Overview: Surface-Enhanced Raman spectroscopy (SERS) is a highly sensitive and high-resolution molecular recognition technique with important applications in many fields. As an emerging low-cost, high-resolution, and highflexibility micro-nano processing method, femtosecond laser direct writing has been widely used in the field of preparing SERS substrates. Compared with traditional processing methods for preparing SERS substrates, femtosecond laser direct writing processing has certain advantages in terms of flexibility, three-dimensional molding, processing material range, processing accuracy, and other aspects. In this review, we classify the processing methods of femtosecond laser preparation of SERS substrates into four categories, including femtosecond laser two-photon metal reduction, femtosecond laser cutting metal, femtosecond laser cutting-sputtering, and femtosecond laser 3D printing. Femtosecond laser two-photon metal reduction uses the two-photon reduction effect to reduce metal cations in metal solutions to metals, such as silver ions in silver nitrate solutions to silver nanoparticles. This method is suitable for the one-step preparation of SERS substrates in closed microchannels. Femtosecond laser cutting metal directly prepares the SERS substrate structure on a metal substrate. This method takes advantage of the high peak power of the femtosecond laser to ablate the surface of the metal sample to obtain a patterned surface structure. At the same time, femtosecond laser ablation produces particle fragments, which are usually redeposited on the patterned surface, resulting in SERS "hot spots". Femtosecond laser direct cutting of metal can prepare SERS substrates in one step, which has the advantages of high processing efficiency and simple processing and is more conducive to the application of large-scale production of practical SERS detection. Femtosecond laser cutting-sputtering is to process any structure on non-metallic substrates such as polymers and then sputtering/evaporating metal nanoparticles on the surface of the structure. This method can prepare transparent and flexible SERS substrates, which are rich in application scenarios. Femtosecond laser 3D printing is to use the three-dimensional processing ability of femtosecond lasers to obtain rich "hot spots" by designing the structure of SERS substrates, and then using template-guided self-assembly technology with different driving forces to deposit/evaporate metal nanoparticles at designated locations. In this paper, we first introduce the current methods for preparing SERS and then conduct a comprehensive review of the processing methods of four femtosecond lasers to prepare SERS substrates. Finally, the advantages and disadvantages of the four femtosecond laser preparation methods for SERS substrate are briefly summarized, and the development prospects of this technology are prospected, aiming to provide it for future related research.

Overview: With the development of modern manufacturing, the size of optical devices is gradually developing towards miniaturization, and integrated optics is also developing to become a topical area of research for many scholars. One of the methods used for producing micro/nano optical devices is femtosecond laser direct writing, a fine three-dimensional processing technique that has been extensively studied by many scholars for its applicability to most materials and can be applied to the fabrication of a wide range of optical devices. Micro/nano-optical devices prepared by femtosecond laser direct writing in crystals have been applied in a broad range of applications in different wavelengths. PMN-PT crystal with relaxed ferroelectric has attracted much attention in recent years for its superior piezoelectric property and large electromechanical coupling coefficient, and its application in the infrared band is more prominent, so the fabrication of the optical devices based on PMN-PT crystal has gradually become a relevant research hotspot. The LIPSS is one of the micro/nano-structures that can be processed by femtosecond laser direct writing. The LIPSS is prevalent in many materials and has been found in metals, semiconductors, dielectrics, etc. Similarly, LIPSS can be induced by femtosecond lasers in PMN-PT crystal. The LIPSS has a wide range of applications in the fields of anti-reflectivity, permanent coloration, and wettability. Nevertheless, the physical processes and the mechanisms involved in the formation of LIPSS have different interpretations in different materials. In this paper, we describe the LIPSS induced by femtosecond laser on the surface of the PMN-PT crystal and characterize it theoretically. We have achieved a change in the period of the LIPSS from 750 nm to 3000 nm after experimenting with different laser parameters. Afterward, we simultaneously obtained the phase transition of the LIPSS in PMN-PT crystal through temperature modulation, and this phase transition can be analyzed by the variation of the Raman spectra. At the same time, we have obtained the Curie temperature for the LIPSS structure that is approximately 10 ℃ lower than that of the PMN-PT crystal and have analyzed the phase transition process through the structural properties of the PMN-PT crystal. The results of our experiments and analyzes on the LIPSS in PMN-PT crystal reported in this paper can provide some experience for the subsequent development of the optical devices related to the LIPSS in PMN-PT crystal.

Overview: Two-photon lithography (TPL) has been a research hotspot in 3D micro/nano writing technology due to its characteristics of high resolution, low thermal influence, a wide range of processed materials, low environmental requirements, and 3D processing capability. It has shown unique advantages in the fields of life science, material engineering, micro/nano optics, microfluidic, micro machinery, and so on. This paper summarizes the research works done by researchers on different writing methods to improve TPL processing efficiency. Single-beam writing is the main method for TPL, which mainly depends on the speed of the scanning device. Single-beam writing has the advantages of simple system and high-quality beam, and it is easy to combine various effects to improve writing results. It mainly includes scanning modes based on the translation stage, galvo, polygon laser scanner, and acousto-optic deflector (AOD) (Fig. 2). All these modes have advantages and disadvantages. As for the scanning speed comparison, polygon laser scanner and AOD have relatively faster writing rates (faster than m/s). Multi-foci parallel lithography can obviously promote efficiency, elevating the speed by dozens or even hundreds of thousands of times, mainly based on spatial light modulator (SLM), digital micromirror device (DMD), microlens array (MLA), diffractive optical elements (DOE), multi-beam interference, and so on (Figs. 3-15). Multi-foci parallel lithography based on SLM is most widely used owing to its high efficiency and ability to flexible and independent control of each single beam, but the refresh rate is still insufficient. DMD has a higher refreshing rate (32 kHz), but the state-of-the-art beam parallelism realized by DMD is severely limited. More parallel beams are further required for improving the processing efficiency. The 2D pattern exposure method based on SLM or DMD can further improve the TPL efficiency with the superiority of generating flexibly designed pattern (Figs. 16-18). However, the 2D projection exposure technology is still difficult to achieve high writing precision, especially the axial resolution. An available method to improve the axial precision is spatially and temporally focusing an ultrafast laser to implement a strong intensity gradient at the spatial focal plane that restricts polymerization within a thin layer. The 3D projection method will be the most efficient writing method in the future, especially in 3D device processing (Figs. 19-20). Researchers used this technique to make hollow tubular and conical helices structures, increasing the processing speed by 600 times. However, the research results show that the current 3D projection can only process simple 3D structures. Further researches on 3D exposure processing of complex structures are expected, which will effectively expand its application in various fields. Authors believe that with the effort of researchers on efficiency improvement gradually, TPL can further highlight its advantages to promote the development of life science, materials engineering, micro-nano optics, and many other fields.