propagating through air was then followed for a range of parameter values taking the self-focusing effect of the medium and the impact of the plasma generated by its breakdown. Approximate values of the machining parameters for clean fabrication are deduced from the calculations, which were used to conduct a laser machining test on brass. [Copyright &y& Elsevier]

Abstract: We demonstrate self-fabrication of a submicrometer-sized void array in fused silica using a 100fs 0.2–3μJ Ti:Sapphire femtosecond laser and a high 0.9 numerical aperture (NA) objective lens. The effect of the focusing conditions of NA, laser energy, and pulse number on the shape of the fabricated void was investigated. The void has a linearly drawn shape in the direction of the laser irradiation when a single pulse is irradiated and an increasing number of incident pulses resulted in the breakup of the long void into multiple spherical ones, leading to a periodically aligned void array. The void shape also varied with the depth of the focus point beneath the fused silica surface, because the amount of self-focusing has a significant effect on the generation of the voids. The void shape was narrower and longer when the laser pulse was focused with the higher NA (up to 0.9) objective lens in the deeper position (up to 70μm) in the fused silica. We also demonstrated a 90° bend waveguide consisting of such long voids and waveguides fabricated by a femtosecond laser. [Copyright &y& Elsevier]

Recent research and development on the femtosecond laser has shown that it is a powerful tool for high precision micromachining. However, the femtosecond laser-material interaction is fast and complex due to its ultrashort pulse duration. Consequently, it is not an easy task to optimize process parameters and to ensure high-quality fabrication. In this context, we propose an in-line monitoring technique based on plasma plume diagnostics. By analyzing the spectra of the femtosecond laser-induced plasma, which was detected and analyzed using a spectrometer, the plasma temperature of the copper sample was estimated. As drilling holes and cutting lines are two common applications in industry, the relationship between the emission light and the geometry of the fabricated structure was investigated based on the time-varying signals from the photodetector. These results will help us to understand the mechanism of femtosecond laser ablation so as to achieve high quality micromachining. [ABSTRACT FROM AUTHOR]

[Display omitted] • We show controlled Voronoi partitions (VPs) by femtosecond laser processing on Au. • The VPs are formed by controlling the laser fluence. • FDTD and TTM are used to explain the formation of emerging structures. Ultrafast and high intensity laser pulses can drive materials into highly non-equilibrium conditions. At a sufficiently high intensity, femtosecond laser pulses can turn a solid material into a superheated solid or a superheated liquid state. For metals with low electron–phonon coupling strength, e.g., gold (Au), a superheated liquid state can lead to the formation of a family of unique surface structures, the so-called Voronoi partitions (VPs). Although chaotic in nature, we demonstrate here a control of VP formation through varying the laser fluence and pulse number, resulting in a rich variety of VPs. To understand the formation of complex surface geometries under laser irradiation, we utilizes a numerical approach that integrates the finite difference time domain of electromagnetic field method with the two-temperature model. By studying the spatio-temporal distribution of electromagnetic and temperature fields and by analyzing the geometric properties of the VPs, we provide a framework for understanding the emerging surface patterns. With VPs, novel structures are generated with promising potential applications, namely, Gecko-skin-like nano-spikes and nano-cauliflower-like surface structures. [ABSTRACT FROM AUTHOR]

An advanced superwicking aluminum material based on a microgroove surface structure textured with both laser-induced periodic surface structures and fine microholes was produced by direct femtosecond laser nano/microstructuring technology. The created material demonstrates excellent wicking performance in a temperature range of 23 to 120 °C. The experiments on wicking dynamics show a record-high velocity of water spreading that achieves about 450 mm/s at 23 °C and 320 mm/s at 120 °C when the spreading water undergoes intensive boiling. The lifetime of classic Washburn capillary flow dynamics shortens as the temperature increases up to 80 °C. The effects of evaporation and boiling on water spreading become significant above 80 °C, resulting in vanishing of Washburn's dynamics. Both the inertial and visco-inertial flow regimes are insignificantly affected by evaporation at temperatures below the boiling point of water. The boiling effect on the inertial regime is small at 120 °C; however, its effect on the visco-inertial regime is essential. The created material with effective wicking performance under water boiling conditions can find applications in Maisotsenko cycle (M-cycle) high-temperature heat/mass exchangers for enhancing power generation efficiency that is an important factor in reducing CO2 emissions and mitigation of the global climate change. [ABSTRACT FROM AUTHOR]

• The AR characteristic of 1-D moth-eye structures has been successfully controlled by adjusting the effective refractive index distribution of micro tapers for THz waves, which is achieved by employing femtosecond laser processing. • The THz-TDS measured and FDTD simulated results showed good agreement for all the samples, which can induce the power reflectance nearly to 0. • The stair profile displayed the most stable reflectance distribution of nearly 0 maintained in the 0.5–1.0-THz range, as well as lower total power reflectance for a broadband THz region from 0.1–1.0 THz. The present study focuses on controlling the antireflective (AR) characteristics of one-dimensional moth eye structures on a high-resistivity silicon substrate by adjusting the profile (i.e., refractive index distribution) of its micro-tapers using femtosecond laser processing. Moth eye structures that comprise periodically arranged tapers with varying sizes and profiles (triangle, parabola, stair, and triangle–stair) were fabricated using femtosecond laser ablation via a fine adjustment of the processing pattern. The AR characteristics of these samples were experimentally evaluated by a standard terahertz (THz) time-domain spectroscopy and modeled by employing the method of Finite-difference time-domain (FDTD). The measurement data agreed well with the simulation results, and the power reflectance of these structures was induced from approximately 30% (bare silicon substrate) to a very low value (less than 5%) for a broad THz band. Furthermore, the profiled structures showed a varying power reflectance distribution with respect to the frequency in the THz region, especially for the low reflection one (<5%) approximately from 0.5 to 1.0 THz. Three samples with different aspect ratios (approximately 1–3) but having the similar refractive index distribution were also fabricated by using an industrial high-power femtosecond laser (40 W) to demonstrate the relationship between the AR characteristics and the aspect ratio of the moth-eye structures, as well as the processing efficiency. The structures displayed a very good AR performance for a broadband THz region. Besides the broad AR band and low reflectance, this adjustability for the profile of laser-fabricated tapers could improve the flexibility of anti-reflection for THz devices, such as THz wave emitters, receivers, and filters, and the antireflective frequencies less than 1 THz suggest that this technology can be used in the next generation of mobile communication. [ABSTRACT FROM AUTHOR]

Strong light localization inside the nanoscale gaps provides remarkable opportunities for creation of various medical and biosensing platforms stimulating an active search for inexpensive and easily scalable fabrication at a sub-100 nm resolution. In this paper, self-organized laser-induced periodic surface structures (LIPSSs) with the shortest ever reported periodicity of 70 ± 10 nm were directly imprinted on the crystalline Si wafer upon its direct femtosecond-laser ablation in isopropanol. Appearance of such a nanoscale morphology was explained by the formation of a periodic topography on the surface of photoexcited Si driven by interference phenomena as well as subsequent down-scaling of the imprinted grating period via Rayleigh-Taylor hydrodynamic instability. The produced deep subwavelength LIPSSs demonstrate strong anisotropic anti-reflection performance, ensuring efficient delivery of the incident far-field radiation to the electromagnetic "hot spots" localized in the Si nanogaps. This allows realization of various optical biosensing platforms operating via strong interactions of quantum emitters with nanoscale light fields. The demonstrated 80-fold enhancement of spontaneous emission from the attached nanolayer of organic dye molecules and in situ optical tracing of catalytic molecular transformations substantiate bare and metal-capped deep subwavelength Si LIPSSs as a promising inexpensive multifunctional biosensing platform.

A wavefront aberration causes focal spot distortion leading to loss of resolution and efficiency in laser processing. Therefore, aberration compensation is important for ensuring sub-micron resolution in laser processing. In this paper, femtosecond laser processing with adaptive optics based on a convolutional neural network was demonstrated. The aberrations existing in the laser processing system were continuously predicted by the trained network with an update period of 36 ms and was compensated by a liquid crystal spatial light modulator. In the experiment, the neural network-based adaptive optics reduced the wavefront error in the laser processing system to most one-ninth. Furthermore, parallel laser processing by a computer-generated hologram displayed on the spatial light modulator was also demonstrated while dynamically compensating the aberrations in the system. [ABSTRACT FROM AUTHOR]

The induction of surface cleavage along the crystalline structure of a zinc oxide substrate (plane orientation: 0001) by femtosecond laser pulses (wavelength: 1030 nm) has been reported; a scanning electron microscope image of the one-pulse (pulse energy: 6–60 μJ) irradiated surface shows very clear marks from broken hexagons. This cleavage process differs from the general laser-induced melt process observed on the surfaces of narrower-bandgap semiconductors and other metal materials. This phenomenon is discussed using a multi-photon absorption model, and the pulse-energy dependence of the cleavage depth (less than 3 μm) is quantitatively analyzed. Laser-induced cleavage is found not to occur under multi-pulse irradiation; when more than four pulses are irradiated upon the same spot, the general laser-induced melt process becomes dominant. This cleavage–melt shift is considered to be caused by the enhancement of absorption due to the initial pulses, which is supported by our measurement of cathodoluminescence. [ABSTRACT FROM AUTHOR]

The ability to create controllable patterns of micro- and nanostructures on the surface of bulk silicon has widespread application potential. In particular, the direct writing of silicon oxide patterns on silicon via femtosecond laser-induced silicon amorphization has attracted considerable attention owing to its simplicity and high efficiency. However, the direct writing of nanoscale resolution is challenging due to the optical diffraction effect. In this study, we propose a highly efficient, one-step method for preparing silicon oxide nanopatterns on silicon. The proposed method combines femtosecond laser-induced silicon amorphization with a subwavelength-scale beam waist of photonic nanojets. We demonstrate the direct writing of arbitrary nanopatterns via contactless scanning, achieving patterns with a minimum feature size of 310 nm and a height of 120 nm. The proposed method shows potential for the fabrication of multifunctional surfaces, silicon-based chips, and silicon photonics. [ABSTRACT FROM AUTHOR]

A superwicking Ti-6Al-4V alloy material with a hierarchical capillary surface structure was fabricated using femtosecond laser. The basic capillary surface structure is an array of micropillars/microholes. For enhancing its capillary action, the surface of the micropillars/microholes is additionally structured by regular fine microgrooves using a technique of laser-induced periodic surface structures (LIPSS), providing an extremely strong capillary action in a temperature range between 23 °C and 80 °C. Due to strong capillary action, a water drop quickly spreads in the wicking surface structure and forms a thin film over a large surface area, resulting in fast evaporation. The maximum water flow velocity after the acceleration stage is found to be 225–250 mm/s. In contrast to other metallic materials with surface capillarity produced by laser processing, the wicking performance of which quickly degrades with time, the wicking functionality of the material created here is long-lasting. Strong and long-lasting wicking properties make the created material suitable for a large variety of practical applications based on liquid-vapor phase change. Potential significant energy savings in air-conditioning and cooling data centers due to application of the material created here can contribute to mitigation of global warming. [ABSTRACT FROM AUTHOR]

• Physical mechanisms and models for fabrication of various materials are concluded. • Latest advancements of optical processing systems and methods are reviewed. • Technologies in high resolution, large-scale 3D and 4D fabrication are reviewed. • Cutting-edge processing approaches of four kinds of optical devices are summarized. The unique advantages of surface and volume processing enabled by femtosecond laser fabrication makes it one of the most powerful tools for manufacturing optical and photonic devices based on complex three-dimensional structures with diverse functions. In this review, we focus on the recent advancements of femtosecond laser fabrication technologies and their versatile applications in different fields (e.g., nanotechnology, soft robotics, optics, and optoelectronics). Herein, the characteristics and development of laser direct writing, spatial light modulator-based fabrication and interference-based approaches are reviewed. Furthermore, typical applications and approaches related to the manufacturing of cutting-edge optical devices including microlens arrays, micro/nano gratings, photonic crystals, and optical fibers are summarized. Finally, the current challenges and emerging trends of this technology are discussed. [ABSTRACT FROM AUTHOR]

• Muscovite dielectric shows diversity in the nano-patterning by single fs pulses. • A single, 515 nm 190 fs pulse leads to a polymer-like response. • This differs from published results for other crystalline optical materials. • Response of muscovite is wavelength dependent, contrast at 800 nm and 515 nm. • Role of mineral water in muscovite continues to emerge as a key material property. Muscovite, a nanolayered transparent crystalline dielectric, is shown to respond like a polymer upon irradiation with a single 190 fs duration laser pulse, at a wavelength of 515 nm, for a range of fluences (1.4–6.4 J/cm2). Laser-pulse-modified sites were characterized using field emission scanning electron microscopy (FESEM). Cavitation and bubbling are observed in the laser processed region similar to previous observations in poly-methyl-methacrylate (PMMA). The diameter of these sites is consistent with the standard model commonly applied in laser ablation studies. These polymer-like results at 515 nm are in contrast to the response of muscovite exposed to a 150-fs single laser pulse at 800 nm for a similar range of fluences. Where a diverse range of topologies was observed as the fluence was increased. Additionally, we also report an absorption band edge at ~4 eV (for the muscovite sheet used in the study). This is lower than the previously reported band gap energy value of 7.8 eV. We propose that the differences observed at the two wavelengths are primarily due to either 2 or 3 photons being required for nonlinear photoionization and the impact of the mineral water content of muscovite (4.7 wt%). [ABSTRACT FROM AUTHOR]

• V O s were introduced in TiO 2 nanotubes by fs-laser processing with anodization. • TiO 2- x nanotubes formed on microcone's surface, leading excellent light absorption. • Self-doping of oxygen vacancies significantly narrowed the bandgap (1.95 eV). • TiO 2- x electrodes performed 15-time photocurrent and double photodegration rate. • The as-prepared photoelectrodes show great potential in photocatalytic application. Titanium dioxide (TiO 2) photoelectrodes that offer high light absorption and efficient charge separation hold great promise in photocatalysis. In this study, a simple and controllable method for fabricating hierarchical TiO 2− x photoelectrodes by coupling femtosecond laser processing and anodization is proposed. The fabricated photoelectrodes consist of microcones (approximately 150,000 per square centimetre) covered with large quantities of nanotubes. The hierarchical structures possess significant light-trapping effect, meanwhile, oxygen vacancies were simultaneously introduced through laser processing with anodization, which can be attributed to the fs-laser mediated lattice phase transformation (polycrystalline and amorphous layer) of titanium. With the synergistic effects of hierarchical structures and oxygen vacancies, the hierarchical TiO 2− x electrode exhibited a narrowed bandgap (1.95 eV) and remarkable light absorption. Especially under visible light, this photoelectrode demonstrated 15-fold photocurrent enhancement and double the usual photodegradation rate of methylene blue. This morphological control and defect introduction method may be extensively used in efficient solar utilisation. [ABSTRACT FROM AUTHOR]

• The ablation morphology of sapphire coated with a 12-nm-thick gold film has been improved, featuring fewer re-solidified molten materials and thermal cracks, as well as regular crater shape. • The material removal rate of sapphire has increased 2.24 times by depositing a nanolayered gold film on sapphire surface under 50-shot and 150 μJ energy. • The effective mechanism of gold film has been theoretically investigated via the energy transfer among incident photons, free electrons and sapphire lattice phonons. • The gold film can enhance energy transfer from incident laser to sapphire lattice phonons and cause homogenous laser energy deposition on the sapphire surface. Sapphire is widely used in civilian and military equipment owing to its superior optical and mechanical properties. Femtosecond laser has been demonstrated to be an effective tool to process sapphire material. However, the direct processing of sapphire by femtosecond laser still meets some challenges, such as poor ablation morphology and low laser energy absorption. In this work, femtosecond laser processing of sapphire coated with a 12-nm-thick gold film (Au-coated sapphire) has been investigated. The experimental results have revealed that the ablation morphology of Au-coated sapphire has been improved, featuring fewer molten materials and thermal cracks, as well as regular crater shape and uniform periodic surface structures. It has also been found that, under 100 shots condition, the threshold fluence of Au-coated sapphire is reduced by about 56% compared to that of uncoated one. Meanwhile, the incubation effect of Au-coated sapphire is stronger than that of uncoated one. We also illustrate that the material removal rate of Au-coated sapphire is increased up to about two times higher than that of uncoated one. In order to reveal the effective mechanism of the gold film in the laser processing of sapphire, the energy transfer process among incident photons, free electrons and sapphire lattice phonons was studied. Our study provides a guidance for improving the laser ablation capacity of sapphire. [ABSTRACT FROM AUTHOR]

• Real-time spectral response guided smart femtosecond laser bone drilling. • Invasive and label-free positioning with second-harmonic-generation green signal. • High-efficiency and damage-free bone drilling with positioning, monitoring and spectral monitoring. Bone drilling is a common surgical procedure of knee arthroplasty, craniotomy, stapedotomy, dental treatment, and laminotomy. Although femtosecond laser processing provides a promising method for precision bone drilling, it is still challenging to achieve thermal accumulation suppression and damage protection for large-scale hole drilling. In this paper, we propose a real-time spectral response guided femtosecond laser bone drilling method with the second-harmonic-generation (SHG) signals for positioning and monitoring. SHG green signal centered at 513.5 nm is generated below laser ablation threshold of bone, which can provide as a label-free detection for initial positioning. Results show that the spectral intensity ratio of green and white signal gradually decreases due to the accelerated removal of hydroxyapatite with the increased laser fluence, while the spectral intensity ratio of red signal and other visible signal can be used to indicate the thermal damage due to the abnormal enhancement of the red signal ranging from 700 to 800 nm when carbonization occurs. Based on the real-time spectral response at different laser fluence, the key laser processing parameters including defocus distance, flow rate of cooling water, and laser fluence are successfully optimized. Finally, a damage-free hole with the average diameter of 3 mm and the depth of 3.6 mm is successfully drilled within 5 min, while the temperature of surrounding hole wall is 37.1 °C. The proposed method provides a direct-viewing, damage-free and high-efficiency way of laser bone drilling with real-time positioning and monitoring. [ABSTRACT FROM AUTHOR]

Capillary flow of water in an array of open nanotextured microgrooves fabricated by femtosecond laser processing of silicon is studied as a function of temperature using high-speed video recording. In a temperature range of 23–80 °C, the produced wicking material provides extremely fast liquid flow with a maximum velocity of 37 cm/s in the initial spreading stage prior to visco-inertial regime. The capillary performance of the material enhances with increasing temperature in the inertial, visco-inertial, and partially in Washburn flow regimes. The classic universal Washburn's regime is observed at all studied temperatures, giving the evidence of its universality at high temperatures as well. The obtained results are of great significance for creating capillary materials for applications in cooling of electronics, energy harvesting, enhancing the critical heat flux of industrial boilers, and Maisotsenko cycle technologies. [ABSTRACT FROM AUTHOR]

This paper compares anisotropic linear optical properties (linear birefringence, linear dichroism, degree of polarization) and performances (absorption coefficient, thermal stability) of two types of birefringent waveplates fabricated in silica glass by femtosecond laser direct writing. The first type of waveplate is based on birefringence induced by self-organized nanogratings imprinted in the glass. One the other hand, the second design is based on birefringence originating from the stress-field formed around the aforementioned nanogratings. In addition to the provided comparison, the manufacturing of stress-engineered half waveplates in the UV-Visible range, and with mm-size clear aperture and negligible excess losses, is reported. Such results contrast with waveplates made of nanogratings, as the later exhibit significantly higher scattering losses and depolarization effects in the UV-Visible range. [ABSTRACT FROM AUTHOR]

Laser-induced periodic surface structures (LIPSS) are often present when processing solid targets with linearly polarized ultrashort laser pulses. The different irradiation parameters to produce them on metals, semiconductors and dielectrics have been studied extensively, identifying suitable regimes to tailor its properties for applications in the fields of optics, medicine, fluidics and tribology, to name a few. One important parameter widely present when exposing the samples to the high intensities provided by these laser pulses in air environment, that generally is not considered, is the formation of a superficial laser-induced oxide layer. In this paper, we fabricate LIPSS on a layer of the oxidation prone hard-coating material chromium nitride in order to investigate the impact of the laser-induced oxide layer on its formation. A variety of complementary surface analytic techniques were employed, revealing morphological, chemical and structural characteristics of well-known high-spatial frequency LIPSS (HSFL) together with a new type of low-spatial frequency LIPSS (LSFL) with an anomalous orientation parallel to the laser polarization. Based on this input, we performed finite-difference time-domain calculations considering a layered system resembling the geometry of the HSFL along with the presence of a laser-induced oxide layer. The simulations support a scenario that the new type of LSFL is formed at the interface between the laser-induced oxide layer and the non-altered material underneath. These findings suggest that LSFL structures parallel to the polarization can be easily induced in materials that are prone to oxidation. [ABSTRACT FROM AUTHOR]

The high-precision patterning of metal halide perovskites (MHPs) is of paramount importance for their device application. Here, we demonstrate the femtosecond (fs)-laser-assisted formation of three-dimensional MHP nanocrystal (NC) patterns with strong blue photoluminescence (PL) inside an oxide glass. Our strategy enables the crystallization and erasing of CsPb(Cl/Br) 3 NCs inside a glass localized around the laser focal area through a combination of fs laser irradiation and thermal treatment processes. These recoverable patterns exhibit a switchable PL associated with the laser-induced defect and the thermal healing of MHP NCs that are benefits from the soft ionic crystal structure and low formation energy of the MHPs. Due to the high stability offered by the protection of the oxide glass matrix, the laser printing of fine-structured MHP micropatterns can be repeated over multiple cycles with a high robustness compared with their colloidal process counterparts. Our results demonstrate a simple strategy for creating emissive patterns inside a stable and transparent solid matrix that could be promising for applications including information storage, three-dimensional displays, anticounterfeit labels, and information security protection.

Natural compound eyes provide the inspiration for developing artificial optical devices that feature a large field of view (FOV). However, the imaging ability of artificial compound eyes is generally based on the large number of ommatidia. The lack of a tunable imaging mechanism significantly limits the practical applications of artificial compound eyes, for instance, distinguishing targets at different distances. Herein, we reported zoom compound eyes that enable variable-focus imaging by integrating a deformable poly(dimethylsiloxane) (PDMS) microlens array (MLA) with a microfluidic chamber. The thin and soft PDMS MLA was fabricated by soft lithography using a hard template prepared by a combined technology of femtosecond laser processing and wet etching. As compared with other mechanical machining strategies, our combined technology features high flexibility, efficiency, and uniformity, as well as designable processing capability, since the size, distribution, and arrangement of the ommatidia can be well controlled during femtosecond laser processing. By tuning the volume of water injected into the chamber, the PDMS MLA can deform from a planar structure to a hemispherical shape, evolving into a tunable compound eye of variable FOV up to 180°. More importantly, the tunable chamber can functionalize as the main zoom lens for tunable imaging, which endows the compound eye with the additional capability of distinguishing targets at different distances. Its focal length can be turned from 3.03 mm to infinity with an angular resolution of 3.86 × 10 -4 rad. This zoom compound eye combines the advantages of monocular eyes and compound eyes together, holding great promise for developing advanced micro-optical devices that enable large FOV and variable-focus imaging.

We report on a novel single-step method to develop steel surfaces with permanent highly hydrophilic and anti-corrosive properties, without employing any chemical coating. It is based on the femtosecond (fs) laser processing in a saturated background gas atmosphere. It is particularly shown that the fs laser microstructuring of steel in the presence of ammonia gas gives rise to pseudoperiodic arrays of microcones exhibiting highly hydrophilic properties, which are stable over time. This is in contrast to the conventional fs laser processing of steel in air, which always provides surfaces with progressively increasing hydrophobicity following irradiation. More importantly, the surfaces subjected to fs laser treatment in ammonia exhibit remarkable anti-corrosion properties, contrary to those processed in air, as well as untreated ones. The combination of two functionalities, namely hydrophilicity and corrosion resistance, together with the facile processing performed directly onto the steel surface, without the need to deposit any coating, opens the way for the laser-based production of high-performance steel components for a variety of applications, including mechanical parts, fluidic components and consumer products. [ABSTRACT FROM AUTHOR]

Laser surface structuring has been demonstrated to be a versatile technology to create various functional materials by modifying solid surface properties. An interesting experimental phenomenon of self‐organized periodic microholes array formation is demonstrated by exposing an aluminum surface to femtosecond laser irradiation. The microholes with a diameter much smaller than focal laser spot size are produced along laser scan paths due to the incubation effect of multiple laser scans, and they spontaneously form a highly ordered microholes array after 80 scans. It is found that the microhole period and diameter are highly dependent on laser fluence, and controllable microholes arrays with different periods and diameters are achieved by adjusting laser fluence. The physical mechanism behind the formation of the microholes array is attributed to femtosecond laser‐induced melting and Marangoni effect. The research provides a novel processing approach to achieve controllable microholes surface materials fabrication at a high speed on metal substrates, which are of great interests for various technological applications. [ABSTRACT FROM AUTHOR]

In this work, a scalable thermal imprinting method that allows to directly fabricate superhydrophobic polymer surfaces with superstrong resistance to harsh cleaning and mechanical abrasion is reported. A titanium (Ti) mold is produced by femtosecond laser processing to possess a hierarchical micro‐ and nanoscale pattern. Through thermal imprinting, this hierarchical pattern onto poly(propylene) sheets is able to be accurately reproduced, rending the polymer surface superhydrophobic. The imprinting method employed uses an aqueous ethanol solution of stearic acid to assist demolding, and periodically uses xylene to limit mold contamination and restore the mold to excellent condition for further imprinting. These strategies allow to repeatedly use the mold over 50 times without degradation. A range of durability tests are further performed, and showed that the produced suprehydrophobility on the poly(propylene) sheets exhibit excellent durability, withstanding brush washing, ultrasound cleaning, and sandpaper abrasion. The reported method can also be used to fabricate superhydrophobic surfaces of various thermoplastics that are broadly utilized in daily life. [ABSTRACT FROM AUTHOR]

This study demonstrated a measurement approach for biomolecules at the picoliter scale, using a newly developed picoliter cuvette inside an optical fiber constructed via near-ultraviolet femtosecond laser drilling. The sensing capacity was estimated to be within 0.4–1.2 pL due to an optical path length of 3–5 microns, as measured by scanning electron microscopy (SEM). The picoliter cuvette exhibited a change in the optical extinction spectrum after addition of biomolecules such as L-cysteine, in conjunction with a gold nanoparticle (GNP) dispersion solution, following a simple measurement configuration involving a small white light source and a compact spectrometer. A linear attenuation of the spectral dip near a wavelength of 520 nm was observed as the L-cysteine concentration was increased at 4 wt% of the GNP mass concentration. The measurement resolution of the concentration using the picoliter cuvette was evaluated at 0.125 mM. The experimental results showed the difference in aggregation processes caused by a different concentration of GNPs. Moreover, they revealed the ability of the picoliter cuvette to verify whether the concentration of GNPs in the liquid sample correspondingly determines homogeneous or inhomogeneous GNP aggregation, as supported by SEM observation and numerical calculations based on Mie theory. [ABSTRACT FROM AUTHOR]

Surface functionalization introduced by precisely-defined surface structures depended on the surface texture and quality. Laser treatment is an advanced, non-contact technique for improving the biomaterials surface characteristics. In this study, femtosecond laser modification was applied to fabricate diverse structures on biodegradable polymer thin films and their ceramic blends. The influences of key laser processing parameters like laser energy and a number of applied laser pulses (N) over laser-treated surfaces were investigated. The modification of surface roughness was determined by atomic force microscopy (AFM). The surface roughness (Rrms) increased from approximately 0.5 to nearly 3 µm. The roughness changed with increasing laser energy and a number of applied laser pulses (N). The induced morphologies with different laser parameters were compared via Scanning electron microscopy (SEM) and confocal microscopy analysis. The chemical composition of exposed surfaces was examined by FTIR, X-ray photoelectron spectroscopy (XPS), and XRD analysis. This work illustrates the capacity of the laser microstructuring method for surface functionalization with possible applications in improvement of cellular attachment and orientation. Cells exhibited an extended shape along laser-modified surface zones compared to non-structured areas and demonstrated parallel alignment to the created structures. We examined laser-material interaction, microstructural outgrowth, and surface-treatment effect. By comparing the experimental results, it can be summarized that considerable processing quality can be obtained with femtosecond laser structuring. [ABSTRACT FROM AUTHOR]

Single crystal perovskites are used in solar cells, photodetectors, and other devices due to their excellent light absorption and carrier transport characteristics. However, for light‐emitting applications, photoluminescence (PL) is usually weak for MAPbBr3 (MA = CH3NH3+) single crystals (MBSCs) compared with their polycrystalline counterpart. Therefore, developing novel techniques to process MBSCs with different morphologies for PL‐related applications is greatly needed. The current strategies for making perovskite crystals are mostly based on bottom‐up method (chemical synthesis and assembling). Here, an easy method to achieve top‐down fabrication of MBSCs, i.e., femtosecond laser processing MBSC surface by controlling the laser parameters is demonstrated. The femtosecond laser processing technology can achieve two orders of magnitude enhancement under ambient conditions in PL. In addition, the processed regions also show three times enhancement in PL under nitrogen environment. It is assumed that this is mainly due to the texture based on photon recycling and light out‐coupling mechanism, and the passivation of surface recombination centers on MBSC. This study not only provides a convenient top‐down strategy to achieve a range of morphological micro‐/nanostructures with enhanced PL on MBSC surface, but also paves the way for applications of MBSCs in light emitting or PL imaging devices. Developing novel techniques to process MAPbBr3 single crystals (MBSCs) with various morphologies for photoluminescence‐related applications is highly necessary. Multi‐morphological micro‐/nanostructures induced by a top‐down fabricating technology, i.e., femtosecond laser processing, are achieved on MBSC surfaces. Dramatically‐enhanced photoluminescence of two orders of magnitude in ambient conditions and three times in nitrogen atmosphere is demonstrated, which is promising for photoluminescence‐related applications. [ABSTRACT FROM AUTHOR]

Laser texturing is an emerging technology for generating surface functionalities on basis of optical, mechanical, or chemical properties. Taking benefit of laser sources with ultrashort (fs) pulse durations features outstanding precision of machining and negligible rims or burrs surrounding the laser-irradiation zone. Consequently, additional mechanical or chemical post-processing steps are usually not required for fs-laser surface texturing (fs-LST). This work aimed to provide a bridge between research in the field of tribology and laser materials processing. The paper reviews the current state-of-the-art in fs-LST, with a focus on the tribological performance (friction and wear) of specific self-organized surface structures (so-called ripples, grooves, and spikes) on steel and titanium alloys. On the titanium alloy, specific sickle-shaped hybrid micro-nanostructures were also observed and tribologically tested. Care is taken to identify accompanying effects affecting the materials hardness, superficial oxidation, nano- and microscale topographies, and the role of additives contained in lubricants, such as commercial engine oil. [ABSTRACT FROM AUTHOR]

Cancer cells undergo dramatic morphology changes when migrating in confined spaces narrower than their diameter during metastasis, and thus it is necessary to understand the deformation mechanism and associated molecular events in order to study tumor progression. To this end, we propose a new biochip with three-dimensional (3D) polymer nanostructures in a closed glass microfluidic chip. "Ship-in-a-bottle" femtosecond laser processing is an exclusive technique to flexibly create 3D small details in biochips. The wavefront correction by the spatial light modulator significantly improves the fabrication resolution of this technique. The device could then accommodate defect-free 3D biomimetic nanoconfigurations for the evaluation of prostate cancer cell migration in confined spaces. Specifically, polymeric channels with widths of ∼900 nm, which is more than one order of magnitude smaller than the cell size, are integrated by femtosecond laser inside glass channels. The cells are responsive to an in-channel gradient of epidermal growth factor and can migrate a distance greater than 20 μm. After migration, the cells suffer partial cytokinesis, followed by fusion of the divided parts back into single cell bodies.

To clarify the effects of micron/submicron hybrid topography on cell morphology and functionalization, we investigated the adhesion and differentiation of human mesenchymal stem cells (hMSCs) to titanium (Ti) surfaces with three different topographies: micron, submicron, and hybrid grooves created using a femtosecond laser. hMSCs cultured on Ti specimens showed high alignment on micron and hybrid surfaces after 6 h of incubation, whereas cells attached to submicron and hybrid surfaces were elongated. An examination of vinculin-positive adhesion plaques indicated that micron grooves affected cellular alignment by modifying the initial cell polarization, whereas submicron grooves affected cellular extension. A superposition effect of topography was evidenced by the highly aligned and elongated morphology of hMSCs grown on the hybrid surface, which promoted osteogenic and chondrogenic differentiation. These findings provide a basis for the design of novel biomaterial surfaces that can control specific cellular functions.

The paper addresses the creation of circular optical properties from a femtosecond laser light beam with a linear polarization in an achiral material (glass) under an orthogonal incidence. In this situation, all aspects of the experiment are achiral and therefore should not give rise to chiral property creation. From that observation, we propose an interpretation that involves the action of a light-induced torque on the matter carrying a light-induced dielectric moment. We found that a direct current (DC) electric field could be produced in the lattice by the femtosecond laser in our conditions and that a non-collinear dielectric moment is created by a nonlinear effect between the DC electric field and the stress field due to the transformation of the material. We reveal that it is possible to break the chiral symmetry of glass using an intense, ultrashort laser light pulse., Competing Interests: The authors declare no conflict of interest.

The synthesis of surfactant-free Au nanoplates is desirable for the development of biocompatible therapeutics/diagnostics. Rapid Δ-function energy deposition by irradiation of aqueous KAuCl4 solution with a 5 s burst of intense shaped laser pulses, followed by slow addition of H2O2, results in selective formation of nanoplates with no additional reagents. The primary mechanism of nanoplate formation is found to be oriented attachment of the spherical seeds, which self-recrystallize to form crystalline Au nanoplates.