Femtosecond Laser Pulse Irradiation Research Articles

Fabrication of Ge2Sb2Te5 crystal micro/nanostructures through single-shot Gaussian-shape femtosecond laser pulse irradiation.

Femtosecond (fs) laser-thin film interaction is one of the most practical methods for fabricating functional nanostructures. However, the details of the interaction mechanism remain unclear. In this study, we demonstrate an abnormal ablation effect on nanofilms by using a tightly focused single fs laser pulse. After the irradiation of a single Gaussian-shaped femtosecond laser pulse, a molten micro/nanopatch at the irradiated central high-power zone is isolated from the surrounding film. The confined localized threshold effect is proposed as the main mechanism for the phase isolation. With this effect, the high refractive index dielectric Ge2Sb2Te5 crystal nanostructures can be fabricated by directed dewetting of the isolated molten micro/nanopatch on Si substrates. After the laser irradiation, the central isolated liquid through an amorphous GST film is transformed into a crystalline state after resolidification. The isolated central micro/nanopatch size can be controlled by the focused spot size and pulse energy, so that the morphologies (size, geometrical morphology, and distribution) of GST nanostructures can be flexibly modulated. Furthermore, separated solid and liquid phase states detected using spatial-temporal-resolved microscopy validates the crucial role of the confined-localized threshold effect in the dewetting effect based on the separated liquid phase.

Femtosecond pulsed laser-induced interconnection of single-walled carbon nanotubes

With the development of electronic information technology, the fabrication of transistors based on bulk (3D) semiconductor materials has encountered bottlenecks. In view of this situation, many researchers have focused on manufacturing two-dimensional nanostructures from one-dimensional nanomaterials to apply to transistors. Based on the experimental platform of femtosecond pulsed laser irradiation system, one-dimensional single-walled carbon nanotubes (SWCNTs) deposited on silica substrate were induced to interconnect, and the optimal process parameters of the interconnection were explored. Then compared with randomly distributed SWCNTs films, the square resistance of SWCNTs films after interconnection was significantly reduced, which provided inspiration for the subsequent preparation of high-performance carbon-based thin film transistors.

Rotationally symmetric colorization of metal surfaces through omnidirectional femtosecond laser-induced periodic surface structures.

Following femtosecond (fs) laser pulse irradiation, the formation of a new type of low-spatial-frequency laser-induced periodic surface structure (LSFL) patterns, namely, omnidirectional LSFLs (OD-LSFLs) with the periodic ordering of their orientations, are investigated on Ni in this Letter. Using a liquid crystal polymer patterned depolarizer, we periodically rotate the polarization of fs laser pulses across the laser spot and create OD-LSFLs by raster scanning fs laser pulses. We also show that the period of the OD-LSFL orientation rotation can be controlled with the defocused distance, and OD-LSFLs can significantly expand the viewing angle of the structural colors in the azimuthal direction without noticeable color degradations.

Enhanced selective solar absorption of surface nanotextured semi-insulating 6H–SiC

Surface treatments were performed on single-crystal semi-insulating 6H–SiC by femtosecond pulsed laser irradiation, aimed at analyzing the effect of the laser-induced periodic surface structures (LIPSSs) on the films’ optical properties. The surface morphology study of the laser-induced nanostructures allows determining the modification threshold fluence of about 0.7 J cm−2 as well as detecting fine (160 nm) and coarse (450–550 nm) ripples according to different values of the laser pulse fluence (ΦP) released to the material. Micro-Raman spectroscopy allows determining the presence of undesired amorphous structural phases when ΦP exceeds 2.15 J cm−2, whereas no compositional variations occur for lower values of ΦP. Samples treated on the entire surface with the pulse fluence conditions to obtain fine ripples were optically tested. Although the long-range order is progressively lost as the accumulated laser fluence increases, the heaviest treated samples show solar absorptance values > 75% and spectral selectivity up to 1.7 projected at the operating temperature of 1000 K, thus pointing out the suitability of fs-laser surface textured 6H–SiC to act as a selective solar absorber for energy conversion devices operating at high temperature.

Realization of ∼10 nm Features on Semiconductor Surfaces via Femtosecond Laser Direct Patterning in Far Field and in Ambient Air.

Direct fabrication of ∼10 nm features by optical means in far field and in ambient air on semiconductor surfaces is significant for next-generation advances nanomanufacturing. We report here a new method that enables the direct formation of 12 nm (λ/66) features on silicon surfaces. It is processed in far field and in ambient air via the irradiation of orthogonally polarized double femtosecond laser beams. The coupling of orthogonally polarized double femtosecond laser beams and the incubation effect due to multiple femtosecond laser pulses irradiation under high repetition rate enable the 12 nm nanostructures creation parallel to the scanning direction, regardless of scanning path.

Nonlinear Optics to Glucose Sensing: Multifunctional Nitrogen and Boron Doped Carbon Dots with Solid‐State Fluorescence in Nanoporous Silica Films

AbstractMultifunctional triple color photoluminescent (PL) nitrogen–boron doped carbon quantum dots (CQDs) with high quantum yield (QY) of 58% are fabricated by one step femtosecond pulsed laser irradiation of a single precursor (2‐aminopyrimidine‐5‐boronic acid) in solution. In situ generated non‐linear and linear emissions are used to monitor CQDs formation which results in enhanced second harmonic generation, two photon absorption (2PA), and linear fluorescence; implying triple mode emission. These CQDs present blue, green, and possible red color rendering which are mostly independent to the respective excitation wavelengths (λ) with large stokes shift of 100 nm. Solid‐state photoluminescence with QY of 46% is achieved by incorporating CQDs into thin transparent nanoporous silica (pSiO2) films (thickness 50 µm) to form a CQDs‐pSiO2 composite which exhibits reverse saturable absorption at λ = 800 nm with 2PA coefficient and excited state absorption cross‐section of 4.94 × 10−10 m W−1 and 6.23 × 10−17 cm2, respectively. CQDs‐pSiO2 is also sensitive to glucose concentration down to 1.0 mg dL−1 in a wide linear range up to 100 mg dL−1. This work therefore demonstrates facile, controllable, and up‐scalable bottom‐up fabrication of CQDs forming multifunctional solid‐state CQDs‐pSiO2 with proven application in optical limiting and glucose sensing.

Research on Charge Transfer Characteristics Induced by Laser Irradiation Monocrystalline Silicon

In order to study the charge characteristics of monocrystalline silicon irradiated by femtosecond pulsed laser, measurement systems of charge and infrared temperature induced by femtosecond pulse laser irradiation monocrystalline silicon are established by using oscilloscope and infrared thermal imager. The monocrystalline silicon (circle disc) irradiated by femtosecond pulse laser are carried out along the thickness direction of monocrystalline silicon. The experimental results show that the thermo-mechanical coupling effect of femtosecond pulsed laser irradiated on monocrystalline silicon will produce polarization in the target. At the initial stage of laser irradiation, the laser damage threshold of the interface between crystal surface and air is low and the charge mainly transfers along the surface of monocrystalline silicon, resulting in a high polarization. With the continuation of laser irradiation, it proves that the charge transfers in monocrystalline silicon gradually, and the polarization degree decreases with the modification of the material internal structure. After laser irradiation for a certain time, the charge transfers tend to be stable. Because the target lies in front of the laser focus, its energy density is insufficient to destroy the macrostructure of the target due to laser-pressure and temperature gradient, the polarization effect tends to be stable, resulting in a stable periodic change of charge transfer and recovery.

Producing anomalous uniform periodic nanostructures on Cr thin films by femtosecond laser irradiation in vacuum.

We report on producing unprecedentedly uniform periodic structures on chromium thin films in vacuum conditions with irradiation of femtosecond laser pulses. In sharp contrast to the observations in air, the achieved surface structures of the ablated groove arrays are surprisingly found to have not only an extraordinarily uniform distribution but also a deep-subwavelength period of 360 nm. The measured both width and depth of the ablated periodic grooves are 150 and 120 nm, respectively, showing a large depth-to-width ratio and sharp-edge profiles. Remarkably, such well-organized nanostructures can be enabled to robustly extend into an infinitely long range via the sample scanning and even have a large-area production with a cylindrical lens. Raman spectral analyses reveal that the regular formation of such nanostructures benefits from avoiding the material oxidation and thermal disturbance of the air plasma on the sample surface.

Ultrafast temporal-spatial dynamics of amorphous-to-crystalline phase transition in Ge2Sb2Te5 thin film triggered by multiple femtosecond laser pulses irradiation

Studies have shown that the crystallization phase state of Ge2Sb2Te5 (GST) can be reversibly modulated by femtosecond (fs) laser multiple pulses, which have excellent applications in reconfigurable multi-level operation fields. In this study, the temporal-spatial crystalline evolution dynamics of amorphous GST film is investigated during two fs laser pulses excitation through a pump-probe shadowgraph imaging technique. A quasi-amorphous phase state, which is different from that in the initial as-deposited amorphous GST, is emerged through the first fs laser pulse excitation with a pulse energy lower than crystallization threshold. The experimental results reveal that a crystallization enhancement effect can be induced through the second pulse excitation based on this quasi-amorphous surface structure. The stimulative cluster generated in the quasi-amorphous reduces the amorphous-to-crystalline phase transition threshold for the second fs laser pulse irradiation. The spatially-resolved phase-transition threshold extension effect in a horizontal direction is proposed with the increasing pulse number to summarize the mechanism of the crystallization enhancement effect. The specific-grain-appearance (coarse grains and fine grains representing different phase transition approach) distributed area induced by single and double fs laser pulses irradiation are experimentally demonstrated corresponding to threshold extension theory.

Study on Femtosecond Laser Processing Characteristics of Nano-Crystalline CVD Diamond Coating

Ultra-short pulse laser interaction with diamond materials has attracted extensive interest in micro- and nano-machining, especially for the fabrication of micro tools, because of the straightforward method and high precision. Thanks to the development of chemical vapor deposition (CVD) technology, high-quality CVD diamonds are employed in more varieties of tools as performance-enhancing coatings. The purpose of the experiments reported here was to explore the machinability of CVD diamond coating under the irradiation of femtosecond (fs) pulsed laser. The factor-control approach was adopted to investigate the influence of scanning speed, single pulse energy and repetition rate on the surface quality and carbon phase transition of CVD diamond coating. The material removal rate and surface roughness were evaluated. The interaction mechanism of scanning speed, single pulse energy, and repetition rate were discussed, and the fs laser ablation threshold of CVD diamond coating was calculated. It was demonstrated that two ablation mechanisms (weak and intensive) were in existence as evidenced by the distinct surface morphologies induced under different processing conditions. A strong dependence on the variation of scanning speed and pulse energy is identified in the examination of surface roughness and removal rate. Lorentzian–Gaussian deconvolution of Raman spectra illustrates that fs laser irradiation yields a strong modification effect on the coating and release the compressive stress in it. Furthermore, a newly defined parameter referring to the fs laser energies applied to unit volume was introduced to depict the degree of ablation and the Taguchi method was used to figure out the significance of different parameters. The ablation threshold of CVD diamond coating at the effective pulses of 90 is calculated to be 0.138 J/cm2.

Nanodeformation in enstatite single crystals: Simulation of micrometeoroid impacts by femtosecond pulsed laser experiments

Space weathering by micrometeoroid bombardment is a cosmic phenomenon on atmosphere-free celestial bodies, a process that is expected to particularly overprint planetesimals and cosmic dust in debris discs. We reproduced micrometeoroid impact craters by femtosecond pulsed laser irradiation on oriented enstatite single crystals (En93Fs7) to investigate the deformation behavior and its orientation dependence. All microcraters show typical bowl shaped morphologies, a glass surface layer with splash like ejecta material and subsurface layering. Although we could reproduce melting and vaporization as typical space weathering effects in the enstatite experiments, there is no formation of agglutinate particles or metallic nanoparticles (npFe0). The shock effects in the deformation layer consist of planar structures like microfractures and cleavages, amorphous lamellae, stacking faults and clinoenstatite lamellae. Their activation and/or orientation depends on the shock direction. In special orientations we observe the activation of glide systems along specific low indexed crystallographic planes. Due to the short timescale and the high strain rates, the most prominent effect is the failure of enstatite by microfracturing along non-rational crystallographic planes. Common deformation mechanisms reported in meteorites like the formation of clinoenstatite lamellae via shearing along [001] (100) occur less frequently. Shear is apparently the dominant mechanism in the formation of the above-mentioned effects and causes also their modification by frictional heating. The wide-spread formation of amorphous lamellae is, for example, interpreted to be the result of this shear heating along planar structures. We interpret this unconventional deformation behavior as a consequence of the small spatial and temporal scale of the experiments, resulting in a short-lived spherical shock wave with high deviatoric stresses in contrast to a long pressure pulse and quasi-hydrostatic compression in large scale impacts that produce typical shock features.

All-solid-state supercapacitors from natural lignin-based composite film by laser direct writing

Lignin as a renewable natural resource has been the focus of numerical interest in applications ranging from pitch to porous carbon material. Herein, a facile approach is reported to transform lignin into porous conductive carbon structures and interdigitated circuits for supercapacitor devices using femtosecond laser direct writing. Comparative studies revealed that the laser irradiation induced tetrahedral amorphous carbon while only graphitic carbon was obtained through pyrolysis. Meanwhile, the composite membrane was easily prepared to further optimize the capacities by mixing functional materials (MoS2) into the lignin/polyacrylonitrile (PAN) composite polymers. A MoS2 decorated porous carbon network material could be fabricated through focused femtosecond pulsed laser irradiation of the corresponding composite membranes. The microstructure and spectroscopic features of these laser induced hybrid carbon materials have been deeply investigated. The supercapacitor based on lignin/PAN has high areal specific capacitances of 6.7 mF cm−2 (0.9 F cm−3) at 10 mV s−1. Moreover, doped microsupercapacitors with MoS2 demonstrated enhanced areal capacitances up to 16 mF cm−2 (2.2 F cm−3) and at 10 mV s−1, respectively. The relatively high areal capacitances indicate that the proposed method is potential for innovative manufacturing energy storage devices based on natural lignin.

Controllable Formation of Si Nanostructures Based on Quasi-Plasmonic Planar Nanostructures Formed by Annular-Shaped Femtosecond Laser Pulse

We demonstrate the feasibility to control the confined electromagnetic field by surface plasmon poloritons (SPPs) excitation and scattering modulation based on the pre-processed Au nanostructures under the irradiation of single-shot femtosecond (fs) laser pulse, and then to control the surface tension-induced molten Si transport due to the directed lateral temperature profile, thus resulting in the formation of Si nanostructures. Annular-shaped fs laser pulse is employed based on nonlinear optical effect of frequency-doubling process to imprint Au planar nanostructures with various morphologies. Because surface plasmon polaritons can be easily excited and controlled on planar nanostructures of noble metals, the localized electromagnetic field can be effectively modulated by controlling the metallic nanostructures. Thus, the quasi-plasmonic Au nanostructures act as precursors for controlling the subsequent electromagnetic field by fs laser irradiation. Upon irradiation of pre-processed Au nanostructures, confined electromagnetic field induces a specific surface tension profile, causing directional transport of molten Si. After solidification, Si nanostructures of controllable morphologies can be fabricated. Simulation results are in agreement with the theoretical mechanism. Moreover, further optimized modulation of nanodome can be achieved by controlling the focus spot control combined with the polarization state control. This paper provides an effective method for enabling a scalable formation of Si nanostructures.

Formation of controllable 1D and 2D periodic surface structures on cobalt by femtosecond double pulse laser irradiation

Controlling the surface morphology at the subwavelength scale is one of the cornerstones of modern nanophotonics. Femtosecond laser-induced surface structuring is a viable technique for the large-scale formation of nano- and microscale structures. A typical example is the formation of one-dimensional laser-induced periodic surface structures (LIPSSs), which can lead to strong modifications of optical and wetting properties of the material surface. Creating two-dimensional (2D) patterned structures, however, is a more challenging and rewarding task. Here, we demonstrate a single step method for fabricating various subwavelength structures on the cobalt (Co) surface using different laser fluences (0.12–0.24 J/cm2) and time delay (0–30 ps) between double pulses. More importantly, we can control the geometry and organization of the formed structures demonstrating spherical, triangular, rhombic, and high spatial frequency LIPSSs using two temporally delayed orthogonally polarized femtosecond laser beams. We show that the laser fluence and delay time between the two beams are the controlling parameters for realizing the different surface morphologies. We provide a numerically supported, phenomenological model to explain the formed 2D structures. Our model employs elements from both the scattered surface-wave interference and the self-organization theories that are commonly used to explain uniform surface structures.

One-step fabrication of bi- and quad-directional femtosecond laser-induced periodic surface structures on metal with a depolarizer

With femtosecond (fs) laser pulse irradiation, a simple but efficient technique to fabricate bi- and quad-directional low spatial frequency laser-induced periodic surface structures (BD- and QD-LSFLs) is suggested on metals. By inserting a quartz wedge achromatic depolarizer prior to the focusing optics, we change the polarization of fs laser pulses periodically in one dimension within the laser spot, and find that this method leads to the creation of BD- and QD-LSFLs by simply raster scanning fs laser pulses on Ni. It is also demonstrated that the interval of orientation change of BD-LSFLs can be manipulated with defocus.

Observation of Second Harmonic Generation in Lightning-Bolt-Like Shaped Nanostructured Metasurface

Second harmonic generation (SHG) was observed on irradiation of a symmetry-broken gold nanostructured surface with an ultrashort pulse of low input power. A metasurface consisting of lightning-bolt-like shaped gold (Au) nanostructures in a periodic arrangement was fabricated on a glass substrate and utilized to excite surface plasmons. The measured extinction spectrum had a dual band maximized at approximately 605 nm and 786 nm under illumination with linearly polarized light. The two-photon induced photoluminescence and SHG intensity were dependent on the polarization states and wavelength of the femtosecond laser pulse irradiation. Furthermore, the SHG intensity peak was linearly dependent on the input power of the incident laser beam, and the spectral property of the emitted second harmonic was correlated to the extinction spectrum of the nanostructured metasurface. The SHG peak over the pumping wavelengths was resonant with a transverse multipolar plasmon mode. The findings provide useful guidelines for efficient production of second harmonic luminescence.

Visualization of vermilion degradation using pump-probe microscopy

Here, we demonstrate the use of pump-probe microscopy for high-resolution studies of vermilion degradation. Vermilion (mostly α-HgS), an important red pigment used in historical paintings, blackens over time, and metallic Hg and β-HgS have been implicated as possible degradation products. Conventional analysis techniques have trouble differentiating α- and β-HgS with sufficiently high spatial resolution. However, pump-probe microscopy can differentiate metallic mercury, α- and β-HgS, and map each distribution on the microscopic scale. We studied artificial degradation of α-HgS; femtosecond-pulsed laser irradiation induces an irreversible phase shift of α- to β-HgS, in which the initial presence of β-HgS grains can increase the rate of conversion in their vicinity. Continuous ultraviolet exposure instead generates both liquid Hg and β-HgS, with a conversion rate that increases with elevated temperatures. Last, we reveal the presence of β-HgS as a natural degradation product in discolored vermilion layers in a 14th century Italian painting.

Reversible surface functionalization of motor proteins for sustainable motility

Kinesins and microtubules are biological motor proteins and rail fibers, respectively. Their combination could be an attractive power source for nanotools. Their movements can be easily reproduced in vitro by the motility assay method. However, the durability of the protein material remains relatively short for use in devices. Herein, regeneration of the active substrate by removing surface-adsorbed kinesins by multi-photon laser ablation and reloading additional kinesin molecules was attempted. The effect of femtosecond laser pulse irradiation to the surface-adsorbed kinesins was investigated by atomic force microscopy and by the microtubule driving performance. The findings of these investigations suggest the successful exchange of kinesins, which leads to improvement of the durability of microtubule driving performance.

Formation of uniform two-dimensional subwavelength structures by delayed triple femtosecond laser pulse irradiation.

The fabrication of subwavelength two-dimensional (2D) structures on metals is of paramount importance to modern nanophotonics. Here we report a method to fabricate 2D conic structures on nickel surfaces using a single beam with three temporally delayed pulses. The 2D structures are fabricated over the entire irradiated region with relatively high uniformity. By controlling the delay between the three pulses, we control the effect of each pulse in creating laser-induced periodic surface structures which enables the control of the 2D structure features, namely, the period and structure dimensions. We explain the results based on the surface plasmon polariton-femtosecond laser interference model.

Circular Depressed Cladding Waveguides in Mechanically Robust, Biocompatible nc-YSZ Transparent Ceramics by fs Laser Pulses

We report on the fabrication of depressed cladding waveguides inscribed by direct femtosecond laser pulses irradiation in transparent nc-yttria stabilized zirconia (YSZ) polycrystalline ceramics. The waveguides consisted of two different circular structures with diameters of 30 and 100 $\mu \text{m}$ , and different lengths. Optical characterization at 633 $\text{}$ and 1568 nm was performed. The obtained modal profiles show dependence on the structure and size of the waveguides. We estimate a laser-induced refractive index contrast of nearly 10 $^{-2}$ , larger than can be explained by induced residual stresses alone. We conjecture that laser interactions lead to localized physical damage, i.e., microvoids cracks and possibly a decreased degree of crystallinity in the laser processed zones, which is supported by Raman measurements. Our results suggest a promising potential use of nc-YSZ in photonic bio-applications.