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With high hardness, high thermal and chemical stability and excellent optical performance, hard materials exhibit great potential applications in various fields, especially in harsh conditions. Femtosecond laser ablation has the capability to fabricate three-dimensional micro/nanostructures in hard materials. However, the low efficiency, low precision and high surface roughness are the main stumbling blocks for femtosecond laser processing of hard materials. So far, etching-assisted femtosecond laser modification has demonstrated to be the efficient strategy to solve the above problems when processing hard materials, including wet etching and dry etching. In this review, femtosecond laser modification that would influence the etching selectivity is introduced. The fundamental and recent applications of the two kinds of etching assisted femtosecond laser modification technologies are summarized. In addition, the challenges and application prospects of these technologies are discussed.

Here, we report embedded spiral zone plates (SZPs) processed by tightly focused femtosecond pulses, inducing a refractive index change in a sapphire crystal. The change in the index is investigated both experimentally and theoretically and found to be 0.93% lower than that before the laser irradiation. Benefiting from the investigation of the photorefraction, a high diffractive efficiency of up to 78% is achieved for the SZPs. In addition, the SZPs are embedded under the surface of the sapphire, which ensures that the components possess stable optical properties in the case of external refractive index changes. The high-performance elements having a high diffractive efficiency and stable optical performance will have tremendous applications in optical communication and optical manipulation. [ABSTRACT FROM AUTHOR]

Micro-ripple and micro-bead structures are formed on a silicon (Si) surface after irradiation with femtosecond laser pulses in nitrogen (N2) atmosphere. Simultaneously, supersaturated nitrogen (N) atoms, with a concentration above 1020 cm−3, are doped into the textured black Si layer via laser ablation. The N-doped Si exhibits strong below-bandgap infrared absorption from 1.1 to 2.5~\mu \textm , which remains nearly unchanged after annealing for 30 min at 873 K. The mechanism of this thermally stable infrared absorption is analyzed by first-principles calculations. According to the transmission electron microscopy results, multiple phases (including single crystalline, nanocrystalline, and amorphous phases) are observed in the laser-irradiated layer. Hall Effect measurements prove that N-dopants induce a low background free-carrier concentration ( \sim 1.67\times 10^16\,\,\text cm^-3 ). Finally, a Schottky-based bulk structure photodiode is made. This broadband photodiode exhibits good thermal stability and a photo-responsivity of 5.3 mA/W for 1.31~\mu \textm at a reverse bias of 10 V. [ABSTRACT FROM PUBLISHER]

Femtosecond laser machining has been widely used for fabricating arbitrary 2.5 dimensional (2.5D) structures. However, it suffers from the problems of low fabrication efficiency and high surface roughness when processing hard materials. To solve these problems, we propose a dry-etching-assisted femtosecond laser machining (DE-FsLM) approach in this paper. The fabrication efficiency could be significantly improved for the formation of complicated 2.5D structures, as the power required for the laser modification of materials is lower than that required for laser ablation. Furthermore, the surface roughness defined by the root-mean-square improved by an order of magnitude because of the flat interfaces of laser-modified regions and untreated areas as well as accurate control during the dry-etching process. As the dry-etching system is compatible with the IC fabrication process, the DE-FsLM technology shows great potential for application in the device integration processing industry. [ABSTRACT FROM AUTHOR]

Gold (Au)-doped-textured silicon (Si) material with a thermostable absorption below bandgap (>50%) is obtained by femtosecond laser irradiation. Although the concentration of Au impurity (1019 cm−3 ) in textured Si is at least four orders of magnitude greater than the solid solubility of Au in crystalline Si, the sheet carrier density (approximately 1010 cm−2) in Au-doped Si is very low due to a self-compensation effect of Au impurity in Si material. The infrared absorption of Au-doped Si is related to laser-induced-structural defects and sub-band absorption of deep energy levels of Au in Si, which is determined by temperature-dependent Hall Effect measurement. Besides supersaturated doping of Au, a gold silicide phase is formed at textured Si surface. [ABSTRACT FROM PUBLISHER]

Femtosecond (fs) laser annealing has been applied to improve the crystalline quality and absorptance below bandgap of ion implanted silicon (Si) with sulfur (S). The doping concentration of S is up to 10^20 atoms/cm3, which is at least four orders of magnitude higher than the solid solubility of S in crystalline Si. According to the Raman spectra, after fs laser irradiation with appropriate laser energy, the crystal quality is evidently better than primary ion implanted sample. Moreover, the optical absorption coefficient at wavelengths of 1100~2400 nm is up to 1.54 \times 10^4 cm ^\mathrm -1 , which is much larger than the contribution of free carriers. Excessive laser energy will induce serious ablation effect and cause the removing of doping layer. We consider that the high absorption is a combination of sub-bandgap transition of S-related localized states, free carrier absorption, ion implantation induced defect absorption and fs laser irradiation induced defect absorption. In the end, a photo-response of 8.4 mA/W is obtained for the photodiode for 1310 nm detection light. [ABSTRACT FROM PUBLISHER]

We report a novel hybrid refractive–diffractive microlens combined with spiral phase for the generation of optical vortex, which is fabricated via femtosecond laser direct writing technology. The unique optical performance of focusing capability is demonstrated. At the focus position, the hollow focus with different integer topological charges is investigated. Moreover, experimental results are supported by finite-element calculation. The novel microlens generating an optical vortex will fulfill important applications in optical manipulation, multistate information encoding, quantum communication, and computation, particularly in the compaction, integration, and simplification of optical vortex generation system. [ABSTRACT FROM PUBLISHER]

Suspended structures have attracted a great deal of attention in the field of micro-technology and nanotechnology for their important functions. However, the fabrication of the suspended structures is typically cumbersome and time-consuming and requires complicated processes, such as multiple deposition, photolithography, and etching. Here, we report a new method of fabricating flexible suspended structures by femtosecond laser direct writing followed by wet chemical alkaline (KOH) etching. Our proposed method will enable convenient and flexible fabrication of suspended structures for a wide variety of applications in the field of micro-technology and nanotechnology. [ABSTRACT FROM PUBLISHER]

Here, we report a sapphire-based Fresnel zone plate (FZP), which is fabricated by femtosecond laser direct writing assisted with subsequent wet etching. With this method, we solved the problem of high surface roughness caused by ultrafast femtosecond laser processing. We have obtained $\sim 12$ -nm average surface roughness smaller than 1/25 of the optical working wavelength. As-formed sapphire FZP also exhibited a well-defined geometry. More importantly, ultraviolet (UV) light focusing and imaging can be easily achieved. Due to the high material hardness, thermal and chemical stabilities of sapphire, such sapphire FZP, may have great potential in UV imaging and focusing under some harsh environments. [ABSTRACT FROM PUBLISHER]

We investigated the strain and surface structural properties of a strain-relaxed Si1–xGex alloy layer with high Ge-content using optical surface second-harmonic generation. Here, the Si1–xGex alloys are heteroepitaxial, and they are deposited onto Si substrates via ultra-high-vacuum-chemical vapor deposition. The in-plane strain and the composition $x$ of the Si1–xGex alloys were determined using Raman spectroscopy. The SH signals generated from three Si1–xGex alloy surfaces versus the rotational angle of the substrate were measured. The SH intensities for the combination of s-input/p-output polarization show fourfold symmetry; however, for s-input/s-output, the SH signals show eightfold symmetry with the rotational angle. Residual strain would induce an enhancement of the isotropic p-output SH component in the Fourier transform coefficient. Finally, the degree of symmetry of the SH signals from these three Si1–xGex alloy surfaces versus the rotational angle was related with the surface pit defects (densities and sizes) and surface roughness. [ABSTRACT FROM AUTHOR]

Micronanostructured silicon surface is irradiated by femtosecond laser pulses at sulfur hexafluoride atmosphere (S-doped black silicon) and photodiodes are successfully fabricated based on this material. From the scanning electronic microscope and atomic force microscope images, the black silicon layer shows micro-nanocomplex structures (microcones covered with nanoparticles). The optoelectrical properties of n+-p junction are formed between S-doped micro-nanostructured silicon-layer and substrate after thermal annealing. For the n+-p photodiode, current–voltage characteristics at different incident light powers have been investigated. The responsivity for 800-nm wavelength is 0.69 A/W at −5 V reverse bias, which is close to that of the usual commercial Si p-i-n photodiode. The present devices are stable and well reproducible. [ABSTRACT FROM PUBLISHER]

Chalcogen-doped microstructural silicon irradiated by femtosecond laser has high near-uniform absorption on a broad spectrum, but the factors leading to infrared absorption are complex and remain an open problem. To clarify the origin of infrared absorption besides hyperdoped Chalcogen atoms, microstructural silicon is fabricated by femtosecond laser under vacuum condition. The relationship between infrared absorption and as-formed new phases (amorphous silicon: $\alpha $ -Si and nanocrystal silicon) is established. It indicates that the infrared absorption is caused by defects related to Urbach states from \alpha $ -Si or nanocrystal Si, and these metastable defects disappeared after a thermal annealing process. From the absorption spectrum of microstructural silicon after etching at different times, it could be figured out that the Urbach states exist in both the surface and subsurface regions of black silicon at a depth of about 2.4 \mu \text{m} . [ABSTRACT FROM PUBLISHER]

Micronanofabrication technologies developed so far pursue faithful conversion from digital models to matter structures. This is vital for microdevices in optics, mechanics, and electronics, where device shape and size matter. However, biotissues grow under rich environmental factor interactions, as demands novel manufacturing approaches for biomimetic and biological fabrication. Here, a concept of dynamic laser prototyping is reported, which is based on a new finding of a multilayered three-dimensional (3D) wrinkling phenomenon. The 3D wrinkling started with formation of a photocrosslinked hydrogel sheet by femtosecond laser direct writing. It was followed by spontaneous self-bending of the sheet, caused by a purposely designed sheet-substrate stress mismatch. The flower blooming process has been successfully mimicked, indicating broader usages of the technology in biotissue-growth-related manufacturing. [ABSTRACT FROM AUTHOR]

A novel formation method of long-period fiber gratings (LPFGs) based on periodic microchannels, which are fabricated by femtosecond laser micromachining and selective chemical etching in conventional single-mode fibers, is proposed and experimentally demonstrated. This kind of grating may be as short as only 3 mm and exhibits low-temperature sensitivity of 9.95 pm/^\circC from 30^\circC to 120^\circC. The strain sensitivities of the resonant wavelength and peak loss are -2.4~nm/m\varepsilon and -3.5~dB/m\varepsilon, respectively. High refractive-index (RI) sensitivity with one order of magnitude larger than that of the conventional LPFGs near RI=1.33 is achieved in general single-mode fibers. [ABSTRACT FROM PUBLISHER]

Excited state dynamics from molecular fluorescent switches of the donor- or acceptor-derived 5-Methoxy-2-(2-pyridyl)thiazole (2-MPT) fluorescent molecules has been studied by ultrafast spectroscopy. The twisted intramolecular charge transfer (TICT) excited state is found to be created within 1 ps. Then the relaxation of TICT excited state causes further twist of the molecular conformation (especially the moiety 2-MPT) and reduces the transition moment. It takes about 600–700 ps to reach a state with ten nanoseconds of lifetime. This final state is nearly a dark state, which is attributed as the main reason for the drastic decrease of the fluorescence quantum field with respect to the moiety 2-MPT. For MPTEA with a strong electron donor in high polarity solvents, TICT's excited state partially transfers to a planar intramolecular charge transfer (PICT) excited state within 10 ps due to the strong dipole-dipole interaction with solvent molecules. PICT's excited state gives a relatively strong emission with a large red shift. The results demonstrate that the excited state dynamics of these molecular fluorescent switches provides deep insight to their photophysical processes, which is important for exploring novel molecular switches as potential candidates for optical biolabels, sensors and logic gates applications. [ABSTRACT FROM PUBLISHER]

A strain-scanning second-harmonic generation technique is proposed for high-sensitivity measurement of weak strain in film surfaces or interfaces. The basic idea is the sequential application of tensile and compressive strains to a strained film sample. From the strain-dependent second-harmonic generation (SHG) intensity, the type of the strain can be easily judged from whether the SHG is enhanced or weakened, and its magnitude can be precisely calibrated by an externally applied strain that is known. Thus, the built-in strain of a SiO2/Si interface could be determined as tensile with a magnitude of 3.07\,\times 10^-4. [ABSTRACT FROM PUBLISHER]

Summary: Femtosecond laser direct writing (FsLDW) has been established as a nano-enabler to solve problems that are otherwise not possible in diversified scientific and industrial fields, because of its unique three-dimensional processing capability, arbitrary-shape designability, and high fabricating accuracy up to tens of nanometers, far beyond the optical diffraction limit. We briefly review the underlying mechanisms for achievement of the superhigh fabrication spatial resolution and surface smoothness, and then discuss the rule and photochemical strategies that are currently exploited for FsLDW. Finally, applications of the delicate nanoprototyping approach in microelectronics, micromechanics, microoptics and microfluidics are introduced. [ABSTRACT FROM AUTHOR]

Birefringence of 3 × 10 − 3 is demonstrated inside cross-sectional regions of 100 μ m, inscribed by axially stretched Bessel-beam-like fs-laser pulses along the c-axis inside sapphire. A high birefringence and retardance of λ / 4 at mid-visible spectral range (green) can be achieved using stretched beams with axial extension of 30–40 μ m. Chosen conditions of laser-writing ensure that there are no formations of self-organized nano-gratings. This method can be adopted for creation of polarization optical elements and fabrication of spatially varying birefringent patterns for optical vortex generation. [ABSTRACT FROM AUTHOR]

Stimuli-Responsive Materials: Smart Compound Eyes Enable Tunable Imaging (Adv. Funct. Keywords: compound eyes; femtosecond laser; smart materials; tunable optical components; variable-focus imaging Compound eyes, femtosecond laser, smart materials, tunable optical components, variable-focus imaging. [Extracted from the article]

Compound eyes are natural multiaperture optical imaging systems and have substantial potential in the field of modern optics. However, both natural and artificial compound eyes are composed of ommatidia with fixed focal lengths, and thus incapable of variable‐focus imaging. In this study, inspired by the tunable crystalline lens of human eyes, smart stimuli‐responsive compound eyes based on the bovine serum album (BSA) protein are fabricated via femtosecond laser direct writing. Due to the swelling and shrinking effect of BSA under different pH conditions, a tunable field of view (FOV, 35°–80°) and variable focal length of ommatidia are achieved. In addition to the direct prototyping of an entire protein‐based compound eye, the ability to flexibly integrate the smart protein ommatidia with a conventional optical lens (an SU‐8 lens in this study) to form a composite compound eye is shown. The composite compound eye achieves nearly 400% of focal length tuning at a fixed FOV. It is anticipated that femtosecond laser fabrication and the integration of smart protein‐based compound eyes may emerge as an enabler for fabricating miniature tunable imaging systems. [ABSTRACT FROM AUTHOR]

With high thermal and mechanical stability, glass artificial compound eyes have great potential applications in wide field‐of‐view (FOV) imaging and fast detection. However, the rapid fabrication of large‐area, high integration, uniform, and well‐designed three‐dimensional (3D) glass compound eyes is still a great challenge. Here, a dry‐etching‐assisted femtosecond laser machining (DE‐FsLM) technology is proposed for fabrication of cm‐sized concave compound eye from a curved sapphire substrate, with which the fabrication efficiency can be improved by over two orders of magnitude compared with direct laser ablation. With high hardness and thermal stabilities, the sapphire concave compound eyes can be used as high‐temperature and hard‐casting templates for the replication of convex compound eyes on K9 glass. The replicated cm2‐size all‐glass compound eye consists of a spherical macrolens (1 cm diameter and 2.3 mm height) and over 190 000 close‐packed ommatidia (≈20 µm diameter and 1.5 µm height). The compound eyes exhibit excellent optical properties with wide FOV (up to 90°) imaging and focusing. These results indicate that the DE‐FsLM and casting replication technology will open new opportunities in micro‐/nanofabrication of hard materials. [ABSTRACT FROM AUTHOR]

We report a systematical study on the liquid assisted femtosecond laser machining of quartz plate in water and under different etching solutions. The ablation features in liquid showed a better structuring quality and improved resolution with 1/3~1/2 smaller features as compared with those made in air. It has been demonstrated that laser induced periodic structures are present to a lesser extent when laser processed in water solutions. The redistribution of oxygen revealed a strong surface modification, which is related to the etching selectivity of laser irradiated regions. Laser ablation in KOH and HF solution showed very different morphology, which relates to the evolution of laser induced plasma on the formation of micro/nano-features in liquid. This work extends laser precision fabrication of hard materials. The mechanism of strong absorption in the regions with permittivity (epsilon) near zero is discussed. [ABSTRACT FROM AUTHOR]