Femtosecond Laser-Induced Phase Transformation on Single-Crystal 6H-SiC

Silicon carbide (SiC) is widely used in many research fields because of its excellent properties. The femtosecond laser has been proven to be an effective method for achieving high-quality and high-efficiency SiC micromachining. In this article, the ablation mechanism irradiated on different surfaces of 6H-SiC by a single pulse under different energies was investigated. The changes in material elements and the geometric spatial distribution of the ablation pit were analyzed using micro-Raman spectroscopy, Energy Dispersive Spectrum (EDS), and an optical microscope, respectively. Moreover, the thresholds for structural transformation and modification zones of 6H-SiC on different surfaces were calculated based on the diameter of the ablation pits created by a femtosecond laser at different single-pulse energies. Experimental results show that the transformation thresholds of the Si surface and the C surface are 5.60 J/cm2 and 6.40 J/cm2, corresponding to the modification thresholds of 2.26 J/cm2 and 2.42 J/cm2, respectively. The Raman and EDS results reveal that there are no phase transformations or material changes on different surfaces of 6H-SiC at low energy, however, decomposition and oxidation occur and then accumulate into dense new phase material under high-energy laser irradiation. We found that the distribution of structural phase transformation is uneven from the center of the spot to the edge. The content of this research reveals the internal evolution mechanism of high-quality laser processing of hard material 6H-SiC. We expect that this research will contribute to the further development of SiC-based MEMS devices.

Design of Laser Activated Antimicrobial Porous Tricalcium Phosphate-Hydroxyapatite Scaffolds for Orthopedic Applications

The field of bone tissue engineering is steadily being improved by novel experimental approaches. Nevertheless, microbial adhesion after scaffold implantation remains a limitation that could lead to the impairment of the regeneration process, or scaffold rejection. The present study introduces a methodology that employs laser-based strategies for the development of antimicrobial interfaces on tricalcium phosphate-hydroxyapatite (TCP-HA) scaffolds. The outer surfaces of the ceramic scaffolds with inner porosity were structured using a femtosecond laser (λ = 800 nm; τ = 70 fs) for developing micropatterns and altering local surface roughness. The pulsed laser deposition of ZnO was used for the subsequent functionalization of both laser-structured and unmodified surfaces. The impact of the fs irradiation was investigated by Raman spectroscopy and X-ray diffraction. The effects of the ZnO-layered ceramic surfaces on initial bacterial adherence were assessed by culturing Staphylococcus aureus on both functionalized and non-functionalized scaffolds. Bacterial metabolic activity and morphology were monitored via the Resazurin assay and microscopic approaches. The presence of ZnO evidently decreased the metabolic activity of bacteria and led to impaired cell morphology. The results from this study have led to the conclusion that the combination of fs laser-structured surface topography and ZnO could yield a potential antimicrobial interface for implants in bone tissue engineering.

A Computational Evaluation of Minimum Feature Size in Projection Two-Photon Lithography for Rapid Sub-100 nm Additive Manufacturing

Two-photon lithography (TPL) is a laser-based additive manufacturing technique that enables the printing of arbitrarily complex cm-scale polymeric 3D structures with sub-micron features. Although various approaches have been investigated to enable the printing of fine features in TPL, it is still challenging to achieve rapid sub-100 nm 3D printing. A key limitation is that the physical phenomena that govern the theoretical and practical limits of the minimum feature size are not well known. Here, we investigate these limits in the projection TPL (P-PTL) process, which is a high-throughput variant of TPL, wherein entire 2D layers are printed at once. We quantify the effects of the projected feature size, optical power, exposure time, and photoinitiator concentration on the printed feature size through finite element modeling of photopolymerization. Simulations are performed rapidly over a vast parameter set exceeding 10,000 combinations through a dynamic programming scheme, which is implemented on high-performance computing resources. We demonstrate that there is no physics-based limit to the minimum feature sizes achievable with a precise and well-calibrated P-TPL system, despite the discrete nature of illumination. However, the practically achievable minimum feature size is limited by the increased sensitivity of the degree of polymer conversion to the processing parameters in the sub-100 nm regime. The insights generated here can serve as a roadmap towards fast, precise, and predictable sub-100 nm 3D printing.

Direct Fabrication of Te-Doped Black Si with an Enhanced Photoelectric Performance by Femtosecond Laser Irradiation under Water

Tellurium (Te)-doped black silicon (Si) with enhanced absorption and photoelectric performance over a broad wavelength range of 0.2-2.5 μm was obtained using femtosecond (fs) laser irradiation in liquid water. Prior to laser irradiation, the Si sample was covered with a Te thin film (thickness 200 nm) over an adhesion layer of Cr (thickness 5 nm). Surface analyses by scanning electron microscopy and three-dimensional confocal microscopy evidence the presence of hierarchical surface structures combining quasi-periodic stripes with a spatial period of about 5 μm and subwavelength laser-induced periodic surface structures directed in directions parallel and perpendicular to the direction of the laser polarization, respectively. Moreover, the incorporation of Te generates intermediate levels within the Si bandgap. The Te-doped black Si shows a significant enhancement of the absorption, which reaches values of about 48% in the UV and visible (0.2-1.1 μm) and 70% in the near-infrared (1.1-2.5 μm) spectral ranges, respectively, due to the synergistic effects of multiscale surface structures and Te incorporation. Moreover, the surface reflectance is reduced to almost zero across the entire spectrum. The Te-doped black Si sample is used to realize a photodetector which displays an impressive photoelectric capability, being characterized by a responsivity of 328 mA/W, and an external quantum efficiency of 49.27% at a voltage bias of -10 V for 1064 nm light illumination, with rising and falling times of 55 and 67 ms, respectively. These figures remarkably outperform the response of unprocessed Si under the same experimental conditions.

Femtosecond Laser Inscribed Excessively Tilted Fiber Grating for Humidity Sensing

We propose a humidity sensor using an excessively tilted fiber grating (Ex-TFG) coated with agarose fabricated using femtosecond laser processing. The processed grating showcases remarkable differentiation between TE and TM modes, achieving an exceptionally narrow bandwidth of approximately 1.5 nm and an impressive modulation depth of up to 15 dB for both modes. We exposed the agarose-coated TFG sensor to various relative humidity levels and monitored the resonance wavelength to test its humidity sensing capability. Our findings demonstrated that the sensor exhibited a rapid response time (2-4 s) and showed a high response sensitivity (18.5 pm/%RH) between the humidity changes and the resonant wavelength shifts. The high sensitivity, linearity, repeatability, low hysteresis, and excellent long-term stability of the TFG humidity sensor, as demonstrated in our experimental results, make it an attractive option for environmental monitoring or biomedical diagnosis.

Preparation of Polytetrafluoroethylene Superhydrophobic Materials by Femtosecond Laser Processing Technology

In recent years, superhydrophobic surfaces have attracted significant attention due to their promising applications, especially in ice prevention, reduction in air resistance, and self-cleaning. This study utilizes femtosecond laser processing technology to prepare different surface microstructures on polytetrafluoroethylene (PTFE) surfaces. Through experiments, it investigates the relationship between the solid-liquid contact ratio and surface hydrophobicity. The shape of water droplets on different microstructure surfaces is simulated using ANSYS, and the relationship between surface microstructures and hydrophobicity is explored in the theoretical model. A superhydrophobic surface with a contact angle of up to 166° was obtained by machining grooves with different spacings in polytetrafluoroethylene sheets with femtosecond laser technology. Due to the micro- and nanostructures on the surface, the oleophobicity of the processed oleophilic PTFE surface is enhanced.

Ultra-Short Laser-Assisted Micro-Structure Formations on Mg/Zn Double-Doped Calcium Phosphate Ceramics for Enhanced Antimicrobial Activity

Bacterial infection is one of the most common and harmful medical issues following the implantation of materials and devices in the body leading to antibiotic resistance of diverse bacterial strains. In this work, a novel approach is presented combining adopted laser-based patterning method in addition to doping (Mg and Zn) metal ions to prepare calcium phosphate ceramic substrate, applicable in medicine, with enhanced surface antimicrobial characteristics. The preparation of tablets containing Mg (8.44 mol%) and Zn (2.63 mol%) β-tricalcium phosphate involved biomimetic precipitation of amorphous calcium phosphate in media of simulated body fluid enriched with Mg2+ and Zn2+ ions as well as the presence of valine as an organic additive, followed by step-wise calcination up to 1000 °C. The results from laser processing showed formation of deeper patterns with increased surface roughness (from 4.9 µm to 9.4 µm) as laser power and velocity increase, keeping constant the hatch sizes of 50 µm. The textured surfaces consist of peaks and valleys arrangement that change the morphology of Escherichia coli cells and decrease of cell viability. Our study reveals the possibilities of the application of ultra-short laser radiation as a potential alternative therapy for controlling the antimicrobial effect of the ceramic surface.

Cultivation and Transplantation of Three-Dimensional Skins with Laser-Processed Biodegradable Membranes

For the treatment of irreversible, extensive skin damage, artificial skins or cultured skins are useful when allogeneic skins are unavailable. However, most of them lack vasculature, causing delayed perfusion and hence delay or failure in engraftment of the tissues. We previously developed a prevascularized three-dimensional (3D) cultured skin based on the layer-by-layer cell coating technique (LbL-3D skin), in which cells are seeded and laminated on a porous polymer membrane for medium supply to the thick cultured tissue. Recent animal studies have demonstrated that LbL-3D skin can achieve rapid perfusion and high graft survival after transplantation. However, there were practical issues with separating LbL-3D skins from the membranes before transplantation and the handling separated LbL-3D skins for transplantation. To address these problems, in this study, we examined the use of biodegradable porous polymer membranes that enabled the transplantation of LbL-3D skins together with the membranes, which could be decomposed after transplantation. Thin films made from poly (lactic-co-glycolic acid) (PLGA) were irradiated with femtosecond laser pulses to create micro through-holes, producing porous membranes. We designed and fabricated culture inserts with the PLGA membranes and cultivated LbL-3D skins with 2 × 106 neonatal normal human dermal fibroblasts and 1 × 104 human umbilical vein endothelial cells in the dermis of 20 cell layers and 1 × 105 neonatal human epidermal keratinocytes in the epidermis. Histological analyses revealed that the skins cultured on the PLGA membranes had thickness of about 400 μm and that there were no defects in the quality of the skins cultured on the PLGA membranes when compared with those cultured on the conventional (nonbiodegradable) commercial membranes. The cultured LbL-3D skins were then transplanted together with the PLGA membranes onto full-thickness excisional wounds in mice. At 7 days posttransplantation onto a mouse, the tissues above and below the membrane were connected through the holes with collagen-positive fibers that appeared to migrate from both the host and donor sides, and favorable reepithelization was observed throughout the transplanted skin region. However, insufficient engraftment was observed in some cases. Thus, further optimization of the membrane conditions would be needed to improve the transplantation outcome.

Formation of Periodic Surface Structures by Multipulse Femtosecond Laser Processing of Au-Coated Ni in Various Fluids

Using multipulse linearly polarized femtosecond laser processing of a Au-coated Ni surface in various liquid media created subwavelength self-organized nanoripples. The thin gold film improved the laser absorptivity, decreasing the ripple generation threshold in liquids. High spatial frequency ripples exhibited lower angular deviation than low spatial frequency ones, but in water the deviation was comparable for both types of ripples. The initiation of nanoripples may precede nanoparticle generation, which is why in hexane several cuboid Au particles were trapped between the ripples. Fast cooling processes freeze ejected molten droplets during the phase explosion and surface reorganization. Grazing incidence X-ray diffraction of samples processed in butanol showed a small shift toward smaller angles for the Ni phase, indicating a lattice expansion due to higher tensile stress. Confocal micro-Raman spectroscopy detected surface graphitization and amorphization in areas laser-treated in ethanol, butanol, and hexane, with the highest carbonization observed in butanol. Presumably, femtosecond laser-induced photolysis triggers the formation of graphite nanocrystallites, and consecutive pulses cause amorphization. Static contact angle measurements showed a general tendency toward hydrophobicity with highest contact angles for rippled areas created in butanol.

Static Hydrophobic Cuprous Oxide Surface Fabricated via One-Step Laser-Induced Oxidation of a Copper Substrate

In this study, we developed a one-step method for fabricating hydrophobic surfaces on copper (Cu) substrates. Cuprous oxide (Cu₂O) with low free energy was successfully formed after low-fluence laser direct irradiation. The formation of Cu₂O enhanced the hydrophobicity of the Cu substrate surface, and the contact angle linearly increased with the proportion of Cu₂O. The Cu₂O fabricated by low-fluence laser treatment showed the same crystal plane orientation as the pristine Cu substrate, implying an epitaxial growth of Cu₂O on a Cu substrate.

Locally Thinned, Core-Shell Nanowire-Integrated Multi-gate MoS2 Transistors for Active Control of Extendable Logic

Field-effect transistor (FET) devices with multi-gate coupled structures usually exhibit special electrical properties and are suitable for fabricating multifunctional devices. Among them, the 1D nanowire gate configuration has become a promising gate design to tailor 2D FET performances. However, due to possible short circuiting induced by nanowire contact and the high requirement for precision manipulation, the integration of multi-nanowires as gates in a single 2D electronic system remains a grand challenge. Herein, local laser--thinned multiple core-shell SiC@SiO₂ nanowires are successfully integrated into MoS₂ transistors as multi-gates for active control of extendable logic applications. Nanowire gates (NGs) locally enhance the carrier transportation, and the use of multiple NGs can achieve designed band structures to tune the performance of the device. For core-shell structures, a semiconducting core is used to introduce a gate bias, and the insulating shell provides protection against short circuiting between NGs, facilitating nanowire assembly. Furthermore, a global control gate is introduced to co-tune the overall electrical characteristics, while active control of logic devices and extendable inputs are achieved based on this model. This work proposes a novel nanowire multi-gate configuration, which provides possibilities for localized, precise control of band structures and the fabrication of highly integrated, multifunctional, and controllable nano-devices.

Evaporative and Wicking Functionalities at Hot Airflows of Laser Nano-/Microstructured Ti-6Al-4V Material

A novel multifunctional material with efficient wicking and evaporative functionalities was fabricated using hierarchical surface nano-/microstructuring by femtosecond laser micromachining. The created material exhibits excellent multifunctional performance. Our experiments in a wind tunnel demonstrate its good wicking and evaporative functionalities under the conditions of high-temperature airflows. An important finding of this work is the significantly enhanced evaporation rate of the created material compared with the free water surface. The obtained results provide a platform for the practical implementation of Maisotsenko-cycle cooling technologies for substantially increasing efficiency in power generation, thermal management, and other evaporation-based technologies. The developed multifunctional material demonstrates long-lasting wicking and evaporative functionalities that are resistant to degradation under high-temperature airflows, indicating its suitability for practical applications.

Femtosecond Laser Induced Lattice Deformation in KTN Crystal

In recent years, many novel optical phenomena have been discovered based on perovskite materials, but the practical applications are limited because of the difficulties of device fabrication. Here, we propose a method to directly induce localized lattice modification inside the potassium tantalate niobate crystal by using the femtosecond laser. This selective modification at the processed regions and the surrounding areas is characterized by two-dimensional Raman spectrum mapping. The spectrum variations corresponding to specific lattice vibration modes demonstrate the lattice structure deformation. In this way, the lattice expansion at the femtosecond laser irradiated regions and the lattice compression at the surrounding areas are revealed. Furthermore, surface morphology measurement confirms this lattice expansion and suggests the extension of lattice structure along the space diagonal direction. Moreover, the existence of an amorphization core is revealed. These modifications on the sample lattice can induce localized changes in physicochemical properties; therefore, this method can realize the fabrication of both linear diffraction and nonlinear frequency conversion devices by utilizing the novel optical responses of perovskite materials.

Femtosecond Laser Processing Technology for Anti-Reflection Surfaces of Hard Materials

The anti-reflection properties of hard material surfaces are of great significance in the fields of infrared imaging, optoelectronic devices, and aerospace. Femtosecond laser processing has drawn a lot of attentions in the field of optics as an innovative, efficient, and green micro-nano processing method. The anti-reflection surface prepared on hard materials by femtosecond laser processing technology has good anti-reflection properties under a broad spectrum with all angles, effectively suppresses reflection, and improves light transmittance/absorption. In this review, the recent advances on femtosecond laser processing of anti-reflection surfaces on hard materials are summarized. The principle of anti-reflection structure and the selection of anti-reflection materials in different applications are elaborated upon. Finally, the limitations and challenges of the current anti-reflection surface are discussed, and the future development trend of the anti-reflection surface are prospected.

Polarization of Femtosecond Laser for Titanium Alloy Nanopatterning Influences Osteoblastic Differentiation

Ultrashort pulse lasers have significant advantages over conventional continuous wave and long pulse lasers for the texturing of metallic surfaces, especially for nanoscale surface structure patterning. Furthermore, ultrafast laser beam polarization allows for the precise control of the spatial alignment of nanotextures imprinted on titanium-based implant surfaces. In this article, we report the biological effect of beam polarization on human mesenchymal stem cell differentiation. We created, on polished titanium-6aluminum-4vanadium (Ti-6Al-4V) plates, a laser-induced periodic surface structure (LIPSS) using linear or azimuthal polarization of infrared beams to generate linear or radial LIPSS, respectively. The main difference between the two surfaces was the microstructural anisotropy of the linear LIPSS and the isotropy of the radial LIPSS. At 7 d post seeding, cells on the radial LIPSS surface showed the highest extracellular fibronectin production. At 14 days, qRT-PCR showed on the same surface an increase in osteogenesis-related genes, such as alkaline phosphatase and osterix. At 21 d, mineralization clusters indicative of final osteoinduction were more abundant on the radial LIPSS. Taken together, we identified that creating more isotropic than linear surfaces enhances cell differentiation, resulting in an improved osseointegration. Thus, the fine tuning of ultrashort pulse lasers may be a promising new route for the functionalization of medical implants.

Deep Subwavelength Laser-Induced Periodic Surface Structures on Silicon as a Novel Multifunctional Biosensing Platform

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.

Superwicking Functionality of Femtosecond Laser Textured Aluminum at High Temperatures

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.

Near-Infrared Femtosecond Laser Ablation of Au-Coated Ni: Effect of Organic Fluids and Water on Crater Morphology, Ablation Efficiency and Hydrodynamic Properties of NiAu Nanoparticles

Scanning electron microscopy (SEM) and profilometry of the crater morphology and ablation efficiency upon femtosecond laser ablation of Au-coated Ni targets in various fluids revealed a pronounced dependence on the ablation medium. For ethanol, a sufficient ablation efficiency was obtained, whereas for 2-butanol a higher efficiency indicated stronger laser-target interaction. Hierarchical features in the crater periphery pointed to asymmetrical energy deposition or a residual effect of the Coulomb-explosion-initiating ablation. Significant beam deviation in 2-butanol caused maximum multiple scattering at the crater bottom. The highest values of microstrain and increased grain size, obtained from Williamson-Hall plots, indicated the superposition of mechanical stress, defect formation and propagation of fatigue cracks in the crater circumference. For n-hexane, deposition of frozen droplets in the outer crater region suggested a femtosecond-laser-induced phase explosion. A maximum ablation depth occurred in water, likely due to its high cooling efficiency. Grazing incidence micro X-ray diffraction (GIXRD) of the used target showed residual carbon and partial surface oxidation. The produced nanoparticle colloids were examined by multiangle dynamic light scattering (DLS), employing larger scattering angles for higher sensitivity toward smaller nanoparticles. The smallest nanoparticles were obtained in 2-butanol and ethanol. In n-hexane, floating carbon flakes originated from femtosecond-laser-induced solvent decomposition.

Femtosecond Laser-Pulse-Induced Surface Cleavage of Zinc Oxide Substrate

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.

Spreading and Drying Dynamics of Water Drop on Hot Surface of Superwicking Ti-6Al-4V Alloy Material Fabricated by Femtosecond Laser

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.

Temperature Effect on Capillary Flow Dynamics in 1D Array of Open Nanotextured Microchannels Produced by Femtosecond Laser on Silicon

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.

Three-Dimensional Laser-Assisted Patterning of Blue-Emissive Metal Halide Perovskite Nanocrystals inside a Glass with Switchable Photoluminescence

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.

Bioinspired Zoom Compound Eyes Enable Variable-Focus Imaging

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.

A Comparison between Nanogratings-Based and Stress-Engineered Waveplates Written by Femtosecond Laser in Silica

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.

The Role of the Laser-Induced Oxide Layer in the Formation of Laser-Induced Periodic Surface Structures

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.

Femtosecond Laser Fabrication of Stable Hydrophilic and Anti-Corrosive Steel Surfaces

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.

Picoliter Cuvette inside an Optical Fiber to Track Gold Nanoparticle Aggregation for Measurement of Biomolecules

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.

Femtosecond Laser Fabrication of Engineered Functional Surfaces Based on Biodegradable Polymer and Biopolymer/Ceramic Composite Thin Films

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 (R_rms) 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.

3D Biomimetic Chips for Cancer Cell Migration in Nanometer-Sized Spaces Using "Ship-in-a-Bottle" Femtosecond Laser Processing

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.

Femtosecond Laser Texturing of Surfaces for Tribological Applications

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.

Parity violation in chiral structure creation under femtosecond laser irradiation in silica glass?

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.

Triangular gold nanoplate growth by oriented attachment of Au seeds generated by strong field laser reduction

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.

Laser-Induced, Polarization Dependent Shape Transformation of Au/Ag Nanoparticles in Glass

Bimetallic, initially spherical Ag/Au nanoparticles in glass prepared by ion implantation have been irradiated with intense femtosecond laser pulses at intensities still below the damage threshold of the material surface. This high-intensity laser processing produces dichroism in the irradiated region, which can be assigned to the observed anisotropic nanoparticle shapes with preferential orientation of the longer particle axis along the direction of laser polarization. In addition, the particle sizes have considerably been increased upon processing.