Recent Advances in Applications of Ultrafast Lasers

Abstract

Ultrafast lasers, characterized by femtosecond and picosecond pulse durations, have revolutionized material processing due to their high energy density and minimal thermal diffusion, and have played a transformative role in precision manufacturing. This review first traces the progression from early ruby lasers to modern titanium–sapphire lasers, highlighting breakthroughs like Kerr-lens mode-locking and chirped pulse amplification. It also examines the interaction mechanisms between ultrafast pulses and various materials, including metals, dielectrics, and semiconductors. Applications of ultrafast lasers in microstructure processing techniques are detailed, such as drilling, cutting, surface ablation, and nano welding, demonstrating the versatility and precision of the technology. Additionally, it covers femtosecond laser direct writing for optical waveguides and the significant advancements in imaging and precision measurement. This review concludes by discussing potential future advancements and industrial applications of ultrafast lasers.

1. Introduction

An extremely short pulse of electromagnetic radiation is called an ultrafast laser. An ultrafast laser, or more accurately, an ultrafast pulse laser, employs mode-locking techniques to produce pulses with durations in the femtosecond (fs) or picosecond (ps) range [1]. A typical ultrashort pulse exhibits a temporal width of 10 to 100 fs (1 fs = 10⁻¹⁵ s) [2]. The quest for ultrashort pulse durations dates back to the 1960s, when Maiman demonstrated the first ruby crystal laser at Hughes Research Laboratories, marking a significant breakthrough in the field of laser technology [3]. In 1972, Ippen reported that passively mode-locked continuous wave (cw) dye lasers could generate pulses as short as 1.5 ps, with the broad spectral bandwidth of the dye laser facilitating the advancement of ultrafast pulse generation, as shown in Figure 1 [4,5].

The maintenance of dye solutions presents significant complexities, which restricts their practical applications [6]. Solid-state lasers represent an effective alternative to address the limitations associated with dye lasers [7]. In 1987, Fork et al. successfully combined pulse broadening and dispersion compensation effects to produce pulses as short as 6 fs [8]. However, generating pulses shorter than 100 fs remains a considerable challenge, and the use of dye lasers can be cumbersome. Notably, as early as 1963, Johnson demonstrated the first nickel-doped magnesium fluoride (Ni-MgF₂) vibrational laser; however, this innovation did not adequately address the requirement for low-temperature cooling [9,10]. In the late 1970s, Allied Corporation discovered that the Alexandrite laser could operate in either pulsed or continuous wave modes when pumped by a lamp, with a tuning range of 700 to 800 nm [11]. Nearly 15 years later, Moulton pioneered a tunable solid-state laser utilizing titanium-doped sapphire Al₂O₃, which significantly expanded the tuning range from 660 nm to 1180 nm, as shown in Figure 1 [12].

A significant impetus for the generation of short pulses was provided by the development of Kerr-lens mode-locking in 1990. This technique involves self-focusing within a titanium-doped sapphire (Ti: Sapphire) crystal, which facilitates the clustering of mode-locked pulses circulating within the laser cavity [13]. In prior studies, the production of pulses significantly shorter than 100 fs was exceedingly complex [14]. Following Spence’s initial report of the self-mode-locking Ti: Sapphire laser in the same year, the same research group subsequently coupled the Ti: Sapphire laser with lens mode-locking to generate 60 fs pulses within the oscillator cavity, as shown in Figure 1 [15]. By incorporating an intracavity pulse compressor, the pulse duration was further reduced to 45 fs. By 1995, the record pulse length had been decreased to 8 fs through the use of chirped dielectric mirrors, as shown in Figure 1 [16]. In 2001, a German research group successfully produced a 5 fs pulse with a wavelength span of one octave, ranging from 600 nm to 1200 nm [17]. Notably, this new method is more user-friendly compared to previous ultrafast laser techniques, and Ti: Sapphire has emerged as the preferred medium for generating femtosecond pulses.

This groundbreaking discovery has facilitated the generation of high-peak-power femtosecond pulses directly from the laser cavity. The advent of chirped pulse amplification (CPA) lasers in the mid-1980s marked the emergence of high-power ultrafast lasers as a transformative tool in various fields. CPA technology has been developed to amplify pulses while preserving their nonlinear effects without damaging the crystal, demonstrating significant potential across a wide range of applications [18,19,20]. Strickland et al. achieved a pivotal advancement in 1985 by overcoming the limitations on pulse amplification imposed by nonlinear phenomena, an accomplishment for which they were awarded the Nobel Prize in Physics in 2018, as shown in Figure 1 [21,22]. Furthermore, the generation of pulses on the order of a few ps or less presented a considerable challenge for researchers both domestically and internationally. In 1983, Dzhibladze et al. introduced the concept of pulsed fiber lasers [23]. By 1990, Chang’s group successfully developed ultrafast fiber lasers with high power by employing highly doped active fiber lasers in conjunction with chirped pulse amplification [24]. The integration of mode-locked Ti: Sapphire lasers with CPA yields exceptionally high-power pulses. Additionally, intense single-cycle pulses have been generated from a Ti: Sapphire laser system utilizing gas-filled hollow core fibers [25,26]. This methodology is extensively utilized in ultrafast laboratories for applications in strong-field physics [27,28]. To enhance industrial applications, Eidam’s research group developed a high-average-power fs fiber laser in 2010, employing CPA and large-mode-area hollow-core photonic fiber technology [29].

The concept of assembling diode lasers in stacks or arrays can be traced back to the era of pulsed single-heterojunction lasers, which were utilized in various military applications during the 1970s. While higher power outputs were achieved, this advancement came at the expense of beam quality. Semiconductor laser technology progressed to the point where double-heterostructure lasers were being developed, leading to a reduction in the threshold for cw emission. The introduction of metallorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) significantly transformed the design methodologies employed in laser technology, as shown in Figure 1 [30,31]. In 1984, Welch et al. presented the first commercial diode-pumped laser, which was generated using GaAlAs with a pump wavelength of 808 nm, producing an output of 100 mW in cw mode [32]. It became evident that diode pumping was considerably more efficient than lamp pumping, and the Nd: YAG laser demonstrated excellent energy storage capabilities, enabling Q-switching to produce high peak powers. In 1987, a 1 W cw diode-pumped Nd: YAG laser was introduced in Missouri, as shown in Figure 1 [33]. However, the operational lifetime of GaAlAs diode lasers significantly decreased below the 780 nm wavelength. Two years later, Professor Isamu Akasaki successfully developed blue light-emitting diodes (LEDs) [34]. Following years of research and development, gallium nitride (GaN) was effectively harnessed for use in LEDs and lasers [35]. In 1996, Shuji Nakamura et al. reported the fabrication of the first cw blue/violet laser based on indium gallium nitride (InGaN), an achievement for which he was awarded the Nobel Prize in Physics in 2018 [36,37].

With the ongoing advancements in science and technology, alongside the extensive application of novel materials, components are evolving towards lightweight, miniaturized, and high-precision designs. The trend of miniaturization, which initially emerged with CMOS electronics, is now permeating various domains, including micro-optics, micro-fluidics, and micro-mechanics [38,39]. In these areas, the novel functionalities of micro- and nanostructures arise from their physical or chemical responses that are influenced by size- and shape-dependent effects. In optical systems, these advancements leverage the enhanced interactions between light and matter within artificially engineered media. Consequently, the demand for high-precision and high-quality material processing is becoming increasingly critical. However, traditional processing methods, such as electrical discharge machining (EDM) and long-pulse laser processing, often fail to achieve a high degree of alignment between design specifications and processing outcomes due to defects in the heat-affected zone. In contrast, ultrafast laser technology has emerged as the optical solution for precision manufacturing.

Ultrafast lasers are characterized by their narrow pulse widths, high energy densities, and short interaction times with materials. On the one hand, ultrafast lasers demonstrate an exceptionally high instantaneous power density, which can include nonlinear absorption phenomena, thereby rendering ultrafast laser processing versatile and applicable to a wide range of materials. On the other hand, the short duration of the laser pulses results in minimal thermal diffusion effects, allowing for the neglect of microcracks and recast layers. This characteristic contributes to a reduction in the generation of debris and facilitates high-precision machining [40].

Ultrafast lasers have rapidly revolutionized materials processing since their initial demonstration in 1987, which was conducted by the groups led by Srinivasan and Stuke [41,42]. In 1989, two years later, Stuke’s group further showcased the clean ablation of transparent materials through the mechanism of multiphoton absorption, as shown in Figure 1 [43].

The ultra-short pulse width of the lasers effectively mitigates the diffusion of heat in the processing area, thereby minimizing the formation of the heat-affected zone and facilitating the ultrahigh precision micro-nano processing of various materials. In 1996, Hirao’s research group achieved a significant advancement in ultrafast laser processing by realizing internal modifications in glass, resulting in an increase in refractive index by approximately 10⁻² to 10⁻³ through multiphoton absorption [44]. Subsequently, in 1997, Kawata’s team integrated the concept of multiphoton absorption from ultrafast lasers into laser stereolithography, thereby categorizing it as additive 3D manufacturing, commonly referred to as 3D printing [45]. The capability for the internal processing of transparent materials and direct 3D micro- and nanofabrication is uniquely enabled by ultrafast lasers, which led to an expansion of research in ultrafast laser processing throughout the 2000s [46,47]. By 2003, Costache identified a novel phenomenon associated with the interaction between ultrafast lasers and materials, as shown in Figure 1 [48]. In the late 2000s, Eversole et al. demonstrated the fabrication of subwavelength structures through near-field enhancement near the nanospheres, as shown in Figure 1 [49]. Furthermore, advancements in the performance of ultrafast laser systems during the 2000s significantly contributed to achieving more reliable processing for practical applications [50]. In 2006, Arai’s group developed a robust, stable, and compact fiber chirped pulse amplifier [51]. Additionally, in 2008, a compact high-power ultrafast laser system based on diode-pumped rare earth-doped laser media was developed for industrial applications, despite the typical pulse width being in the picosecond range [52]. Nevertheless, ultrafast lasers remain invaluable tools for research and manufacturing. The application of ultrashort pulse lasers in manufacturing seeks to address the limitations of traditional technologies, particularly at the nanometer and micron scales, as shown in Figure 1 [53,54,55].

The field of ultrafast laser technology is undergoing rapid advancements. This review aims to provide a comprehensive overview of the current state of the field, serving as a foundational resource for readers interested in exploring these themes further. Since its inception in the 1930s, ultrafast laser processing has emerged as a reliable and potent tool for commercial and industrial applications, with basic research in this area becoming increasingly vigorous. Moreover, advancements in laser technology have led to the development of lasers capable of emitting ultrashort pulses with durations measured in fs [56]. This capability has transformed the landscape of laser materials processing and has been a focal point of research since the initial demonstration of laser technology, thereby unlocking new avenues for the advancement of optical and photonic devices. Indeed, ultrafast lasers find applications across a diverse range of industries, including microscopic imaging, as shown in Figure 1 [57], laser direct writing [58], spectroscopy [59], pulsed laser deposition [60], micro welding [61], remote sensing [62], and numerous others [63,64], as shown in Figure 2.

2. Applications of Ultrafast Lasers in Microstructure Processing

The evolution of laser technologies, particularly the development of ultrafast lasers, has significantly enhanced the application of lasers in microstructure processing. The high energy density associated with ultrafast lasers facilitates the processing of a wide variety of materials, including those that are difficult to manipulate using traditional techniques, such as photolithography and ion etching [65]. Compared to alternative methods, ultrafast laser processing is relatively uncomplicated, requiring fewer procedural steps and imposing less stringent environmental conditions. By directly focusing the laser on the material’s surface, processing can be accomplished without the necessity of preparing a mask or engaging in other preparatory steps. Furthermore, in the fabrication of large-area array microstructures, the laser beam can be scanned along a predetermined path using a galvanometer to define the processing area. Alternatively, a motion platform may be employed, allowing the beam to remain stationary while the sample is moved along the platform to achieve the processing outcome [66,67].

The previously mentioned advantages of ultrafast lasers have resulted in their widespread application across various fields. One notable area is optics, where ultrafast lasers are utilized in the fabrication of micro-nano-optical components, such as microlens arrays and optical waveguides, among other devices. These applications illustrate that the microstructures produced exhibit high surface quality and precise geometric dimensions, thereby demonstrating that ultrafast laser technology is a versatile and capable of processing a diverse array of structures.

2.1. Conventional Ultrafast Laser Processing

2.1.1. Drilling

Micropores, which are prevalent micro-nanostructures, are employed in the fuel injector nozzles of internal combustion engines [68,69] and in the heat dissipation mechanisms of turbine blades [70,71]. The principle underlying laser micropore processing involves the formation of a focused spot by concentrating a laser beam with high energy density through a focusing lens, which subsequently interacts with the material at the focal point. This interaction causes a rapid increase in the material’s temperature, reaching its melting point, at which vaporization occurs, thereby facilitating the removal of the material. Notably, the entire process of laser–material interaction is non-contact, thereby mitigating the risk of equipment damage. The size of the focused spot can be adjusted based on the characteristics of the focusing lens, typically ranging from tens of microns. This adaptability allows laser micropore processing to achieve micron-level accuracy, ensuring high precision in manufactured devices. However, the varying pulse widths of the laser produce distinct machining effects on the surface of the material. As early as 1996, Chichkov et al. conducted a study on the interaction of lasers with matter by examining the surface processing effects of three different pulse widths, nanosecond, picosecond, and femtosecond, using a scanning electron microscope [72], as depicted in Figure 3. As illustrated in Figure 3c, the femtosecond laser yielded a morphology characterized by smoother and more refined features. The micropores obtained through ultrafast femtosecond laser processing were found to exhibit superior contour roundness and material removal efficacy when compared to those produced by long-pulse laser processing, thereby substantiating the advantages of ultrafast femtosecond laser processing for micropore fabrication.

In the field of ultrafast laser drilling, Abeln delineated four distinct categories of laser drilling strategies: single pulse, percussion, trepanning, and helical drilling [73]. These categories are depicted in Figure 4. Among these classifications, single pulse drilling is most appropriate for materials with minimal thickness or shallow micropores, while percussion drilling is better suited for thicker materials. Conversely, trepanning and helical drilling are particularly effective for micropores with larger diameters.

In addition to advancement in pulse width and processing strategies, researchers have also made substantial progress in enhancing the perforation effects of femtosecond lasers. This progress has been achieved through ongoing investigations into various processing parameters, including laser power, repetition rate, pulse width, focusing conditions, and the processing environment. For instance, Li et al. conducted experiments utilizing a copper foil collector to examine the influence of pulse energy, repetition rate, and defocusing amount on the quality of perforations [74]. Similarly, Lu et al. explored the effect of the focusing position and laser power on the morphology of the micropores created by femtosecond lasers [75]. Their research indicated that the interplay between the self-focusing effect and plasma dispersion results in a forward displacement of the focal point towards the lens. Furthermore, it was found that micropores created with high-power lasers could be modified using low-power lasers, which effectively reduced the taper of the micropores and enhanced the shape of the exit hole without altering the diameter of the entrance hole. The study revealed that both pulse energy and repetition frequency significantly influenced the morphology of the holes, with the taper being minimized at a defocusing distance of 1.8 mm. In a separate investigation, Jeong et al. utilized a custom-built femtosecond Yb: KGW laser to assess the effects of pulse energy and the number of pulses on the geometry of the micropores [76], successfully fabricating hourglass-shaped micropores in diamond samples. In a latest study, Obata et al. achieved high-quality and high-efficiency micropore drilling using a GHz burst-mode femtosecond laser based on the principle of laser-induced plasma-assisted ablation [77]. Furthermore, this research also illustrated the substantial benefits of this approach in relation to drilling quality, efficiency, and processing resolution by contrasting its outcomes with those of conventional single-pulse drilling.

The processing environment of femtosecond lasers significantly influences the quality of the micropores produced. Joudkazis et al. demonstrated that debris generated during processing in a vacuum environment is more readily expelled, resulting in micropores with a reduced recast layer and enhanced hole precision [78]. In a vacuum setting, the propagation of energy and the ejection of plasma are optimized, as the laser beam experiences less interference from air ionization. Consequently, it is possible to reduce the ambient air pressure when creating micropores characterized by large depth-to-diameter ratios. In the study on the effects of water-assisted micropore processing through the contact of water with the lower surface of silicon wafers, Laakso et al. found that water-assisted ultrafast laser drilling effectively minimized the area of thermal damage [79]. Li et al. investigated the use of alcohol-assisted femtosecond laser processing for the fabrication of 6H-SiC micropores, revealing that the creation of micropores was more efficient under alcohol-assisted conditions compared to a vacuum environment [80].

2.1.2. Cutting

Ultrafast lasers are increasingly utilized not only for micropore machining, but also for micro-nano cutting applications. The advantages of ultrafast laser cutting include a reduction in cutting apparatus, decreased contamination, and the formation of circular contours on individual dies, which collectively enhance the mechanical strength of the dies [40]. Within the microelectronics industry, cutting technology is employed to separate integrated circuit chips from wafers. Historically, the industry has predominantly utilized wafers with thicknesses exceeding 150 μm, which were effectively processed using mechanical wear-cutting diamond-coated wire blades. However, as the demand for wafers measuring 50 μm or less continues to rise, these thinner wafers are susceptible to breakage due to the mechanical stresses induced by the wire cutting method. In contrast, ultrafast laser cutting, as a non-contact cutting technology, significantly mitigates the risk of damage to wafers during the cutting process. Research has shown that the quality of laser cutting is highly dependent on the pulse width [81]. When the pulse width exceeds one nanosecond, the quality of the cut is compromised due to thermal melting and the redeposition of the molten layer [82]. Therefore, it is essential to utilize ultrafast lasers for high-quality cutting, leveraging the minimal thermal effects associated with ultrafast processing. Additionally, the narrow kerf line produced by ultrafast dicing allows for the placement of a greater number of circuit chips on photolithographically processed wafers, thereby reducing the average cost per chip.

The cutting of highly hard materials, such as diamond, presents significant challenges that necessitate the application of unconventional methods. Ultrafast laser cutting has emerged as a promising technique for the efficient and precise cutting of high-hardness materials. Ogawa et al. demonstrated the efficacy of this method by processing a binder-free polycrystalline diamond micro milling cutter using a femtosecond laser [83]. The resulting tool molding process is illustrated in Figure 5a,b. The material removal rate achieved through femtosecond laser processing in this experiment reached a throughput of 0.004mm³/s, thereby satisfying industrial demands. For X-ray diamond lens processing, in a pioneering study, Polikarpov and his colleagues successfully fabricated single-crystal diamond planar refractive lenses by laser cutting on 300 μm thick diamond plates [84]. Subsequently, they employed a femtosecond laser to cut diamond and improved the technique to realize higher precision cutting of an X-ray planar compound refractive lens with a thickness of 600 μm [85], as depicted in Figure 5c, and the technical indexes are summarized in

Table 1. Technical indexes of femtosecond laser cutting.

Application of Cutting	Wavelength	Pulse Width	Repetition Rate	Pulse Energy	Scanning Speed
Ball end mill cutting [83]	1045 nm	350 fs	200 kHz	<50 μJ	>1 m/s
X-ray diamond lens cutting [85]	1030 nm	400 fs	200 kHz	20 W	0.1 mm/s

.

2.1.3. Surface Ablation

One of the most significant applications of ultrafast lasers is surface ablation, which has demonstrated remarkable precision and efficiency in surface processing with specific functions.

Superhydrophobic surfaces provide several benefits, including anti-icing, anti-corrosion, and self-cleaning properties, making them applicable across various fields. The surface energy of different materials is influenced by their chemical composition, and by modifying the parameters of laser processing, it is possible to construct micro- and nanostructures on the surface of specific materials, thereby achieving a superhydrophobic state directly. Yang et al. employed a femtosecond laser to etch micro-nanostructures on the surface of silicone rubber [86]. They discovered a method for preparing a superhydrophobic silicone rubber surface with high elasticity and controllable adhesion. This surface was capable of realizing the motion control of liquid droplets. Hong et al. employed a femtosecond laser to microprocess a ceramic surface [87]. The superhydrophobic ceramic surface prepared after polydimethylsiloxane (PDMS) modification exhibited excellent hydrophobicity and delayed icing ability. Su et al. employed femtosecond laser processing in conjunction with chemical surface modification to fabricate a novel hump-structured bifacial membrane on the surface of an aluminum foil [88], as illustrated in Figure 6. The prepared biomimetic hump bifacial membrane exhibited a higher mist water collection rate compared to the planar bifacial membrane. The bionic hump bifacial membrane demonstrated a 250% improvement in fog water collection efficiency compared to the planar bifacial membrane. Additionally, it was capable of capturing horizontally oriented droplets, offering a novel and effective solution for surface droplet collection. Song et al. constructed controllable triple-wettable surfaces based on the femtosecond laser direct-write technique [89], which could switch between the lotus leaf superhydrophobic effect, the rice leaf anisotropic effect, and the rose petal effect. This study represents a significant advancement beyond conventional double wettability, providing compelling evidence for the expansion of controllable wettable surfaces into new applications, including droplet control and transportation.

Another important area of research is the texturization of the surface of silicon materials using femtosecond lasers. Researchers have discovered that the efficiency of photovoltaic devices, such as solar cells, can be enhanced by the removal of a polycrystalline layer with a thickness of approximately 100 nanometers from the surface of solar cells manufactured from silicon [90]. In an optimization study, researchers discovered that silicon textured with a femtosecond laser in SF₆ could be utilized to manufacture thin-film solar cells due to its thinner absorber layer (approximately 20 μm), which is capable of absorbing a greater number of photons [91]. Consequently, the use of laser-textured solar cells enables the realization of enhanced light collection and solar cell energy conversion. This study also demonstrated that the absorption of laser-textured solar cells approaches the theoretical limit of solar cell absorption.

Laser-induced periodic surface structures (LIPSS) represent a method of forming micro- and nanoscale periodic structures through the laser processing of material surfaces. When a femtosecond laser radiates a material surface, the electric field of the laser interacts with the electrons in the material, resulting in the generation of periodic fluctuations in the electron density. These fluctuations subsequently influence the propagation and scattering of the laser within the material, ultimately resulting in the formation of periodic surface structures. Han et al. investigated the ultrafast dynamics of high-spatial-frequency LIPSS on a silicon surface by using a co-linear pumped probe imaging method [92]. Jiang et al. have successfully prepared large-area periodic nanowire structures in indium tin oxide (ITO) films by LIPSS and femtosecond laser direct writing [93]. These nanowires exhibit favorable electrical and optical properties. For the morphology of LIPSS, previous studies have demonstrated that the orientation of the periodic structures is typically parallel or perpendicular to the direction of laser polarization [94]. In contrast, Zhang et al. demonstrated that the limitation of laser polarization state imposed by the liquid vortex and flow generated by ablation in liquid can be overcome, resulting in the formation of circular and crisscross LIPSS [95]. This finding paves the way for the future development of diverse LIPSS morphologies through ultrafast laser ablation.

Conventional femtosecond laser ablation techniques are limited by the optical diffraction limit, which restricts their ability to form structures at the nanoscale. Lin et al. used a microsphere focusing technique to successfully fabricate high-resolution surface nanostructures on Sb₂S₃ thin films, achieving a smallest feature size of 30 nm [96]. This technique holds significant potential for future advancements in laser nanolithography.

Table 2. Technical indexes of femtosecond laser surface ablation.

Type of Surface Ablation	Wavelength	Pulse Width	Repetition Rate	Pulse Energy	Scanning Speed
Superhydrophobic silicone rubber surface [86]	800 nm	100 fs	1 kHz	136.2 J/cm2	2 mm/s
Superhydrophobic ceramic surface [87]	800 nm	100 fs	1 kHz	17–23 mW	1 mm/s
Fog water collection surface [88]	800 nm	104 fs	1 kHz	/	/
Triple wettable surface [89]	800 nm	104 fs	1 kHz	200 mW	30 mm/s
Surface-textured solar cells [90]	/	600 fs	/	8 kJ/m2	/
Thin-film textured silicon solar cells [91]	800 nm	/	/	4 kJ/m2	/
Large-area periodic nanowire structures [93]	1030 nm	190 fs	1 kHz	1 mJ	3 mm/s
Circular and crisscross LIPSS [95]	1045 nm	457 fs	100 kHz	50–700 mW	0.5–1 mm/s
Microsphere focusing ablation [96]	800 nm	/	76 MHz	0.38 mJ/cm2	100 μm/s

summarizes the technical indexes of femtosecond laser surface ablation.

2.1.4. Nano Welding

Ultrafast laser microstructure welding is a technique that involves melting the material interface and eliminating sample gaps through ultrafast laser–matter interaction. This is followed by the formation of a strong bonding effect between microstructures. In the field of welding micro- and nanostructures, the primary focus is on joining conductive materials. Huang et al. were among the first to demonstrate femtosecond laser welding of Ag microwire and Cu substrate [97]. Their findings highlighted the effectiveness of high pulse repetition frequency and low pulse energy in minimizing the formation of brittle intermetallic compounds and weld defects. Specifically, the use of high repetition frequencies and low pulse energy effectively reduced the formation of these undesirable compounds and defects, while also decreasing the ablation of metallic materials.

In the welding of conductive nanowires, femtosecond laser irradiation of silver nanowires also induces a localized plasma resonance, which generates localized high temperatures that can be used for nanowire joining, cutting, and remodeling. Furthermore, the nanowire structure outside the localized plasma resonance is not damaged by the high temperature because the overall temperature reaches equilibrium within 10⁻¹² s due to thermal diffusion. The researchers demonstrated the welding of Ag-Ag homogeneous metal nanowires [98], ZnO-ZnO homogeneous semiconductor nanowires [99], and Ag-TiO₂ heterogeneous nanowires [100], respectively, as shown in Figure 7a–e. Similarly, Lin et al. employed femtosecond laser irradiation of the junctions between Au nanowires and TiO₂ sinkers to induce localized plasma absorption enhancement at the junctions [61]. This resulted in the formation of junctions with high mechanical strength and high electrical conductivity, as illustrated in Figure 7f.

The technical indexes of femtosecond laser nano welding are shown in

Table 3. Technical indexes of femtosecond laser nano welding with 800 nm wavelength and 1 kHz repetition rate.

Type of Nano Welding	Pulse Width	Pulse Energy	Pulse Number /Irradiation Time
Ag nanowire and the Cu substrate [97]	35 fs	1.02 J/cm2	500–2000 pulses
Ag-Ag nanowires [98]	35 fs	~90 mJ/cm2	9–10 s
ZnO-ZnO nanowires [99]	35 fs	77.6 mJ/cm2	30 s
Ag-TiO2 nanowires [100]	50 fs	17.5 mJ/cm2	10 s
Au nanowires and TiO2 sinkers [61]	<50 fs	18.3–21.5 mJ/cm2	5 s

.

2.2. Femtosecond Laser Direct Writing

2.2.1. Optical Waveguide

Femtosecond Laser Direct Writing (FsLDW) is a technology that employs the use of ultrafast pulses and the ultra-intense transient energy of femtosecond lasers for the fabrication of micro- and nanostructures. The fundamental principle of this technology is the focusing of a femtosecond laser beam into the interior of a material. This process triggers nonlinear optical effects in the material and possesses a multitude of exceptional attributes, including processing precision that surpasses the diffraction limit, an extensive range of processable materials, and nonlinear multiphoton absorption, which confer upon it a distinctive edge in three-dimensional micro- and nanofabrication.

Davis et al. found that when a focused femtosecond laser is used to irradiate glass materials [44], such as pure silicon and germanium-doped silicon, the laser causes line damage on the surface or inside of the material, which will lead to an increase in the refractive index of the material in the damaged region. However, researchers have found that controlling laser parameters can improve the morphology of the damaged area, which can be used to guide the propagation of light. Subsequently, when a femtosecond laser irradiates materials, it usually causes the variation of the positive refractive index of glass or polymer and the negative refractive index change of crystal. Furthermore, Tan et al. enhanced the FsLDW technique by incorporating a simultaneous spatiotemporal focusing technique [101]. This resulted in the realization of true 3D isotropic micromachining and the resolution asymmetry observed in conventional FsLDW was overcome.

Accordingly, such characteristics therefore allow monolithic and sophisticated waveguide fabrication in a very flexible manner. Optical waveguides formed by ultrafast lasers in transparent dielectric materials are generally classified into the following two types according to the refractive index change: Type-I waveguides, which exhibit an increased refractive index, and Type-II waveguides, which exhibit a decreased refractive index.

In the case of Type-I optical waveguides, Nasu et al. prepared a two-dimensional waveguide with a length of 500 μm using a femtosecond laser in a planar light waveguide chip made of boron–phosphorus-doped glass [102], as shown in Figure 8a. The loss at each coupling point was only 0.1 dB, and the transmission loss of the waveguide was 0.34 dB/cm. In addition, they prepared a three-dimensional waveguide with a length of approximately 2 mm [103], as illustrated in Figure 8b. The additional losses of the transverse electric mode TE and the transverse magnetic mode TM at a wavelength of 1550 nm are 2.7 dB and 2.8 dB, respectively. Lindenmann et al. prepared photonic wires for optical interconnections in intra-chip or inter-chip configurations [104], as illustrated in Figure 8c. The average insertion loss of photonic wires in the C-band is 1.6 dB, and their typical dimensions are 1–2 μm. The spacing can be precisely controlled to within 5 μm. This method is well suited to the mass production of high-density photonic wires.

In the case of Type-II optical waveguides, researchers employed femtosecond lasers to fabricate bilinear waveguides within lithium tantalate (LiTaO₃) crystals [105]. However, it was observed that the waveguides could only guide the TM mode after annealing, and that the transmission loss was considerable, reaching approximately 10 dB/cm. He et al. prepared bilinear waveguides in potassium tantalum niobate (KTN) crystals, with a transmission loss of only 0.9 dB/cm for the 632.8 nm TE light [106]. In general, a Type-II waveguide can only guide the optical transmission of a single polarization mode, which presents certain limitations in interconnection.

An optical waveguide is composed of a low refractive index cladding layer and its wrapped high refractive index core layer. According to different types of refractive index changes in laser direct writing, femtosecond laser direct-write waveguides can be categorized into four types: directly written waveguides, stress-induced waveguides, depressed cladding waveguides, and ablated ridge waveguides [107], as illustrated in Figure 9.

The technical indexes among the different types of waveguides mentioned above are shown in

Table 4. Technical indexes of FsLDW.

Type of Waveguide	Wavelength	Pulse Width	Repetition Rate	Pulse Energy	Scanning Speed
Simultaneous spatiotemporal focusing FsLDW [101]	1030 nm	/	10 kHz	6.2–9.5 μJ	200 μm/s
2D waveguide [102]	775 nm	150 fs	1 kHz	182 nJ	10 μm/s
3D waveguide [103]	775 nm	150 fs	1 kHz	48 mW	/
Photonic wires [104]	780 nm	150 fs	100 MHz	/	/
LiTaO3 crystal cladding and dual-line waveguides [105]	800 nm	120 fs	1 kHz	2.9 μJ	20–500 μm/s
KTN crystal dual-line waveguides [106]	1030 nm	400 fs	25 kHz	1.40 μJ	500 μm/s

.

2.2.2. Nanodevices Based on Optical Waveguides

Currently, in the field of micro-nano photonics, optical waveguide-based nanodevices are mainly optical couplers and optical beam splitters.

As a pivotal element in the system, the optical waveguide coupler is designed to facilitate the transmission of optical signals from one waveguide to another. This enables the distribution, merging, and exchange of optical signals, thereby enhancing the overall functionality of the system. In terms of important results for optical couplers, Minoshima et al. prepared two-dimensional X-shaped and parallel waveguide couplers in glass using the FsLDW technique [108,109]. The splitting ratio could be flexibly regulated by adjusting the angle, length, or spacing of the waveguide. In a separate study, Suzuki et al. prepared a three-dimensional 3 × 3 directional couplers in glass, but due to the precision of the motion platform and the processing errors caused by other factors, the field energy in each branch of the coupler was not considered [110]. Skryabin et al. prepared planar 1 × 2, 2 × 2, and 3D 3 × 3 couplers in Tm³⁺:YAG crystals by using a femtosecond laser to fabricate a concave cladding waveguide [111], as shown in Figure 10. Pospiech et al. combined a femtosecond laser with a spatial light modulator to create a high-efficiency processing technique [112]. Marshall et al. also used a femtosecond laser direct-write method to prepare a directional coupler that can be used for optical quantum circuits [113,114]. Two key performance parameters of the coupler are the coupling ratio and insertion loss. The current ultrafast laser processing method enables precise control of the coupling ratio; however, further research is required to reduce the insertion loss.

Optical waveguide beam splitters represent a crucial component in the functionality of an optical chip. Its ability to distribute and process optical signals through the splitting of an input beam into multiple output beams makes it a highly versatile device. Furthermore, its capacity to create intricate optical path designs in a compact form has garnered significant interest within the fields of optical communication and photonics research. Homoelle et al. fabricated a Y-shaped beam splitter in pure quartz using a femtosecond laser, and the beam splitting ratio was close to 1:1 when the angle of the pinch angle was 0.5° [115]. For the deepening of this technology, researchers prepared Y-shaped waveguide beam splitters in lithium tantalate [116] and sapphire crystals (as illustrated in Figure 11a,b) [117], with beam splitting ratios approaching 1:1 and polarization-independent light-guiding characteristics. Liu et al. prepared 1 × 2, 1 × 4, and 1 × 8 multibeam beam splitters [118]. However, due to the instability of the laser power and pulse width, as well as the fluctuation of the sample motion velocity, the energy was not uniformly distributed in each branch. Lv et al. prepared 1 × 2 and 1 × 4 waveguide beam splitters in lithium niobate crystals [119]. Subsequently, they prepared uniformly split 1 × 2 and 1 × 3 waveguide beam splitters in lithium niobate crystals by using the method of preparing a Type-II waveguide [120], as illustrated in Figure 11c,d. Ajates et al. prepared waveguide beam splitters with different three-dimensional structures in lithium niobate crystals using a femtosecond laser [121]. They then compared the effects of each structure on the transmission loss and mode evolution, and developed a numerical model that can accurately predict the refractive index distribution generated by the femtosecond laser in the crystal. This is an important tool for optimizing the structure and designing new structures. In a broad sense, the demultiplexer is also defined as a beam splitter, which is used to separate different wavelengths of input light. Liu et al. fabricated a demultiplexer capable of separating the wavelengths of 532 nm and 1064 nm in YAG crystals by the FsLDW technique [122], and the separation effect is insensitive to polarization. The creation of these nanodevices has illustrated the potential of FsLDW technique for utilization in micro-nano photonics.

Table 5. Technical indexes of FsLDW for nanodevices.

Type of FsLDW Nanodevices	Wavelength	Pulse Width	Repetition Rate	Pulse Energy	Scanning Speed
X-shaped waveguide coupler [108]	800 nm	80 fs	4 MHz	20 nJ	10 mm/s
Directional waveguide coupler [109]	400 nm	25 fs	80 MHz	2.8 nJ	3–10 μm/s
3 × 3 directional coupler [110]	/	67 fs	5.85 MHz	200 mW	8 mm/s
2 × 2, 1 × 2 and 3 × 3 directional couplers [111]	1030 nm	240 fs	1 MHz	450 nJ	1 mm/s
Waveguide coupler created by SLM [112]	/	420 fs	1 MHz	1.6 μJ	0.1–1 mm/s
Quantum directional coupler [113]	800 nm	120 fs	1 kHz	/	/
Y beam splitter in silica [115]	820 nm	60 fs	1 kHz	1 μJ	30 μm/s
1:1 beam splitters in lithium tantalate crystal [116]	795 nm	120 fs	1 kHz	0.4 μJ	0.5 mm/s
1:1 beam splitters in sapphire crystal [117]	795 nm	120 fs	1 kHz	1.2 μJ	0.5 mm/s
1 × 2, 1 × 4, and 1 × 8 beam splitters [118]	800 nm	125 fs	150 kHz	52 mW	50 μm/s
1 × 2 and 1 × 4 beam splitters in lithium niobate crystals [119]	1031 nm	420 fs	5 kHz	4.9 μJ	4 mm/s
Uniformly split 1 × 2 and 1 × 3 beam splitters in lithium niobate crystals [120]	795 nm	120 fs	1 kHz	2 μJ	750 μm/s
3D waveguide beam splitters [121]	795 nm	~120 fs	1 kHz	1.9 μJ	0.35 mm/s
Polarization-insensitive demultiplexer [122]	800 nm	90 fs	1 kHz	0.28 μJ	0.6 mm/s

shows the technical indexes of FsLDW for nanodevices.

2.2.3. SPIDER

Segmented Planar Imaging Detector for Electro-Reconnaissance (SPIDER) represents an advanced interferometric imaging system that integrates lightweight electro-optical components with sophisticated signal processing techniques to generate high-resolution images of extended scenes [123,124,125]. The system employs a two-aperture geometrical configuration, as depicted in Figure 12a [126]. The operational mode of the photonic integrated interference imaging system is illustrated in the schematic diagrams presented in Figure 12c–e. In this mode, the microlens array captures the spatial optical signal from the target and couples it to the photonic integrated circuit (PIC). The optical signal is then transmitted through the optical waveguide on the PIC, where it is split and multiplexed by the array waveguide grating. Subsequently, the optical phase modulator adjusts the two optical waves to satisfy the conditions necessary for interference. Following the output of coherent light from the PIC, the signal enters the 90° optical mixer and balanced detector, facilitating for light wave interference and signal measurement, The target image is ultimately reconstructed through digital signal processing [127,128]. Furthermore, in 2013, Richard et al. proposed a model for the SPIDER payload integrated with a spacecraft bus, thereby establishing a foundational framework for both domestic and international scholars interested in pursuing further research in this area [129].

To further investigate the reconstruction image quality of the SPIDER system, researchers both domestically and internationally have conducted extensive studies. In 2013, Richard L. et al. presented findings on wide-field-of-view (FOV) SPIDER imaging, using waveguide arrays positioned behind each lenslet. The two-satellite scene, measuring 12 m × 30 m, exceeded the FOV_tile of 20 m. These tile images encompassed different portions of the scene, which were subsequently stitched together to create a wide-FOV SPIDER mosaic image, as illustrated in Figure 13a [129]. Four years later, the same research group developed a testing apparatus to simulate the far-field imaging capabilities of SPIDER, as shown in Figure 13b. The reconstructed image closely approximated the target, demonstrating a significant improvement in image quality compared to the direct inverse fast Fourier transform of the test data [130]. Furtherly, the same authors revised the previous optical testbed in 2018 to generate and reconstruct a two-dimensional extended scene following phase error correction; the reconstructed image presented in Figure 13c closely resembles the simulated image [131].

In 2021, Zhang et al. proposed a novel lenslet array arrangement and a lenslet pairing approach, as shown in Figure 14a. This innovative lenslet array configuration incorporates additional lenslet arrays oriented in various directions, distinguishing it from the existing radial lenslet spokes [132]. The image restoration and reconstruction simulation results, presented in Figure 14, demonstrate that the method discussed in this paper significantly enhances spatial frequency coverage and positively impacts the quality of directly restored images. Furthermore, image reconstruction based on compressive sensing theory has been shown to improve the quality of interferometric imaging [130,133]. Additionally, an alternative approach for achieving superior image restoration quality has been introduced by Song et al., who employed a multimode interference (MMI) coupler in place of an orthogonal detector. They proposed a three-point configuration method to arrange the lenslet and calculate spatial spectrum values from the output currents [128]. The first row of Figure 14b illustrates the lenslet arrangement, while the second row of Figure 14b indicates that the MMI-SPIDER system provides more comprehensive coverage of the spatial spectrum compared to the traditional SPIDER system, resulting in enhanced image quality within the MMI-SPIDER imaging framework.

2.3. Nano Conductive Structure Processing

The incorporation of nano conductive structures into the design of integrated circuits, displays, and memory devices has become a crucial aspect of modern technology. These structures offer conductive properties that are discernible at the micron and nanometer scales, enabling significant advancements in the performance and functionality of these electronic components. Moreover, the potential of nano conductive structures has been recognized in the context of lithium-ion batteries and supercapacitors as well. Their incorporation into these energy storage devices can enhance the conductivity and energy density of the electrode material, ultimately improving the overall performance and efficiency of the system. Additionally, their use in highly sensitive sensors for detecting minute physical or chemical changes holds immense promise in a multitude of applications. Currently, the main methods for the preparation of nano conductive structures using ultrafast lasers are photodynamic organization, multiphoton reduction, and femtosecond laser sintering.

For the photodynamic assembly method, Wang et al. demonstrated the self-assembly of silver nanoparticles by a tightly focused femtosecond laser [134]. The photodynamic organization method refers to the processing of nanostructures in a highly concentrated metal colloid by using light-driven force to trap the metal seeds. The assembly of silver particles with an average diameter of approximately 2.5 nm was observed at the center of the laser focus. These particles were subsequently sintered onto the substrate by the laser. The processing resolution of this method exceeded the optical diffraction limit of 190 nm, allowing for the realization of different electrode patterns on the substrate by changing the path of the femtosecond laser. This is illustrated in Figure 15.

For the multiphoton reduction method, it primarily employs the interaction of a femtosecond laser with ultrashort pulse width and high peak power with the antecedent solution to reduce metal ions to metal atoms. Tanaka et al. prepared silver nanoparticles in silver nitrate solution using the multiphoton reduction method with a processing resolution of 1.02 μm, and the particles stacked up to form a nanowire, thereby establishing interconnection between the electrodes [135]. The measured resistivity of the silver wire was 5.30 × 10⁻⁸ Ω·m, which is only 3.3 times that of silver.

The femtosecond laser sintering method is employed to prepare two-dimensional materials through the intense laser excitation of metal nanoparticles by the plasma resonance effect. Silver and copper are among the most commonly chosen materials for the ultrashort pulse sintering method due to their good electrical conductivity. Huang et al. utilized a femtosecond laser with a frequency of 76 MHz to sinter copper electrodes reductively on a polyimide thin film [136]. The scanning speed was optimized to yield copper wires with a width of 5.5 μm and a porosity of 9.89%, with the purity of the copper wires reaching 91.42% and the resistivity being approximately 1.3 × 10⁻⁷ Ω·m.

The technical indexes of nano conductive structures by femtosecond laser processing are summarized in

Table 6. Technical indexes of nano conductive structures by femtosecond laser processing.

Type of Nano Conductive Structure Processing	Wavelength	Pulse Width	Repetition Rate	Pulse Energy	Scanning Speed
Photodynamic organization [134]	800 nm	120 fs	/	/	/
Multiphoton reduction [135]	800 nm	80 fs	80 MHz	18.8–29.8 mW	/
Femtosecond laser sintering [136]	1030 nm	100 fs	76 MHz	0.17 nJ	5 mm/s

.

2.4. Processing and Applications of Microlens Array

2.4.1. Processing

Microlens arrays are a type of optical element consisting of multiple tiny lens units in a specific arrangement. They offer several advantages, including miniaturization, lightweight construction, arraying, and integration, and are widely used in various optical systems. By adjusting the shape, focal length, and arrangement of each microlens, microlens arrays can achieve specific optical functions, such as beam shaping, beam homogenization, and fiber coupling. Microlens arrays are most commonly employed to enhance the light collection efficiency of charge-coupled device (CCD) arrays. They collect and focus light that would otherwise fall within the non-sensitive area of the CCD. In 2009, Wu et al. utilized femtosecond laser two-photon polymerization direct-write technology to fabricate aspheric microlenses and lens arrays with 100% filling factor [137]; as illustrated in Figure 16a,b, the relative error of the lens profile was less than 0.2%. This work demonstrates that ultrafast laser technology can be employed to fabricate microlens arrays with high precision, simplicity, and speed. Currently, ultrafast laser microlens array processing can be classified into three methods: two-photon polymerization, wet etch-assisted ultrafast laser ablation, and dry etch-assisted ultrafast laser ablation.

Figure 15. Schematic of self-assembly of silver nanoparticles. Reprinted with permission from [138]. Copyright (2015) IOP Publishing.

Figure 15. Schematic of self-assembly of silver nanoparticles. Reprinted with permission from [138]. Copyright (2015) IOP Publishing.

In recent years, a series of microlens arrays with excellent technical specifications have been designed and fabricated. Lin et al. employed two-photon polymerization to fabricate a three-dimensional curved compound eye structure comprising an array of microlenses by point-by-point photopolymerization of a femtosecond laser on a photoresist surface [138]. Theoretically, it can achieve a field of view angle of 107.48° × 97.97°. Liu et al. present a method for the rapid preparation of ultra-smooth sapphire concave microlens arrays by dry etch-assisted femtosecond laser processing [139]. The technique involves femtosecond laser modification and subsequent inductively coupled plasma etching. Atomic force microscopy measurements revealed that the surface roughness of the microlens arrays was approximately 1.1 nm. Qin et al. employed a time-shaped femtosecond laser to facilitate chemical etching, thereby enabling the fabrication of microlenses, as illustrated in Figure 16 [140]. A 35 fs, 800 nm wavelength femtosecond laser system was employed to modify the laser pulse into a time-delayed pulse train, consisting of two sub-pulses combined and focused on the sample by a Mach–Zehnder interferometer. The sample was then treated with an HF solution. By controlling the pulse delay, microlens arrays with NA in the range of 0.1 to 0.65 can be fabricated.

Figure 16. SEM images of the morphology of a microlens array. (a) Morphology of a square microlens array. (b) Morphology of a hexagon microlens array. (c) Enlarged view of a part of (a), and (d) enlarged view of part (b). Dotted boxes of the same color represent local magnification [140]. Copyright (2021) optica publishing group.

Figure 16. SEM images of the morphology of a microlens array. (a) Morphology of a square microlens array. (b) Morphology of a hexagon microlens array. (c) Enlarged view of a part of (a), and (d) enlarged view of part (b). Dotted boxes of the same color represent local magnification [140]. Copyright (2021) optica publishing group.

Traditional microlens arrays are typically characterized by a fixed focal length. However, in recent years, the development of intelligent devices has led to a need for greater flexibility and the potential for extended applications. Tunable microlens arrays have emerged as a solution to these challenges, offering enhanced functionality and versatility. This has led to a surge in research interest in this area. Ma et al. employed a novel approach to fabricate tunable microlens arrays based on bovine serum albumin [141]. They utilized the programmable 3D processing capabilities of FsLDW technology to directly create these arrays. This microlens array is capable of realizing an adjustable field of view (35–80°) and variable focal length through the expansion and contraction effects of albumin under different pH conditions. Hu et al. employed femtosecond laser-assisted etching to fabricate integrated microlens arrays within three-dimensional glass microfluidic channels [142]. By introducing fluids with varying refractive indices into the microchip channel, the focal length and focal spot size can be continuously adjusted.

The technical indexes of different kinds of microlens arrays fabricated by femtosecond laser processing are illustrated in

Table 7. Technical indexes of microlens array femtosecond laser processing.

Type of Microlens Arrays	Wavelength	Pulse Width	Repetition Rate	Pulse Energy	Irradiation Time
100% fill-factor microlens arrays [137]	790 nm	120 fs	80 MHz	6 mW	/
3D curved compound-eye structure [138]	800 nm	100 fs	80 MHz	48 mW	/
Sapphire concave microlens arrays [139]	343 nm	290 fs	200 kHz	8 μJ	3.2 h
Chemical processing [140]	800 nm	35 fs	1 kHz	1.5	2 h
Tunable albumin microlens arrays [141]	800 nm	120 fs	80 MHz	7–20 mW	500–1000 μs/voxel
Tunable microfluidic microlens arrays [142]	1045 nm	360 fs	200 kHz	5 mW	65 s/microlens

.

2.4.2. Application: Snapshot Hyperspectral Imaging

One of the important applications of microlens arrays is snapshot hyperspectral imaging. A snapshot light-field imaging spectrometer based on a microlens array was developed by Su et al. [143]. The system is capable of capturing spatial-spectral data cubes in a single exposure without a scanning process. The study demonstrates that the microlens array has significant advantages in optical efficiency and resolution over pinhole arrays. Jason constructed a snapshot spectrometer based on a microlens array, which enabled the average spectral resolution to be adjusted from 22.66 nm (80 × 80 × 22) to 13.94 nm (88 × 88 × 46) [144]. This system has a wavelength range of 485 to 660 nm and provides high-resolution hyperspectral fluorescence imaging in less than one second of the exposure time. Yu et al. proposed a microlens array snapshot hyperspectral microscope system for biomedical applications [145]. The system is capable of the rapid acquisition of three-dimensional data cubes at a detector frame rate with a spatial resolution of 2.5 μm and 180 channels of 100 nm spectral sampling in the spectral range of 400–800 nm with a spectral resolution of approximately 0.56 nm. Combined with commercial camera lenses, Zhang et al. propose a low-cost, small-volume, portable snapshot imaging spectrometer device that can simultaneously acquire both spectral and spatial information in a single “snapshot” [146]. As shown in Figure 17, the core element is an array of 60 × 60 microlenses, each with an aperture of 125 μm × 125 μm. The spectrometer has a resolution of approximately 10 nm, while a data cube measuring 21 × 29 × 40 is generated within the 400–800 nm wavelength range.

3. Imaging and Precision Measurement

3.1. Remote Sensing

Two distinct categories of remote sensing techniques that employ ultrafast lasers have been identified: the remote detection of atmospheric conditions utilizing femtosecond laser pulse filamentation, and long-range imaging achieved through time-of-flight measurements.

The propagation of femtosecond laser pulses within the atmosphere leads to the formation of high-intensity, high-plasma-density filaments [147]. This phenomenon can be attributed to the interaction between nonlinear light intensity effects and plasma dynamics. These filaments are capable of propagating over considerable distances in the atmosphere, generating intense spectral signals that can be employed for the detection of atmospheric constituents. The interaction between optical filaments and atmospheric molecules results in the generation of fluorescence spectral lines, which exhibit distinct spectral fingerprints. These fingerprints can be utilized to identify and quantify various atmospheric components [148]. Moreover, light filaments can produce supercontinuum spectral lasers that encompass the atmospheric optical transmission window, thereby facilitating the detection of multiple atmospheric components. Kasparian et al. have introduced an innovative ground-based femtosecond intense laser remote sensing system [149]. This system, which sends a 100 fs 3-TW laser pulse into the atmosphere, employs filament supercontinuum lidar technology for the detection of atmospheric environmental parameters, including atmospheric composition, temperature, humidity, and even wind speed. The experimental setup and measurement results of the filament-based supercontinuum laser for atmospheric environment detection over distances ranging from several kilometers to tens of kilometers are illustrated in Figure 18. Subsequent investigations by the research team have confirmed that the system is also capable of detecting additional data, such as particle size and density, temperature, and relative humidity within cloud formations [150].

Recent advancements in single-photon detection technology have significantly enhanced the capabilities of time-of-flight-based ultrafast laser remote sensing imaging. By employing an ultrafast laser to scan the target point-by-point, it becomes apparent that at considerable distances, the absorption and scattering of the echo by the atmosphere can diminish the intensity of the returned light to levels comparable to a single photon. This necessitates the utilization of single-photon detectors for detection purposes. Single-photon detection technology not only provides remarkable sensitivity to single photons but also achieves picosecond time resolution, thereby offering superior detection performance and improved distance resolution in remote sensing imaging. Building upon this technology, Li et al. advanced the test system and optimized single-photon detector performance, successfully implementing ultrafast laser remote sensing imaging at distances of 45 km [151] and 201.5 km [62], as illustrated in Figure 19a,b. Additionally, Zeng’s team further improved the single-photon detection capacity of the ultrafast laser remote sensing imaging system by incorporating a single-pixel camera, enabling the detection of cooperative targets at 100 km and non-cooperative targets at 3 km [152]. Different technical indexes of ultrafast laser remote sensing are shown in

Table 8. Technical indexes of ultrafast laser remote sensing.

Application	Wavelength	Pulse Width	Repetition Rate	Pulse Energy
Gas composition detection [149]	800 nm	70 fs	10 Hz	350 mJ
45 km remote sensing imaging [151]	1550 nm	500 ps	100 kHz	120 mW
201.5 km remote sensing imaging [62]	1550 nm	600 ps	500 kHz	600 mW
Sensing by a single-pixel camera [152]	532 nm	100 ps	10 kHz	/

.

3.2. Microscopic Imaging

Optical microscopes enable the exploration of the microscopic world. However, the diffraction limit restricts conventional optical microscopes from achieving resolution on the order of wavelengths. Given the increasing research demands in materials science and nanoscience, there is a critical need for the development of microscopic imaging techniques that provide both high spatial and temporal resolution. The emergence of ultrafast lasers presents a promising approach to overcoming these limitations. The femtosecond pump–probe technique represents a significant advancement in ultrafast time-resolved measurement techniques. The technique employs two femtosecond pulsed lasers with a time delay. The higher-energy, time-advanced pulse serves as the pump light for excitation of the sample, while the lower-energy, time-advanced pulse serves as the probe light for detecting the dynamics of the sample after excitation. By modifying the time delay of the probe light, it is possible to record the response of the sample at different time points, thereby obtaining time-resolved information about the sample.

Researchers have integrated femtosecond pump-probe technique with other techniques to develop a range of microimaging techniques that exceed the diffraction-limited resolution. Zhou et al. observed the spatiotemporal evolution of textures on gold films by using a high-numerical-aperture microscope and introducing a femtosecond pump–probe technique, thereby achieving microimaging with a spatial resolution of 300 nm and a temporal resolution of 1 ps [57], as shown in Figure 20. Massaro et al. proposed a novel structured light pump–probe microscope and used it to study the carrier dynamics of silicon nanowires [153]. They employed pump light with a central wavelength of 800 nm to excite the carrier motion of silicon nanowires and used the frequency-doubled 400 nm light as the probe light, reconstructing three images with different initial phases. The spatial resolution of the imaging reached 114 nm, exceeding the diffraction limit. Xu et al. combined the femtosecond pump–probe technique with Fourier transform contouring based on single-frame structured light streaks and proposed and realized a wide-field spatiotemporal microscope with single-pulse structured light probing without multi-step phase shifting [154]. This microscope utilizes the Fourier transform contouring algorithm to reconstruct three-dimensional height information on the sample surface with only one raw image of the structured light illumination at the time of imaging. Ultrafast evolutionary imaging of the light-induced abrupt field in carbon nanotubes was first proposed and realized by Barwick et al. [155]. By energy filtering these modulated pulsed electrons, the near-field electric field distribution can be directly imaged in space. The electric field dynamics processes on the femtosecond time scale are then obtained by modulating the delay time of the pump–probe pulse. Additionally, researchers combined the ultrahigh temporal resolution of femtosecond lasers with the ultrahigh spatial resolution of photoemission electron microscopy, thereby achieving simultaneous ultrahigh temporal and spatial resolution capabilities [156]. The temporal resolution is determined by an ultrafast laser system, while the spatial resolution is determined by an electron microscope system, which can reach 10 nm in a phase contrast-corrected system. In addition to these, some researchers have also combined ultrafast laser with near-field scanning optical microscopes [157], atomic force microscopes [158], and scanning tunneling microscopes [159], respectively, in order to achieve femtosecond-level temporal resolution and spatial super-resolution. These studies provide novel tools and methodologies for elucidating and regulating the dynamical behavior of micro- and nanoscale matter.

Table 9. Technical indexes of femtosecond laser-based microscope.

Type of Microscope	Wavelength	Pulse Width	Repetition Rate	Pulse Energy
High-NA microscope [57]	800 nm	50 fs	100 Hz	3.5 mJ
Structured light pump–probe microscope [153]	800 nm	90 fs	79 MHz	23 pJ (after conversion)
Fourier-transform wide-field spatiotemporal microscope [154]	800 nm	120 fs	10 Hz	3 mJ
Photon-induced near-field electron microscopy [155]	1038 nm	220 fs	500 kHz	14 mJ/cm2

shows the technical indexes of femtosecond laser-based microscopes.

3.3. Spectral Analysis

Recent studies have highlighted the significant potential of femtosecond laser spectroscopy, particularly femtosecond laser-induced breakdown spectroscopy (LIBS) [160]. This approach offers numerous advantages over traditional spectral measurement techniques. It is applicable to a diverse range of substances, including solids, liquids, and gasses, and can analyze multiple elements simultaneously, making it particularly advantageous for complex sample analysis [161]. Consequently, femtosecond laser-induced breakdown spectroscopy (fs-LIBS) has found applications across various fields, including environmental monitoring, industrial production, and biomedicine. The use of femtosecond lasers in LIBS techniques provides several benefits, including an improved signal-to-noise ratio, accelerated detection speeds, enhanced accuracy, and superior molecular detection capabilities.

In recent years, fs-LIBS has demonstrated extensive applicability and significant potential across various fields. By utilizing ultrashort laser pulses, researchers have achieved high-precision analysis and detection of different materials. The following sections will highlight several representative studies, showcasing the applications of fs-LIBS in gas composition detection, elemental and isotopic analysis, thin film thickness measurement, and biological research. Kotzagianni et al. utilized a 100 fs laser pulse to ablate a mixed gas flame of air and methane [162], as shown in Figure 21. Spectral intensity was found to exhibit a linear relationship with methane content, indicating that fs-LIBS was capable of directly measuring the air-to-methane equivalence ratio. This discovery offers a novel method for the detection of gas composition. In terms of elemental and isotopic detection, Elhassan et al. ablated coins using 500 fs laser pulses to quantitatively analyze the elements contained therein and demonstrated the potential application of fs-LIBS in currency detection [163]. Additionally, Hou et al. employed femtosecond laser ablation molecular isotope spectroscopy to detect the isotopic spectra of zirconium (Zr), thereby demonstrating the effectiveness of the technique for accurate measurements [164]. Banerjee et al. utilized 170 fs of ultraviolet laser pulses to probe the thickness of a thin layer of chromium on a silicon substrate, thereby achieving a high degree of lateral resolution with high accuracy [165]. In the field of biology, Baudel et al. successfully employed fs-LIBS to detect trace minerals in various bacteria and to visualize the distinctions between strains of bacteria through the multidimensional space established by elemental concentrations [166]. This demonstrated the advantages of the technique in distinguishing between different strains of bacteria. Collectively, these research results illustrate the extensive applicability and significant potential of femtosecond laser spectroscopy in numerous fields.

The technical indexes of fs-LIBS are shown in

Table 10. Technical indexes of femtosecond laser-induced breakdown spectroscopy.

Application of fs-LIBS	Wavelength	Pulse Width	Repetition Rate	Pulse Energy
Gas composition detection [162]	800 nm	100 fs	10 Hz	5.2 mJ
Element detection [163]	248 nm	500 fs	/	13.5 mJ
Isotope analysis [164]	343 nm	500 fs	1 kHz	160 μJ
Accurate thickness measurement [165]	266 nm	170 fs	1 kHz	/
Microbiological discrimination [166]	810 nm	120 fs	/	3.8 mJ

.

4. Conclusions and Prospect

Over the past few years, ultrafast laser processing technology has made significant advances, becoming an important tool in the field of micro- and nanofabrication. The unique properties of ultrafast lasers, which include extremely short pulse widths and very high peak powers, have led to superior performance in micro- and nanofabrication, including high precision, low heat-affected zones, and high material adaptability. Ultrafast laser direct-write processing of optical waveguides represents a powerful fabrication tool for fields such as integrated photonics and optical communications. Ultrafast laser-prepared two-dimensional structures and microlens arrays open up new possibilities in fields such as optoelectronics and optical imaging. The use of ultrafast lasers in applications such as remote sensing, microscopic imaging, and spectroscopy offers new perspectives and tools for fields such as environmental monitoring, biomedicine, and materials science.

As ultrafast laser technology continues to develop, we will witness an increase in the number of innovative applications. In terms of processing technology, the improvement of processing accuracy, processing efficiency, and processing range will be a significant area of research. In addition, the challenge of integrating ultrafast laser technology with other technologies, such as microelectronics, nanotechnology, and biotechnology, to achieve higher-level system integration will also be a significant issue in the future. Ultrafast laser processing technology is poised to realize a multitude of possibilities and opportunities in the future. It is anticipated that further breakthroughs and innovations will be made in future research and applications.