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Infrared antireflection of a zinc sulfide (ZnS) surface is important to improve performance of infrared detector systems. In this paper, double-pulse femtosecond laser micro-machining is proposed to fabricate a subwavelength quasi-periodic array (SQA) on ZnS substrate for infrared antireflection. The SQA consisting of approximately 30 million holes within a 2 × 2 cm2 area is uniformly formed in a short time. The double-pulse beam can effectively suppress the surface plasma shielding effect, resulting in obtaining a larger array depth. Further, the SQA depth is tunable by changing pulse energy and pulse delay, and can be used to readily regulate the infrared transmittance spectra as well as hydrophobicity. Additionally, the optical field intensity distributions of the SQA simulated by the rigorous coupled-wave analysis method indicate the modulation effect by the array depth. Finally, the infrared imaging quality captured through an infrared window embedded SQA is evaluated by a self-built infrared detection system. [ABSTRACT FROM AUTHOR]

As a special kind of Bragg grating, phase-shifted fiber Bragg grating (PS-FBG) has attracted extensive attention because of its extremely narrow transmission window and excellent sensing performance. The main purpose of this manuscript is to discuss the PS-FBG with special sensing characteristics and explore the influence of different inscription technologies on the sensing characteristics of PS-FBG by comparing the existing inscription methods. The sensing characteristics, advantages and disadvantages of PS-FBG with different structures are analyzed. [ABSTRACT FROM AUTHOR]

Silver nanowires (AgNWs) conductors with fine flexibility and high conductivity have been considered to have a promising perspective in the area of flexible transparent electrode. However, many challenges still exist in improving the conductivity of AgNWs films, such as high temperature and time‐consuming. Herein, an efficient and rapid method to weld AgNWs by spatial light modulated femtosecond (fs) laser is proposed. The Gaussian laser beam is modulated into a line‐shaped laser beam with the length of 773 μm. Compared with traditional spherical lens focus irradiation welding, the welding efficiency can be improved up to tens of times. The mechanism of fs laser welding is mainly to use high‐energy laser beam irradiation to generate local field enhancement which causes local high temperature at the nanowire junction. After laser welding, the sheet resistance of AgNWs films can be reduced to 14.7 Ω sq−1 at the transmittance of 89.5% without substrate damage. The welded AgNWs films exhibit excellent electrical conductivity even after 10 000 bending cycles. Moreover, the welded AgNWs films are used to make a flexible transparent heater, showing superior sheet resistance uniformity. [ABSTRACT FROM AUTHOR]

In this research, amplitude Fresnel zone plates (FZPs) were fabricated on fused silica glass with femtosecond laser. The focusing characteristics including focal length, diffraction efficiency, geometric concentrator ratio, spot energy uniformity, depth of focus and resolution were studied by simulations and experiments. To optimize the focusing characteristics of FZP, we proposed an optimized machining process, an extended FZP and an fractal FZP. The optimized machining process increased the diffraction efficiency of the amplitude FZP from 6.96% to 8.14%, the extended FZP broke through the limit of minimum line width in processing and improved the resolution, and the fractal FZP increased the focus depth from 4 mm to 10 mm. In addition, we wrote a FZP on the end face of multi mode fiber, the divergent annular beam converged and formed an focus ring at the focal plane. This research will provide broad applications in fiber tweezers, beam coupling and optical communication. [ABSTRACT FROM AUTHOR]

Femtosecond laser ablation is a flexible method for manufacturing gallium arsenide (GaAs) micro-optical components. However, its machining efficiency and surface quality do not satisfy industry requirements. Wet etching assisted femtosecond laser ablation is proposed to solve these problems. After corrosion, protrusions and particles are selectively corroded. X-ray diffraction and energy dispersive X-ray spectroscopy results show that oxides are formed after laser ablation, and the crystal cell size of GaAs increases. Thus the contact area between hydrofluoric acid atoms and the GaAs crystal cell increases, the ratio of modified-to-unmodified corrosion rate is as high as 10:1. A one-dimensional grating, a conventional Fresnel zone plate (FZP), and a fractal FZP are fabricated using this method. Experiments show that the GaAs micro-optical component can diffract and focus infrared light with a wavelength of 1.5 µm well. The process offers a way to achieve high efficiency and quality manufacturing of GaAs microstructures. [ABSTRACT FROM AUTHOR]

Herein, an effective method is proposed for the fabrication of sequential colors on copper by adjusting femtosecond laser defocusing distance. The sequential evolution tendency of induced colors by adjusting laser defocusing distance is conducted. It is revealed that such a change of surface optical response is originated from the gradual evolution of three different kinds of structures. The angle‐resolved spectrum of the colored surface is investigated. In addition, the simultaneously derived hydrophobicity of the colored surface is explored. [ABSTRACT FROM AUTHOR]

With the alignment of the fiber core systems containing dual-CCDs and high-precision electric displacement platform, twisted long period fiber gratings (T-LPFGs) were fabricated in two different twisted SMF-28 fibers by femtosecond laser. The torsion characteristics of the T-LPFGs were experimentally and theoretical investigated and demonstrated in this study. The achieved torsion sensitivity is 117.4 pm/(rad/m) in the torsion range −105–0 rad/m with a linearity of 0.9995. Experimental results show that compared with the ordinary long period fiber gratings, the resonance wavelength of the gratings presents an opposite symmetrical shift depending on the twisting direction after the applied torsion is removed. In addition, high sensitivity could be obtained, which is very suitable for the applications in the torsion sensor. These results are important for the design of new torsion sensors based on T-LPFGs fabricated by femtosecond laser. [ABSTRACT FROM AUTHOR]

The high-quality, high-efficiency micro-hole drilling of structural ceramics to improve the thermal conductivity of hot-end parts or achieve high-density electronic packaging is still a technical challenge for conventional processing techniques. Recently, the laser drilling method (LDM) has become the preferred processing tool for structural ceramics, and it plays an irreplaceable role in the industrialized processing of group holes on structural ceramic surfaces. A variety of LDMs such as long pulsed laser drilling, short pulsed laser drilling, ultrafast pulsed laser drilling, liquid-assisted laser drilling, combined pulse laser drilling have been developed to achieved high-quality and high-efficiency micro-hole drilling through controlling the laser–matter interaction. This article reviews the characteristics of different LDMs and systematically compares the morphology, diameter, circularity, taper angle, cross-section, heat affect zone, recast layer, cracks, roughness, micro–nano structure, photothermal effect and photochemical reaction of the drilling. Additionally, exactly what processing parameters and ambient environments are optimal for precise and efficient laser drilling and their recent advancements were analyzed. Finally, a summary and outlook of the LDM technology are also highlighted. [ABSTRACT FROM AUTHOR]

In order to improve the performance of silver nanowire (AgNW) flexible transparent conductive films (FTCFs), including the conductivity, uniformity, and reliability, the welding of high repetition rate femtosecond (fs) laser is applied in this work. Fs laser irradiation can produce local enhancement of electric field, which induce melting at the gap of the AgNWs and enhance electrical conductivity of nanowire networks. The overall resistivity of the laser-welded AgNW FTCFs reduced significantly and the transparency changed slightly. Meanwhile, PET substrates were not damaged during the laser welding procedure in particular parameters. The AgNW FTCFs can achieve a nonuniformity factor of the sheet resistance as 4.6% at an average sheet resistance of 16.1 Ω/sq and transmittance of 91%. The laser-welded AgNW FTCFs also exhibited excellent reliability against mechanical bending over 10,000 cycles. The welding process may open up a new approach for improvement of FTCFs photoelectric property and can be applied in the fabrication of silver nanostructures for flexible optoelectronic and integration of functional devices. [ABSTRACT FROM AUTHOR]

• Copper-coated fiber Bragg grating was developed. • Multilayer FBG with nickel, gold, copper showed high strength and sensitivity. • Multilayer FBG can be applied in extremely high-temperature environments. In this study, a copper coating was deposited on the surface of a fiber Bragg grating (FBG) via electroplating. We compared the temperature responses of the uncoated and copper-coated FBGs from 20 °C to 1000 °C. Under high-temperature conditions, copper diffused into the FBG to form copper crystals, and the copper coating gradually underwent high-temperature oxidation. This reduced the sensitivity and spectral consistency of the copper-coated FBG. A multilayer metal-coated FBG was prepared with an inner layer of nickel, outer layer of gold, and copper layer in the middle. The multilayer FBG exhibited a temperature sensitivity of 23.1–23.53 pm/°C. Furthermore, it showed the coating-enhancement effect and good linearity after multiple thermal cycles. The multilayer metal-coated FBG not only shows high-temperature sensitization but also solves the problem of bare FBGs being extremely fragile after experiencing high temperatures. This study provides a feasible solution for the application of FBGs in extremely high-temperature environments. [ABSTRACT FROM AUTHOR]

[Display omitted] • Broadband light absorption surface is obtained from cross-scale micro-/nanostructures fabricated by femtosecond laser. • Light absorption mechanism is numerically and experimentally studied. • Photo-thermal-electric conversion efficiency of a Seebeck device is improved with such surface integrated on. • Performance parameters and device design of a thermoelectric photodetector are explored. The limited sensitivity and inherent long response time restrict the widespread applicability of thermally-based photodetectors. Herein, a copper surface endowed with optical absorption enhancement by femtosecond laser fabrication is introduced, to improve the sensitivity and power prediction speed of thermoelectric photodetectors. Cross-scale microstructures including periodic micro-taper array and randomly dispersed particles are obtained on the surface, and their formation mechanism is elucidated. The surface exhibits an average reflectivity of 8% over the wavelength range of 200–2500 nm. This broadband absorption is mainly due to the local light field enhancement by the unevenly oxidized and size-dispersed particles, while the periodic micro-taper array further enhances the absorption by geometric light trapping effect. When integrated into a thermoelectric device, a comprehensive parameter analysis including sensitivity (45.053 mV/W), response time (5.47 s), precision of time-dependent prediction (2 s to obtain R 2 ⩾ 0.999), and precision of measuring at different points (3.97%) is performed. Our study offers a practical approach to produce cross-scale light absorption microstructure, paving the way for its promising advances in photothermal-based optoelectronic applications. [ABSTRACT FROM AUTHOR]

• A simple and robust method for fabricating patterned superhydrophobic surfaces by femtosecond laser micromachining is presented. • The wettability and the adhesion force of the patterned surface are tuned by changing the width of the linear pattern. • Based on the modified Furmidge equation, the relationship between the adhesion and the liquid-solid interface width is explained. • The patterned surface has the ability to manage fluid motion, transportation and mixing. Patterned superhydrophobic surfaces have a wide range of applications in terms of microfluidic devices. In this work, a simple and robust method is proposed to fabricate patterned superhydrophobic surfaces by two combined steps including femtosecond laser microfabrication and immersion treatment. The obtained surface exhibits contact angles of 170.0°, 167.6°, and 169.5°, and sliding angles of 5.8°, 6.0°, and 3.9° for 12 μL, 18 μL, and 30 μL water droplets, respectively. The high-adhesion hydrophobic linear pattern combined with the surrounding low-adhesion superhydrophobic surface exhibits controllable wettability and pronounced liquid-sliding anisotropy. Since the contact angle and the adhesion force can be controlled by adjusting the width of the linear pattern, the droplet motion can be directionally modulated. As the line width increases, the sliding angle in parallel direction can be regulated from 18.9° to 90°, from 15.3° to 47.6°, and from 8.6° to 25.0° for 12 μL, 18 μL, and 30 μL water droplets, respectively. Based on the modified Furmidge equation, the relationship between adhesion and liquid-solid interface width can be reasonably explained. Furthermore, the potential applications of the patterned surfaces for droplet transport and mixing are explored. The wettability of the superhydrophobic surfaces to liquids of different types and pH is also investigated. The work presented here has the potential to advance femtosecond laser scanning as a simple, precise, and controllable method for fabricating patterned superhydrophobic surfaces, and to accelerate their applications in parallel reactors, chemical micro-assays, drug delivery systems and so on. [ABSTRACT FROM AUTHOR]

Beam shaping has been widely used in laser processing. Compared with Bessel beam, Bessel-like beam has unique advantages of fabricating high-aspect-ratio holes due to its tunable and longer non-diffraction length. However, the theoretical and experimental optimization of Bessel-like beams for laser manufacture has remained to be investigated. In this paper, we established a theoretical model of the intensity distribution and peak energy density of the Bessel-like beams based on a simplified phase of arc axicon. It was found that the longer propagation length and the lower peak energy density of the Bessel-like beams, contributed to the longer ablation length and higher ablation precision in the silica glass. The ablation length of the optimized Bessel-like beams was at least 1.6 times longer than that of the Bessel beam, which furtherly improved the fabrication efficiency of microdrilling. • Bessel-like beams were generated by an arc axicon. • The intensity distribution and peak energy density of Bessel-like beams were investigated. • The optimized Bessel-like beams ablated longer length and higher precision in silica glass. • Through-microholes array showed considerable precision and uniformity. [ABSTRACT FROM AUTHOR]

In this paper, one-step fabrication of fusiform hierarchical micro/nanostructures on copper surface by femtosecond laser is proposed. According to the preferential valley ablation, Marangoni effect based on melt hydrodynamics and the coupling of the incident light and surface plasmon polaritons, the formation mechanism of the fusiform hierarchical micro/nanostructures is explained. And the effect of laser parameters including the defocusing distance, laser power, and scanning speed on the surface morphology is investigated. Also, the quantitative analysis of the fusiform tapers is carried out. It is demonstrated that the three-dimensional size of the fusiform hierarchical micro/nanostructures could be controlled by adjusting laser parameters. In addition, the potential applications of the fusiform hierarchical micro/nanostructures in microliter droplet directional transportation and light absorption are explored. • One step fabrication of fusiform hierarchical micro/nanostructures on Cu by fs laser is proposed. • The formation mechanism of the micro/nanostructures is explained by Marangoni effect and surface plasmons model. • Effect of laser parameters on the three-dimensional micro/nanostructures' size is investigated. • The potential applications of the micro/nanostructures in droplet transportation and light absorption are explored. [ABSTRACT FROM AUTHOR]

With the rapid development of communication technology, frequency selective surface (FSS) operating at higher frequencies is required and it is necessary to design and fabricate new FSS. In this paper, a broadband double-layer FSS is proposed which is composed of one layer of foam, two metal layers, and four layers of substrate. The simulation result shows that a high transmission in the band of 60.03–89.33 GHz (covering the E band) with a relative bandwidth up to 29 % is achieved. In addition, the effects of the key structural parameters, oblique incidence angle, and TE/TM polarization on the transmission performance are further explored. The results indicate that the proposed FSS has good oblique incidence angular stability and polarization insensitivity up to 60°. Finally, the proposed FSS is fabricated by the femtosecond laser and measured by the waveguide method. Experimental results show that the machined sample has high precision and the machining error is within 1 µm, and good agreement between the simulated and measured results is observed. Therefore, the proposed FSS could be used as spatial filters or sub-reflectors of multiband antennas and the corresponding processing method could be extended to other millimeter wave functional devices, such as absorbers, radomes, etc. [ABSTRACT FROM AUTHOR]

In this study, a facile and detailed strategy to fabricate superhydrophobic aluminum surfaces with controllable adhesion by femtosecond laser ablation is presented. The influences of key femtosecond laser processing parameters including the scanning speed, laser power and interval on the wetting properties of the laser-ablated surfaces are investigated. It is demonstrated that the adhesion between water and superhydrophobic surface can be effectively tuned from extremely low adhesion to high adhesion by adjusting laser processing parameters. At the same time, the mechanism is discussed for the changes of the wetting behaviors of the laser-ablated surfaces. These superhydrophobic surfaces with tunable adhesion have many potential applications, such as self-cleaning surface, oil–water separation, anti-icing surface and liquid transportation. [ABSTRACT FROM AUTHOR]

In this study, the temperature effects on hole geometry of the PMMA during micro-holes drilling by femtosecond laser has been studied under various pulse energy and number of pulse. The laser-induced hole’s diameter is considerably increased by 73% as the temperature rises from 20 °C to 90 °C. Remarkable enhancement in the removal volume of micro-hole is also observed under high temperature. The possible mechanism for such changes is discussed in detail on account of optical absorption enhancement and higher density of surface plasma. The atomic percentage of oxygen obviously increases with the increase of temperature, which is beneficial to femtosecond laser fabrication of PMMA micro-hole. The spatter area of micro-hole has been found to tremendously extend with the increase of temperature, which is due to recoil pressure effect. These results demonstrate that temperature plays a crucial role to tailor micro-hole fabrication by femtosecond laser. [ABSTRACT FROM AUTHOR]

A highly sensitive torsion sensor based on long period fiber grating (LPFG) fabricated by 800 nm femtosecond laser pulses is proposed and demonstrated. LPFG with an attenuation depth of ∼14 dB is achieved within the wavelength range of 1425–1575 nm. The experiment results show that the LP 02 and LP 03 resonant wavelengths experience red-shift when the twist direction is clockwise while they occur blue-shift in the twist counterclockwise direction as the twist rate increases. However, the LP 04 resonant wavelength is always shifted toward shorter wavelength independently of the twist directions and higher twist sensitivity is observed. In addition, the loss peak amplitude of LPFG shows a tendency to decrease with the twist rate increases whether the LPFG is twisted clockwise or counterclockwise. Meanwhile, the resonant wavelength occurs splitting phenomenon in the case of higher twist rate as well as the high order resonant wavelength performs more significantly. Additionally, the sensor shows a twist sensitivity as high as 118.7 pm/(rad/m) in the range of −105 to −52.5 rad/m and that of 181.7 pm/(rad/m) in the range of 52.5–105 rad/m. [ABSTRACT FROM AUTHOR]

• Bioinspired from Cicada wing exhibiting high transparency at wide viewing angles. • Antireflective subwavelength nanostructures (ASS) fabricated by femtosecond laser. • ASS designed for high transmittance by rigorous coupled wave analysis. • Broadband antireflective arrays with hydrophobicity for high transparency glass. In nature, the Cicada wing exhibiting a high transparency at wide viewing angles can be bioinspired for the formation of broadband antireflective subwavelength nanostructures (ASS), which can significantly improve the transmittance of optical devices. In this study, a femtosecond laser micro-machining is developed to fabricate a biomimic nanostructure array on optical glass for antireflection. The underlying mechanism for the transmittance enhancement and geometry design of the artificial structure is thoroughly explored by applying a rigorous coupled-wave analysis method both in the visible and the near-infrared band. In addition, the height, diameter, and period of the ASS experimentally induced by the femtosecond laser are adjusted by controlling the laser-processing parameters of pulse energy and scanning velocity. Accordingly, the proposed bioinspired nanostructures featuring a wide-angle omnidirectional antireflection in the visible band, as well as hydrophobicity properties, are anticipated to be widely used in glass displays applications. [ABSTRACT FROM AUTHOR]

• Femtosecond laser Bessel beam micromachining fabricate antireflective microstructures. • Fabry-Perot and Mie resonances in the multi-resonant surface work together to reduce reflection. • The proposed method promots antireflective surfaces for wide civil and military applications. Surface antireflection of zinc sulfide (ZnS) is crucial to improve the performance of infrared windows. In this study, femtosecond laser Bessel beam micromachining is proposed to fabricate antireflective microstructures (ARMs) on ZnS to suppress surface Fresnel reflection. The ARM consisting of approximately one hundred thousand holes within 30 × 30 mm2 is uniformly spread on the ZnS surface. The Fabry-Perot (FP) and Mie resonances in the multi-resonant surface work together to reduce reflection and are enhanced with increasing ARM depth. The ARM depth is tunable by adjusting the pulse energy and is utilized to facilely regulate the reflectance spectra as well as wettability. Finally, the fabricated ARM with wide-angle antireflection and hydrophobicity proved the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

In this work, a low-temperature and low-pressure method was proposed for the bonding strength enhancement of SiC chips and substrates using micro/nanostructures fabricated by femtosecond laser. It is demonstrated that a reliable bond was achieved at 180 °C without assistant pressure. When the sintering temperature reached 250 °C and the assistant pressure was 10 MPa, the average shear strength reached 45 MPa, which was more than 3 times that of the flat substrate. The fracture surface morphology and the cross-section of the sample were characterized by SEM to analyze the enhancement mechanism of shear strength. The shear strength under different sintering conditions was also discussed. The thermomechanical coupled finite element simulation results demonstrate that the power module possessed excellent thermal capability and reliable lifetime under a temperature shock environment (−50 °C–250 °C). It is proved that the measured temperature distribution of the joint is in agreement with the simulation results. This method has great application prospects in microelectronic packaging, especially for wide-bandgap power semiconductors. [ABSTRACT FROM AUTHOR]

• High temperature characteristics of overexposed and unexposed FBGs are compared. • Overexposed FBGs shown better high temperature stability than the unexposed FBGs. • Overexposed FBGs can withstand a high temperature of up to 1000 °C. • The wavelength hysteresis and spectral repeatability is improved by air cooling treatment. The high temperature properties of fiber Bragg gratings (FBGs) fabricated by overexposure using femtosecond laser phase mask technology are investigated. It is found that the overexposed FBG (FBG-II) has exhibited a temperature resistance of up to1000 °C and a reflection intensity of over 12 dB after air cooling treatment at 1000 °C for 5 h, as well as showing better high temperature stability than the unexposed gratings (FBG-I). Moreover, the temperature stability of FBG-II can be enhanced obviously through the rapid air cooling treatment after high temperature annealing, as well as the higher the air cooling rate, the more significant the effect. In addition, the wavelength hysteresis and spectral repeatability of the FBG-IIs can be significantly improved through the method. This research essentially provides a new approach to enhance the thermal properties of FBG and shows great potential in a harsh environment. [ABSTRACT FROM AUTHOR]

• 1. Bessel Femtosecond laser fabricates antireflective subwavelength structures (ASS). • 2. ASS designed and optimized for high transmittance by rigorous coupled wave analysis. • 3. Broadband and wide-angle ASS with anti-laser damage properties and hydrophobicity. Enhancing infrared window transmittance of zinc sulfide (ZnS) is an effective way to improve the infrared detection imaging quality. In this study, the antireflective subwavelength structure (ASS) is designed and fabricated on ZnS substrate, aiming to suppress surface Fresnel reflection and achieve high transmittance within the far-infrared band. The structure parameters of ASS such as height, period, and fill factor are initially set based on the effective medium approximation theory, and are optimized via the rigorous coupled-wave analysis (RCWA) method. Moreover, the antireflective ASS mechanism is explored by using the local light field selective enhancements in structures for capturing incident photons. In addition, a femtosecond laser Bessel beam is used to experimentally fabricate the designed ASS on the ZnS substrate for obtaining large array height, resulting in transmittance enhancement in the far-infrared region. Ultimately, the broadband and wide-angle ASS with the anti-laser damage properties and hydrophobicity is achieved on the optical window. Hence, the clarity of the infrared image captured by the infrared detection system is improved. [ABSTRACT FROM AUTHOR]

A fiberized refractive index (RI) Mach–Zehnder interferometer (MZI) based on a micro V-shaped slot in fiber core is fabricated in a standard single mode optical fiber by femtosecond laser machining and chemical etching. The interference fringe of the MZI would shift with the variation of the ambient RI. The experimental results show that the RI sensitivity for the sensor is about −10 4 nm/RIU (refractive index unit) within the RI range between 1.332 and 1.352. Its robust structure and high sensitivity will have attractive potential application in chemical, biological and environmental monitoring. [ABSTRACT FROM AUTHOR]

In order to achieve high precision laser patterning of silver nanowires (AgNWs) network on polyethylene terephthalate (PET) substrate, the ablation morphology of AgNWs on PET flexible substrate by femtosecond (fs) laser is analyzed. The ablation threshold of AgNWs and PET are evaluated by investigating the dependency of the diameters versus laser pulse energy. The single-pulse ablation threshold of PET and AgNWs are 1.50 J/cm2 and 0.55 J/cm2, respectively. The ablation mechanism of AgNWs is discussed in terms of laser-induced surface plasmon polariton. By controlling the energy of laser irradiation, AgNWs can be effectively removed with slightly damage to the thermally sensitive PET substrate. Furthermore, The AgNWs can be welded in an energy range below the ablation threshold of AgNWs. This study provides a deeper understanding for fs patterning of AgNWs network and represents a promising strategy for fabricating high performance flexible AgNWs-based electronics. Unlabelled Image • The factors influencing femtosecond laser processing of AgNWs network are analyzed. • The ablation mechanism of AgNWs is discussed in terms of surface plasmon polariton. • The ablation threshold of AgNWs and PET are evaluated, respectively. • AgNWs can be removed by femtosecond laser with slightly damage to PET substrate. [ABSTRACT FROM AUTHOR]

• Femtosecond laser induced second harmonic generation (SHG) in ZnS. • The 513 nm SHG signals are demonstrating from 1026 nm femtosecond laser. • The defocus can be detected by relationship between SHG power and sample step. • The proposed approach is promising for femtosecond laser machining applications. Second harmonic generation (SHG) has been observed from zinc sulfide (ZnS) under linearly polarized and circularly polarized femtosecond laser excitations. The output power of SHG signal at 513 nm exhibits a quadratic relationship to the input power of femtosecond laser at 1026 nm, which can figure out the effective SHG response of 2.12 pm/V in ZnS. Additionally, the total, vertical, and horizontal SHG signals all demonstrate well defined cosine-like curves with approximate periods of 40°, indicating a textured crystallized material rather than randomly orientated crystals. Finally, the relative position between the laser focus point and ZnS surface could be detected by the relationship between the SHG power and sample step, due to the regulation of the SHG signal by the power density of incident laser, which is also determined by the relative position. This work may pave the way towards nonlinear optical architectures and femtosecond laser machining. [ABSTRACT FROM AUTHOR]

• Antireflective subwavelength structures (ASS) on surface of zinc sulfide (ZnS) for infrared transmittance enhancement by femtosecond laser. • Concave ASS on ZnS designed and optimized for high transmittance by finite difference time domain (FDTD) and effective medium theory (EMT). • Broadband antireflective conical and pyramidal arrays on ZnS with robust mechanical characteristics for application in infrared optical windows. The surface optical loss in optical windows is primarily attributed to Fresnel reflection at the interface between air and substrate of high refractive index materials such as zinc sulfide (ZnS). In this study, the concave antireflective subwavelength structures (ASS) on ZnS have been numerically and experimentally investigated to obtain high transmittance over a wide bandwidth in the far-infrared range from 7 μm to 14 μm. The mechanism for transmittance enhancement is explored by localized field selective enhancement in structures based on Wood-Rayleigh law, also being confirmed by the light energy flowing in Poynting vector distribution. The geometric parameters including shapes, period, and depth of as-designed array have been specifically optimized by finite-difference time-domain (FDTD) method and effective medium theory (EMT). Hereby, three-dimensional direct femtosecond laser writing (3D fs-DLW) is employed to fabricate the ASS in inverted conical and pyramidal array with the largest transmittance reaching to 85.2% in 9 μm wavelength and robust mechanical characteristics (hardness >2.39 GPa), expecting to be of great potential applications in infrared optical windows. [ABSTRACT FROM AUTHOR]

• Convex grid-patterned microstructures are processed by fs laser and chemical etching. • According to Raman spectra, the formation of the microstructures is investigated. • The effect of experimental parameters on the microstructures' 3D size is discussed. • The hydrophobic property of a surface with such microstructures is explored. The convex grid-patterned microstructures were fabricated on a silicon substrate by using femtosecond laser assisted with chemical etching in this study. Along the femtosecond laser scanning traces, the isosceles trapezoidal microstructures were generated. According to Raman spectra and elemental distribution, the formation of the convex microstructures is investigated. Also, the effect of experimental parameters including laser power, scanning speed, etching duration and etching temperature on the microstructures' three-dimensional size is discussed in detail. In addition, the hydrophobic property of a surface with large-area convex grid-patterned microstructures was explored. [ABSTRACT FROM AUTHOR]